Universal mechanical response of polypropylene under cyclic deformation

Experimental data are reported on isotactic polypropylene in uniaxial tensile (i) cyclic tests (oscillations between maximum strain 0.15 and the zero minimum stress) with various crosshead speeds and (ii) relaxation tests at temperatures ranging from room temperature to 100 ° C. Observations in cyclic tests show that maximum stresses decrease, while minimum strains grow with the number of cycles. A universal character is revealed of the dependence of minimum strain on the number of cycles: this function is independent of temperature and weakly affected by strain rate. To rationalize this observation, constitutive equations are proposed in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers and a scenario is suggested for damage accumulation. Two types of damage are distinguished which are induced by (i) lamellar fragmentation in the crystalline phase and (ii) nucleation and growth of micro-voids in the amorphous matrix. The stress-strain relations involve five adjustable parameters that are found by fitting observations (50 cycles of loading-retraction) at room temperature and α-transition point. Numerical simulation demonstrates that the model correctly describes experimental data and can predict observations in tests with a large number of cycles.