Numerical simulation of the interaction of laser-generated Rayleigh waves with subsurface cracks

In this article, the interaction of a laser-generated Rayleigh wave with a subsurface crack has been simulated in detail. The finite element method (FEM) was used to model a subsurface hollow with a rectangular shape, which was used to represent the subsurface crack. By studying the contributions of all of the reflected and mode-converted signals from the subsurface crack, the arrival time of those signals and the mechanism underlying their occurrence could be fully explained. The first negative peak of the reflected waves originated from the top corner of the subsurface crack. The arrival time of this peak first decreased and then increased as the depth of the subsurface crack increased, which was due to the mode conversion when the diffracted bulk mode interacted with the free surface. However, when the depth of the crack remained the same and the height of the subsurface crack increased, the arrival time of the first negative peak of the reflected waves remained the same but the arrival time of the following reflected peaks maintained a linear relationship with the height of the crack. Furthermore, the sources and mechanism of the reflected waves were studied. It was found that the interval time of the peaks was dependent on the height of the crack, and a method for measuring the height of subsurface cracks has been proposed in this paper. The results of the simulation enable the quantitative measurement of the size of subsurface cracks and the height measuring method has the potential to measure the height of subsurface defects and structures.