Overall constitutive description of symmetric layered media based on scattering of oblique SH waves

TL;DR

This paper explores how to model the scattering of oblique SH waves in layered media using Willis-type constitutive matrices, revealing limitations and proposing wave vector-dependent formulations for accurate representation.

Contribution

It introduces a method to determine overall constitutive parameters for layered media and demonstrates the necessity of wave vector dependence in Willis-type models for accurate scattering reproduction.

Findings

Willis-type tensors with wave vector independence cannot fully reproduce scattering data.

Wave vector-dependent formulations can match the scattering matrix exactly.

Analytical limits restrict the forms of spatially dispersive constitutive tensors.

Abstract

This papers investigates the scattering of oblique shear horizontal (SH) waves off finite periodic media made of elastic and viscoelastic layers. It further considers whether a Willis-type constitutive matrix (in temporal and spatial Fourier domain) may reproduce the scattering matrix (SM) of such a system. In answering this question the procedure to determine the relevant overall constitutive parameters for such a medium is presented. To do this, first the general form of the dispersion relation and impedances for oblique SH propagation in such coupled Willis-type media are developed. The band structure and scattering of layered media are calculated using the transfer matrix (TM) method. The dispersion relation may be derived based on the eigensolutions of an infinite periodic domain. The wave impedances associated with the exterior surfaces of a finite thickness slab are extracted from the scattering of such a system. Based on reciprocity and available symmetries of the structure and each constituent layer, the general form of the dispersion and impedances may be simplified. The overall quantities may be extracted by equating the scattering data from TM with those expected from a Willis-type medium. It becomes evident that a Willis-type coupled constitutive tensor with components that are assumed independent of wave vector is unable to reproduce all oblique scattering data. Therefore, non-unique wave vector dependent formulations are introduced, whose SM matches that of the layered media exactly. It is further shown that the dependence of the overall constitutive tensors of such systems on the wave vector is not removable even at very small frequencies and incidence angles and that analytical considerations significantly limit the potential forms of the spatially dispersive constitutive tensors.