V. Auxetic materials

  • What are auxetic materials ?

Auxetics are materials that have a negative Poisson's ratio. When stretched, they become thicker perpendicular to the applied force. This occurs due to their hinge-like structures, which flex when stretched. Auxetic materials can be single molecules or a particular structure of macroscopic matter. Such materials are expected to have mechanical properties such as high energy absorption and fracture resistance. Auxetics may be useful in applications such as body armor, packing material, knee and elbow pads, robust shock absorbing material.

external image Auxetic_Hexagon.svg

  • How do Auxetic Materials Work? Poisson's ratio is the ratio of the contractile lateral strain to the tensile axial strain for a material stretched axially, and is typically around +0.2 to +0.4. However, when we look into classical elasticity theory we find that the Poisson's ratios of isotropic materials can not only take negative values, but can have a range of negative values twice that of positive ones.A study of the structure of materials, and how it deforms, demonstrates that auxetic properties are entirely feasible. Figure 1 shows a 2D structure consisting of a regular array of rectangular nodules connected by fibrils. Deformation of the structure is by `hinging' of the fibrils. For the `open' geometry shown in figure 1a, the cells elongate along the direction of stretch and contract transversely in response to stretching the network, giving a positive v. However, modify the structure to adopt a `re-entrant' geometry, figure 1b, and the network now undergoes elongation both along and transverse to the direction of applied load. In other words, this is an auxetic structure.