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PCN CLASSROOM TRAINING HANDBOOK VISUAL TESTING FOREWORD This Classroom Training Handbook is published by PCN (a subsidiary of the British Institute of NDT) and covers the PCN examination syllabus for the Visual Testing NDT method. It is essential reading for candidates in the PCN level 2 and level 3 certification examinations described in PCN/GEN appendix ES, and is intended for use by instructors and students on PCN approved training courses covering this method, although it may be found equally useful by those preparing themselves for other examinations covering Visual Testing. Copies of the handbook may be obtained from organisations conducting PCN approved courses of training, or direct from PCN Certification Services at The British Institute of NDT, 1 Spencer Parade, Northampton NN1 5AA, England (telephone +44 (0) 1604 259056).

CONTENTS Overview

V1

Physics of light

V2

The eye and vision

V3

Environmental conditions

V4

Light sources

V5

Optical aids

V6

Measuring equipment

V’7

Temperature measurement

V8

Surface conditions

V9

Inspection procedures

V10

Recording & reporting

V11

Definitions

V12

UNIT VI • OVERVIEW Definition of visual inspection Visual inspection is the monitoring of specific parameters by visual and optical assessments of test objects and surfaces using the visible portion of the electromagnetic spectrum. Inspection may be by the use of the eye alone or can be enhanced using optical systems such as magnifiers and microscopes. A variety of equipment is available to the visual inspector including mirrors and gauges, which can be used for profile assessment, borescopes and endoscopes, which are used on parts with limited access, and video and computer enhancement systems. Although a visual test is a test in itself, it also forms an integral part of many of the other nondestructive testing methods. For example, magnetic particle and penetrant inspection requires visual observation and assessment of the detected indication; radiographs require visual inspection for the interpretation of results; ultrasonic inspection requires the visual assessment of the trace on a CRT. Visual inspection applications Visual inspection has applications in virtually every industry. It is used for inspecting exposed or accessible surfaces of opaque objects, such as the surface of a finished steel part, and for inspecting the interior of transparent objects, such as the inside of a glass object. The methods employed are similar for all visual inspections in that the eye, sometimes assisted by optical systems, picks up and transmits information to the brain, which assimilates the information and is able to make a judgement with reference to previous experience. Primary processing inspection This is visual inspection of the raw materials used in a process, and of the plant and manufacturing processes being employed. This means checking raw materials to ensure conformity to specification and checking manufacturing conditions at each stage. For example, in the metal industry, furnace conditions, ladle lining, and ingot mould conditions would be checked. During processing, melt condition, slag cover, stream condition, turbulence, splashing etc., could all he monitored. Knowledge of the industry is essential for monitoring and interpreting process information, so that deviations are spotted immediately. These observations may influence the choice of route for further processing. Other examples are inspections on mould stripping, ingot, concast bloom or casting condition, soaking pit and roll condition and observation of stock rolling for problems. Secondary and finishing process inspection The inspection and monitoring of dies and components for forging, extrusion and drawing, the cutting and joining in fabrication, the forming by welding, soldering, brazing, spinning and press forming are some t) f the metal manufacturing processes monitored by visual inspection. Finishing processes such as final shaping, cleaning, heat treatment, plating, coatings, and assembling

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etc., require visual inspection. Visual inspection often plays a major role and in some cases is the only method of assessing process performance. Visual inspection can be done alone or before other tests (including other non-destructive tests) are made. Other manufacturing industries, for example plastics, composites, electronics, food and textile industries, employ a similar pattern of inspection. In-service inspection Visual inspection is a vital part of in-service inspection, which may or may not be backed up by other NDT methods. The examinations made are for fatigue cracks, creep failure, corrosion, erosion, abrasion, mechanical damage, wear and tear, distortion and poor workmanship. The ability to differentiate between different damage mechanisms, which may, to an untrained inspector, have similar appearances, often results in incorrect diagnoses. The trained, experienced visual inspector can elicit a great deal of information, and could use this information to decide which other tests to perform in order to confirm a diagnosis of the problem. Conditions for visual inspection Visual inspection must take place in a clean, comfortable environment with adequate lighting. There should be reasonable access to the parts to be inspected and attention should be paid to safety, working position, and atmospheric conditions. The test piece should be clean and free from protective coatings. Any equipment to be used should be checked for accuracy and its operation understood by the inspector. The procedures to be followed should be documented, all relevant specifications should be to hand, and results of any observations should be clearly presented.

Standards for visual inspection The International Organisation for Standardisation (ISO) and the Comité Européen de Normalisation (CEN) are drafting generic standards covering the general principles of visual testing, as well as equipment and terminology associated with this NDT method. In addition, many application standards have recognised the importance of visual inspection.

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UNIT V2• PHYSICS OF LIGHT Radiant energy-energy transmitted by electromagnetic waves Light Visible light is defined as radiant energy capable of exciting the human retina and creating a visual sensation. It is the portion of the electromagnetic spectrum with wavelengths between 380-770 mm. At these wavelengths radiant energy makes visible anything from which it is emitted or reflected in sufficient quantity to activate the receptors of the eye. Light can be quantified in many ways: Luminous flux The luminous energy emitted per second from a light source. The unit of luminous flux is the lumen (lm). As the lumen is a measure of energy per unit time it must he related to the Wait (W). The energy of a light source depends on its wavelength, but as a rough guide I W equals 621 lumens of green light (wavelength 5.54 x I0-10 m). Luminous intensity This is the luminous flux emitted per unit solid angle. Solid angles are measured in steradian (sr), and therefore the unit of luminous intensity is the lumen per steradian or lmsr1 . A practical unit of luminous intensity is the candela (cd), where I candela = I lumen steradian-1 , or, a light source with intensity 1 candela emits I lumen per steradian.

Solid angle S ?

A

? Flux

Luminous intensity Luminance The luminance of a surface is the luminous flux per unit area coming from that surface. It is measured in lumen per metre2 , or 1m.m-2 . Illuminance Considering a surface on which light falls, illuminance is defined as the luminous flux per unit area

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falling on a surface. It is measured in lux (lx), one lux being the illuminance of a surface one metre from a light source of one candela. Luminous efficiency The luminous efficiency of a light source is the ratio of total luminous flux (lumens) to total radiant energy output. Overall luminous efficiency is the ratio of the total luminous flux to total energy input.

Properties of light Light can be reflected, refracted, diffracted, and polarised. These are properties of waves, and it is thought that light can he described as a wave motion travelling in straight lines at a speed of 2.99 x l0-8 ms’ (in a vacuum). A wave has a wavelength, frequency and velocity. Wavelength is usually measured in nanometres (I x 10-9 m), and the frequency in Hertz (Hz) or cycles per second. Velocity = wavelength x frequency.

Wavelength Direction of motion Amplitude

Direction of vibration

Transverse wave The visible spectrum consists of all the colours from violet through to red, i.e. violet, indigo, blue, green, yellow, orange, red. White light is all the colours combined together. The colours represent different wavelengths, red being the longer and violet the shorter. The shorter wavelength, higher frequency waves are more energetic than longer, tower frequency waves.

Frequency 1019

? -rays

1016 X-rays

1015 1014 UV

IR

1012

108 Hz Radio waves

Visible

Electromagnetic spectrum Reflection A reflected ray is one turned back from a surface. There are two laws of reflection:

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1.

The reflected ray, the incident ray and the normal to the reflected surface all tie in the same plane.

2.

The incident angle and reflected angle are equal.

N

A

B

i r X

Y O

Silvering

A

B

O Mirror

Regular reflection

A

O Paper

Diffuse reflection

Diffuse reflection

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Reflection can be regular or diffuse. On a polished smooth surface all rays incident at a given angle are reflected hack at that angle, i.e. parallel rays are reflected back parallel to each other. This is regular reflection. On the other hand, on a rough surface such as paper, the light is reflected in different directions. At each point on the surface the laws of reflection are obeyed, but the angle of incidence of each ray is different. This is known as diffuse reflection.

Refraction When a tight ray is incident on the surface of a transparent object, only a small percentage is reflected, the rest of the light continuing through the new medium in a new direction. The amount that the light is bent depends upon the two mediums and the incident angle. Snell’s Law states that for a given boundary, the ratio of the sine of the incident angle to the sine of the refracted angle is a constant. The constant is known as the refractive index. As for reflection, the incident and the refracted rays and the normal at the point of incidence all lie in the same plane.

A

C

Air

i Glass

sin i/sin r = constant

r Refracted ray B

Refraction

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NOTES: Coherent light sources –sources, which have the same wavelength and amplitude of vibration and are in phase with each other.

Diffraction Diffraction is when light waves pass around an object and spread beyond the limits of the geometric shadow.

Nearly straight-line Incident waves propagation

Diffraction

Incident waves

Diffraction Interference When two or more coherent light beams reach the same area, their effect is additive. This leads to the phenomenon known as interference and appears as a pattern of light and dark bands showing where the wave-fronts have either reinforced or cancelled each other. The reinforcement is called constructive interference and the cancelling effect is called destructive interference.

S

A

O

B

Q P

Double split showing interference of overlapping beams Polarised light This is light in which the electromagnetic vibration is only in one plane. Inverse square law Light obeys the inverse square law, which states that at double the distance the intensity will fall to a quarter of the original intensity i.e. I1 D22 = I2 D12

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Ordinary unpolarised P light(Vibration in all direction)

b

a

Q

Polarised lightvibrates only in one direction (and normal to it)

Q P

a

b

No light passes, as polarised light passing through disc P cannot pass through disc Q

Polarisation Theories of Light There are several theories describing radiant energy: a.

Corpuscular Theory by Sir Isaac Newton - luminous bodies emits radiant energy in particles, which are intermittently ejected and travel in straight lines. The particles act on the eye to stimulate the optic nerves to produce light sensation.

b. Wave Theory by Christian Huygens - states light results from molecular vibrations in luminous material, which is transmitted through a hypothetical medium in wave-like movements. c.

Electromagnetic Theory by James Clerk Maxwell - suggests luminous bodies emit light in the form of radiant energy, which is propagated in the form of electromagnetic waves.

d. Quantum Theory by Planck - this is an update on the corpuscular theory. Energy is emitted and absorbed in discrete quantities (quanta). These packages of energy are known as photons. e.

Unified Theory by De Broglie and Heisenberg - based on the premise that every moving particle has an associated waveform, i.e. light can be thought of as both a particle and a wave.

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At present, the true nature of light is not well understood and there is still much debate as to whether or not the Quantum and Unified Theories are valid.

Blackbody radiation A black body is a body, which completely absorbs any heat or light radiation falling upon it. Blackbody conditions relate to either an absorber or a reflector in which a true black body will, in the case of the absorber, totally absorb all incoming radiation, irrespective of wavelength. For a reflector or emitter, a black body can be a perfect radiator. There are various laws, which attempt to relate wavelength and intensity of absorbed or emitted radiation to the temperature (or energy) of a black body.

Planek’s Radiation Law relates to an equation to produce a series of energy curves from a blackbody source. Wier Radiation Law produces a simplified Planck’s equation applying to the visible region (380-770 nm). Stefan Boltzmann Law is a modification of the Planck’s equation. The law states that the total radiant power/per unit area varies as the fourth power of the absolute temperature.

