L - 02 - Calibration of The Testing System [PDF]

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NATIONAL TRAINING COURSE ON ULTRASONIC TESTING (UT) LEVEL 2 as per ISO 9712 April 2019

“REAWAKENING OF TECHNOLOGY FOR SECURE TOMORROW”

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Ultrasonic Testing LEVEL 2 (as per ISO 9712)

Calibration of the Testing System Sanjaya Perera (B.Sc(Maths).Sp(Hons)) Scientific Officer/NDT Inspector

RT, MT, PT Level 3(As per ISO 9712/CBNDT, Sri Lanka) UT Level 2(As per ISO 9712/CBNDT, Sri Lanka) PT, MT, UT, RT Level 2(As per SNT-TC-1A/ISNT-NCB, India) DIR Level 2(As per ISO 9712/JPK, Malaysia/BAM, German/IAEA) VT & TT Level 2(As per ISO 9712/Russia/ IAEA) [email protected]

Purpose of Calibration • Calibration of the equipment is most important if accurate testing and reliable results are to be obtained. • In Ultrasonic testing calibration means the verification and adjusting of ultrasonic equipment characteristics so that reliable and reproducible test results are obtained. 5/3/2019

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• The calibration procedure used in ultrasonic testing can be classified into: 1) Equipment characteristics verification. 2) Range calibration. 3) Reference level or sensitivity setting. • Codes usually specify the required instrument capability. • The frequency of calibration is based on practical field experience and is often mandated through consensus codes and standards. 5/3/2019

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Calibration & Reference Blocks • Test blocks whose dimensions have been established and sanctioned by any of the various groups concerned with material testing standards are called standard test blocks. • In ultrasonic pulse echo testing technique, test blocks containing notches, slots or drilled holes are used as calibration & reference blocks: 5/3/2019

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• Calibration Blocks i. Determine the operating characteristics of the flaw detector and probes ii. Establish reproducible test conditions • Reference Blocks i. Compare the height or location of the echo from a flaw in the test specimen to that from an artificial flaw in the test block.

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• Some times the same test block may be used as a calibration or reference block.

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IIW V1 Calibration Block

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• The most versatile calibration block is the block made from medium carbon ferritic and normalized steel described by the International Institute of Welding (I.I.W) and proposed by the International Standard Organization (I.S.O.). This block, called the I.I.W. VI block.

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This block is generally used for: (i) The calibration of the time base using normal beam probes and angle beam probes. (ii) The determination of probe index and probe angle of angle probes. HINT: The angle beam transducer is subject to wear in normal use. This wear can change the probe index and the probe angle. 5/3/2019

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• Also this IIW V1 block is used for the checking of performance characteristics of the ultrasonic flaw detector such as; – – – – – – –

Time base linearity/ Horizontal Linearity Screen height linearity/ Amplitude Linearity Amplitude control linearity Resolving power Penetrative power Dead zone check Pulse length

• The setting of sensitivity. • The comparison of various materials due to their different acoustic velocities. 5/3/2019

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Horizontal Linearity • The horizontal linearity or time base linearity is a measure of the degree of difference between an actual distance and a distance read out on the CRT/ LCD. • The I.I.W blocks or any block of similar material and finish may be used to measure horizontal linearity. • The choice of thickness is determined by the requirement that a longitudinal wave probe placed on the block produces several back-wall echoes within the chosen range. 5/3/2019

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• For checking the linearity two of the back-wall echoes should be set to coincide with appropriate scale divisions. • The position of each of the remaining echoes is then carefully noted. • The maximum tolerance is 1% for the range chosen. • Non-linearity of the time base is a real problem with modern flaw detectors and the most common cause of apparent non-linearity is the poor calibration of time base by the operator. • An important precaution to take during the assessment of time base linearity is that time base readings are taken as each signal is brought to a common amplitude. This is usually about ½ FSH. 5/3/2019

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Vertical Linearity • The screen height linearity/ vertical linearity or amplitude linearity is a measure of the degree of proportionality between an input to the amplifier and echo height displayed on the CRT/LCD. • To know whether the amplifier of the flaw detector amplifies a small signal in the same ratio as it amplifies a large signal, i.e. whether the amplifier is linear or not, You need to consider at least two pulses. • The ratio of the two pulses will remain the same as the gain of the instrument is changed over its operational range. MWSP/UT level 1/Feb 2017 14 5/3/2019

• Here, two signals are set to 80% and 40% of full screen height (FSH). The gain setting is then decreased by 6 dB, which should decrease the signal amplitudes by 50%, resulting in the signals dropping to 40% and 20% FSH, respectively.

