Nonlinear ultrasonic testing method is one of few techniques that are effective for evaluating early material degradation and micro-crack initiation by analysing the nonlinear effects of ultrasonic waves. This study presents a noncontact nonlinear ultrasonic testing method based on lasers for closed surface crack inspection. A pulsed laser grating source by grating mask is applied to generate narrowband surface acoustic wave of a chosen frequency. A nonlinear numerical simulation based on Finite Element Method (FEM) is developed to simulate the generation of higher harmonics by closed surface crack. The simulation results show that the acoustic nonlinearity parameter increases with the micro closed crack length, while decreases with the micro closed crack buried depth. Moreover, the detection capability of this method is verified by an experiment. The experimental results show that the proposed laser-based nonlinear ultrasonic testing method can provide a fully noncontact and coupling-free measurement method for closed surface crack inspection.

非线性超声测试方法是通过分析超声波的非线性效应而有效评估早期材料降解和微裂纹萌生的少数技术之一。这项研究提出了一种基于激光的非接触式非线性超声测试方法，用于闭合表面裂纹检查。施加通过光栅掩模的脉冲激光光栅源以产生选定频率的窄带表面声波。建立了基于有限元方法（FEM）的非线性数值模拟，以模拟由闭合表面裂纹产生的高次谐波。仿真结果表明，声非线性参数随微裂纹闭合长度的增加而增大，而随微裂纹闭合深度的减小而减小。另外，通过实验验证了该方法的检测能力。