Omniscan Techniques for Crack Sizing

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OmniScan MX Phased Array Techniques for Crack Sizing

ID Creeping Wave, Shear-Wave Tip Diffraction and High Angle Refracted Longitudinal Wave Technique (HALT)

Chris Magruder Olympus NDT 281 922 930012559 Gulf Fwy [email protected] Houston, TX 77034 www.rd-tech.com

Chris Magruder Technical Support - Olympus NDT

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Table of Contents

Overview ...................................................................................................................................................3

Equipment Used.......................................................................................................................................4

OmniScan Setup for ID Creeping Wave and High-Angle L-wave Sectorial Scan ............................5

The Basics .................................................................................................................................................6

55–70-Degree Refracted Longitudinal Sectorial Scan..........................................................................7

ID Creeping Wave—Collateral Echo 1 (CE1) Wave Physics ..............................................................9

ID Creeping Wave—Collateral Echo 2 (CE2) Wave Physics ............................................................10

OmniScan ID Creeping Wave Calibration..........................................................................................11

OmniScan ID Creeping Wave Calibration (CE1 and CE2) ..............................................................12

OmniScan Setup for Shear-Wave Tip Diffraction for Crack Sizing ................................................14

Shear-Wave Wedge-Delay (TOF or Zero Offset) Calibration for Tip Diffraction .........................16

Shear-Wave Sensitivity (ACG) Calibration ........................................................................................17

Shear-Wave TCG/DAC Calibration ....................................................................................................18

Crack-Sizing Technique – Evaluation Procedure (ID Creeper, HALT, and Tip Diffraction).......19

Example of Deep ID-Connected Crack – 85 % Through-Wall Dimension......................................24

Example of Shallow ID-Connected Crack – 33 % Through-Wall Dimension.................................27

OmniScan MX PA 16:128 vs. 32:128 — Power of the Pulsers ..........................................................30

Conclusion ..............................................................................................................................................31


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Overview The objective of this project is to demonstrate the benefits of the OmniScan MX Phased Array instrument in crack-sizing accuracy using the following crack-sizing techniques.

• ID Creeping Wave (30-70-70) • High Angle L-Wave Technique (HALT) • Shear-Wave Tip Diffraction

Although these inspections have been performed for many years using conventional single-channel ultrasonic flaw detectors, the results and accuracy are dependent on the skill and experience level of the inspector. The advantage of the phased array technique is that it allows an inspector to perform these same techniques with much greater accuracy and less dependency on the skill of the operator. In fact, several of the techniques are combined together using refracted longitudinal sectorial scans and shear-wave sectorial scans. Where conventional single-channel UT requires separate probes and independent calibrations, the phased array sectorial scans combine several of these techniques and allow them to be performed and analyzed simultaneously. Although the sizing techniques are explained step by step, this report assumes the operator is capable of performing basic calibration and setup functions for the sectorial scans including focal law wizard, wedge delay, sensitivity, TCG/DAC calibrations, etc. These functions are described in the report but for detailed step-by-step procedures consult the OmniScan PA software manual for version 1.4. Normally welds requiring a high level of sizing accuracy are left in the “As welded” condition during construction. To use this technique the probe must be capable of moving over the flaw sufficiently to perform the echo-dynamic measurement of CE1 and to optimize a tip signal using a high-angle L-wave. This technique usually requires flat-topping the weld. All cracks documented in this report are in carbon-steel materials 25.4 mm (1 inch) thick. Different material types and thickness ranges require phased array probes of different frequencies, element pitch, and aperture for optimized results. Consult the Olympus Phased Array Probe catalog for more details. With the use of an OmniScan probe-splitter adapter, both the ID creeping wave and shear-wave sectorial scan probes can be used simultaneously and do not require switching between setup files, changing probes, or rebooting the OmniScan. This greatly aids the speed and improves characterization as all inspection methods are available in real-time to the operator. The calibration blocks and crack samples used for this project were designed and provided by the Davis NDE Advanced UT Crack Sizing Program and are of similar design to those used in the EPRI IGSCC training program. Special thanks to Mark Davis for his assistance and support on this project.