Spectral emissivity This is the ratio of output of any wavelength to that of a blackbody at the same temperature and wavelength. When the spectral emissivity is the same for all wavelengths it is known as a grey body.

Colour temperature The colour temperature of a radiator is the temperature at which a black body must operate so that its output is the closest approximation to a perfect colour match with the output of the selective radiator. Significant errors usually arise when using colour temperatures for incandescent (glowing) source assessments. Colour temperatures associated with light sources (other than incandescent) are correlated colour temperatures, not true temperatures.

Measurement of light

Although the human eye is the main detector for white light, instrument detectors such as photoconductive cells, photodiodes, phototransistors and photographic film etc. can measure the radiant properties of light and provide accurate data not available by human eye assessment.

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The measurement of light is called photometry with the measurement instruments being called photometers and radiometers and are either portable or laboratory based, the latter being the most accurate. A radiometer is an instrument used to detect and measure electromagnetic radiant energy. A photometer is a specific type of radiometer, which measures only the visible part of the radiant energy.

Measurement of visible light Photometers are used to measure light energy within the visible spectrum and can measure luminous intensity, luminous flux, illuminance, luminance, light distribution, light reflection and light transmittance. A spectroscope measures the spectral distribution of colours or selective wavelengths. Apparent differences in the intensity of various light sources can be due to differences in the ability of the measuring instrument to detect different wavelengths (spectral response). This needs to be considered when any measurement of visible light is being made. There are many types of photometers. For example, one type incorporates a photometer for visible light and radiometer for ultraviolet light. The choice of photometer will depend on the intensity of the light source, the wavelengths of the source, the accuracy required, whether testing is indoor or outdoors etc.

Measurement of ultraviolet light Ultraviolet Light, known also as black light, is not visible to the human eye, but can be made visible by using fluorescent dyes. These dyes absorb the ultraviolet radiation and emit the absorbed energy as light of wavelengths usually in the yellow-green portion of the spectrum. Ultraviolet radiation is defined as the part of the electromagnetic spectrum having wavelengths from 100-400 nm i.e. just beyond the violet end of the visible spectrum. It is divided into 3 types, UV-A, UV-B, and UV-C. UV-A is the portion between 315-400 nm; UV-B is the portion between 280-3 15 nm; UV-C is the portion between 100-280 nm. Radiometers are used to measure radiant power over a wide range of wavelengths, including ultraviolet light. These instruments measure ultraviolet light in microwatts per cm2 .

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Unit V3 The Eye And Vision The eye Sclera (white)

Conjunctiva

Choroids (blood Vessels) Ciliary muscle Cornea Pupil Aqueous humour Ins

Vitreous humour Blind spot

Retina (light sensitive)

Optic nerve

Components of the human eye in cross section The eye operates in a similar way to a camera. Light passes through the transparent cornea and enters the inner part of the eye through the pupil. The size of the pupil and thus the amount of light entering the eye is controlled by the iris. The lens focuses light rays from an object onto the retina, at the rear of the eyeball. The tight rays are then converted from light energy to electrical signals by groups of receptor cells called rods and tones. Rods are not colour sensitive hut detect the intensity of the light and can respond at very low levels, whereas cones are sensitive to different colours, responding according to the colour’s specific wavelength, but need a much higher intensity of light to respond. These signals are passed along the optic nerve to the brain which then processes the information and forms the picture we see. To enable the picture to he formed and identified, the brain requires process data, which is gained by training and experience. The eye is a complex receptor organ, the mechanism of which is only partly understood. When the eye views a scene it views it in two stages: 1.

Entire field vision called preattentive processing.

2.

Localised focus on a specific object in the field.

Both of the above are processed by the brain to produce the pictures we see. It is thought that the eye and brain simplify the various tight patterns into spots, lines, shadows, edges, colours, orientation and position within the entire field. This data is then compared with data previously collected and stored in the brain’s long-term memory. This enables images to be identified and compared and allows differentiation between pattern changes and colour changes. Different objects can be easily identified but problems arise when attempting to identify objects of similar shapes, characters and colours. When an unspecified object is being sought, the full field of vision requires careful examination but when a specified object is of a known characteristic, it can be shown that only half the field requires inspection and so is identified more quickly.

Webers Law is used by psychophysicists and states that: 1.

Single points and lines are more important than their relationship to each other.

2.

Closed forms or objects appear to stand out more than open forms.

These laws mean that the brain picks out certain items in preference to others, e.g. the brain will pay more attention to a closed, solid object than to open shapes. Each item within a field of view is coded and stored in a specific part of the brain and withdrawn

when required to produce a complete picture. Sometimes the incorrect item is withdrawn and positioned in the wrong place, thus creating an optical illusion and causing the real picture to be misunderstood. This also accounts in part for the differences between what people see and the true picture.

Vision Vision acuity Vision acuity is the term used to express the spatial resolving power of the eye. It involves near vision and far vision to cover and identify what is seen and will be dependent on physical, medical and physiological conditions and differs from person to person. The limitation of unaided normal vision with average viewing conditions is a disc approximately 0.25 mm diameter and a line 0.025 mm wide at a distance of about 150 mm, the nearest the eye can focus. All the rays of light must focus on the retina for perfect vision. Some people have eye defects in which the rays of light cannot focus on the retina, creating an out of focus image. Near sightedness is when the eye lens focuses the light rays in front of the retina. Far sightedness is when the lens of the eye focuses light rays beyond the retina. Both of these conditions can be corrected by placing a suitable eye-glass lens 17-21 mm from the retina. The majority of visual tests require near vision acuity within 400 mm. Far vision examination is carried out at 6. m. The charts used for the vision examination should be white with letters printed clearly in black. The lighting conditions should be as specified, e.g. the room lighting should he 800 lx and the background luminance of the chart should be 85 ? 5 cd m2

Near vision examination This is carried out with or without eyeglasses, usually at a distance of 300-400 mm using suitable reading charts - the chart used by opticians and ophthalmologists is a standard test chart devised by the British College of Ophthalmologists. A Jaeger chart can be used instead. This is a standard reading chart with text sizes ranging from Jaeger 1(11 – very small) to Jaeger 20 (J20 - large). It is an easy test to administer as it involves reading a small paragraph at a specified distance.

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Vision can also be checked with certain machines. These produce patterns of all shapes and sizes and are used to perform a quick check or screen the eyesight. If someone fails an eye test using this type of machine, their vision is then checked more accurately using one of the standard reading charts. The test specified by PCN in order to he eligible for examinations in NDT requires that: “All candidates shall have natural or corrected vision to be capable of reading from a standard test chart, Jaeger No.] letters or Times Roman N4 or equivalent at a distance of not less than 30 cm.” Candidates will also need to pass the Ishihara colour perception test (see below). ASNT specify the following: “the applicant is capable of reading a minimum of Jaeger No.2 or equivalent type and size of letter at a distance of not less than 12 inches on a standard Jaeger test chart. The ability to perceive an Ortho -Rater minimum of 8 or a similar test pattern is also acceptable. This test should he administered annually. Candidates must also he capable of differentiating different shades of colours - this colour vision must be tested every 3 years.

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UNIT V3 • THE EYE AND VISION

Far vision examination As for near vision examination, standard test charts or machines can be used. If using a Chart, the test is carried out at 6 m with the charts containing varying sizes of letters.

Colour vision There are applications where colour vision is a major factor in the correct assessment of an object. A person’s ability to correctly identify all the colours is therefore important. If there are colour deficiencies (colour blindness), then important details can he missed or misinterpreted. Colour deficiencies and true colour blindness can be either inherited or acquired from specific medical problems. Ten per cent of the male population have some sort of colour deficiency. The most common hereditary defect is an inability to distinguish clearly between red and green, and occurs in approximately 2% of males hut very rarely in females. Acquired colour deficiencies can be due to many illnesses and varies in severity depending on cause. Ageing can also affect colour vision.

Colour vision testing Colour vision testing can be carried out with: 1.

An anomaloscope, which allows mixing of colours.

2.

Charts with different coloured spots. A commonly used test is one devised by S Ishihara, which is a set of charts using coloured dot patterns.

3.

Caps with 15 changes in colour hue to check for red blindness and green blindness.

Phototopic vision Phototopic vision (foveal vision) is when the eye is adapted to light vision - after a few minutes of exposure to more than 3.0 cdm2 . As the cones work effectively at higher light intensities, phototopic vision is mediated mainly by the colour sensitive cones and so colour vision is clear.

Scotopic vision Scotopic vision (parafoveal vision) is when the eye becomes dark adapted to low levels of illumination of below 3.0 x 10-5 cdtm-2 . This requires a considerable time of 30-45 minutes, depending upon the initial light exposure values. Only the rods are sensitive to low light intensities, therefore differences in intensity can be detected but colour vision is poor or absent.

The effects of health on vision A person’s health affects their sight ability, a variable that may he difficult to detect but which produces inconsistent or incorrect visual assessments. Diabetes impairs normal vision years after it first appears, and may produce a gradual loss of vision due to cataracts, which is a loss of transparency in the lens of the eye. Glaucoma is the build up of

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pressure within the eye starting with blindness and destruction of the eye. parents are afflicted with the disease. per year, by an optician and, if present,

slight vision impairment and, when severe, results in total This is genetic and capable of appearing in persons whose Detection is by frequent eye pressure checks, at least once can be controlled with drugs.

Standard eye testing only detects certain physical abnormalities, such as long sightedness or short sightedness, colour blindness etc., hut there are many other eye-related factors which are not assessed and which may influence visual ability.

Perception

UNIT V3 • THE EYE AND VISION

Perception is the difference between physical reality and the view that we think we see. It is how an observer’s brain interprets the data it is being given. The Muller-Lyer illusion demonstrates one of the problems. Different individuals perceive the same view in different ways and it is important to know why and how these differences occur. The difference between observers depends upon pre-programming of the brain by training and experience and the mental and physical state at the time of observation.

Lighting The eye is capable of adaptation to variable lighting conditions. Time is required and it can take up to 30 minutes to achieve full sensitivity with changes from high to low levels of lighting and vice versa. The adaptation time is also influenced by disease, fatigue, chemical emissions and drugs and generally becomes more sluggish with age. These, together with environmental factors of heat, noise, dust and posture, can produce images far removed from the actual physical object being inspected. Distance and three-dimensional perception When a person views an image or scene, each eye records the view from a slightly different angle producing two views of the same object or scene. This enables the brain to produce threedimensional images of objects and produces a perception of depth and distance when viewing a scene. An important consideration is the expectation factor or the pre-programmed condition of what is expected in the inspection and is influenced by either a negative or positive pre-emption, i.e. if a person is told to look for defects in an object, defects may be found even if none are present.

Effects of fatigue and health on perception Seeing is an active process, where changing images are constantly being processed and interpreted by the brain. Fatigue reduces an observer’s efficiency and visual ability. There are many diseases which, will impair the sight and general ill health will reduce

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In the Müller-Lyer illusion, the shafts of two arrows are the o same length - contrary to appearances the brain’s processing ability. These problems will all lead to inaccurate interpretation of physical data. For visual inspection to have the highest degree of detectability we require a set of conditions which, whilst desirable, arc often not achievable and will generally be a compromise, except under laboratory or test room conditions. Probably the major factors that, are controllable are training, experience, lighting, environmental conditions and equipment. As outlined above, accurate observation of physical data needs ideal environmental conditions and training/experience to overcome the problems associated with perception.

