Transformation design of in-plane elastic cylindrical cloaks, concentrators and lenses

Abstract

We analyse the elastic properties of a class of cylindrical cloaks deduced from linear geometric transforms ${\bf x} \to {\bf x}'$ in the framework of the Milton-Briane- Willis cloaking theory [New Journal of Physics 8, 248, 2006]. More precisely, we assume that the mapping between displacement fields ${\bf u}({\bf x}) \to {\bf u}'({\bf x}')$ is such that ${\bf u}'({\bf x}') = {\bf A}^{-t}{\bf u}({\bf x})$, where ${\bf A}$ is either the transformation gradient $F_{ij} = \partial x'_i/ \partial x_j$ or the second order identity tensor ${\bf I}$. The nature of the cloaks under review can be three-fold: some of them are neutral for a source located a couple of wavelengths away; other lead to either a mirage effect or a field confinement when the source is located inside the concealment region or within their coated region (some act as elastic concentrators squeezing the wavelength of a pressure or shear polarized incident plane wave in their core); a last category of cloaks is classified as an elastic counterpart of electromagnetic perfect cylindrical lenses. The former two categories require either rank-4 elastic tensor and rank-2 density tensor and additional rank-3 and 2 positive definite tensors $({\bf A} = {\bf F})$ or a rank 4 elasticity tensor and a scalar density $({\bf A} = {\bf I})$ with spatially varying positive values. However, the latter example further requires that all rank-4, 3 and 2 tensors be negative definite $({\bf A} = {\bf F})$ or that the elasticity tensor be negative definite (and non fully symmetric) as well as a negative scalar density $({\bf A} = {\bf I})$. We provide some illustrative numerical examples with the Finite Element package Comsol Multiphysics when ${\bf A}$ is the identity.