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

本课题旨在建立低周和高周疲劳条件下疲劳损伤过程中疲劳裂纹起始点和起裂周期的预测方法。对弯曲边黄铜板进行了疲劳试验、应变测量和超声波测量。整体应变分布与由应力-应变曲线估算的结果吻合较好。局部应变的分布与整体应变的分布相似，但随着加载循环次数的增加，局部应变的空间波动增大。试件的表面粗糙度与局部应变分布的逆符合得很好。通过超声测量得到的传递函数的平均梯度随循环次数的增加而单调变化，但二维分布并未显示出随裂纹出现的系统性变化。对深缺口铝试件进行了低周疲劳试验和超声测试。随着循环次数的增加，底部回波的傅里叶谱的峰值强度和传递函数的平均梯度在凹口根部附近单调变化，但频谱的峰值强度没有变化。利用这些超声参数计算了判别分析中的马氏广义距离分析，并将其应用于裂纹的预测。

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.