Search for Forbidden Directions of Elastic Surface-Wave Propagation in Anisotropic Crystals

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Abstract
A detailed study of elastic surface‐wave propagation on a variety of free surfaces on many anisotropic crystals has been made. Included in the search were cases of propagation on nonsymmetry as well as symmetry planes. To date, no combination of surface, direction of progagation, and crystal has been found for which an unattenuated surface wave satisfying the traction‐free boundary conditions cannot be propagated. Solutions are shown for examples which occur within previously reported ``forbidden'' ranges and for cases in which the free surface is a nonsymmetry plane. The ``forbidden'' ranges appear now to be those in which the penetration of the surface wave becomes relatively deep. A fast and general computer program has been developed for this study to allow arbitrary choice of propagation conditions. The direction of energy flow is computed in the general case by effectively constructing the normal to an appropriate segment of the slowness surface. Solutions have been found for pseudo‐surface waves in various ranges of directions. These are second surface‐type waves which can propagate almost unattenuated.

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