Ultrasonic flaw sizing is an important issue for predicting the remaining life of industrial plants. The Time-of-Flight Diffraction (TOFD) method is recognized as one of the most accurate flaw height sizing methods, however, it is not applicable to thick austenitic steels due to the high-attenuation of crack tip diffraction echo used for flaw sizing. In this study, applicability of time-frequency analysis of Short Time Fourier Transform (STFT) and Continuous Wavelet Transform (CWT) to the TOFD method was examined. We first conducted a basic test using a stainless steel specimen with an artificial penetration slit. We found that the frequency components of three signals used for the sizing (lateral wave, diffraction echo and back surface echo) were different. This feature can be effectively used for signal type classification for observed signals. Next, we proposed a new flaw sizing method using a specific frequency component extracted by STFT or CWT. We confirmed that the sizing accuracy of the proposed method is similar to the traditional method using raw data. Finally, a weld joint stainless steel specimen with stress corrosion cracking was prepared for the feasibility study. The crack tip diffraction echo can be separated from other types of signals using the proposed method and the SN ratio of detected signal was increased by this method.

抄録全体を表示