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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.
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