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Equipment Used

• Olympus OmniScan PA MX 16:128 Phased Array Acquisition System • Olympus OmniScan PA MX 32:128 Phased Array Acquisition System • Olympus OmniScan Software version 1.4 (Mult-ichannel option enabled) • Olympus 5L64 Phased Array Probe (5 MHz, 0.6 mm element pitch, 64 elements) • Olympus 5L16 Phased Array Probe (5 MHz, 0.6 mm element pitch, 16 elements) • Olympus SA2N55S Phased Array Wedge • Olympus SA2N60L Phased Array Wedge • Olympus SA1N60L Phased Array Wedge • Olympus SA1N60S Phased Array Wedge • Olympus Probe Splitter AAUX202A (allows two PA probes on OmniScan MX PA)


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OmniScan Setup for ID Creeping Wave and High-Angle L-wave Sectorial Scan The OmniScan set up for the ID creeping wave requires configuring a refracted longitudinal (RL) sectorial scan from 55 degrees to 70 degrees at a one-degree resolution. Smaller resolutions do not significantly improve the sizing accuracy or improve the resolution for this application. 12 mm to 25 mm thickness range in carbon and stainless steels For the 32:128 OmniScan, use the 5L64 probe and the SA2N60L wedge. For the 16:128 OmniScan, use the 5L16 probe and the SA1N60L wedge. All cracks documented in this report are in 25.4 mm (1 inch) thick material. Different material types and thickness ranges require phased array probes of different frequencies, element pitch, and aperture for optimized results. Consult the Olympus Phased Array Probe catalog for more details. The beam is focused at approximately ½ to ¾ the thickness of the material. There are comments later in this report describing under what conditions it is beneficial to refocus the beam at a different depth once the crack is determined to extend to the upper third, middle third, or lower third of the material volume. This requires recalibrating the wedge delay and will be addressed later. In the UT -> Advanced -> Points parameter, change the default value of 340 to 640. This will optimize the resolution on the A-scan display and assist the operator in determining peaks, tips, facets, etc. The maximum point quantity available for any A-scan is 999 points. In the Display -> Rulers -> UT Unit, change the default value of True Depth to Half Path. This application is extremely difficult to complete when working in true-depth mode and the benefits of the uncorrected displays are explained in the CE1 echo-dynamics calibration and flaw-interpretation sections of this report. Using the calibration wizards, perform wedge delay and sensitivity calibration on any 1.5–2 mm side- drilled hole at the approximate depth of the material. This step is critical and accurate results are only possible when using a calibrated sectorial scan. The focal law calculator will not provide the accuracy required without fine-tuning the wedge delay and equalizing the sensitivity of the RL sectorial scan. The TCG/DAC is not required for the RL sectorial-scan channel. The TCG/DAC is only required on the shear-wave sectorial- and linear-scan channels and is critical on these channels for accurate results. The RL sectorial scan is used only for depth and height measurements and requires adjusting the gain on the relative focal law for each flaw. This component of this sizing technique does not require amplitude calibration beyond ACG calibration on one side-drilled hole in the range of the material thickness.


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The Basics Any RL (refracted longitudinal) beam in the range of 55 degrees to 75 degrees produces the CE1 and CE2 shear wave components at slightly different angles, signal/noise, and velocities. The conventional ID Creeping Wave UT technique primarily uses the 70-degree refracted L-wave. The phased array technique allows us to see several aspects of the crack by visualizing all the RL angles between 55 and 70 degrees and provides a CE1 and CE2 component on each of these angles as well. This provides an advantage over the conventional UT because the crack can be seen in several aspects from the same probe position. It also makes it easier to differentiate CE1 and CE2 from geometric reflectors and other signals present on the A-scan and sectorial scan. These are described in detail in the ID Creeping-Wave section of this report. The four basic components of the phased array ID Creeping-Wave Sizing Technique L-wave signal from the crack tip L-wave signal from the crack base Collateral Echo 1 (CE1 30-70-70 signal) Collateral Echo 2 (CE2 ID Creeper)


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55–70-Degree Refracted Longitudinal Sectorial Scan Two of the four relative signals required in this application are direct longitudinal signals from the base and the tip of the crack. The tip of the crack will always be at a higher angle or focal law than the base. With the phased array sectorial scan, these two signals can be seen at the same time from the same probe position and optimized by moving the probe forward and backward perpendicular to the crack. For the purposes of this application, all direct L-waves are analyzed in the first leg of the inspection. In general, RL angle beams are limited to the first leg and lose most of their energy when striking any surface making “skipping” impractical. Identifying the deepest ligament or crack tip from the high angle L-wave is the most critical part of this technique. Extreme care should be taken to ensure that you are analyzing a true tip and not a facet or branch of a larger flaw. All measurements taken from the L-wave signals are based on peaked signals. Use of the envelope feature in the OmniScan is essential when performing through-wall measurements on phased array crack tips. This is critical. Only use peaked signals for tip measurements. Properly peaked signal in green gate with A-scan envelope measuring crack tip at 13.97 mm.

