ADVANCED ULTRASONIC FLAW SIZING OF STRESS CORROSION CRACKING IN THICK STAINLESS STEEL

Abstract: The TOFD (Time-of-Flight Diffraction) method is the one of the most accurate flaw sizing methods. This study aimed to increase the sophistication of the TOFD system by utilizing time-frequency analysis (wavelet transform) for flaw sizing on thick stainless steel welds. A wavelet algorithm was implemented into our TOFD system, and a preliminary test was conducted for a 46.2 mm-thick stainless steel specimen with an artificial penetration slit (0.8mm width, 8.4-9.9mm height). As a result, we found that the frequency component of the three main echoes observed used for sizing (lateral wave, diffraction echo and back surface echo) were different. This feature can be effectively used for signal type classification during flaw size testing. The crack depth was measured by using the extracted specific frequency component of TOFD signals by wavelet transform. We confirmed that the sizing accuracy of the proposed method is similar to the traditional method using raw data. Finally, a weld joint austenitic stainless steel specimen with stress corrosion cracking was prepared for a feasibility study. Crack tip diffraction echoes can be separated from other waves by the proposed method, and the S/N ratio of the detected signal was enhanced by our method. Overall, the method proposed in this study makes relevant contributions to the flaw sizing of thick stainless steel (especially at welds)