If similar checks across the full vertical range (0-100% FSH) are within tolerance, the machine is considered to be linear in the vertical direction. 5/3/2019

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Time base linearity/ Horizontal Linearity ASTM E 317; 1994 Use appropriate block and receive noninterfering back echoes. Set reflections at 50% of full scale (FS/FSH) for its position measurement. Set position of 3rd and 9th signals at 20% and 80% of scale divisions. Read and record scale position of each other multiple. Data are plotted. Screen height linearity/ Amplitude linearity BS 4331; 1972 Obtain reflected signal from 1.5mm dia in standards block. Set the signal 80% of FSH. Note the value of calibrated gain control in dB. Increase gain by 2 dB. Signal should increase to 100% FSH. From initial setting decrease gain by 6 dB. The signal should be 40% FSH. Reducing further 12 db, signal should be 10% FSH. Reducing further 6 dB, signal should be 5% FSH.

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Resolving Power/ Resolution • The resolution of a flaw detector is the ability to resolve minor difference in distance and direction. • To determine the resolution of a flaw detector I.I.W VI block is used with normal beam probes.

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• A7 Block

Resolved

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The RC Block is used to determine the resolution of angle beam transducers per the requirements of AWS. Engraved Index markers are provided for 45, 60, and 70 degree refracted angle beams. 5/3/2019

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Typical Resolution Values 5/3/2019

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• Resolving power depends on damping and pulse width. • Pulse width can be measured by using Normal probe or Angle probe.

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Pulse Width • Also known as pulse duration and pulse length. • Calibrate the time base and obtain an echo of the 1.5mm hole in a V1 block. Adjust the GAIN until signal from the hole is 100% FSH. Measure the width of the signal in millimeters at 10% FSH. RF mode is recommended.

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Penetrative Power A longitudinal wave probe is placed on the plastic insert/ Perspex of the I.I.W VI block having a thickness of 23 mm and the gain for the instrument is set to its maximum. The number of multiple echoes and the amplitude of the last echo are noted and are used to express the maximum penetrative power of the set and the probe.

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Dead Zone Dead Zone is defined as the region in a material adjoining the surface of entry from which no direct echoes from discontinuities can be detected. For a single crystal probe the length of the initial pulse is the dead zone, for any signal from a reflector at a shorter distance than this will be concealed in the initial pulse.

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Signal to Noise Ratio (SNR) • Obtain a signal from the 1.5mm hole in V1 block and set it to 20% FSH. Note the dB setting. • Increase signal height on the attenuator/ gain control until the grass at the same range reaches 20% FSH. • Note the difference between the two dB readings. • The first reading gives an idea of the sensitivity of the probe and flaw detector. • The difference between the two readings gives the signal to noise ratio. 5/3/2019

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IIW V2 Calibration Block

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• This is a more compact form of the VI block suitable for site use. • Particularly suitable for short near field lengths and the time base calibration of small diameter normal and angle probes.

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Time Base/ Range Calibration using Normal probe and IIW V1 block

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Exercise 01: Calibrate the range for 50mm, 100mm and 200mm for the normal probe. ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... 5/3/2019

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Angle Probe- Probe Index • The point at which the centre of the beam leaves the probe and enters the test material is called the Probe Index or Emission Point. • As the probe surface wears down, the probe index can change. • It should be checked regularly. • Probe index can be find by using IIW V1 block or V2 block.

