Observation of elastic bound modes in the continuum in architected beams

TL;DR

This paper experimentally demonstrates elastic bound modes in the continuum within architected beams, enabling wave energy confinement in compact regions with potential applications in sensors and resonators.

Contribution

It introduces a novel design method for achieving elastic BICs in compact regions of beams using equilibrium constraints and out-of-phase motion.

Findings

Multiple BICs identified in the compact region.

Finite element simulations confirm BIC predictions.

Laser Doppler vibrometry verifies experimental BIC observation.

Abstract

We report the experimental observation of an elastic bound mode in the continuum (BIC) in a compact region of an architected beam. We consider a long slender beam with rigid masses attached at periodic intervals, with a compact segment bounded by four protruding side beams. The key idea is to seek a mode where the side beams move out-of-phase with the compact region, thereby nullifying the forces and moments outside this region and resulting in a bound mode. The structure is modeled using Euler-Bernoulli beam theory and the side beams are designed by imposing equilibrium constraints required for a BIC. Multiple BICs are found in the compact region, and for each BIC, we find a one-parameter family of BIC supporting side beam designs. The predictions are verified by three-dimensional finite element simulations, followed by their experimental observation using laser Doppler vibrometry in a macro-scale structure. Our approach allows to achieve BICs in an arbitrary sized compact region of the architected beam. Our findings may open avenues for confining elastic wave energy in compact regions for applications in sensors and resonators.