The operator must familiarize himself or herself with both the A-scan of a single focal law, and the corrected or uncorrected sectorial-scan image when performing analysis of the L-waves to benefit from the phased array technique. All the information from an individual A-scan or focal law is present in the sectorial scan but has been color-coded based on amplitude. In other words, with a little practice, by looking at a sectorial scan, the operator should be able to visualize each A-scan without the A-scan display.


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Below is the 55–70 degree sectorial scan visualizing the tip and base of the crack on different focal laws from the same probe position. The spatial separation between the tip and base of the crack in the uncorrected sectorial scan below is the through-wall dimension of the crack. This is also the same basic technique as the AATT (Absolute Arrival Time Technique) but instead of using one angle from different probe positions, the phased array allows both the base and tip to be seen at the same time from different focal laws or angles. The signal in the green gate on the A-scan is peaked and measured using the BD (green gate) reading and is the through-wall dimension of the crack (in this case 13.17 mm or 46 % through-wall dimension).

Volume corrected 55–70-degree RL sectorial scan visualizing tip of crack at 21.38 mm and base of crack at 24.87 mm. In this case the crack is 21.38 mm or 9 % through-wall dimension.


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ID Creeping Wave—Collateral Echo 1 (CE1) Wave Physics The CE1 signal is usually referred to as the 30-70-70 signal. The CE1 is produced when the direct shear wave strikes the ID at a critical angle and the mode converts to a 70-degree longitudinal wave. This mode-converted L-wave strikes the face of a shallow to mid-wall crack and reflects a 70-degree refracted longitudinal-wave signal back to the probe receiver. With any RL beam, an associated direct shear-wave signal of approximately 30 degrees is also produced. This shear-wave beam is called Collateral Echo 1 (CE1). The main purpose of CE1 is to move the probe across the flaw and measure the echo-dynamics or “signal-walk” distance to provide an approximation of the through-wall dimension of the flaw (20%, 40%, 60%, or 80%). This is done on the 70-, 60-, and 55-degree focal laws for all calibration notches and a chart is developed to use for comparison against the echo-dynamic measurement of the crack. The deeper the crack, the more echo-dynamic or signal-walk distance on the A-scan.

Echo-dynamic measurements of the CE1 signal are performed on the 70-degree focal law for the 20%, 40%, 60%, and 80% calibration notches. A benefit of the phased array technique is that a relationship to the calibration notches can also be trended using the 55- and 60-degree focal laws (or any other focal laws in the RL sectorial scan). In other words, based on the amount of echo-dynamic travel or “walk” that a flaw produces on CE1, an estimation of crack depth can be made when comparing this information to the echo-dynamic chart created using the 20%, 40%, 60%, and 80% calibration notches.

Echo-dynamic measurement of a 40 % through-wall crack. The U(r-m) displays the delta between the measure and reference cursor on the ultrasound axis where the signal rises and falls below 20 % threshold of gate A (red gate).


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ID Creeping Wave—Collateral Echo 2 (CE2) Wave Physics The IDCR (Inside-Diameter Creeping Wave) is produced using a high-angle refracted longitudinal wave in the range of approximately 55–70 degrees. In addition to the CE1 signal, an OD Creeping Wave at an angle slightly above the 70-degree L-wave releases another indirect shear-wave component at approximately 31.5 degrees. This angle changes slightly for focal laws below 70 degrees. This indirect shear wave strikes the ID surface and the mode converts to a longitudinal wave that moves or “creeps” along the ID surface towards the base of the crack. This signal is called Collateral Echo 2 (CE2). Presence of CE2 on a crack confirms that the crack is propagating or connected to the inside diameter. An embedded mid-wall crack or OD- connected crack will have no CE2 component as the ID creeper passes under the flaw. The CE2 signal is a short-lived energy that travels along the ID surface to detect the base of an ID-connected flaw. CE2 has a very short echo-dynamic pattern and is extremely sensitive to very shallow cracks. A low amplitude signal at a signal-to-noise ratio of 3 or 4 to 1 is normal for analysis on CE2. Again, the presence of this signal confirms that the flaw is propagating from the ID. The absence of this signal indicates that the flaw is not ID connected. No other analysis on CE2 is required. The CE2 signal will pass over most counter-bore configurations and geometric reflectors on the ID surface. Inadequate penetration and pipe-to-pipe mismatch will also produce a large amplitude CE2 signal and care should be taken not to confuse these conditions with the base of a crack. The primary purpose of CE2 is to confirm that the crack is ID-connected. The only analysis required is the confirmation that CE2 is present or absent at any amplitude. Another difference in the phased array technique is that the CE1 signal is found to be much stronger than the CE2 across all focal laws in the RL sectorial scan. In conventional UT, the CE2 or ID creeper usually has a much higher amplitude regardless of the crack’s through-wall dimension.

