Torsional surface wave propagation in a transversely isotropic FG substrate with piezoelectric over-layer within surface/interface theory

Propagation of the torsional surface waves in a medium consisting of a functionally graded (FG) substrate bonded to a thin piezoelectric over-layer has been analytically formulated in the mathematical framework of surface/interface elasticity theory. In the cases where the wavelength and/or the thickness of the over-layer are comparable to the surface/interface characteristic length, then the surface/interface effects are not negligible. It is assumed that the over-layer is made of hexagonal 622 crystals with a single axis of rotational symmetry coinciding with the axis of polarization. The half-space is made of an FG transversely isotropic material in which the elasticity tensor and the mass density vary linearly with depth. Accounting for the surface/interface effects, the pertinent dispersion relation is derived analytically and verified for five different limiting cases of the proposed problem. The effect of the inhomogeneity parameters of the FG half-space and the surface/interface parameters on the dispersion relation is studied numerically, and the results are compared with those obtained from the classical theory.