Elastic bound modes in the continuum in architected beams

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 with use of Euler-Bernoulli beam theory and the side beams are designed by our imposing equilibrium constraints required for a BIC. Multiple bound modes in the continuum 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 by laser Doppler vibrometry in a macroscale structure. Our approach allows one to achieve bound modes in the continuum 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.