激光超声能够激发宽频带的表面波,可实现金属材料表面微缺陷的定位分析与深度检测。根据表面波在缺陷处的透射/反射阈值现象,提出了测量表面缺陷深度的临界频率法;基于光弹激发原理搭建了激光超声检测平台,得到了铝合金样品的B扫成像,实现了表面缺陷的定位;结合小波变换分析了表面缺陷的透射与反射波形能量分布,测得透射/反射阈值θ₀=1/4,进而采用临界频率法实现了表面微缺陷的深度估计。此外,还分析了激发点与探测点距离、待测样品材料对透射/反射阈值的影响。结果表明,基于激光超声临界频率法可以实现表面微缺陷的定位及深度检测,且透射/反射阈值大小与表面波传播距离、待测样品材质无关。

Laser ultrasonic can realize the localization and depth detection of microdefects on the metal surface owing to broadband surface waves. In this paper, a critical frequency method for measuring the depth of a microdefect is proposed according to the transmission/reflection threshold phenomenon. Based on the principle of photoelastic excitation, a laser ultrasonic testing platform is built and B-scanning images of aluminum alloy samples and the localization of microdefects are obtained by using the platform. Spectral energy distribution of the transmission and reflection waves of a microdefect is analyzed by combining the wavelet transform, and the threshold value θ0 is measured as 1/4 when the transmission/reflection threshold phenomenon occurs. At last, the depth estimation of a microdefect is realized by the critical frequency method. In addition, the influences of sample material and the distance between the excitation points and the detection points are also analyzed. Experimental results show that the critical frequency method based on the laser ultrasonic can realize the depth detection of microdefects on the metal surface, and the transmission/reflection threshold value is independent of the propagation distance and the sample material.