Design and additive manufacturing of multistable mechanical metamaterials with tunable stiffness*

Mechanical metamaterials are an emerging class of materials whose intricate microstructures can be carefully engineered to program mechanical properties and promote reversible transitions between multiple stable states of energy. This work presents the design of a mechanical metamaterial based on an assembly of bistable von Mises truss building blocks. First, we show that coupling two von Mises trusses induces geometric frustration which in turn influences the change of effective stiffness in the system when switching from one stable state to the other. Based on a simple analytical model, we characterize the stiffness variation as a function of the geometric parameters and highlight three possible scenarios: (1) increased; (2) decreased or (3) constant stiffness between the stable states. We then combine the von Mises trusses to build a metamaterial unit cell that can be tessellated to form complex 3d shapes. To validate the concept, we fabricate our multistable metamaterial out of PLA and TPU via fused filament fabrication and evaluate its mechanical response by measuring experimentally its effective elastic modulus in both stable states under compression. Lastly, a range of applications for this unit cell are explored, with the intention of expanding its potential to include a panel capable of adjusting its bending stiffness and a surface that can undergo morphing transformations.