Irreversible deformation of isotactic polypropylene in the pre-yield regime

In the modeling of the mechanical response of a polymer over a large strain range, the nonlinear viscoelastic and viscoplastic behavior must be considered. For many polymers, nonlinear behavior is observed at low loads, e.g. by a stress-dependence of the creep compliance for stresses above 2 MPa in case of the polypropylene used in this study. Additionally, plastic deformation has been observed at strains below the yield point for several polymers. In this study, the irreversible deformation by cavitation and shear yielding of polypropylene are characterized in the pre-yield regime in uniaxial tensile tests using digital image correlation. The recovery of strain after unloading at a prescribed strain level is measured and used to identify the evolution of the plastic strain during uniaxial tension. An experimental technique for simultaneous determination of the true stress–true strain curve and the degree of stress whitening, which relates to the amount of cavitation, is introduced and the initiation of cavitation is compared to the plastic deformation detected in strain recovery at various temperatures.