Can Micro-Chiral Cellular Structures Be Auxetic?

Auxetic materials with negative Poisson’s ratio have recently attracted the interest of the scientific community. This is because these materials have the potential to deliver specific functions thanks to a discretization of their constitutive matter that is intended as an aggregation of macro-scaled cellular structures characterized by specific geometries. Many auxetic structures with different geometries and auxetic response have been proposed. Downscaling the cellular structures (e.g. microscale) has the potential to improve the overall auxetic properties of the material, yet there is not enough research on the behavior of these structures at the microscale. This paper studies the auxetic behavior of a modified hexachiral structure formed by microscale elements. First an optimization study was performed to define the most appropriate geometrical dimensions. Then the auxeticity of the proposed cell was assessed carefully introducing a new Coefficient of Auxeticity which allowed to capture complex mechanical responses (e.g. including cellular rotations) as those faced with the proposed design. The proposed miniaturized design shows an outstanding stable auxetic response up to 20% strain of the micro-cell. It is believed that this study has the potential to pave the way to a new class of auxetic materials.