NDT31-Eddy Current Testing (ET) [PDF]

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Eddy Current Testing (ET) NDT31

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Eddy Current Testing (ET) NDT31 Contents Section

Subject

Preliminary pages Contents Standards and Associated Reading COSHH, H&S, Cautions and Warnings Introduction to NDT Methods NDT Certification Schemes

1

1.1 1.2 1.3 1.4

2

2.1 2.2 2.3 2.4

3

3.1 3.2 3.3 3.4 3.5 3.6

4

4.1 4.2

5

5.1

6

6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14

NDT31-50316b Contents

Introduction

The SI units of measurement History of eddy current testing Definition of non-destructive testing (NDT) Choice of method

Principles

Electricity Magnetism Alternating current theory Eddy currents

Equipment

Circuits Simple circuits Instruments Adjustments Probes Calibration blocks

Practices

Documentation Applications

AC Theory

Capacitive reactance

Phase Analysis

Signal/noise separation Phase analysis Idealised impedance diagram Normalised impedance Conductivity Magnetic permeability Thickness Frequency Probe diameter Characteristic parameter Characteristic frequency Skin effect Phase discrimination Suppression of undersired effects

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6.15

7

7.1 7.2 7.3 7.4 7.5

8

8.1 8.2 8.3 8.4 8.5 8.6

9

Multifrequency testing

Instrumentation

Cathode ray oscilloscopes Send-receive coils Hall effect probes Dynamic testing Frequency response

Material Sorting

Conductivity meters Conductivity effects Electromagnetic sorting bridges Bridge sorting variables Automatic gates Standards

Crack Detection

9.1 9.2 9.3 9.4 9.5

Universal crack detectors Surface coils Crack detection Weld testing Rotating probes

10

Tube Testing

11

Eddy Current for Welding Inspection

10.1 10.2 10.3 10.4 10.5 10.6 10.7 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 11.10 11.11 11.12 11.13 11.14

Manufactured tube testing Condenser tube inspection Probes Test frequency Coil size Signal patterns Reference standards Introduction Eddy current application overview Basic eddy current theory Generation of eddy currents Principles governing the generation of eddy currents Fundametal properties of eddy current flow Electrical circuits and probe impedance Resistance and reactance Inductive reactance Capactive reactance Impedance Inductance (L) Eddy current weld testing Probe/coil arrangements

Appendix 1 Appendix 2 Appendix 3

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Preface These notes are provided as training reference material and to meet the study requirements for examination on the NDT course to which they relate. They do not form an authoritative document, nor should they be used as a reference for NDT inspection or used as the basis for decision making on NDT matters. The standards listed are correct at time of printing and should be consulted for technical matters. NOTE: These training notes are not subject to amendment after issue.

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Standards and Associated Reading EN ISO 1330-1

Non Destructive Testing – Terminology Part 1: List of general terms

EN ISO 1330-2

Non Destructive Testing – Terminology Part 2: Terms common to NDT methods

EN ISO 12718

Non Destructive Testing – Eddy Current Testing - Terminology

EN ISO 15549

Non Destructive Testing – Eddy Current Testing – General Principles

EN ISO 15548-1

Non Destructive Testing – Equipment for Eddy Current examination – Part 1 – Instrument characteristics and verification

EN ISO 15548-2

Non Destructive Testing – Equipment for Eddy Current examination – Part 2 Probe characteristics and verification

EN ISO 15548-3

Non Destructive Testing –Equipment for Eddy Current Examination– Part 3 System characteristics and verification

EN ISO 17643

Non Destructive Examination of Welds – Eddy Current examination of welds by complex plane analysis

EN ISO 17635

Non-destructive testing of welds. General rules for metallic materials

M 38

Guide to compilation of instructions and reports for the inservice and non-destructive testing of aerospace products

ISO 27831-1

Metallic and other inorganic coatings – cleaning and preparation of metallic surfaces. Part 1. Ferrous Metals and alloys

ISO 27831-2

Metallic and other inorganic coatings – cleaning and preparation of metallic surfaces. Part 1. Non-Ferrous Metals and alloys

ISO 9712

Non-destructive testing. Qualification and certification of personnel

EN 4179

Aerospace series. Qualification and approval of personnel for non-destructive testing

Special Techniques EN ISO 2360

Non-conductive coatings on non-magnetic electrically conductive basis materials – measurement of coating thickness: Amplitude sensitive eddy-current method

EN ISO 21968

Non-magnetic metallic coatings on metallic and non-metallic basis materials – measurement of coating thickness: Phase sensitive eddy-current method

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Aerospace prEN 2002

Aerospace Series – Test methods for metallic materials – Part 20: Eddy Current test on pipes under pressure

Welds EN ISO 17643

Non-destructive examination of welds – Eddy current examination of welds by complex plane analysis

Tubes & Pipes EN 1971

Copper & Copper Alloys – Eddy current test for measuring defects on seamless round copper and copper ally tubes – Part 1: Testing with an encircling coil on the outer surface – Part 2: Test with an internal probe on the inner surface

EN ISO 10893

NDT of Steel Tubes – Part 1: Automated electromagnetic testing of seamless and welded (except submerged arc-welded) steel tubes for the verification of hydraulic leak tightness – Part 2: Automated eddy current testing of seamless and welded (except submerged arc-welded) steel tubes for the detection of imperfections

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Associated Reading IAEA. Training Course Series 48. Training Guidelines for Non Destructive Testing Techniques: Eddy Current Testing at Level 2. http://www.ndted.org/EducationResources/CommunityCollege/EddyCurrents/cc_ec_index.htm 

Mathematics and Formulae in NDT. Edited by Dr. R Halmshaw. Obtainable from the British Institute of Non-Destructive Testing

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COSHH, H&S, Caution and Warnings Relevant to TWI Training & Examination Services Introduction The use of chemicals in NDT is regulated by law under the Control of Substances Hazardous to Health (COSHH) Regulations 2005. These regulations require the School to assess and control the risk of health damage from every kind of substance used in training. Students are also required by the law to co-operate with the School’s risk management efforts and to comply with the control measures adopted. Hazard Data Sheets The School holds Manufacturers Safety Data Sheets for every substance in use. Copies are readily available for students to read before using any product. The Data Sheets contain information on:       

The trade name of the product; eg Magnaglo, Ardrox, etc. Hazardous ingredients of the products. The effect of those ingredients on peoples health. The hazard category of the substance; eg irritant, harmful, corrosive or toxic, etc. Special precautions for use; eg the correct Personal Protective Equipment (PPE) to wear. Instructions for First Aid. Advice on disposal.

EH40 – Occupational Exposure Limits What is Exposure? 

Exposure to a substance is uptake into the body. The exposure routes are by:



Breathing fume, dust, gas or mist. Skin contact. Injection into the skin. Swallowing.

  

Many thousands of substances are used at work but only about 500 substances have Workplace Exposure Limits (WELs). Until 2005 it had been normal for HSE to publish a new edition of EH40, or at least an amendment, each year. However with increasing use of the website facilities the HSE no longer always publishes a revised hardcopy edition, or amendment. The web based list which became applicable from 1st October 2007 can now be found at http://www.hse.gov.uk/coshh/table1.pdf

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Introduction to Non-Destructive Testing Non-destructive testing (NDT) is the ability to examine a material (usually for discontinuities) without degrading it or permanently altering the article being tested, as opposed to destructive testing which renders the product virtually useless after testing. Other advantages of NDT over destructive testing are that every item can be examined with no adverse consequences, materials can be examined for conditions internally and at the surface and, most importantly, parts can be examined whilst in service, giving a good balance between cost effectiveness and quality control. NDT is used in almost every industry with the majority of applications coming from the aerospace, power generation, automotive, rail, oil & gas, petrochemical and pipeline markets, safety being the main priority of these industries. When properly applied, NDT saves money, time, materials and lives. NDT as it is known today has been developing since around the 1920s, with the methods used today taking shape later and vast technological advancements being made during the Second World War. The basic principal methods are:      

Visual testing (VT). Penetrant testing (PT). Magnetic particle testing (MT). Eddy current testing (ET). Ultrasonic testing (UT). Radiographic testing (RT).

In all NDT methods, the interpretation of results is critical. Much depends on the skill and experience of the technician, although properly formulated test techniques and procedures will improve accuracy and consistency. Visual testing (VT) With sufficient lighting and access, visual techniques provide simple, rapid methods of testing whilst also being the least expensive. Close visual testing (CVT) refers to viewing directly with the eye (with or without magnification) whereas remote visual inspection (RVI) refers to the use of optical devices such as the boroscope and the fibrescope. Visual testing begins with the eye; however, the first boroscopes used a hollow tube and a mirror with a small lamp at the end to investigate the bores of rifles and cannons for problems and discontinuities. In the 1950s, the lamps were replaced by glass fibre bundles which were used to transmit the light. These became known as fibrescopes which were also less rigid, increasing the capabilities of testing. With usage expanding, many users began to suffer from eye fatigue which led to the development of video technology. This was first used in the 1970s and relies on electronics to transmit the images rather than fibreoptics. Further enhancements to video technology include pan, tilt and zoom lenses, and mounting cameras to platforms and wheels, all allowing more parts to be tested and better images for improved inspection. Video devices also allow recordings of inspections to be taken, meaning permanent records can be kept. This has a number of advantages such as enabling other inspectors to observe the test as it was performed and allowing further review and evaluation. Penetrant testing (PT) Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of excess penetrant, the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used.

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Advantages

Disadvantages

Applicable to non-ferromagnetics

Only detects defects open to the surface

Able to test large parts with a portable kit

Careful surface preparation required

Batch testing

Not applicable to porous materials

Applicable to small parts with complex geometry

Temperature dependent

Simple, cheap, easy to interpret

Cannot retest indefinitely

Sensitivity

Compatibility of chemicals

History of penetrant testing A very early surface inspection technique involved the rubbing of carbon black on glazed pottery. The carbon black would settle in surface cracks, rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the oil and whiting method. In this method, heavy oil, commonly available in railway workshops, was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels. A different (though related) method was introduced in the 1940s. The surface under examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not for finding defects. Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with Switzer Bros, Cleveland, OH, USA. More effective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or colour contrast dye penetrant method, is still used quite extensively today. In the 1940s, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as black light), rendering indications from cracks and other surface flaws more readily visible to inspectors. UV lights have become increasingly portable with hand held UV torches now readily available.

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Magnetic particle testing (MT) Magnetic particle testing is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages

Disadvantages

Will detect some sub-surface defects

Ferromagnetic materials only

Rapid and simple to understand

Requirement to test in two directions

Pre-cleaning not as critical as with dye penetrant testing (PT)

Demagnetisation may be required

Will work through thin coatings

Oddly-shaped parts difficult to test

Cheap equipment

Not suited to batch testing

Direct test method

Can damage the component under test

History of magnetic particle testing The origins of MT can be traced to the 1860s when cannon barrels were tested for defects by first magnetising the barrel and then running a compass down the length of the barrel. By monitoring the needle of the compass, defects within the barrel could be detected. This form of NDT became much more common after the First World War, in the 1920s, when William Hoke discovered that flaws in magnetised materials created distortions in the magnetic field. When a fine ferromagnetic powder was applied to the parts, it was observed that they built up around the defects, providing a visible indication of their location. Magnetic particle testing superseded the oil and chalk method in the 1930s as it proved far more sensitive to surface breaking flaws. Today it is still preferred to the penetrant method on ferromagnetic material and much of the equipment being used then is very similar to that of today, with the only advances coming in the form of fluorescent coating to increase the visibility of indications and more portable devices being used. In the early days, battery packs and direct current were the norm and it was some years before alternating current proved acceptable. Magnetism The phenomenon called magnetism is said to have been discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron. The use of magnets in navigation goes back to Viking times or maybe earlier, where it was found that rods of magnetised material, when freely suspended, would always point in a north-south direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, followed by Michael Faraday (1791-1867), whose experiments revealed that magnetic and electrical energy could be interchanged.

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Historical perspective Electromagnetic testing – the interaction of magnetic fields with circulating electrical currents - had its origin in 1831 when Michael Faraday discovered electromagnetic induction. He induced current flow in a secondary coil by switching a battery on and off. D E Hughes performed the first recorded eddy current test in 1879. He was able to distinguish between different metals by noting a change in excitation frequency resulting from effects of test material resistivity and magnetic permeability. Introduction to electromagnetic testing Many electromagnetic induction or eddy current comparators were patented in the period from 1952. Innumerable examples of comparator tests were reported in the literature and in patents. Many involved simple comparator coils into which round bars or other test objects were placed, producing simple changes in the amplitudes of test signals, or unbalancing simple bridge circuits. In nearly all cases, particularly where ferromagnetic test materials were involved, no quantitative analyses of test objects dimensions, properties, or discontinuities were possible with such instruments. Often, difficulties were encountered in reproducing test results. Some test circuits were adjusted or balanced to optimise signal differences between a known good test object and a known defective test object for each group of objects to be tested. Little or no correlation could then be obtained between various types of specimens, each type having been compared to an arbitrarily selected specimen of the same specific type. Developments in electromagnetic induction tests Rapid technological developments in many fields before and during the Second World War (1939-45) contributed both to the demand for NDT and to the development of advanced test methods. Radar and sonar systems allowed the viewing of test data on the screens of cathode-ray tubes or oscilloscopes. Developments in electronic instrumentation and magnetic sensors used both for degaussing ships and for actuating magnetic mines brought a resurgence of activity. Eddy current testing (ET) Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages

Disadvantages

Sensitive to surface defects

Very susceptible to permeability changes

Can detect through several layers

Only on conductive materials

Can detect through surface coatings

Will not detect defects parallel to surface

Accurate conductivity measurements

Not suitable for large areas and/or complex geometries

Can be automated

Signal interpretation required

Little pre-cleaning required

No permanent record (unless automated)

Portability

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History of eddy current testing The principles of eddy currents arose in 1831 with Faraday’s discovery of electromagnetic induction; eddy current testing methods have their origins in a period just after the First World War, when materials with a high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering – they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in magnetic materials. The first eddy current testing devices for NDT were in 1879 by Hughes, who used the principles of eddy currents to conduct metallurgical sorting tests and the stray flux tube and bar tests. It was left to Dr Friedrich Förster in the late 1940s to develop the modern day eddy current testing equipment and formulate the theories which govern their use. The introduction by Förster of sophisticated, stable, quantitative test equipment and of practical methods for analysis of quantitative test signals on the complex plane was by far the most important factor contributing to the rapid development and acceptance of electromagnetic induction and eddy current testing. Förster is rightly identified as the father of modern eddy current testing. By 1950, he had developed a precise theory for many basic types of eddy current tests, including both absolute and differential or comparator test systems and probe or fork coil systems used with thin sheets and extended surfaces. Continued advances in research and development, advanced electronics and digital equipment have led to eddy currents becoming one of the most versatile of the surface methods of inspection. Eddy current methods have developed into a wide range of uses and are recognised as being the forerunner of NDT techniques today. From the mid1980s, microprocessor-based eddy current testing instruments were developed which had many advantages for inspectors. Modern electronics have made instruments more user friendly, providing reduced noise levels which made certain test applications very difficult, but also improving methods of signal presentation and recording capabilities. Applications for microcomputer chips abound, from giving lift-off suppression in simple crack detection to providing signal processing for immediate analysis of condenser tube inspection. As with other testing methods, improvements to the equipment have been made to increase its portability and computer-based systems now allow easy data manipulation and signal processing. Eddy current testing is now a widely used and understood inspection method for flaw detection as well as for thickness and conductivity measurements. Ultrasonic testing (UT) Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will return to the probe in a time period other than that expected of a faultfree specimen.

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Advantages

Disadvantages

Sensitive to cracks at various orientations

No permanent record (unless automated)

Portability

Not easily applied to complex geometries and rough surfaces

Safety

Unsuited to coarse grained materials

Able to penetrate thick sections

Reliant upon defect orientation

Measures depth and through-wall extent

History of ultrasonic testing In Medieval times craftsmen casting bells for churches were aware that a properly cast bell rang true when struck and that a bell with flaws would give out a false note. This principle was used by wheel-tappers inspecting rolling stock on the railways; they struck wheels with a hammer and listened to the note given out. A loose tyre sounded wrong. The origin of modern ultrasonic testing (UT) is the discovery by the Curie brothers in 1880 that quartz crystals cut in a certain way produce an electric potential when subjected to pressure - the piezo-electric effect, from the Greek piedzein (to press or strike). In 1881 Lippman theorised that the effect might work in reverse, and that quartz crystals might change shape if an electric current was applied to them. He found that this was so and experimented further. Crystals of quartz vibrate when alternating currents are applied to them. Crystal microphones in a modern stereo rely on this principle. When the Titanic sank in 1912, the Admiralty tried to find a way of locating icebergs by sending out sound waves and listening for an echo. They experimented further with sound to detect submarines during the First World War. Between the wars, marine echo sounding was developed and in the Second World War ASDIC (Anti-Submarine Detection Investigation Committee) was extensively used in the Battle of the Atlantic against the U-boats. In 1929, the Russian physicist Sokolov experimented with through-transmission techniques, passing vibrations through metals to find flaws; this work was taken up by the Germans. In the 1930s the cathode ray tube was developed and miniaturised in the Second World War to fit small airborne radar sets into aircraft. It made the UT set as we know it possible. Around 1931 Mulhauser obtained a patent for a system using two probes to detect flaws in solids and following this Firestone (1940) and Simons (1945) developed pulsed UT using a pulse-echo technique. In the years after the Second World War, researchers in Japan began to experiment on the use of ultrasound for medical diagnostic purposes. Working largely in isolation until the 1950s, the Japanese developed techniques for the detection of gallstones, breast masses, and tumours. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation. The first flaw detector was made by Sproule in 1942 while he was working for the Scottish firm Kelvin & Hughes. Similar work was carried out by Firestone in the USA and by German physicists. Sproule went on to develop the shear-wave probe.

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Initially UT was limited to testing aircraft, but in the 1950s it was extensively used in the building of power stations in Britain for examining thick steel components safely and cheaply. UT was found to have several advantages over radiography in heavy industrial applications:    

No health hazards were associated with radiography, and a UT technician could work next to welders and other employees without endangering them of holding up work. It was efficient in detecting toe cracks in boilers – a major cause of explosions and lack of fusion in boiler tubes. It could find planar defects, like laminations, which were sometimes missed by radiography. A UT check on a thick component took no more time than a similar check on a thin component as opposed to long exposure times in radiography.

Over the next twenty years, improvements focused on accurate detection and sizing of the flaws with limited success, until 1977 when Silk first discovered an accurate measurement and display of the top and bottom edges of a discontinuity with the timeof-flight diffraction (TOFD) technique. Advances in computing technology have now expanded the use of TOFD as real time analyses of results are now available. It was also during the 1970s that industries focused on reducing the size and weight of ultrasonic flaw detectors and making them more portable. This was achieved by using semiconductor technology and during the 1990s microchips were introduced into the devices to allow calibration parameters and signal traces to be stored. LCD display panels and digital technology have also contributed to reducing the size and weight of ultrasonic flaw detectors. With the development of ultrasonic phased array and increased computing power, the future for ultrasonic inspection is very exciting. Ultrasound used for testing The main use of ultrasonic inspection in the human and the animal world is for detecting objects and measuring distance. A pulse of ultrasound (a squeak from a bat or a pulse from an ultrasonic source) hits an object and is reflected back to its source like an echo. From the time it takes to travel to the object and back, the distance of the object from the sound source can be calculated. That is how bats fly in the dark and how dolphins navigate through water. It is also how warships detected and attacked submarines in the Second World War. Wearing a blindfold, you can determine if you are in a very large hall or an ordinary room by clapping your hands sharply; a large hall will give back a distinct echo, but an ordinary room will not. A bat’s echo location is more precise: the bat gives out and can sense short wavelengths of ultrasound and these give a sharper echo than we can detect. In UT a sound pulse is sent into a solid object and an echo returns from any flaws in that object or from the other side of the object. An echo is returned from a solid-air interface or any solid-non-solid interface in the object being examined. We can send ultrasonic pulses into material by making a piezo-electric crystal vibrate in a probe. The pulses can travel in a compression, shear or transverse mode. This is the basis of ultrasonic testing. However, the information from the returning echoes must be presented for interpretation. It is for this purpose that the UT set, or flaw detector as it is frequently called, contains a cathode ray tube. In the majority of UT sets, the information is presented on the screen in a display called the A Scan. The bottom of the CRT screen is a time base made to represent a distance say 100mm. An echo from the backwall comes up on the screen as a signal, the amplitude of which represents the amount of sound returning to the probe. By seeing how far the signal comes along the screen we can measure the thickness of the material we are examining.

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If that material contains a flaw, sound is reflected back from the flaw and appears on the screen as a signal in front of the backwall echo (BWE) as the sound reflected from the flaw has not had so far to travel as that from the backwall. BWE BWE

BWE BWE

Defect Defect

Ultrasonic signals Anything that sends back sound energy to a probe to cause a signal on the screen is called a reflector. By measuring the distance from the edge of the CRT screen to the signal, we can calculate how far down in the material the reflector lies. Radiographic testing (RT) Radiography monitors the varying transmission of ionising radiation through a material with the aid of photographic film or fluorescent screens to detect changes in density and thickness. It will locate internal and surface-breaking defects. Advantages

Disadvantages

Gives a permanent record, the radiograph

Radiation health hazard

Detects internal flaws

Can be sensitive to defect orientation and so can miss planar flaws

Detects volumetric flaws readily

Limited ability to detect fine cracks

Can be used on most materials

Access is required to both sides of the object

Can check for correct assembly

Skilled radiographic interpretation is required

Gives a direct image of flaws

Relatively slow method of inspection

Fluoroscopy can give real time imaging

High capital cost High running cost

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History of radiographic testing X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923) who was a Professor at Wϋrzburg University in Germany. Whilst performing experiments in which he passed an electric current through a Crookes tube (an evacuated glass tube with an anode and a cathode), he found that when a high voltage was applied, the tube produced a fluorescent glow. Roentgen noticed that some nearby photographic plates became fogged. This caused Roentgen to conclude that a new type of ray was being emitted from the tube. He believed that unknown rays were passing from the tube and through the plates. He found that the new ray could pass through most substances. Roentgen also discovered that the ray could pass through the tissue of humans, but not bones and metal objects. One of Roentgen's first experiments late in 1895 was a film of the hand of his wife. Shortly after the discovery of X-rays, another form of penetrating rays was discovered. In 1896 French scientist Henri Becquerel discovered natural radioactivity. Many scientists of the period were working with cathode rays, and other scientists were gathering evidence on the theory that the atom could be subdivided. Some of the new research showed that certain types of atoms disintegrate by themselves. It was Becquerel who discovered this phenomenon while investigating the properties of fluorescent minerals. One of the minerals Becquerel worked with was a uranium compound. On a day when it was too cloudy to expose his samples to direct sunlight, Becquerel stored some of the compound in a drawer with photographic plates. Later when he developed these plates, he discovered that they were fogged (indicating exposure to light). Becquerel wondered what would have caused this fogging. He knew he had wrapped the plates tightly before using them, so the fogging was not due to stray light; in addition, he noticed that only the plates that were in the drawer with the uranium compound were fogged. Becquerel concluded that the uranium compound gave off a type of radiation that could penetrate heavy paper and expose photographic film. Becquerel continued to test samples of uranium compounds and determined that the source of radiation was the element uranium. Becquerel did not pursue his discovery of radioactivity, but others did. While working in France at the time of Becquerel's discovery, Polish scientist Marie Curie became very interested in his work. She suspected that a uranium ore known as pitch-blende contained other radioactive elements. Marie and her husband, French scientist Pierre Curie, started looking for these other elements. In 1898, the Curies discovered another radio-active element in pitchblende, and named it polonium in honour of Marie’s native homeland. Later that year, the Curies discovered another

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radioactive element which they named ‘radium’, or shining element. Both polonium and radium were more radioactive than uranium. Due to her lifelong research in this field, Marie Curie is widely credited with the discovery of gamma radiation and the introduction of the new term: radio-active. Since these discoveries, many other radioactive elements have been discovered or produced. Radiography in the form of NDT took shape in the early 1920s when H H Lester began testing on different materials. Radium became the initial industrial gamma ray source. The material allowed castings up to 10 to 12 inches thick to be radiographed. During the Second World War, industrial radiography grew tremendously as part of the Navy's shipbuilding programme. In 1946, man-made gamma ray sources from elements such as cobalt and iridium became available. These new sources were far stronger than radium and much less expensive. The man-made sources rapidly replaced radium, and the use of gamma rays increased quickly in industrial radiography. William D Coolidge's name is inseparably linked with the X-ray tube popularly called the Coolidge tube. This invention completely revolutionised the generation of X-rays and remains the model upon which all X-ray tubes for medical applications are patterned. He invented ductile tungsten, the filament material still used in such lamps. He was awarded 83 patents. Although the theories and practices have changed very little, radiographic equipment has developed. These developments include better images through higher quality films and also lighter, more portable equipment. In addition to conventional film radiography, digital radiographic systems are now widespread within the NDT industry. The use of photostimulable phosphor (PSP) bearing imaging plates with photomultipliers to capture image signals and analogue-to-digital converters (ADC) are used extensively in computed radiography (CR). Direct radiography (DR) systems are also used based upon complementary metal oxide sensor (CMOS) technology and TFT (thin film transistors). These systems have the ability to directly convert light into digital format; additionally, they may be coupled with a scintillator which coats CMOS and charged couple device (CCD) sensors. The scintillator converts photon energy to light before the sensor and ADC converts to digital format. Systems which use scintillators in this way are often referred to as indirect systems. Quality issues of any digital system are based upon the effective pixel size and the signal-to-noise ratio (SNR). The benefits of using digital systems are the speed of inspection and the absence of chemical processing requirements and wet film; however, the initial equipment costs will be high.

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NDT Certification Schemes CSWIP – Certification Scheme for Personnel Managed by TWI Certification Ltd (TWICL), a TWI Group company formed in 1993 to separate TWI’s activities in the field of personnel and company certification thus ensuring continued compliance with international standards for certification bodies and is accredited by UKAS to BS EN ISO 17024. TWICL establishes and implements certification schemes, approves training courses, and authorises examination bodies and assessors in a large variety of inspection fields, including; non-destructive testing (NDT), welding and plant inspectors, welding supervisors, welding coordination, plastic welders, underwater inspectors, integrity management, general inspection of offshore facilities, cathodic protection, heat treatment. TWI Certification Ltd Granta Park, Great Abington, Cambridge CB21 6AL, United Kingdom Tel: +44 (0) 1223 899000 Fax: +44 (0) 1223 892588 Email: [email protected] Website: www.cswip.com

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PCN – Personal Certification in Non-destructive testing Managed and marketed by the British Institute of Non-Destructive Testing (BINDT) which owns and operates the PCN Certification Scheme, it offeres a UKAS accreditied certification of competence for NDT and condition monitoring in a variety of product sectors. The British Institute of Non-Destructive Testing Certification Services Division, Newton Building, St. Georges Avenue, Northampton, NN2 6JB, United Kingdom Tel: +44 (0)1604 893811 Fax: +44 (0)1604 892868 Email: [email protected] Website: http://www.bindt.org/Certification/General_Information Both schemes offer NDT certification conforming to BS EN ISO 9712; Qualification and Certification of NDT personnel, this superseding EN473. The PCN Scheme What follows is a summary of the general requirements for qualification and PCN certification of NDT personnel as described in PCN/GEN Issue 5 Revision R. PCN Certification is a scheme which covers the qualification of NDT inspection staff to meet the requirements of European and International Standards. Typically a standard or procedure will call for the Inspector to be certified in accordance with BS EN ISO 9712 and/or PCN requirements. The PCN Gen Document describes how the PCN system works. The points below cover extracts from this document which are major items, the full document can be viewed on the BINDT website – www.bindt.org/certification/PCN.

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References PCN documents PSL/4 PSL/8A PSL/30 PSL/31 PSL/42 PSL/44 PSL/49 PSL/51 PSL/57C PSL/67 PSL/70 CP9 CP16 CP17 CP19 CP22 CP25 CP27

Examination availability PCN documents – issue status Log of pre-certification experience Use of PCN & UKAS logo Log of pre-certification on-the-job training Vision requirements Examination exemptions for holders of certification other than PCN Acceptable certification for persons supervising PCN candidates gaining experience prior to certification Application for certification, experience gained post examination Supplementary 56 day waiver Request for L2 certificate issue to a L3 holder Requirements for BINDT authorised qualifying bodies Renewal and recertification of PCN Levels 1 & 2 certificates Renewal and recertification of PCN Level 3 certificates Informal access to authorised qualifying bodies by third parties Marking and grading PCN examinations Guidelines for the preparation of NDT procedures and instructions in PCN examinations Code of ethics for PCN certificate holders

PCN/GEN Appendix Z1 – NDT Training Syllabi Levels of PCN certification Level 1 personnel are qualified to carry out NDT operations according to written instructions under the supervision of appropriately qualified Level 2 or 3 personnel. Within the scope of the competence defined on the certificate, Level 1 personnel may be authorised by the employer to perform the following in accordance with NDT instructions:    

Set up equipment. Carry out the test. Record and classify the results in terms of written criteria. Report the results.

Level 1 personnel have not demonstrated competence in the choice of test method or technique to be used, nor for the assessment, characterisation or interpretation of test results.

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Level 2 personnel have demonstrated competence to perform and supervise nondestructive testing according to established or recognised procedures. Within the scope of the competence defined on the certificate, Level 2 personnel may be authorised by the employer to:          

Select the NDT technique for the test method to be used. Define the limitations of application of the testing method. Translate NDT standards and specifications into NDT instructions. Set up and verify equipment settings. Perform and supervise tests. Interpret and evaluate results according to applicable standards, specifications. Prepare written NDT instructions. Carry out and supervise all Level 1 duties. Provide guidance for personnel at or below Level 2. Organise and report the results of non-destructive tests.

codes

or

Level 3 personnel are qualified to direct any NDT operation for which they are certificated and may be authorised by the employer to:      

Assume full responsibility for a test facility or examination centre and staff. Establish, review for editorial and technical correctness and validate NDT instructions and procedures. Interpret codes, standards, specifications and procedures. Designate the particular test methods, techniques and procedures to be used. Within the scope and limitations of any certification held carry out all Level 1 and 2 duties and; Provide guidance and supervision at all levels.

Level 3 personnel have demonstrated:  



Competence to interpret and evaluate test results in terms of existing codes, standards and specifications. Possession of the required level of knowledge in applicable materials, fabrication and product technology sufficient to enable the selection of NDT methods and techniques and to assist in the establishment of test criteria where none are otherwise available. General familiarity with other NDT methods.

Level 3 certificated personnel may be authorised to carry out, manage and supervise PCN qualification examinations on behalf of the British Institute of NDT. Where Level 3 duties require the individual to apply routine NDT by a method(s) within a particular product or industry sector, the British Institute of NDT strongly recommends that industry demand that this person should hold and maintain Level 2 certification in the applicable methods and sectors.

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Training Table 1 Minimum required duration of training. NDT method

Level 1 hours

Level 2 hours1

Level 3 hours

ET

40

40

40

PT

16

24

24

MT

16

24

32

RT

40

80

72

RI

N/A

56

N/A

UT

40

80

72

VT

16

24

24

BRS

16

N/A

N/A

RPS

N/A

24

N\A

Basic knowledge

(Direct access to Level 3 examination parts A- C)

80

Note 1. Direct access to Level 2 requires the total number of hours shown in Table 1 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 1 for Levels 1-3. Up to one third of the total specified in this table may take the form of OTJ training documented using form PSL/42 provided it is verifiable and covered practical application of the syllabus detailed in CEN ISO/TR 25107:2006.

Industrial NDT experience   

Industrial NDT experience in the appropriate sector may be acquired prior to or following success in the qualification examination. In the event that the experience is sought following successful examination, the results of the examination shall remain valid for up to two years. Documentary evidence (in a form acceptable to the British Institute of NDT, ie. on PCN form PSL/30) of experience satisfying the following requirements shall be confirmed by the employer and submitted to BINDT AQB prior to examination, or directly to BINDT prior to the award of PCN certification in the event that experience is gained after examination.

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Table 2 Minimum duration of experience for certification. Experience, months NDT method

Level 1

Level 2

Level 3

ET

3

9

18

MT

1

3

12

PT

1

3

12

RT

3

9

18

UT

3

9

18

RI

N/A

6

N/A

VT

1

3

12

Work experience in months is based on a nominal 40-hour week or the legal week of work. When an individual is working in excess of 40h/week, he may be credited with experience based on the total hours, but he shall be required to produce evidence of this experience. Direct access to Level 2 requires the total number of hours shown in Table 2 for Levels 1 and 2, and direct access to Level 3 requires the total number of hours shown in Table 2 for Levels 1-3

Qualification examination Table 3 Numbers of general questions. NDT method

Level 1

Level 2

ET

40

40

PT

30

40

MT

30

40

RT

40

40

RI

N/A

40

UT

40

40

VT

30

40

BRS

30

N/A

RPS

N/A

20 plus 4 narrative

Note:

All Level 1 specific theory papers have 30 questions. All Level 2 specific theory papers have 36 questions.

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Re-examination a

A candidate who fails to obtain the pass grade for any examination part (general, specific or practical) may be re-examined twice in the failed part(s), provided the reexamination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

b

A candidate who achieves a passing grade of 70% in each of the examination parts (general, specific or practical) but whose average score is less than the required 80% may be re-examined a maximum of two times in any or all of the examination parts in order to achieve an overall average score of 80%, provided the re-examination takes place not sooner than one month, unless further training acceptable to BINDT is satisfactorily completed, nor later than twelve months after the original examination.

c

A candidate who fails all permitted re-examinations shall apply for and take the initial examination according to the procedure established for new candidates.

d

A candidate whose examination results have not been accepted for reason of fraud or unethical behaviour shall wait at least twelve months before re-applying for examination.

Summary The PCN scheme is managed and administered by the British Institute of NDT (BINDT) on behalf of its stakeholders. It meets or exceeds the criteria of BS EN ISO 9712. There are 6 appendices covering various industry and product sectors, 1 2 3 4 5 6

Aerospace. Castings. Welds. Wrought Products and Forgings. Pre and in-service inspection (multi sector). Railway.

There are many additional supporting documents varying from vision requirements PSL44 to renewal and recertification (Levels 1 and 2 – CP16; Level 3 – CP17) and so on. The document defines many terms used in certification of NDT personnel (PCN Gen Section 3) The certification body (BINDT) meets the requirements of BS EN ISO 17024 (PCN Gen section 5)

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BINDT approves authorised qualifying bodies (AQBs) to carry out the examinations (PCN Gen Section 5) a b c d e f g h i j k l

The document sets out the Levels of PCN certification and what each level of personnel is qualified to do (PCN Gen section 6). There are 3 Levels of PCN certification. Candidates for examination must have successfully completed a BINDT validated course of training at a BINDT authorised training organisation (PCN Gen Section 7). Table 1 shows the minimum required duration of training for all Levels and methods plus a section of notes. Table 2 gives the minimum duration of experience for each Level and method. A candidate is required to have a vision test of colour perception and a near vision test (Jaeger Number 1 or N4.5). PCN Gen Section a – the near vision test to be taken annually. Examination applications are made directly with the AQB. PCN Level 1s and 2 initial exams comprise general; specific and practical parts. Table 3 shows the number of general questions at Levels 1 and 2 examinations. There are 30 specific questions on the Level 1 papers. There are 36 questions on the Level 2 specific papers. A variety of practical samples are tested depending on the method and sector. A Level 3 examination comprises a basic and a method examination – however the basic examination needs to be passed only once. Table 4 shows the number of basic examination questions. Table 5 shows the number of Level 3 examination questions.

