Change in ultrasonic waveform in fatigue damaging process and its application to damage evaluation

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This project is concerned with establishment of a method for prediction of initiation point and period of the fatigue crack in fatigue damaging process under low-cycle and high-cycle fatigue conditions. Brass plates with curved sides were subjected to the fatigue testing, the strain measurement and the ultrasonic measurement. The global strain distribution was in good agreement with that estimated from the stress-strain curve. The local strain took a similar distribution to than of the global strain overall, but a spatial fluctuation of the local strain increased with increasing number of the loading cycle. The surface roughness of the specimen was in good agreement with an inverse of the local strain distribution. The average gradient of the transfer function obtained by the ultrasonic measurement changed monotonically with increasing cycle number, but the two dimensional distribution revealed no systematic change following crack appearance. The aluminum specimens with a deep notch were subjected to the low-cycle fatigue testing and the ultrasonic measurement. The peak intensity of the Fourier spectrum of the bottom echo and the average gradient of the transfer function changed monotonically around the notch root with increasing cycle number, but no change in peak intensity of the spectrum. Mahalanobis' generalized distance analysis, one of discriminant analyzes, was calculated with these ultrasonic parameters, and was employed for prediction of crack appearance successively.