激光超声检测带过渡圆角平板表面缺陷的数值研究

为了研究激光激发出的超声波在带过渡圆角的金属平板上的传播规律和检测表面缺陷的方法，采用有限元法模拟了该类平板中的激光超声现象，分析了表面波在圆角区域的传播规律和与表面缺陷的作用过程。数值结果表明:激光激发出纵波、横波和Rayleigh波等，其中Rayleigh波主要存在于表面mm量级，并且在过渡圆角处发生模式转换生成了直达波R′和模式转换波R_R等多种表面波；经过过渡区域后的声波在表面缺陷处发生了反射和透射现象，通过B扫图可以检测缺陷的位置。随着缺陷深度的增加，表面波的透射系数不断减小，且透射波R_t和R_st存在0.5 μs左右的到达时间差，该时间差与缺陷深度近似成线性正相关。数值结果为激光超声检测带过渡圆角的平板表面缺陷提供了有价值的参考。

Numerical study on surface defects detection of plate with transition fillet by laser ultrasound

In order to study the propagation law of laser-excited ultrasonic waves on metal plate with transitional fillet and the method of detecting surface defects,the finite element method was used to simulate the laser ultrasonic phenomenon in the plate,and the propagation process of surface wave in the rounded area and its interaction with defects were analyzed.The numerical result shows that,the laser excites the longitudinal wave,the transverse wave and the Rayleigh wave,and the Rayleigh wave mainly exists on the surface of millimeter magnitude.At the transition fillet,these surface waves are converted in mode,and various surface waves such as direct wave R′and infrasound source generated wave RR are generated;the waves passing through the transition area are reflected and transmitted at the surface defects,and the position of the defect can be detected by the B-scan.As the depth of the defect increases,the transmission coefficient constantly decreases,and there is a difference in arrival time of about 0.5μs between the transmitted waves Rt and Rst,which is linearly positively correlated with the depth of the defect.The numerical results provide valuable references for laser ultrasonic detection of plate surface defects with transition fillet.