DEVIATION FROM CLASSICAL ELASTICITY IN THE ACOUSTIC RESPONSE OF AUXETIC FOAMS

Abstract

ABSTRACT Acoustic properties of an auxetic foam were measured to validate a previously proposed lower bound for Poisson's ratio, ν ≥ 1/5, inferred from classical two-parameter elasticity theory. This limit differs from the commonly reported lower limit of −1 for isotropic materials in the linear elastic range. For a foam measured to have a value of ν > 0.2, agreement was found between the measured flexural resonance frequency of a disk sample and the theoretical value. On the other hand, for an auxetic sample (ν < 0), the prediction from the two-parameter theory was significantly in error. Thus, for materials having ν < 1/5 (auxetic foams, as well as very hard solids such as diamond, germanium, and fused quartz), the equations of classical elasticity are invalid.