Design of dual-material lattice structures with compression-torsion bistability

Lattice structures with bistability can maintain additional equilibrium states after external loads are removed. While conventional bistable structures typically realize compression and torsional bistabilities under compression and twisting loads, this paper introduces a new design of bistable structures that exhibit torsional bistability under uniaxial compression. The proposed structure is composed of two co-axis polygonal prisms connected by struts. Torsional bistability is achieved by opposite rotations of two co-axial polygonal prisms under compression, which result in snap-through instabilities in the connecting struts. An analytical model and numerical simulations demonstrate that dual-material design for the inclined and connecting struts is required to induce bistability. A parameter study is conducted to illustrate the effect of the volume fractions of soft material in struts and the modulus ratios between two materials on bistability characteristics. One such design is practically fabricated by multi-material 3D printing, and experiments are conducted to demonstrate the existence of the second equilibrium state. Furthermore, a lattice structure is designed by periodically arraying the unit structures, which exhibit multiple stable states due to existence a sequence of bistable states arising in a layer-by-layer manner. This compression-torsion bistability breaks the restriction on the direction of the bistable deformation and the applied load, and provides inspiration for novel functional designs in the areas of logic gates and deformation control.