Cyclic viscoplasticity of semicrystalline polymers

Observations are reported on three grades of isotactic polypropylene in uniaxial cyclic tensile tests with various maximum strains. Constitutive equations are derived for the viscoplastic behavior of a semicrystalline polymer. A characteristic feature of the model is that the rate-of-strain tensor for plastic flow is split into the sum of two components, one of which is proportional to the deviator of the stress tensor, while the other is proportional to the rate-of-strain tensor. The stress-strain relations involve four adjustable parameters that are found by fitting the experimental data. Numerical simulation demonstrates that the model correctly predicts the mechanical response of semicrystalline polymers in cyclic tests.