A Poisson's ratio sign-switching mechanical metamaterial with tunable stiffness

An innovative Poisson's ratio (PR) sign-switching stiffness-changing mechanical metamaterial with potential applicability in automotive and construction industries is presented. The metamaterial that is designed by modifying and combining hexagonal and re-entrant unit cells is fabricated via additive manufacturing and explored using experiments and finite element analysis (FEA). The metamaterial shows excellent specific energy absorption (SEA) properties in comparison with the hexagonal lattice structure, exhibiting approximately 25% higher SEA. Parametric studies reveal that the auxeticity of the metamaterial and its PR sign-changing properties can be tuned by modifying the design parameters. Also, the load-carrying capacity of the metamaterial can be tuned along with its stiffness-changing properties by modifying the unit cell parameters. The geometric parameters of the internal concave unit cells significantly affect the auxeticity of the structure and the strains in which PR sign-switching and effective Young's modulus change take place. Finally, an enhanced form of the metamaterial is presented and analyzed via FEA, showing superior effective Young's modulus change ratio and improved stability under in-plane compression. The parametric studies show that the proposed metamaterial exhibits stiffness-changing property under transverse compression and PR sign-switching behaviour under transverse tension.