Exploiting geometric frustration in coupled von mises trusses to program multifunctional mechanical metamaterials

Multistable mechanical metamaterials are an emerging class of materials whose intricate internal structure can be engineered to program mechanical properties and promote reversible transitions between multiple stable states of energy. In this work, the design of a mechanical metamaterial based on an assembly of bistable von Mises trusses is presented. It is shown that coupling two von Mises trusses induces geometric frustration, which leads to an asymmetry between the stable states. Then the von Mises trusses are combined to build a unit cell that can change effective stiffness in compression when switching states. Based on a semi-analytical model, the stiffness variation is characterized as a function of the geometric parameters and three possible scenarios are highlighted: 1) increased, 2) decreased, or 3) constant stiffness between the stable states. To validate the concept, the multistable metamaterials out of polylactic acid and thermoplastic polyurethane via fused filament fabrication are fabricated, and their mechanical response is evaluated by measuring experimentally the effective stiffness in both stable states under compression. This unit cell also features modularity, enabling reversible assembly and post-fabrication tunability. Finally, a range of applications are explored, including sandwich panels capable of changing their compressive and bending stiffness as well as their surface morphology.