Confirmation of ID-connected crack


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OmniScan ID Creeping Wave Calibration This calibration block for this sizing technique is made from the same material and thickness as the cracked specimens. Through wall notches at 80 %, 60 %, 40 %, and 20% are cut into the material with sufficient distance between them to ensure the 55–70-degree RL sectorial scan can visualize the base and tip of each notch without interference from the corners or adjacent notches. A larger calibration block, to include notches at 90%, 80%, 70%, etc., can produce more accurate echo-dynamic curves but are generally not necessary for 5 % through-wall accuracy in material thicknesses up to 25.4 mm. ID Creeping Wave 25.4 mm Calibration Block designed by Mark Davis, Inc.

The 60 % and 40 % notches are on the other side of the calibration block. The L-wave tip signal should be ±0.003 inch on each of the four calibration target tips. The L-wave base signal should be 25.4 mm on the base of each notch. The CE1 echo-dynamic is measured and recorded on all notches. CE2 is present at varying amplitudes on all reflectors as they all represent ID-connected targets. All readings and measurements should be taken from the digital readings available on the top of the OmniScan display. These measurements are most commonly the result of the highest amplitude or shortest time of flight of the signal in the gate. Gate positions are constantly adjusted throughout the procedure to isolate signals for measurement. Do not read measurements directly off of the rulers in the A-scan and S-scan displays as this will not allow the accuracy that justifies this technique.

80% 20%

60% 40%


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OmniScan ID Creeping Wave Calibration (CE1 and CE2) The calibration for this technique requires that the 55–70 RL sectorial scan L-waves have a calibrated wedge delay or time of flight with a precision of ±0.003 in. All focal laws must also have been calibrated for ACG on one side-drilled hole at the approximate target depth (usually within 75 % to 125 % of the material thickness). TCG or DAC for the ID creeper and L-waves is not required as it is for the shear-wave tip diffraction. The IDCR or CE2 does not require any calibration. You should note the amplitude of the CE2 signal during calibration but it will be present on all calibration targets and its amplitude is not a critical parameter. Only its presence or absence is relevant in determining if the flaw is ID-connected. Calibrating CE1 requires building an echo-dynamic chart for comparison to the echo dynamic of the flaw. CE1 measurements and analysis are performed on the 70-degree focal law only.

1. Select the 70-degree focal law and place the probe on the 40 % calibration notch and optimize the CE1 signal by moving the probe toward and away from the notch while observing the A-scan envelope. With CE1 peaked, adjust the gain so that CE1 is at 100 % screen-height. This will be the CE1 reference sensitivity for all echo-dynamic measurements on the other remaining calibration notches and on the crack. For the examples listed in this report the reference sensitivity measured on the 40 % notch is 10 dB.

At the reference sensitivity, CE1 will be the largest amplitude signal on the screen and easily distinguished from CE2, both tip and base L-wave signals, and geometric reflectors.

2. Set the red gate A threshold to 20 % and ensure that it is long enough to capture the entire

echo- dynamic measurement from where the signal rises above 20 %, peaks, and falls back below 20 % as the probe is moved over the calibration notch.

3. Display the Um-r reading and position the Ultrasound measure and reference cursors where

the envelope signal breaks the gate on the rise and fall points of the echo dynamic. Record the Um-r measurement and repeat on the 20 %, 60 %, and 80 % notches.


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4. Repeat the process for every calibration notch. Record the echo-dynamic measurements on all

4 calibration notches using the 70-degree focal law as in the chart below.

CE1 Echo-dynamic measurements for 25.4 mm calibration block 80 % 60 % 40 % 20 % 70 degree 34.7 mm 32 mm 25.4 mm 0 mm

5. At this point the calibration for the 55–70-degree RL sectorial scan is complete and includes

calibrated time of flight for the L-waves, a CE1 echo-dynamic chart for comparison against the flaw data, and a CE2 signal-present confirmation from every calibration notch.

