Experimental determination of viscoelastic properties of a highly compressible porous materials imbibed with water

¹ Department of Machine Elements and Tribology, University Politehnica of Bucharest, Bucharest, Romania
² CNRS INSA-Lyon, LaMCoS, Université de Lyon, Villeurbanne, France

^a Corresponding author: mihaela.radu@upb.ro

Received: 4 July 2014
Accepted: 17 June 2015

Abstract

The experimental study of a highly compressible porous material, made of cellulose fibers, and imbibed with water, during stress relaxation tests on a rheometer, shows that the material relaxes when kept at constant strain as long as there are open pores. Using the relaxation curves, the behaviour under compression can be divided into two distinct zones, a poro-elastic one and a purely-elastic one, bounded by the zero-porosity limit. In the poro-elastic zone, the “non-hookean” stress-strain curve is fitted using Spriggs’ expression of the elastic modulus dependence with porosity. Therefore, the elastic modulus of the porous material is expressed as an exponential function with two parameters: the elastic modulus of the fibers and the initial porosity of the material. For the purely – elastic zone the variation of stress is expressed as a sum of the residual stress obtained from the continuity equation and a Hookean stress-strain variation of a thinner material with the elastic modulus of the fibers – determined previously. The relaxation curves are fitted with a relationship between the decay stress and time, approximated with three terms of Prony series expansion of the generalized Maxwell model.