QUASI-BULK RAYLEIGH-WAVES IN SEMIINFINITE MEDIA OF ARBITRARY ANISOTROPY

Specific features of quasi-bulk elastic waves arising in the neighbour hood of the direction of propagation of an exceptional bulk wave are d iscussed. A quasi-bulk surface wave is characterized by that it penetr ates into the substrate a distance far larger than the sound wavelengt h, and an exceptional bulk wave is a homogeneous mode with group veloc ity parallel to the crystal boundary and satisfying the condition of a mechanically free surface. Approximate expressions for the penetratio n depth, partial amplitudes, and phase velocity of quasi-bulk surface waves are derived without any assumption about the crystallographic sy mmetry of the medium. In particular it is found that independently of the symmetry of the crystal the penetration depth is inversely proport ional to the square of angles specifying deflection from the geometry of propagation corresponding to the existence of an exceptional wave, The value of the ''gap'' between the phase velocity of a quasi-bulk wa ve and the associated limiting velocity of bulk modes is proportional to the fourth power of the values of these angles. The obtained expres sions clearly demonstrate an intimate connection between the occurrenc e of quasi-bulk solutions and the non-existence of the subsonic Raylei gh wave for the direction of propagation of the exceptional bulk wave.