For thicker components it may be beneficial to build the above chart on CE1 signals using the 55, 60, and 60 degree focal laws. For materials under 25.4 mm, the separation in these echo-dynamic measurements is not significant and does not provide useful information.


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OmniScan Setup for Shear-Wave Tip Diffraction for Crack Sizing The shear-wave sectorial-scan channel compliments the RL sectorial scan in several ways. One way is that it helps characterize the flaw and identify conditions that can make the ID creeping wave mislead the inspector. Complex root geometry, sharp or steep counter bore in the weld bevel, pipe or plate mismatch, etc. can be easily sorted out with an understanding of the sectorial scan. In addition, stacked flaws, embedded cracks, pre-service defects, and other types of flaws all have a specific signature in the phased array sectorial scan that can help the operator in determining flaw type and through-wall dimension. Shear-wave tip diffraction alone will not provide the same level of sizing accuracy except in shallow flaws. In mid-wall flaws and deep flaws, shear- wave tip diffraction in this application is approximately ±2 mm. In addition to the 45–70-degree shear-wave sectorial scan, linear scans can be programmed and calibrated to aid in characterization and sizing tip diffraction. A complete 4-channel setup to cover most phased array applications includes the following channels: Group 1 Sectorial Scan 45–70-degree shear wave (full volume) Group 2 Linear Scan 70-degree shear wave (lower ½ of weld) Group 3 Linear Scan 60-degree shear wave (full volume) Group 4 Linear Scan 52-degree shear wave (upper ½ of weld) Up to 8 channels can be programmed on an OmniScan MX PA and three channels can be displayed simultaneously. The preferred inspection method is to display only the sectorial scan for detection and switch between linear-scan channels to optimize and characterize defects.

The primary OmniScan display view for this application is the S-scan and A-scan.


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Use of the OmniScan Y adapter AAUX202A allows multiple probes to be connected and active on the same setup file or inspection. This permits both the longitudinal-wave sectorial-scan probe/wedge and shear-wave probe/wedge to be used simultaneously for both manual and automated applications. Without the use of the multiple-probe adapters, the longitudinal- and shear-wave channels must be on separate setup files and the probe/wedge changed in between techniques.

Shear-Wave Tip Diffraction signal on 61-degree focal law of 45–70-degree sector scan.


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Shear-Wave Wedge-Delay (TOF or Zero Offset) Calibration for Tip Diffraction As with the basic calibration rules of any code, every focal law or A-scan within the 45–70-degree sectorial scan must meet the same criteria as a single-channel conventional flaw detector for linearity, sensitivity, wedge delay, or time of flight. It must be capable of an independent DAC/TCG on each focal law to cover the inspection range. For the 32:128 OmniScan, use the 5L64 probe and the SA2N55S wedge. For the 16:128 OmniScan, use the 5L16 probe and the SA1N60S wedge. The use of wizards in the OmniScan software allows these functions to be performed simultaneously on all the focal laws within a channel or group. Again, the end result of these calibrations is that each component or focal law is the direct equivalent of a single-channel conventional A-scan for linearity, sensitivity, time of flight, and TCG/DAC. If you do not calibrate this channel for wedge delay, sensitivity, and a TCG/DAC, you will not be able to achieve the flaw-sizing accuracy and defect characterization that is detailed in this report. All measurements on side-drilled holes used for wedge-delay calibration (time of flight or zero offset) should have an accuracy of ±0.003 in. or .100 mm. This is critical to achieving ±1-mm sizing accuracy in this thickness range. Wedge delay or zero offset is calibrated using the OmniScan calibration wizard and an IIW block or any side-drilled hole at a known depth in the range of the material thickness. A side-drilled hole allows for calibrating wedge delay on all focal laws simultaneously. As the probe is moved over the side-drilled hole, the 45–70-degree beams are exposed to the hole and an independent beam offset is calculated for each focal law. As the A-scan tracks the hole in the gated area, the depth of each focal law is available in the DA reading window. In other words, every angle from 45–70-degrees in one-degree increments is calibrated on a side-drilled hole and an independent wedge lay in µs is saved in the software.