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UNIT V4 • ENVIRONMENTAL CONDITIONS The environment in which visual inspection is to he carried out requires careful attention. The following considerations are the most important. 1.

Cleanliness

2.

Lighting

3.

Access

4.

Atmospheric conditions (temperature, humidity etc.)

5.

Safety

Cleanliness Cleanliness is a basic requirement for accurate visual inspection. The subject under examination must be clean to the extent required for accurate visual inspection. In some cases, e.g. critical inspection, the test-piece must be extremely clean and free from dirt, grease, scale, flux and all protective coatings. Methods of cleaning the test-piece are discussed more fully in Unit V9 Surface Conditions. The examination area must also be clean to avoid contamination of the test sample and interference with test equipment. These clean conditions are easily achieved in a test house or laborato ry, but if an in-service inspection of a large piece of plant is being made, the environment is fixed and the inspector will normally have to do the test under difficult conditions, e.g. on bridges or on rigs offshore. Plastic sheeting is often used to protect the surface from a contaminating environment.

Lighting The background of the area in which testing is carried out has to be given careful consideration. Reflections and shadows from the structure, walls, windows, ceiling and floor will all affect visual perception and influence the outcome of the test. The test area should be correctly lit - the recommended lighting ratio is about 3:1 between test-piece and dark background and 1:3 between test-piece and light background. The human eye works most efficiently in daylight, but this is not always achievable. When working in conditions other than daylight, the eye can work effectively hut must be given time to adapt. For visual inspection, brightness or adequate illumination of the test surface is a major factor and this depends on the reflectivity and intensity of the light source. Excessive brightness interferes with the ability to see clearly and therefore impairs critical inspection. The light intensity must be concentrated with a recommended minimum value of 160 lx for general inspection and 5001x for critical examination. Specific standards may require higher minimum values.

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The different types of light sources commonly used in visual testing are described in V5 – Light Sources

Access and viewing distance If unaided visual inspection is to be carried out accurately, observations must be made within 300 mm. Direct access is not always available, for example, inside small-bore pipework, small chambers, inside combustion chambers of internal combustion and jet engines and environments which are hostile due to radiation, chemical or heat hazards.

NOTES: The improper use of any light source can damage human vision. Where access is restricted, inspection can be made using special equipment. See Unit V6 – Optical Aids.

Viewing angles and distances The eye’s resolving power is dependent on the angle and distance from the test surface. The average eye at 300mm can resolve the angular separation of two points on a test-piece down to 0.01670 (1 minute of arc). This means that the best resolution is about 0.09 mm at 300 mm and 0. 18mm at 600mm. It is recommended that direct vision testing should he carried out between 250 mm and 20 600mm and the angle between the eye and test surface not less than 300 .

Atmospheric conditions Temperature and humidity should be moderate in order for working conditions to he as 30 comfortable as possible. There are rules from the Health & Safety Executive regarding the minimum temperatures allowed for working. The minimum is 160 C after the first hour. There are also suggestions regarding overcrowding and ventilation. Bad conditions could affect the inspector’s perception and lead to inaccuracies. Laboratories or test houses can be made reasonably comfortable hut when the inspection has to he made outdoors or in a workshop then the environment is not controllable and the inspector has to do the best assessment he can, in some cases noting down the atmospheric conditions at the time of the inspection.

Safety The test area must he made as safe as possible. Many precautions are common sense, but special mention should be made concerning the use of solvents for cleaning and the safety of different light sources. Fatigue is also an important factor Perception is affected by stress and fatigue. It is obviously unsafe for an inspector to work when he is overtired and prone to accidents.

Solvents There are regulations covering the storage and use of solvents. Many solvents are inflammable and present a fire hazard, and some are toxic. Safety data sheets containing information such as the Occupational Exposure Limit and Flash Point should be available and appropriate precautions taken. Most solvents are covered by the COSHH regulations and many can present a long-term health hazard.

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Light sources The safety of different light sources varies considerably. It is important that the inspector is aware of these differences and takes appropriate measures to protect himself and other workers. Before commencing visual inspection it is essential tha t the hazards of 80 light sources be clearly understood to prevent temporary or permanent damage to the sensitive parts of the eye. Manufacturers’ instructions and literature must be consulted before working near or with any artificial light or radiation source. The use of high-intensity light sources, both natural and artificial, creates a potential eye hazard. The human eye operates most effectively in an area illuminated with sunlight, with its characteristic spectrum distribution and intensities that are significantly different from some artificial sources. Light sources for visual inspection require careful selection to produce as near to optimum conditions as possible for the human eye.

NOTES: Photochemical – a chemical that responds to light The effects of long-term exposure to natural light (normal daylight illumination) have been studied and there is evidence that photochemical changes occur in the eye, with a long- and short-term impairment of vision. Natural sunlight has components other than visible light, e.g. ultraviolet and infrared, which may have biological effects, the lengthening and shortening of daylight with the time of year affects all biological systems, including vision, and there is also some evidence that chronic over-exposure to sunlight weakens the immune system. Exposure to light before and during inspection can result in incorrect visual assessments. Excessive exposure to light during visual inspection may result in the failure to make the correct assessment, similar to insufficient illumination. In industry, the visual inspector may encounter many different sources of visible light and other forms of radiation. There are different hazards associated with different sources depending on their output, and so it is important to know the hazards associated with each type of radiation.

Reaction to a very bright visible light source This is probably the greatest danger to the eye, with the normal reaction being to look away from the source or close the eyes. A very intense light will cause burning over a large area of the retina. This damage can be irreversible and permanent. There are very few guidelines for the safe use of visible light.

UV hazards (ultraviolet light) Ultraviolet (UV) is the invisible short wavelength radiation just beyond the short wavelength end of the visible spectrum and covers the wavelengths 100 nm to 400 nm. The responses or effects on the eye are known to produce the following, even with very short exposure: a.

Possible cause of cataracts.

b.

Reddening of th e skin (erytherma).

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c.

Lens fluorescence, eyestrain and headaches.

d. e.

Fatigue. Welder’s flash (keratoconjunctivitis).

Long-term exposure (chronic) is known to produce an increased risk of skin cancer and skin ageing. The cancer melanoma can be life threatening. Values indicating maximum recommended exposures have been determined. Ultraviolet absorbing eye protection, face wear and gloves are available and should be worn when exposure would exceed these values.

Infrared hazards Infrared radiation is invisible radiation beyond the red end of the visible spectrum – it can produce hyperthermia (overheating), which can he lethal to cells of living tissue. Consequently, infrared radiation entering the eye can produce damage. It also causes burning, the natural response being to move away from the source.

Laser hazards Laser light is high intensity and will cause loss of vision from burning of the retina similar to looking at the sun. Laser hazard controls are mainly common sense: a.

Use eye protection.

b.

Do not enter the laser beam path.

c.

Limit the primary and secondary beam from occupied areas.

If an individual is accidentally exposed to laser light, the retina is likely to become damaged. However, tong-term visual impairment is unusual, as only a small area of the retina will have suffered.

Photosensitisers Photosensitisers are substances, which can enhance the effect of and consequently the damage done by, light on human tissues. A large number of commonly used drugs, food additives and cosmetics are known to be phototoxic or photoallergenic agents. Some of these substances can sensitise organs beneath the skin and, as the longer wavelengths of light can penetrate into these areas, damage can be done at lower intensities than expected. It should also be remembered that people vary in their genetic susceptibility to damage by light, sonic people being much more sensitive than others.

Thermal factors Visible and infrared radiation enters the eye and is absorbed by the retina. Only a small part is absorbed by the visual pigments, the rest of the energy being transmitted as heat to the retina, which causes a rise in temperature. If the intensity is high enough, this can lead to burning. The human retina is normally subject to irradiance below I microwatt per mm2, except for occasional brief exposures to the sun, arc lamps, quartz halogen lamps etc. Normally, only when arc or

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filaments are magnified can this produce burning of part of the retina. Lower exposures produce short-term depression in phototopic (daylight) sensitivity and a marked longer-term loss of scotopic (dark adapted) vision.

Blue hazards The blue hazard is based on the fact that the retina can be damaged by blue light at intensities that do not increase retinal temperature sufficient to cause a thermal hazard. It has been found that blue light can produce 10-100 times more retinal damage than longer visible wavelengths. Precautions should therefore be taken to limit exposure to excessive blue light.

Eye protection filters Eye protection filters for various groups of workers are standardised as shades and specified for particular applications, e.g. welding and steel making operations. The shades filter the most intense wavelengths from a particular source hut otherwise allow adequate vision. Applicable normative documents BS 2092 1987- Specification for eye protection for industrial and non-industrial use. BS 6967 1988 - Glossary of terms for personal eye protection. BS EN 169 - Specification for filters for personal eye protection equipment used in welding and similar operations. BS EN 170- Specifications for UV filters used in personal eye protection equipment. BS EN 171 - Specifications for infrared filters used in personal eye protection equipment.

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UNIT V5 • LIGHT SOURCES Notes: Monochromatic – light of only a single wavelength. In practice, there is no such thing as monochromatic light, but sources exist which emit a very limited band of wavelengths. The light required for visual testing can be provided by a number of sources. The source should be chosen according to the application of the visual test or it may be that the source is specified in a procedure. Background lighting conditions can also vary. A bright background with too much glare will interfere with the inspection, as will a background, which is too dark or casts shadows over the test area. The following light sources are the most frequently used for visual inspection.

Daylight Daylight is the best light possible as it provides optimum wavelength distribution for practice; there is no such the human eye. It is not always feasible to use daylight, and an overcast day gives much thing as monochromatic better results than bright sunshine. It is the best light for photography. Filters may be used to produce a light of a specific wavelength where monochromatic light is required.

Flash light This is a tungsten filament bulb wit a battery supply of up to 2 V.A flash light is portable, robust, and easy to use.

Hand lamp This is a low-voltage mains operated lamp, with a tungsten filament bulb. A transformer is used to produce voltages of less than I0 V. It is portable and easy to use.

Desk top/angle poise lamps These lamps have tungsten filament bulbs and use mains voltage. They are very useful as they can he adjusted to all angles and positioned to give the best possible view.

Fluorescent lamps These are gas discharge tubes, either straight or circular, which can be battery operated or use mains voltages. Their usefulness is limited, but they give a soft light, uniform over a large area.

Sodium and mercury vapour lamps These are gas discharge lamps. They emit only certain (selective) wavelengths, so are usually only used when there is no other source available.

Stroboscopic light sources A method of using a synchronised pulse of tight to inspect rapid moving machinery. This makes rotary/moving components appear still, so that they can be inspected more accurately.

Ultraviolet light Ultraviolet lamps are used with fluorescent material producing a secondary light emission of a longer wavelength within the visible spectrum. The fluorescence produces an excellent contrast

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usually yellow/green against a dark violet/black background. Various fluorescent dyes exist and the colour emitted depends on the chemicals chosen. The optimum fluorescence is governed by the wavelength of the UV light and its intensity.

Halogen lamp This is the tungsten-halogen filament lamp. The presence of a small amount of iodine helps to prevent evaporation of the tungsten filament - this leads to longer life for the filament and allows the lamp to he run at higher temperatures, giving a very white light. They give the greatest tight output of all types of bulb.

