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 R_R 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 0.5 μs between the transmitted waves R_t and R_st, 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.