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• To find the probe index place the probe on a V1 block and obtain an echo from the 100mm radius and establish at more than 50% FSH using the gain control. Maximize the echo by moving the probe forwards and backwards. Note down the coincide reading as a probe index with the reference slot in V1 block. 5/3/2019

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25mm

• The probe is placed either facing the 25 mm quadrant or the 50 mm of quadrant V2 block to obtain echoes at 25 mm or 50 mm on the CRT screen. The probe is moved to and fro to maximize the echo. When the echo amplitude is a maximum, the probe index is obtained by extending the centre mark of the millimeter scale on the block on to the probe. MWSP/UT level 1/Feb 2017 5/3/2019

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Angle Probe- Probe Angle • Using V1; – To determine the probe angle, the probe is moved forward and backward according to its angle either at position "a" (35° to 60° ), "b" (60° to 75°) or "c" (75 ° to 80° ) until the amplitude of the echo from the perspex insert or 1.5 mm diameter hole is maximum. The angle of the probe is the one at which the index of the probe meets the angle scale on the block when the echo amplitude is maximum.

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Angle Probe- Probe Angle • Using V2; – To determine the actual probe angle, the probe index is placed against the appropriate probe angle inscribed on the block with the beam directed towards the 5 mm diameter hole. The probe is moved forward and backward until the echo is a maximum. An estimate of the probe angle is then made by noting the probe index position with respect to the angles inscribed on the block.

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Time Base/ Range Calibration using Angle probe and IIW V2 block

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Exercise 02: Calibrate the range for 250mm for the angle probe using V2 block. ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... ............................................................................................... 5/3/2019

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Calibration in Curved Work Pieces • Sensitivity – Two factors contribute to the reduction in sensitivity when a test specimen with a curved surface is ultrasonically tested. i. Widening or divergence of the transmitted beam ii. Reduction in the contact area between the probe and the test specimen

SOLUTION Use an adapter block or shoe to increase the contact area 5/3/2019

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Calibration in Curved Work Pieces • Skip distance and Beam path correction – The range calibration for a test with angle probes is usually made with IIW calibration blocks. – The skip-distances and beam-path-length respectively are increased by factors 'fp' and 'fs', which depend on the probe angle and the ratio of wall thickness 'd' to the outside diameter 'D' (i.e. d/D).

𝑃𝑃𝑟𝑟 = 𝑓𝑓𝑝𝑝 × 𝑃𝑃𝑒𝑒 5/3/2019

𝑆𝑆𝑟𝑟 = 𝑓𝑓𝑠𝑠 × 𝑆𝑆𝑒𝑒

Pr -increased full skip distance for curved surface Pe -full skip distance in a plate of same thickness Sr -increased beam path length Se –beam path length in a plate of same thickness UT Level2/MWSP/NCNDT

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Take d=30mm, D=1500mm and Probe angle= 70o

fp =1.3

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Take d=30mm, D=1500mm and Probe angle= 70o

fs =1.28

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Area Amplitude Blocks • Area-amplitude blocks provide artificial flaws of different sizes at the same depth. Eight blocks made from the same (50 mm) diameter round stock, each 3-3/4" (95.25 mm) in height, constitute a set of area-amplitude blocks. • Similar area-amplitude blocks made from square stock are sometimes known as Alcoa series-A blocks. • Each block has a 3/4" (19.6 mm) deep flat bottom hole drilled in the centre of the bottom surface. The hole diameters vary from 1/64“ (0.4 mm) to 8/64" (3.17 mm). 5/3/2019

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• The amplitude of the echo from a flat bottom hole in the far field of a normal beam probe is proportional to the area of the bottom of the hole. Therefore these blocks can be used to check linearity of a pulse echo inspection system and to relate echo amplitude to the area.

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Distance Amplitude Blocks • Distance-amplitude blocks provide artificial flaws of a given size at various depths (metal distance). • Also known as Alcoa series-B blocks or Hitt blocks.

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Normal Beam Sensitivity Calibration • The sensitivity of the system is adjusted based on a standard reference reflector. • Both side-drilled holes (SDHs) and flat-bottomed holes (FBHs) are used for this purpose. – distance amplitude set and area amplitude set Hole sizes are the same throughout this set but distance changes.

Hole sizes vary in this set but distances remain the same.