Miscellaneous Borescopes, endoscopes, fibre optics and microscopes require light sources which range from battery-operated on portable equipment to mains-operated on fixed or laboratory-based equipment. The mains output of 240 V maybe used at full value for high-pressure, high- intensity light sources. When low-intensity sources are used, the voltage is reduced about 6 V. The following table shows the maximum working distances to produce 500 lx at the test surface. Source

Distance

Flash light

250mm

60W bulb

250mm

75W bulb

380 mm

1OOW bulb

460 mm

Measurements should be made with a photometer to produce a profile of the lighting in the inspection area. The glare index can also be calculated by reference to published tables.

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UNIT V6 • OPTICAL AIDS Visual inspection is rarely done by the eye alone. There are many optical aids which may be used to assist an inspection including: 1.

Magnifiers and microscopes

2.

Mirrors

3.

Borescopes and endoscopes

4.

Fibre optics

5.

Video cameras

6.

Special equipment, including imaging and computer-based systems

Magnifiers and microscopes Magnifiers range from I.5x magnifying glasses/lens to the limit of optical microscopy. Hand-held magnifiers normally cover the range up to lOx magnification. Above this magnification the short working distance becomes a problem and low-powered microscopes and macroscopes are used. These may he ocular or binocular, wide field and or stereoscopic. Low-powered microscopes often have one or two objectives to give two magnifications up to 40x. Medium-powered microscopes may have two or more objectives with magnification between 20x and 1OOx. High-powered microscopes have a number of objectives, often up to six, which give a magnification range of 50x to 2000x. With these microscopes specially prepared surfaces, sections or replicas are required. Often these microscopes have the facility for polarisation, phase contrast and interference examinations. Stereoscopic microscopes have a typical magnification range from ‘7x to 150x, with a useful upper limit of about 60x. This type of microscope allows the specimen to be moved around and gives a three-dimensional view. The microscopes may be either of a transmitted light or a reflected light type. The former is used for transparent samples; hut opaque samples require reflected light. Transmitted light microscopes operate with the light source behind the specimen with light passing through the transparent sample. Reflected light microscopes pass light through the objective by a light reflector on to the surface of opaque samples which reflects the light hack through the objective and to the eyepiece.

Hand-held lenses Low-powered hand-held lenses, up to about 1Ox magnifications, are used to magnify fine small detail

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to enable a better assessment to be made. The hand lens is moved close to the surface to be inspected and then slowly moved away until the surface is in focus. The distance from the lens to the eye will he variable so and should be around 300 mm, the distance for near vision. Continuous adjustment is often required to focus specific parts of the detail. The uses of hand-held lenses are infinite with most visual inspection of fine detail benefiting from their use. They are widely used in many industries, from metal components to fabric inspection.

Angle-poise mounted magnifiers With magnification of about 1Ox, the equipment often incorporates a light source, typically a circular fluorescent tube producing a uniform illumination in the inspection zone. NOTES: ES 5165- Guide to the selection of low-power magnifiers for visual inspection Components are transported to the test bench and either manipulated on the bench with the magnifier adjusted to produce the desired focus or manipulated under the magnification at a fixed distance. The working distance of the eye from the lens varies hut with most magnifiers should be about the normal reading distance of approximately 300 mm. Angle-poise magnifiers would be used in a fixed inspection station within a machine shop, test house or inspection department. Typical inspection includes machine tools, small components, and fabric inspection.

Low-power microscope These groups of microscopes cover a large range from low-power stereoscope of up to 20x magnification to monocular and binocular equipment up to 50x magnification. The majority of equipment is bench mounted and is also portable, with some types capable of mounting on large components. In most cases the microscopes consist of a stage, objective lens, eyepiece lens and a light source.

Mode of operation The working distance between stage and objective lens is restricted and may therefore require small sections to be taken from large pieces. The sample may require special preparation, sections from translucent material for mounting on to glass slides for examination by transmitted light. Opaque materials can only be examined by reflected light and may require some form of preparation depending on the features requiring examination. Once a sample is prepared it is placed on the microscope stage, the light source switched on, the eyepieces adjusted (binocular type) and the stage slowly racked up by means of the coarse focus until an image is obtained whilst viewing through the eyepiece lens. Critical focusing is achieved by the use of the fine focus.

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Applications These microscopes are used for routine testing/inspection of component surfaces, structure, metals, ceramics, plants, tissues, electronic component materials, fabrics, liquid, fractures, fibres, etc.

Measuring microscopes and special microscopes These are microscopes used to measure specific parameters and are used for small detail which requires accurate measuring, e.g. from surface finish to fabrics and hardness impressions of Brinell and Vickers hardness testing. Our illustrative example is the Brinell microscope used to measure the diameter of the indentation made by a steel ball on the surface of the test piece. The microscopes incorporate a measuring scale which can normally he adjusted to obtain a sharp focus of the scale of the surface to be measured. A light source is usually incorporated in the form of a small tungsten filament bulb operating from a small battery and should be checked for illumination by switching on prior to the operation. The microscope is then checked against the calibration scale to ensure the measurements obtained from the test are correct and within any specified limits. The Brinell microscope is used to measure the ball impression diameter in two directions after the above checks have been made. The dimensions obtained are then converted by means of a chart to give a Brinell hardness number, which is an indication of the material’s hardness. Special microscopes and magnifiers include: Surface comparator magnifiers - used to check surface finish. Measuring magnifier -this is a magnifier incorporating a measuring scale available in a range of units.

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UNIT V6• OPTICAL AIDS Shop microscope - about 40x magnification. Used for a range of inspection from plated and painted surfaces to defective components and surface wear. Laboratory microscope this is a conventional compound microscope. A great range of magnification, field coverage and resolution is available. Magnification can range from 100x to 2000x. It is designed principally for transmitted light as it tends to be used with transparent or semitransparent materials. Metallurgical microscope - this is very similar to a laboratory microscope but will have vertical illumination so that opaque samples can be viewed using reflected light. Brinell microscope - See example above.

Mirrors Can be made with minors allowing viewing behind or underneath objects or components with flexibility to obtain optimum viewing angles. Mirrors are available in various shapes, sizes and curvature configurations (convex, concave, parabolic), with adjustable and telescopic handles. The use of mirrors requires a degree of practice to reflect the light and obtain the desired reflection.

Borescopes and endoscopes The borescope, which is a self-illuminated telescope, originates from an early development of equipment to explore inside human bodies without having to operate. The original equipment was called an endoscope, derived from the Greek words for inside view and this is the term now used in Britain for flexible borescopes. Cystoscopes (a tube incorporating a lens and light source) were developed for examination of the human bladder and are the basis for borescopes used in visual inspection. Types of borescopes Rigid borescopes The rigid borescope was originally developed to inspect the bores of rifles and gun barrels. The image at the eyepiece is produced by an objective lens, prism, relay lenses and eyepiece and may have either fixed or adjustable focusing, the latter having a greater advantage over the fixed focus type.

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Mini Borescope

Objective

Relay

Ocular

Thin lens relay system

relay pair

relay pair

Rod lens relay system

relay lens

relay lens

Hybrid lens relay system

relay set

relay set

Typical borescope lens system

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A rigid mini-borescope contains a single solid fibre to replace the lenses. The fibre is about 1 mm in diameter and the lens aperture equal to a pin-hole camera, resulting in an infinite depth of field. Focusing adjustments can help in overcoming and compensating for variability in eyesight and expands the depth of field, therefore producing sharper images The following is a list of special borescopes used for special examination: Panoramic borescopes - this has a scanning mirror mounted in front of the objective lens system. This gives a wide range of vision and allows the rapid inspection of the 60 insides of cylinders, pipes etc.

Typical borescope showing available angles of view

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Water- or gas-proof borescopes - for high-temperature applications inside engines etc. Can be used in liquid or gas environments. Angulated borescopes - having various bends~, permitting inspection of areas not normally accessible by a rigid straight borescope. In Right-angled borescopes - used for looking around corners. Wide-field borescopes - up to 1200 field of view. Miniature borescope - down to 1.75 mm diameter. Periscope borescopes - used to see above or over objects. Ultraviolet borescopes - used in fluorescent inspection (e.g. magnetic particle inspection and penetrant testing). Comes complete with UV light source, filters etc. Calibrated borescopes - used in special examinations. The external tube is calibrated in order to indicate depth of insertion during tests.

Setting up a rigid borescope 1.

Place a protractor on a board and position the borescope parallel to the 00 line with the lens directly over the centre mark.

2.

Ensure that the protractor centre is behind the lens window between 25 to 50 mm away.

3.

Sight through the instrument and, using marks on the edge of the protractor, mark

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the field of view, left and right hand edge and centre. Readings from the protractor give direction and field of view. The angle of view ranges from 200 to 3600 .

Endoscopes or Fibrescopes Endoscopes are flexible systems using fibre optics, which are used in a similar manner to borescopes. They are used extensively in medicine and many engineering applications.

Fibre optics Fibre optics use very thin flexible glass fibre filaments between 9-30 microns in diameter. These filaments are capable of transmitting light within the boundaries of the fibre by internal reflections, the light following the path of the fibre irrespective of its shape. This property allows the light or image to he transmi tted around bends and curves without additional optical equipment. The fibre consists of a core of high quality optical glass with a case of glass of different refractive index, which acts as a mirror. The fibres are very small in cross-section and transmit very little light; therefore the fibres are grouped together in bundles, many thousands at a time, to produce the required level of illumination. Max dia (mm)

Working length (mm)

Direction of view (DOV)

Field of view (FOV)

Depth of field (mm)

292 333 76 161 247 333

Lateral (0900 ) Direct (0000 ) Direct (0000 )

500 600 100 , 300, 600 or 800

4.10

121 207 292 379

Direct (0000 )

300 or 500

5.48

290 440 590 290 440 590 290 440 590 740 240 440 650 240 440

Direct (0000 )

550

5 – infinity

550

5 – infinity

4.10 4.10

7.98

2 – Infinity 4 – Infinity 0 10 FOV 80-infinity 300 FOV 10-infinity 500 /600 FOV (direct) 4 –4 infinity 500 /600 FOV (lateral) 2 – lateral 800 FOV 2 – infinity

Fore-oblique (0450 )

Lateral (0900 )

Direct (0000 )

Fore-oblique (0450 )

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240 340 440 550 760 960 1280

Lateral (0900 )

300

340 9.98

11098

0

5 - infinity 5 - infinity

340

Retro (110 )

55

240 440 440

Direct (0000 )

550

240 340 440 550 760 960 340

Lateral (0900 )

340

10 - infinity

0

Fore-oblique (0450 )

0

Lateral (090 )

3300

10 - infinity

0

10 - infinity

330

The application of transmitting light and receiving images requires the use of two separate bunches of fibre, one to transmit (the light guide) and one to receive (the image guide). The fibre filaments for light guides are about 30 micron in diameter, and are used in bundles, the light guide bundle. The fibres for the image guide have a diameter of about 9-17 microns, smaller than the light guide fibres, because the diameter of the fibres is one of the factors which will affect the resolution. An objective lens is attached to one end of the fibres to focus the picture, which is transmitted by fibre optics to the eyepiece and can then be adjusted to produce a sharp image.

