Non linear strain rate dependency and unloading behavior of semi-crystalline polymers

The classical viscoelastic constitutive equations do not simulate correctly the mechanical behavior of semi crystalline polymers, particularly in the case of the strain rate jumping or unloading to Zero stress after tensile testing. The main experimental observations of mechanical response of polyethylene and polypropylene at small strains are reviewed. A physical model, which account for the deformation-induced evolution of the material structure, is developed and compared to the experimental observations. It is concluded that this model is the most adequate approach for the simulation of the mechanical behavior and the structure development of semi-crystalline polymers under loading and unloading.