Tunable isotropy on the mechanical properties of wavy hexachiral metamaterials: Numerical simulation and additive manufacturing

Hexachiral auxetic metamaterials can exhibit unconventional behaviors of negative Poisson’s ratio that can achieve superior mechanical properties including energy absorption and indentation resistance. The thickness ratio between the wavy ligament and center ring, which is denoted as t ∗ , has a critical influence on the isotropic mechanical properties of the hexachiral structures. This work aims to investigate the effect of t ∗ on the isotropic auxeticity of wavy hexachiral structures under tensile and compressive loadings through simulations and experiments. A parametric study on hexachiral unit cells with t ∗ values ranging from 0.25 to 1.5 was conducted with finite element analysis. With a decrease in t ∗ values, the deformation mechanism showed improved coiling of the wavy hexachiral structure with thinner ligaments and thicker center rings, which exhibits better bending and rotational deformations, respectively. The wavy hexachiral structure with the t ∗ value of 0.33 achieved the lowest average effective Poisson’s ratio of −0.94 and anisotropic factor of 0.00023. The tunable isotropic auxeticity is validated by the experimental Poisson’s ratios and deformations of the wavy hexachiral structures fabricated by laser additive manufacturing up to 3.33% tensile strain and 20% compressive strain. These findings provide insights on the design of mechanical isotropy of chiral metamaterials for additive manufacturing.