Table 4 Number of basic examination questions. Part

Examination

Number of questions

A

Materials technology and science, including typical defects in a wide range of products including castings welds and wrought products.

30

B

Qualification and certification procedure in accordance with this document

10

C

15 general questions at Level 2 standard for each of four NDT methods chosen by the candidate, including at least one volumetric NDT method (UT or RT).

60

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Table 5 Main method examination. Part

Subject

Number of questions

D

Level 3 knowledge relating to the test method applied

30

E

Application of the NDT method in the sector concerned, including the applicable codes, standards, and specifications. This may be an open book examination in relation to codes, standards, and specifications.

20

F

Drafting of one or more NDT procedures in the relevant sector. The applicable codes, standards, and specifications shall be available to the candidate.

m A pass is obtained where each part is 70% or over with an average grade of 80% or over. n A PCN certificate is valid for 5 years. o Renewal and recertification requirements are covered in CP16 for Level 1 and Level 2 and CP17 for Level 3.

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Section 1 Introduction

1

Introduction This section covers the syllabus for PCN approval in eddy current testing of aircraft components and structures. It also provides a basis for the more advanced concepts used in tube testing, material sorting and weld testing which are covered in section 2. The text for this course is laid out in a manner which it is hoped will make it easier to follow than conventional course texts. In general, right hand pages are used for text and left hand pages for flow charts, diagrams and tables. Looking across the page to the right of a particular diagram you should find the relevant text. We have left plenty of space on the pages to encourage you to add notes from the lectures. The flow charts, we hope, you will find useful in following the progress of the course lectures. In eddy current methods there are many concepts and models that are difficult to comprehend unless they can be put into the context of the subject as a whole. Because we are using flow charts there is no index. Each flow chart splits a subject title into several subheadings, given with a decimal notation for the paragraph number. Therefore the number 2.2.31 means paragraph number 31, under subheading number 2 of subject title 2. This makes it easier for us to change the text. We hope it does not confuse you.

1.1

The SI units of measurement Before we start you may care to study the units of measurement on the facing page. The United Kingdom adheres to a treaty signed at the General Conference on Weights and Measures, which has established a Systèmes Internationales of units. Eventually these units will replace all existing Imperial and CGS units. Certainly not all of these units are of relevance to this course but the Table will be a useful reference. We shall also be using scientific notation, which is useful shorthand for writing numbers with a great number of zeros. For example: 7.0 x 10³ = 7000 7.0 x 10ˉ³ = 0.0007 But m.sˉ¹ = m/s m.sˉ² = m/s m.s² = m x s² But don’t worry, if in doubt write the numbers out in full.

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1.2

History of eddy current testing Eddy current testing methods have their origins in a period just after the First World War, when materials with high magnetic permeability were being developed for electrical power transformer cores and motor armatures. Eddy currents are a considerable nuisance in electrical engineering - they dissipate heat and efforts to reduce their effect led to a discovery that they could be used to detect material changes and cracks in the magnetic materials. The first eddy current testing devices for NDT were by Huges in 1879. It was left to Frederick Forster in the late 1940s to develop the modern eddy current testing equipment and formulate the theories which govern their use. Since then, eddy current methods have developed into a wide range of uses and are recognised as being the front-runner in NDT techniques today. Modern electronics have not only reduced the noise levels which made certain test applications very difficult but they have also improved the methods of signal presentation. Microcomputer chips abound, from giving lift-off suppression in simple crack detectors to providing signal processing for immediate analysis of condenser tube inspections.

1.3

Definition of non-destructive testing (NDT) Non-destructive testing includes physical testing methods for detecting flaws in a material or component in a manner which does not in any way harm the service life of the material or component. The basic principal methods are:      

Visual testing (VT). Penetrant testing (PT). Magnetic particle testing (MT). Eddy current testing (ET). Ultrasonic testing (UT). Radiographic testing (RT).

In all the NDT methods, results can be misinterpreted easily. For example, MT may reveal strong indications along the weld toe that are impossible to distinguish from toe cracks. Much depends on the skill of the operator, although properly formulated test techniques and procedures will improve test accuracy and consistency. Some NDT methods can be destructive. There are, for example, many corrosive liquids used in penetrants and contrast aids.

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Table 1.1 SI units of measurement Base quantities Length Mass Time Electric current Thermodynamic temperature Luminous intensity Amount of substance

metre kilogram second ampere kelvin candela mole

Symbol m kg sec A K cd mol

Derived units Frequency Force Pressure and stress Work and energy Power Quantity of electricity e.m.f. and potential difference Electric capacitance Electric resistance Electric conductance Magnetic flux Magnetic flux density Inductance Luminous flux Illumination

hertz newton pascal joule watt coulomb volt

Hz N Pa J W C V

1Hz=1secˉ¹ 1N= 1kg.m/sec² 1Pa=1N/m² 1J=1N/m 1W=1J/sec 1C=1A/sec 1V=1W/A

farad ohm siemens weber tesla henry lumen lux

F Ω S Wb T H lm lx

1F=1A.sec/V 1Ω=1V/A 1S=1Ωˉ¹ 1Wb=1V/sec 1T=1Wb/m ² 1H=1V.sec/A 1lm=cd/sec 1lx=1lm/m ²

Other accepted units Volume Mass Energy

litre tonne electron volt

l t eV

1l=1dm³ 1t=10³kg Approx 1.60219 x 10ˉ¹9

Prefixes 10¹² 10 10 10³ 10² 10 10ˉ¹ 10ˉ² 10ˉ³ 10ˉ6 10ˉ9 10ˉ¹² 10ˉ¹5 10ˉ¹8

tera giga mega kilo hector deca deci centi milli nicro nano pico femto atto

Symbol T G M k h d d c m µ n p f a

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Penetrant testing (PT) Penetrant testing locates surface-breaking discontinuities by covering the item with a penetrating liquid, which is drawn into the discontinuity by capillary action. After removal of the excess penetrant the indication is made visible by application of a developer. Colour contrast or fluorescent systems may be used. Advantages

Disadvantages

Applicable to non-ferromagnetics

Will only detect defects open to the surface Careful surface preparation required

Able to test large parts with a portable kit Batch testing

Not applicable to porous materials

Applicable to small parts with complex geometry Simple, cheap, easy to interpret

Temperature dependant

Good Sensitivity to surface defects

Compatibility of chemicals

Cannot retest indefinitely

Magnetic particle testing (MT) Magnetic particle testing is used to locate surface and slightly sub-surface discontinuities in ferromagnetic materials by introducing a magnetic flux into the material. Advantages

Disadvantages

Will detect some sub-surface defects

Ferromagnetic materials only

Rapid and simple to understand

Requirement to test in two directions

Pre-cleaning not as critical as with dye penetrant inspection (DPI) Will work through thin coatings

Demagnetisation may be required Odd shaped parts difficult to test

Cheap rugged equipment

Not suited to batch testing

Direct test method

Can damage the component under test

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Eddy current testing (ET) Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between those currents and the material. Eddy currents are generated by coils in the test probe and monitored simultaneously by measuring the coils electrical impedance. As it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the material must be an electrical conductor. Advantages

Disadvantages

Sensitive to surface defects

Very susceptible to permeability changes

Can detect through several layers

Only on conductive materials

Can detect through surface coatings

Will not detect defects parallel to surface

Accurate conductivity measurements Can be automated

Not suitable for large areas and/or complex geometries Signal interpretation required

Little pre-cleaning required

No permanent record (unless automated)

Portability

Radiography testing (RT) Radiography testing monitors the varying transmission of ionising radiation through a material with the aid of photographic film, fluorescent screens or digitally using (a) Computed Radiography with phosphor photostimulable screens or (b) Direct Radiography with Digital Detector Devices and Arrays, to detect changes in density and thickness. It will locate internal and surfacebreaking defects. Advantages

Disadvantages

Gives a permanent record, the radiograph

There is a radiation health hazard

Detects internal flaws

Can be sensitive to defect orientation and so can miss planar flaws Has limited ability to detect fine cracks

Detects volumetric flaws readily Can be used on most materials

Gives a direct image of flaws

Access is required to both sides of the object Skilled radiographic interpretation is required Is a relatively slow method of inspection

Fluoroscopy can give real time imaging

Has a high capital cost

Can check for correct assembly

Has a high running cost

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Ultrasonic testing (UT) - pulse echo Ultrasonic testing measures the time for high frequency (0.5-50MHz) pulses of ultrasound to travel through the inspection material. If a discontinuity is present, the ultrasound will be reflected to the probe in a time period other than would be expected of a fault free specimen. Advantages

Disadvantages

Sensitive to cracks at various orientations

No permanent record (unless automated)

Portability Safety

Not easily applied to complex geometries and rough surfaces. Unsuited to coarse grained materials

Able to penetrate thick sections

Reliant upon defect orientation

Measures depth and through-wall extent

1.4

Choice of method Before deciding on a particular NDT inspection method it is advantageous to have certain information:      

Reason for inspection. (To detect cracks, to sort between materials, to check assembly, etc.). Likely orientation of planar discontinuities, if they are the answer to the above question. Type of material. Likely position of discontinuities. Geometry and thickness of object to be tested. Accessibility.

This information can be derived from:  

Product knowledge. Previous failures.

Accuracy of critical sizing of indications varies from method to method. Liquid penetrant testing The length of a surface-breaking discontinuity can be determined readily but the depth dimensions can only be assessed subjectively by observing the amount of bleed out. Magnetic particle testing The length of a discontinuity can be determined from the indication but no assessment of discontinuity depth can be made. Eddy current testing The length of a discontinuity can be determined. The depth of a discontinuity or material thinning can be determined by amplitude measurement, phase measurement or both but the techniques for critical sizing are somewhat subjective.

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Ultrasonic testing The length and position of a discontinuity can be determined. Depth measurements are more difficult but crack tip diffraction or time-of-flight techniques can give good results. Radiography testing The length and plan view position can be determined. Through-thickness positioning requires additional angulated exposures to be taken. The throughthickness dimension of discontinuities cannot readily be determined.

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Section 2 Principles

2

Principles

2.1

Electricity Electricity refers to the flow of electrons through simple materials and devices. The name is derived from the Greek word Elektra, the name given to an exotic mineral, which when rubbed with a cloth, builds up a static charge which creates sparks. It was Benjamin Franklin and his hazardous experiments with flying a kite into thunder clouds, who hit on the idea that electricity could be described as something flowing through a conductor from positive to negative electrodes. We now assign the phenomenon of electricity to the flow of electrons which is of course from the negative to the positive electrode, but Franklin’s concept still remains. In fact the flow of electricity through semiconductors is somewhat different in manner from the flow of electricity through metals, where free electrons exist. We say that the flow of current is a semiconductor and is due to the displacement of positive ‘holes’, which of course is the director of Franklin’s electric current. Electricity is very dangerous to life. Currents of only a few fractions of an amp can set the heart muscles into fibrillation; a condition which stops the circulation of blood due to irregular and shallow heartbeats. Fortunately we are covered in a skin of very high electrical resistance and quite high voltages are needed to break down the barrier. However, eddy current testing instruments are electrical instruments and if they run off the mains power supply they will carry 240 volts. So be careful and for goodness’ sake do not poke around near cathode ray tubes when they are switched on. Some of those coloured bits on the circuit board may be capacitors charge with two or three thousand volts.

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a

b

c Figure 2.1: a Hydrogen atom; b Copper atom; c Experiment with pith balls and glass rod.

2.1.1

Electrons The basic building block of all matter is the atom. The nature of the atom and the electromagnetic forces within it determine the characteristics of matter. There are 118 different elements known to make up matter and each one has a characteristic atom. The simplest atom is hydrogen, which has a nucleus of one proton, or positively charged particle and one neutron, a neutral particle, orbited by one electron, a negatively charged particle (Figure 2.1a). The angular momentum of the orbiting electron is exactly balanced by the electrostatic forces between its negative charge and the positive charge on the nucleus.

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As the atoms become larger and the number of charged particles increases, so the electrons arrange themselves in fixed orbits or shells called K,L,M,N,O and P. The outer shell is the valence shell and it is the number of electrons in this shell which determines the electrical and chemical properties of an atom. Copper has only one valance electron. This can be lost easily and for this reason copper is a good conductor of electricity (Figure 2.1b). 2.1.2

Electrostatics Electrostatics is the study of electrical forces which exist between charge particles. In their most fundamental form these forces hold the electron in orbit around the nucleus of an atom. The origin of these forces is a mystery but we do know what their effects are. For example, we know that like charges repel and unlike charges attract. By convention the electrostatic force lines are drawn pointing away from the positive charge and towards the negative charge (Figure 2.1c). The effects of electrostatic fields can be demonstrated using pith balls and a glass rod. The glass rod is first charged positive by rubbing it with a silk cloth. This removes the electrons by friction. The rod is then brought close to the balls, which although initially of neutral charge, will become polarised so that they are both attracted to the rod. As soon as the rod touches the balls, electrons are removed from both so that they become positive and repel each other. Electrostatic charge is caused by electrons. An excess of electrons will create a negative charge. A deficiency of electrons will create a positive charge. The amount of electrostatic charge is measured in coulombs. One coulomb = 6.25 x 1018 electrons.

Figure 2.2 A DC circuit.

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Figure 2.3 Dry cell.

2.1.3

Direct current If a positive charge is placed at one end of the conductor and a negative charge at the other, then electrons will flow along the conductor creating an electric current. This current will continue only until the charges have been neutralised (Figure 2.2). An electric circuit is a complete path around which electrons can flow. If the circuit is broken, then the electrons cannot flow and the circuit becomes an open circuit. To generate a continuous supply of electrons, a battery is needed. The battery relies on the chemical action between two different metals called electrodes immersed in a salt or acid solution called an electrolyte. The conductor provides a supply of electrons to conduct the current. The load provides the pressure against which the electromotive force of the battery must push the electrons, otherwise the circuit will short.

2.1.4

Battery A battery is a means of applying a potential difference across a circuit to push electrons around it. The simple battery shown is a primary cell and cannot be recharged. Other types including lead acid and nickel-cadmium batteries can be recharged and are therefore secondary cells (Figure 2.3).

2.1.5

Ampere (A) The ampere is the unit of measurement of current flow. 1 ampere = 1 coulomb of electrons passing any point in one second.

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In SI units it is a base quantity and therefore defined in absolute terms as that current which when flowing along two infinitely long parallel conductors, one metre apart in free space, exert any attraction of 2 x 10ˉ7 Newtons per metre. 2.1.6

Volt (V) The volt is a measure of pressure, forcing electrons around a circuit. A potential difference is created between opposite charges at either end of a conductor. The greater the difference, the greater the pressure which forces the electrons along. The voltage can occur without current flow in what we call an open circuit. The supply voltage is called the electromotive force. In SI units, the potential difference is one volt between two points of a conducting wire carrying a constant current of one ampere, when the power dissipated between them is one watt.

2.1.7

Resistance (R) The opposition to current flow in a DC circuit is called the resistance. It is rather like friction in mechanics. It opposes the flow of electrons and generates heat.

Figure 2.4 Ohm’s law.

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Figure 2.5 Power formulae.

Figure 2.6 series circuit.

2.1.8

Figure 2.7 parallel circuit.

Ohm’s law Ohm discovered that the amount of current flowing through a material varies directly with the applied voltage and inversely with the resistance of the material.

R is in Ohms (Ω). V is in volts. I is in amps. A simple way of remembering Ohm’s law is to draw it in circular form (Figure 2.4). Quantities on either side of the vertical line are multiplied, while quantities below the horizontal line are divided into quantities above it.

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To use the circle, simply cover the segment you want to find and the position of the remaining letters tells you the procedure to follow. 2.1.9

Power formula Power is the rate at which work is done. In a DC circuit, work is done whenever electrons are set in motion. Therefore in an open circuit, where electrons cannot flow, no work is done even through there is an electromotive force applied from the battery. P = I X V. P is in watts. I is in amps. V is in volts. By using Ohm’s law to substitute the variables, the power formulae (Figure 2.5) can also be written as:



OR



2.1.10 Series circuits A series circuit (Figure 2.6) contains only one path along which the current can flow. It is governed by three laws: Individual resistances in a series circuit add up to the total circuit resistance: R = R1 + R2…RN Current has the same value at any point within a series circuit. Individual voltages across resistances in a series circuit add up to the total applied voltage. 2.1.11 Parallel circuits A parallel circuit (Figure 2.7) has two or more paths for the current to flow along. It is also governed by three laws: 1 2 3

Total voltage of parallel circuit is the same across each branch of that circuit. Total current in a parallel circuit is equal to the sum of the individual branch circuits. Total resistance in a parallel circuit is always less than the value of the smallest resistive branch.

1 1 1 1    R R1 R2 RN

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Figure 2.8 Series string of lights.

Figure 2.9 Parallel string of lights.

Figure 2.10 Systematics showing examples of an electrics circuit in a car.

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2.1.12 Parallel and series circuits. Parallel circuits are an advantage in lighting a Christmas tree with several bulbs. When they are connected in series and one filament is blown then all the lights will go out. When connected in parallel, current will continue to flow to the bulbs even if one of the filaments is blown (Figures 2.8 and 2.9). 2.1.13 Car electrics circuits Three parallel branches of a car electrics circuit are shown (Figure 2.10), feeding current to the head-lamp, spark plugs and fan. The twelve volt battery consisting of six two volt cells in series supplies a voltage against the car chassis. We can analyse the circuits by taking measurements with a universal meter where circuits are accessible and calculating voltages or amperages to give information about circuit components which are not accessible. For example, to find the resistance of the head-light filament we could measure the current by connecting an ammeter across the open switch and dividing this value into the voltage across the bulb. We know this is twelve volts as there are no other loads in this branch. To find the voltage across the coil in the spark plug branch, the voltage across the dropping resistor could be measure and subtracted from twelve volts. Similarly, when the fan is not accessible, the current in the fan motor branch could be measured by connecting an ammeter across the open switch and measuring the voltage across the speed control. The fan voltage could then be calculated by subtracting the speed control voltage from twelve volts. The speed motor has three switches, the one without a resistance corresponding to the fastest fan speed. The spark plug branch is designed so that when one of the set of points in the distributor is closed, current rapidly builds up in the coil creating a strong magnetic field. When the points open, this field collapses suddenly, creating a high voltage and therefore arc in the spark plug. The buffer capacitor is placed across the points to prevent similar spark occurring there since it prevents the coil’s inductive voltage reaching the points.

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Figure 2.11 Meter controls.

Figure 2.12 Capacitor.

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2.1.14 Resistor Resistors are used to control the amount of current in a circuit. Two variable resistors, usually called potentiometers or ‘pots’ are shown which set the zero and control the sensitivity of the meter (Figure 2.11). As the resistance in the parallel potentiometer increases so a greater proportion of the circuit current will flow through the meter decreasing its sensitivity. The series potentiometer will zero the meter. 2.1.15 Capacitor A capacitor or condenser is a device for storing electric charge (Figure 2.12). It consists of two parallel plates separated by a dielectric material. If the plates are connected to the terminals of a battery, the positive terminal will take electrons from one plate and the negative terminal will push electrons onto the other plate. A voltage will build up across the capacitor which will eventually equal the electromotive force of the battery and the capacitor will be fully charged. The amount of charge that a capacitor can take is measured by a quantity called capacitance: C=

Q V

C is the capacitance in farads. Q is the amount of charge in coulombs. V is the voltage. The farad is a very large unit and so common capacitors are rated in microfarads or picofarads. The capacitance depends on three factors: 1 2 3

The size of the capacitor plates. The greater areas of the plates facing each other, the more charge they can hold. The distance between the plates. The closer they are together, the greater the capacitance. The nature of the dielectric material that separates the capacitor plates. Not only does the dielectric prevent charge breaking down the barrier between the plates, but also the dielectric helps the capacitor to store charge. For example, glass will allow the capacitor to store eight times more charge than air, when it is placed between the plates.

Capacitors have a very wide range of uses where a large transient current is needed, for example, in spot welders, flash guns, ignitions systems and dc magnetic particle inspection equipment.

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For eddy current testing we are interested in variable capacitors which are used in alternating circuits to adjust the phase between voltage and current or create resonance. 2.1.16 Conductance (G) Conductance is a measure of the ability of a material to conduct electricity and is the inverse of resistance: G=

1 R

G is in Siemens. R is in ohms. 2.1.17 Resistivity (ρ) Resistivity is a measure of how easy current will flow through a material. If the resistivity is very high then there are few free electrons available to conduct the current and the electrons have difficulty in passing obstacles such as atoms, discontinuities and impurities in the material. A great deal of heat will be generated depending upon the voltage pushing the electrons along (materials of this nature are called insulators). Conversely a very low resistivity allows more current to flow and is a characteristic of copper and aluminium. Materials of this nature are called conductors:

10 ρ is in micro-ohms • cm. is the length in cm. A is the cross-sectional area of the circuit in cm². R is in ohms. 2.1.18 Conductivity (ợ) Conductivity is the inverse of resistivity:



1 x 10 8 

ợ is in Siemens/m. ρ is in micro-ohms •·cm. 2.2

Magnetism The phenomenon called magnetism was discovered in the ancient Greek city of Magnesia, where naturally occurring magnets were found to attract iron.

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The use of magnets in navigation goes back to the eleventh century, where it was found that rods of magnetised material, when freely suspended, would always point in a North-South direction. The end of the rod which pointed towards the North Pole star became known as the North Pole and consequently the other end became the South Pole. Hans Christian Oersted (1777-1851) discovered the connection between electricity and magnetism, to be followed by Michael Faraday (1791-1867) whose experiments revealed that magnetic and electrical energy could be interchanged. The region surrounding a permanent magnet or electric current will deflect a small magnet or compass in curved lines known as the lines of magnetic force or flux. By convention, in the case of permanent magnets, the magnetic flux flows from south to north internally and north to south externally. In the case of a conductor, the direction of flux flow is determined by the right hand rule. This study of magnetism is of vital importance in electrical engineering, electronics and computers. In eddy current testing we are interested in magnetism both in the way that it couples the current in the test coils to the eddy current field in the testpiece and in the dramatic test signals created in ferromagnetic materials that can obliterate defect signals.

Figure 2.13 Magnetic domains.

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Figure 2.14 Magnetisation curve.

2.2.1

Paramagnetism and diamagnetism All matter is made up of magnetic as well as electrical forces. These forces make up the atoms which are the fundamental building blocks of matter and give matter its substance. Within the atom there are magnetic forces which are the result of the spinning and orbiting motions of the electrons. If an external field is applied, then according to Lenz’s law, any magnetic moments should align themselves to oppose the applied field. This is the case with diamagnetic elements, a group which includes copper. The effect is so very slight as to be negligible. In a paramagnetic element, the balance of magnetic moments which exist in a diamagnetic is offset because there are more electrons spinning or orbiting in one direction than there are in another. This gives rise to a resultant magnetic moment with a north and South Pole which will align itself parallel with any external magnetic field. The effect is very weak because the thermal excitation in the atoms prevent anything but a very weak alignment. Paramagnetics, a group of elements which includes aluminium, can therefore be regarded as nonmagnetic.

2.2.2

Ferromagnetism Ferromagnetism is a term used to describe certain materials which exhibit strong magnetic behaviour. We say that they have high magnetic permeability. The three most common ferromagnetic elements are iron, cobalt and nickel, but there are others, for example, gadolinium, which is important for electronics. Within the crystal lattices of ferromagnetics there exist magnetic domains. Within each domain, the magnetic dipoles, as we call a north-south pole pair, are in parallel alignment with the same pole pointing in one direction.

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One end of the domain will therefore have a strong north pole and the other a south pole. The magnetic circuits created by the domains are aligned to reduce flux leakage to a minimum. They therefore adopt a parallel but opposing arrangement or lay across each other. Between the domains exists a domain wall across which the magnetic dipoles twist like a corkscrew. In the ground state, the domains have no preferred orientations and the ferromagnetic is unmagnetised. If an external magnetic field is applied in the direction of the magnetic dipoles in domain A, then this domain will grow at the expense of domain B by twisting over the dipoles in the domain wall (Figure 2.13). This change is elastic. If the external field is removed the domains will return to the ground state. As the external field continues to increase, the domains walls become detached from dislocations which they tend to follow in the crystal lattice and will latch themselves to achieve eventually a state of magnetic saturation. The shape of the magnetisation curve is a characteristic of a ferrogmagnetic element or material. The steeper the curve, the easier it is to magnetise.

Figure 2.15 Magnetic hysteresis.

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Figure 2.16 Hysteresis loops for iron and steel.

2.2.3

Magnetic permeability (µ) The ease with which a material conducts magnetic flux is called its magnetic permeability:



B H

µ is the absolute magnetic permeability in Henry/metre. B is the magnetic flux density in teslas. H is the magnetic field strength in A/m. For air and non-magnetic materials, µ is constant and denoted by µo. µo = 4π x 10ˉ7 teslas or Henries/metre. For ferromagnetic materials it varies considerably according to the value of H. For convenience we use relative permeability µr:

r 

 0

Relative permeability is therefore a dimensionless ratio which relates the permeability of the material to that of air.

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2.2.4

Magnetic hysteresis When a ferromagnetic is placed in an alternating magnetic fiend (H), the variation in the density of flux lines (B) through it, gives rise to magnetic hysteresis (Figure 2.15). The word hysteresis is derived from the Greek for delayed and is used to describe one thing lagging behind another. The variation of B with H follows a hysteresis loop and is a characteristic of the ferromagnetic material. Let us take it from the point where all the domains in the ferromagnetic are aligned with the applied field. This is called the saturation point. As H falls to zero, B is reduced to a value given by the remanence point. This is due to a degree of plasticity in the domain alignment which prevents them from returning to random orientations. It gives ferromagnetics their permanent magnetism. H has to be applied in the opposing direction to a value given by the coercive force to knock the magnetic domains out of alignment and reduce B to zero. Further increases in H in this direction will then take the domains to saturation once more, but with the polarity reversed. The shape of the hysteresis loop is an important characteristic of ferromagnetic materials and can be used to grade sort them on the basis of their hardness’s. Steels are magnetically hard, while irons are magnetically soft (Figure 2.16). The slope of the axes of the two hysteresis loops show that the steel is more difficult to magnetise than the iron, but that once magnetised, it is more difficult to demagnetise. The steel would therefore make a better permanent magnet despite having a smaller remanence. It is much larger coercive force will make steel much more difficult to demagnetise by shaking and knocking. On the other hand the much smaller magnetisations and demagnetisation forces operating in the iron reduce the energy losses called hysteresis losses and make it more suitable for the cores of coils and transformers.

Figure 2.17 Right hand rule.

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Figure 2.18 Magnetic field through a coil.

Figure 2.19 Car motor ignition.

2.2.5

Electromagnetism Whenever electric current flows along a conductor, a magnetic field is set up around the conductor in a plane with its axis parallel with the flow of electrons. The magnetic field is there only when electrons are flowing. The direction of the flow of magnetism is given by the right hand rule (Figure 2.17). If the thumb of the right hand is extended in the direction in which the conventional current is flowing, then the direction of the magnetic flow is given by the fingers. Remember that the electron flow is in the opposite direction to the conventional current flow.

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The larger the current, the stronger the magnetic flow. The further away from the conductor, the weaker the magnetic flow. The magnetic flow can be detected by placing a compass needle near the conductor. It will align itself with the North Pole pointing in the direction of the magnetic flow. 2.2.6

Coils If a current-carrying wire is looped into several turns, the magnetic field around each turn link together, giving rise to a strong magnetic field through what is now a coil (Figure 2.18). This magnetic field behaves like a bar magnet and will attract ferromagnetic objects. The polarity of the coil ends is determined by a rule which shows that when the coil is viewed end-on, if the conventional current is flowing clockwise, that will be the South Pole. If the conventional current is flowing anti-clockwise, that end of the coil will be the North Pole. The intensity of the magnetic field through the coil is a product of the coil current and the number of coil turns. Coils can be used to control electrical switches called relays. One typical application is the ignition switch of a car engine (Figure 2.19). Here a small control current passes through a coil. When it is switched on the ferromagnetic plunger moves through the coil and closes the contacts of the starter motor circuit. This carries an extremely high load, of perhaps one hundred amperes and it would be extremely hazardous to connect this directly to the key switch.

Figure 2.20 Magnetic circuit.

2.2.7

Magnetic circuits A magnetic circuit (Figure 2.20) can be made by analogy to an electric circuit by replacing the battery with a coil and the conductor with a ferromagnetic. The electromotive force then becomes the magnetomotive force and is measure by multiplying the coil current by the number of turns. The amperage becomes the magnetic flux or linkage and is measured in webers.

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The ration between magnetomotive force and flux is constant as is the ration of electromotive force to current in an electric circuit. This magnetic equivalent of Ohm’s law is called Bosanquet’s law and the magnetic equivalent of resistance is called reluctance, and is given by:

 Where F = Magnetomotive force (Mmf).  = Magnetic flux. 2.2.8

Magnetic flux density (B) This is also called the magnetic induction and the SI unit of measurement is the tesla:

B

 A

B is in the flux density in teslas. Φ is the magnetic flux in webers. A is the cross-sectioned area of the magnetic circuit in m². One tesla is the magnetic flux density of a uniform field that produces a torque of 1N/m on a plane current loop carrying one ampere and having a projected area of 1m² in a plan perpendicular to the field. 2.2.9

Magnetic field strength (H) The SI unit of magnetic field strength is the ampere per metre:

H

m.m.f. D

H is in A/m. mmf is in A • turns (Mmf is the magnetomotive force, also known as magnetic potential and is analogous to emf or voltage in electricity). D is the axial distance in metres. It is the magnetic field strength in the interior of an elongated, uniformly would solenoid which is excited with a linear current density in its winding of one ampere per metre of axial distance. 2.2.10 Inductors Coils have an effect on the current which is passing through them and are therefore called inductors. The magnetic field which they create acts as a store of energy, which has been taken from the electrical current. As long as the current is not changing, the magnetic field is in a steady state and it has no effect on the current. If the current is building up, the current finds itself building up the magnetic field as well and its flow is opposed. This results in the current taking a longer time to build up in a circuit containing an inductor than in a circuit containing a resistor only.

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As the current in the inductor decreases, the magnetic field is reconverted to electrical energy and so slows the rate at which the current in the inductor decays. It can be said therefore, that inductors act to oppose any change in the current through them. 2.2.11 Inductance (L) The ability of a coil to store magnetic energy and oppose changes in the current is called inductance:

L ϒ N A l

= = = = =

Inductance in henrys. A geometric factor. Number of coil turns. Coil’s planar surface area in mm². Coil’s axial length.

The henry is a very large unit. Eddy current coils have inductances of a few microhenrys. Inductance is a property of only those electrical circuits where the current is varying. The opposition to current flow generates a voltage or self-inductance in the circuit, but it can also generate a voltage in a neighbouring circuit through mutual-inductance. The latter is the transformer principle. 2.3

Alternating current theory Alternating currents are continually reversing. The electrons will be flowing along a circuit in one direction, slowing down until they are stationery then flowing in the opposite directions until they reach a maximum velocity (current) before slowing down again and reversing once more. This alternation occurs in a regular period termed at the frequency. The current from the mains supply alternates approximately 50 times a second or at 50 hertz. In eddy current testing, the frequency of the currents is of vital importance and may range from 10Hz-10MHz (10 megahertz or 10 million cycles per second). The change in the current with time can be represented by a sine wave model. When the capacitors or inductors are placed in an AC circuit we find that the voltage and current waves do not coincide. We say they are out of phase. To analyse these phase differences we use vector diagrams.

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Figure 2.21 Sine waves.

Figure 2.22 Root mean square.

2.3.1

Sine waves If a pen rotates in a circle around a sheet of paper which is moving under it at a constant velocity, we find that the pen describes a sine wave (Figure 2.21). The characteristics of a sine wave are: 1 2 3 4 5

2.3.2

One cycle is equivalent to one revolution of the circle or 360° or 2 π radians. The amplitude of the current (Φ) is proportional to sine of the included angle (ợ). The rate of change in the current is at a maximum where it crosses the datum and zero where it reaches the peak values. The positive and negative peak values are equal but opposite. Each cycle has a constant period determined by the frequency.

Root mean square Since alternating currents are reversing between equal but opposite peak values it is not possible to measure their mean value. In practice the root mean square (RMS) (Figure 2.22) value is measured, which is defined as that value of steady current which would dissipate heat at the same rate in a given resistance.

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The power is dissipated in heat is given by: P = l²R If the resistance is constant, the average power (Pa) will be given by

2.

By plotting the squares of the current values we can find an average, since negative as well as positive values become positive. To measure the square of the current we use a moving iron ammeter. This type of ammeter consists of two iron rods which are forced apart as they are magnetised. Their level of magnetisation is proportional to the current and therefore the force between them is roughly proportional to the square of the current. The meter is calibrated to read the root of the mean to the square values and is therefore non-linear.

Figure 2.23 Faraday’s experiment.

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Figure 2.24 Reactance and resistance.

2.3.3

Faraday’s laws Faraday discovered the inductive effects of rapid changes in the magnetic field. When current is abruptly switched off in an electrical circuit it will induce an electromotive force which, if magnetically coupled to another electrical circuit, will create a current in that circuit. When the battery is disconnected in circuit A, the light in circuit B flashes for an instant. Similarly when the battery is reconnected and the current is building up in circuit A, so the bulb in circuit B flashes. While current is flowing steadily in circuit A, the light in B is off (Figure 2.23). The two circuits are not linked electrically but the magnetic field around circuit A does link through circuit B. Faraday went on to define two laws: Whenever a magnetic field linking a circuit is changed, it sets up an electromotive force. The amplitude of this induced electromotive force is proportional to the rate of change of magnetic flux.

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2.3.4

Lenz’s law This law states that the electromotive force induced by the variation in magnetic flux is always in such a direction that if it produces a current, the magnetic effect of that current opposes the flux variation responsible for both the electromotive force and the current.

2.3.5

Resistance and reactance The resistance in an AC circuit represents a loss of electrical energy as heat, as it does in a dc circuit. In an AC however, there are two other components which oppose the flow of current and these are called reactances (Figure 2.24). One is the capacitive reactance, which creates a voltage across a capacitor and the other is the inductive reactance which creates a voltage across an inductor. The capacitor converts current into electrostatic energy and the inductor converts current into magnetic energy. As the energy is reconverted to current when the polarity of the circuit current reverses, neither of the reactances represents an actual loss in electrical energy. Ohm’s law can be applied to the reactances. The ratio of voltage to current across each component is constant:

XC  Xc Vc XL VL I

Vc I

= = = = =

XL 

VL I

Capacitive reactance in ohms. Voltage across the capacitor. Inductive reactance in ohms. Voltage across the inductor. Circuit current.

There is a complication. The voltages and currents in an AC circuit are sinusoidal waves and therefore have a phase as well as amplitude. Across the resistor, voltage and current are in phase. Across a capacitor the current leads the voltage by a quarter cycle (π/2). This can be explained as follows: When the voltage in the circuit is at maximum, so is the charge in the capacitor. It is therefore not charging and the current is zero. When the voltage starts to fall the capacitor is completely discharged and the voltage is zero. Across an inductor, the voltage leads the current by a quarter cycle (π/2). This can be explained as follows: When the current in the circuit is at a maximum, the rate of change in the magnetic flux in the coil is zero and therefore the self-induced voltage is zero. When the current in the circuit is at zero so the rate of change in the magnetic flux is at a maximum and so therefore is the self-induced voltage. When the current is building up in the positive direction, so the induced voltage will be slowing down in the positive directions. When the current is building up in the negative direction, so the voltage is slowing down in the negative direction.

