Static elastic cloaking, low frequency elastic wave transparency and neutral inclusions

TL;DR

This paper explores how to design shell materials around a core in elastic media to achieve static cloaking and low frequency wave transparency, introducing the concept of strong neutral inclusions as a practical solution.

Contribution

It establishes the connection between static elastic cloaking, low frequency scattering, and neutral inclusions, introducing strong neutral inclusions as a new, physically realizable approach.

Findings

Anisotropic shells are required for static cloaking.

Isotropic materials can reduce low frequency scattering.

Strong neutral inclusions enable perfect static cloaking with standard materials.

Abstract

New connections between static elastic cloaking, low frequency elastic wave scattering and neutral inclusions are established in the context of two dimensional elasticity. A cylindrical core surrounded by a cylindrical shell is embedded in a uniform elastic matrix. Given the core and matrix properties, we answer the questions of how to select the shell material such that (i) it acts as a static elastic cloak, and (ii) it eliminates low frequency scattering of incident elastic waves. It is shown that static cloaking (i) requires an anisotropic shell, whereas scattering reduction (ii) can be satisfied more simply with isotropic materials. Implicit solutions for the shell material are obtained by considering the core-shell composite cylinder as a neutral elastic inclusion. Two types of neutral inclusion are distinguished, \textit{weak} and \textit{strong} with the former equivalent to low frequency transparency {and the classical Christensen and Lo generalised self-consistent result for in-plane shear from 1979. Our introduction of the \textit{strong neutral inclusion} is an important extension of this result in that we show that standard anisotropic shells can act as perfect static cloaks, contrasting previous work that has employed "unphysical" materials.} The relationships between low frequency transparency, static cloaking and neutral inclusions provide the material designer with options for achieving elastic cloaking in the quasi-static limit.