Scattering of ultrasonic waves by a crack which is oriented parallel to a free surface is analyzed. Incident longitudinal and transverse body waves, as well as incident Rayleigh surface waves, are considered. The presence of a parallel subsurface crack gives rise to scattered surfaces waves whose amplitude-spectra show distinct resonance peaks, due to resonance of the layer between the crack and the free surface, particularly when this layer is thin. Scattering of body waves for normal incidence, both from the side of the free surface and from the interior of the solid, is investigated in some detail. The ratio of the amplitude spectra of the surface wave displacements for these two cases provides a measure of the distance d from the crack to the free surface. The length a of the crack can subsequently be obtained from a master curve which plots the first resonance frequency versus d/a. The amplitude spectra for scattered surface waves due to an incident surface wave also show resonance peaks, but these are part of a more complicated pattern of peaks and valleys, and they are therefore more difficult to classify.
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