Mechanics of Auxetic Materials

Abstract

Poisson’s ratio is a mechanical property that represents the lateral behavior of materials under an axial load. In contrast to typical natural materials with a positive Poisson’s ratio, auxetic materials have a negative Poisson’s ratio (NPR) characterized by unilateral shrinkage or expansion against axial compressive or tensile loadings, respectively. Here, based on a comparison between conventional and auxetic materials, a review of auxetic materials that exhibit various types of deformation mechanisms and characteristics induced by a negative Poisson’s ratio is provided. First, the deformation mechanisms in auxetic materials resulting in lateral expansion under tensile loads are described. They are classified according to their deformation mechanism or the structural motif enabling the auxetic behavior including re-entrant structures, rotating unit structures, chiral structures, fibril/nodule structures, buckling-induced structures, helical yarn structures, Miura-folded structures, and crumpled structures. Then, the expected properties of auxetic materials in several aspects are discussed. The mechanical response of these materials can be drastically changed depending on the amount of applied loads, and auxetic materials are expected to have unusual, possibly enhanced geometrical and mechanical characteristics such as synclastic curvature in bending, deformation-dependent permeability, high shear stiffness, indentation resistance, and fracture toughness, and improved damping and sound absorption properties. Finally, some representative potential applications of auxetic materials are illustrated with a short discussion on the limitations and outlook of auxetic materials.