Cyclic viscoplasticity of solid polymers: The effects of strain rate and amplitude of deformation

Observations are reported on polypropylene random copolymer in uniaxial cyclic tensile tests with various strain rates (ranging from 1.7 × 10^-4 to 8.3 × 10^-3 s^-1). Each cycle of deformation involves tension up to the maximal strain ε{lunate}_max (from 0.05 to 0.20) and retraction down to the zero stress. The study focuses on deformation programs with 10-50 cycles in each test. A constitutive model is derived for the viscoplastic behavior of a solid polymer at three-dimensional cyclic deformations with small strains. Material constants in the stress-strain relations are found by fitting the experimental data. Good agreement is demonstrated between the observations and the results of numerical simulation.