In-plane mechanical properties of a re-entrant hexagonal and chiral hybrid unit-cell metamaterials

Abstract

Negative Poisson's ratio (NPR) lattice metamaterials have considerable potential for aerospace, medical treatment, and piezoelectricity applications. However, a metamaterial with a high tensile stiffness in the vertical direction and can achieve substantial displacement in its horizontal direction is lacking. In this work, a novel hybrid unit cell comprising a re-entrant hexagonal cell and a tetra-chiral cell connected in parallel is designed. NPR and tunable stiffness are achieved in such a geometry, which can produce significant deformation in the x-direction and has good tensile stiffness in the y-direction. Based on the Eulerian–Bernoulli beam theory, the mechanical properties of the unit cell are derived. Poisson’s ratio in the xy plane and elastic modulus in the x and y directions are established. Using the finite element (FE) method, we analyzed this hybrid unite cell. Samples with identical geometric variables to those of theoretical models are fabricated by 3D printing. Their Poisson’s ratios and elastic modulus in the x and y directions are tested to verify the theoretical and FE results. Results indicate that the Poisson's ratio of metamaterial is negative. The cell-wall thickness has more influence on elastic properties than the aspect ratio of the cell wall. This study can provide new ideas for the engineering applications of metamaterials.