Application of borescopes and endoscopes The many variants of the borescope are used to inspect the internal condition and integrity of pipework, combustion chambers, gas cylinders, small tanks, chambers and vessels where unaided visual inspection is not practical. Borescopes are widely used in the automotive industry, to examine engine cylinders without having to take the engine apart. In machine shops, they are used to test the internal surface conditions of many components. They are used in the nuclear and chemical industries for remote observation, so that an inspector can remain in a safe area while examining a more dangerous environment. This is particularly important in tube inspection in power stations, chemical plants, etc.

Video systems As remote inspection requirements become ever more complex and demanding, the systems design engineer seeks more sophisticated solutions to inspection problems. This often leads to the use of video systems, using either real or virtual images. A real image is composed of real light waves, which can be projected onto a screen or captured on film or video tape. A virtual image is only an apparent image and so cannot be directly captured, but the sophisticated electronics in a video

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system allows the virtual image to be converted into a real picture. Video systems work on the principle that a picture can be thought of as being composed

of a large number of very small dots (picture elements or pixels). These dots can be any shade of brightness from white (brightest) through shades of grey to black (darkest). When observing the screen from normal viewing distance, these dots merge to form a continuous picture. This technique is used in television, where information on the degree of brightness of each pixel is sent from the transmitting end to the receiving end, where a reproduction of the original scene is formed. The basic equipment required for a video system is the video camera, a TV monitor and cables to relay electrical information between them. Additions to the system can include light sources, a control unit, and signal processing/analysing equipment. An analyzer makes it possible to store or to freeze images. Stored images can be processed to improve upon the real image in order to enhance inspection and detection of discontinuities in the object inspected.

Television camera The optical image of the scene to be televised is focused, via a zoom lens, onto the target of the camera tube. The target is coated in photo -conductive or photo -emissive material, and this generates a pattern of electrical voltages at the back of the target, with the voltage at any point being proportional to the brightness of the corresponding image point. The target is scanned by an electron beam generated in the camera tube - moving left to right across the target and rapidly back to left again, then left to right and rapidly back and so on. The beam starts at the top of the target and works down to the bottom, and returns rapidly to the top and begins the process again. As the target (videcon) is being scanned the voltages representing detail in the image are transferred to the output terminal of the camera tube. A video camera operates in the same way as a TV camera, but is usually of a much simpler construction.

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Another method of televising an image is by the use of a semiconductor pick-up device in place of the tube. Each small chip contains thousands of silicon photodiodes, with each diode storing a degree of charge dependent upon the amount of light falling on it. Each diode represents a pixel and can be electronically read and converted to corresponding image in a picture tube.

Picture tube - cathode ray tube A cathode ray tube is used to convert the signals from the camera hack to an image. The tube contains an electron beam, which is fired at a screen coated in a material, which emits light when struck by electrons. The beam density is controlled at each point by the picture signal input from the camera, i.e. the voltages representing each point of the

Specifications for Olympus Videoscopes Photos & Tables courtesy of Olympus C6 series scene determine the electron density hitting the screen, and therefore the brightness, at the corresponding point on the screen, and so a reproduction of the original image is built up on the screen.

Image quality

The television camera tube is a very important component in the system since it must produce highresolution pictures. As mentioned above, any image can be regarded as a series of very small dots or elements. The best resolution is obtained with the highest number of image picture elements as possible, therefore the higher the number of pixels, the better the resolution on the target. The quality of the picture on the screen is governed by scattered and reflected light within the tube, all of which reduce image contrast. The visual interpretation of the images/pictures on the screen is governed by contrast, brightness and the resolution or number of lines in the picture. The greater

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the number of lines, the better the resolution. Therefore with the screen size, number of lines in the picture and the magnification, it is possible to calculate the smallest resolvable detail.

Effect of magnification An increase in the size of the image, which is then projected electronically onto a screen, improves the resolution of the smallest detail without having to resort to improving the resolution of the monitor. The disadvantages are that increasing magnification of the test-piece also magnifies any movement in the camera system and may also affect the depth of field available.

Depth of field Depth of field is the range over which the camera/lens produces satisfactory definition which is in focus. It can he expected that the depth of field will decrease with increase in magnification.

Applications Cameras can operate in a range of diverse applications. They can be used alone, with zoom and telescopic lenses, or in conjunction with optical fibres to produce very small endoscopes. Video cameras can also he fitted to visual equipment such as magnifiers, microscopes etc. Inspection of pipework and vessels which may appear difficult, if not impossible, can now be performed by remote control video equipment employing small cameras and lighting systems. The equipment is complex, usually consisting of a video endoscope incorporating a camera, fibre optic lighting and control systems, all of which are controlled from a distance. The cameras can be front view, wide angle or side view or a combination of these kinds. Fibre optics are used to transmit light to the working head from a remote source. In general, cable lengths are limited to 30 metres since electronic problems occur with longer lengths. The camera is only a small part of the total system, which requires control systems, energy and light sources, monitoring systems and recording systems, the latter often 80 being a video recorder. The camera system can be, pushed through a tube, pulled through a tube or lowered into a tube. If pushed, the cable must be able to carry all the systems to operate the camera and rigid enough to push the system, and yet it has to he flexible enough to negotiate bends up to about 450, Cable reels with hand cranking systems are used to pull inspection systems through pipe work. Cables for camera systems require sheaves and guides at all changes in section in order to avoid damage or sticking.

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UNIT V7 • MEASURING EQUIPMENT Measurement A measurement can be defined as a quantity, condition, or property that can be determined objectively. There should be a defined procedure to accomplish the measurement. The measurement process is always a comparison of the measured with a reference quantity of the same kind. Where it is not possible for direct contact measurement to be made, then a converted measurement may be used, e.g. radiation measured at a distance can be converted to give source qualities. Measuring equipment used in inspection covers a large range of types of gauges, rules, micrometers etc. BS 5781 and ISO 10012-1 cover the quality assurance requirements for measuring equipment. It is intended that visual inspection is restricted to the following list in view of the complexity of the subject. 1.

Rules.

2.

Tapes.

3.

Square and templates.

4.

Protractors.

5.

Callipers (internal and external).

6.

Verniers.

7.

Micrometers

8.

Clock gauges.

9.

Slip gauges.

10.

Shadow graphs.

Measurement and calibration system A supplier or purchaser needs maximum confidence that a product meets the specified requirements in terms of its physical attributes, e.g. correct dimensions. Confidence is gained by using a calibration system to attempt to ensure that all measuring equipment is accurate to within stated tolerances. A calibration system is necessary for a supplier to be assured of the correct measurements of his own products. A calibration system is also necessary for a purchaser who wishes to verify conformance of the supplied product.

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A calibration system should ensure that all measuring equipment has the capability of making measurements within defined tolerances. These tolerances must be appropriate to the measuring equipment, e.g. it is not realistic to specify a 0.01 mm tolerance over the length of a I m steel rule. A calibration system should attempt to prevent inaccuracies occurring, or at least ensure that any deficiency in measuring equipment is detected before use. A calibration system should allow time for any detected deficiencies to be corrected. A calibration system should be reviewed periodically and systematically. In most circumstances, measurement standards and measuring equipment used should be traceable back to national or international standards. The calibration system should demonstrate that this is ultimately the case.

Frequency of calibration Where practical, all measuring equipment and calibration standards should be identified to indicate their calibration status, e.g. by the use of labels or tags. Where it is not practical to identify equipment in this way, the calibration procedure should identify how the equipment is controlled to meet the specified requirements. The frequency of checking and calibration depends on various factors a.

Type of measuring equipment.

b.

Amount and degree of use.

c.

Purpose of measurement.

d.

Stability of measuring equipment.

Calibration records To be in accordance with BS 5781, calibration records should include the following information: a.

The description of equipment and unique identification.

b.

The date on which each calibration was performed.

c.

The results obtained from calibration.

d.

The planned calibration interval. Plus, where appropriate, the following additional information:

e.

The designated permissible limits of error.

f.

The reference to calibration procedures.

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g.

The source of calibration used to establish traceability.

h.

The environmental condition for calibration and the measurement data as measured and as corrected to reference conditions.

i.

A statement of the cumulative effect of uncertainties on the data obtained in the calibration.

j.

Details of any maintenance (servicing, adjustment, repairs) or modifications that could affect the calibration status.

k.

Any limitations in use.

l.

Equipment descriptions.

Rules Thin steel rules are used by the visual inspector for accuracy, i.e. less parallax error, good dimensional stability and small width of graticules. Measurement markings may he metric, imperial or both. The smallest increment is usually 0.5 mm or 1/64 inch.

Tape rules These are manufactured from steel with measurements painted or engraved. Measurements can he metric, imperial or both, the smallest increment is usually 1 mm and they are available from about I m to 1OOm in length. Tape rules should be checked for accuracy at a specified temperature and tension.

Squares or templates These are devices used to check profiles. They should he made from a stable material, preferably steel. The square, for checking right angles., is placed on the component requiring checking and viewed for contact against the surface usually with a light source behind so that any non-contact area can he seen. Templates are used in a similar manner. It may he required that the gap between the template and the surface be measured a tapered gauge with engraved graduations is often used for this.

Protractors Usually manufactured from a clear stable plastic, with engraved markings in degrees. They are used to measure angles which are actual, projected or transferred.

Callipers There are two types of calliper, (1) for internal use - used mainly to measure bore diameters, and (2) for external use - used mainly for measuring external diameters. Callipers may be used for direct measurements or may be used to compare dimensions on one component, or compare dimensions between components.

Vernier callipers A vernier is a small movable auxiliary scale attached to, and sliding in contact with, a scale of graduation. It is usually graduated in 9/10 ths of the main scale to enable readings to be made to a

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fraction (usually one tenth) of a division on the fine scale. “Verniers are either digital or direct reading which are capable of measuring dimensions to an accuracy of 0.01 mm, or 0.0005”. The vernier calliper has the vernier attached to the callipers. The callipers exist in a variety of forms hut the contact points are commonly chisel shaped. Typical uses are the thickness measurements of plate; pipe etc., inside diameters of bores, wire diameters and depths of blind holes. Inside jaws

4

0

5

6

10 cm

outside jaws

Slide Callipers Here the 4th vernier mark coincides with scale mark 0.01 cm 0.02 cm 0.03 cm x = 0.04 cm 6 cm

5 cm

Scale

5

Vernier

How to read a vernier

Micrometers A standard micrometer is a U-shaped gauge in which the gap between the measuring surfaces is adjustable by an accurate screwed barrel whose end forms one face. The measured gap is read off a scale uncovered by the barrel. Metric micrometers are read to the nearest 0.001 mm; imperial micrometers are read to the nearest 0.00005”. The anvils of the micrometer may he flat, rounded, pointed or chisel shaped depending on the application. They are typically used for measuring thickness and diameters. They are also available in other forms, e.g. for use as a depth gauge.

Metric Micrometer screw gauge

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Clock gauges These are length measuring instruments in which the linear displacement of the anvil is magnified and displayed on a dial gauge by the deflection of a needle pointer or pointers. The smallest increment depends on the particular clock gauge, but some metric clock gauges can be read to the nearest 0.001 mm; imperial gauges to the nearest 0.00005”. There a variety of designs available, some are U-shaped and used similar to a micrometer, although the anvils are spring loaded. One specific design has a needle which can be used to measure small dimensional differences in surface profile.

