Session: 01-02-02: Acoustic Metamaterials

Paper Number: 147394

147394 - Bound Modes in the Continuum (Bics) in Two-Dimensional Elastic Meta-Structures 

We report the experimental observation of a new class of localized modes, namely bound modes in the continuum (BICs) in two dimensional elastic architected structures. BICs have the unique property that their frequencies lie in passband and their amplitude outside a compact region goes to zero. Such compact modes in the frequency passband eliminates the requirement of energy bandgap to confine the energy. We will illustrate the existence of two distinct types of BICs. The first is a hinge localized mode in square lattice, while the second is a BIC at a hexagon-re-entrant-hexagon lattice.

In a square lattice, we seek topological corner localized modes by starting with a structure having chiral and four-fold rotation symmetry. The unit cell of this lattice transitions from topologically trivial to non-trivial based on the relative values of the inter and intra cell spring stiffness values. We break the four fold symmetry, while maintaining two fold and chiral symmetries to achieve corner localized modes at the hinge interface of trivial and non-trivial lattices. The frequency of the center localized modes can be tuned by changing the interface stiffness. These theoretical predictions are verified by three dimensional finite element simulations, followed by their validation on a macro-scale fabricated sample using  laser Doppler vibrometry.

Next, we consider in-plane waves in a structure comprising of two identical hexagonal spring mass lattice connected by a layer of re-entrant lattice based spring mass system. The masses interact with other masses in the lattice via axial and rotational springs. The mode shapes of this structure show the  existence of bound modes in the re-entrant part, whose frequency lies in frequency passband of both the hexagon and re-entrant lattices. The predictions are using three dimensional finite element analysis and a structure is fabricated using TPU. We observe the BIC in the $3D$ structure using a high speed camera. Piezo-electric transducers are used to excite in-plane waves while digital image correlation techniques are used to determine the resulting displacement fields. Finally, the BIC confined in the re-entrant part of this structure can be turned to a bulk mode by deforming this part to a hexagon. The shape changing can be done by pulling the lattice from the far field. The modal analysis of the deformed lattice shows the existence of a bulk mode at the localized mode frequency. Such switching of a BICs to bulk mode is useful to storing and releasing wave energy at will, for potential application in wave-based sensors and actuators.

Presenting Author: Adib Rahman Kansas State University

Presenting Author Biography: Adib Rahman completed his BSc in Mechanical Engineering from Bangladesh University of Engineering and Technology (BUET) in 2019. After that, he started PhD in Mechanical Engineering at Kansas State University in 2021. His research focus is computational mechanics and metamaterials.

Authors:

Adib Rahman Kansas State University
Raj Kumar Pal Kansas State University