In this theoretical study, we investigate the propagation of Love waves in a layered structure consisting of two different homogenous piezoelectric materials, an upper layer and a substrate. A functionally graded piezoelectric material (FGPM) buffer layer is in between the upper layer and the substrate. We employ the power series technique to solve the governing differential equations with variable coefficients. The influence of the gradient coefficients of FGPM and the layer thicknesses on the dispersion relations, the electro-mechanical coupling factor, and the stress distributions of Love waves in this structure are investigated. We demonstrate that the low gradient coefficient raises the significant variation of the phase velocity within a certain range of ratios of upper layer thickness to equivalent thickness. The electro-mechanical coupling factor can be increased when the equivalent thickness equals one or two wavelengths, and the discontinuity of the interlaminar stress can be eliminated by the FGPM buffer layer. The theoretical results set guidelines not only for the design of high-performance surface acoustic wave (SAW) devices using the FGPM buffer layer, but also for the measurement of material properties in such FGPM layered structures using Love waves.

An analytical study on transverse surface waves propagating in a functionally graded material carrying a piezoelectric layer is carried out by Wentzel-Kramers-Brillouin technique. Dispersion relations for the existence of the waves are obtained for material gradient of arbitrary functions and the effects of material gradient on wave propagation are quantified. Numerical examples show the presence of material gradient significantly affects long waves but has only negligible effects on short waves. Depending on whether the surface wave velocity is smaller or greater than the bulk-shear-wave velocity in the piezoelectric layer, two different types of dispersion behavior are discussed.

The propagation behavior of Love waves in a functionally graded material layered non-piezoelectric half-space with initial stress is taken into account. The Wentzel-Kramers-Brillouin (WKB) technique is adopted for the theoretical derivations. The analytical solutions are obtained for the dispersion relations and the distributions of the mechanical displacement and stress along the thickness direction in the layered structure. First, these solutions are used to study the effects of the initial stress on the dispersion relations and the group and phase velocities, then the influences of the initial stress on the distributions of the mechanical displacement and shear stresses along the thickness direction are discussed in detail. Numerical results obtained indicate that the phase velocity of the Love waves increases with the increase in the magnitude of the initial tensile stress, while decreases with the increase in the magnitude of the initial compression stress. The effects on the dispersion relations of the Love wave propagation are negligible as the magnitudes of the initial stress are less than 100MPa. Some other results are obtained for the distributions of field quantities along thickness direction. The results obtained are not only meaningful for the design of functionally graded structures with high performance but also effective for the evaluation of residual stress distribution in the layered structures.

The propagation of transverse surface waves in a piezoelectric material carrying a prestressed metal layer of finite thickness is investigated analytically. The dispersion equation is solved in closed form for class 6mm piezoelectric materials and the conditions for the existence of various wave modes are obtained. Numerical examples are presented for an aluminum, gold, or zinc layer on a PZT-4 ceramic substrate. The presence of initial stress can extend or shorten the range of phase velocity within which a nonleaky but dispersive mode exists. The dependence of phase velocity on initial stress opens a new window for designing acoustic wave devices.