Shear Wave UT – UT Imaging

UT imaging can be used to obtain two- and three-dimensional pictures of cracks, or other types of flaws, in a pressure vessel component from an automated ultrasonic examination. An ultrasonic transducer (either longitudinal or shear wave) is moved over the vessel’s surface with a scanning fixture. The X-Y coordinates of the transducer locations on the vessel and the UT signals obtained by the transducer are recorded on a magnetic disk as the transducer is moved over the vessel’s surface. A computer subsequently constructs an image of the crack (or other type of flaw) based on the UT signal amplitude or distance data correlated with the location of the transducer.

Both plan view and cross-sectional UT images can be obtained for permanent records to very accurately determine the location, orientation, and size of a crack.

These images are especially useful to a fitness-for-service analysis, that is critically dependent upon an accurate knowledge of the crack’s dimensions with respect to the maximum principal membrane stress (see Section 750).

UT imaging can also be used to “map” internal corrosion from the I.D. surface of a vessel, using a longitudinal wave UT transducer. The image constructed by the computer will depict the I.D. surface contour of the vessel, which represents the internal corrosion loss. This can be most useful when it is desired to determine the size of locally corroded areas with a remaining wall thickness that is possibly less than the minimum required, to evaluate if repairs are necessary when it is preferred not to open the vessel for an internal inspection.

It should be noted that UT imaging does not improve upon the accuracy of the UT data itself, and, therefore, it is not a substitute for using the optimum UT procedures for flaw detection and sizing discussed above for manual UT examinations. However, the UT image can be very beneficial for analyzing the UT data and giving a permanent record of flaw location, orientation, and size. Characteristics of a flaw can become readily apparent in a UT image. The image can be very important for evaluating the significance of a flaw that could be missed with a manual UT examination.

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