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Figure 2.25 Coil’s equivalent circuit.

Figure 2.26 Impedance diagram.

2.3.6

Impedance (Z) The application of Ohm’s law to an AC circuit gives the formula:

Z

V I

Z is the circuit impedance in ohms. V is the voltage. I is the current. The impedance is a vector quantity, which is described by an amplitude and a phase. In eddy current testing, the most important impedance is that which exists across a test coil (Figure 2.25). The coil can be regarded as an indicative reactance and resistance in series. The capacitive reactance of the coil is negligible. The impedance (Figure 2.26) in this case has two components which are vectors that are at right angles to each other.

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Let us say that we know that the inductive reactance of the coil is 4ohms and its resistance is 3ohms. If the two quantities were scaler we would simply add them together to find the impedance. However, they are vectors and must be added together vectorily as described in Section 2.3.8. To add the two vectors we can draw an impedance diagram as shown, from which we find that the impedance is 5ohms. Alternatively we could find the impedance vector by using Pythagoras’s theorem and solving the equation:

You may sometimes see this written in the form Z = R + j XL Where j = √-1 and is the mathematical operator which rotates the XL vector through 90°. The angle ợ gives the angle between the voltage and current phases in the coil. This is because the voltage is in phase with the current in resistance and 90° lead of the current in the inductive reactance. The voltage vector can therefore be substituted for the impedance vector. The angle ợ can be solved from:

arctan  

XL R

Let us now say that the inductive reactance of the coil is reduced to 3ohms because of the increased eddy currents in the coil’s magnetic field which dissipate more heat and therefore increase the coil’s resistance to 4ohms. The coil’s impedance is still 5ohms, but the phase angle between voltage and current has changed by 160°. There has been a phase change but no amplitude change. A simple meter reading circuit would miss this change. Similar hypothetical changes in the coil’s impedance could be used to show changes in the amplitude of the impedance but not in the phase between voltage and current. Normally of course we are dealing with combinations of inductive and resistive components that can be described by movements in a point at the end of the impedance vector to any position on the impedance diagram. A diagram of this sort can be displayed on a cathode ray tube and is called a vector point or more colloquially, a flying dot or spot display. A complete phase and amplitude analysis can then be made of the coil impedance. Other facts which we have already realised can be described on the impedance diagram. If the coil has no resistance, then it is a pure inductor, the impedance equals the inductive reactance and the voltage leads the current by 90°. If the coil has no inductive reactance, then the coil is a pure resistor, the impedance will equal the resistance and the voltage and current are in phase. As described in section 2.3.7 this occurs when dc is passed through the coil.

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2.3.7

Frequency The inductive reactance and the capacitive reactance depend upon the frequency of the ac current, as can be seen from the following equations:

2 XL Xc f L c

= = = = =





1 2

Inductive reactance. Capacitive reactance. Test ac frequency. Circuit inductance. Circuit capitance.

Figure 2.27 Vectors.

2.3.8

Vectors Some physical quantities are described by a single number. These are scalar quantities. Examples of scalar quantities are speed, temperature and weight. Others have a directional quantity as well and cannot be described by a single number. These are vector quantities (Figure 2.27). Examples include velocity, force and coil impedance. If we represent a vector as a point in space and it moves to another point in space we say it undergoes a displacement. Displacement is a vector quantity, because it is to be described completely we must know its magnitude and its direction. Another feature of vectors is that if two vectors are to be equal, they must have the same magnitude and the same direction. Vectors which have the same magnitude but not the same direction cannot be equal. A typical vector problem is shown. An aeroplane flies 20km in a direction 60°N of east, then 30km straight east then 10km straight north. Where will it end up?

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We can plot the vectors as components in a rectangular (cartesian) co-ordinate system on a scaled diagram. The resultant vector R and its direction can be measured from the diagram or calculated. For eddy current testing we use vector diagrams to describe impedance in a coil. Let’s simulate the vector problem we have just solved in an impedance diagram. To begin with the equivalent circuit for a coil, we have an inductive reactance and resistance in series. The voltage across the resistance is in phase with the current so we shall replace the x co-ordinate with this. The voltage across the inductive reactance leads the current by 90° and we shall replace the y coordinate with this. Initially the voltage across the whole coil is 20 millivolts and leads the current by 60°. The voltage across the resistance then increases by 30 millivolts and across the inductive reactance increases by 10 millivolts. The voltage across the whole coil now becomes 48.4 millivolts and it leads the current by 34.3°. 2.4

Eddy currents Eddy currents are electrical currents induced in metals by alternating magnetic fields. They are closed loops of current which circulate in a plane perpendicular to the magnetic flux except at the surface, where they will flow parallel with that surface. For eddy current testing, the magnetic fields are generated by a coil carrying high frequency AC. When the coil is brought into close proximity with a metal, the alternating magnetic field induces the eddy currents. The eddy currents are encircled by their own magnetic fields which are in a direction to oppose the field from the coil which is generating them. They therefore have a choking effect on the coil current. The choking effect, which is reflected in the coil’s impedance, is monitored by the eddy current instrument. Changes in the eddy current field due to changes in the metals properties near the surface, cause changes in the coil’s impedance. These are the test signals. It is difficult to understand the process without the conceptual models of the physicist. These are enshrined in the classical laws of Faraday and Lenz and in Maxwell’s equations. The following sections describe the factors which affect the eddy current field.

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Table 2.1 Conductivities. Materials Silver Copper Gold Aluminium Al-6101 Al-5052 7075-T6 Magnesium Phosphor bronze Cartridge brass Admiralty brass Tungsten Nickel 98Cu-2Ni 70Cu-30Ni 70Cu-22Ni Iron Platinum Tantalum Carbon steel Chromium steel Cobalt steel Stainless steel 501 Stainless steel 410 Stainless steel 304 Lead Monel Zirconium Titanium

2.4.1

IACS% 105 100 75 61 56 35 32 37 15 28 24 30 23 35 4.5 5.7 18 16 14 9.5 6.1 6.3 4.5 3 2.5 8.4 3.6 3.4 3.1

µΩ•cm 1.6 1.7 2.35 5.3 3.1 4.93 5.3 4.6 10.5 6.2 7.0 5.65 7.98 4.99 37 30 9.7 10.6 12.45 18 29 28 40 57 70 20.6 48.2 50 54.8 Resistivity

Electrical conductivity (ợ) Conductivity is a measure of the ease with which electrons flow in a material and will therefore determine the eddy current density. Conductivity is the inverse of resistivity. Some tables of material properties will list one, some tables will list the other and this can be very confusing. Resistivity is usually given in µΩ•cm and conductivity in mΩ•mm² or siemens/m. To add to the confusion, in eddy current testing, conductivities are usually measured in IACS. This is the International Annealed copper standard which ranks pure annealed copper as 100% IACS and air as 0%. The conversion factors are: 100%IACS = 58m/Ω mm² = 5.8 10



/

/

1m/Ω mm² = 106 siemens/m

1siemen / m 

10 8 1 cm

Conductivity depends on a number of material properties. It will depend upon its composition, temperature, hardness, temperature history and cold working. Any discontinuity within the material matrix which obstructs the free flow of electrons will reduce the conductivity. This is why increasing alloy composition will reduce conductivity. Eddy current testing therefore makes a useful sorter of mixed alloys, particularly of aluminium-magnesium alloys.

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Conductivity is affected by heat treatment of the material. This feature can be used in assessing the fitness-for-purpose of aluminium aircraft components damaged in engine burn-outs and tyre bursts. The hardening of the aluminium alloy increases its conductivity. However, it must be remembered that at very high temperatures this effect can be reversed. Table 2.2 Magnetic permeabilities. Material 0.1%C steel

0.34% C steel

Annealed Normalised Cast

Max.rel. µ 2420 1950 2100

Annealed Normalised Cast

1200 970 840

Mn steel

2.4.2

1300

Spheroidal graphite

Pearlitic Annealed

290 1150

Grey iron

As-cast Annealed

315 1560

Magnetic permeability Permeability has a dominant effect on eddy currents. The noise created by permeability changes in ferrous welds makes the eddy current technique a difficult method to apply to weld inspection. Another problem lies in the inspection of non-magnetic condenser tubes, where ferrous baffle plates can often give a noise level high enough to obliterate defect signals from the tube wall. Recent advances in eddy current testing do seem to be overcoming these problems. As well as introducing high levels of noise to the eddy current test, permeability also reduces the depth of penetration of the eddy currents to the extent that only surface discontinuities can be detected. The permeability effects can magnetically. Beyond saturation This can only be accomplished in current test coil sits between two

be removed by saturating the material a ferromagnetic behaves as a paramagnetic. pipe and bar testing systems, where the eddy powerful DC coils that encircle the pipe or bar.

The magnetic permeability is reduced to unity if the ferromagnetic is heated above its curie point. For mild steel, this lies at about 720°C. Saturationmagnetisation is not necessary therefore when testing hot bar and billet with eddy currents. Magnetic and Relative Permeability are further described in Section 2.2.3 with maximum values of relative permeabilities given in Table 2.2 above.

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Measurement of magnetic permeability does provide useful information about ferromagnetic materials. This is the basis of ferrous segregators and electromagnetic sorting bridges. Permeability is affected by:     

Thermal processing history. Mechanical working. Internal stresses. Temperature. Chemical composition.

The equipment used in these material sorters is based upon the principles of eddy current testing, but because it is the inductive effects of magnetic materials in the test coil and not the eddy current effect which dominates over any given test signal, the methods are referred to as electromagnetic testing and not eddy current testing. See section 8 for further details. A typical use for the instrument is to sort out forgings in a batch which have been case-hardened. The method is surprisingly sensitive to even minor changes in the case depth.

Figure 2.28 Standard depth of penetration.

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Figure 2.29 Skin of currents around a slot.

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Figure 2.30 Standard depths of penetration.

2.4.3

Frequency The most important test variable is the frequency of the current sent through the test coil. Eddy current testing is done at frequencies from a few hertz to several megahertz. The most important effect of test frequency is upon the depth of penetration (Figure 2.28) of the eddy current field. As the frequency increases so the depth of penetration decreases. This is known as skin effect (Figure 2.29) and it can be defined by the formula:



500 f..

Where  is the standard depth of penetration in mm. f is the frequency in hertz. σ is the conductivity in m/   mm2.  is the permeability. When Eddy Currents flow in a Conducting material magnetic fields are produced that oppose the primary magnetic field, thus reducing the resultant magnetic flux and causing a reduction in current flow as the depth below the surface increases. This is known as the ‘skin effect’. The standard depth of penetration is defined as the depth below the surface (Figure 2.30) at which the intensity of the eddy current field has been reduced

1 to a value of e of its intensity at the surface. The function of e is the base of the natural logarithms. It is equal to 2.718 when taken to three decimal places. Therefore at the standard depth of penetration, the eddy current field intensity is at approximately one third of its surface value (36.8%).

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What a bizarre way of setting a standard, you may say. Well the intensity of eddy current field falls exponentially with increasing depth. The equation for a curve describing this decay is of the form: Intensity =

8

-depth

The intensity never actually reaches zero, so we take value beyond which the effect of eddy currents on the test coil is small. The standard depth of penetration acts as a good reference point to base frequencies used for finding subsurface discontinuities. Remember however, that there are eddy currents at greater depths that may affect coil impedances and that with thin wall tubes and solid bars it is their geometry that determines the depth of penetration, not the formula. Always use calibration blocks with discontinuities at known depths when setting sensitivities for low frequency testing. The reason for the exponential decay of eddy current intensity with increasing depth is that each layer of eddy current partially shields the next deeper layer from the coil’s magnetic field. The rate of decay in intensity falls as the depth increases and the eddy current intensity decreases, so that in theory the intensity reaches zero only at infinity. The formula only applies to the skin depth. High frequency eddy current testers are made sensitive to surface breaking slots well in excess of the standard depth of penetration because the eddy currents flow around the slot sides and tip. The standard depth of penetration is also dependent upon the conductivity and permeability of the material. An increase in the conductivity increases the intensity of the eddy currents at the surface, creating a greater shield against the coil’s magnetic field. The rate of decay therefore increases. Permeability has a very strong effect. Unless it can be removed from a ferromagnetic by magnetic saturation, the eddy currents are going to be contained to within a few microns of the surface.

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2.4.4

Figure 2.31 Edge effect.

Figure 2.32 Lift-off.

Figure 2.33 Fill factor.

Figure 2.34 Discontinuities.

Edge effect Edge effect (Figure 2.31) is the name given to the eddy current test noise caused by contours and edges to the test surface. Signals from cracks emanating from an edge can be difficult to detect unless the edge effect can first be cancelled or zeroed out on the meter.

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Surface probes are often held in fixtures or jigs that will keep the probe at a fixed distance from the edge, as it is scanned parallel with the edge. The edge effect is therefore kept constant. Where a ferromagnetic material abuts the edge, the edge effect is much stronger and it is necessary to use shielded probes where the coil’s external magnetic field can be constrained within a ferrite housing. For tube testing with different encircling coils, a few millimetres at each end of the tube cannot be tested because of the edge effect. 2.4.5

Lift-off Lift-off (Figure 2.32) is the term given to the eddy current test response to lifting a surface coil from the test surface. As the coil moves away, the magnetic coupling to the eddy current field weakens very rapidly. Small movements give a pronounced effect. The noise generated by the test coil as it scans a round surface would be too high unless measures are taken to lessen lift-off. These measures include tuning the coil with a capacitor and rotating the lift-off plane in the impedance diagram in a manner which reduces the lift-off effect. On the other hand, the lift-off effect can be used to measure the thickness of non-conductive paint coatings on a metal substrate.

2.4.6

Fill factor Fill factor (Figure 2.33) is the lift-off equivalent when using encircling coils. It is a measure of magnetic coupling between tube and coil. For an internal bobbin coil, the fill factor is measured as the square of the ratio of the coil diameter over tube diameter. for example:

ɳ For an encircling coil, the fill factor is the square of the ration of the tube diameter over the coil diameter. for example:

ɳ The fill factor can never exceed 1.0 and is more usually about 0.7. At high test speeds, large fill factors will inevitably result in damage to the coil. A fill factor below 0.6 will result in a low sensitivity. There is an exception in the case of electromagnetic sorting bridges. The test frequencies are low and the test signals are caused by the inductive effects of the ferromagnetic testpieces. Fill factor is then of less importance. 2.4.7

Discontinuities Only cracks and laminations which distort the eddy current field will give rise to eddy current test signals. Laminations parallel with the test surface will not be detected (Figure 2.34). A surface crack will increase the resistive path of the eddy currents and deflect them downwards so that their magnetic fields have less effect on the coil. Changes in both the inductive reactance and resistance of the coil can then be expected.

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Section 3 Equipment

3

Equipment

3.1

Circuits The circuits used in the eddy current test instruments are designed to amplify the very small changes in the coil current while keeping noise to a minimum. Although it is not necessary to know of the complexities of modern electronics it is both useful and interesting to know something of the principles. Early high frequency crack detectors have much in common with radio receivers. The coil is analogous to the radio aerial. The bridge circuit however, has always formed the basis of low frequency equipment.

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a

b

Figure 3.1 Series resonance curve: a

Simple circuit;

b

Double circuit.

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3.2

Simple circuits A very simple circuit for detecting changes in the coil impedance would consist of an oscillator to supply high frequency sinusoidal currents to the coil and a voltmeter connected across the coil, Figure 3.1a. The meter would be zeroed with the probe on the test surface so that the eddy current field affecting the coil’s impedance is in a steady state. As the probe crosses a crack, the eddy currents flow around the crack tip. The coil impedance changes creating a deflection in the voltmeter. The probe coil is in an absolute arrangement with the instrument circuit. Alternatively there could be a double arrangement, Figure 3.1b. The oscillator feeds current to a separate driver coil. In the steady state an E.M.F. is induced in the receiver coil by the driver coil and the eddy current field. A change in the eddy current field will again cause a change in the impedance of the receiver coil that will be recorded by the meter. These circuits would not make practical eddy current test instruments because the voltage changes due to quite major cracks would only be of the order of 0.1%.

3.2.1

Resonance circuits Resonance circuits are tuned circuits in which the coil’s inductive reactance is in resonance (Figure 3.3) with the capacitive reactance of a capacitor placed in the circuit. Small changes in the coil impedance can then be made to create large changes in the coil voltage. Resonance occurs in an AC circuit when the capacitive reactance equals the inductive reactance: XL = XC

2fL 

f

1 2fC

1 2 LC

f is frequency. C = capacitance (farads, F). L = Inductance (henrys, H). Resonance occurs at a unique frequency and for most practical purposes; this is done in the kHz-MHz range.

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Taking the example of an L-C-R series circuit, the variable capacitor XC is adjusted so that the oscillator frequency is the resonance frequency of the circuit:

Z  R2  ( X L  X C )2 Z R XL Xc

= = = =

Impedance of circuit (ohms). resistance (ohms). Inductive reactance (ohms). Capacitive reactance (ohms).

At resonance Z = R. By plotting XL, XC and R for various frequencies, it can be seen that at resonance, Z is at a minimum and therefore the voltage is at a minimum (Figure 3.4). A slight change in the coil impedance will displace the resonance frequency from the oscillator frequency and the circuit voltage will increase dramatically. High frequency eddy current testers usually have one absolute coil tuned in parallel with a fixed capacitor and do not have selectable frequencies. To maintain a reasonably constant coil impedance, the frequency for testing materials of low conductivity, for example austenitic stainless steel, may be up to 2MHz whereas for materials of too high conductivity, for example aluminium, may be no more than 500kHz. To test ferrous materials which have low conductivity but high permeability, the higher test frequency is used but with a probe at lower inductance.

Figure 3.2 Wheatstone bridge.

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Figure 3.3 Eddy current test AC bridge circuits.

Figure 3.4 Phase sensitive circuits.

3.2.2

Bridge circuits Most eddy current test instruments use AC bridge circuits to detect the very slight changes in the impedance of the test coil. These are modified forms of the Wheatstone Bridge (Figure 3.2) which is a classroom instrument used to measure resistances to a high degree of accuracy. Resistor R is adjusted until the meter reads zero. If current is not flowing through the meter then the potential at A equals the potential at C:

V AB VCB  V AD VCD VAB = VCB. VAD = VCD.

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Since the meter is zero, the current through P must be the same as the current through R and the current through Q must equal the current through X:

VBC I c Q V I P and AB  A  VCD I c X VAD I A R Q X=RX P Where: X is the unknown. R is adjustable. P and Q are the ratio arms that set the resolution of the bridge. In AC bridges (Figure 3.3), the resistors are replaced with impedances. These introduce voltage phases as well as amplitudes into the balancing. A typical crack detector may have an absolute test coil in one arm and a load in the other, or alternatively, one half of a differential coil in one arm and the other half of the differential coil in the other. For the sorting bridge, one arm contains a coil with the reference standard, the other arm the coil with the testpiece of unknown properties. The X and R controls are used to bring the bridges into balance by affecting both the amplitude and phase of the voltage through the meter. If the meter is replaced by a cathode ray tube, the sinusoidal voltages from the standard and test coils are adjusted until they are exactly 180° out of phase. The trace then appears as a horizontal line. 3.2.3

Phase-sensitive circuits Meters normally detect only changes in the amplitude of the coil voltage. They can however be made sensitive to changes in the phase of the voltage as well as by using a double bridge arrangement, Figure 3.4. The primary bridge circuit containing the test coils shown in this case as differential coils, are connected to a second phase-sensitive bridge which also receives the reference voltage. The meter is so arranged that it only receives current through the diodes which is in phase with the reference voltage. The signal voltage may for example change in phase only without a change in amplitude by moving to the right on the A-scan. The meter will respond because the proportion of current now entering the meter is increased. Phase-sensitive instruments are essential in low frequency work because of the effect of subsurface discontinuities upon the eddy current phase.

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3.3

Instruments The instruments used in eddy current testing range from pocket-sized paint thickness gauges to computer-controlled automated test systems. We shall concentrate on the meter reading and cathode ray tube display types.

Figure 3.5 Moving coil ammeter.

Figure 3.6 Lift off compensation.

3.3.1

Meter reading instruments Most eddy current testing instruments are meter reading. They are simple to use and the meter can be calibrated to measure conductivity, crack severity, paint thickness or many other test variables. For the level of sensitivity required, meters have to be of moving coil type (Figure 3.5). These measures mean values of the current. Since the mean value of an AC current is zero, the current has first to be rectified before measurement.

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The moving coil is rotated inside a magnetic field by the interaction between the current in the coil and the magnetic field between the magnets. The direction of the mechanical force is given by Fleming’s right hand rule and is against the coil spring. The great the current in the coil, the greater the force. Moving coil ammeters have a slow response due to the inertia in the spring. The meter will not respond fully to short eddy current signals generated as the probe scans the surface. For this reason, light-emitting diodes are incorporated, set to illuminate at predetermined levels. The diodes respond immediately. 3.3.2

Lift-off control Meter reading instruments that are used for crack detection have a lift-off control to deaden the effect of probe movement when scanning. Lift-off compensation (Figure 3.6) can be accomplished in a number of ways, which are best understood with reference to the impedance diagram. A simple sequence for setting the lift-off compensation is as follows: Figure 3.6 is the impedance diagram for an eddy current test circuit containing a coil, a variable capacitor and a variable resistor in series. When the coil is placed on the test surface the impedance meter reads 6 (OA). When the coil is placed on a thin sheet of cardboard the impedance meter reads 8(OB). A locus AB produced therefore represents the lift-off plane. The second Figure in Figure 3.6 shows the changes in impedance with adjustments to the lift-off control (variable capacitor) and zero control (variable resistor). Adjustments 1 2 3 4 5

With coil on the cardboard, increase LIFT-OFF until meter reads 6(OC). With coil on the test surface, the meter will now read 6.2(OD). Decrease ZERO until meter reads 6 once more (OE). With coil on the cardboard, the meter will read 5.8(OF). Decrease LIFT-OFF until meter reads 6 once more (OG). With coil on the test surface, the meter will read 5.9(OH). Increase ZERO until meter reads 6 once more (OI). Repeat the sequence of adjustments until the meter reads 6 both on the test surface and on the cardboard.

The locus AB will have moved to A1B1 on the circle of radius 6 as shown in the third Figure in Figure 3.6.

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Figure 3.7 Cathode ray storage scope.

Figure 3.8 CRT displays.

Figure 3.9 Vector point display.

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3.3.3

Cathode ray tubes Cathode ray tubes (Figure 3.7) can be used to analyse changes in the phase and amplitude of the eddy current test signal. An electron gun fires a beam of electrons between electrostatic plates, the X and Y plates. The electrons, which carry a negative charge, can be deflected upwards by putting a positive charge on the upper Y plate or to the left by putting a positive charge on the left hand X plate. The point written onto the phosphor screen by the electrons can therefore be moved to any position on the screen. In storage scopes (Figure 3.7) the illuminated spot on the phosphor screen can be retained when the electron beam is moved or switched off by flooding the screen with low speed electrons. These do not illuminate the screen but only continue to excite the phosphors, which have been hit by the high speed electrons from the electron gun. To erase the screen, the flood current is switched off.

3.3.4

A-scan display A-scan display shown on the left in Figure 3.8. A time base can be created between the X plates by applying a sawtooth-shaped pulse. This sends the electron beam from left to right and then almost instantly back to left to begin another sweep. If this is repeated one hundred times each second, then a continuous horizontal line will appear across the screen. Its length corresponds to 1/100th of a second. Time base sweeps of as little as one microsecond can be achieved so that extremely short transient signals can be seen. These signals are sent to the Y plates.

3.3.5

Ellipsoid display Ellipsoid display shown on the right in Figure 3.8. If two unrectified sinusoidal voltages are sent simultaneously to the X and Y plates, then an ellipsoid is formed on the cathode ray tube screen. The two voltages must have the same frequency. The phase and amplitudes of the two voltages will affect the shape of the ellipsoid. It can vary from a straight line when the voltages are in phase to a circle if the voltages are 90o out of phase. The tilt of the ellipsoid is affected by the relative amplitudes of the two voltages.

3.3.6

Vector point display Modern eddy current test instruments use a cathode ray tube display which simulates the impedance diagram, Figure 3.9. The signal voltage is first rectified and then split into sine and cosine components about an arbitrarily selected phase angle. The sine and cosine functions are arrived at electronically and of course give two components to the signal voltage which are 90o to each other. These can be regarded as the XL and R axes of the impedance diagram although their actual vector directions are controlled by the phase rotation control. With the cathode ray tube connected across a bridge circuit, an absolute coil is one arm and a load coil is the other, the variable capacitor (X) and variable resistor (R) are adjusted to bring the bridge to a state of balance. In this state, no current is entering the CRT and rotation of the phase control will have no effect on the vector point. Most instruments allow automatic balancing of the bridge circuit.

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The bridge should be balanced with the probe down on the test surface. Lifting the probe up will show the lift-off plane. This is usually rotated until it moves off to the left of the screen. The probe can then be moved over slots and towards the sides of a slotted testblock to give the crack and edge effect signals. The sensitivity control is used to alter the amplitudes of the signals. The frequency control will alter the phase angle between the signals. 3.4

Probes Eddy current test probes come in many forms. When selecting a probe there is the coil arrangement to consider and its effect on sensitivity. The coil size is constrained by high inductive reactances at high frequencies. Surface probes may need to be shaped to reach confined spaces. Encircling probes and internal bobbin probes should fit the tube as closely as possible. Finally, the probe has to match the circuitry of the instrument. There is not the ability to interchange like that is found in ultrasonic test equipment. Often it is necessary to make special probes and a probe-making facility becomes necessary where eddy current testing is used on a wide range of component shapes.

Figure 3.10 Coil arrangements.

3.4.1

Coil arrangements The coil arrangements (Figure 3.10) can be classified into four types. Single coils have the same coil both to drive the eddy currents and receive signals due to changes in the eddy current flow. The meter or cathode ray tube monitors the voltage across the coil. The circuit is suitable for the simple high frequency crack detectors where signals are confined to amplitude changes and noise from the subsurface eddy current field is negligible. The double coil arrangement has one coil to drive the eddy currents and another coil to receive the test signals. The voltage in the receiver coil is induced by eddy currents and the current in the driver coil. It is much less than the voltage in the driver coil alone and there is a higher signal to noise ratio.

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Differential coils are commonly used in tube testing. The coil is in two different halves, wound in opposition. The inductive reactance in one half is equal but opposite to the inductive reactance in the other half. Bipolar signals are produced when a discontinuity comes through the coil. The wavelength of the signal is dependent upon the separation of the coil halves and the speed of travel of the discontinuity. Signals are therefore suitable for modulation analysis, where only signals of a certain wavelength are allowed through the filters. Differential coils do not respond to gradual changes in tube dimension that would generate unacceptable levels of noise in absolute coils. However, they detect only the ends of continuous uniform defects lying parallel with probe travel. If the defect ends correspond with tube ends, differential coils will miss the defect entirely. By having a separate drive coil from the differential receiver coil in a double differential coil arrangement, noise levels are further reduced. The choice of differential or absolute coils for tube testing is a difficult one. Differential coils are less prone to temperature drift and ignore gradual changes in tube dimensions. Absolute coils give signals that are easier to interpret and do not miss longitudinal defects throughout the tube length. A combination of the two may be necessary: differential coils for a primary tester that will detect defective tubing and absolute coils for a fuller analysis.

Figure 3.11 Surface probe.

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Figure 3.12 Encircling shape.

Figure 3.13 Internal bobbine probe.

3.4.2

Surface probes Surface probes (Figure 3.11) induce an eddy current field which is parallel with the test surface. The field circulates about the probe and so there is good sensitivity to planar discontinuities in any plane except the one which is parallel with the surface. Laminar defects remain undetected. The simplest surface probes are pencil probes. These are used at high frequencies to detect surface breaking flaws. The coil is only a few millimetres long and is wrapped around a ferrite core to increase the flux density. In shielded pencil probes the coil is in a ferrite housing that pulls in the coil’s external field to reduce edge effects. The ferrite tip may be protected by stick PTFE tape. The bolt-hole probe is designed for insertion into a fastener or bolthole. The coil lies perpendicular to the hole bore and the split end will accommodate a small change in the hole diameter. The holder for the probe allows rotation at fixed depths within the hole. Manual manipulation of a probe in fastener holes using static eddy current testers is laborious and has largely been superseded by rotating probes and dynamic testers. Lower frequency probes have larger coil diameters and usually double differential coil arrangements. A ferrite housing is essential if the field width is to be kept reasonable. These larger probes are called pancake probes. Ring or doughnut probes are low frequency probes designed for testing around steel fasteners in a wing skin without the need of removing the fastener for a bolt-hole probe inspection. The ferrite core of the pancake probe has been removed and the ferromagnetism of the fasteners is relied upon to draw the coil flux down into the skin.

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3.4.3

Encircling probes For tube, rod and wiring testing, the coil is wrapped around the aperture of the probe and as close to the surface as possible (Figure 3.12). The fill factor (See Section 2.4.6) should be no less than 0.7 if sensitivity is to be kept high. Guiding the tube or bar through the coil at high speeds is difficult and this restricts the maximum fill factor that is attainable without danger of damage to the probe aperture. Electromagnetic sorting bridges have large coils. Fill factor is not critical as it is the inductive effects of the ferromagnetic testpiece inside the coil that dominates the coil impedance and not the eddy current flow.

3.4.4

Internal bobbin probes To inspect condenser tubes in heat exchangers, the probe (Figure 3.13) must be inserted into the tube as there is no access to the outside. Low fill factors of the order of 0.65 are necessary because of probe jams in dented tubes. The probe is first fired with compressed air to the tube end and then retrieved at a constant speed of 200-300mm/sec while the test signals are recorded.

3.4.5

Remote Field Eddy Current (RFET) RFET uses an Eddy Current send-receive type probe technique for tube testing (usually from the tube inner) that operates in both differential and absolute modes simultaneously such that localised defects can be detected with the differential mode and gradual defects with the absolute mode. The detector coils are separated by the equivalent of two or three times the tube diameter and are equally sensitive to internal and external indications with tube wall loss being measured through variations in phase.

3.5

Calibration blocks Calibration blocks are a vital part of eddy current testing. The tests rely on the appropriate design of calibration blocks and reference standards to an extent greater than any other NDT method. Eddy current fields are too complex for any quantitative assessments of signals. Signals can only be compared with those from known discontinuities. Cracks must be compared with slots thinning with stepped wedges, tube wall defects with through drilled holes and conductivity measurements with IACS testblocks.

Figure 3.14 HF slotted calibration block.

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Figure 3.15 Ring probe calibration block.

3.5.1

Slotted calibration blocks High frequency surface crack detectors are calibrated on blocks of the test material which contain 0.5 and 1mm deep spark eroded slots. Aluminium, mild steel and austenitic stainless steel blocks (Figure 3.14) are readily supplied. For meter reading instruments, the zero and lift-off are set with the probe on the block, away from the edge or slots. The probe is then scanned over the 0.5mm slot to obtain the greatest meter deflection and then held steady while the meter sensitivity is adjusted to give a 40% deflection of full scale. Proceeding to the 1mm slot, the sensitivity is adjusted to give an 80% deflection. Signal deflections from the testpiece can now be compared with those from the slots. A threshold may be set at 25% of full scale deflection and signals above this investigated. On no account should measurements of crack depth be based on comparisons with the reference deflections. Crack morphology will differ greatly from that of the slot. Low frequency eddy current instruments for detecting subsurface cracks must be calibrated with the slots at the required depth below the surface. This may be accomplished by placing a plate of the test material over the slotted block. The frequency should be set to give a standard depth of penetration which is about 110% of the thickness of the cover plate. Depth of penetration has to be traded off against test sensitivity. It should be just enough to reach the subsurface slots but not so great as to give poor signal to noise separation. Moreover, if the eddy current field penetrates too far, noise may be picked up from features below the layer of interest. Special blocks have been designed for calibrating ring probes for detecting cracks emanating from steel fasteners in a wing spar (Figure 3.15). The frequency is first set to give a standard depth of penetration greater than the skin thickness. The lift-off and zero of the instrument are set with the ring probe over a flaw-free fastener. Then the ring probe is moved to a fastener with one slot and the sensitivity adjusted to give a 50% full scale meter deflection. Then finally to a fastener with two slots to give a 100% full scale meter deflection. When moved to the testpiece, noise levels due to variations between the permeabilities of the fasteners are very high. This makes inspection difficult. In all cases, the calibration block sets the sensitivity level only. Lift-off and zero have to be reset when the probe is moved to the testpiece.

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Figure 3.16 Step wedges.

Figure 3.17 Tube standards.

3.5.2

Step wedges The meter deflection can be sent to indicate wall thinning in a thin metal plate. The frequency is set to give a standard depth of penetration just beyond the plate thickness but not so great as to be affected by deeper metal substrates. The illustrated step wedge (Figure 3.16) could be used to indicate 50% thinning in a 2mm thick wing skin by setting the zero, 50 and 100% full scale meter deflections on the 2.0, 1.5 and 1.0mm steps. Remember that the eddy current fields respond to volume changes rather than changes in the residual wall thickness. A deep conical shaped pit may give no greater meter deflection than a shallow but flat area of thinning. To assess the depth of thinning, two methods can be used that both involve the construction on graph paper of calibration curves that note the response of the meter to known changes in thickness. In the first, a curve is constructed at a fixed frequency that gives field penetration just below the wall being measured. This is suited to larger areas of thinning. In the second, the frequency is adjusted and its value noted at which thinning to known depths get a response on the meter. This is more suited to pitting. In either case, the method, although providing useful information, cannot give a reliable level of accuracy.

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3.5.3

Tube standards Manufactured tube is usually tested for through defects that may cause leaks (Figure 3.17). The through drilled hole therefore gives a suitable reference signal. For condenser tube inspection, corrosion on the inner tube surfaces has to be distinguished from corrosion on the outer tube surface. This is done by setting up the instrument on tubes containing machined slots or flats. The frequency is set to give a 90° phase difference between the two surfaces as they appear on the cathode ray tube display. This can be done because an internal groove will appear as a change in fill factor to an internal bobbin probe, while an external groove will appear as a change in wall thickness. The f90 frequency as it is called can be found by analysis of the impedance diagram. It is approximately 110% of one standard depth of penetration. The use of Impedance diagrams is covered in further detail in Section 6, Phase Analysis. The use of calibration blocks in tube testing is covered in greater detail within Section 10 of the course notes.

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Section 4 Practices

4

Practices

4.1

Documentation Proper documentation of non-destructive tests is essential if they are to have a meaningful role in quality control. For eddy current testing this is even more important because the specifications and procedures which do exist tend to be ambiguous and the tests must be tied down to more specific requirements, including applications to products, manufacturing processes and in-service inspection.

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Eddy current methods Technique sheet Technique no: Component identification: Area of examination: Purpose of examination: Equipment required:

Instrument: Probes: Calibration blocks:

Preparation of component: Examination procedure: a) Instrument calibration 1. Initial setting 2. Sensitivity setting 3. Alarm threshold setting b) Test procedure Probe position

Setting-up procedure

c) Acceptance standard

Reporting procedure: Additional information:

Prepared by:

Qualification:

Date:

Approved by:

Qualification:

Date:

Figure 4.1 Technique sheet.

