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.
Auxetic foams exhibit the unusual property of a negative Poisson's ratio and hence expand laterally when stretched. In this, the first of two papers, a systematic study is made of the mechanical properties of a range of foams fabricated to have different negative Poisson's ratios. The stress-strain behaviour, in both tension and compression, is compared with conventional foams. Values for the Young's moduli and Poisson's ratios in the three principal directions, in both tension and compression, are obtained. Negative Poisson's ratios ranging between -0.2 and -0.9 in tension, and between -0.1 and -0.8 in compression have been obtained. The effect of densification, during loading is significantly enhanced in auxetic foams, and this significantly affects the stress-strain curves. The creep resistance of the foams is also examined.
Auxetic materials with a negative Poisson's ratio have the fascinating property of expanding laterally when stretched longitudinally. This property is fundamentally important to the properties of a material, and significant changes in the value of the ratio have important effects on mechanical performance. This paper examines the variation in static and dynamic shear moduli on a range of auxetic foams and also examines variation in loss tangent. It is shown that both the static, elastic shear modulus and the dynamic loss tangent may increase with a negative Poisson's ratio. Such changes are attributed to modifications of the cell structure of the foam, not to changes in the material of the foam, during conversion to an auxetic foam. Such changes may have important applications in cushioning, packaging, sound absorption and protection.
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