Kapelewski J., Lila B.

The purpose of the paper is to outline a systematic unified nonlocal treatment of some acoustic mode conversion effects associated with 2D pseudo-arrays located at solid-solid interface. It will be illustrated, in some detail, for the instructive case of Stoneley-type (SW-type), wave incident on a periodically nonhomogeneous portion of an interface and partly transformed into bulk modes (longitudinal or shear), steered and focussed in the component solid, considered to be that being tested. An analytical model scheme is developed, using a variational method combined with the T-matrix approach, appropriate for the 2D periodic pseudo-array treated, with taking into account 3D profiles of particular elements. One important advantage of the model as applied to quasielastic dynamics of the system in consideration, is that the boundary condition at boundaries of particular array elementswith their solid substrate are incorporated into the field equations through making use of position-dependent material constants. The treatment includes also a possible predeformation of the medium superimposed on the vibration. It is assumed in the model that the vibration does not influence the perturbation and that the predeformation is quasielastic one, so the equations of motion can be linearized. The problem then comes back to the classical one, but with modified local elastic constants. The approach is essentially not restricted to small perturbation of the interface structure, and can be easily extended to allow for statistically distributed component of sub-interface gradient of the substrate solid parameters, and the effect of a homogeneous top layer being a possible part of the structure in consideration, as normally produced in the implantation process. Some analytical results of the model for chosen profiles of the array elements will be demonstrated in the report.