Abstract
The surface properties of a solid can sometimes be different from the bulk properties because of near surface gradients in such variables as chemical composition, metallurgical microstructure, degree of cold work, etc. Since surface or Rayleigh acoustic waves sample the surface elasticity to a depth comparable to the wavelength, these waves can be used to determine such a property gradient in a nondestructive way by measuring the velocity of propagation as a function of frequency. A theoretical expression relating the frequency dependence of the wave velocity to the gradient in elastic constants and density has been derived using perturbation theory, and several cases of practical interest have been calculated. Experiments on quench-hardened steel in which the property gradient is the hardness have been carried out to verify the theory.
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