Numerical simulation of the reflected acoustic wave components in the near field of surface defects

The interaction of a laser generated surface acoustic wave with a surface crack has been simulated in detail by the finite element method, where a surface notch of rectangular shape has been introduced to represent the fatigue crack for the convenience of modelling. It is shown that four distinct reflected components are present in the captured waveforms; the first component is the direct reflection of a compressive pulse from the left side of the notch, while the second one is assumed to be the Rayleigh wave converted from the laser generated compressive wave when it impacts the left face of the notch. The arrival time of the third peak has demonstrated clearly that it comes from the direct reflection of an initial Rayleigh wave on the near side of the notch. The origin of the last component has been assumed to be the mode conversion occurring at the base of the notch, which is based on the fact that the deeper the notch the longer the arrival time of the fourth peak.