The effect of pre-stressing on the equi-biaxial fatigue life of EPDM

Abstract

The present paper presents a simple framework to model continuous volumetric damage in elastomers. The formulation predicts phenomenologically the growth of microscopic cavities, and can be applied to both static and fatigue loading conditions. This first version of the approach cannot handle cavitation and is limited to small values of porosities. The derivation is based on the use of a simple scalar damage parameter, the irreversible volume change, and takes naturally into account the change in stiffness through the explicit dependence of the material parameters on the damage variable. The thermodynamic force which drives the volume change contains the hydrostatic stress and also a contribution due to stiffness evolution. As a first application, a damage compressible neo-Hookean constitutive equation is derived and a simple example is studied. INTRODUCTIONRubber-like materials are usually considered as incompressible. However, under multiaxial or fatigue loading conditions, cavitation and cavities growth take place, and lead to damage and finally to fracture (Farris 1968;Le Cam et al. 2004;Le Gorju 2007). Special experiments can be carried out to exhibit this behaviour as proposed by Thomas (1958), Gent andWang (1991) or Legorju-Jago and Bathias (2002). For modelling, on the one hand the cavitation phenomenon under hydrostatic loading conditions is studied considering the stability conditions for the sudden growth of microscopic cavities in the incompressible bulk (see Ball (1982), Horgan and Abeyaratne (1986) for example). On the other hand, several phenomenological approaches have been proposed to predict the growth of pre-existing cavities; the corresponding models incorporate damage variables into compressible hyperelastic approaches (see Boyce and Arruda (2000) for a short review) to quantify the irreversible change of porosity (Andrieux et al. 1997;Dorfmann et al. 2002;Layouni et al. 2003;Li et al. 2007). These models can also be extended to cavitation by adapting the rate equation of the damage variable (Dorfmann 2003). Nevertheless, they are limited to small values of the porosity.In the present paper, similarly to Andrieux et al. (1997), we propose a simple theoretical framework to model the compressibility induced by damage in hyperelastic materials. Our approach is phenomenological and is restricted to small values of porosity, such that the growing cavities do not interfere. The scalar damage variable is the irreversible volume change and its influence on the stiffness of the material is taken into account through the material parameters. The rate equation chosen here is not adapted to sudden volume change (cavitation) but only to continuous volume change (damage by continuous growth of cavities).The derivation of the model is described in the next section, the emphasize being laid on the determination of the thermodynamic force which drives the volume change. Then, a very simple constitutive equation which generalizes the compressible neo-Hookean model is considered to illustrate the rel...