Effect of water content on the constitutive response of a cellulose foam

Abstract

Cellulose foams can undergo a large one-way actuation strain when taken from a dry, pre-compressed state to a damp state by the addition of water. In order to develop a predictive capability for the 3D actuation of a foam-based structure with a non-uniform water concentration, it is first necessary to understand the mechanical properties of the foam (including actuation strain) as a function of water content. Here, we report the anisotropic constitutive response of a cellulose foam as a function of water content. Over the range of strain rate and water content considered, the cellulose foam is adequately approximated by a visco-plastic, transversely isotropic, compressible constitutive model. Unloading from the plastic state by a stress drop equal to the initial yield stress leads to reversed plastic flow: this is an extreme form of kinematic hardening.