Slip gauges

Slip gauges A slip gauge is an accurately ground and lapped block plate or bar with known accurate dimensions between two lapped surfaces. Slip gauges are mainly used for calibration checks on micrometers, vernier callipers and similar measuring equipment. More than one slip gauge should he used during calibration to check the linearity of the equipment.

Shadow graphs The shadow graph is an instrument used to project the profile of a component onto a screen with an increase In magnification of 5-100x. This allows the profile to he measured or compared with a template to check the accuracy of the component to the tolerance requirements specified on a drawing. It is used to check cutter tool profiles for wear where the machined component profile is critical, e.g. for checking the radii in the Charpy ‘V’ notch test-pieces.

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UNIT V8 • TEMPERATURE MEASUREMENT Radiation pyrometers The measurement of high temperatures often requires an instrument which is not in contact with the hot body. These instruments measure the radiation emitted by the hot body. Radiation pyrometers, the instruments used to measure the emitted radiation under black body conditions, follow the Stefan Boltzrnann Law which may be stated as an increase of 1% in the absolute temperature of a radiating body results in an increase of 4% in the energy emitted. Since this only applies to black body conditions similar to energy emitted from a chamber at uniform temperature, corrections are required for hot bodies in the open. Radiation pyrometers use lenses or minors to concentrate the emitted energy onto a thermocouple which then generate s an electro-motive force (EMF) and is measured on a calibrated millivoltmeter. The bottom end of the temperature measurement is about 5000 C, with no upper limit.

A.

Non-focusing type

Thermocouple

B. Fery type

Thermocouple M. v. meter

M

x

Adjustment Mirror Lens

Lamp

Eyepiece

Red glass

Image R B

Ammeter

Low reading

Bridge Circuit G

Correct

R B High

Disappearing filament pyrometer. In these pyrometers the intensity of the light from the hot body is compared with the intensity of light from some standard source, and both are matched in the instrument to one specific wavelength, usually red light.

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Optical pyrometers Optical pyrometers compare the intensity of light from the hot source with a bulb filament. The current is applied to the bulb filament until a match is achieved or the filament disappears. The comparison is carried out in the red light part of the spectrum and temperature can be read off direct. The method is only effective at temperatures above 5001.

Temperature indicating sticks Temperature sticks are crayon or chalk material with a calibrated specified temperature at which they will melt and are available over a range of +380 C to +13700 C with an accuracy of +/- 1%. The stick is placed in contact with the surface of the component and will melt at the temperature marked on the stick. A typical application is in welding to check preheat and post heating temperatures. Some sticks do not melt, but change colour at the indicated temperature instead.

Temperature indicating pellets Pellets have an advantage over sticks in that they have a longer indication period and are used in prolonged heating applications in which a temperature stick could fade with time or are not accessible. One of the uses is to check oven and muffle furnace chambers. The pellets are placed within the oven at selected positions to check temperature distribution. When the temperature is reached the pellets begin to melt. After cooling down, the different positions are checked to see which pellets melted and which pellets failed to reach temperature.

Liquid temperature indicators Liquid indicators, which change colour at specific temperatures, are available for component surface temperature checks where temperature sticks may be unsatisfactory. The liquid is brushed on before the operation and used on highly polished surfaces or for marking large areas which require viewing from a distance.

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UNIT V9 • SURFACE CONDITIONS There are a variety of material attributes leading to surface conditions which can affect the efficacy of a visual examination, these include: cleanliness, colour, condition, geometry, size, temperature, surface texture, type of material and any surface coatings present.

Cleanliness of test surface The component or part to he tested must he adequately cleaned prior to inspection. A dirty surface will make the surface finish appear different, obstructs visual assessment of a surface and could mask defects. The cleaning method most suitable to use depends on various factors including the properties of the test-piece, the contaminants to he removed, skill required, access and cost. Examples: a large fixed component will not be able to be dipped in a tank of solvent; hut could be cleaned by brush. Aluminium will react with alkalis, so basic cleaning agents cannot be used with aluminium components. Certain solvents will dissolve the test-piece or may cause corrosion. Abrasive cleaning may damage the surface of the-test piece. Methods of cleaning The surfaces may have adherent materials or surface contamination requiring different methods for removal. The main cleaning methods are as follows: 1.

Dry abrasive blasting

2.

Wet blasting

3.

Wire brushing

4.

Grinding

5.

Scraping

6.

Needle gunning

7.

Flame cleaning

8.

Paint stripper

9.

Vapour degreasing

10.

Solvent cleaning

11.

Detergent cleaning

Dry abrasive blasting Abrasive blasting either dry or wet, is carried out with a concentrated stream of small abrasive particles. The abrasive can be either metallic or mineral and includes sand, slag, steel or chilled iron

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grit and shot, slag and bead. These are blasted at the surface to remove surface scale, rust or paint that is adherent to the surface. Grease and oil must be removed prior to blasting. The action of abrasive blasting results in a clean surface which may, depending upon the abrasive used, produce a rough surface finish which may he required for adhesion purposes if a subsequent coating is to be used. When used on steel, some abrasives (especially shot) plastically deform and work harden the surface. Small surface-breaking defects may he peened over and hidden from view. Wet blasting Wet blasting methods are good for removing chlorides from surfaces and are good for the removal of toxic coatings, e.g. red lead paint films, because they do not create dust. However, all wet blasting methods have similar disadvantages over dry abrasive blasting, including: the availability and drainage of water, the production and disposal of sludge (particularly with abrasive injection), the extra cost of supplying and mixing a substrate inhibitor and the problem of drying a large surface area.

High-pressure pure water blasting Operates at pressures up to 35,000 p.s.i. which can be extremely dangerous. The advantages of this method are as follows: ?

Simple to operate.

?

Highly flexible and mobile in use.

?

Suitable for removing soluble contamination.

?

Will remove millscale at high pressures.

High-pressure water plus abrasive injection Operates at pressures up to 20,000 p.s.i. which can he extremely dangerous. The advantages of this method are the same as for high-pressure pure water blasting, but will also remove firmly held contamination and will leave a surface profile.

Low-pressure water plus abrasive injection Operates at 100 p.s.i. It is claimed that this technique is very controllable and will remove one coat of paint of a multi-coat system if required. Disadvantages include high cost and low efficiency.

Steam cleaning, with or without abrasive injection Operates at approximately 100 p.s.i.. This method is ideal for surfaces contaminated with oil, grease, etc.. Disadvantages include high cost and low efficiency.

Air blasting with water injection Water with or without an inhibitor is injected into an air/abrasive stream.

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Wire brushing Hand and power wire brushing is an effective method of removing the majority of the less adherent materials but not as effective as abrasive blasting for scaled and painted surfaces.

Grinding The action of grinding removes the surface metal/material in a localised area and is useful for spot dressing. Full surface grinding is when all the surface is dressed with machine grinding producing a uniform surface compared to hand grinding which tends to produce undulations. Belt or disc sanding is a less severe method than grinding for surface dressing. Some materials, i.e. those that can be hardened, are susceptible to grinding cracks caused by the heat generated by the friction and the subsequent rapid cooling related to area effects.

Scraping Hand or power tool cleaning with scrapers remove lightly adherent material from the surface without significant metal removal, except when employed on soft materials.

Needle gunning The needle gun consists of numerous air-operated reciprocating needles and are used to clean areas difficult to reach by other methods, welds and rivet heads.

Flame cleaning The method of applying an oxyacetylene flame to the steel surface to be cleaned is an efficient method of removing rust, millscale and other contamination. The effectiveness of the process is due to a combination of factors:

Differential expansion The millscale, on contact with the intense heat, expands at a faster rate than the steel to which it is attached and flakes off.

Dehydration Rust is a combination of iron oxide and moisture. As the moisture is rapidly driven off, the rust is dehydrated and converted to a dry powder which can then be removed by wire brushing.

Heat penetration The heat from the flame penetrates all the surface irregularities and removes all traces of moisture, oil, grease etc. The flame cleaning of any form of fastener, e.g. rivets or bolts, should be avoided as a loss of mechanical strength may be caused.

Paint strippers One of the most effective methods of removing paint coatings without mechanical damage to the base material is paint stripper or paint remover. The painted surface is softened and may then be removed by either scraping or washing. Paint strippers are solvents and blends of solvents specially formulated to remove different types of paint. They do not clean dirt, scale, grease etc. effectively.

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Vapour degreasing True vapour degreasing is the immersion of the component for cleaning in a solvent vapour of 1, 1, 1 trichloroethane or similar. This is regarded as the most effective method for the removal of grease, oil and semi -adherent surface debris. This method requires specialist equipment and is most useful for components in a factory house situation. It requires a special tank/heater and condensation tubes to prevent the vapour from spilling out of the tank.

Solvent cleaning Solvents which are capable of breaking down grease and oil-based surface contamination are very effective in removing lightly adherent surface contamination. Application by immersion, brushing or wiping requires to be thorough and may require repeated applications on heavy deposits. No special equipment is required, although good ventilation is essential for safety. This method can be used on large or small test-pieces and can be employed in or out of doors. Care must be used to ensure that the solvent does not react with the test material.

Detergent cleaning Detergents are either alkaline or acid based and are generally used to remove light surface contamination. Grease or oil-based deposits, especially heavy deposits, are very difficult to remove. Detergent cleaning is a relatively safe method of cleaning.

Surface profile and finish This section deals with the shape of a surface and its texture. Defects can be difficult to identify in a rough surface and rough surfaces can cause problems with magnification. The roughness of a surface is governed by the peak to trough height, the density of texture, mainly peaks and troughs and shape of the undulations. The peak to trough height (amplitude) or appearance of a surface may be assessed by a number of methods, including the use of a surface profile needle gauge, surface replica tape or a surface comparator.

Roughness

Waviness

Error of form

Amplitudes are often measured on blast surfaces prior to coating application, but for most other surface assessments a judgement is made by eye, sometimes with the aid of a comparator. Comparators are available for a variety of surface textures including machining and blasting. Replication is a method of copying the surface condition of components for analysis remote from the test item. Methods of replication range from plaster casting to cellulose acetate films, the latter being used for metallographic examinations.

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A common method for grading a surface texture, especially on machined or ground components, is to use the arithmetical mean deviation from a reference line drawn midway between the peaks to trough height. The values, measured in microns, are suffixed with the R ? parameter. This measurement was previously known as the CLA or Centre Line Average.

(P)

Mean height =

Area (P) L

L Areas r r 2 1

r Ra = 1

s

1

s

2

r

s

s

1000 V L v (V v = vertical magnification) 2

1

2

X

L

Another method is to use R? , which is the 10 point height parameter, which is defined as the average distance between the five highest peaks and the five deepest valleys within the sample length, measured from a line parallel to the reference line. The surface finish of a test item immediately after processing will typically be as follows:

As Cast Cast surfaces vary from a poor to excellent finish. The degree of fine detail that can be detected is dependent upon method of casting and condition of the moulds or masters. Surface roughness ranges from about 1.0 ? m R? on die castings to 25 ? m R? for sand castings. ? Sand casting - generally rough surface finish. ? Shell mould - smooth, good surface finish. ? Die casting - smooth, very good finish. ? Investment casting - depends on the master, but can be a very good finish.

