We study the responses of laser-generated acoustic waves to localized reversible/irreversible modifications of microscopic asperities on crack surfaces during crack closure, which is an essential process in nonlinear photoacoustic/photothermal crack detection techniques. Our laser ultrasonics technique involves optical measurement of the transmission and mode conversion of the laser-generated surface acoustic waves caused by the crack. Reversible/irreversible modifications of asperities can be achieved via non-contact photothermal loading of the crack. Three photothermal loading cycles were realized in individual succession and were monitored using the laser ultrasonics technique at various experimental locations along the crack. In our experiments, each photothermal loading cycle includes multiple successive subcycles, in which the material is first heated and then cooled to its equilibrium temperature, thereby initiating local closing, followed by opening of the crack. Each subcycle is monitored twice using the laser ultrasonics technique, once each at the end of heating and cooling. Furthermore, each successive subcycle is accomplished at a higher power of heating laser than that of the previous subcycle. Significant differences in the peak-to-peak amplitude of the surface skimming longitudinal acoustic wave, which is excited by mode conversion of the Rayleigh wave by the crack, are revealed during the first cycle of photothermal loading. These differences clearly indicate a partial irreversibility of the mechanical processes occurring in the crack surfaces during subcycles of the first cycle. In a larger temporal scale, irreversible modification of crack surfaces is observed from the significant difference between the experimental results of the first photothermal loading cycle and the subsequent two cycles, whereas a reversible response of the crack surface to thermoelastic loading is observed from the similarity of the measurements accumulated during the two subsequent cycles.

中文翻译：

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光热载荷下真实裂纹可逆/不可逆改性的激光超声监测

我们研究了激光产生的声波对裂纹闭合过程中裂纹表面微观凹凸的局部可逆/不可逆修改的响应，这是非线性光声/光热裂纹检测技术中的一个重要过程。我们的激光超声技术涉及对裂纹引起的激光产生的表面声波的传输和模式转换的光学测量。可以通过裂纹的非接触光热加载来实现粗糙度的可逆/不可逆修改。三个光热加载循环分别实现，并在沿裂纹的不同实验位置使用激光超声技术进行监测。在我们的实验中，每个光热加载循环包括多个连续的子循环，其中材料首先被加热，然后冷却到其平衡温度，从而开始局部闭合，随后裂纹打开。每个子循环使用激光超声波技术监测两次，在加热和冷却结束时各监测一次。此外，每个连续的子循环都是在比前一个子循环更高的加热激光功率下完成的。在光热加载的第一个循环期间，揭示了由裂纹对瑞利波的模式转换激发的表面撇去纵向声波的峰峰值幅度的显着差异。这些差异清楚地表明，在第一次循环的子循环期间，裂纹表面发生的机械过程存在部分不可逆性。在更大的时间尺度上，