The Time-Dependent Poisson's Ratio of Viscoelastic Cellular Materials Can Increase or Decrease

In viscoelastic materials, the Poisson's ratio is not a material constant but can depend upon time. For polymeric solids, the shear modulus relaxes much more than the bulk modulus, therefore, the Poisson's ratio v(t) is an increasing function of time. This article demonstrates that such time dependence is not a necessary consequence of the theory of viscoelasticity. Composite microstructures of cellular solids are presented which result in v(t) which decreases with time.