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Shear-Wave Sensitivity (ACG) Calibration Sensitivity or ACG (Angle-Corrected Gain) is calibrated using the OmniScan calibration wizard and a side-drilled hole at a known depth. Typical ACG calibration is done using 1.5- or 2 mm- side- drilled holes such as 2 mm hole found at 15 mm from the surface on IIW block or similar block with multiple side drilled holes at different depths covering the thickness range of the material. As the probe is moved over the side-drilled hole, the 45–70-degree beams (at one-degree increments) of the focal laws are exposed to the hole and a curve is created that displays the amplitude of the hole on each focal law. An independent gain offset is calculated and added to the hardware gain. The end result of this calibration is that focal laws 45–70 will detect the hole with amplitude of 80 %. Without an independent focal-law gain, it would be impossible to set the correct sensitivity for each angle of inspection. One side of the array would be too sensitive and the other side not sensitive enough. Sectorial-scan curve prior to calibration.

Sectorial-scan curve equalized after calibration. An independent gain offset is calculated and added to the UT gain so that all focal laws detect the side-drilled hole at 80 % amplitude.


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Shear-Wave TCG/DAC Calibration Continuing the same process over a series of side-drilled holes or ID/OD notches creates a DAC/TCG (Distance-amplitude correction and time-corrected gain). This corrects calibration reflectors at different depths or sound paths so they are all detected at 80 % amplitude. As the probe is moved exposing all focal laws to the calibration reflectors, the calibration wizard, in the OmniScan, stores an independent gain offset for every A-scan at every TCG/DAC point. The end result of the sensitivity, wedge delay, and TCG calibrations is that the side-drilled holes or notches at different metal paths can be detected at the same amplitude and correct time of flight. Every A-scan on every channel has to be the equivalent of a single-channel conventional flaw detector. To do this quickly and accurately requires using the software tools and a familiarity with the OmniScan user interface.


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Crack-Sizing Technique – Evaluation Procedure (ID Creeper, HALT, and Tip Diffraction)

1. Select the ID creeping-wave channel and add 6 dB to the reference sensitivity. Display the S-scan and A-scan view and select the 70-degree focal law to be displayed on the A-scan. Do not use the Smart A-scan (Highest %) feature for this application.

2. Move the probe towards and away from the flaw until CE1 is peaked and observe the

presence and/or absence of CE2 and the crack base and tip L-wave signals in the uncorrected sectorial scan. Move the probe side to side, on and off of the flaw to distinguish geometric reflectors on the ID surface from CE1, CE2, and the base and tip L-wave signals.

3. With the gain at +6 dB above reference sensitivity, record the presence or absence of CE2.

The presence of CE2 indicates that the crack is connected to the ID. The absence of CE2 indicates that the crack is not connected to the ID. This assists the inspector in determining if the flaw is embedded, stacked, etc.

4. Lower the gain-to-reference sensitivity. The A-scan should still display the 70L focal law. At reference sensitivity, CE1 should be the highest amplitude signal on the A-scan at over 100 % amplitude. This is true even on shallow cracks. This is different from the conventional UT technique where CE2 usually has much higher amplitude than CE1.

CE2 present at +6 dB over reference sensitivity. Confirms crack is ID-connected.


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5. Using the envelope feature on the 70-degree A-scan, record the echo-dynamic travel of CE1

by moving the probe forward over the flaw until the envelope signal rises, peaks, and falls back below 20 % amplitude threshold in gate A (red gate).

6. In the 70-degree A-scan display, place the Amplitude Reference cursor (red cursor) at the point where the envelope signal rises through gate A at 20 %.

7. In the 70-degree A-scan display, place the Amplitude Measure cursor (green cursor) at the

point where the envelope signal falls through gate A at 20 %.

8. In the Readings menu, display the U(r-m) reading and record the value. This is the echo dynamic of the flaw. Compare this with the echo-dynamic chart that was created with the calibration block. Based on the reading, an estimation of the crack depth is now possible. You should be able to determine if this is a shallow, mid-wall, or deep-crack. In the example above the echo dynamic is 29.32 mm, therefore between a 40–60 % through-wall dimension is suspected. This information would correlate to a crack depth from between 10–15 mm from the surface. Stacked flaws or embedded flaws in the same thickness range do not have echo-dynamic measurements supporting an ID-connected crack.

CE1 Echo-dynamic measurements for 25.4 mm calibration block

80 % 60 % 40 % 20 % 70 degree 34.7 mm 32 mm 25.4 mm 0 mm

9. Repeat this process on the 55-, 60-, and 65-degree focal laws if the measurements have been

recorded on the calibration block. On material 25 mm and thinner, this is not necessary or

CE1 at reference displaying echo dynamic of 70L focal law over the crack.


