A constitutive model for semi-crystalline polymers at high temperature and finite plastic strain: Application to PA6 and PE biaxial stretching

This paper deals with the mechanical behavior of semi-crystalline polymer films at high temperature (90 °C) in finite plastic strains. Due to the complex morphology of such materials, standard elastoplastic formulations are not suitable for describing in a unified way their uniaxial and biaxial tensile behaviors which involve specific physical phenomena. According to physical considerations concerning the mesoscopic semi-crystalline structure of such polymers, a constitutive model has been specially developed, with the idea of a three-phase morphology in relation to the average distance between crystalline blocks. A quite classical constitutive law is used to describe the mechanical (hyper)-elastic and/or plastic behavior of each phase (an eight-chain “network” model is employed). As a key of the work, a “multi-axial factor” is introduced, in the context of perfect plasticity, so as to relate the threshold stress and the deformation mode, considering the effect of entanglements. Uniaxial and equi-biaxial tensile experimental tests have been performed using two materials: polyamide 6 and polyethylene. Model parameters are calibrated using uniaxial stress–strain experimental curves. Then, the biaxial simulation curves are shown to be in very good agreement with the corresponding experimental results.