Three-dimensional double negative mechanical metamaterials with tailored anisotropy

Mechanical metamaterials, developed artificially, possess remarkable properties, though these often conflict and are difficult to balance. Previous research on double-negative (negative stiffness and negative Poisson's ratio) metamaterials has been limited to two-dimensional structures or complex fabrication methods, restricting their functionality and applications. To overcome these challenges, we propose a three-dimensional double-negative mechanical metamaterial design, comprising negative stiffness cells and frameworks. This design employs cube arrangements, rotation angles, selective connections, and guide rod assemblies to create 3D structures that maintain localised negative stiffness. The overall negative Poisson's ratio is achieved through coercive deformation between frameworks. Specimens are rapidly fabricated through one-time 3D printing and facile assembly. Based on this strategy, a series of 3D orthotropic isotropic and anisotropic metamaterials are proposed, which have a large tunable design space including parameters such as compressive force, Poisson's ratio and anisotropy. In addition, these metamaterials exhibit high elasticity and mechanical hysteresis during cyclic loading and unloading. These properties were verified by simulations and experiments. These metamaterials have potential applications in helmets, crash beams, landing gears, and other orientation-dependent protection systems.