Multi-mechanism models for semi-crystalline polymer: Constitutive relations and finite element implementation

Two multi-phase models called 2M1C and 2M2C which take the degree of crystallinity into account were investigated with the help of tensile and creep test database of Polyamide 6 (PA6) material. Multi-phase models are based on the localization of the stress and the homogenization of the local viscoplastic strains. The two models differ mainly by the supplementary coupling introduced in the 2M1C model by means of the flow expression. The first objective of this work is to select which one of the two models is more appropriate for the description of the PA6 mechanical behaviour. The constitutive relations are presented and the finite element (FE) implementation is discussed. The creep strain rate versus applied stress curves are well simulated by both models. The only difference can be detected at the phase level where the creep strain and the creep strain rates differ in their evolution. The models can be extended to any semi-crystalline polymer. Experimental data from the literature are selected to assess the models capabilities to account for the crystallisation kinetics controlled by the strain level.