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Techniques

An NDT technique is a way of using an NDT method within the constraints of a procedure. It is definitive in approach and does not allow the operator to exercise choice in carrying out the test. The NDT procedure is a more general document, which describes how, where, when and why NDT is to be applied. The technique is prepared according to the procedure, in the light of past experience and a knowledge of the defects sought. A good technique will provide coverage, be concise and give clear instructions. It may have to be modified if experience indicates improvements, even to the extent of changing the test method. The document must therefore allow for subsequent amendments and be part of a system in which amendments can be released to all concerned. The technique must be approved by someone in authority who is suitably qualified and experienced in the specific NDT technique and who will have a sound working knowledge of NDT and product technology including the product application and defects sought by the test. A suitably qualified and experienced Eddy Current NDT Technician (Level 2) may prepare the Technique sheet and also carry out the test as required. Technique writing requires discipline. The blank form (Figure 4.1) shows the main subject areas to be covered but the actual document may need to be extended to several pages to include diagrams of calibration blocks and special probes, and the extent of probe test coverage.

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Eddy current methods Test report - Practical exercises report sheet Course no:

Date:

Test operator(s): Component:

Number

Equipment: Instrument: Probes: Test frequency(ies): Sensitivity setting: Defect threshold level: Lift-off setting: Scanning procedure: DIAGRAM SHOWING LOCATION AND LENGTH OF DISCONTINUITIES

Name: Qualification:

Signature:

Figure 4.2 Test report.

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4.1.1

Test reports Any NDT report should: 1 2 3 4

5

Be properly documented with a report number and date (Figure 4.2). Refer to a technique (Figure 4.1) which will give details of the test operation. Contain enough information for the test to be repeated under identical conditions. It should give details of the equipment used, calibration standards and where possible instrument serial numbers. Record the results of the test. Where a diagram is used this should show the datum used to locate flaws. Major defects such as cracks should be measured and their lengths given with perhaps the maximum signal amplitude as an indication of crack depth. Spurious and non-relevant indications must not be recorded. Show the signature of the test operator, as well as his name and qualifications.

If no defects are present, then words should be chosen carefully. Phrases such as no significant defects are ambiguous. All defects are significant because they are defined as those flaws which create a substantial risk of failure. They are therefore outside of specification. Phrases like acceptable to specification or no indications are preferable. 4.2

Applications Eddy current testing has an ever-expanding repertoire of applications. The problem lies in isolating the discontinuities which may be signals in one application but noise in another.

4.2.1

Crack detection Eddy current crack detection equipment can be divided into high frequency instruments for finding surface breaking cracks in ferrous and non-ferrous materials and low frequency instruments for finding cracks in non-ferrous materials. Detection of subsurface cracks in ferrous materials in possible but only when it has been saturated magnetically to remove permeability effects. This is a complex affair and is only practicable in automated tube testing systems. Eddy current test are the most sensitive of all NDT methods to surface cracks. High frequencies of the order of 2MHz give high resolution, but the probes are small and covering large surface areas takes a long time. Low frequency crack detectors need larger probes to accommodate for suitable coil inductances. The frequency setting is critical and is in the range 100100kHz depending on the depth of penetration that is required. Subsurface eddy current fields are influenced more by phase changes than amplitude changes and therefore phase sensing circuits are essential. Although traditionally they were meter reading instruments, the trend in crack detectors is towards instruments with cathode ray tube displays for their added versatility.

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4.2.2

Tube and wire testing Automated eddy current test systems have been developed for the inspection of tube, bar and wire at speeds of up to 3 metres per second. Once the operator has calibrated the instrument using a tube or wire with known flaws, the test installation runs automatically, ejecting defective pieces from the production line or marking them with paint. The mechanical handling equipment for the test pieces becomes so complex that the actual eddy current test instrumentation may appear an insignificant part. Facilities for magnetic saturation and demagnetisation of ferrous tubes and wires increase the capital costs considerably. The constant test speeds and differential coils allow for modulation of the test signal with the speed and then filtering to remove noise. Unfortunately, when using differential coils, it is possible to pass through tubes with consistent defects throughout their whole length, without detection. Because of the edge effect, tube ends cannot be detected. Extrusion defects along the centre of bar cannot be detected either because the eddy current field from an encircling coil is at zero intensity at the centre of a solid cylinder.

4.2.3

Condenser tube inspection This application is currently receiving a great deal of attention in connection with the heat exchangers of pressurised water reactors. Tube thinning is the main defect and by selecting what is known as the f90 frequency, signals from thinning on the outside surface can be set 90o out of phase from signals from thinning on the inside surface. By recording the X and Y signals from the impedance diagram on a two-channel strip chart recorder, the extent of thinning can be ascertained at test speeds of 200-300mm per second. A major problem is caused by the baffle plates which separate the condenser tubes. The tubes are non-magnetic, stainless steel, cupro-nickel or more recently, titanium. The baffle plates are ferrous and the permeability signal is enough to obliterate signals from thinning between the tube and baffle plate. To alleviate this problem, instruments have been developed to operate with two frequencies simultaneously. The separate signal phases are then mixed in a manner which removes unwanted permeability effects. Inspection is usually done with differential coils because they do not drift with temperature changes. The signal interpretation is more difficult and it is often necessary to do supplementary tests with absolute coils. Other recent developments include the use of computers to analyse X and Y channels for defect signals. The inspection can then be done in real time.

4.2.4

Material sorting Ferrous segregators and electromagnetic sorting bridges are useful tools in sorting steels which have been hardened. Conductivity meters can be used to sort aluminium and copper alloys, both for compositional variations and hardness variations.

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Great care has to be taken to ensure that the variation being detected is the relevant one. For example, the change in the conductivity of an aluminium may be due to a change in composition or a change in its hardness. Because eddy current fields penetrate below the surface of the test material, the method does provide a better sample of material properties than many other material sorting methods and more importantly it is very rapid. 4.2.5

Weld testing Simple high frequency eddy current testers have been used for some time to detect toe cracks in ferrous welds. The method has the advantage in being able to detect cracks through paint layers. The disadvantages lie in the high noise levels caused by permeability changes in the weld and lift-off noise from rough cap surfaces. Recent devices have to some extent overcome these problems. They are being used to supplement magnetic particle inspections under water, to distinguish strong spurious indications from toe cracks. The equipment uses a cathode ray tube with a vector point display and special coil arrangements.

4.2.6

Coating thickness measurement The high near surface resolution of eddy current tests makes it useful for measuring coatings, metallic and paint, on metal substrates.

4.2.7

Ferrite Testing Ferrite testing is undertaken to determine the ferrite content (usually as a percentage) in austenitic stainless steel, duplex steel welds and cladding to ensure that the residual ferrite content is within a specific range that is compatible with the mechanical strength requirements and corrosion resistant properties needed. The ferrite meter is an eddy current conductivity meter typically used on welded and clad vessels used in the petrochemical and process plant industries.

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Table 4.1 Logarithms of numbers 10-49.

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Table 4.2 Logarithms of numbers 50-99.

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Table 4.3 Antilogarithms 00-49.

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Table 4.4 Antilogarithms 50-99.

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Table 4.5 functions of angles 1-45°.

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Section 5 AC Theory

5

AC Theory Alternating current theory was introduced during section 1 of this course. The reason for phase leads or lags between voltage and current in an AC circuit is now explained. For those who are used to mathematical concepts, equations are introduced for eddy current flow, as is the j notation as a shorthand way of operating vector quantities. The effects of inductors and capacitors in AC networks are investigated.

Figure 5.1 Voltage (A, B, C, D, E and F) and current across a capacitor.

Figure 5.2 Voltage (A, B, C, D, E and F) and current across an inductor.

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5.1

Capacitive reactance Across a capacitor we are concerned with the build-up of electric charge on the plates of two electrodes which face each other. The rate of build-up of charge on the two plates will depend upon the voltage in the circuit. We therefore use the voltage as our reference and construct the current wave onto it. In the diagram of an alternating current (Figure 5.1), the voltage is changing at its greatest rate at A, C and E. At these points the current away from the electrodes of the capacitor will be at its greatest. At B, D and F the voltage change is zero and therefore the current will be zero. From A to B the voltage increasing in the positive direction and therefore the current will be positive. From B to C the voltage is decreasing in the positive direction and therefore the current will be negative. We can therefore draw the current wave on to the voltage wave and show that in an AC circuit which has only a capacitive reactance, the current leads the voltage by 90o.

5.1.1

Inductive reactance In an inductor (Figure 5.2), when the current changes there is a self-induced EMF which by Lenz’s law acts in opposition EMF is at a maximum. At B, D and F the rate of change of current is zero and therefore the induced EMF is zero. According to Lenz’s law, at A the current is going to positive and therefore the induced EMF will be negative. Similarly at C the current is going to negative and therefore the induced EMF will be positive. The induced EMF opposes the applied EMF and therefore the voltage. The current lags the voltage by 90o. In real AC circuits, there are combinations of capacitive and inductive reactances and of resistances. The resistances in effect loses electrical energy as heat. The capacitance temporarily stores energy in an electrostatic field and the inductance temporarily stores it is a magnetic field. Both reactances return the energy into electricity but cause a displacement between the voltage and current. In eddy current testing we are mainly interested in coils. These have an inductance and resistance and even a very small capacitive reactance can become quite significant at high frequencies.

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Figure 5.3 Eddy current induction.

5.1.2

Equations for eddy current flow Equations for eddy current induction is shown in Figure 5.3. For a proper analysis of eddy current effects it is necessary to express the variables as mathematical equations. In this course we are interested in the practical effects, for which only a superficial knowledge of the mathematical analyses is necessary. We can start with a look at Faraday’s laws. The value from current that is varying sinusoidally with time at any instant given by: = o sin (  t) Where: = instantaneous value of the current at time, t. o = peak value of the current.  = angular velocity = 2πx frequency in hertz. Oersted discovered that the amount of magnetic flux in a current carrying coil is given by:

= N Where:  = flux. N = number of turns in the coil. = coil current. Faraday’s laws state that there is a voltage induced whenever the flux changes and that its magnitude is dependent upon that rate of change. Since the voltage opposes the change, according to Lenz’s law: V=–

d dt

Where V = induced voltage.

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Figure 5.4 AC series circuit.

Figure 5.5 AC Parallel L-R circuit.

5.1.3

AC Series circuit The voltage and current phase relationships in an AC circuit (Figure 5.4) in which the capacitor, inductor and resistor are in series have been dealt with in section 1.

Since

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The impedance (Z) is given by:

R 2  ( XL  X C )2

Z=

and arctan Where  is the phase angle between the voltage and current. It XL is greater and XC then the circuit is effectively inductive and current lags the voltage. If XC is greater than XL then the circuit is effectively capacitive and the current leads the voltage. If XL = XC then the circuit is effectively resistive and is in a state of resonance. We dealt with series resonance in Section 1. To reiterate, at resonance the impedance of the circuit falls to a minimum and is equal to the resistance. The energy oscillating between electrostatic energy and magnetic energy in the capacitor and inductor respectively. Series resonance circuits act as acceptor filters. Frequencies outside the bandwidth of the filter are rejected. This is used in radio communications to tune the radio to a particular frequency. 5.1.4

Parallel L–R circuits Unlike the series circuit, the supply voltage is now taken as common to all branches and it is the current which is divided into the networks (Figure 5.5). and the voltage and current are in phase in the resistive branch.



and the voltage leads the current by 90o in the inductive branch.

From the vector diagram.





=

+





Since Z =

1

1

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Figure 5.6 Parellel L-C circuit.

Figure 5.7 Parallel resonance.

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5.1.5

Parallel L-C circuit Current in the capacitive branch is given by

Current in the inductive branch is given by

and is lagging the voltage by 90°.

From the vector diagram (Figure 5.6), the current voltage may be leading or lagging the voltage, depending upon whether the inductive reactance or the capacitive reactance is the greater.



1 1

1

When XL = XC and the circuit is in resonance, then Z becomes infinite. 5.1.6

Parallel circuit resonance In the case of a coil, which has both inductance and resistance and is in parallel with a capacitance, we can see what happens as the frequency of the supply current increases. When the frequency is zero (DC condition) the coil reactance is zero so that only the coil resistance limits the current. The capacitive reactance is infinite and therefore lC is zero. As the frequency increase so the coil reactance increases and the current decrease, lagging at a progressively greater angle from the voltage. On the other hand, the capacitive reactance decreases so that increases but always remains 90° leading the voltage. At some frequency XL = XC and resonance will occur, the circuit impedance will reach a maximum and the circuit current a minimum. At resonance, current is oscillating between the inductance and capacitance. Only a small current is needed from the supply to make good resistive losses in the coil. A parallel resonance network (Figure 5.7) acts as a rejecter of resonance frequencies in the circuit.

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Figure 5.8 Addition of vectors.

5.1.7

j Operator j is a mathematical operator which rotates a vector clockwise through 90° without changing its magnitude. For example, we can split the impedance vector into the inductive reactance and resistance components such that: XL = Zsin Φ and R = Zcos Φ the where  is the phase angle between impedance and resistance. For phase angle has been rotated through 90 degrees and this can be denoted by j. Thus a commonly used shorthand version of the formula|:

R 2  X L2

Z=

is Z = R + j Where the underline indicates that we are dealing with vector and not scalar quantities. From the diagram it can be seen that two operations of j (=j2) rotate the phasor through 180o and it in effect becomes -1 j2 = -1



1

An application of the j operator is shown in Figure 5.8. The resulting vector of adding a+jb and c+jd is given by (a+c) + j (b+d). Similarly the resulting vector from subtracting the vectors is given by (c-a) + j (d-b).

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Section 6 Phase Analysis

6

Phase Analysis The signals generated in an eddy current test are vector and not scalar quantities. That is to say, they are properly described by two quantities, their amplitude and phase and not just by one quantity, their amplitude. Phase analysis of the test signal using a cathode ray oscilloscope instead of mere amplitude measurements with a meter; allow a greater level of differentiation between relevant signals and unwanted noise. The effect of eddy currents on the coil impedance is described on the impedance plane diagram and various analytical standards are introduced. Methods of suppressing undesired noise are described.

Figure 6.1 Signal and noise separation.

6.1

Signal/noise separation One of the most difficult problems in an eddy current test is the separation of signals from non-relevant noise. Many tests are impossible because signals from flaws cannot be distinguished from background noise. This is particularly true of testing ferrous welds, where magnetic permeability effects can obliterate crack signals.

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There are three conventional approaches to the problem: Firstly, amplitude analysis uses the amplitude of the incoming signal. It may give a deflection on a meter or strip-chart recorder. There are severe limitations to this method. Non- relevant signals from lift-off and edge effect may exceed in amplitude those from the crack. Secondly, phase analysis uses the phase as well as the amplitude of the signal. The phase displacement between the output or reference signal and the input or incoming signal is analysed with a cathode ray oscilloscope in an A-scan, ellipsoid, or vector point display. The A-scan displays the signal on a time base. The ellipsoid is created by the interference of the output signal across the Xplates with the input signal across the Y-plates. The vector point display divides the signal into real and imaginery components that are sent to the X- and Yplates of the oscilloscope. Discrimination is still not possible if the non-relevant signal has both the same phase and amplitude as the relevant one. Finally, the frequency of signal may form the basis of an analysis. It must first be modulated and this is done with the test speed. Either the testpiece passes the probe at a fixed speed as is usually the case in tube testing or the probe scans the test surface at a constant speed, as is the case in rotating probes for bolt hole inspection. Non-relevant signals due to minor dimensional changes will have a long wavelength and low frequency which can be filtered out from relevant, relatively high frequency signals from flaws. The test speed must be chosen carefully for the desired discrimination between signal and noise and must be constant. Dynamic testing using signals modulated with the test speed combined with phase analysis of the filtered signals provides the most sensitive method of eddy current testing (Figure 6.1). 6.2

Phase analysis Eddy currents have surprisingly well ordered effects on the amplitude and phase of coil voltages. Thanks mainly to the efforts of Dr Forster in the immediately post-war years; these effects have been rationalised using mathematics. Computers can now be used with a good level of accuracy to predict the eddy current test responses to simulated defects. Although well beyond the scope of this text and arguably often taken beyond the needs of NDT, impedance analysis is a fascinating subject, allowing greater insight into the nature of eddy currents and will be dealt with in a simplified version here.

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Figure 6.2 Idealised impedance.

Figure 6.3 Normalised impedance.

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6.3

Idealised impedance diagram The reduction of a complex electrical circuit into a simple equivalent circuit is a common method of analysis. We can, for example, regard the test coil as a primary winding of a transformer and the eddy current field as the secondary winding. The eddy currents therefore load the current in the test coil as the output voltage loads the input voltage of a transformer and simple electrical analysis can be done (Figure 6.2). in the testpiece. This can be transferred The eddy currents meet a resistance back to the primary circuit by multiplying by the turns ratio squared. If we regard the eddy current field as a one turn coil only, then the new resistance will appear in parallel with the coil as a shunt. Ignoring any other resistance or capacitance in the circuit, the impedance Z is given by:

1

Z=

 1   NP R E

2

  1      X   L

2

When the coil is in air, When

=  and Z will equal XL.

= O and the testpiece is a perfect conductor, then Z = O. If the

conductance of the shunt and

are equal then

The impedance therefore describes a semicircle. 6.4



√

Normalised impedance The circuit impedance depends upon probe parameters as well as frequency and the eddy current field. The coil diameter, coil length, number of turns and coil material all have an effect. Therefore impedance analysis could only be done for individual coils and separate diagrams would have to be constructed for each coil. To overcome this problem, normalised impedance diagrams are used (Figure 6.3). The effect of the coil parameters is removed by dividing the impedance components by the inductive reactance of the coil in air when it is away from any eddy current field. The coil wire and cable resistance must also be subtracted from the resistance component, RL=R-RDC and

and

are

dimensionless and independent of the coil inductance and resistance.

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Figure 6.4 Impedance diagram for a surface coil.

Figure 6.5 Impedance diagram for a surface coil.

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6.5

Conductivity Figure 6.5 shows how the lift-off impedance planes on a normalised impedance graph come from unity in the empty coil condition down to the test surface along a locus of points which show increasing conductivity in a downward direction. With the coil in air, the relative impedance is at 1. As the coil comes down onto the metal the impedance moves along one of the lift-off impedance planes until it meets the locus of increasing conductivity. Metals of different conductivity can therefore be identified according to the direction of their lift-off impedance plane. Notice that the separation between lift-off and conductivity is greatest near the knee of the curve. Lift-off effects and conductivity changes are almost inseparable at the top of the curve. The length of the lift-off planes is greater on good conductors than on poor conductors. The lift-off planes may be calibrated to measure paint thickness on metal substrates. A high degree of resolution will be attainable on good conductors.

6.6

Magnetic permeability Very small increases in permeability send the (Figure 6.5). Permeability variations of only 1-5 found in austenitic stainless steels but for low likely to be 500-1000. It is evident therefore dominate changes in the coil impedance.

coil impedance up the graph (relative permeability) may be carbon steels the variation is that permeability effects will

On ferromagnetics which are good conductors, the permeability and conductivity impedance planes are almost parallel whereas on ferromagnetics of low conductivity they are at right angles to each other.

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Figure 6.6 Impedance diagram for a surface coil.

Figure 6.7 Lift-off plane at two frequencies.

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Figure 6.8 Impedance diagram for a surface coil.

6.7

Thickness As the thickness of the test material decreases, so its resistance will increase and the impedance will be expected to move up the curve (Figure 6.6). However, we have to take into account skin effect and therefore only a finite thickness will have an effect. This thickness will depend upon the frequency of the alternating magnetic field. On a material of a particular conductivity and at a particular test frequency, we can expect the impedance to leave the conductivity plane as the thickness approaches the standard depth which will affect the coil impedance. As the thickness decreases, the impedance moves in a characteristic spiral that it in consequence of skin depth and phase lag. The lift-off and thickness impedance planes are at 180o apart on good conductors and nearly parallel on poor conductors.

6.8

Frequency Frequency has a similar effect to conductivity. As it increases so the impedance moves down the graph. At low frequencies, the depth of penetration is greater and the resistance of the testpiece has a more significant effect on coil impedance. Generally, frequencies are selected to operate near the knee of the curve. Frequency is the most important variable that can be controlled in the test. It determines the phase angles between different impedance planes and therefore the ease with which different effects can be discriminated.

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The example (Figure 6.7 and 6.8) shows how the lift-off planes at different conductivities are more easily discriminated at high frequencies than at low frequencies. However, at high frequencies there is more noise due to pronounced lift-off signals and increased skin effect. (Figure 6.8).

Figure 6.9 Impendance diagram for a surface coil.

Figure 6.10 Impedance diagram for a surface coil.

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Figure 6.11 Impedance diagram for an encircling coil.

6.9

Probe diameter Probe diameter has the same effect as frequency (Figure 6.9). As it increases so the impedance moves down the curve. Therefore when working at low frequencies it helps to use a large diameter probe to bring the impedance close to the knee of the curve.

6.10

Characteristic parameter Various methods have been developed to combine all the effects of conductivity, permeability, thickness, frequency and coil parameters in one impedance diagram (Figure 6.10). One method uses the characteristic parameter PC. Where:

i.d.  o.d. 4 r = mean coil radius in metres =  = angular velocity = 2 radians. 

 = absolute permeability (for non-magnetics = 4 10

 = electrical conductivity in siemens/metre.

henry/metre).

For non-magnetic the equation may be written as 7.9 10



Where:  = resistivity in



 cm.



r = mean radius in mm. The solid lines represent PC values increasing from zero to infinity, while holding the coil at a constant lift-off from the surface.

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Test conditions with the same characteristic parameter will operate at the same point on the impedance diagram. It shows, for example, that lift-off can be best discriminated from other effects at high test frequencies. 6.11

Characteristic frequency Similar in function to the characteristic parameter, the characteristic frequency was derived by Forster for the setting of test parameters in testing tubes and cylinders. The characteristic frequency is the frequency for which the Bessel function solutions to Maxwell’s equations equals one. Maxwell’s equation describe electromagnetic induction and the Bessel functions are a way of solving them (Figure 6.11). For thick wall tubes and cylinders



For thin wall tubes

5066

5066 °

Where: Do = external diameter, cm. t = wall thickness.

Where: Do = external diameter, cm.  = conductivity in m/   mm2

The equation for thick wall tubes applies when the wall thickness is much greater than δ. For thin wall tubes it applies when the wall thickness is much smaller than δ. (ie standard depth of penetration, mm). Forster’s similarity law states that two eddy current changes will have a similar effect on the coil impedance if their f/fg ratios are the same.

Figure 6.12 Eddy current standard depth of penetration.

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Figure 6.13 Eddy current distribution in solid cylinders.

,



Figure 6.14 Eddy current phase lag with depth.

6.12

Skin effect This term is used to describe the concentration of eddy currents at the surface of the metal, just beneath the test coil. Eddy currents are closed loops of current that flow perpendicular to magnetic flux from the coil but are deflected parallel with the surface contours.

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Each layer of eddy current shields the layer of eddy currents below it, thus reducing the strength of the magnetic field. The intensity of the eddy currents at any particular depth will depend upon the intensity of the eddy current above it, which leads to an exponential decay that can be expressed as: sin



Where: lo = eddy current intensity at the surface. ld = eddy current intensity at depth d. δ = depth at which intensity of eddy currents is reduced to

1 of its value at the e

surface ie standard depth of penetration (Figure 6.12). d = eddy current depth; e=constant 2.718 (Natural logarithm base). For infinitely thick material and where fields are planar, that is to say induced by large diameter long coils, then the depth at which the intensity of the eddy 1 (approximately 37%) of its value at the surface is given by: currents is e



500 f

Where: δ = standard depth of penetration, mm.  = conductivity, m/   mm2.  = permeability. f = frequency, Hz. (See also Section 2.4.3 – Frequency). At 2δ the intensity is surface.

1 1 (13.5%) and at 3δ (5%) of the intensity at the 2 e e3

This formula is applicable only under strict conditions. For thin wall tubes the current intensity drops less quickly and for solid cylinders inside encircling coils it is always zero at the centre (Figure 6.13). Although the eddy currents may be restricted within thin strata in a component, the magnetic field may extend beyond to generate further skins of eddy current flow. This effect has significant application in, for example, testing wing spars underneath the skin of an aircraft.

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The phase angle of the eddy currents also changes with depth (Figure 6.14). ∝ sin or thick material phase lag

d d radians x 57o.  

At  the phase lag is 57°. At 2  it is 114°. This phenomenon allows us to distinguish defects on the inside from those on the outside surfaces of a tube by selecting frequencies at which there is a 90° separation.

Figure 6.15 Phase discrimination between lift off and thickness.

Figure 6.16 Suppression of undesired effects.

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6.13

Phase discrimination Successful eddy current test rely to a large extent on adjusting the test frequency to get as wide a phase angle as possible between impedance planes due to unwanted noise and impedance planes due to relevant signals. The impedance diagrams show how a particular frequency has been chosen so that lift-off and thickness variations (Figure 6.15) are at almost 90° to each other. This occurs when

t is approximately 0.8. The sensitivity is increased 

until the vector point display covers the relevant part of the impedance diagram that includes the nominal thickness being measured. The whole diagram is then rotated on the display so that the lift-off plane is horizontal and moves off to the left of the screen. Y movements now correspond to changes in thickness and X movements to unwanted lift-off effects. 6.14

Suppression of undesired effects In eddy current tests using meters and bridge circuits, the undesired effects (Figure 6.16) can be suppressed by selecting a null point for the bridge on the impedance diagram which is at the centre of curvature of a curved impedance plane. Of course impedance planes are rarely circular and so the suppression can only work for a limited number of impedance changes. Another problem is that only one undesired effect can be suppressed when two or even three effects may be contributing noise to the test. The diagrams show bridge null points that have been selected in the right hand diagram to suppress lift-off effects to measure conductivity and in the left hand diagram to suppress conductivity effects in order to measure permeability. A preset lift-off of this nature is used in conductivity meters. Notice however that the lift-off will work satisfactorily only over a limited range of conductivity values. Very high values of conductivity or very low values will require a different null point.

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Figure 6.17 Impedance signals at different frequencies.

Figure 6.18 Dual frequency mixing.

6.15

Multifrequency testing The impedance plane diagrams (Figure 6.17) show that the phase angles between impedance planes vary with frequency. The phase angles between liftoff, cracks and permeability may be at one set of values at one frequency and at another set of values at a different frequency. If two test frequencies are used simultaneously in the test coil and displayed separately on the vector point display, then it may be possible to subtract unwanted signals. This is the basis of multifrequency tests. Tests with two frequencies are now common but even five frequencies can be processed and analysed with the help of a computer.

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Firstly two frequencies are selected to give good phase discrimination. The one frequency is likely to be about ten times the other. The phase and sensitivity of the impedances at each frequency are then adjusted independently on the vector point display. In the example shown (Figure 6.18), it is the permeability effect that is to be removed. When the permeability plane at one frequency is 180° out of phase with the permeability plane at the other, then they can be mixed to give a third vector point which has only lift-off (L) and crack planes (C). This can then be adjusted in the usual way so that the lift-off plane is horizontal and off to the left of the screen. In summary, in single frequency tests, one variable can be suppressed using phase analysis. With two frequencies, two variables can be suppressed. With three frequencies, three variables and so on (Figure 6.18). The more frequencies there are, the more complex the electronics and the greater the possibility of extraneous non-relevant signals due to cross-talk between the frequencies.

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Section 7 Instrumentation

7

Instrumentation The instruments used for phase analysis have developed quickly in recent years. Solid state display instruments are improving the portability of eddy current testing equipment. Cathode Ray Tube (CRT) instruments are still in common use and are described in Section 7.1. In the probes, innovative coil arrangements are helping to improve signal to noise separation and induce eddy current fields with improved sensitivity to defects with certain orientations. Dynamic test systems in which signals are modulated by the test speed provide a means of filtering unwanted noise from the test.

Figure 7.1 Cathode ray oscilloscope schematic.

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Figure 7.2 Lissajous Figure.

7.1

Cathode ray oscilloscopes Phase analysis is carried out with cathode ray oscilloscopes (Figure 7.1). The instruments used in eddy current testing are adapted from those used to investigate networks in electronic and telecommunication engineering. Special purpose instruments in rugged cases and with battery operation are available and because they are easily portable are used for site or field applications. Cathode ray oscilloscopes can be used to measure voltage, current, time, frequency and phase. They have a very high impedance and only a slight loading effect on the circuits to which they are connected. They are capable of resolving signals from a few millivolts to a few hundred volts at frequencies up to 1GHz. The cathode ray tube was dealt with in Section 3.3.3. A schematic design of a cathode ray oscilloscope to give an A-scan vector point or ellipsoid display is shown. For an A-scan the input signal from the bridge circuit that contains the test coil comes in through the Y-amplifier. The attenuator is present to reduce the input signals in discrete measured amounts if measurements are to be made of signal amplitude. To the X-amplifier is fed a saw-tooth waveform from the time-base generator. This drives the beam horizontally across the screen at a fixed repetition rate. To synchronise the flyback of the electron beam to zero, with the input signal so that the next input signal superimposed on the next time base sweep, the pulse generator is also connected to the Y-amplifier. For vector point displays, the time base generator is not needed and the Xamplifier is driven from another signal source. Both the X and Y signals are rectified and averaged. They are derived by splitting the bridge signal into sine and cosine components electronically using the phase angle between the coil voltage and a reference voltage. The sine and cosine components are of course at 90° to each other and form the axes of the impedance diagram display.

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If the X and Y signals are both sine waves of the same frequency, a Lissajous Figure is obtained (Figure 7.2). The sensitivities of the amplifiers are adjusted so that the height of the variation of the beam in the Y-direction is equal to the width of the variation of the beam in the X-direction. If both amplifiers drive the beam positive at the same time and negative at the same time, they are in phase and a straight line is seen. If they are 90o out of phase, a circle forms and if 180o out of phase a straight line across the screen in the other direction is formed. If Vx sine ( t  ) is the voltage applied to the X-amplifier and Vy sin (t ) is the voltage applied to the Y-amplifier, then when the voltage at the Y-amplifier is zero,  t  0 and the voltage across the Y-amplifier is Vx and is given by OB.

OA Vx sin    sin  OB Vx The ratio would therefore give the phase angle between the two signals. Lissajous figures were commonly used in tube and wire testing, where test parameters can be selected so that the ellipse would only open up in the presence of defects.

Figure 7.3 Send-receive coils.

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Figure 7.4 Hall effect transducer.

7.2

Send-receive coils Send-receive or double coil arrangements are used particularly at the lower test frequencies to reduce temperature drift (Figure 7.3). They can be found in both encircling and surface probes. The magnetomotive force generated by the oscillator in the primary winding  . The current is relatively constant despite temperature changes equal because of the high resistance in the circuit. The receiver coils are two coils wound in opposite directions. The secondary circuit is connected to a high impedance amplifier and therefore the induced voltage (proportional to Np  ocos  t) is not affected by coil resistance either. In the presence of a conductor the receiver coils receive two signals. One is transmitted from the primary coil and the other is reflected from the eddy currents in the conductor. The reflected signal is picked up by the receiver coil near the conductor, but not by the other coil which is too far away. This state of imbalance creates the signal.

7.3

Hall effect probes Detector-coils have the disadvantage of being affected by the frequency as well as intensity of the eddy currents. Hall effect probes will detect eddy current fields and will give a voltage signal which is not frequency dependent. Moreover they can be made very small in size. The hall effect is caused when a magnetic field passes through a special semiconductor material transducer across which a current is flowing (Figure 7.4). The current trajectory is deflected so that electrons tend to build up to one side of the element creating a voltage. This voltage is proportional to the vertical component of the magnetic field.

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Figure 7.5 Filters.

7.4

Dynamic testing Many eddy current tests are carried out at a constant test speed so that signals can be modulated and then filtered to improve the signal to noise separation. Applications include tube and wire testing and the rotating probes used in fastener hole inspection. They normally employ differential coils that will generate bipolar signals that can be filtered. The filter arrangement illustrated will only accept one frequency. The input signal first meets an acceptor filter which is a series circuit of capacitance and inductance set to resonate at the required frequency. It therefore has low impedance at this frequency, whereas other frequencies are rejected or diminished depending upon the bandwidth of the filter. The bandwidth is defined between the signal intensities to which they fall to

1 2

of peak and is

dependent upon the Q factor (ratio of reactive power to active power) of the series circuit. The second rejector filter, where the capacitance and inductance are resonating in parallel at the required frequency, prevents more of the unwanted frequencies reaching the amplifier. At resonance, the parallel resonance circuit is at maximum impedance and therefore effectively forces the signal into the amplifier (Figure 7.5). 7.5

Frequency response Measures the time required to respond to a signal and is particularly important when testing at high speeds. It is defined as the frequency at which the output signal fall to

1 2

of the maximum.

Therefore if the two halves of an encircling differential coil are d mm wide and d the test speed is s mm/sec then the duration of each signal is sec and the s s Hz if sensitivity is to be maintained. This frequency response will need to be d should be regarded as a minimum. A frequency response of twice the signal time is preferable.

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Cathode ray tubes have a very high frequency response but it can be reduced drastically with high levels of input signals. Meters give a poor frequency response and should therefore be supplemented with a light emitting diode to give a visible alarm if signals exceed the threshold levels. The frequency response of chart records varies from 100Hz for ink pen types to 1kHz for ultraviolet marker types.

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Section 8 Material Sorting

8

Material Sorting There are two properties of metals that can be used for material sorting with eddy current test equipment. The first is electrical conductivity and the second is magnetic permeability. The measurements are purely relative however, relative to some standard for calibration, which must be chosen carefully. Drawing conclusions about the composition or metallurgical characteristics of a testpiece from an eddy test is very difficult because of the diversity of variables which affect conductivity and permeability.

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Figure 8.1 Circuit diagram for a conductivity meter.

Figure 8.2 Resistivity variations with aluminium alloy contents.

Figure 8.3 Cold age hardened Al alloys.

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Figure 8.4 Heat damage to aluminium.

Copyright © TWI Ltd

8.1

Conductivity meters Conductivity measurements are quite straightforward and do not normally require a phase analysis. A typical bridge circuit with a meter is shown. The bridge is unbalanced as previously described to suppress the effects of lift-off. There are number of causes of error however. If the frequency is low and the material section being tested is thin, then the measurement will be affected by thickness variations and the presence of a different conductor in the substrate. The thickness should be at least 3  . If the frequency is high then surface inhomogeneities, for example thin oxide layers, will interfere with the measurement. Although the double probes used in most conductivity meters are insensitive to temperature changes, the conductivity of the test material and the reference standards will be sensitive to ambient temperature (Figure 8.1).

1  1 (1  T ) 2 Where

 is the thermal coefficient of resistivity.

The conductivities of aluminium reference standards used for calibration have been known to drift over a period of years due to ageing. Surface curvature, edge conductivity readings.

effect

and

other

discontinuities

will

all

affect

A ferrite meter is an example of a conductivity meter and its application is described in Section 4.2.7. 8.2

Conductivity effects Conductivity can vary with a number of factors. Some of the more useful are shown (Figures 8.2, 8.3 and 8.4). In particular, conductivity measurements can be useful in detecting heat damage in aluminium. Cold working tends to decrease conductivity by introducing dislocations into the metal lattice. Cold working has a very marked effect on measurements taken on austenitic stainless steels, but this is due to increasing permeability rather than changes in conductivity.

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Figure 8.5 Frequency selection.

Figure 8.6 Fundamental and harmonic spead bands.

Figure 8.7 Bainte formation in austenitic.

