Multi-cycle viscoplastic deformation of polypropylene

Experimental data are reported on isotactic polypropylene in tensile cyclic tests with a strain-controlled program (150 cycles) and various maximum strains. A model is developed in cyclic viscoplasticity of semicrystalline polymers. The constitutive equations describe the mechanical response along each individual cycle of loadingunloading. Material constants in the stressstrain relations are found by fitting observations during several first cycles. For cyclic deformation with a large number of cycles, phenomenological equations are introduced to account for the effect of plastic flow and damage accumulation on adjustable parameters. It is demonstrated that the model qualitatively predicts changes in maximum stress and minimum strain per cycle with number of cycles. The stressstrain relations are applied to assess growth of residual strain under cyclic loading with large (tens of thousand) number of cycles.