Hot worked Hot worked surfaces invariably have an oxide layer and, in some instances, grain boundary penetration of oxide which is part of the high-temperature scaling mechanism. Examination for

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fine detail on such surfaces is not possible without dressing (grinding, sanding etc.) either a small area or the whole surface, in order to remove the oxides. Surface roughness on hot rolled surfaces is between 12.5 to 25 ? m Ra , and 3.2 to 12.5 ? m for Ra forged material.

? Hot rolled - scaled finish, surface quality depends upon the finishing temperature. ? Hot rolled de-scaled - surface condition depends upon roll surface condition. ? Cold rolled - generally very good surface finish. ? Hot forged open scaled, moderate surface finish. ? Closed die forging - scaled good surface finish. ? Cold forged (cold heading) - usually very good.

Machined Machining introduces plastic deformation into the machined surface which tends to smear the surface layer, so masking fine detail. Machined surfaces vary from 0.05 to 1.6

? m Ra

? Rough machined - coarse surface finish. ? Fine - good surface finish. ? Polished - very good/excellent surface finish. ? Mirror excellent surface finish.

Sawn

? m Ra.

The roughness of a sawn surface falls within the range of 3.2 to 25 ? Circular - rough surface finish. ? Bandsaw - fine surface finish.

Ground Depends upon the grit used for grinding; tine to coarse.

Polished The surface produced may he by mechanical methods, fixed abrasion, slurry/paste, or electropolished. Excellent surface finish, typically from 0.02 to 1.0

? m Ra

Blasted Finish depends on the hardness of surface, abrasive characteristics (density, size and shape etc.), and angle of impingement, but amplitudes vary from a few microns to greater than 100 ? m.

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Temperature The temperature of objects for visual inspection would normally he at room temperature I a (approximately 200C). However, there are instances where this is not possible and may create dimensional problems and possibly a health hazard. Objects with temperatures below freezing or above about 600C can inflict damage on unprotected hands thereby producing a health risk. Dimensional stability is also affected since calibration is usually at 200C and a component temperature that varies from this will produce inaccuracies in both component and measuring equipment. Visual inspection of very hot objects from a distance is a frequent event in manufacturing to observe the condition of plant and product. Furnace hearth and ladle lining conditions are examined between casts, soaking pit and reheating furnace every shift for refractory damage or burner problems. During the manufacturing process, inspection of ingots takes place above 9000C, and of hot rolled and forged products between 9600C and 1 2500C. In-service inspection of engines, hollers etc. takes place at the elevated working temperature.

Surface coating The visual examination of a surface coating is a subject by itself where specialist knowledge would be required. Within the scope of this unit, surface coatings are relevant because they interfere with the visual examination of the metal substrate. There are many types of coating such as organic paints and metallic coatings. Anodised surfaces may also be encountered; this is a very thin surface finish, 1-2 microns, consisting of an oxidised layer which can easily he destroyed. Surfaces should be visually examined prior to coating for surface defects and also for the correct finish for coating adhesion and coating durability purposes. It is worth noting that the degree of surface preparation often governs the service life of the coating. Sometimes the visual examination of a coated surface is a requirement in order to detect and measure corrosion. To determine the full extent of corrosion the surface coating must be removed in the vicinity of the problem area. It must not be assumed that organic paints can all be removed with a solvent. There are many types of paint and some can only be successfully removed with abrasive blasting or heating, scraping and power brushing.

Colour A strong contrast of colour and patterns such as black and white, red and blue or red and green should be avoided as this may cause physiological problems and interfere with perception. The colour of a surface is considered in terms of its three chief qualities: hue, lightness, and saturation.

Hue Colours are divided into groups having the same hue, i.e. into reds, yellows, greens, blues, purples etc.. In ordinary speech this quality is often vaguely called colour.

Lightness The lightness of a colour is determined by the proportion of light which it reflects, irrespective of hue and saturation. Corresponding terms used are value and reflectance value.

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Saturation Colours of similar hue and lightness may differ in colourfulness or intensity of colour. This quality, termed saturation, may be defined as the intensity of any particular hue when compared with a neutral grey of similar lightness, the spectrum colours being the most intense or of highest saturation. The terms chroma and intensity are also used in a similar sense.

The Munsell system The Munsell system of colour coding shows in a convenient manner the relationship between the three chief qualities of colour. The circular band represents the hues in proper order, the vertical axis is the scale of value and the paths protruding outwards from the centre represent the degree of chroma which increases in intensity in the direction of the arrow.

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UNIT V 10 • INSPECTION PROCEDURES Inspection within a quality system Quality assurance is a system which attempts to ensure the quality of a product or service, i.e. the customer gets what was required or expected. Quality systems should cover all operations associated with that product or service to assure quality. At some stage in the system, the product (either finished or partly finished), raw materials, or plant will require inspection. The inspection of many products or materials includes visual testing, non-destructive testing, destructive testing and/or performance testing. Visual inspection plays an important part in the testing of a product/material to ensure its quality. A part does not have to be perfect to be acceptable. Allowable defects and concessions should he documented. A part is considered acceptable if it passes the specification’s requirements or is within the limits of any concessions.

Documentation is an important part of a quality system and will typically include instructions, procedures, drawings, appraisals, customer purchase orders, inspection reports, test results, nonconformity reports, concessions and standards.

Inspection verses quality control Inspection is defined in BS 4778 Part I Quality Vocabulary as, ‘activities such as measuring, examining, testing, gauging one or more characteristics of a product or service and comparing these with specified requirements to determine conformity.’ Inspection is one of the important elements within a system for quality assurance which requires continuing evaluation in the same way as the other elements, e.g. planning, design/specifications, production etc. Inspection is not quality assurance and it is not quality control, hut inspection is a method used by both. Quality control is defined in BS 4778 Part I as, ‘the operational techniques and activities that are used to fulfill requirements for quality.’ Quality control is involved with the monitoring of a process and eliminating the causes of any deficient output with any process~, or any phase during a contract, which has an effect on quality. The information obtained from inspection, as defined above, is used for quality control. Quality control deals with the actual measurement of quality performance, this performance is compared against what is required, and action is taken on the difference. Quality control asks the question “is the work/action being performed correctly?”

Normative documents - definitions 1.

Normative document: Document that provides rules, guidelines or characteristics for activities or their results.

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The term normative document is a generic term that covers such documents as standards, technical specifications, codes of practice and regulations. [ISO GUIDE 21 2.

Standard: Document, established by consensus and approved by a recognized body, that provides, for common and repeated use, rules, guidelines or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context. I 1St) GUIDE 21

3.

Code of practice: Document that recommends practices or procedures for the design, manufacture, installation, maintenance or utilization of equipment, structures or products. A code of practice maybe a standard a part of a standard or independent of a standard. [ISO GUIDE 2]

4.

Specification: The document that prescribes the requirements with which the product or service has to conform. A specification should refer to or include drawings, patterns or other relevant documents and should also indicate the means and the criteria where by conformity can he checked. [BS 4778 PART 1]

5.

NDT Procedure: A written description of all essential parameters and precautions to he observed when applying an NDT technique to a specific test, following an established standard, code or specification. [PCN/GEN/92]

6.

NDT Instruction: A written description of the precise steps to be followed in testing to an established standard, code, specification or NDT procedure. [PCN/GEN/92]

Information required to perform inspection The visual inspector requires certain specific information before inspection can commence. Ideally, a written procedure and/or a written instruction should be provided which makes it totally clear as to what is required from the inspection, these should be written taking into consideration the qualification of the user Visual inspection procedures should at least cover the following information: a.

Scope.

b.

References.

c.

Definitions.

d.

Safety.

e.

Personnel.

f.

Identification and datum’s.

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g.

Surface condition or preparation.

h.

Extent of inspection.

i.

Equipment to use.

j.

Equipment calibration and checks.

k.

Lighting requirements.

l.

Inspection technique detail.

m.

Evaluation of results.

n.

Acceptance and rejection criteria to use.

o.

Reporting and recording requirements.

Sampling methods Before the commencement of inspection, the method of sampling must be decided. One hundred percent visual inspection is used when the highest quality integrity is required and the cost of this inspection is not prohibitive. By one hundred percent inspection we mean that every component is examined, instead of a random sample. Usually selective sampling is used in order to reduce the costs and time involved. Selective sampling may take the form of a specific number randomly selected from a batch, or a production sequence selection, e.g. every hundredth component is examined. In this case only a small percentage of the components are inspected and it is very important that the method of selective sampling chosen will adequately reflect the true number of defects in a production process. Probability selection curves are sometimes used to determine this sampling criteria. NOTES: Probability selection curves these are graphs which give statistical information about a process, allowing the most effective and representative sampling method to be chosen

Acceptance/rejection criteria Inspected parts may be divided into defective and non-defective. Defective parts can be subdivided into reject or repair. Large expensive parts are more likely to be repaired than small, cheaper parts, hut the type and extent of the defects has to be considered. The inspection will normally be performed in accordance with specification requirements defining tolerances and limitations for that component or system. There are not many standard specifications specifically for visual inspection, but many application specifications expect some visual inspection in their routines. Every industry, manufacturing process and inspection authority has specific acceptance/ rejection criteria for the product or system. To correctly interpret the results from an inspection against acceptance/rejection criteria often requires a detailed knowledge of flaws and the process if flaws are being evaluated.

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An example of acceptance/rejection criteria for welds is shown below: NOTES: See ISO 5817 and the following for examples (many sources exist) Defect Type

Inspection

Acceptance Criteria

External profile

Visual

Internal profile

Visual and Radiographic

Root penetration Root concavity

Visual Visual

Excess weld metal (reinforcement) shall he uniform and not more than 3 mm in height. It shall merge smoothly with the parent metal and shall extend beyond the original joint preparation by not more than 3 mm on each side. In no area shall the weld face be lower than the adjacent pipe surface. The root bead or any concavity shall merge smoothly into the adjacent surfaces hut at no point shall the weld he thinner than the wall thickness of the thinnest plate. Not to exceed 2 .5 mm. Length not to exceed 25% of total weld length. Depth not to exceed 2.0mm

Root undercut Shrinkage groove

Visual

Cracks

Visual

Cracks

Visual and Radiographic

Cap undercut

Visual

Elongated linear porosity in root run (hollow bead) Lack of inter-run fusion

Radiographic

Length not to exceed 30mm in any continuous weld length of 300 mm or not to exceed 1/10 of the total length of the weld when this is less than 300 mm. Depth not to exceed 1.5mm Length not to exceed 30mm in any continuous weld length of 300 mm or not to exceed 1/10 of the total length of the weld when this is less than 300 mm. Not permitted The toes of the welds shall be smoothly and gradually into the parent metal. Length not to exceed 60mm in any continuous weld length of 300mm or not to exceed 20% of the total length of the weld when it is less than 300mm. Depth not to exceed 1.5mm. Length not to exceed 60mm in any continuous weld length of 300mm or not to exceed 20% of the total

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Lack of inter-run fusion Lack of side wall fusion Elongated inclusions

length of the weld when less than 300mm.

