Numerical study on coated metal surface crack by laser ultrasonic detection

In order to study the application of ultrasonic wave induced by line-source pulse laser on surface crack detection of the coated metal plate, finite element simulation method was used to establish the model of metal plate with cracks with and without nickel coating. Rayleigh wave excited by laser and the propagation process of Rayleigh wave were simulated. Through theoretical analysis of waveform at the receiving point, the relationship of coating thickness, crack depth and Rayleigh wave time-frequency domain signal was obtained. The numerical results show that Rayleigh wave velocity varies with the thickness of coating thickness h. When there are cracks on the surface, the arriving time difference Δt between reflection Rayleigh wave and shear Rayleigh wave of the model without coating has linear relationship with crack depth h_c. Δt of the model with coating has linear relationship with crack depth h_c at different segmentations. The numerical results provide reference for actual measurement of the surface crack depth of metal sheets with coating.