基于激光激发/检测的非线性激光超声裂纹检测技术的核心在于,使用机械/激光辐照的方式在样品/裂纹上施加载荷,并使裂纹实现周期性开合。但真实裂纹的形貌复杂、裂纹壁粗糙等因素都会对结果产生影响。针对这一问题,用一黑玻璃平面与一平凸透镜的凸面模拟两裂纹壁,通过改变两者之间的距离模拟真实裂纹在载荷施加时的闭合过程。采用脉冲点激光源作为超声激发源,在黑玻璃中激发多模态的超声波,用激光测振仪探测超声,定量推进黑玻璃模拟裂纹的闭合过程,记录并研究不同推进距离下各模态信号峰峰值的变化。实验结果表明:在黑玻璃和平凸透镜完全不接触的状态下推动黑玻璃,随着黑玻璃推进距离的增大,黑玻璃与平凸透镜之间的挤压力度增大,直达纵波信号峰峰值和直达横波信号峰峰值呈现增大的趋势;在黑玻璃与平凸透镜挤压力度较小时,纵波转横波的模式转换信号峰峰值先增大后减小,而反射纵波信号峰峰值有减小的趋势。

The core of the nonlinear laser ultrasonic crack detection technology based on laser excitation/detection lies in the fact that it makes the crack to be periodically opened and closed by using mechanical/laser irradiation to apply a load on the sample/crack. However, many factors such as the complex crack morphology and crack wall roughness affect the results. Aiming at this problem, a black glass plane and a convex surface of a plano-convex lens are used to simulate crack walls, by changing the relative position between the black glass and the lens to simulate the real crack closure process. The pulse point laser source is used as the ultrasonic excitation source to excite the ultrasonic wave on the black glass surface, the ultrasonic wave is detected by the laser vibrometer to quantitatively promote the closure process of the black glass simulated crack, and the peak-to-peak value of each modal signal under different push distances is recorded. The experimental results show that the black glass is pushed in a state where the black glass and the convex lens are not in contact at all. With the increase of the black glass push distance, the pressing force between the black glass and the plano-convex lens increases, and the peak-to-peak values of the direct longitudinal wave signal and transverse wave signal have a tendency to become larger. When the pressing force between the black glass and the plano-convex lens is small, the peak-to-peak value of the mode-transition signal of the longitudinal wave to the transverse wave first increases and then decreases, and the peak-to-peak value of the reflected longitudinal wave signal decreases.