Mechanical metamaterials with negative stiffness under compressive, tensile, and shear loadings

Mechanical metamaterials are rationally designed structures with engineered architectures that, in theory, can lead to any desirable mechanical properties. Recently, great interest has especially been generated toward those with negative mechanical properties. In this paper, we propose structures that can show negative stiffness under shear, tensile, and compressive loads. The principal working mechanism of the proposed structures is the use of embedded pre-buckled and pre-twisted elements that exhibit bi-stable behaviour. In the tensile- and compression-based models, structures with constant and gradient wall thicknesses were considered. The proposed structures exhibited repeated stress jumps and drops during loading, indicating their potential for advanced energy absorption applications. Moreover, the proposed structures demonstrated negative stiffness multiple times over specific ranges of displacement which can be beneficial for multi-stage actuators and passive switches.