Graded anisotropic metamaterials for elastic wave mode conversion

TL;DR

This paper introduces a novel approach using graded anisotropic metamaterials to enable efficient broadband elastic wave mode conversion across interfaces with large stiffness contrast, validated through simulations and experiments.

Contribution

It presents a new design methodology employing functionally graded anisotropic metamaterials for broadband elastic wave mode conversion, including experimental validation.

Findings

Achieved broadband mode conversion in the 1-10 kHz range

Demonstrated effective impedance mismatch mitigation

Validated design with additively manufactured specimen

Abstract

Efficient transmission of elastic waves across interfaces is central to several applications, including medical imaging, seismic isolation, and transducer design. Interfaces with abrupt changes in the material properties significantly impede wave transmission, leading to reflections. This limitation, known as impedance mismatch, becomes even more prominent for mode conversion between different wave types due to polarization mismatch. In this study, we investigate a mechanism employing two-dimensional functionally graded anisotropic metamaterials to facilitate longitudinal--shear mode conversion as waves propagate from a stiff to a compliant medium. By embedding density and anisotropic shape gradients within the functionally graded metamaterial, polarization-induced impedance mismatch is mitigated and efficient mode conversion is enabled. We use unit cell dispersion analysis to tailor the frequency range for mode conversion through gradation in the dispersion behavior and coupling between modes. Using frequency-domain finite element analysis, we demonstrate broadband mode conversion across interfaces with large stiffness contrast operating in the 1--10 kHz range. We then experimentally validate and quantify mode conversion through full-field velocity measurements on an additively manufactured specimen. We further apply the methodology to design a device capable of converting radial--tangential wave modes.