Frequency-Dependent Scattering of Laser-Generated Rayleigh Waves for Surface Crack Characterization

Wideband laser-generated Rayleigh waves have been extensively exploited for the identification and characterization of surface cracks, most of which, nonetheless, are of a nature of either numerical simulation or experimental observation. Earlier, an elastodynamic reciprocity theorem-based theoretical model was proposed by the authors [1, 2], aimed at scrutinizing the interaction of narrowband Rayleigh-Lamb waves with a surface or subsurface crack. In this study, the model is expanded to a wideband scenario to analytically explore the interaction of a laser-generated Rayleigh wave with a surface crack as well as the resultant crack-scattered Rayleigh wavefield. A dimensionless parameter is formulated based on the closed-form solution to the magnitude of the narrowband scattered Rayleigh wavefield, revealing that the scattering effect of a surface crack on the Rayleigh waves is frequency-dependent and a characteristic frequency exists, at which the scattered Rayleigh wavefield manifests the strongest intensity. For the wideband laser-generated Rayleigh wave, such dependence can be calibrated by a spectral damage indicator (SDI), which facilitates evaluation of the severity of the surface crack. Proof-of-concept simulation is performed to demonstrate the frequency-dependent scattering and its use for characterizing surface cracks. Quantitative agreement between the analytical and numerical validates the accuracy of the proposed model and SDI. Results advance the use of the laser-generated Rayleigh waves for the identification and evaluation of surface cracks.