Conventional ultrasonic imaging uses frequencies up to 10 MHz or perhaps a little higher. Scanning acoustic microscopy extends the frequencies used to the GHz region, with a corresponding improvement in resolution. The technique is now beginning to be applied to problems in the study of materials. The advantages over other kinds of microscopy lie in the unique ways that ultrasonic waves interact with elastic solids. The development of diffusion bonds and plastic deformation at a stressed notch have been studied using transmission. In reflection, the contrast must be understood in terms of its variation with defocus. This enables images of grain structure in polycrystalline materials to be accounted for. The depth sampled corresponds to a Rayleigh wave profile. Damping mechanisms in the material, due, for example, to dislocations, can affect the contrast. Enhanced contrast is found from surface-breaking discontinuities, such as fine microcracks.
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