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8.3

Electromagnetic sorting bridges Electromagnetic (EM) sorting bridges offer a rapid method of sorting ferromagnetic materials. Although the magnetic permeability effects predominate in these tests there is some sensitivity to conductivity changes as well. A bridge circuit is used with two test coils. One contains a standard, the other a testpiece. A CRT with A-scan display shows a sine wave that is the resultant from summing vectorily the sine waves generate in the coils containing the standard and testpiece respectively. If the standard and testpiece are identical then the resultant is a straight line. A degree of difference will produce a fundamental wave the shape of which will be affected by amplitude and phase differences in the two waves. Moreover, the presence of harmonics over and above the fundamental frequency can also be an important distinguishing feature. The sensitivity is adjusted to give the required level of distinction between the grades of material that are being sorted. This is attained by experimentation. Too high a sensitivity will show up differences in every testpiece. Ideally the largest spread bands should be symmetrical about a vertical line on the screen. However, harmonics can be as equally important in distinguishing spread bands as the fundamental waves (Figures 8.5 and 8.6). Better resolution of permeability from conductivity is obtained at low test frequencies. At high frequencies, conductivity and permeability effects may cancel each other out. Conductivity effects are generally the result of compositional changes and can be important. At low frequencies they tend to change the harmonics rather than the fundamental. The permeability of the test material is dependent upon the applied magnetic field strength. On the magnetisation curve, at high levels of (H) the permeability falls and this should be avoided in bridge sorting. Among the variables which affect the EM bridges are: 1 2 3 4 5

Thermal processing. Mechanical processing. Chemical composition. Internal stresses. Temperature.

Internal stresses reduce the permeability as a result of magnetostriction. The notable exception is austenitic stainless steel, where working leads to the formation of magnetic bainite (Figure 8.7) which increases the permeability.

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Figure 8.8 Automatic bridge sorter.

8.4

Bridge sorting variables Analysis of the wave forms produced, with attention given to the harmonics as well as the fundamental waves, can differentiate materials on the basis of chemical composition, hardness, structure and dimension. In carbon steels, increasing carbon content decreases the conductivity and the permeability but compositional changes are generally overshadowed by heat treatment. In low alloy steels there is a significant fall in permeability with increase in alloy composition. The assessment of case hardening depth is a very important application, but the properties of the core material must remain constant. It is easier to quantify the measurements on induction or flame hardened testpieces than on carburised or nitrided cases, because the former involve changes in the metal structure only and not the composition.

8.5

Automatic gates Bridge sorters can be automated (Figure 8.8) to receive test components on a conveyor and sort them into receiver bins. High test speeds can be attained and because every component can be inspected, the system, when built into a production line, provides a very useful tool in quality control.

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In the system illustrated, a standard is held stationary in the reference standard coil while the testpieces move continuously through the test coil. A reading is taken when the testpiece is in the centre of the coil. This corresponds to the point of inversion of the sine waves on the CRT. The test speed must be slow enough for at least 2-3 cycles of the magnetising current to give an impedance signal. For this reason long coils have been developed for systems in which the testpiece drops through the coil. Many automated systems use a vector point display, with the screen divided into quadrants and monitored. An electronic counter counts the number of impedances which occur in each quadrant. At the end of the batch of testpieces the Figures can be analysed statistically to indicate the quality of the batch. 8.6

Standards The choice of suitable standards for material sorting is vital. They should be of the same size and shape, with identical composition and heat treatment and similar surface finish. They should be demagnetised and attention should be given to any stresses which may be set up due to cold working. Allowance should be made for temperature increases due to the induced currents. Finally, play safe and have two or three standards available.

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Section 9 Crack Detection

9

Crack Detection Eddy current tests provide the most sensitive of all the non-destructive methods for detecting cracks. However, there are considerations of crack orientation to the eddy current flow to be taken into account and there is very little penetration, particularly in ferrous materials, below the surface. Moreover, it is very easy to misinterpret signals. Two recent developments in weld testing and bolt hole inspection show how eddy current test techniques can be developed to overcome problems with test noise.

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Figure 9.1 CRT with flying dot display.

Figure 9.2 Surface coil.

Figure 9.3 Directional properties of a pancake probe.

Figure 9.4 Coil arrangement for scanning rivets.

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9.1

Universal crack detectors For use over a wide range of applications a CRT is used with a vector point display (Figure 9.1). It is connected to a bridge circuit with the test probe containing an absolute coil in one of the arms or a differential coil in two of the arms. The test frequency has to be varied to accommodate different test conditions. Not only is depth of penetration a factor to be considered but also the phase discrimination between relevant impedance planes and unwanted effects have to be maximised. A frequency range of 1kHz-10MHz is common. Most instruments have automatic bridge balancing. The sensitivity control affects the bridge output signal and not the coil current. It may be calibrated in decibels, a scale in which an increase of 6dB is equivalent to doubling the signal amplitude. Some instruments do have controls for the coil current. This compensates for gross imbalances in the bridge when using coils at very low or very high frequencies. The quadrature components of the bridge are generated as sine and cosine phasors of the voltage using the current as datum and dialling in a value for the phase angle from the phase rotation control. The phase rotation control does not give an absolute value therefore. It needs a reference which is often taken as the lift-off plane set horizontally off to the right of the screen.

9.2

Surface coils Bridge circuits require two similar coils. If one senses the testpieces, then it is an absolute coil. If both sense the testpiece it is a differential coil. Differential coils are not sensitive to gradual changes and temperature drift (Figure 9.2). Ferrite cores are needed to increase the inductances of very small coils; they provide a small surface contact area and resist wear. The increase in inductance increases the

XL ratio of the coil and therefore reduces the relative importance R

of the temperature effects upon R. When using the flatter pancake probes (Figure 9.3), the directional properties of the coil become important. As well as being insensitive to laminar flaws, there is zero sensitivity at the coil centre. There is little sensitivity to flaws parallel with the coil winding, but there is maximum sensitivity to flaws across the coil winding. Gap probes have a magnetic field which shapes the eddy current flow to cross laminar defects. Many ingenious coil arrangements have been developed for special applications. The one shown has four receiver coils dispersed equidistantly around a central send coil. In the balanced defect-free condition, a uniform eddy current field is set up around the send coil. When the coils pass along rivets in an aircraft skin (Figure 9.4), distortions are created which form a regular pattern of signals with the receive coils. When a crack is present, however, an increased distortion is created. By suitable selection of test parameters the vector point movement on the impedance diagram due to normal rivet distortions can be clearly distinguished from a movement due to crack distortions.

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Figure 9.5 Slot depth.

Figure 9.6 Impedance display of weld toe.

Figure 9.7 Rotating probes.

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9.3

Crack detection Surface breaking cracks are not affected by skin depth. They deflect the eddy currents down and around the tip of the crack. They therefore increase the resistive path of the eddy currents and change the orientation of the magnetic field generated by the eddy currents. These have an effect on both the coil’s inductive reactance and resistance. A phase shift can be detected which rotates the impedance plane slightly in a clockwise direction as the cracks become deeper. An interesting phenomenon is observed when comparing edge effect with a slot signal. The edge effect is along a different impedance plane. As another edge is brought up to the coil, therefore simulating a very wide slot which gradually gets narrower, so the edge effect plane moves up towards the slot original. Subsurface cracks rotate the impedance plane clockwise. This phase rotation can be used to assess crack depth below the surface (Figure 9.5). Very tight cracks may leak the current but when all is said and done, of all the NDT methods, eddy current test are the most sensitive to cracks.

9.4

Weld testing The testing of ferrous welds is made difficult by the roughness of the weld cap and changes in the magnetic permeability along the heat affected zone. Eddy current methods offer significant advantages over magnetic particle inspection, however, because paint layers do not have to be removed and troublesome spurious indication in the weld toe can be identified. The permeability variations in the weld toe can be very great due to hardening in the heat affected zone. Conventional vector point displays drift to such an extent therefore, that crack signals become impossible to identify. A recently developed eddy current testing device, however, uses a special coil arrangement to balance out permeability effects. It displays impedance signals from cracks in a manner that is readily discernible from noise due to probe movement to and fro across the weld toe (Figure 9.6).

9.5

Rotating probes Inspection of fastener holes with bolt hole probes is a laborious process. Manual rotation of the probe gives rise to high levels of noise due to wobble. A much more efficient method uses a differential coil that creates a signal that is modulating with the probe rotating (Figure 9.7) at constant high speed inside the hole. An A-scan is created on the CRT for every rotation of the coil inside the hole. By marking it off in degrees from a datum coinciding with a marker on the probe, the angular position of the crack is indicated. By modulating the signal much unwanted noise can be filtered away. The two halves of the differential coil are wrapped in a Figure eight over the forked ferrite core. Cracks often propagate right through the fastener hole and if the halves of the differential coil were opposite each other they would both pass through crack simultaneously and there would not be a signal.

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To adjust test frequency and set the phase so that the impedance plane of one unwanted signal is horizontal and does not appear on the A-scan, a vector point is used as an alternative display on the CRT.

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Section 10 Tube Testing

10

Tube Testing Eddy current testing has been used to inspect tubes from the very early days in the development of the method. It is rapid, can be made sensitive to a wide range of defect conditions and the equipment can be fully automated. There are two distinct applications; one is for manufactured tube on line and the other is for condenser (and heat exchangers) tubes in situ. The latter in particular has captured much attention of late because of its importance in the maintenance of nuclear (and petrochemical) plant.

Figure 10.1 Tube tester with DC saturation.

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Figure 10.2 Condenser tube tester.

10.1

Manufactured tube testing Eddy currents have been used in the testing of tube since the 1930s. Most of Dr Forster’s pioneering work was done on the theoretical analysis of eddy current fields in tubes, cylinders and wires. His mathematical solutions were proven by experimenting with glass tubes filled with mercury containing plastic inserts to simulate discontinuities. The equipment used today is highly automated, often computer-controlled and capable of test speeds of up to 6m/sec. The block diagram shows a circuit plan for eddy current test system for inspecting ferrous tubes. The permeability effects of the ferrous material have to be overcome to make the tests sensitive to subsurface flaws. This is accomplished by including DC magnetic saturation coils in the test head,Figure 10.1. The test probe has a double differential coil arrangement. The primary coils also generate a reference voltage that is used to discriminate the phase of the secondary coil voltage.

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A Lissajous display is used to calibrate the instrument on a standard tube with reference flaws. Adjustments are made to test frequency, signal amplitude and phase to give the best discrimination between signals and noise. When a defect signal is detected it appears on a strip chart recorder to provide a permanent record. The signal also triggers the paint gun to mark the position of the defect and operate the sorting gate. 10.2

Condenser tube inspection To test condenser tubes (Figure 10.2) in heat exchangers, an internal probe is fired to the end of the tube and retracted at a constant speed of about 200mm/sec. Signal analysis is more complex, particularly in view of the presence of ferrous baffle plates that separate the condenser tubes. Their ferromagnetic properties give a level of noise that may obliterate test signals from corrosion between the plate and tube. Efforts have been made to design multi frequency tests that overcome this problem. The test results of a condenser tube inspection have in the past been recorded as X and Y deflections on a two channel strip chart recorder. The several hundred metres of recorded chart from one heat exchanger are then inspected visually. There can then be a problem in identifying the defective tube in the heat exchanger are then inspected visually. There can then be a problem in identifying the defective tube in the heat exchanger when inspecting the results of several hundred tube tests. This has led to the development of a real time testing system that uses a computer to analyse the test results. A backup recording on a video provides a permanent record.

10.3

Probes For testing manufactured tubes up to 50mm in diameter, encircling coils are used. Beyond 50mm the sensitivity to all but gross defects are reduced and surface coils are needed that orbit the tube. Encircling coils are not sensitive to purely circumferential planar flaws or laminar flaws. The depth of penetration is determined by test frequency except in the case of solid cylinders, where the intensity of eddy current is always zero at the centre despite the test frequency. Most tube tests are carried out with the differential coil arrangements. These are not sensitive to temperature drift and gradual insignificant changes in the tube dimension and are less affected by tube wobble than absolute coils. However, they only detect the ends of longitudinal flaws and will miss entirely uniform longitudinal flaws that extend the whole tube length. This problem has exercised the minds of equipment developers for many years. Where continuous defects can arise, for example in seam welded tubes, then it is evidently not worth the risk in having differential encircling coils. Absolute encircling coils and surface coils, however, are much noisier and test speeds are greatly reduced. To test ferrous tubes for internal defects the tube wall must be saturated magnetically. This can only be done with dc energised fields that are superimposed upon the AC fields of the test coils. Furthermore, the tube has to be demagnetised afterwards. The conventional diminishing AC field will only demagnetise the surface because of skin effect and so a slowly reversing DC field is necessary. This reduces test speeds considerably.

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Figure 10.3 Forster curve for tube.

Figure 10.4 f90 frequency.

Figure 10.5 Phase angle changes at f90 frequency.

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10.4

Test frequency The test frequency is the most important variable used in controlling the eddy current test. It determines the depth of penetration of the eddy current field and the phase discrimination between noise and signals. For setting the frequency when testing manufactured tube for through defects the Forster curves are often employed (Figure 10.3). These were derived by plotting the

f ration on a normalised impedance diagram. f is the test fg

frequency and fg is the characteristic frequency defined by:

5066 Where:  = Relative permeability.

 Do t

= Conductivity, m  / mm 2. = External diameter, cm. = Wall thickness, cm.

A similar formula is used to define the characteristic frequency of solid cylinders.

fg 

5066 d 2

Where d is the diameter of the cylinder, cm. According to Forster’s similarity law, geometrically similar defects result in the same eddy current effect if their

f ratios are the same. If for example, a fg

particular defect in one tube gives a particular eddy current signal, it will give the same signal in a tube of different diameter if the

f ratio is adjusted to the fg

same value. Balanced sensitivity to defects, conductivity changes and dimensional changes are obtained on the knee of the

f curve, where the ratio is approximately fg

equal to six for solid cylinders. To distinguish ferromagnetic inclusions, then an

f of about two provides an fg

almost 90o separation between permeability effects and conductivity changes.

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For testing condenser tubes with internal bore probes, it is important to distinguish between thinning of the tube wall due to corrosion on the inside surface, from that due to corrosion on the outside surface. This is done at the f90 frequency, Figure 10.4 and 10.5. To an internal coil, thinning from the outside of the tube is seen as a reduction in wall thickness, while thinning from the inside of the tube is seen as a reduction in fill factor. A frequency can be selected where these two effects have impedance planes at 90o to each other. It occurs when the nominal wall thickness is approximately 1.1 of the standard depth of penetration:

f90 

3 kHz t2

Where: = Resistivity in  t

.cm.

= Wall thickness in mm.

As can be seen in the normalised impedance diagram, the phase angle between the internal and external slots varies from a few degrees at low frequencies, to also 180o at high frequencies. The impedance plane for a hole occurs between the slots. It coincides with the impedance plane, when thinning from both the inside and outside surfaces meet and the tube wall disappears. There is a relationship between amount of thinning and the phase angle that can be used to determine the residual wall thickness.

Figure 10.6 Coil dimensions.

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Figure 10.7 Tube inspection signal patterns.

10.5

Coil size The closer the coil fits the tube, the higher will be the magnetic coupling between coil and tube and therefore the greater the sensitivity of the test. A tight fit cannot be used because either the tube must be free to move inside the coil or the coil inside the tube. The fill factor is used as a measure of coupling. 2

D    t  Dc

  for encircling probes 

D    c  Dt

  for internal probes 

2

Where: Dc = coil diameter. Dt = tube diameter.  =fill factor. Probe damage is a constant problem where tubes have to be fed through encircling coils at high speed. Internal probes often get stuck inside condenser tubes due to dents. For these applications, fill factors no better than 0.7 are used. Ideally as in the diagram shown (Figure 10.6), the fill factor should be such that the gap between coil and tube should be approximately half the wall thickness. With differential coils, the distance between the differentially wound halves of the coils should be considered because along with the test speed, it will determine the frequency of signals. These must not exceed the frequency response of the instrument and will determine the bandwidths of the filters.

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10.6

Signal patterns The diagram illustrates the signal patters which may be derived from passing an absolute and a differential coil probe through a stainless steel condenser tube (Figure 10.7). The signals are derived form an internal slot, an external slot, a through drilled hole, the ferrous baffle plate, a magnetite deposit and a dent. The differential coil gives characteristic petal-shaped impedance patterns. As the leading coil passes the defect, the vector point extends around one petal, coming back to the origin when the defect is between the coils, before extending around the petal in the opposite quadrant. Although differential signals are more difficult to analyse, there is no drift in the balanced vector point that can be expected when using absolute coils. Condenser tube inspection is conducted at the f90 frequency. The impedance diagram is rotated so that the impedance planes for slots on the inside surface are horizontal and slots on the outside surface are vertical. X and Y movements in the vector point are then recorded on a two channel strip chart recorder. By comparing the pen movements on the X and Y channels, the different flaws can be distinguished.

10.7

Reference standards Eddy current instruments for testing tubes must be calibrated with tubes containing reference flaws. These are usually machined flats, longitudinal EDM (electrodischarge machined) slots, circumferential EDM notches and drilled holes. These reference standards should be easy to fabricate, reproducible, precisely sized and should closely resemble the natural flaw. Drilled holes are more commonly used where through defects that will cause leaking are sought. Machined flats are more suitable for detecting thinning. If used to set the f90 frequency, they will need to be on both the inside and outside surfaces.

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Section 11 Eddy Current for Welding Inspection

11

Eddy Current for Welding Inspection

11.1

Introduction Traditionally surface crack detection in ferritic steel welds with eddy current techniques has been difficult due to the change in material properties in the heat affected zone. These typically produce signals much larger than crack signals. Sophisticated probe design and construction, combined with modern electronic equipment, have largely overcome the traditional problems and now enable the advantages of eddy current techniques to be applied to in-service inspection of ferritic steel structures in the as-we!ded conditions. Specifically, the advantage of the technique is that under quantifiable conditions an inspection may now be carried out through corrosion protection systems. This means the costly removal and replacement of the protective coating is now not necessary. An additional advantage is that, on detection of surface breaking defects, the amplitude of the signals obtained, given the appropriate corrections for coating thickness, geometry etc. can be compared directly with the slots in the calibration block, therefore enabling decisions on appropriate remedial action to be taken immediately. The general principles of Eddy Current, Non Destructive Testing are described in BS EN ISO 15549.

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11.2

Eddy current application overview Eddy current testing is based on inducing electrical currents in the material to be inspected and observing the interaction between these currents and the material. The process is as follows:

Figure 11.1 Test material – conductor.

1 2 3 4 5

When a changing magnetic field intersects an electrical conductor, eddy currents are induced according to Faraday’s and Ohm’s Laws. Consider this to be the excitation or primary magnetic field. The induced electrical currents (known as eddy currents because of their closed circulatory path) generate their own magnetic field. Consider this to be the secondary magnetic field. This secondary magnetic field opposes the primary magnetic field and an equilibrium results. The primary field is changed – therefore the electrical properties of the coil are changed – specifically the property known as the Electrical Impedance. By monitoring the changes in coil impedance the electrical, magnetic and geometric properties of the component can be measured.

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Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux, Figure 11.1. They normally travel parallel to the coils windings and parallel to the surface. The shape of the induced eddy currents reflects the shape of the coils. Coils parallel to the surface will induce circular eddy currents. In the weld probe the coils sit on their rim resulting in an oval shape eddy current field. Eddy current flow is restricted to the area affected by the primary magnetic field. The depth of penetration of the induced eddy currents depends on a number of variables;   

Electrical resistivity or electrical conductivity (electrical resistivity and electrical conductivity are reciprocal of each other). Magnetic permeability. Test Frequency.

Phase lag is a key parameter in eddy current testing. Phase lag depends on the same material properties as that governing standard depth of penetration.

Phase lag β



X 50√ρ/fμ

radians

Where x is the distance below the surface in mm. At one standard depth of penetration the phase lag is 57° at two standard depths of penetration the phase lag would be 114°.

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11.3

Basic eddy current theory The basic equipment required to produce eddy currents consists of:   

Source of an alternating current (AC) called an oscillator. Probe containing a coil – usually of insulated copper wire. Volt meter to measure the voltage (potential drop) across the coil.

Figure 11.2 Basic Eddy current Test Equipment

The oscillator usually is capable of generating a time varying (Alternating in direction – usually sinusoidal) current at frequencies ranging from about 1,000 cycles per second (1kHz) to about 2,000,000 cycles per second (2MHz). Special applications may generate higher or lower frequencies or even use pulsed currents. The probe coil has many variables and must be specific to the application. These variables include:    

Wire diameter. Number of turns. Coil diameter. Length of coil.

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There are several configurations of surface probe. These, again, must be considered specific to the application. In general terms, surface probes may be one of the following:   

Single coil (Self-Inductance). An excitation coil with a separate receiving (sensing) coil. (Send-Receive). An excitation coil with a Hall-Effect sensing detector. (Magnetic Reaction).

These are illustrated below: Voltmeter

Voltmeter

Voltmeter

Oscillator

Oscillator

Oscillator

Excitation coil

Excitation coil Hall Effect Sensor

Coil Test Material Self-Inductance

Sensing coil Test Material Send-Receive

Test Material Magnetic Reaction

Figure 11.3 Surface probe types.

The voltmeter measures changes in the voltage across the coil. These changes may be the result of: Changes in electrical conditions and material properties such as:     

Electrical conductivity (resistivity). Magnetic permeability. Geometry of the component. Material dimensions. Relative position between the coil and the material being tested.

This voltage change consists of both an amplitude variation and a phase variation relative to the current passing through the coil. 11.4

Generation of eddy currents Magnetic field around a coil When an electric current flows through a conductor, a magnetic field is set up around the conductor in a direction at 90 to the electric current. This is explained by maxwell’s right hand rule. If the thumb of the right hand is extended in the direction in which the current is flowing, then the direction of the magnetic field is represented by the fingers.

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Figure 11.4 Right hand rule.

When the conductor is ferromagnetic, strong magnetic flux lines are created, also in the direction of the fingers, this is called circular magnetism. Circular magnetism is not polar and cannot be detected externally on a round symmetrical specimen. Now, if the original conductor carrying the current is bent into a loop, the magnetic field around the conductor will pass through the loop in one direction. Associated with the magnetic field is the magnetic flux density. This is the number of lines of force or maxwells, as they are called in cgs units, per unit area. The unit of flux density in SI units is the Tesla (T). A Tesla is 1weber per square metre (Wb/m2). 

1 weber is 100,000,000 maxwells or lines of force.

The Tesla replaces the Gauss. 

1 Gauss is 1 magnetic line of force per cm2.

There are 10,000 (10kG) Gauss in 1 Tesla. 

Or 10

Gauss = 1 Tesla.

Flux density is in the same direction as the magnetic field and its magnitude depends on its position and amplitude of the current flowing through the conductor. Flux density is therefore a field vector quantity and is given the symbol B.

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Figure 11.5 Current flow along a straight conductor.

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Figure 11.6 Magnetic flux distribution – single turn coil.

Flux density B varies linearly with electric current in the coil ie. if coil current doubles the flux density doubles everywhere. The total magnetic flux Φp contained within the loop is the product of B and the area of the coil. The unit of magnetic flux in the SI system is the weber (Wb).

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Figure 11.7 Longitudinal magnetic flux generated from a current carrying coil.

The field within the loop has direction and one side will be a north pole and the other a south pole. By increasing the number of loops, a coil, or solenoid, is created and the strength of the field passing through the coil is proportional to the current passing through the conductor in amperes multiplied by the number of turns in the solenoid. When a ferromagnetic material is placed in an energised coil, the magnetic field is concentrated in the specimen. One end of the specimen is a north pole and the other south pole. This is called longitudinal magnetism. Longitudinal magnetism has polarity and is therefore readily detectable. Only one type of field can exist in a material at one time; the stronger will wipe out the weaker. Normally in magnetic particle inspection, circular tests are carried out before longitudinal ones.

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Current Flow

N – North Pole – Anti-clockwise

Current Flow S – South Pole – Clockwise Figure 11.8 Looking at the ends of the coil – direction of current flow.

11.5

Principles governing the generation of eddy currents The three major principles or laws governing the generation of eddy currents are:   

Ohm’s Law. Faraday’s Law. Lenz’s Law.

Ohm’s law Ohm discovered that the amount of current flowing through a material varies directly with the applied voltage and inversely with the resistance of the material.

I

V R

Where: R is in Ohms () V is in volts (V) I is in amps (A) A simple way of remembering Ohm’s law is to draw it in circular form. Quantities on either side of the vertical line are multiplied, while quantities below the horizontal line are divided into quantities above it. To use the circle, simply cover the segment you want to find and the position of the remaining letter tells you the procedure to follow.

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V I

R

Figure 11.9 Ohm’s law in circular form.

In any electrical circuit, current flow is governed by Ohm’s Law and is equal to the driving (primary circuit) voltage divided by primary circuit impedance. In an electrical circuit, Impedance is defined as the total opposition to flow of alternating current (AC). Impedance represents the combination of those electrical properties that affect the flow of current through the circuit. The application of Ohm’s Law to an alternating current (AC) circuit gives the formula:

Z

V I

Where: Z is the circuit impedance in Ohms (). V is the voltage in volts (V). I is the current in amps (A).

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Figure 11.10 Test material – conductor.

The eddy current coil is part of the primary circuit. From Oersted’s discovery, a magnetic flux Φp exists around a current carrying coil proportional to the number of turns in the coil (Np) and the current (Ip). Faraday’s laws Faraday discovered the inductive effects of rapid changes in the magnetic field. When current is abruptly switched off in an electrical circuit it will induce an electromotive force which, if magnetically coupled to another electrical circuit, will create a current in that circuit. In Figure 11.11 - When the battery is disconnected in circuit A, the light in circuit B flashes for an instant. Similarly when the battery is reconnected and the current is building up in circuit A, so the bulb in circuit B flashes. While current is flowing steadily in circuit A, the light in B is off. The two circuits are not linked electrically but the magnetic field around circuit A does link through circuit B.

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Faraday went on to define two laws: 1

Whenever a magnetic field linking a circuit is changed, it sets up an electromotive force.

2

The amplitude of this induced electromotive force is proportional to the rate of change.

Figure 11.11 Faraday’s experiment.

Lenz’s law This law states that the electromotive force (emf or voltage) induced by the variation in magnetic flux is always in such a direction that if it produces a current (Is) the magnetic effect of that current opposes the flux variation (Φp) responsible for both the electromotive force and the current. Summary Magnetic flux is created by passing alternating current (AC) through the test coil. When this coil is brought into close proximity to a conducting material, eddy currents are induced. The flow of eddy currents results in resistive (Ohmic) losses. In addition, the magnetic flux associated with the eddy currents opposes the coil’s magnetic flux, thereby decreasing the net magnetic flux. This results in a change of coil impedance and subsequent voltage drop across the coil. It is the opposition between the primary (coil) and secondary (eddy currents) fields that provide the basis for extracting information during eddy current testing. 11.6

Fundamental Properties of eddy current Flow   

Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux generated by the probe coil. They normally travel parallel to the coil’s windings and parallel to the surface of the component being tested. Eddy current flow is limited to the area of influence of the inducing magnetic field, see Figue 11.12.

Frequency The most important test variable is the frequency of the current sent through the test coil. Eddy current testing is conducted at frequencies from a few hertz (Hz) to several megahertz (MHz).

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The most important effect of test frequency is upon the depth of penetration of the eddy current field. As the frequency increases so the depth of penetration decreases. The phenomenon known as skin effect is described as follows:

Figure 11.12 Standard depth of penetration.

Eddy currents induced by a changing magnetic field concentrate near the surface of the test material adjacent to the excitation coil. The depth of penetration decreases with increasing test frequency and is a function of electrical conductivity (σ) and magnetic permeability (µ) of the test material. The eddy currents flowing in the test material at any depth produce magnetic fields, which oppose the primary magnetic field produced by the excitation coil. The net magnetic field is therefore reduced thus decreasing the current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil’s magnetic field thereby weakening the magnetic field at greater depths and reducing induced eddy currents. Skin Effect can be defined by the formula: Standard depth of penetration: δ = 50

/

mm or δ =

mm

Where: ρ is electrical resistivity. Units are microhm-centimetres (µΩ.cm). σ is the electrical conductivity in siemens/metre. f is the test frequency in hertz (Hz). µr is relative magnetic permeability, no units – dimensionless. µ is the absolute permeability of the material, Henry/metre.

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For air and non-magnetic materials, µ is constant and is denoted by µ0. µ0. = 4π x 10-7 teslas or Henries/metre For ferromagnetic materials it varies considerably according to the value of H, the magnetic field strength. For convenience we use relative permeability µr Where: µr = µ / µ0. Relative permeability is therefore a dimensionless permeability of the material to that of air.

ratio,

relating

the

Or use the following formula:



660

Where:  is the standard depth of penetration in mm. f is the frequency in hertz.  is the conductivity in IACS (International Annealed Copper Standard).  is the relative permeability.

/ . IACS = 5.8 10 Iron = ~ 18% IACS. Low Alloy Steel = ~ 11% IACS. BS EN ISO 12718 gives an alternative formula for .



1 f 

Where:  is in cm.  = 3.14. f is the frequency in hertz.  is the conductivity in % IACS.  = 4  x 10-7 H/m. The standard depth of penetration is defined as the depth below the surface at which the intensity of the eddy current field has been reduced to a value of

of

its intensity at the surface. The function e is the base of natural logarithms. It is equal to 2.718 when taken to three decimal places. Therefore at the standard depth of penetration, the eddy current field intensity is at approximately one third of its surface value. (37%). Phase lag is a key parameter in eddy current testing. Phase lag depends on the same material properties as that governing standard depth of penetration.

Phase lag β



√ /

NDT31-50316b Eddy Current for Welding Inspection



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Where x is the distance below the surface in mm. At one standard depth of penetration, the phase lag is 570° at two standard depths of penetration the phase lag would be 1140°. Standard Depths of Penetration as a function of frequency for various test material are illustrated below:

Figure 11.13 Standard depths of penetration.

Sensitivity to defects depends on eddy current density at the expected defect location. Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (2), eddy current density has decreased to (1/e)2 or 13.5% of the current density at the surface. At three standard depths of penetration (3), the eddy current density is down to 5% of the surface density. However, keep in mind that these values only apply to thick materials (thickness greater than 5) and planar magnetic excitation fields. Please also note that the magnetic flux is attenuated across the test material but not completely. Although the currents are restricted to flow within specimen boundaries, the magnetic field extends into the air space beyond. This allows the inspection of multi-layer components such as aircraft wings. The sensitivity to a sub-surface defect depends on the current density at that depth. It is therefore important to know the effective depth of penetration. The effective depth of penetration is arbitrarily defined as the depth at which eddy current density decreases to 5% of the surface density. For large probes and thick samples this depth is about three standard depths of penetration (3). Unfortunately for most components and practical probe sizes, this depth will be less than 3, the eddy currents being attenuated more than predicted by the skin depth equations.

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11.7

Electrical circuits and probe impedance Eddy current testing applications consist of monitoring the flow and distribution of eddy currents in test material. This is achieved indirectly by monitoring probe (coil) impedance during the inspection. It is therefore necessary that an appreciation of impedance and associated electrical factors Is gained. Resistance (R) The opposition to current flow in direct (DC) and alternating (AC) circuits is called the resistance. It is rather like friction in mechanics. It opposes the flow of electrons and generates heat.

Where: R = Resistance in Ohms ().  = Resistivity in micro-ohms cm. = Length of conductor. A = Cross-sectional area of conductor. Ohm’s Law may be applied:

R

V I

Where: V is the voltage drop across the resistor (Volts). I is the current through the resistor (Amps).

900 Voltage

Current 0

0

1800

0

3600

2700 Figure 11.14 Voltage and current through a resistor.

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11.8

Resistance and reactance The resistance in an AC circuit represents a loss of electrical energy as heat, as it does in a DC circuit. In an AC circuit however, there are two other components which oppose the flow of current and these are called reactances. One is the capacitive reactance, which creates a voltage across a capacitor and the other is the inductive reactance which creates a voltage across an inductor (coil). The capacitor converts current into electrostatic energy and the inductor converts current into magnetic energy. As the energy is reconverted to current when the polarity of the circuit current reverses, neither of the reactances represents an actual loss in electrical energy. The effect of the capacitance and inductance in the circuit is to push the voltage and current out of phase with each other, either lagging or leading as follows: a) b) c)

In an AC circuit with only resistance, current and voltage are in phase (Figure 11.14). In an AC circuit with only inductance, current and voltage are out of phase by 90, with voltage leading current (Figure 11.15). In an AC circuit with only capacitance, current and voltage are out of phase by 90 with voltage lagging current (Figure 11.16).

An aid to memorising these is:

C

I

Capacitance – current leads voltage 11.9

V

I

L

Voltage leads current in Inductance

Inductive reactance Opposition to changes in alternating current (AC) flow through a coil is called inductive reactance. The symbol for Reactance is (X). For inductive reactance the symbol is (XL). Inductive reactance is calculated using one of the following formulae: XL = ωL OR XL = 2πfL - unit is ohms (Ω) Where: f is the frequency of alternating current (Hz). ω is the angular frequency in radians/second.

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Figure 11.15 Voltage Current across an Inductor.

11.10

Capacitive reactance Opposition to changes in alternating current (AC) across a capacitor is called capacitive reactance. The symbol for Reactance is (X) for capacitive reactance the symbol is (Xc). Eddy current coil capacitive reactance is normally negligible, however, capacitance can be important when considering the impedance of probe cables.

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Capacitive reactance is calculated using:

XC 

1 unit is ohms   2fC

Where: f is the frequency of alternating current (Hz). C is the capacitance – unit is the farad.

Figure 11.16 Voltage and current across a capacitor.

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11.11

Impedance The total opposition to alternating current (AC) flow is called Impedance. The symbol for impedance is Z. For a coil impedance is calculated using:

X  R 2  XL

2

XL Z XT = (XL- XC)

R

XC

Figure 11.17 Impedance may be represented in a vector diagram.

11.12

Inductance (L) The ability of a coil to store magnetic energy and oppose changes in the current is called inductance:

L R

N2 A I

Where: L is the inductance in henrys. R is the geometric factor. N is the number of coil turns. A is the coil’s planar surface area in mm2. I is the coil’s axial length. The henry is a very large unit. Eddy current coils have inductances of a few micro-henrys (µH). Inductance is a property of only those electrical circuits where the current is varying. The opposition to current flow generates a voltage or self-inductance in the circuit but it can also generate a voltage in a neighbouring circuit through mutual-inductance. The latter is the transformer principle.

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The self-inductance of a coil is proportional to the square of the coil windings ( ) and planar surface area (A) and inversely proportional to coil length (l). 11.13

Eddy current weld testing When considering the use of Eddy current Techniques for coated welds there are a number of variables to assess prior to choosing specific pieces of equipment. These are as follows:    

Suitable Eddy current probes/coils. Material. Coatings. Weld Geometry caused by the weld profile.

With reference to Figure 11.18 coated weld section, the variables are reasonably self evident. The coating thickness varies considerably, the thickest section being on the bottom toe of the weld, exactly where we would expect our in-service defects such as fatigue cracks to occur.

1 2 3 4 5 6

8

7

'Lift-off' signal corresponding with coating thickness.

3&6 4 7&8 1&2

'Lift-off' signal horizontal

5 0 Figure 11.18 Coated weld section – variation in sensitivity dure to application of protective coatings.

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The thickness will also change along the length of the weld as the geometry changes. The K-Node found Offshore is used as a typical example, see Figure 11.19.

Figure 11.19 Typical K node configuration.

