Exploiting self-contact in mechanical metamaterials for new discrete functionalities

The rational geometric and topological organization of mechanical metamaterials allows for unconventional responses to external loads. These materials exhibit a coupling between deformation in orthogonal directions governed by Poisson's ratio, which, in turn, can be precisely adjusted through the deliberate selection of specific geometrical parameters within the metamaterial's structure. Although certain structural motifs may even enable a negative Poisson's ratio, it is conventionally assumed to remain constant or near-constant during deformation. In this study, we introduce a novel design concept that enables metamaterials to switch the sign of Poisson's ratio during loading, specifically at predetermined compressive strain levels, by harnessing self-contact between individual elements of the metamaterial. Through the integration of finite element simulations and experimental testing, we establish a direct correlation between the geometrical parameters of the unit cell and the mechanical response of the metamaterial. This correlation enables us to engineer samples with desired functionality and we present a discrete scales demonstrator that exploits this switchable behavior.