Synergistic energy absorption mechanisms of a liquid crystal elastomer-based metamaterial*

Architected materials (or metamaterials) have been designed to trap energy through elastic buckling instabilities, but it has a fixed energy absorption capability regardless of strain rates. Liquid crystal elastomers (LCEs) are highly dissipative materials compared to conventional elastomers. In this work, we applied LCEs to amplify the energy absorption capabilities of a mechanical metamaterial that experiences snap-through buckling under compressive loading. Compression tests showed that the energy absorption density of LCE unit cell increased with increasing strain rate, such that at a nominal strain rate of ~0.5/s, the energy absorption of the LCE structure was 4 times greater than the analogous PDMS (elastomeric) structure. Furthermore, the energy absorption density increased with stacking number of LCE unit cells. We measured an ~76% greater energy absorption density for a 4-layer stack of the LCE unit cells than a single unit cell. We also applied finite element simulations to calculate the energy dissipated by material viscoelasticity and the energy stored by snap buckling. In vertically stacked LCE structures, the unit cell structures did not buckle simultaneously. At lower strain rates, the buckling of some layers caused other layers to recover (i.e., straighten) then buckle again after the collapse of the preceding layers. This load-unloading cycle increased the viscoelastic dissipation without changing the stored energy. To further promote this dissipation mechanism, we varied the thickness of the beams in each layer to ensure sequential buckling of the different layers. In total, the simulations showed a synergistic interaction between the viscoelastic behavior of the material and the snap buckling of the metamaterial that greatly enhanced energy absorption.