The influence of the amorphous matrix on the plastic hardening at large strain of semicrystalline polymers

The stress-strain behaviour of polyethylene, determined from mechanical tes ts at constant true strain rate, is characterized by high strain hardening in uniaxial tension and strain softening in simple shear. The strong influe nce of the deformation path is correlated with the different crystalline te xtures developed. This behaviour is analysed on a theoretical basis in term s of the specific response of a polycrystalline viscoplastic aggregate and of a rubber-like hyperelastic network. For the 100% crystalline model (by C anova), the tensile hardening is overestimated with respect to experiment, but the shear softening is correctly predicted. In contrast, for the 100% a morphous model (by van der Giessen), the tensile hardening is smaller, but the shear response cannot reproduce the softening. Therefore, the discussio n is focused on a composite model which mixes the constitutive equations of the crystalline phase and of the amorphous phase through the self-consiste nt scheme of Herve and Zaoui. Originally established for elastic multiphase materials, this model is applied here to the nonlinear case of polyethylen e by means of an incremental tangent computation. The stress-strain curves predicted by this approach can be fitted with very good precision to the ex perimental curves in tension and shear with reasonable values of the micros tructural parameters.