Experimental demonstration of Willis coupling for elastic torsional waves

Abstract

While Willis coupling has been originally proposed as the analog bianisotropy for elastic waves in a bulk solid, its realizations are currently limited to airborne sound and flexural waves on an elastic beam. Here, we experimentally demonstrate Willis coupling for elastic torsional waves on a thin beam. The design is achieved by breaking mirror symmetry for a metamaterial atom with local resonance for the particular type of elastic waves, which also serves as a universal approach in generating Willis coupling. The investigations will be useful for non-destructive testing on pipes and beam structures with torsional waves.