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Ultrasonic Weld Flaw Characterization and Analysis
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Ultrasonic Weld Flaw Analysis Without a Computer

Essentially all widely specified ultrasonic inspection codes and standards for piping, pressure vessels and other weld fabricated structural coponents require rejection of weld flaws which can be identified as cracks or crack-like in nature. Code acceptance/rejection criteria are based upon the flaw signal amplitude compared to that produced by a notch or side drilled hole in a calibration reference block.

Unfortunately, flaw signal amplitude alone cannot be used to identify the nature of the reflector which produces it. In this case an accept/reject decision is based purely on UT signal amplitude which can lead to rejection of benign flaws (slag, porosity) and unnecessary weld repairs. Of much greater concern, however, are the more serious cracks and crack-like (lack of penetration, fusion) defects which, if not correctly identified and remain in a structure after fabrication, could lead to future problems.

A number of investigators have demonstrated that pattern recognition techniques can be successfully applied for analyzing the nature of reflectors (flaws) identified during the ultrasonic inspection of welds. These flaw signature methods apply feature analysis to the digitized RF echo signal. By means of appropriate computer software, time domain measurements can be made by use of the fast fourier transform. However, use of only the time domain flaw echo features of rise time, pulse duration, and fall time on weldments has been shown capable of attaining a 75 to 90 percent reliability in sorting flaws into cracklike and non-crack type categories.

Figure 1.
Time Domain Features of Crack-Like and Non-Crack-Like Flaws

NDTech has developed a manual measuring technique for analysis of flaw echoes appearing on the CRT of the pulse echo ultrasonic flaw detector. This approach permits measurement pulse duration with adequate accuracy and successful application for field use. NDTech is in a position to train those interested in applying this test method.

References:

Cross, N.O., "Ultrasonic Weld Flaw Signature Analysis", ASNT Fall Conference, Atlanta, GA, October 15, 1981.
Rose, J. L., Nestleroth, J. B., and Poplawski, E. G., "Flaw Classification in Welded Plates Using a Microprocessor Controlled Flaw Detector", NDT International, August 1980, pp. 154-164.
Mucciardi, A. N., Dau, G. T., "Detection and Sizing of Feedwater Nozzle Cracks," Electric Power Research Institute, EPRI-NP-1234-SR Special Report, December, 1979.

Figure 1. Time Domain Features of Crack-Like and Non-Crack-Like Flaws

Figure 1.
Time Domain Features of Crack-Like and Non-Crack-Like Flaws