Shear Wave UT – Amplitude Based Sizing

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Figure 700-20 illustrates how the depth of a crack is determined using amplitude based sizing techniques. The crack was detected by shear wave UT as shown in Figure 700-19. The occurrence of a relatively high amplitude peak in the oscilloscope display at a distance corresponding to the thickness of the shell confirms that the crack has initiated at the I.D. surface.

Amplitude Based Sizing Using Shear Wave UT

The peak will normally decrease in amplitude as the transducer is moved closer to the weld, as shown for the position of Transducer B in Figure 700-20, when the “corner” reflection attributable to the base of the crack at the I.D. surface is lost. The amplitude of the peak reflected from the crack can be quite low, and it should be increased to approximately 80 percent of screen height (12 dB) by adjusting the sensitivity control on the ultrasonic instrument.

Amplitude of the peak will remain relatively constant as the transducer is moved closer to the weld. Only a portion of the ultrasonic wave will be reflected by the crack as the transducer is moved further towards the weld, and some of the wave passes over the tip of the crack. It is generally assumed that the tip of the crack is located by a decrease in peak amplitude to 50% of the maximum after adjustment of the sensitivity control (i.e., 6 dB drop), which is shown for the position of Transducer C. The depth of the crack is determined by the distance that the transducer is moved along the surface from the indication of the base of the crack (position A) to the indication of the tip of the crack (position C).

Amplitude base sizing is reasonably satisfactory for crack depths that exceed the diameter of the transducer. However, other characteristics of the crack (such as changes in orientation, roughness, and width) can affect the amplitude of the peak, which can lead to errors in sizing.

Amplitude based sizing is generally not satisfactory for cracks shallower than the diameter of the transducer. This is the situation with most vessel inspections. A 1/8-inch deep crack may not significantly affect the integrity and Reliability of a vessel, whereas a ¾-inch crack would require repair before the vessel is returned to service. Amplitude based sizing tends to oversize small cracks and undersize large ones, and, therefore, may not accurately discriminate between the shallow superficial crack and the deeper one that could jeopardize safe operation.

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