Anisotropic Plastic Hardening in Semi-Crystalline Polymers

Abstract

The stress-strain behaviour of polyethylene was determined under uniaxial tension and simple shear. It was shown that the yield stress is correctly modelled by the J2 flow theory, but that the subsequent plastic flow follows a more complex behaviour, with a gradual strain hardening in tension and a marked plastic softening under simple shear. The X-ray diffraction analysis of plastically deformed specimens show that uniaxial tension induces a very acute orientation of the cristalline lamellae with the c axis (chain axis) along the tensile direction. Unlikely, simple shear gives rise to the rotation of the crystalline domains along a bimodal path which promotes a strain-softening. It is shown that these results agree with a microscopic deformation model based on the highly anisotropic pencil glide of polymeric chains and on the practical absence of intrinsic plastic hardening for the active glide system.