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useful as the echo-dynamic curves between these angles are very similar and do not provide significantly different data. On thicker samples the echo dynamics between these angles will be greater and comparison to the calibration block on these additional angles will help quantify the depth of the crack prior to searching for the tip signal.

10. Using the envelope feature on the 70-degree A-scan, move the probe towards and away from

the crack to the position where CE1 is peaked.

11. With CE1 peaked, hold the probe perfectly still and add 18 dB to the reference sensitivity. DON’T MOVE THE PROBE. From the probe position where CE1 is peaked, both the crack tip (65–70 degree focal law) and the crack base (55–65-degree focal law) are visible in the uncorrected sectorial scan. The tip always appears on a higher focal law in the uncorrected sectorial scan than the base or corner-trap signal. The spatial separation between these two signals in True Depth is the equivalent sizing technique as the Absolute Arrival Time or AATT.

12. CE1 peaks when it is skipping into the face of the crack on the second ½ skip. This means that

the probability that CE1 has a center beam directly in the middle of the flaw is high. From this position it is natural that the lower angle 55-60 degree focal laws will be aimed at the crack base or corner trap and the 60–70-degree focal laws will be aimed toward the crack tip.

13. For material thicknesses too large for the base and tip to be seen simultaneously, a connected

planer crack will be detected and visible at varying amplitude and time of flight on all focal laws.

At ref +18 dB both the crack base and tip are easily detected. CE1 is also the highest amplitude signal on the display.

The tip will require peaking and optimization and at this point the signal is for reference only. Do not assume this is the true depth yet.


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14. Slowly and carefully move the probe in and out from the weld, observing the L-wave signal from the tip of the crack. Observe the tip signal walking in and out of the low-level noise that is present. Observe the A-scan and sectorial scan and pay attention to the spatial separation between the base signal and the tip signal.

15. Adjust the gain so you are evaluating the tip signal above 30 % amplitude with at least 2 or 3 to

1 signal-to-noise ratio. It is not unusual for the tip signal to decrease in amplitude and then increase in amplitude as the beams are exposed to different aspects and ligaments of the crack.

Your ability to size cracks accurately in this application depends on your ability to identify and peak the tips and separate them from base material and wedge noise. Extreme care should be taken to differentiate low-level noise, wedge echoes, and base-material responses from a small amplitude crack-tip signal that is “walking” in the A-scan. Once you believe you have identified the deepest tip or facet of the crack, rapidly move the probe forward and backward and observe the tip “walking” on the A-scan. The noise, low-level base-metal indications, and standing waves will remain at the same time of flight and the crack tip will walk forward and backward through them. This phenomenon can only be visualized dynamically while the probe is moving.

16. After confirming that you have identified the tip, place gate B (green gate) over the peaked-tip signal and record the BD reading. This is the true-depth value of the crack tip (13.17 mm or 48 % through-wall dimension).

Do not take measurements off the ruler bars. Use the DB or DA reading and position the gate over the tip signal.

Tip signal peaked in green gate B. This is the crack depth or through-wall dimension.


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17. Everything beyond this point is for secondary confirmation and to ensure that the flaw is a single-connected crack and not a stacked flaw, or an embedded flaw on top of a geometric reflector from the ID such as mismatch, counter bore, root concavity, or excessive root reinforcement.

18. Switch channels to the shear-wave sectorial scan and place the reference cursor (red cursor)

on the ultrasound axis in the sectorial scan at the depth of the signal recorded from the L-wave tip. This is the position that you have determined as the maximum through-wall dimension and the sectorial scan will ensure that no other tip signals can be detected above the reference cursor on the ultrasound axis of the sectorial scan.

45–70-degree sectorial scan detecting deepest crack tip at 61 degrees. Notice multiple branches and ligaments visible in the A-scan and S-scan.

19. Repeat this process on both sides of the weld and repeat using the linear-scan channels as further confirmation. If the flaw tip is best detected on the 61-degree focal law of the sectorial scan it is natural that the 60-degree linear scan will provide a dynamic side view with optimum- flaw orientation to the beam.

At this point the technique has provided over 5 levels of characterization/detection/sizing that should all support each other. No other single factor should be weighted more heavily than the High Angle L-wave on the crack tip in step 16.

