Mechanical Performance Of Materials Research Articles

Indentation Resilience of Conventional and Auxetic Foams

Materials with a negative Poisson's ratio (auxetic materials) demonstrate the fascinating property of becoming fatter when stretched. This phenomenon is fundamentally important to the properties of a material and a significant change in the value of the ratio has significant effects on the material's mechanical performance. This article describes foams with a negative Poisson's ratio that show better indentation resilience than conventional foams with a positive Poisson's ratio. The unusual mechanical properties of auxetic foams are attributed to the deformation characteristics of re-entrant (concave polyhedral) cell structures.

Nonlinear Analysis of the Poisson's Ratio of Negative Poisson's Ratio Foams

This article contains an analytic study of Poisson's ratio of re-entrant foam materials with negative Poisson's ratio. These materials get fatter when stretched and thinner when compressed. The Poisson effect is so fundamentally important to the properties of a material that a large change in the value of the ratio will have significant effects on the material's mechanical performance. Isotropic foam structures with negative Poisson's ratio have been fabricated through a permanent volumetric transformation. The cells were converted from the convex polyhedral shape of conventional foam cells to a concave or "reentrant" shape. Mechanical behavior of a re-entrant open cell foam material will differ from that of a conventional foam in ways not addressed by existing theoretical treatment. Poisson's ratio as a function of strain is obtained by modeling the three-dimensional unit cell as an idealized polyhedron unit cell. Poisson's ratio is predicted to approach the isotropic limit of −1 with increasing permanent volumetric compression ratio of idealized cells, in comparison with experimental values as small as −0.8.