Design and experimental analysis of superelastic auxetic metamaterials

Metamaterials are composed of structures that react with complex modes of deformation to simple loading. They may, for instance, be used to create auxetic behavior: the ability of a metamaterial to expand laterally when subjected to tension as opposed to the Poisson type contraction exhibited by conventional materials. While the design of metamaterials composed of a linear-elastic base material is already challenging, adding the non-linear behavior of superelastic NiTi to the mix potentially offers intriguing new possibilities to create metamaterials with tailor-made, non-linear load-deformation characteristics. In this contribution, we report on an exploratory study on the design of 2D superelastic metamaterials cut out of superelastic NiTi sheet material. The focus is on the effect of localization phenomena that can occur in tension and (partly) in bending, and how they affect the metamaterial’s local vs. macroscopic behavior. We perform dedicated mechanical experiments on bow tie structures with varying geometries and on a simple chiral structure, using high-resolution digital image correlation (DIC) to document strain distributions across the complex 2D structures. Complementary Finite Element simulations are performed with a material model for superelasticity. The Finite element results are predominantly used to predict optimum geometry parameter sets to maximize auxeticity; they are directly compared to the DIC results of the corresponding validation experiments. Our results highlight the potential of tunable auxetic superelastic metamaterials for use in damping applications. They moreover demonstrate that localization phenomena need to be taken into account during the design stages of these novel types of complex materials.