A range of reflection acoustic micrographs that demonstrate how different contrast mechanisms can lead to images useful in materials science are discussed. The most important class of mechamism affecting contrast involves disturbance of the leaky Rayleigh wave propagating along the surface of the specimen. Pictures of very fine cracks (considerably smaller than the acoustic wavelength) and regions of plastic deformation can be obtained using this mechanism. The ability of the scanning acoustic microscope (SAM) to image plastic deformation is a particularly important application to materials science because there are serious limitations to existing techniques. Useful images may also be obtained in situations where Rayleigh-wave excitation does not predominate; in such cases interference between bulk waves can be responsible for the contrast observed. The conditions necessary for the production of various types of image are discussed, in terms of the elastic properties of and the boundary conditions close to the feature of interest.
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