Move the probe back and forth and side to side to ensure no tip signals are detected above the depth of the recorded L-wave sector scan in step 16 above. It is normal if shear-wave tips are only detected within several mm beneath the recorded L-wave depth.

The linear 55-degree channel detecting the same crack tip at the same depth as the sectorial scan.


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Example of Deep ID-Connected Crack – 85 % Through-Wall Dimension CE1 Echo dynamic at reference sensitivity = 31.94 mm (Suspect 70-90 % through-wall dimension)

+18 dB – CE2 present (ID connected) – L-wave crack tip and crack base detected.


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L-wave peaked on tip of crack (green gate) and L-wave base signal present (red gate). Crack depth = 3.58 mm or 86 % through-wall dimension.

Shear-wave tip diffraction prove up using 45–70-degree sectorial scan.


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Shear-wave tip diffraction prove up using 55-degree linear scan. Easily characterized as one continuous crack and not stacked defects or embedded flaw on top of geometric reflector.

Shear-wave tip diffraction prove up using 65-degree linear scan.


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Example of Shallow ID-Connected Crack – 33 % Through-Wall Dimension CE1 echo dynamic at reference sensitivity = 19.77 mm (30-40% through-wall dimension)

+18 dB – CE2 present (ID connected) – L-wave crack tip and crack base present


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L-wave peaked on crack tip and L-wave base signal present. Crack depth = 17.02 mm or 33% through-wall dimension.

Shear-wave tip diffraction prove up using 45–70-degree sectorial scan.


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Shear-wave tip diffraction prove up using 55-degree linear scan.

Shear-wave tip diffraction prove up using 55-degree linear scan from opposite side of crack.


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OmniScan MX PA 16:128 vs. 32:128 — Power of the Pulsers In both 55-degree longitudinal A-scan displays below, the signal in the red gate A is the same crack tip peaked at 14 mm in depth and 23 mm in sound path. The sensitivity on both A-scans is adjusted to 80 % amplitude to compare signal-to-noise ratio and crack-tip signal characterization. The 16-pulser A-scan requires an additional 8.4 dB to bring the crack tip to 80% amplitude. The 32-pulser instrument with approximately the same aperture (within 3 mm) allows a much clearer tip signal and higher signal-to-noise ratio for improved sizing accuracy. The 32-pulser instrument can focus twice the energy at the selected focal depth as the 16-pulser version of this application resulting in more efficient beam steering and clearer tips that are easier to identify. Increasing the number of elements or pulsers for a given aperture is even more beneficial when performing this technique on austenitic materials and welds, inconel cladded components and dissimilar metal welds. In the 16-pulser A-scan displays below, the multiple signals and higher noise make it more difficult to correctly identify and peak the true-tip signal of the crack. These multiple signals can confuse the inspector. It would be a common mistake to confuse the small signal identified by the green arrow with the true peaked-tip signal identified by the blue arrows. 5 MHz 32 elements × .6 mm pitch probe = 19 mm aperture (OmniScan MX PA 32:128)

5 MHz 16 elements × 1 mm pitch probe = 16 mm aperture (OmniScan MX PA 16:128)


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Conclusion The benefits of the phased array technology are enormous for reducing the dependency on the skill of the inspector and increasing the accuracy in crack sizing, defect characterization, and recognizing geometric conditions that can confuse the operator and result in inaccurate inspection results. Combining the ID creeping wave, high angle L-waves, and shear-wave tip diffraction on 2-4 sectorial-scan and linear-scan channels, this technique provides over 5 levels of crack-depth verification that should all support each other. In addition to ID-connected cracks, these techniques are also useful in the detection, characterization, and sizing of embedded and surface-connected flaws as well. Within this procedure there are components of many typical phased array applications for pre-service construction and in-service inspection techniques. These applications can by automated with the use of semi-mechanized one-line encoded scanners, and/or computer based full mechanized bi-directional scanners using the same OmniScan instrument. Use of encoders, acquisition of bi-directional data, and the ability to perform analysis using off-line computer software greatly enhances the inspection results and documentation capability of all phased array and conventional UT applications. Training for this application is available through the Davis NDE Advanced UT Crack Sizing Training Program. Information for this course and other phased array training courses is available at: http://www.olympusndt.com/en/training-academy/ Please forward questions and comments regarding this inspection technique to [email protected] or [email protected] The old way The new way

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Omniscan Techniques for Crack Sizing