A two-phase model for the diffuso-mechanical behaviour of semicrystalline polymers in gaseous environment

The increasing use of semicrystalline polymers (SCPs) in complex petroleum structures which are subjected to high variations of temperature and gas pressure requires the development of reliable and predictive constitutive models. For that purpose a diffuso-elastoviscoplastic model is presented here by considering a two-phase representation, classically used in porous media, for the representative volume element (RVE) of SCP in gaseous environment. One phase is crystalline (skeleton), and the other corresponds to the mixture (fluid) of gas and free amorphous (the latter is considered penetrable by gas). The modelling is described within the framework of the thermodynamics of irreversible processes with internal variables. General balance equations are established by considering the above RVE. The general diffuso-mechanical macroscopic continuum formulation obtained is applied to elastoviscoplastic behaviour. The implementation of this two-phase diffuso-elastoviscoplastic model in Abaqus™ software via a user subroutine allows to study the interactions between crystal, free amorphous and gas during a rapid decompression test. More precisely, the evolution of pressure in the two-phase polyvinylidene fluoride SCP is given during a decompression test with carbon dioxide. Some results are discussed. The presented model is supposed to facilitate the introduction of damage criterion in order to describe the blistering occurring during a decompression.