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Sensitivity setting with Alcoa A

As a result of the reduced FBH surface area "seen" by the sound beam, the amplitude decreases as the FBH diameter . However, the location of the screen signal does not change horizontally since the sound path remains constant. 5/3/2019

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Sensitivity setting with Alcoa B

When the region to be tested involves relatively thick sections, the calibration process needs to determine the effective reduction in sound energy with increasing distance. For straight beam applications, this is often done using distance amplitude blocks. 5/3/2019

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Distance Amplitude Correction (DAC)

• Normal probes

– Use distance amplitude blocks – For normal beam probes the DAC curve need not be constructed when the thickness of material is less than 2 inches (50 mm). This correction is only needed for thicknesses greater than 2 inches (50 mm).

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Distance Amplitude Correction (DAC)

• Angle probes

– Use reference blocks with a side drilled hole – The primary reference level (PRL) is set for an angle probe by adjusting the signal from the drilled reference target to an amplitude of 80% of FSH and marking the position of the echo peak on the screen. – The probe is then moved to other locations (positions 2, 3, and 4) and the signal amplitude is marked on the screen for each position. – A curve is drawn joining these points. This is the Distance Amplitude Correction (DAC) curve. Lines may also be drawn at 50% or 20% of this reference level. 5/3/2019

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Distance Gain Size diagram (DGS) • Another method which is used to set the sensitivity. • DGS diagrams are drawn by comparing the echoes from small reflectors, namely different diameter flat bottom holes located at various distances from the probe, with the echo of a large reflector such as a backwall, also at different distances from the probe. 5/3/2019

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• EXAMPLE A steel forging of 200 mm thickness is being tested with a normal probe having a frequency of 2 MHz and a crystal diameter of 10 mm. An assumedly flat bottom hole (FBH) is detected at a depth of 100mm and its echo amplitude is displayed on the CRT screen. It is required to find out the size of this FBH by evaluating the echo amplitude. Step 1 Calculate Nsteel=D2/4𝜆𝜆, Nsteel=D2f/4𝑽𝑽, Nsteel=8.5mm Step 2 Distance (D) for back wall= 200/8.5 =23.5 Step 3 Distance (D) for FBH= 100/8.5 =11.7 5/3/2019

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23.5dB 5/3/2019

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Step 4 In DGS diagram, draw a line perpendicular to D-axis at 23.5 and extend it to meet the back wall line. Step 5 From the point of intersection as in (step 4), which is also called the reference point, draw a line perpendicular to the G-axis meeting the G-axis at 23.5 dB. Step 6 Draw also a line perpendicular to the D axis at 11.7. Step 7 Adjust the echo height of the reference reflector (backwall) to 80% of FSH. Note the corresponding value of the dB control. Let it be, for example, 14 dB 54 5/3/2019and denoted by G . 1 UT Level2/MWSP/NCNDT

Step 8 Increase the gain until the echo from the FBH also reaches the reference height, i.e. 80% of FSH. Say the setting of gain control for this is 34 dB. Call it G2. Step 9 Thus the echo of the FBH needs an additional increase in gain of 20 dB (G2 – G1) in order to reach the same screen height as the back wall echo. Step 10 Locate a point 20 dB below the dB value (as in step 5) corresponding to reference point (23.5 dB). This is the point 43.5 dB on the G-axis. Step 11 Draw a line at 43.5 dB and perpendicular to G-axis. 55 the line in (step 6) at a point. 5/3/2019Extend this line to meet UT Level2/MWSP/NCNDT

23.5dB 43.5dB 5/3/2019

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Step 8 Note the S-value corresponding to this point of intersection. This comes out to 0.3. Step 9 Multiply this value of S in (xv) by the crystal diameter to get the diameter of the FBH. This comes out to be 3 mm. Step 10 The FBH detected at a depth of 100 mm has been evaluated to have a diameter of 3 mm.

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Coupling Medium • Proper coupling medium or couplant should be used between the probe and the test specimen to improve the transmission of ultrasonic energy by eliminating air between the two. • Commonly used couplants in ultrasonic testing are glycerine, water, oils, petroleum greases, silicon grease, wall-paper paste and various commercial paste. 5/3/2019

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THANK YOU

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