It is therefore necessary to ensure that the technique chosen is capable of the following:   



11.14

Evaluating the material to be tested. Measuring the coating thickness in order that the full extent of the problem is quantified and evaluating the constituents of the coating. The sensitivity of the equipment is capable of being adjusted in order to compensate for the maximum coating thickness noted in the previous exercise. The resolution of the equipment is sufficient to distinguish between the signals generated by the defects sought and the background noise caused by the surface conditions (profile and/or roughness) of the weld and adjacent areas.

Probes/coil arrangements The first consideration must be access. Is it possible to get to the area of interest? Let us look at the vast range of coils used in everyday applications and try to work our way through them until suitable probes/coil arrangements are identified.

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Types of inspection probes/coils In general, we can categorise probes into two distinct applications:   

The surface probe, see Figure 11.20. Encircling or through probe, see Figure 11.21. Inside coil, see Figure 11.22.

Test coil arrangements For our purposes we can sub-divide these as follows:  

Single coil (Absolute), see Figure 11.23. Differential coil, see Figure 11.24.

The relative advantages or disadvantages and applications of each type of coil arrangement is dealt with elsewhere in the notes so for the purpose of this exercise we shall only consider surface probes. We are immediately drawn to the pencil probe. It is very versatile. It may be formed into numerous shapes and sizes to meet most weld configurations. The basic components of the Pencil Probe are as follows: 

Single Coil, Absolute Arrangement.

In this arrangement the same coil is used to induce eddy currents in the component and to sense the component's reaction on the eddy currents. The single coil will test only the area under the coil and does not compare itself with a reference standard. These probes generally have small coils and operate at relatively high frequencies. The pencil probes we shall assess have ferrite cores. These are used to induce a greater magnetic flux and eddy current field. Let us draw up a specification and/or check list for evaluation of the probes:  



Material Evaluation: Is the probe suitable for this purpose? Coating Thickness Measurements: Is the probe capable of measuring the coating thickness to be found on components to be tested? The varying constituents of the coatings must also be subject to some thought. Are any of the layers which make up the coating conductive? What effect, if any, will these conductive layers have on the coating thickness checks? Surface Crack Detection: Are we capable of detecting surface breaking defects in carbon steel? What size of defect are we capable of detecting and under what circumstances? Are we capable of detecting these defects under typical coatings found in the field? In other words we must define the limitations of the instrumentation/probe coil combination and systematically build up a reasonable specification and testing procedure for the instrumentation/probe coil combination which will allow reproducibility of test and results.

We have developed a series of practical exercises to try and demonstrate the various topics discussed previously. It may also be possible to quantify some of the limitations of the system by completing the exercises.

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Figure 11.20: a

Schematic of eddy current surface probe;

b

Surface probe and the effect of non-conductive coating thickness on eddy current distribution in the test material.

Figure 11.21 Schematic of eddy current encircling coil probe showing the primary excitation coil and the secondard pick up coil.

Figure 11.22 Schematic of eddy current inside coil bobbin probe showing defect detection in a non-ferrous tube.

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Figure 11.23 Schematic of eddy current single coil probe showing the effect of a crack on eddy current distribution (right) compared to a defect free distribution (left).

TEST INSTRUMENT Figure 11.24 Schematic of eddy current single coil self-comparison differential probe.

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Appendix 1

Sample Instruction - Amplitude Analysis, Full Length, Internal Defects. Scope: This instruction is documented as the process by which Condenser Tubes are inspected over their Full Length for Internal Defects using the Eddy Current Amplitude Analysis method with a differential coil. The signals are recorded/printed out and the defects classified. Document reference and status: TWI/ET/Tubes/AAFLID/1/UK Component identification number: Training Bundle, AAFLID/TB1/UK Description (incl material and dimensions): 90/10 Copper Nickel Tubes 1.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect Internal defects Area to be tested: Full Length of 1.5m tubes Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Differential bobbin probe, 11mm diameter. Part No. PID110N05R20K. This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.42m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with thru holes to simulate internal metal loss. Part No. CEGB, ESI Type A. See Figure A1-1. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper width. Tools - Flexible measuring tape.

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Copyright © TWI Ltd

Pre-test: Ppreparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency: 20KHz. X Y gain: Set 1:1. X Y position: Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence: As required. Phase: Set to 0° initially. Lo pass filter: 50. Balance: Off. Flaw detector initial calibration Phase: Set so that defects along Y axis are initially negative going. Sensitivity: Set to achieve an 80% FSH vertical deflection from simulated defect No. 6 (8 x 0.65mm holes). Chart recorder settings – channel 1 only Chart speed 15mm/sec. Pen position Pen is to be positioned centrally (five main-scale division from top and five main-scales from bottom). Amplitude 5v up and 5v down. Chart recorder final sensitivity Phase Check defect signals are initially negative going. Amplitude Set to achieve an 80% deflection from simulated defect No. 6 (8 x 0.65mm holes).

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Copyright © TWI Ltd

Detailed instructions for application of test (continued) 1

Calibration recording: With calibration as above, make a recording/print-out of the 6 sets of simulated defects in Calibration tube type A. Probe should be withdrawn at a steady rate and the trace should show outlet and inlet signals at either end of calibration trace. Identify trace as ‘Calibration In’.

2

Inspection: Carry out an inspection of first tube in a similar manner, ensuring that scan speed is constant and that both outlet and inlet signals are produced at either end of calibration trace.

3

Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded.

4

Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position.

5

Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

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Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: Cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-conformance statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed.

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A1-4

Copyright © TWI Ltd

Recording and classifying results: Action to be taken when defects are detected or no fault found: Classify calibration tube - Using the report sheet attached, enter the signal amplitudes obtained from the calibration tube. The smallest amplitude, (obtained from the single hole) should be classified as class limit 1 and the others as class limit 2, 3A, 3B, 4 and 5 respectively. Defect classification - All defect indications appearing on the chart recorder paper trace, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Assess class of defect by referring to calibration tube class limits. Defects > 8 cross drilled holes = class 6. All tubes with defects equal to or greater than class 4 are to be retested and reported separately. Ensure ‘calibrations ‘in’ and ‘out’ are repeated for this retest. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, and defect amplitude, classification and positional information with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe

PCN/Level 2/ET/Wrought Tubes

Tubby Pipe

23rd July 2013

Authorising person's details: Ted Brass

PCN/Level 3/ET/Wrought Tubes

Ted Brass

23rd July 2013

8

6

3

4

2

1

D=0.6 14.2mm OD

0.42m

Figure A1.1 Calibration tube CEGB ESI Type A.

NDT31-50316b Sample Instructions

A1-5

Copyright © TWI Ltd

Eddy Current Tubes Inspection Results Recording Sheet Full Length Test – Differential Mode – Internal Defects Name:

Date:

EQUIPMENT USED Flaw Detector: Serial No:

Class Limit

Sample:

CALIBRATION Amplitude

Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle:

Reference Tube:

DEFECT SIGNAL TUBE No.

Amp. mm.

COMMENTS Class

Location

RETESTS

NDT31-50316b Sample Instructions

A1-6

Copyright © TWI Ltd

Appendix 1B – Sample Instruction - Amplitude Analysis, Inlet End, Internal Defects Scope: This instruction is documented as the process by which Condenser Tubes are inspected at the Inlet end for Internal Erosion/Thinning using the Eddy Current Amplitude Analysis method with an absolute coil. The signals are recorded/printed out and assessed against a calibration tube graph of amplitude and thinning. Document reference and status: TWI/ET/Tubes/AAIEID/2/UK Component identification number: Training Bundle, AAIEID/TB2/UK Description (incl material and dimensions): 90/10 Copper Nickel Tubes 0.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect internal material loss in the tubes, coincident with the ends of the inserts, due to erosion. Area to be tested: Inlet ends only, in vicinity of where 6" or 7.5" venture inserts may have been fitted. Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Absolute bobbin probe, 11mm diameter. Part No. PID110N05R20K This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.7m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with tapered external annuli to simulate internal metal loss. Part No. CEGB, ESI Type D, see Figure A1.2. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper width. Tools - Flexible measuring tape.

NDT31-50316b Sample Instructions

A1-7

Copyright © TWI Ltd

Pre-test: preparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency: 10KHz. X Y gain: Set 1:1. X Y position: Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence: As required. Phase: Set initially to 0°. Hi pass filter: Off. Lo pass filter: 30. Balance: 120μΗ. Flaw detector initial calibration Phase and Set to achieve a 50% FSH vertical deflection from simulated defect Sensitivity: 50% material loss. Chart recorder settings Chart speed: 15mm/sec. Pen position: Pen is to be positioned centrally (five main-scale division from top and five main-scales from bottom. Amplitude: 5v up and 5v down. Chart recorder final sensitivity Set to achieve an 50% deflection from simulated defect 50% material loss.

NDT31-50316b Sample Instructions

A1-8

Copyright © TWI Ltd

Detailed instructions for application of test (continued) 1 Calibration recording: With calibration as above, make a recording/print-out of the 5 sets of simulated defects in Calibration tube type D. Probe should be withdrawn at a steady rate and the trace should show outlet and inlet signals at either end of calibration trace. Identify trace as ‘Calibration In’. 2 Inspection: Ensure null balance obtained in each tube prior to scan. Carry out an inspection of first tube in a similar manner, ensuring that scan speed is constant and that both outlet and inlet signals are produced at either end of calibration trace. 3 Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded. 4 Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position. 5 Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

NDT31-50316b Sample Instructions

A1-9

Copyright © TWI Ltd

Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: Cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-Conformance Statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed.

NDT31-50316b Sample Instructions

A1-10

Copyright © TWI Ltd

Recording and classifying results: Action to be taken when defects are detected or no fault found: Calibration tube – Using the report sheet shown in Figure A1.2, enter the signal amplitudes obtained from 10, 20, 30, 40 and 50% material losses. Produce a graph of the calibration tube’s amplitude against percentage thinning. Inspection defects - All defect indications appearing on the chart recorder paper trace, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Assess the amount of material loss for each defect using the graph produced from the calibration tube. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, and defect amplitude, characterisation, classification and positional information, with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe

PCN/Level 2/ET/Wrought Tubes

Tubby Pipe

23rd July 2013

Authorising person's details: Ted Brass

PCN/Level 3/ET/Wrought Tubes

Ted Brass

23rd July 2013

10%

20%

30%

40%

50%

0.7m

Figure A1.2 Calibration Tube CEGB ESI Type D.

NDT31-50316b Sample Instructions

A1-11

Copyright © TWI Ltd

Eddy Current Tubes Inspection Results Recording Sheet Inlet End Test – Absolute Mode – Internal Defects Name:

Date:

EQUIPMENT USED

Sample:

CALIBRATION

Flaw Detector:

% Thinning

Amplitude

Serial No: Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle:

Reference Tube:

TUBE No.

DEFECT SIGNAL Amp. mm.

NDT31-50316b Sample Instructions

% Thinning

COMMENTS

Location

A1-12

Copyright © TWI Ltd

Appendix C – Sample instruction - phase analysis, full length, external defects Scope: This instruction is documented as the process by which Condenser Tubes are inspected over their Full Length for External Defects using the Eddy Current Phase Analysis method with a differential coil. The defect signals are assessed against a graph of the calibration tube, phase angle and thinning. Printer is used to give positional information. Document reference and status: TWI/ET/Tubes/PAFLED/1/UK Component identification number: Training Bundle, PAFLED TB1/UK Description (incl material and dimensions): 90/10 Copper Nickel tubes 1.5m long, 14.2mm diameter, 6 off Drawing attached on last page – Yes / No (circle as applicable) Purpose of the test: To detect external defects. Area to be tested: Full length of 1.5m tubes. Personnel: The minimum requirements for training certification and authorization of NDT operators. (method / sector / scheme, including job-specific training if necessary), All personnel carrying out this instruction shall be qualified to PCN/Level 1/Wrought Tubes and carry company approval as a minimum. Safety requirements: All safety instructions contained in the TWI Health and Safety Manual, which apply to tube inspections, are to be complied with. Equipment to be used: (together with identification and settings) Instrument - Nortec 500D – or similar Impedance Phase display instrument capable of attaining the parameters of this inspection. Probe - Air cored Differential bobbin probe, 11mm diameter. Part No. PID110N05R20K. This probe will give a nominal fill factor of 84%. Any alternative probe should give a minimum fill factor of 70%. Calibration tube - Calibration Tube, 0.7m long, 14.2mm O/D, made from 90/10 Cu/Ni Brass, with External annuli to simulate external metal loss. Part No. CEGB, ESI Type B, see Figure A1.3. Chart recorder - Astro Med Dash 2EZ+ and recorder/printer with 80mm paper. Tools - Protractor and Flexible measuring tape.

NDT31-50316b Sample Instructions

A1-13

Copyright © TWI Ltd

Pre-test: preparation of the test area: Ensure all equipment pre-use checks are carried out in accordance with manufacturers instructions. Ensure all equipment calibration certificates are valid and all electrical safety checks are completed. Visual: Carry out a visual inspection of tubes and ascertain that they are in a fit condition for eddy current examination. Ensure bores of inspection tubes are clean and free of any silt deposits. Ensure also that bores of tubes are not damaged or dented to an extent that might restrict probe travel. Note and report such tubes. Every effort is to be made to ensure probe does not become stuck. Ensure all tubes are identified for position. It is normal practice to number the tubes downwards in vertical rows. Detailed instructions for application of test A detailed clear written description in the application of the NDT technique (with reference to sketches if appropriate): Flaw detector initial settings Frequency 30KHz. X Y Gain Set 1:1. X Y Position Set X and Y so that the null point is in the centre of the trace, (five main-scale division from top and five main-scales from bottom). Persistence Permanent. Phase Set initially to 0°. Hi Pass Filter Off. Lo Pass Filter 50. Balance Off. Flaw detector calibration Phase: Set to achieve a 90degree phase display from the 100% thinning simulated defect. Sensitivity: Set to achieve an 80% FSH vertical deflection from the 100% defect. Chart recorder settings Chart speed 15mm/sec. Pen position Central. Amptiude 5v up and 5v down. Chart recorder final sensitivity Set to achieve 80% FSH from the 100% defect.

NDT31-50316b Sample Instructions

A1-14

Copyright © TWI Ltd

Detailed instructions for application of test (continued) 1 Calibration recording - With calibration as above, obtain signals from the 10, 30 and 50% simulated defects and measure the phase angles of each defect in turn. The phase angle is measured from the extrpulated peak signal using the flaw dectors phase control see figure A1.4. Adjust amplitude to approximately 80% FSH in turn in order to make a correct assessment of phase angles. 2 Inspection - Carry out an inspection of first tube in a similar manner. Defect signals should be maximised and phase angles recorded, noting defect position, from chart recorder. 3 Inspection recording: Monitor impedance display during examination and ensure that any defect indications have been successfully recorded. 4 Tube identification: Repeat item 2 and 3 above on remaining tubes, identifying each trace with its respective tube position. 5 Recheck calibration: Ensure a recording/print-out is produced at the end of the inspection run. Identify as ‘Calibration Out’. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... .........................................................................................................................

NDT31-50316b Sample Instructions

A1-15

Copyright © TWI Ltd

Detailed instructions for application of test (continued) ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... Post test: cleaning and preservation of test object: Ensure tubes are left in a clean and unblocked state. Non-Conformance Statement: Instructing the operator on actions to be taken in the event that this instruction cannot be applied: If for any reason the parameters of this inspection cannot be complied with, then the inspection is to be halted and the supervisor informed.

NDT31-50316b Sample Instructions

A1-16

Copyright © TWI Ltd

Recording and classifying results: Action to be taken when defects are detected or no fault found: Calibration tube - Using the report sheet attached, enter the signal phase angles obtained from the calibration tube. Draw a graph of the calibration tube defects – phase angle against percentage thinning. Inspection defects - All indications, which cannot be attributed to entry, exit or steel support plate indications, are to be considered as defects and reported. Phase angles are to be measured and the amount of thinning assessed by using the calibration tube graph. Reporting the results: Reporting format and essential data required for the report: All defects are to be reported in accordance with TWI reporting procedure using a copy of the recording sheet attached to this instruction. Ensure all fields are correctly annotated with equipment details, tube identification, defect phase angles and percentage thinning. Give positional information with reference to a datum. Defective tubes are to be clearly marked, isolated where possible and supervisor informed. Tube entry restrictions: Note and report any tube restrictions, which prevented full inspection. Where introduction of probe into tube was impossible due to blockage, then tube to be annotated as ‘UTE’ – unable to enter. Originating person’s details: Tubby Pipe

PCN/Level 2/ET/Wrought Tubes

Tubby Pip

23rd July 2013

Authorising person's details: Ted Brass

PCN/Level 3/ET/Wrought Tubes

Ted Brass

23rd July 2013

NDT31-50316b Sample Instructions

A1-17

Copyright © TWI Ltd

Calibration Tube Type B – Cupro-Nickel 90/10 External Annuli and Thru Holes to simulate OD defects. 8x0.85mm thru holes

10%

10%

50%

30%

70%

100% 50%

490mm

Figure A1.3 Calibration Tube CEGB ESI Type B.

0°

90°

Figure A1.4 showing typical phase measurement.

NDT31-50316b Sample Instructions

A1-18

Copyright © TWI Ltd

Eddy Current Tubes Inspection Results Recording Sheet Full length Test – Differential Mode - External Defects Name:

Date:

EQUIPMENT USED

Sample:

CALIBRATION

Flaw Detector:

% Thinning

Phase Angle

Serial No: Recorder: Serial No.: Probe: Test Frequency: Gain Setting: Phase angle:

TUBE No.

Reference Tube:

Phase Angle

NDT31-50316b Sample Instructions

DEFECT SIGNAL % Thinning Location

A1-19

COMMENTS

Copyright © TWI Ltd

Appendix 2

ESTestMaker Questions 1

An eddy current test system closely approximates a transformer. In this approximation, what would the second coil be represented by? a b c d

2

By convention, the direction of a magnetic line of force is represented by an arrow on a line. The arrow would point in the direction: a b c d

3

Electrical contact. Specimen conductivity. An alternating magnetic field. Induced electrical current.

Which of the following is not a mandatory component in a basic eddy current test apparatus? a b c d

7

A mythical quantity. An imaginary but useful concept. Equal to 1gh mass when converted by Einstein’s equation. 1 micron diameter and 10 microns long.

Which of the following conditions is not necessary for eddy current testing? a b c d

6

A dry cell battery. A generator or alternator. A microphone. An electric motor.

The magnetic line of force is: a b c d

5

In which a unit north pole would be moved. In which a unit south pole would be moved. Perpendicular to the plane of the line. Indicated by the thumb in the left hand rule.

Which of the following is not an example of electromechanical energy conversion devices? a b c d

4

The induced eddy currents. The eddy current probe. The test sample. A Hall detector used as a receiver.

An AC source. A coil (probe). An impedance plane. A volt meter.

Which of the following is not a probe configuration used in eddy current testing? a b c d

Self inductance (single coil). Send-receive (2 coils). Magnetic reaction (coil and hall detector). Semi-conductor reaction (2 hall detectors).

NDT31-50316b ESTestMaker Questions

A2-1

Copyright © TWI Ltd

8

The sense or direction of a magnetic field around a conductor is most commonly determined using: a b c d

9

Tesla or Webers per square metre (Wb/m2) are units of: a b c d

10

Core permeability. Number of coil turns. Current in the coil. All of the above.

A voltage is induced in a region of space when there exists a changing magnetic field. This is a statement of: a b c d

14

25T. 5T. 2.5T. 2.25Wb/m2.

An increase in which of the following would result in the increase of magnetic flux density (B) in a solenoid? a b c d

13

Halves. Remains unchanged. Doubles. Quadruples.

If the magnetic flux density for a given location and orientation near a current carrying conductor is 5 Wb/m2, what is it when the current is cut by half? a b c d

12

Eddy current. Impedance. Reluctance. Magnetic flux density.

If the electric current in a coil is doubled the magnetic flux density: a b c d

11

Lenz’s Law. Ohm’s Law. A Rowland Ring. The right hand rule.

Faraday’s Law. Oersted’s Law. Helmholtz’s Theorem. Ohm’s Law

Lenz’s Law states: a b c d

An alternating magnetic field induces an alternating voltage. The magnitude of induced current is a function of magnetic flux through a circuit. The induced EMF is opposite to the change causing it. I = B A cos  where B=flux density, A = circuit area and  = the angle between B and the circuit area A.

NDT31-50316b ESTestMaker Questions

A2-2

Copyright © TWI Ltd

15

The back EMF opposing the inducing EMF is a result of: a b c d

16

The principal cause of magnetism in a naturally magnetic substance is: a b c d

17

1 kHz. 1 standard depth of penetration (e). 3 standard depths of penetration (3e). It is not possible to estimate.

When gap between plates of the same material is being measured, the probe should be placed on the thinner of the two plates when possible. Why? a b c d

21

No current flow in the test piece. Dc being induced in the test piece. AC being induced in the test piece. A short circuit.

When performing thickness or gap testing, what should the operating frequency be? a b c d

20

Permeability. Flux density. Pole strength. Field intensity.

Moving a direct current carrying conductor up and down near a conductive test piece will result in: a b c d

19

Hysteresis. The weak nuclear force. Uncompensated electron spin. Graviton concentration in the Domain wall.

The number of lines of magnetic flux divided by a unit area is the: a b c d

18

The Hall effect. Eddy current flow. Geo-magnetic reversals. Weak nuclear forces.

The frequency needed would be too low otherwise. To minimize depth of penetration problems. So results are linear. To increase signal to noise ratio.

The relationship between electric current flow, electromotive force and resistance to electric current flow is described by: a b c d

Lenz’s law. Ohm’s law. Faraday’s rule. The ampere-ohm equation.

NDT31-50316b ESTestMaker Questions

A2-3

Copyright © TWI Ltd

22

Another term for voltage is: a b c d

23

When determining resistivity of a sample of an aluminium alloy, why is it recommended you do not tough the sample with your fingers? a b c d

24

A V block is used to maintain parallelism. Curved calibration standards are used. Lower operating frequencies are used. Both a and b.

When eddy current probes used for restivitiy readings are required to be used on small surfaces (eg bolt heads), what can be done to overcome edge effects? a b c d

28

Variations in alloy. Variations in heat treatment time/temperature. Variations in fabrication stresses. All of the above.

What is done to correct for reduced field coupling when making conductivity measurements on curved surface? a b c d

27

Linear. Logarithmic. Exponential. Zero, that is why it is chosen as the standard.

In field applications, specific conductivity values are not used; instead a range of conductivities can be expected from a finished product. Why is this so? a b c d

26

Oil of the skin increases resistivity. Oil of the skin decreases resistivity. Sample temperature can be changed. Oils on the test surface from the fingers will produce an unwanted lift-off.

Conductivity changes for annealed copper (100 IACS) as a function of temperature change are: a b c d

25

Electromotive force. Magnetomotive force. Potential drop. Both a and c.

Use field collimators. Use correction factors from a pre-made edge-distance curve. Both a and b. Use higher frequencies.

When does material thickness affect the results of a conductivity test? When: a b c d

Eddy current effective penetration is greater than material thickness. Conductivity is very high. The material is backed by a higher conductivity material. Lift-off is a result of a surface roughness.

NDT31-50316b ESTestMaker Questions

A2-4

Copyright © TWI Ltd

29

If temperature of a test piece increases what other eddy current parameter will likely increase? a b c d

30

Lift-off compensating probes place a compensating coil around the sensing coil. The purpose of this is: a b c d

31

Localised heating caused by eddy currents. Skin depth effect. Decrease in magnetic flux. Permeability of the test piece.

To eliminate probe wobble using a two frequency multifrequency set up, what function listed below would be incorrect? a b c d

35

Gap. Pencil. Spring. Spinning.

The main factor limiting sensitivity to subsurface defects is: a b c d

34

Smaller than the drive coils. Wound in opposition to each other. Arranged to provide a zero net voltage in air. All of the above.

Laminations or disbanding would most likely require you use a (n) probe. a b c d

33

To rotate the defect signal relative to the lift-off signal. Allow shallow defects to be detected on rough surfaces. Both a and b. None of the above.

In the reflection type send-receive coil, the receive coils are: a b c d

32

Conductivity. Resistivity. Frequency. Lift-off.

Adjust signal amplitudes at the two frequencies to be equal. Adjust phase at the two frequencies to be 90ø apart. Add the two signals together. Both a and c are incorrect.

Eddy current information is often digitized for transmission and processing. What is the best resolution possible using 8 bit conversion? a b c d

0.5%. 1.0%. 5.0%. 8.0%.

NDT31-50316b ESTestMaker Questions

A2-5

Copyright © TWI Ltd

36

Characterising eddy current responses by patterns rather than specific signal responses is termed: a b c d

37

What are the charge carriers used by hall effect devices? a b c d

38

Skin depth. Effective depth of penetration. Stand depth of penetration. Saturation depth.

Nonlinear distortion characterised by the appearance of harmonics of the fundamental output when the input wave was sinusoidal is called: a b c d

42

Depth of penetration. Critical distance. Exponential distance. Coating thickness.

The depth beyond which a test system can no longer detect further increase in specimen thickness is the: a b c d

41

Lift-off. Wobulation. Coil clearance. Shimmy.

The distance in a test specimen that eddy current intensity has decreased 37% of their surface value is the: a b c d

40

Electrons. Positrons. Holes. Both a and c.

The effect that produces signal variations due to variation in coil spacing due to lateral motion of test specimen when passing through an encircling coil is? a b c d

39

Spectrum analysis. Signature analysis. Waveform analysis. Pattern recognition.

Harmonic distortion. Amplitude distortion. RF noise. Both a and b.

Conductance is an electrical quantity which can also be defined as the reciprocal of: a b c d

Inductance. Resistance. Resistivity. Reluctance.

NDT31-50316b ESTestMaker Questions

A2-6

Copyright © TWI Ltd

43

Resistivity of a material is a function of: a b c d

44

A change in signal voltage resulting from EMF produced by the relative motion between test piece and coil is a result of the: a b c d

45

ohm. ohms. ohms. ohms.

R = Ro + T. R = Ro + dT. R = Ro (1 + a dT). None of the above.

Given copper at 20oC. With resistivity 5.9  ohm-cm and thermal coefficient of resistivity of 0.0039, what is the resistivity when the copper is warmed to 40°C.? a b c d

49

1 2 4 8

Which equation would be used to calculate the resistance of a length of conductor at room temperature other than standard temperature? a b c d

48

Resistor, series. Resistor, parallel. Capacitor, series. Capacitor, parallel.

If the resistance in a 1cm long wire is 2 ohms when it has 0.1cm diameter, what will the resistance be in a wire of the same length and material but only 0.05cm diameter? a b c d

47

Edge effect. Speed effect. Harmonic distortion. Fill factor.

In order to use a galvanometer (which normally measures currents in the range of milliamps) as an ammeter measuring 10-20 amps you would put put a in with the galvanometer: a b c d

46

A material’s cross-sectional area. A material’s length. Overall resistance. None of the above.

2.90  ohm-cm. 5.80  ohm-cm. 6.25  ohm-cm. 11.60  ohm-cm.

When an eddy current probe is brought near a conductive sample the net magnetic flux in the system: a b c d

Increases. Decreases. Remains unchanged. Drops to zero when the part is contacted.

NDT31-50316b ESTestMaker Questions

A2-7

Copyright © TWI Ltd

50

Eddy current density in a sample is: a b c d

51

Strictly speaking, the standard skin depth equation; J/Jo = (e^- β) sin (wt- β), is true for only: a b c d

52

Rods with diameters greater than 2δ. Rods with radius greater than 2 δ. All conditions. No condition, a slight current density will always exist.

Xδ. x/δ. δ /x. 57 x/δ.

Phase lag of eddy currents in a sample is dependent on: a b c d

56

66%. 37%. 14%. 9%.

Phase lag in degrees would be represented by (where x - depth, δ = standard depth of penetration). a b c d

55

that at

When inspecting a rod with an encircling coil the eddy current density at the centre of the rod is zero for δ = standard depth of penetration). a b c d

54

Thick material and planar magnetic fields. Tubular products. Thin plate inspection. All of the above.

At 2 standard depths of penetration, eddy current density is about the surface: a b c d

53

Proportional to the conductivity of the sample. Proportional to the permeability of the sample. Inversely proportional to the depth from the surface of the sample. All of the above.

Depth into the sample. Resistivity of the test piece. Relative magnetic permeability of the sample. All of the above.

Eddy current flow in a test sample is accomplished indirectly by monitoring: a b c d

Current changes in the sample. Resistivity changes in the sample. Impedance changes in the coil. Coil resonance.

NDT31-50316b ESTestMaker Questions

A2-8

Copyright © TWI Ltd

57

The equation 2πfL =: a b c d

58

The equation 1/2πfC =: a b c d

59

Pulse-echo. Impedance. Send-receive. Sing-a-round.

When the eddy current test system is represented by the transformer the sample can be considered the secondary winding with: a b c d

63

Simple addition. Simple subtraction. Vector addition. A weighted average.

The method of eddy current testing that uses a dedicated coil to induce eddy currents in a test piece and another coil to detect eddy current variations in the test piece is the method. a b c d

62

Impedance. Resistance. Reactance. Reluctance.

In an AC circuit the total voltage across a resistor and an inductor in series is found by: a b c d

61

Inductive reactance. Capacitance. Capacitive reactance. Total impedance (electrical).

The vector sum quantity of resistance and reactance in an AC current is: a b c d

60

Inductive reactance. Inductance. Capacitive reactance. Total impedance (electric).

A single turn. 10 turns. Zero turns. None of the above, it is not possible to determine.

On a normalised impedance curve which of the following parameters would move the operating point up the curve when increased? a b c d

Resistivity of sample. Operating frequency. Sample conductivity. Lift off.

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An increase in tube wall or plate thickness will move the operating point on the impedance curve: a b c d

65

An increase in test frequency will move the operating point on the impedance curve: a b c d

66

Upward. Downward. To a point inside the curve. To a point outside the curve.

Increasing which of the following parameters will move the operating point up on the impedance curve? a b c d

70

The primary coil. The secondary coil. The receive coil. Both b and c.

An increase in electrical resistivity of a sample will move the operating point on the impedance curve: a b c d

69

Changes in voltage across the primary coil. Changes in current across the primary coil. Two separate coils. A single multiplexed coil.

In the send-receive method of eddy current testing the variations in eddy current flow due to flaws in the test piece are monitored by: a b c d

68

Up. Down. To a point inside the original curve. To a point outside the original curve.

The send-receive method of eddy current testing uses: a b c d

67

Up. Down. Inside the curve. Outside the curve.

Resistivity. Thickness (of tube or plate). Frequency. Diameter of a surface probe.

In the impedance method of eddy current testing the impedance phase Ө (in degrees) is calculated from (w is the angular frequency, L is inductance, R is resistance): a b c d

Ө Ө Ө Ө

= = = =

Arcsin (wL/R). Arccos (R/Lw). Arctan (wL/R). Arcsin (R2+L2)^½.

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The effect of sample and test parameters can be illustrated using: a b c d

72

Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50kHz; what is the coil inductive reactance? a b c d

73

20.2 ohms. 20.2 microhenries. 44.7 ohms. Not possible to determine with information given.

Given a probe with 50 ohms resistance and 40  H inductance, when operated next to a copper sample at 20kHz the probe impedance is 55 ohms and impedance phase Ө is 40o, what is the inductive reactance of the probe when operating on the sample? a b c d

76

1.59 ohms. 2.51 ohms. 6.3 ohms. 10 ohms.

Given a coil with 20ohms and 60 microhenries inductance in air and operated at 50 kHz, when brought next to an inconel sample the probe impedance is 28.5 ohms and impedance phase Ө is 45o, what is the probe’s inductive reactance? a b c d

75

0.4 ohms. 1.6 ohms. 3.9 ohms. 15.7 ohms.

Given a coil with 2ohms resistance and 20  H inductance and operated at 20kHz, what is the coil’s inductive reactance? a b c d

74

Magnetographs. Impedance diagrams. Polar projections. Polarised light.

55 ohms. 42.1 ohms. 35.3 ohms. 5 ohms.

If given total impedance of a probe operating on a test sample and know the impedance phase angle, what equation is used to determine the inductive reactance of the probe? a b c d

Xp Xp Xp Xp

= = = =

2πfL. Zp cos Ө Zp tan Ө Xp sin Ө

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Voltage changes across the probe due to a defect in most eddy current inspections are on the order of: a b c d

78

Balancing is required in the eddy current instrument to: a b c d

79

Nonlinear voltage output with change in probe impedance. Increased sensitivity. Reduced lift-off effects. All of the above.

In the L-C circuit used by simple meter crack detectors, the circuit is operated: a b c d

83

Amplitude. Phase. Both a and b. No form.

The result of operating an eddy current test instrument at a point other than balance point is: a b c d

82

Resistivity. Lift-off. Resonance. Permeability.

When a simple bridge made up of 4 impedance arms, the voltage in adjacent arms of the bridge must be equal in: a b c d

81

Allow resonance. Avoid resonance. Set meter type instruments to zero. Eliminate the voltage difference between two coils.

The most troublesome parameter in eddy current testing is: a b c d

80

1%. 10%. 100%. 1000%.

Independent of operating frequency. At the resonance frequency. Very near resonance frequency. Both b and c.

The reactive power of inductance and capacitance in a tuned L-C circuit are: a b c d

Equal. Maximum. Minimum. Zero.

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Crack detector type ECT instruments based on resonant circuits detecting surface defects on low resistivity materials such as aluminium would have operating frequencies in the range. a b c d

85

The purpose of multifrequency ECT technique is: a b c d

86

c d

Have very low response rates (8Hz). Are usually used as a playback recording instrument for hardcopies of specific signals. Do not have the ability to locate defects and provide length information about the defect. All of the above.

Instrument frequency response is limited by: a b c d

90

Be impedance matched to ECT instrument. Be phase locked to the ECT instrument. Have an equal or higher frequency response. Have a lower frequency response that the ECT instrument.

X-Y recorders: a b

89

Mixing modules. Filters. Phasors. Frequency selectors.

Recording of eddy current signals from ECT instruments requires that the recording instrument: a b c d

88

To increase frequency response of instruments. Elimination of the effects of undesirable parameters. Increase sensitivity to non-surface breaking defects. To allow inspection with phased array probes.

Multifrequency instruments have the same controls and functions as general purpose ECT instruments with the addition of: a b c d

87

DC. 60-100Hz. 10-100kHz. 10-100MHz.

Probe size. Operating frequency of the probe. Probe motion (inspection speed). None of the above.

Most eddy current instruments use some form of options are available for lift-off compensation. a b c d

for balancing but several

Inductor. AC bridge. DC bridge. Potentiometer.

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The impedance of probes used in eddy current testing can vary over a range. Instruments must be able to balance over this range. Most instruments can handle prove impedances between: a b c d

92

A parallel L-C circuit used in crack detectors has an inductive of 150 ohms. The capacitive reactance would be about under normal operating conditions. a b c d

93

One coil. Two coils. More than two coils. A DC magnetic field.

Absolute probe. Differential probe. Spring probe. Pencil probe.

The purpose of spring loading an eddy current probe against the test material is: a b c d

97

interact(s) with the test material.

When two similar coils on the AC bridge of the eddy current instrument sense with the test material the probe is a (n): a b c d

96

Using a resonance test frequency. Multiple coil probes. Testing under liquid nitrogen. None of the above.

An absolute probe requires a b c d

95

0.1 ohms. 75 ohms. 150 ohms. Not possible to know.

