Detailed features of acoustic images of subsurface cracks were studied and it was found that the minima of the amplitude of a reflected converging beam are due to excitation of the leaky Lamb waves with negative group velocities near the cutoff condition. This finding is useful in depth measurement of subsurface cracks or delaminations. By applying imaging and V(z) curve analysis, the effect of various surface finishes on the bending strength of silicon nitride samples was investigated. It was found that the attenuation of the leaky Rayleigh waves is more sensitive than the velocity to the bending strength. It was also found that the length of the identation cracks measured in acoustic images is smaller for samples with lower strength. These findings provide some insight on the role of subsurface defects caused by the surface finish.
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