Shear Wave UT – Time Based Sizing
Time based sizing generally provides greater accuracy for determining depth, especially when the crack is relatively small with respect to the diameter of the transducer. Therefore, time based sizing should be used whenever a fitness-for-service analysis is made to evaluate the integrity and reliability of a vessel.
Several variations of time based sizing have been developed, but only the general concept is discussed here. Different vendors may use different techniques, but they are all significantly more complicated to apply than amplitude based sizing. All time based sizing techniques are highly dependent on the skill and expertise of the technicians performing the examination, and only the most qualified vendors who can appropriately demonstrate the accuracy of their procedures should be used.
Figure 700-21 illustrates one shear wave UT technique that can be used to determine the depth of a crack. The ultrasonic wave propagates through the material as a wave front having a width at the surface equal to the diameter of the transducer. The width of the wave front spreads somewhat as it propagates, but this can be minimized by the use of high frequencies and large diameter transducers. Two peaks, referred to as a doublet, can be observed in the oscilloscope display for the position of the transducer.
A peak with a relatively high amplitude (Peak R) will be developed by the corner reflection of the ultrasonic wave from the base of the crack at the I.D. surface. This reflected peak will normally have maximum amplitude at the distance in the oscilloscope display corresponding to the thickness of the vessel shell.
The portion of the ultrasonic wave that passes over the tip of the crack is diffracted, which results in the formation of a second peak (Peak D) with an appreciably lower amplitude (referred to as a tip-diffracted satellite pulse). The amplitude of the tipdiffracted peak can be maximized for better observation by moving the transducer, which moves the reflected peak from the distance in the oscilloscope display that corresponds to the thickness of the shell and decreases its amplitude. Nevertheless, the sensitivity control on the ultrasonic instrument usually has to be adjusted quite high to be able to see the tip-diffracted pulse, and, therefore, it can be difficult to distinguish from the background noise. One aid to distinguishing the tip-diffracted peak from the noise is that the tip-diffracted peak will move in unison with the reflected peak (i.e., as a doublet) across the oscilloscope display as the transducer is moved.
The separation between the reflected and diffracted peaks is constant whenever they are observed together, regardless of the position of the transducer, and is the result of the difference in the “time-of-flight” of the ultrasonic wave from the base and tip of the crack. Therefore, the distance in the oscilloscope display of the separation of the peaks indicates the depth of the crack (D´ in Figure 700-21).
Other techniques for time base sizing can be used, and, in fact, may be superior for some types of cracks. The actual technique that is used is based to a large extent on the vendor’s knowledge and experience. The vendor should be required to demonstrate the accuracy of the technique by examining a test block containing cracks similar to those in the pressure vessel that is being evaluated. Consult Company Inspection Groups, or a specialist for recommendations.
Categories: In-Service Inspection | Tags: Shear Wave | Leave a comment