Porosity (other than elongated porosity in root run)

Radiographic

Inclusions (other than elongated) e.g. equiaxed

Radiographic

Burn-through

Visual

Wormhole

Radiographic

Root alignment

Visual

Stray arcs

Visual

Weld proximity

Visual

An isolated pore greater than 25% of the wall thickness or 3 mm whichever is the smallest in any direction, shall he considered unacceptable. Size of an inclusion not to exceed 3 mm in any dimension. Total length of inclusions not to exceed 15 mm in any continuous weld length of 300 mm and not more than five inclusions of maximum width in this 300 mm length. Adjacent inclusions shall he separated by a minimum distance of 20mm. Not to exceed 5 mm in any dimension and only one in any continuous weld length of 300 mm. Not to exceed 6 mm in length or 1.5 mm In diameter. Misalignment at the root shall not exceed 10% of the metal thickness or 2.5mm whichever is the smaller. Stray arcs on the surface of the parent plate should he avoided, any that do occur shall be subject to an acceptable repair procedure. The toes of adjacent welds including branches, fittings and attachments should be separated by a minimum of twice the wall thickness, or 25 mm, whichever is the least

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UNIT V 11 • RECORDING & REPOR TING Reporting The visual test results should he reported in a manner appropriate to the test being performed. The procedures for reporting should be decided before the start of the test and often takes the form of a written report sheet. This maybe accompanied by permanent images such as photographs, video tape, replicas etc.. Sometimes a very detailed report is required for a part, for example a critical part or a failed part. This may include the results of all testing performed with photographs, test sheets etc.. This report may not only include the results of visual tests but of other non- destructive and destructive tests. The details of a visual test which should be reported are: 1.

Conditions. This would include lighting conditions, cleaning methods and surface preparation, temperature etc..

2.

Equipment. This would list all the equipment used, with details of type of equipment, rating, any identification numbers etc..

3.

Identification of the part being tested.

4.

Identification of test, e.g. test number.

5.

Date of test.

6.

Personnel performing the test.

7.

Specifications for test (British Standard, customer specification etc.).

8.

Instruction sheet number and/or procedure number.

9.

Results of test, e.g. type of defects found, defect dimensions, defect locations, relevant measurements of the test item such as thickness or diameter, any unusual observations.

10.

All photographs, video tapes, replicas if applicable. These must he marked with appropriate test numbers, measurements, and method/conditions of recording.

11.

Conclusions. This can be pass/fail, in specification/not in specification, corrective action needed, written comments etc.. The visual inspector may be required to discuss the results of the tests with an engineer or quality manager to form a conclusion and comment on any further action required.

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Recording observations During visual inspection the observations and interpretation of what is apparently seen by the inspector will require reporting by either a pictorial or written description. In many situations the preferred method is a picture with text since this will have equal impact but not necessarily the same interpretation by different people. When people make observations, their interpretation is subjective (and will differ from person to person). Visual inspection requires some method of recording results in order to make the observations more objective (not differing from person to person). Cameras, microscopes, borescopes and endoscopes can be used with adapters, giving them the ability to record observations permanently on video tapes and still photographs.

Photography Photography is an excellent method of recording specific visual results, but there are pitfalls to avoid if good results are to be obtained. The following is a guide to the requirements for producing consistent results. The camera should be of SLR 35mm or large format, which enables selective focusing of the subject. This should incorporate either a standard lens of 50 mm, a wide-angle lens 28-35 mm, a telephoto 90-135 mm, all ideally with macro focusing. Close-up lenses (diopters 1, 2 & 3), bellows or extension tubes are used to increase the macro focusing facility. The type of film requires careful consideration. Fine-grained slow films can only be used when there is no movement in the system since exposure times tend to he long where maximum depth of field is required. Past films traditionally are coarse-grained and therefore may not provide the definition required when enlarged since the grain size is also enlarged. This results in poor definition compared with fine-grained films. A polaroid camera and film can be used instead of the usual 35 mm camera. It requires special adapters and does not give high quality photographs but it produces instant results. Lighting of the subject has to be from the correct orientation with correct colour and intensity. Direct lighting is usually not possible, particularly in close-up photography, therefore the position of light sources require mobility so that the required illumination can be achieved. The area/subject to be photographed requires even illumination, free from shadows and glare, with the best results obtained with diffused light or bounced light/flash. Flash units illumination requires special care to reduce unwanted reflection by bouncing the flash from a ceiling. A white background or umbrella produces a more diffused illumination and often eliminates very dark, deep shadows. Oblique lighting is sometimes desirable to enhance relief detail, and requires positioning whilst actually viewing the subject through the camera. Photographing small objects requires careful selection of the background colour and texture. A general rule is that light coloured objects require light backgrounds whilst dark coloured objects require dark backgrounds. Photography of dark coloured objects on white backgrounds often produces an over dark object on the photograph, with possible loss of detail.

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A measurement scale system should ideally be included on the photograph to enable size perception to be made when viewing the photograph.

Video systems Video systems can he used either alone or with borescopes for observations in visual inspection. If the system is complete with some sort of video recording device (e.g. VCR) then a permanent record of the inspection can be made. This has certain advantages over photography in that a complete (and dynamic) picture is recorded. Video recording of a specific feature of the test or of all the testing event provides a permanent record which can be viewed later. The quality of the video picture is dependent upon the video camera and associated equipment. The quality, particularly the picture sharpness, can be improved by replacing the optical system in a borescope by a miniature video camera in the form of a solidsta te chip or light sensor which transmits the image electronically to the remainder of the video recording system and monitor. The video recording system consists basically of a VCR, but specialist equipment is available which will improve the image and give the best quality recording possible. These systems rely on very advanced electronics. The advantages of using video for recording is that it is possible to magnify the video image and therefore improve detail making assessment easier Videos of events allow re-fins and freeze frames to study and evaluate any specific feature.

Thermal printout A video camera may he linked to a printer to produce a thermal printout image. This acts like a photocopier, producing an instant image from the video. Quality can be variable depending on the settings of the thermal printer.

Replication This is a permanent record of the surface of a part. Replication is used to copy surface conditions such as impact damage, wear, corrosion, and cracking. The material used to form the replica determines the tine quality resolvable, with nitrocellulose and cellulose acetate having the best replication ability. These materials are used when a microscopic examination is anticipated. Other materials used to produce replicas are plaster, plastic materials, Plasticine, varnishes, clays, silicone rubber, tapes, etc.. Silicone rubber has the ability to flow into cracks, crevices and pits. Examination of the replica allows detailed inspection and measurement of specific features or profiles, e.g. depths of pits could be measured using gauges, micrometers or a low-power magnifier with graticules.

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UNIT V 12 • DEFINITIONS Black body

A theoretical object that radiates more total power of any wavelength than any other source at the same temperature.

Black light

A term used for ultraviolet radiation.

Blind spot

Part of the eye’s retina without any rods or cones, where the optic nerve enters the eye.

Blue hazard

Over-heating damage to the retina caused by exposure to high frequency visible light.

Borescope

A device of lenses and a light source for the inspection of inaccessible cavities. The word originates from its use in inspection of gun bores.

Candela

The SI unit of luminous intensity.

Colour

The sensation produced at the human eye by rays of light of different wavelengths.

Colour blindness

The inability to assess all the individual wavelengths of white light correctly. The temperature of a black body which radiates predominantly the same wavelength as the source being described.

Colour temperature

Cone

A receptor in the retina which dominates response when the light level is high.

Contrast

The difference between the amounts of light reflected or transmitted by an object

Dark adaptation

The process by which the retina of the eye adapts to light of less than 0.034 candela per sq meter.

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Depth of field

The distance over which the image is in focus.

Diffraction

The phenomenon in which waves spread out beyond the geometric shadow of an obstacle.

Directional lighting

Lighting from a preferred direction or angle.

Direct viewing

Viewing an object in the immediate location.

Endoscope

A flexible device for inside viewing or inspection.

Far vision

Viewing an object beyond arms length.

Fibre optics

The transmission of light through fibres of glass, quartz, or plastics.

Field view

The range of area where things can he seen through lens or aperture.

Glare

Excessive brightness which interferes with clear vision and judgement.

Grey body

A radiator whose spectral emissivity is uniform for all wavelengths.

Hue

The perception of colour which discriminates different colours as a result of their wavelength.

Illuminance

The density of luminous flux measured in lux (SI unit).

Incandescent

Emitting visible radiation as a result of heating.

Infrared

Electromagnetic radiation in the wavelength range 0.75 to 1000 gin approximately.

Interference

Interaction between two or more waves of the same frequency emitted

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from coherent sources. Laser

An acronym - Light Amplification by the Stimulated Emission of Radiation.

Lumen

SI unit of luminous flux measurement equivalent to candela x steradian.

Luminance

The measure of the brightness of a surface. Measured in candelas per square metre.

Lux

SI unit for illuminance. Equivalent to lumens per square metre.

Machine vision

An automated system which acquires, processes and analyses images.

Monochromatic

Light of only one wavelength or, in practice, a very narrow band of wavelengths.

Near vision

Vision of objects generally within arms length.

Near sightedness

Adequate vision acuity to view objects within arms length.

Peripheral vision

The seeing of objects outside the central vision field.

Phase contrast

A technique in microscopy where phase differences are converted to differences of light intensity, thereby giving contrasts in the image wherever a change in thickness or refractive index occurs.

Photochemical

A chemical which changes when exposed to light.

Photoconductive cell

A cell using photoconductive material, often cadmium sulphide, between electrodes.

Photoconductivity

The property of certain materials to change their electrical conductivity under the influence of light.

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Photodiode

A semiconductor diode fitted with a small lens which can focus light on the p-n junction. They can be used as light sensors.

Photometry

The science and practice of measuring light.

Photopic vision

Vision adapted to daylight. Also known as Foveal vision and light adapted vision.

Phototransistor

A 3-electrode photosensitive semiconductor device.

Pixel

A light point on a screen of a digital image.

Polarisation

Nonrandom orientation of electric and magnetic fields of electromagnetic waves.

Psychophysics

Interaction between vision performance and the physical or psychological factors. The luminous flux radiated per unit area. Measured in lumens per square metre.

Radiance

Radiation

The emission of energy as electromagnetic waves or particles.

Radiometer

An instrument for measuring radiant power of specified wavelengths or frequencies.

Reflectance

The ratio of reflected wave energy to incident wave energy.

Refraction

The change in direction of a wave front which occurs when the wave travels through a material boundary.

Resolving power

The ability of vision or other detection systems to separate two points.

Rods

A receptor in the retina that

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response to low levels of luminance. They are not colour sensitive. Scotopic vision

Dark adapted vision using only the rods of the retina. Also known as parafoveal vision

SI units

Systeme International d’Unites, an internationally agreed system of units.

Spectral response

The comparative response to light of constant intensity but different wavelengths.

Spectroscope

General term for an instrument which produces or records different wavelengths of light.

Spectrum

Representation of radiant energy in adjacent bands.

Specular

Minor like.

Specular reflection

When reflected and incident waves form equal angles.

Ultraviolet radiation

Electromagnetic radiation between 100 and 400 nm between visible light and X-rays.

Vision

Perception by eyesight

Visual acuity

A term used to express the spatial resolving power of the eye. It is measured by determining the minimum angle of separation which has to be subtended at the eye between two points before they can be seen as two separate points.

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