Compression-Twist Coupling Mechanical Metamaterials with Programmed Bistability

Mechanical metamaterials are artificial materials that control their macroscopic properties using repetitive units rather than chemical constituents. Through rational design and spatial arrangement of the unit cells, mechanical metamaterials can realize a range of counterintuitive properties on a larger scale. In this work, a type of mechanical metamaterial unit cell is proposed, exhibiting both compression-twist coupling behavior and bistability that can be programmed. The design involves linking two cylindrical frames with topology-designed inclined beams. Under uniaxial loading, the structure undergoes a compression-twist deformation, along with buckling at two joints of the inclined beams. Through a rational design of the unit's geometric parameters, the structure can retain its deformed state once the applied displacement surpasses a specified threshold, showing a programmed bistable characteristic. We investigated the influence of the involved parameters on the mechanical response of the unit cells numerically, which agrees well with our experimental results. Since the inclined beams dominate the elastic deformation of unit cells, the two cylindrical frames are almost independent of the bistable response and can therefore be designed in any shape for various arrangements of unit cells in multi-dimensional space.