A New Technique for Surface Acoustic Wave Speed Measurement in Laterally Inhomogeneous Materials
The technique of measuring surface acoustic wave (SAW) speed of a solid by an acoustic microscope has been known to the nondestructive evaluation (NDE) community for last twenty five years. A number of research papers [1–17] have been published on this subject. The technique is based on generating the V(z) curve, also known as the acoustic material signature of the solid under inspection. The V(z) curve is generated by moving the microscope lens vertically, normal to the plane of the solid and recording the reflected signal. The voltage produced by the reflected signal oscillates between a series of maxima and minima as the lens-specimen distance is changed. This is because the phases of the two rays, the central ray and the nonspecularly reflected critical ray which are received by the receiver, vary as the lens-specimen distance is altered. The constructive interference between the two rays gives maxima or peaks and the destructive interference creates minima or dips in the V(z) curve. The spacing between two successive minima is related to the SAW speed of the specimen and thus the SAW speed is measured. This technique works very well for a laterally homogeneous specimen. However, for a laterally nonhomogeneous material predicting the material property variation by the V(z) technique becomes very cumbersome. Because in this case V(z) curves are to be experimentally generated at different locations of the specimen to predict the nonmonotonic SAW speed variation in the specimen.
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