Compensation for undesirable material and coupling variations can be achieved by: a b c d

94

50-75 ohms. 0.1-100k ohms. 10-200 ohms. 5-500 ohms.

Greater wear protection. To maintain constant capacitance. To minimise lift-off. To prevent bearkhausen noise.

The purpose of the ferromagnetic core used in a gap probe is to: a b c d

Shape the magnetic field. Saturate the test piece with magnetism. Compensate for lift-off. Reduce heating effects caused by eddy current.

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In the send-receive probe arrangement where the driver and receiver coil are on opposite sides of a plate, signal variation will result from: a b c d

99

Maximum response to defects detected by eddy currents are obtained when: a b c d

100

Allow operators to set lift-off horizontal. Avoid resonance on impedance graph displays. Allow study of test specimen variations without concern from probe variations. Establish a common ground for international discussions of eddy current testing.

Decrease in sensitivity resulting from increasing lift-off is more pronounced for: a b c d

104

Phase angel increases. Amplitude increases. Both a and b. None of the above, no significant change occurs to the defect signal.

The reason for normalising probe impedance is to: a b c d

103

r. 1/r. r^/½. No relationship exists.

For a given sized defect, what significant defect signal change occurs when testing a plate using the through transmission (send-receive) method and the defect occurs first 25% of the wall thickness from the transmit coil, then 50% and 75%? a b c d

102

Eddy current flow is parallel to the maximum dimension of the defect. Eddy current flow is perpendicular to the maximum dimension of the defect. The defect cause probe resonance. Lift-off is eliminated.

Eddy current flow and its associated magnetic flux are a function of position under the coil. The relationship could best be described as being proportional to (r=radial distance from coil centre): a b c d

101

Material variations (eg Voids) in the test material. Coil-to-coil spacing. Proximity variations of test piece to coils (lift-off). Both a and b.

Large diameter probes. Small diameter probes. Deeper defects. Both b and c.

As a general rule, probe diameter should be selected so that it is: a b c d

Greater than or equal to the expected defect length. Less than or equal to the expected defect length. Less than or equal to the expected defect depth. Twice the minimum allowable defect length.

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At high operating frequencies, the effective coil diameter (sensing diameter) is approximately equal to: a b c d

106

Permeability changes are of greater concern in eddy current testing because: a b c d

107

Depth. Size. Orientation. None of the above (ie all are frequency dependant).

Using a typical impedance type EC machine with storage monitor, electrical resistivity determinations are made by: a b c d

111

Lift-off vector and the defect to sound specimen vector. Total voltage vector and the resistive voltage vector. About double the phase leg. Both a and c.

Which defect parameter will not affect the probe frequency you select to locate a defect? a b c d

110

Skin depth and phase lag effects. Resonance effect. Test specimen capacitance effect. All of the above.

The phase angle used to estimate defect depth is the angle between the: a b c d

109

They can cause parts to fail but cannot be detected. Small changes in permeability cause large impedance changes. Small changes in permeability can obscure other test variables. Both b and c.

The reversal swirl that is observed on a normalised impedance graph showing the effects of decreasing thickness is a result of: a b c d

108

0.5 coil diameters. The actual coil diameter. 2 coil diameter. The skin depth.

Observing resonance effects. Comparison to reference samples. Taking measurements at two different frequencies. None of the above, impedance instruments cannot be used for resistivity measurements.

When given a plate sample for resistivity determination, test frequency should be selected such that skin depth is at least: a b c d

Equal to the plate thickness. One third the plate thickness. One tenth the plate thickness. Twice the plate thickness.

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Frequency for plate thickness determinations of thin sections can be approximated by ; where  resistivity (  ohm-cm), t=thickness (mm) and δ=standard depth of penetration (mm). a b c d

113

Operating at about 80% of the resonant frequency. Using a lower inductance probe. Reduce cable length. Any or all of the above.

1mm. 6mm. 12mm. 18mm.

Which of the following is not an advantage of the eddy current test method? a b c d

118

Operating at about 80% of the resonant frequency. Using a lower inductance probe. Reduce cable length. Any or all of the above.

A practical depth limit for flaw detection and location using eddy current test methods is about: a b c d

117

That is as low as possible. That is as high as is practical. Giving only one skin depth of penetration. One half the resonant frequency.

Most impedance eddy current instruments will not operate at resonance. This situation is remedied by: a b c d

116

1.6  /t2 (kHz). t/  (Hz). 3t2/  (kHz).  t (kHz).

Measuring the thickness of conductive layer on another conductor (neither being magnetic) requires: a b c d

115

= = = =

Thickness determination of a non-conductive coating on a conductive (nonmagnetic) material is done using a frequency: a b c d

114

f f f f

100% volumetric inspection is possible (within limits). Speed. Clean smooth surfaces not required. No couplant required.

When performing an eddy current test and you encountered a signal that could be a crack, permeability change or restivity change, you would: a b c d

Change the frequency. Rotate the phase to put the lift-off vertical. Increase gain and look for roughness of signal. Use MPI instead of eddy current testing.

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The f90 for tubing and plate are found using similar but different equations. These equations were determined: a b c d

120

Problems with ferromagnetic indications occurring in material that is not ferromagnetic can be overcome by: a b c d

121

Defect depth. Wall thickness. Both a and b. Not important.

Long gradual defects can be missed by using a b c d

125

Reduced frequency range. Increased probe-cable capacitance. Decreasing sensitivity to the far surface defects. Bobbin breakdown.

Coil spacing on differential probes for general inspection purposes of tubing is usually: a b c d

124

Encircling probes cannot be made bigger. Fill factor becomes too difficult to regulate for large encircling probes. Higher defect sensitivity can be achieved using surface probes. Both b and c.

To increase sensitivity to near surface defects using a bobbin style probe coil length and thickness are reduced. This however results in: a b c d

123

Using a saturating permanent magnet. Retesting at a lower frequency. Retesting at a higher frequency. Both a and b.

Encircling probes (or internal probes) are likely to be replaced by surface probes for tubing with a diameter greater than 50mm. The reason for this is: a b c d

122

Empirically. By computer simulations. From characteristic frequency (fg). From the phase lag equation.

probes.

Encircling. Differential. Bobbin. Absolute.

Which of the following is an advantage of the differential probe compared to the absolute? a b c d

Sensitive to gradual dimensional changes. Low sensitivity to probe wobble. Easily interpreted signals. All of the above.

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Effects of temperature drift are reduced by using: a b c d

127

The main reason an eddy current coil can detect support plates in heat exchangers when testing tubes from the inside diameter is: a b c d

128

12.2. 11.6. 11.1. 10.9.

An encircling coil is used on a 12mm diameter solid rod. What is the fill-factor if the average coil diameter is 13mm? a b c d

132

25Hz. 250Hz. 250kHz. 250MHz.

If a probe for internal tube testing has an average coil diameter of 11mm, what size would the tube inside diameter be to give a 0.9 fill-factor? a b c d

131

Decreased signal to noise ratio. Decreased signal amplitude. Both a and b. None of the above, probe impedance matching to instrument impedance is not important.

Assuming resistance is negligible and probe inductance is 80  henries, for a cable with 5 x 10^-9 farads capacitance, what is resonance frequency? a b c d

130

Support plates are always ferro-magnetic. Support plates are always the same material as the tube. Magnetic flux is not restricted by the tube wall. Support plates act as resonance amplifiers in the circuit.

A probe whose operating impedance is not between 20-200 ohms will most likely in: a b c d

129

Differential probes. Probe pre-heat. Liquid nitrogen baths. Gap probes.

0.80. 0.85. 0.92. 1.08.

Impedance diagrams for cylinders are not the simple semi circular shapes used for plate. This is a result of: a b c d

Skin effect. Phase lag. Leakage fields. Both a and b.

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A tube being tested by an internal probe has an ID to OD ration of 0.8. Under what conditions does this appear to be a thin wall tube? a b c d

134

Test frequency for solid cylinders, maximum sensitivity to defects, resistivity and dimensions is obtained when f/fg=: a b c d

135

1630Hz. 2.3kHz. 70kHz. 128kHz.

What is the f90 for an encircling coil used on aluminium tubing, P =5.1  ohm-cm, wall thickness 5mm, diameter 40mm? a b c d

139

Internal coil inspection of tubing. External coil inspection of tubing. Pancake coil inspection of plate. Both a and b.

Given a brass tube 20mm diameter (OD) with a 3mm wall and the resistivity of brass is 7.0  ohm-cm, what is the f90 for testing this tubing? a b c d

138

0.08. 0.9. 1.1. 3.

The equation f90 = 3  /t2 applies to: a b c d

137

2. 6. 100. 400.

The f90 frequency has been found empirically from the ratio of thickness and skin depth. For testing tubing this ratio is: a b c d

136

Higher operating frequency. Lower operating frequency. When fill factor is 1. When fill factor is reduced.

612Hz. 3.1kHz. 14.7kHz. 61.2kHz.

When tube testing at f90 (internal absolute probe), if ID wall loss moves the operating point for an absolute coil in a negative X direction, a shallow OD defect would move the operating point: a b c d

+X. –Y. +Y. Both -X and -Y in equal proportions.

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When tube testing (internal absolute probe) at f90 and setting OD wall loss to move +Y on the scope, what is the probably source of a +X moving signal? a b c d

141

When interpreting eddy current signals by quadrature components on strip charts the X channel information is used for: a b c d

142

Tooling or handling equipment. Impurities in the melt. Working below the curie temperature. Oxidation.

Ferromagnetic deposits and inclusions are usually: a b c d

146

Signals are too large making small defects hard to see. No magnetite occurs to use as a reference. Elimination frequencies are too high. Elimination frequencies are too low.

Ferromagnetic inclusions on or in normally non magnetic aluminium will arise due to: a b c d

145

You re-inspect the area at 2f90. You re-inspect the area at 4f90. You re-inspect the area at 0.1f90. Both a and b.

Vectorial addition of signals at conductive non-magnetic support plates is not usually viable because: a b c d

144

Analysing defect type. Analysing defect depth. An analysis threshold. Both a and b.

To eliminate magnetic deposits as a possible cause of defect signals (ie a nonrelevant indication) it is recommended that: a b c d

143

ID wall loss. Through hole. Dent. Support plate.

Non detectable. Non-relevant or false indications. More critical than their signals indicate. Eliminated by small saturating magnets within the coil.

In multifrequency instruments 2 or more operating frequencies are input to a probe simultaneously. What output must be adjusted to permit effective vectorial addition? a b c d

Gain. Phase. Frequency difference. Both a and b.

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What condition can be eliminated using multifrequency eddy current technique? a b c d

148

Metal hardness can be indicated by eddy current testing. This is accomplished by: a b c d

149

Sample curvature. Ambient temperature variation. Coatings. All of the above.

Relative permeability is measured in which units? a b c d

153

Austenitic stainless steel. Titanium. Tungsten. Annealed aluminium.

Which of the following can cause variability in resistivity readings taken for the purpose of sorting? a b c d

152

Brass à = 0.0046. Copper à = 0.0050. Titanium à = 0.0400. Platinum à = 0.0040.

Degree of cold working of which material can be determined by eddy current methods monitoring for permeability changes instead of resistivity changes? a b c d

151

Indirect measurement of effects on restivity. Amplitude measurement. Multifrequency technique. Both b and c.

Which of the following will have the largest resistivity change with change in temperature (à = thermal coefficient): a b c d

150

Denting and pilgering. Magnetic deposits. Support plates. All of the above.

No units (dimensionless ratio). Webers/Ampere-metre. Webers/metre2. Amperes/metre.

The amount of reverse magnetising force required to eliminate the residual magnetic flux in a ferromagnetic material is: a b c d

5.5 kilgauss. The coercive force. The de-saturating force. Hysteresis.

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Which of the following series of stainless steels is not likely to exhibit an increase in relative permeability with increasing cold working? a b c d

155

In order to facilitate testing of magnetic materials without the interference of permeability changes you would: a b c d

156

100 Hz. 50 kHz. 250 kHz. 320 kHz.

What test frequency has a standard dept of penetration of 1mm for a plate material with resistivity of 130  ohm-cm and relative magnetic permeability of 500? a b c d

159

20mm. 10mm. 0.1mm. 0.05mm.

If a plate material has a resistivity of 65  ohm-cm and relative magnetic permeability of 50, what test frequency should you use to achieve f90 at a depth of 0.2mm? a b c d

158

Heat and hold the part over the curie temperature for testing. Use saturating magnets as part of the probe. Both a and b. Stress relieve the part prior to testing.

If testing a material and you have set up acceptable conditions for phase separation of 90o for 1mm sample depth when relative magnetic permeability is 1, what depth would the 90° separation occur at if relative magnetic permeability changed to 20? a b c d

157

301. 302. 304. 316.

650Hz. 1.20kHz. 240khz. 320kHz.

Magnetic saturation techniques for EC testing that use DC saturation coils are limited to the amount of saturation achieved by: a b c d

Test frequency. Heating of the saturation coil. The size of battery used. The voltage that can be safely used.

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What is a resistivity of 6.2  ohm-cm as a % IACS? a b c d

161

Given the permeability of free space is 4πX10^-7 Wb/A/m and the permeability of an iron bar is 7X10^-4 Wb/A/m, what is the relative permeability of the iron? a b c d

162

Potential differences. Eddy currents. Electron flow. Strawberry fields.

Two insulated wires are wound on a plastic rod such that they are positioned close to each other but not touching. The ends of one wire are connected battery; the ends of the other are connected to a galvanometer. If the connected to the battery has 1 amp flowing through it, what will galvanometer read? a b c d

166

Heat. Magnetic field strength. Mechanical force or torque. All of the above

Electric fields are the same as: a b c d

165

Precise crack length determinations. Crack extension rate determination. Crack width determination. Both a and b.

Eddy current generation to determine material properties use detection of variations in: a b c d

164

12.56. 87.9. 280. 557.

Small eddy current sensors in the vicinity of cracks could be used for: a b c d

163

27.7. 13.1. 9.8. 6.2.

very to a wire the

0 A. 1 A. Just a little less than 1 amp. Just a bit more than 1 amp.

The time constant of the circuit is a ratio inductance to resistance (L/R). This accounts for: a b c d

Generation of eddy currents. Phase lag of induced currents. Voltage amplitude. Self inductance.

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In an eddy current test set-up, magnetic lines of flux from the probe which fail to couple the test piece: a b c d

168

Complex numbers are often used in the analysis of eddy current test systems. Complex numbers have 2 components, they are: a b c d

169

0°. 45°. 90°. 180°. is plotted on the ordinate (vertical axis).

The imaginary component. Inductive reactance. Resistance. Both a and b.

Surface coil eddy current transducers are: a b c d

173

Pure inductance. Pure resistance. All conditions. No conditions.

In an R-L circuit a b c d

172

in an AC circuit.

The phase angle between applied voltage and resultant current in an AC circuit of pure inductance is: a b c d

171

Real and imaginary. Whole and natural. Absolute and integer. Real and unreal.

Voltage and current will be in phase for a b c d

170

Carry no information. Cause self inductance in the magnetising coil. Are responsive to the spacing of coil and test piece. Both b and c.

Always used in the absolute mode. Always flat. Always used on flat surfaces. None of the above.

Measurement of the thickness of a non conductive coating would utilise the effect. a b c d

Skin. Lift-off. Hall. Bassel.

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The inductance in the excitation coil is proportional to the diameter square (D2) and the number of turns squared (N2). The voltage induced in the pickup coil is proportional to: a b c d

175

The purpose of small diameter and high frequency probes for determining thickness of thin coatings on conducting substrates is to: a b c d

176

Increased penetrating ability. Decreased coupling ability. A path of low magnetic reluctance. Both a and c.

Shielding obtained by eddy current skin effect differs from magnetic methods of shielding in which way? a b c d

180

Maintain constant lift-off. Ensure the coil axis is perpendicular to the test surface. Prevent the probe from scratching the test piece. Shape the magnetising field.

Magnetic shielding technique provides the magnetic field lines of the eddy current probe with: a b c d

179

Test piece conductivity and thickness. Test frequency. Proximity of coil to test piece. All of the above.

The purpose of curved wear pieces (shoes) to guide surface probe assemblies is to: a b c d

178

Minimise the eddy current field in the substrate. Maximise the eddy current field in the substrate. Maximise the field in the non-conductive coating. Minimise lift-off effect

The magnetic flux density around an empty test coil is reduced by increases in when testing non-magnetic materials: a b c d

177

N and D. N2 and D. N2 and D2. N and D2.

Skin effect methods amplify the magnetic fields. Skin effect methods attenuate the field rather than change the path. Magnetic methods only work on ferromagnetic test pieces. Skin effect methods are the same as magnetic methods.

Maximum test sensitivity is obtained at which point on the signal locus of the complex plane? a b c d

Maximum displacement to the right. Maximum displacement to the left. Maximum vertical displacement. Minimum vertical displacement.

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What f/fg ration is recommend for testing thin wall non-magnetic tubing for cracks, alloy variations or wall thickness variations? a b c d

182

When using an external encircling coil the frequency ration f/fg to obtain maximum sensitivity to all test variables will be greatest for which variety of heavy wall tube? a b c d

183

Eddy current density on the inner wall is too low for crack detection. There is no sensitivity to ferromagnetic inclusions. No discrimination between inner and outer wall is possible. Variation in wall thickness and cracks look the same.

Which of the following is the direct cause of eddy currents in a test piece placed in an encircling transducer? a b c d

187

The energising coil. The pickup coil. Both a and b. The surrounding air.

Phase angle differences of eddy currents greater than about 100° is not recommended for tube testing with encircling coils because: a b c d

186

Increases near surface sensitivity. Reduce magnetic permeability’s of ferromagnetic test materials. Increase magnetic permeability’s of ferromagnetic test materials. Eliminate probe wobble signals.

When both a primary (energising) and secondary (pickup) coil are used as an encircling coil probe, the time varying flux in the test piece induces an AC voltage in: a b c d

185

Solid bars. Wall thickness to outside tube radius = 0.5. Wall thickness to outside tube radius = 0.01. None of the above, f/g is constant for all encircling coil tests.

What is the purpose of DC magnetic bias in eddy current testing? a b c d

184

0.1. 1.0. 3.6. 10.

Induced voltages form the AC magnetic field. Back EMF within the transducer. Resistivity of the test piece. The magnetic field opposing the transducer’s field.

The limit frequency is: a b c d

The optimum test frequency. The maximum limit test frequency. The minimum limit test frequency. None of the above.

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All other conditions being equal for a bar tested in an encircling coil system, an increase in relative permeability of the bar tested would result in: a b c d

189

Locus curves for diameter changes on the test piece are not straight lines on the normalised impedance plane. Why is this so? a b c d

190

d

Real and imaginary components are interchanges. Real component is rotated by 180°. Vertical and horizontal scales are increased by the magnitude of the relative permeability. Complex impedance plane presentation cannot be used when testing ferromagnetic material.

When testing ferromagnetic bars with an encircling coil, the effects of changes in are reduced or eliminated by DC magnetic saturation. a b c d

193

Greater penetration afforded permits better determination of bulk properties. The angle between diameter and conductivity locii is greater. The angle between diameter and conductivity locii is 90°. None of the above, frequency ratio should be less than 4 for such work.

The complex impedance plane presentation for testing a ferromagnetic bar should be changed in what way from the same test on a non-ferromagnetic bar? The: a b c

192

Due to changes in the Bessel function constant. Diameter changes affect the test frequency ratio. Because of the skin effect. Relative permeability of air change.

Separation of diameter and conductivity effects is better carried out at frequency ratios greater than 4 because: a b c d

191

Decreased secondary coil voltage. Increased secondary coil voltage. No change in secondary coil voltage. None of the above, the premise of the question is incorrect as testing of bars with relative permeability over 1 is not possible.

Resistivity. Diameter. Relative magnetic permeability. Fill factor.

Defect effects from tests in the mercury cylinder can be applied to ferromagnetic materials for practical applications provided: a b c d

Phase is rotated 90°. Mercury resistance is subtracted from the results. Voltages from the mercury tests are multiplied by the relative magnetic permeability of the ferromagnetic material. All of the above.

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The through-transmission technique is used for testing of sheet and foil under certain conditions: a b c d

195

For two separate objects with different relative permeabilities and resistivities, equivalent eddy current tests can be performed by adjusting test frequencies. This is explained by: a b c d

196

Suitably shaped insulators inside a mercury filled tube. Suitably shaped conductors inside a water filled tube. Saw cuts in the material to be tested. EDM notches in the material to be tested.

The curve traced on X-Y storage monitor as an active coil is brought up to a sample of 1100 aluminium (100% pure) is called the: a b c d

199

f/fg ratios are equal. Fill factors are equal. Both a and b. None of the above, signals could never look the same for magnetic and nonmagnetic materials.

The best method of measuring the effects of a specific discontinuity totally within a test specimen but at different depths and orientations is by using: a b c d

198

The similarity law for eddy current testing. Maxwell’s Law. Lenz’s Law. Newton’s First Law of Electromagnetics.

Eddy current tests using encircling coils would provide similar test coil impedances or voltage signals for tests on 100mm diameter aluminium rod and 2mm diameter steel wire if: a b c d

197

When test surfaces are not excessively large. When both surfaces are accessible. When the sheet is not multi-layered. Both a and b.

Reference curve. Coil lift-off locus. Aluminium standard arc (ASA). Eddy current curve.

When a metal sheet is inserted into a through transmission probe arrangement, the transmission coefficient phasor: a b c d

Remains unchanged. Changes in magnitude and phase. Changes in real and imaginary values. Both b and c.

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In a through transmission test of sheet products, why might a metered output monitor the product of thickness and conductivity (absolute measurement method)? a b c d

201

For a non-magnetic foil thickness D, conductivity σ, the effective coil distance is found from Aeff = (253,000/fg σ D). Effective coil distance will decrease if: a b c d

202

Increase coil to part spacing. Increase coil to diameter. Decrease coil to part spacing. Decrease coil to diameter.

Sensitivity of conductivity measurement with the probe coil is: a b c d

206

A straight line connecting the zero lift-off point to the empty coil value. Bent slightly left towards increasing f/fg values. Bent slightly right towards increasing f/fg values. Bent slightly right towards decreasing f/fg values.

In plate testing, to minimise effects of lift-off variations you would: a b c d

205

To increase effective coil distance. To decrease effective coil distance. Unpredictable. Not noticeable.

The lift-off locus is: a b c d

204

Thickness decreases. Resistivity increases. Both a and b. None of the above.

The effect of increasing coil diameter on the effective coil distance is: a b c d

203

Changes in either parameter results in the same change in transmission coefficient. Conductivity of a sheet can be assumed to always be constant. Thickness of a sheet can always be assumed to be constant. Because it is not possible to arrange the frequency ratio to provide maximum sensitivity.

Proportional to the coil’s geometric field gradient. A function of the specimen thickness. A function of the effective coil distance. All of the above.

The apparent impedance curve for two different metals of the same thickness will be the same if: a b c d

Different probe diameters are used. Frequencies are adjusted so σ f is equal (where σ is conductivity and f frequency). Lift-off is adjusted to compensate for skin effects. All of the above.

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A practical f/fg ratio for thickness measurements would be in the range of 1-7.5. This would provide maximum sensitivity to: a b c d

208

In eddy current tests to determine non-conductive coating thicknesses, probe diameter and operating frequency are selected to minimise the effects of what parameter? a b c d

209

= = = =

σ1D1 + σ2D2. σ1σ2 + D1D2. σ1D1)(σ2D2). σ1D1)2 + (σ2D2)2.

A difference in conductivities between the two materials. Use of a lift-off compensating probe. A resonance circuit be used. All of the above.

Angle between crack direction and lift-off effect increases. Magnitude of crack effect decreases. Lift-off effect increases. All of the above.

When inspecting spheres with an encircling coil, what is the equivalent effect of increasing the coil length? a b c d

213

D D D D

To discern very shall cracks using a surface coil you would use a relatively high frequency-conductivity product (σ f). Which of the following would then be true? a b c d

212

σ σ σ σ

Determining plating thickness of a conducting non-magnetic material on another conducting non-magnetic material requires: a b c d

211

Conductivity. Density. Lift-off. Both a and b.

If a sheet was composed of 2 metallic layers with thicknesses D1 and D2 and conductivities σ1 and σ2, what would the equivalent product be when tested by through transmission? a b c d

210

Conductivity. Resistivity. Lift-off. Both a and b.

A frequency increase. A frequency decrease. An increase in material conductivity. A decrease in fill factor.

A part with a length to diameter ration to 1 tested in an encircling coil: a b c d

Cannot be tested by eddy current methods. Results in demagnetisation effects. Gives greatly reduced apparent magnetic permeability. Both b and c.

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When using surface coils for crack detection, shallow cracks and lift-off cannot be separated unless: a b c d

215

The result of using a longer encircling test coil to test a spherical object as compared to a short coil or hemispherical coil would be: a b c d

216

The magnetic field due to skip off the opposite wall phase lagged. The electric field due to skip off the opposite wall phase lagged. The exciter passing the defect. Mode conversion.

In the range of about 3-13mm wall thickness, what frequency range would be used for low frequency remote field eddy current testing of ferromagnetic tubing? a b c d

220

Ferromagnetic tubes. Laminates sheets of tungsten carbide. Paint coatings on aluminium boat hulls. Riveted joints on aircraft fuselage.

Signals received in the remote field eddy current set-up give two response off large defects, one occurs due to the receiver coil passes the defect. What causes the other signal? a b c d

219

0.1 coil diameter. 1/2 the inside pipe diameter. 2 inside pipe diameters. There is no direct coupling zone in remote field eddy current testing.

Remote field eddy current testing is a technique commonly used on: a b c d

218

Increased sensitivity. Reduced fill factor. Improved phase discrimination of cracks and conductivity changes. All of the above.

In remote field eddy current testing, how far does the direct coupling zone extend from the exciter coil? a b c d

217

Lift-off compensating probes are used. Frequency is high enough. Frequency is low enough. Both a and b.

10-300Hz. 500Hz-2kHz. 2-10kHz. 10-50kHz.

When eddy currents are used for sorting techniques it is usual to establish impedance values from: a b c d

Probe characteristics. Samples of known materials. Published information. Trial and error methods.

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Applying a DC electric field to a ferromagnetic coil is done for what purpose? a b c d

222

Sorting of materials by impedance values of an eddy current probe require: a b c d

223

Altered lattice structure inhibits electron flow. Electrons move to lower energy states when alloyed. Electrons move to neutral energy states when alloyed. Both a and b.

Work hardened aluminium has a higher resistivity than annealed aluminium for what reason? a b c d

227

Non-magnetic coatings applied to magnetic bases. Alloys. Superconductors. Isotopic variations of the metal.

Alloying metals added to pure base metals result in decreasing conductivity of the initial value of the pure base metal. Why does this occur even with alloying metals having higher conductivity than the base metal? a b c d

226

Its curved shape. Its vertical direction of movement. Both a and b. Edge effect on the magnetic material would follow the lift-off trace exactly.

Substitutional solid solutions and interstitial solid solutions of metals are forms of: a b c d

225

Relative permeability of all parts to be fixed at 1. Specimen thickness exceeds depth of eddy current penetration. Conductivity of all parts tested be within 10% of each other. Use of the characteristic frequency for test frequency.

In general, the edge effect seen as a probe is moved towards edge of a magnetic test piece as compared to a non-magnetic test piece would be recognised by what feature? a b c d

224

Reduce background noise. Improve signal to noise ratio. Eliminate permeability variations that might affect eddy current coil response. All of the above.

Changes in alloy content. Disruptions in lattice structure. Excitation states of electrons are higher. None of the above, the premise of the question is wrong, resistivity is a constant for a given alloy content regards of worked state.

Which of the following will increase conductivity of an alloy? a b c d

Solution heat treating. Precipitation or aging. Annealing. Cold working.

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What would the effect on conductivity signal be as radius of curvature of the test piece is decreased? a b c d

229

Although specimen and standard may be within the recommended 5oC temperature difference for resistivity measurement, why might the value determined still be incorrect? a b c d

230

A minute increase. A significant increase. A decrease. No change.

Resistivity measurements standards: a b c d

234

Increased resistivity. Decreased resistivity. Both a and b. None of the above (no effect).

What is the effect of ferromagnetic materials on the inductance of an eddy current test coil? a b c d

233

50o below melting point At TC (critical temperature). Room temperature. In a range from -50 to + 15oC.

What is the effect on eddy current determined properties of aluminium alloys that have been annealed for an excessive amount of time? a b c d

232

Measurement temperature and the temperature the standard was originally established at are different. A different probe is used that was used to establish the standard. Test frequency is too high. The specimen is work hardened.

Natural aging of aluminium alloys occurs at what temperature? a b c d

231

Signal amplitude increases. Conductivity measured would decrease from the true value. Conductivity measured would increase above the true value. No effect would be noticed.

made

on

bulk

material

and

without

reference

Cannot be made by any methods known. Use very high frequency eddy current probes. Do not use eddy current methods. Use magnetostrictive effects.

When a ferromagnetic material has a magnetising force applied to it, the magnetic flux that builds within the material lags the applied force. The same lag occurs upon the reduction in magnetising force. What is the lag called? a b c d

Permeability. Hysteresis. Barkhausen effect. Phase shift.

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Which of the following is not a method used to manufacture notches in calibration standards used for eddy current tests of tubing? a b c d

236

What is main advantage of foil calibration standards over affixed coatings calibration pieces? a b c d

237

Simplicity. Economic. Behaves like a crack. Provided a good indication of sensitivity.

What is the advantage of artificial defects made by the EDM process? a b c d

241

Electric discharging machining. Ion milling. TEM (tunnelling electron microscopy). Saw cuts.

What is not one of the advantages of drilled holes being used as reference standard? a b c d

240

Establish acceptance criteria. Verify accuracy of a test. Provide traceability of a test. All of the above.

The most common and reliable method of manufacturing artificial cracks for eddy current standard is by: a b c d

239

Robustness. Accuracy. Resilience. Calibration on curved surfaces.

What is the purpose of calibration reference standard? a b c d

238

Electric discharge machining. Electrophoresis. Milling. Saw cuts.

Speed. Cost. Accuracy. None of the above, EDM is not used to make artificial defects.

A significant disadvantage of using a natural crack as a calibration standard is accurately sizing it. What is the only reliable direct sizing method to determine nature crack depth? a b c d

Time of flight diffraction. X-ray. Potential drop. Cutting the specimen open and optically sizing it under a microscope.

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What is used to regulate the consistency of the manufacturing of calibration standards? a b c d

243

Which of the following is not a means of suppressing an undesired eddy current test signal? a b c d

244

Image stability. Resolution. Cost. Size and power consumption.

How many thresholds must be set on the CRT display of an eddy current instrument in a box gate alarm system? a b c d

248

Operator response. Meter movement (rise time). Operating frequency. Defect type or coating thickness.

What is the most significant drawback of dot matrix displays of EC signals compared to CRT displays? a b c d

247

External, stray magnetic and electric fields. Electrical noise generated within the EC instrument. Mechanical vibrations of test coil or material. All of the above.

What limits the scanning speed when using meter display eddy current instruments? a b c d

246

Varying phase rotation. Reducing receiver gain. Varying bridge balance point. Tuning reactive components in the probe bridge circuit.

Which of the following noise sources can be filters with the appropriate electronics in an eddy current instrument? a b c d

245

Eddy current instruments calibrated to national standards. Codes and specifications. Licensed metrology labs. Level 3 technicians.

2. 3. 4. 8.

What would a polar co-ordinate based phase-gate look like? a b c d

Single line. Box. Pie-slice. Sinusoid.

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Multifrequency can discriminate signals at the same depth because: a b c d

250

Compared to single frequency units, multifrequency eddy current instrument circuits are: a b c d

251

Magnitude of magnetic fields. Direction of magnetic fields. Magnitude and direction of electric fields. Both a and b.

N-type semi conductors use what form of charge carrier? a b c d

255

D/A converter. A/D converter. Motherboard. Parallel interface.

Hall detectors are used to sense magnetic fields. They detect: a b c d

254

CPU. Analogue-to-digital converter. Digital-to-analogue converter. Retro-virus.

A circuit block that uses an analogue voltage as an input and outputs, a proportional binary value is a (n): a b c d

253

The same except for signal separation circuitry. The same except for signal separation and combining circuitry. The same in every way. Equipped with better filters and signal averaging circuits.

A circuit block that accepts a binary number and translates it to an analogue voltage or current proportional to the binary number is a (n): a b c d

252

Sensitivities are greater at lower frequencies. The phase will be different at different frequencies. Ferrites are independent of frequency. Sensitivity is the same for defects but reduced for geometry changes as frequency increases.

Electrons. Positrons. Holes. Quarks.

P-type semiconductors use a b c d

as charge carriers.

Electrons. Holes. Protons. Positrons.

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The ideal signal voltage in a Hall detector element in the absence of a magnetic field is: a b c d

257

The magnitude of the Hall voltage is: a b c d

258

Stabilised DC supplies are needed. Excitation AC current must be constant for all frequencies. Both a and b. None of the above, Hall detectors can be used on any EC instrument.

Eddy current test systems using Hall detectors can accomplish differential tests by: a b c d

262

Magnitude of magnetic field. Direction of magnetic field. Rate of change of total flux linkage. Both a and b.

Instrumentation for systems using Hall detectors instead of pickup coils are different in what respect? a b c d

261

Single pass inspections of large surfaces. Improving depth resolution. Increased depth of penetration. Increasing frequency response.

Which of the following are Hall effect detectors not sensitive to? a b c d

260

Proportional to the external magnetic field. Proportional to the content current in the element. Both a and b. Fixed only by the direction of the magnetic field.

Linear multichannel Hall detector arrays are ideal for: a b c d

259

Zero. Maximum. A loc minimum. Determined by ambient temperature.

Using two Hall detectors. Using two superimposed excitation frequencies. Having the excitation coil double as a pickup coil. No method presently known.

When using Hall detectors, how are sensitivities to relatively great depths achieved? a b c d

Increasing Hall detector size. Increasing test frequency. Increasing excitation coil size. Both a and b.

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What are slip rings used for in eddy current inspection systems? a b c d

264

To avoid rotating parts, probes or test piece, what system would be used to inspect round bar stock? a b c d

265

The probe is water cooled. The probe is set back from the test surface at least 3cm. Extra windings and diameter are used in probe construction. Both b and c.

Seamless pipe and tubing are often made from billets made from continuous cast blooms. The rounds, as the billets are called, are test by eddy current to detect what types of defect? a b c d

269

Ultrasonic probes will depolarise test hot surfaces. No stream of water coupling is needed for ECT. UT cannot be done on steels above the curie temperature. UT mechanical waves cannot penetrate the surface scale.

Eddy current testing of hot billets (1,100oC) can be done provided what precautions are taken? a b c d

268

Orthogonal winding transducer. Multi-pancake probe. Zig-zag probe. Transverse compensating probe.

In what way is eddy current testing more suitable to high speed production tests on hot metals than ultrasonics? a b c d

267

Encircling probes. Circumferential array of probes. Hall effect exciters. Both a and b.

A differential transducer with the two windings around perpendicular to each other used to detect both longitudinal and transverse cracks is called a (n): a b c d

266

Electrical contacts in rotating heads. Clutch mechanisms in probe pushers. To allow ease of motion by encircling probes. To allow ease of motion by bobbin probes.

Cracks. Ovalities. Laps. All of the above.

When ECT is used to test thickness of coatings having a tolerance range, what is the minimum number of calibration specimens required to calibrate the instrument? a b c d

2. 3. 4. 2 or 4 depending on if the coating is conductive or not.

