The closure and modification of a real crack in a black glass sample caused by laser irradiation were observed by laser ultrasonic method. The ultrasonic generated by point pulsed laser was detected using optical deflection method. The crack was irradiated by a continuous heating beam and closed gradually by increasing the power of the heating beam. At the same position of the crack, the power of the heating beam was increased gradually and the peak-to-peak value of transmitted surface acoustic wave and longitudinal wave mode converted by surface acoustic wave was recorded at each heating power both when the crack was heated and cooled after heating. The whole process is repeated three times. Experiment results show that great differences of the peak-to-peak values exist between the crack with heating and without heating. Also, the significant difference of the experiment result between the first time and the last two times was observed, with the latter two results whereas being very similar. The results indicate that irreversible modification could happen to the crack during heating and the modification becomes unnoticeable after several times of heating because the crack become more stable.%采用激光超声方法观测了黑玻璃样品上真实裂纹在光热调制下的闭合和改变.以脉冲点激光源作为超声激发源,用光偏转方法检测超声,以一束连续加热光照射在裂纹上,在裂纹同一个位置,逐渐增大加热光功率,记录裂纹处在各加热光功率下被辐照时以及辐照后冷却两种状态下的透射表面波和模式转换信号峰峰值,整个过程重复三次.试验观察到各模态超声波峰峰值在不同功率加热光辐照下/辐照后的显著变化;此外,观察到第一次试验结果与后两次有较大差距,而后两次试验结果一致.结果表明,裂纹在闭合过程中会发生不可恢复的改变.
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