Bragg scattering by subsurface cracks and grooves

Abstract

Bragg scattering of bulk waves from shallow periodic grooves has been proven experimentally to be an effective method for generation of Rayleigh waves in solids. To study this phenomenon, a double layer of alternately positioned periodic cracks is investigated. The system looks like two aligned half-spaces with periodic grooves: cracks correspond to the main grooves’ faces; the mechanical connection of the grooves’ side faces makes the difference between these systems. A null-space expansion of the wave field is proposed that minimizes the resulting stress. The convenient BIS expansion method is applied in this extensive numerical investigation. For wide cracks, a mode can propagate in the system that resembles the Rayleigh wave because cracks make the substrate surface almost stress free. The efficiency of generation of this mode is studied as a function of crack period and width, the distance between crack layers (corresponding to the groove depth), and the angle of incidence of either shear or longitudinal waves. The results show that up to 25% of the incident bulk wave power can be transformed into surface waves in an optimal case with appropriate wave beamwidth. [Work supported by PAN-NIST Polish-US MS-Curie Joint Fund.]