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Truly effective sorting of aluminium alloys by eddy current determination of resistivity is not possible because: a b c d

271

When online testing of ERW welds in steel pipe using eddy current testing, what problem occurs if the inspection is performed too far from the induction heating coils used for normalising the weld? a b c d

272

The paint is conductive. Edge effects are causing wrong readings. Different metals are causing wrong readings. The lighter one has a void defect.

What is the most effective way of assessing heat or fire damage to heat-treatable aluminium on aircraft? a b c d

275

Increased conductivity. Decreased hardness. Both a and b. No effect.

You are given 2 plates of identical size (50x50x10mm) both painted with a thin coating of black acrylic paint of the same thickness. Eddy current test indicate both have a conductivity of 37% IACS, yet one is nearly twice as heavy as the other. How is this possible? a b c d

274

Noise results as the metal cools below the curie point. The induction coils cannot be used as primary coils the inspection. Too much warpage occurs and lift-off is excessive. Near surface defects are masked by the spherodizing effects within the grain structure.

What is the effect of over-aging on aluminium heat treatable alloys? a b c d

273

Variations due to heat treatment overlap ranges of conductivity. Grain realignments result when eddy currents flow. Defect free areas can never be found in aluminium. Differences in meters used are never standardised.

Eddy current conductivity tests. Ultrasonic velocity tests. Brinnel hardness tests. Thermography.

Alpha-case forms on titanium and its alloys at elevated temperatures. Eddy currents are used to establish the depth of case. What is the cause of the formation of alpha-case? a b c d

Oxygen diffusion from the heated surface. Carbide migration. Active cathodic protection by-products. Passive anodic protection by-products.

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When a single channel strip charge recorder is used with eddy current testing of bolt holes using spinning probes, what 2 parameters are recorded? a b c d

277

Why are cadmium plated steel bolts used as fasteners on aircraft? a b c d

278

Both surface and subsurface defects are found. Even automotive engineers can perform the tests. The speed at which tests can be performed. The application to determine heat treatment quality.

Eddy current test methods are more sensitive than x-rays for detection of aircraft structures. a b c d

282

Paint thickness determinations. Subsurface corrosion detection in multilayer structures. Conductivity determination for alloy sorting. All of the above.

What is the biggest advantage eddy current test methods have that make them the most frequently used NDT method in the automotive industry? a b c d

281

Elimination of frequency dependent phase angle. Improved crack detection by suppressing lift-off output. Increased signal to noise ratio. Frequency control without affecting balance.

Low frequency eddy current (100Hz to 5kHz) is commonly used in aircraft inspections for: a b c d

280

To ensure maximum shear strength. To provide corrosion resistance to the steel bolt. To provide a galvanically similar surface next to any aluminium to reduce corrosion of aluminium. Both b and c.

Some CRT display eddy current instruments allow X and Y gains to be adjusted independently. Increasing Y gain and reducing X (eg Y = 0.2 V/div, = 2.0 V/div) accomplishes what? a b c d

279

Y (vertical) output of signal vs. depth along hole axis. X (horizontal) output of signal vs. depth along hole axis. Y (vertical) output of signal vs. time. Phase of signal vs. position of probe along the hole axis.

in

Corrosion. Missing fasteners. Fatigue cracks. Overloading cracks.

Finned copper tubing used in air conditioning units has smooth land areas at regular intervals along the tube. What is the purpose of these land areas? a b c d

Increase tube rigidity. The locations at which the tube is roll expanded into the tube supports. Increase heat transfer rate by wall thinning. Calibration points for differential coil eddy current inspections.

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During evaluation of an indication in a heat exchanger tube, the probe is moved back and forth over the defect. It is noted that the indication has changed position along the length of the tube. What is the likely source? A: a b c d

284

In eddy current inspections of chiller tubes, freeze cracks located at freeze bulges are often not possible to detect using conventional differential probes because: a b c d

285

c d

A single probe operating at more than one frequency. A single probe operated at one frequency then rescanning the flaw at a different frequency with the same probe. Two or more probes in tandem each at a different frequency. Any of the above constitutes multifrequency ECT.

Multifrequency instruments may be one of two types; simultaneously frequency or alternate frequency. Which is not an advantage of the simultaneous frequency systems? a b c d

289

Physics of a testing media. Characteristics of the test object. Geometry of the test. All of the above.

Multifrequency eddy current testing utilises: a b

288

Wobble, electrical noise. Dimensional variables, material variables. Internal variables, external variables. External variables, internal variables.

In order to do computer modelling of eddy current fields you must provide: a b c d

287

Bulges and cracks have the same phase. The bulge signal is so large it masks the crack. The crack only occurs under the support plate. Both a and c.

Generally in multifrequency techniques for in-situ boiler tube inspections, high frequencies are used to suppress while low frequencies are used to suppress . a b c d

286

Magnetic deposit. Spiral fret. Active cracking. Probe wire has loosened at the connector.

No unnecessary saturation in separation stages. Wide passband of x and y outputs. Low cost of equipment. Permits high inspection speeds.

Increasing temperature of a dielectric (insulating) materials has what effect? a b c d

Increased resistivity. Increased conductivity. Destabilisation of isotypes. No effect on any properties (that’s why they are called insulators).

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What is the advantage of eddy current testing over the potential drop method for sizing surface cracks? a b c d

291

For practical applications of surface probes on curved surfaces: a b c d

292

Over the shallowest notch. Over the deepest notch. Over a defect free area. In air away from the test piece.

Why are eddy current coils not made using iron wire? a b c d

296

Negative Y spike. Positive Y spike. Ellilpse. Angulated line (ie Not horizontal).

When an eddy current is balanced for surface testing for flaws, where is the probe placed? a b c d

295

High resistivity indications. High penetration of eddy currents. Cyclic variations in magnetic permeability. Electrical noise.

In the 1960s a non-storage type oscilloscope was used for eddy current tests. The defect free specimen gave a horizontal line. A defective specimen gave a (n): a b c d

294

Curvature should be small within the region directly below the cross-sections of the coil. Frequency of operation should be as low as possible. Perspex supports should be arranged to fit the curvature inspected. All of the above.

How does hysteresis manifest itself when testing ferromagnetic materials? a b c d

293

Accuracy. It is non-contacting. Cost. There is no advantage.

To avoid hysteresis effects. To make mathematical calculations easier. To prevent excessive heat build-up. For cathodic breakdown considerations.

The higher the value of inductance for a given frequency the greater the degree of: a b c d

Balance ability. Sensitivity. Q factor. Capacitive reactance.

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The transmit-receive or transformer style probe provides: a b c d

298

Inductance increases improve eddy current sensitivity. Why is increasing coil area not a preferred method of increasing sensitivity even though inductance is increased? a b c d

299

Lift-off does not decrease sensitivity. Conductivity and thickness can be measured simultaneously. Temperature changes do not affect conductivity readings. All of the above.

What degree of accuracy can be expected when using eddy currents to determine paint thickness 10  m thick? a b c d

303

Thin samples. Thick samples. Rough surfaces. Ferromagnetic samples.

The through transmission method has the advantage that: a b c d

302

Coils are wound in opposition to each other. Coils are operated at cancelling frequencies. One coil has an air core and the other has an iron core. A subtractive circuit is incorporate into the eddy current instrument.

Phase adjustment on simple conductivity meter instruments is especially useful for what conditions? a b c d

301

It makes the coil too bulky. Resolution of defects is decreased. Solid cores cannot be used. Iron cores must be used.

How does the differential (or auto-comparator) coil provide insensitivity to gradual changes? a b c d

300

Improved s/n ratio. Increased sensitivity to deeper defects. Both a and b. No advantage over single coil probes.

0.01  m. 0.1  m. 1.0  m. 5.0  m.

Tubes with a diameter of more than about 50mm are more effectively tested using than encircling probes. a b c d

Internal axial coils. Surface probes or arrays. Differential probes. Forked probes.

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Multifrequency techniques are performed using: a b c d

305

What is the purpose of pulsed saturation eddy current testing? a b c d

306

Insulators. Semi-conductors. Superconductors. Gold.

On the normalised impedance plane showing the effects of changing conductivity (σ) the coil’s normalised resistance is zero under what condition? a b c d

310

2 δ. 3 δ. 5 δ. 10 δ.

As conductivity of a material approaches infinity its resistive losses approach zero. What type of material exhibits such extremes? a b c d

309

Pulsed eddy current testing. Remote field eddy current testing. Multifrequency eddy current testing. Both a and b.

Using a shielded ferrite coil and the pulsed eddy current technique, penetration of measurable currents in a metal sample can be increased to (where δ the standard depth of penetration): a b c d

308

To allow sequenced multifrequency application. To achieve greater penetration in ferromagnetic materials. Synchronization of gates. It allows time for ring to die down and so improves far wall resolution.

Large DC saturation units for eddy current inspection of ferromagnetic tubing are often required. What technique can be used to avoid use of these heaving DC saturation units? a b c d

307

Absolute coils. Differential coils. Both a and b. Special multifrequency coils only.

σ is zero. σ is infinite. Both a and b. When it equal the normalised inductive reactance.

What does an increase in operating frequency do to the probe coil inductance? a b c d

It increases. It decreases. It may increase or decrease depending where you are on the locus. None of the above.

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The heating of a ferromagnetic part that occurs when the AC field works to align the magnetic domains into a preferred magnetic orientation is reduced by: a b c d

312

The coil to specimen impedance Z can be defined by (where Zc is coil impedance and Zs is specimen impedance): a b c d

313

Monitor effects of any temperature changes. Monitor instrument drift. Monitor probe degradation. All of the above.

In what way does computer acquisition and analysis of eddy current signals (particularly heat exchanger tubing) out-perform humans? a b c d

317

Coil width. Number of turns. Intended operating frequency. Both a and b.

During an eddy current inspection of heat exchanger tubing, what is the purpose of recording a calibration signal with each tube inspected? a b c d

316

Multifrequency techniques. Magnetic focusing probes. Spring loaded probes. Using remote field eddy current techniques

What are the main limiting parameters for a single coil probes dimensions? a b c d

315

(Zc X Zs)/(Zc + Zs). (Zc + Zs)/(Zc X Zs). Zc – Zs. None of the above.

The only way to reduce or eliminate the edge effect is by: a b c d

314

Performing eddy current tests under water. Performing eddy current tests where air temperature is below 0øc. Pre-aligning the domains with DC saturation. Multifrequency eddy currents.

Detection. Reproducibility. Accuracy. All of the above.

The analytical method that consists in correlating changes in amplitude, phase and/or quadrature components of a complex test signal voltage to electromagnetic conditions in the test piece is: a b c d

Phase analysis. Impedance analysis. Differential analysis. Absolute analysis.

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An instrumentation technique that discriminates between variables in the test piece by different phase-angle changes these variables produce in the test signal is: a b c d

319

The ration of the square of the diameter of a cylindrical test piece to the square of the average diameter of the test coil is the: a b c d

320

Skin effect. Doppler shift. Edge effect. Phase shift.

The property of a test system that allows separation of signals from defects on close proximity to each other is: a b c d

324

Absolute probe. Axial probe. Differential probe. Multipancake probe.

The phenomenon whereby depth of penetration decreases with increasing frequency is called: a b c d

323

Optimum frequency. Test frequency. Limit frequency. Characteristic frequency.

Two or more coils in electrical series opposition arranged so EM conditions not common to the areas of the specimen being tested produce a bridge imbalance is a (n): a b c d

322

Flux ratio. Fill factor. Physical impedance. Test ratio.

The frequency providing the highest signal-to-noise ratio for detection of an individual property of the test piece is the: a b c d

321

Impedance analysis. Phase analysis. Modulation analysis. Frequency analysis.

Phase separation. Amplitude discrimination. Defect resolution. Multifrequency demodulation.

The time required for a test system to return to its original state after it has received a signal is the: a b c d

Dead time. Recovery time. Recoil time. System delay.

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Current flow that is time constant in both direction and amplitude is: a b c d

326

The method whereby desirable frequency signals are separated from undesirable frequency signals from the modulating envelope of the carrier frequency signal is called: a b c d

327

Acceptance limits. Reject level. Test criteria. Group level options.

Differential coils are, in some areas, also called a b c d

331

Difficulty in interpreting signals. Over sensitivity to wobble. Reduced sensitivity to outside wall defects. Insensitivity to circumferential cracks.

Test levels used in ECT that establish the group into which a material under test belongs are termed: a b c d

330

Only one or two parameters are subject to change. It must be ferromagnetic. Composition must be uniform throughout. Size and shape must always be small with simple geometric symmetry.

What is the disadvantage of the multi-pancake probe used for internal tube inspections as compared to the axial bobbin type probe? a b c d

329

Phase analysis. Modulation analysis. Filtering. Fast fourier transformer.

In order that useful results be obtained from an eddy current test, what must be true about the test specimen? a b c d

328

Direct current. Eddy current. Induced current. Boring.

ID coils. Bucking coils. Annular coils. Tandem coils.

A test level above or below which test specimens are found to be unacceptable is called? a b c d

The cut-off level. The rejection level. The acceptance threshold. Both a and b.

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A network that passes electromagnetic wave energy over a described range of frequencies and attenuates energy at all other frequencies is a (n): a b c d

333

The slope of the induction curve at zero magnetising force as the test piece is being taken from its demagnetised state is the: a b c d

334

0.735. 0.819. 0.907. 0.956.

Given a tube with a 15mm OD and 1.5mm wall, what size (average diameter) coil is used to obtain an 85% fill factor for an internal inspection? a b c d

338

A two way sort. A three way sort. Threshold sorting. Standard deviation testing.

Given an encircling coil with an average coil diameter of 10.5mm and testing a tube 10mm OD with a 1mm thick wall, what is the fill factor of this set up? a b c d

337

Flux density. Flux leakage. Magnetic history. Permeability.

An electromagnetic sorting based on a signal response from the material under test above or below a level established by two or more calibration standards is: a b c d

336

Virgin permeability. Initial permeability. Maximum permeability. Effective permeability.

The magnetic condition of a ferromagnetic part based on its previous exposures to magnetic fields if the part’s: a b c d

335

Filter. Gate. Inductor. Grate.

14mm. 13mm. 12mm. 11mm.

What is the standard depth of penetration of 304 stainless steel (68.96  ohmcm) having 60% cold work applied (  rel=2) tested at 20kHz? a b c d

1.1mm. 2.1mm. 3.1mm. 4.4mm.

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What is the standard depth of penetration for 301 stainless steel having been 25% cold worked (71 a b c d

340

1.3mm. 2.3mm. 3.4mm. 4.6mm.

Given a standard depth of penetration of 1.3mm exists for a 10kHz test of navelbrass (6.63 a b c d

341

Ampere. Faraday. Förster. Linqvist.

Phase relative to current in the coil. Amplitude. Both a and b. None of the above.

The right hand rule for determining magnetic field direction around a current carrying conductor assumes: a b c d

345

Resistance. Resistivity. Probe electrical impedance. Specimen thickness.

The voltage changes used to determine various parameters in eddy current testing consist of changes in: a b c d

344

1.3mm. 3.9mm. 5.2mm. 6.5mm.

Electromagnetic induction, on which ECT has its foundations, was first discovered by a b c d

343

 ohm-cm), what is the effective depth of penetration?

The quantity actually monitored by an eddy current probe is: a b c d

342

 ohm-cm,  rel=10) tested at 10kHz?

Conventional current flow. Modern theory current flow. Only alternating current flow. Non-geomagnetic

The left hand rule for determining the magnetic field around a current carrying conductor assumes: a b c d

Conventional current flow. Modern theory current flow. The conductor is in the shape of a helix. An antiparellel universe.

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The product of the magnetic flux density in a loop of a current carrying coil times the area of that coil gives: a b c d

347

Magnetic induction or the force per unit pole in a magnetic field is the magnetic analog of: a b c d

348

Electric fields. Magnetic fields. Both a and b. None of the above, energy conversion by electromechanics is not possible.

Faraday’s Law states that the magnitude of the induced voltage in a circuit is: a b c d

352

Decrease its impedance. Wobble. Resonate. Increase its operating frequency.

Electromechanical energy conversion is possible due to: a b c d

351

Probe. Probe and a generator combination. Test sample. None of the above.

As an operating eddy current probe (a coil) is brought near a conductive sample the induction of eddy currents in the sample causes the probe to: a b c d

350

Electric intensity. Electric impedance. Electric resistance. Electromotive force.

An eddy current test system can be considered a form of transformer. As such, the secondary side would be the: a b c d

349

Eddy current intensity. A dimensionless value equal to infinity. Total magnetic flux outside the coil. Total magnetic flux inside the coil.

Equal to the rate of change of the magnetic flux through it. Inversely proportional to the rate of change of the magnetic flux through it. Opposite in sign to the inducing field. Of the same sign as the inducing field.

An alternating voltage in a coil brought near a sample that has a finite impedance will result in: a b c d

A counter EMF. Induced eddy current flow. Both a and b. None of the above.

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353

The intensity of a magnetic field that a unit magnetic pole experiences of a force of one dyne is one: a b c d

354

A single magnetic line of flux is given the unit: a b c d

355

Abvolts. Coulombs. Electro-stats. Hertz.

If 20 coulombs of charge passes a point in 5 seconds, the electric current value would be: a b c d

360

Proportional to pole strength. Inversely proportional to the square of the distance separating them. Both a and b. The opposite of a and b.

The product of current in amperes times time in seconds gives units of: a b c d

359

Hysteresis. Eddy currents. Magnetisation. Permeability.

The force between point magnetic poles is: a b c d

358

Wb/m2. Gauss. Maxwell’s/cm2. All of the above.

Alignment of the magnetic domains in iron by an external field result in: a b c d

357

Dyne. Oersted. Maxwell. Tesla.

Magnetic flux density is expressed in: a b c d

356

Oersted. Telsa. Ohm-com. Gauss.

4 amperes. 100 amperes. 0.8 amperes. 20 amperes.

The purpose of using a radial magnetic field around the current carrying coil in a galvanometer instead of a parallel magnetic field is: a b c d

To reduce resistance. To increase heat dissipation. To maintain a simple direct proportionality between current and coil rotation. For ease of construction of the instrument.

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

Given a wire made of copper with resistivity 1.724 ohm-cm, that is 1cm in length, and has a cross-sectional area of 1cm2, what is the resistance of this section of wire? a b c d

362

Resistance of a piece of wire is a function of: a b c d

363

Negative. Positive. Zero. Unity.

A negative thermal coefficient of resistivity would be characteristic of: a b c d

367

Inherent resistivity. Length and cross-sectional area. Temperature. All of the above.

The temperature coefficient of resistance of a pure metallic conductor is always: a b c d

366

AC power transformers. Carbon composite materials. Eddy current testing. Ultrasonic testing.

Which of the following will have an effect on the electrical resistance of a wire? a b c d

365

Wire length. Cross sectional area of the wire. Resistivity of the material the wire is made of. All of the above.

Eddy currents are an undesirable feature in: a b c d

364

1 ohm. Micro-ohm. 1.724 ohms. 2.972 ohms.

All pure metals. Some semi-conductors. Insulators. Materials conductivity > 100% IACS.

In a nonmagnetic material the back EMF produced by the induced eddy currents has what effect on the probe? a b c d

Reduced coil impedance. Reduced coil current. Increase coil current. Both a and b.

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368

The decrease in eddy current density with increasing depth from the surface is: a b c d

369

The time dependent component of the skin depth equation indicates: a b c d

370

Large diameter. Long. Zig-zag. Either a or b depending on whether plate or tube testing is being done.

57o. 90o. 114o. 180o.

Phase lag in the test sample for a void at 1 standard depth of penetration is: a b c d

374

probes are needed.

The phase lag, in units of degrees, for an eddy current signal displayed on a typical impedance plane scope for a void originating 1 standard depth of penetration below the surface would be: a b c d

373

0.5. 2. 5. 25.

To ensure planar shaped magnetic field a b c d

372

Flux density decreases with depth. Current density decreases with depth. Phase lag of the signal with depth. All of the above.

For the calculation for eddy current density to apply, a sample should be relatively thick. The minimum thickness to allow the simple equation to apply is about δ (where δ is the standard depth of penetration). a b c d

371

Linear. Exponential. Logarithimic. Sinusoidal.

1 radian. 90o. Both a and b. None of the above, it cannot be determined from the given information.

For the purpose of determining electrical characteristics of a coil/sample combination, capacitance can be an important factor in: a b c d

The sample. The probe cables. The probe coil. All of the above.

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The inductive reactance component of an eddy current probe coil’s impedance will with increasing AC frequency: a b c d

376

In the eddy current probe circuit the capacitive component of its impedance is degrees out of phase with its inductive component: a b c d

377

Move up the curve. Move down the curve. Trace smaller semi-circles. Trace larger semi-circles.

The impedance method of eddy current testing uses: a b c d

381

Voltage amplitude and phase representation. Repairing broken solder joints. Fusing near surface defects. Terminating technicians who make incorrect evaluations.

On the ideal impedance diagram the effect of reducing mutual coupling between probe and sample would be to have the impedance point: a b c d

380

Arcsin (R/x). Arccos (R/x). Arctan (R/x). Arctan (x/R).

In eddy current terminology phasors are used for: a b c d

379

0o. 90o. 180o. 270o.

The phase of the impedance in an AC circuit is found from: a b c d

378

Increase. Decrease. Remain unchanged. React unpredictably.

Two coils. Changes in voltage across the primary coil. Changes in voltage across the secondary coil. Spring loaded probes only.

As the diameter of the eddy current probe increases, the operating point on the normalised impedance curve moves (for a surface probe ie not for tube testing). a b c d

Up. Down. In. Out.

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Variations in the flow of eddy currents caused by flaws in the test piece are monitored as voltage fluctuations in the secondary coil in the: a b c d

383

When a probe/sample combination is modelled as an equivalent circuit, the secondary circuit load equivalent would be considered a (n): a b c d

384

L L L L

   

D2. D. 1/D. 1/D2

An increase in probe diameter will move the operating point on the impedance curve: a b c d

388

Up. Down. Inside the original curve. Outside the original curve

What best describes probe inductance as a function of probe diameter? (  indicates proportional to): a b c d

387

Short circuit. Open circuit. Resonance circuit. None of the above.

All other factors constant, increasing lift-off will move the operating point on the impedance curve: a b c d

386

Resistive load in parallel with the coil’s inductive reactance. Inductive load in parallel with the coil’s inductive reactance. Capacitive load in series with the coil’s inductive reactance. Short circuit.

Using the analogy of the coil/sample as a transformer circuit, when the coil is held far from the sample we can approximate a (n): a b c d

385

Send-receive method of ECT. Impedance method of ECT. Resonance method of ECT. Potential drop method.

Up. Down. To a point inside the original curve. To a point outside the original curve.

An inductive and a resistance impedance change in the test coil resulting when an operating eddy current probe is moved near a conductive test sample is represented on a (n): a b c d

Standard penetration chart. Phase correction graph. E meter. Impedance graph display.

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The decrease in semicircle radius of the impedance curve display when lift-off increases indicates: a b c d

390

Given a coil with 50 ohm resistance and 50 microhenries inductance and operated at 50 kHz; what is the impedance phase angle? a b c d

391

The AC signal is too difficult to analyse. DC is more energy efficient. To allow phase rotation. So both electronic and mechanical balancing can be used.



H inductance and operated at 20 Given a coil with 2 ohms resistances and 20 kHz, what is the impedance phase angel (in degrees)? a b c d

394

Balance button. Video filter. AC to DC converter. Amplifier.

Conversion of the AC unbalance voltage signal to a DC signal retaining amplitude and phase characteristics is done for what reason? a b c d

393

0o. 5.6o. 17.4o. 90o.

The most significant instrument component required to detect the small variation in probe impedance or voltage caused by detecting defects in eddy current testing is the . a b c d

392

A smaller change in coil impedance. Quantum effects. Increased resistivity. An approximate short circuit.

-14.2°. 38.6°. 44.4°. 51.4°.

The impedance phase angle of a probe operating next to a copper test sample is 40o. What is the inductive reactance of the probe in this situation if the total impedance measured is 30 ohms? a b c d

19.3 ohms. 22.9 ohms. 25.2 ohms. Not possible to determine with information given.

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Given a probe operating at 0.5MHz next to a brass sample, total probe impedance is measured at 47.2 ohms, if the impedance phase angel is 45o what is the resistive load of the sample? a b c d

396

Given the resistive load of a probe/sample circuit as 5.1 ohms and the resistance of the probe when operated in air as 15 ohms, what would the impedance phase angle be if total impedance of this circuit was 24.5 ohms? a b c d

397

c d

45o. 90o. 180o. 270o.

Internal filtering to decrease instrument or system noise results in: a b c d

401

Phase change in the bridge circuit. An impedance change in the bridge circuit. Current flow in the previously balance bridge circuit. All of the above.

Quadrature components of the bridge AC output are generated by sampling the sinusoidal signal at two positions apart on the waveform: a b c d

400

Detecting impedance changes between coils. Detecting impedance changes between a single coil and a reference impendance. Both a and b. None of the above.

The typical figure 8 pattern that occurs with a differential probe moving over a defect is a result of: a b c d

399

22o. 35o. 55o. 68o.

In eddy current instruments, bridge circuits are used for: a b

398

Same as the inductive reactance in the probe. 33.3 ohms. 47.2 ohms. Not possible to determine from information given.

Decreasing frequency response of the instrument. Decreasing maximum inspection speed. Increased s/n ration. All of the above.

Most eddy current instruments can tolerate an impedance mismatch in the AC bridge on the order of: a b c d

0%. 5%. 50%. Any amount.

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402

In the L-C bridge circuit used by simple meter crack detectors, the capacitor is connected in parallel with the in the bridge circuit. a b c d

403

At the resonant frequency of an L-C circuit, output voltage for a given measurement: a b c d

404

Gain, lift-off, balance. Gain, lift-off, frequency. Gain, lift-off, filter. Lift-off, balance, frequency.

In resonant circuit crack detectors, the lift-off control actually varies: a b c d

408

Not selectable. Infinitely variable. Limited to the khz range. Limited to the MHz range.

Resonant circuit crack detectors have a meter output and 3 controls: a b c d

407

5-10 ohms. 40-60 ohms. 100-200 ohms. 10-200 ohms.

Test frequencies for crack detectors operating at or close to resonant frequency are: a b c d

406

Zero. Minimum. Maximum. Not useful.

On most eddy current instruments using the impedance method, the AC bridge circuits can usually balance coils having impedances in a range of: a b c d

405

Probe coil. Resistor in the arm adjacent to the probe coil. Resistor in the arm opposite the probe coil. Oscillator generator.

Amplifier gain. Operating frequency (by less than 25%). Bridge resistance. None of the above.

Which of the following systems has the advantage of being unaffected by temperature variations? a b c d

General purpose instruments. Send-receive instruments. Resonant circuit instruments. None of the above can eliminate temperature drift.

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In send-receive ECT systems, probes with 2 receive coils have those coils would in opposition. The purpose for this to: a b c d

410

Now obsolete, the ellipse and slit methods of eddy current testing: a b c d

411

c d

0.9. 0.866. 0.707. 0.5.

Given a parallel L-C circuit with a probe inductance of 80 x 10^-6 Henries and operated at resonance frequency, 252kHz, what is the cable capacitance? a b c d

415

Lower than general purpose ECT instruments. Proportional to recording speed (length of tape past the record head per unit time). Inversely proportional to recording speed. Based on tape thickness.

Frequency response of an instrument is based on the fact that the output signal of an instrument will be less than the input signal as inspection speed increases. Instrument frequency response is defined as the frequency where output signal is -3dB from the input. This would relate to a volt signal out for a 1 volt signal input: a b c d

414

FM shielding. AM shielding. Relative motion between the coil and sample. Two send and two receive coils.

FM tape recorders have often been used to store eddy current signals for subsequent retrieval. Frequency response for these instruments is: a b

413

Used the AC signal, without conversion to DC, for analysis. Were mainly for sorting materials. Were used to measure large (>5%) coil impedance variations. All of the above.

Modulation analysis is a specialised ECT method that requires: a b c d

412

Eliminate thermal draft. Permit phase discrimination. Allow no net voltage in the receive coils when both sense the same material. Both and b.

126.5 ohms. 80 x 10^-6 farads. 5 x 10^-9 farads. Cannot be determined.

Given a parallel L-C circuit with cable capacitance 5 x 10^-9 farads and operating at a resonance frequency of 2252kHz, what is the inductive reactance of the probe? a b c d

5 x 10^-9 henries. 126.5 ohms. 253 ohms. Cannot be determined.

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When selecting an eddy current instrument for a particular project you need to know: a b c d

417

Resonance frequency can be determined for a parallel L-C circuit by: a b c d

418

c d

Lift-off compensation. Temperature compensation. Test frequency not affecting relative impedance of the coils. Both a and b.

Band pass filters. Ferrite cups. Annular arrays. Lift-off compensating coils.

In an absolute probe configuration, a second coil, apart from the sensing coil, is required for: a b c d

422

Provides greater inductance from a given coil size. Provides increased field coupling for small surface area in contact with test material. Temperature compensation. To increase distance from coil to test surface to allow wear protection.

To reduce the effective sensing diameter of surface probes operating at relatively low frequencies, the use of is recommended: a b c d

421

=1/2(π)(LC)^½. =(2I(π)LC)^ ½. =L/C. =2(π)L/C.

Mounting a disc of metal, having similar properties to the test material, next to the reference coil in an absolute probe has the advantage of: a b c d

420

fr fr fr fr

Which of the following is not a reason for using a ferrite core on the sensing coil of a pencil probe? a b

419

Test frequency and type of lift-off compensation. Type of output signals (eg meter or scope). Instrument type (impedance, send-receive, crack detector, etc.). All of the above.

Bridge nulling. Lift-off compensation. Temperature compensation. All of the above.

The effective probe diameter extends to about a b c d

beyond the coil diameter.

1mm. 1 coil radius. 1 coil diameter. 4 skin depths.

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423

Ferrite cups can be used to obtain a b c d

424

b c d

½ the standard depth of penetration. The skin depth (δ). Twice the skin depth. The effective depth of penetration.

Varying frequency for a probe on a given specimen will move the operating point down the impedance graph with increasing frequency. If the specimen is not thick, a reversal swirl occurs forming a knee. This is a result of: a b c d

429

Up the curve. Down the curve. Horizontally left. Horizontally right.

For a thick specimen, test frequency should be selected to provide good separation from lift-off variations. This is facilitated by setting frequency so that the greatest expected defect depth is at: a b c d

428

Dividing the inductive reactance component by the coil’s inductive reactance in air (XL/Xo). Subtracting the coil/cable resistance in air. Both a and b. None of the above.

All other parameters constant, an increase of permeability in the test piece causes the operating point on a normalised impedance curve to move: a b c d

427

Frequency. Coil diameter and core materials. Number of turns. Length of coil.

Normalising probe impedance for impedance graph displays is accomplished by: a

426

A concentrated field. Right angle current changes. Reduced lift-off noise. Higher frequencies.

Which of the following is not a probe parameter affecting impedance results? a b c d

425

without affecting depth of penetration.

Skin depth and phase lag effects. Instruments instability. Capacitive effect. None of the above.

The characteristic parameter, Pc, used by Deeds and Dodd is primarily a modelling tool. Test conditions with the same characteristic parameter have the same: a b c d

Probe parameters. Material parameters. Operating point on the normalised impedance graph. Probe and instrument parameters.

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430

If lift-off is arranged on the eddy current storage monitor so the signal moves from right to left as the probe is moved away from the sample, an increase in sample thickness would conventionally move: a b c d

431

Maximum frequency you would use for determining thickness of a non-conductive coating on a conductor would be: a b c d

432

d

provides

equal

discrimination

for

resistivity

Ensure test sample and standards are at a uniform temperature. Perform all such tests in liquid nitrogen. Ensure the probe used has large inductive reactance compared to coil resistance (xl/rc>50). Both a and c.

The most significant difficulty in determining thickness of conductive coatings on conductors is that: a b c d

435

At the top of the curve. At the bottom of the curve. Near the knee of the curve. Anywhere on the curve measurements.

To prevent error in resistivity determinations caused by temperature, you should: a b c

434

1MHz. 500kHz. 1000Hz. Limited by probe to instrument impedance matching, cable resonance and cable noise.

When making resistivity measurements on unknown samples, the frequency used is selected such that the operating point on the impedance graph is: a b c d

433

Down. Up. Right. Left.

Variations in base material as well as coating material will affect the signal. Probes must be specially designed. The test cannot be done if an air gap exists between the two conductive materials. Both b and c.

The problem with overcoming probe-cable resonance by operating above 1.2fg (fr-resonance frequency) is: a b c d

Phase discrimination. Greatly reduced sensitivity. Arcing from probe to test piece. None of the above, operating at 1.2 times the resonance frequency is the preferred option.

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436

What is the effective diameter of a surface probe with a 5mm diameter coil used on a sample with p = 72  ohm-cm and operated at 2MHz. (p is resistivity): a b c d

437

The ration of thickness to skin depth t/δ that provides a 90o separation between lift-off and thickness change is empirically derived. It is found to be about for plate testing: a b c d

438

Its approach signal. Amplitude. The rougher signal quality. All of the above provide evidence of flaws.

The best way to distinguish between localised resistivity changes and a real defect is: a b c d

442

½ Ө. Ө. 2 Ө. 3 Ө.

A very shallow surface defect can be distinguished from lift-off by: a b c d

441

Work hardened 7075-T6 (AL alloy). Fe304 deposits on heat exchanger tubing. EDM notches in 304 stainless steel. Segregation in Austenitic stainless steel.

The phase angle (as measured from the lift-off signal) of a shallow surface or sub-surface defect is related to the eddy current phase lag á=x/δ (radius), where x = flaw depth and δ = skin depth. The phase angle seen on the storage monitor is approximately: a b c d

440

0.1. 0.8. 1.6. 4.

Which is not a source of ferromagnetic indications? a b c d

439

5.5mm. 6.2mm. 7.5mm. 10mm.

Retest the area with a smaller probe at the same frequency. Retest the area at 1.3 of the test frequency. Retest the area at 3 times the test frequency. All of the above.

Encircling or bobbin style probes used for tube testing require careful design of coil size to optimise sensitivity and coupling. Coil length and coil depth should be about: a b c d

Equal Equal A 3-1 Equal

to wall thickness. to the shortest allowable defect. ratio. to 1 skin depth at the f90 frequency.

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443

The reference coil in a bobbin style probe can be mounted concentrically inside the test coil and the probe still be considered and absolute probe because: a b c d

444

When using a bobbin type differential probe, sensitivity to near surface defects can be improved by: a b c d

445

Sufficient wall loss has occurred at the point of maximum deterioration. The leading and trailing edges are abrupt. A sufficiently low frequency is used. A fill factor of greater than 0.9 is used.

Ferromagnetic materials can affect probe impedance. These ferrogmagnetic materials: a b c d

449

Gap probes. Absolute probes. Differential probes. Bobbin style internal probes.

If a defect is longer than the spacing between the coils on a differential coil, the defect can only be recognised as such if: a b c d

448

Shape of the defect. Length of the defect. Coil configuration of the probe. All of the above.

Insensitivity to gradual changes in dimensions or properties is both an advantage and disadvantage, depending on the situation. This feature is exhibited by: a b c d

447

Wrapping coils in opposition. Decreasing coil spacing. Increasing coil spacing. Turning up the gain.

Symmetry of a differential signal as the probe is moved over a defect will depend on: a b c d

446

The AC bridge doesn’t know the difference. It is used in conjunction with an external reference coil inside a calibration tube. The fill factor for the reference coil is