Finite viscoplasticity of semicrystalline polymers

A series of uniaxial tensile tests is performed on isotactic polypropylene at room temperature with cross-head speeds ranging from 5 to 100 mm/min. Prior to mechanical testing, injection-molded samples are annealed for 24 h at the temperature 160°C that ensures that the specimens can be drawn up to the Hencky strain of 0.6 without neckling. A constitutive model is derived for the viscoplastic behavior of a semicrystalline polymer at finite strains. The stress-strain relations are determined by five adjustable parameters that are found by matching the observations. Fair agreement is demonstrated between the experimental data and the results of numerical simulation. It is shown that the material parameters in the constitutive equations are strongly affected by the strain rate.