Geometric and viscoelastic bias in bistable mechanical metamaterials

Abstract This study evaluates the effects of print configuration (geometric bias) and material properties (viscoelastic bias) on the snap-through behavior of additively manufactured bistable mechanical metamaterials, sometimes called architected materials or lattice structures. Bistability arises from the reversible buckling of a sinusoidal shaped beam, which allows these structures to exhibit two stable states, i.e., expanded and collapsed. Previous studies of additively manufactured bistable structures have noted a mismatch in the mechanical behavior when transitioning from one stable state to the other during quasi-static tension and compression tests. However, these studies do not systematically study the source of this mismatch. Herein, we identify both geometric and viscoelastic biasing as the cause of this potentially significant mismatch. Bistable metamaterials were 3D printed via fused filament fabrication (FFF) using thermoplastic polyurethane (TPU) for the bistable beams and polylactic acid (PLA) for the supporting frames. Three initial beam configurations were evaluated: collapsed, expanded, and flat (modular). We quantified differences in the snap-through response of the structures resulting from both geometric and viscoelastic biasing. Results indicated that geometric biasing reduced the force required to transition the structure towards its print configuration from 6N to around 3N. In addition, structures that were viscoelastically biased by aging in one stable state could be reprogrammed through additional aging in the opposite stable state. Results from a finite element analysis (FEA) using stiffness proportional damping of the TPU beams agreed well with experimental results for geometric biasing. The understanding gained herein enables bistable structures to be tailored to applications requiring energy dissipation through large deformations.