A bandgap switchable elastic metamaterial using shape memory alloys

Adaptive elastic metamaterials are generally tunable but not switchable. Here, the word “switchable” means switching between different bandgap mechanisms, such as from the local resonance bandgaps to the Bragg scattering bandgaps and vice versa. In this work, to achieve switchable bandgaps, we report a new class of elastic metamaterials whose transmission properties can be significantly tuned by curved two-way shape memory alloy (SMA) resonators. The proposed switchable metamaterial possesses bandgaps capable of being switched back and forth between the Bragg scattering and localized resonance ones. Without thermally activating the curved SMA resonators, the metamaterial beam behaves as a phononic crystal beam whose bandgaps are formed by a periodic array of concentrated masses. By heating the SMAs, the SMAs recover their original curved shape and lift the concentrated masses to form the local resonance bandgaps. The reversible dramatic variation in shape and the stiffness of the SMA resonators allows the bandgaps to be switchable and of course tunable. In addition, the equivalent spring stiffness of a curved beam at two possible directions for the first two modes is derived based on Castigliano's second theorem and is experimentally validated. Compared to SMA springs, the curved shape SMAs allow the generation of high-order local resonance bandgaps. If thermally activated at different temperatures, the negative effective mass density can further be tunable. To the author's knowledge, this work is the first theoretical and careful experimental investigation realizing switchable metamaterials using the two-way shape memory alloys.