Shear Wave UT – Crack Detection

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Shear wave UT is very useful for detecting cracks that have developed during service. Figure 700-19 illustrates how shear wave UT, calibrated according to Figure 700-18, can be used to detect a crack in the heat affected zone of a weld joint. This crack has started at the back surface of the workpiece (I.D. of the vessel), and has propagated towards the front (O.D.) surface.

Shear wave UT is performed by moving the transducer along the O.D. surface of the vessel towards the weld joint. No reflection will be observed in the oscilloscope display with the position of Transducer A in Figure 700-19, because the ultrasonic shear wave is reflected by the I.D. surface away from the transducer.

Shear Wave UT Calibrated According to Figure 700-18 for Detecting a Crack in Heat Affected Zone of Weld Joint

A relatively strong reflection from the base of the crack will be observed in the oscilloscope display for the position of Transducer C, because the base of the crack at the I.D. surface of the vessel acts like a notch in the back surface of a test block. The amplitude of this peak can exceed DAC, and it will appear at a distance in the oscilloscope display corresponding to the thickness of the vessel shell at this location.

When the transducer is moved closer to the weld, as depicted by the position of Transducer D, the amplitude of the peak will usually decrease significantly, and it will appear in the oscilloscope display at a distance corresponding to a depth less than the thickness of the vessel shell. The crack does not always provide a good surface for reflecting the ultrasonic shear wave back to the transducer for detection, because it is usually not oriented perpendicular to the wave. However, cracks normally have a rough (or faceted) texture that will reflect at least a small portion of the ultrasonic wave. The amplitude of this reflection can be very low and difficult to distinguish from the “noise” that results from the relatively high gain setting that is required to obtain satisfactory peak heights from the drilled holes in the test block during calibration. ASME Code, Section V, Article 4, now requires recording all reflections with amplitudes of 20% DAC and greater as flaws. This is a significant increase in the sensitivity of crack detection from the previously used 50% DAC recording level.

Considerable skill and experience is required for a technician to properly interpret reflections with amplitudes this low as an indication of a crack. Cracks that give very low amplitude reflections with ultrasonic shear waves at 45 degrees can give stronger reflections at 60 degrees or 70 degrees. Ultrasonic waves at the greater angles will be more nearly perpendicular to a crack propagating through the vessel shell. Therefore, it is advisable to perform shear wave UT at two or more angles, to maximize the probability of detecting the cracks.

When the transducer is moved still closer to the weld, the ultrasonic shear wave will eventually pass above the tip of the crack, as depicted for Transducer E. The wave will once again be reflected by the I.D. surface of the vessel away from the transducer, and the peak will disappear. Disappearance of the peak can give a rough indication of the depth of a crack, but it should never be used by itself to evaluate the integrity and reliability of a vessel. Other shear wave UT techniques are available that provide much more accurate crack depth and size data.

It is possible for the ultrasonic shear wave reflected by the I.D. surface of the vessel to be reflected subsequently by the crack, as depicted for the position of Transducer B. The peak for this type of reflection from the crack will appear in the oscilloscope display at a distance that is greater than the thickness of the vessel shell. It will then be necessary for the technician to use geometry to determine the actual depth at which the wave is reflected by the crack. The actual I.D. surface of a vessel will not always be a good enough reflector to produce reflections of this type from cracks, because this surface can be roughened considerably by corrosion.

Very fine cracks, such as those resulting from stress corrosion, and cracks that are filled with a corrosion scale may permit a portion of the ultrasonic wave to propagate through. This wave propagation will further reduce the amplitudes of the peaks in the oscilloscope display attributable to these types of cracks, and, therefore, they can be difficult to detect. Proper calibration of shear wave UT with test blocks containing side drilled holes will not guarantee that cracks will always be detected. A special test block containing cracks of the type that may have developed in the pressure vessel should be used to demonstrate that the shear wave UT procedure being used is capable of detecting the cracks, and that the technician has the expertise to properly interpret the data. This concept is gradually being adopted by ASNT and ASME. Contact CRTC Engineering analysis for information and assistance.
Using shear wave UT to detect cracks in nozzle welds or other vessel components with complex geometries can be more difficult than for the previous example. The technician performing the examination will have to know the actual geometry and dimensions of the component, to determine the angles for the ultrasonic shear waves that are required to probe the locations where cracks are most likely to develop. In addition, more complex component geometries can have surfaces that reflect the waves back to the transducer. The technician will have to be able to distinguish these reflections from the surfaces of the vessel component from reflections coming from cracks.

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