Effect of crystalline structure on the mechanical response of polypropylene under cyclic deformation

To evaluate the influence of crystalline structure on the mechanical behavior of polypropylene (PP), uniaxial tensile cyclic tests with a mixed program (oscillations between fixed maximum strains and the zero minimum stress) were performed on isotactic PP (iPP) manufactured by the Ziegler-Natta catalysis method, metallocene catalyzed PP (mPP), and annealed mPP. Although the stress-strain diagrams of iPP and mPP under tension are quite similar, their responses under unloading differ markedly. The residual strain (measured as the strain under retraction down to the zero stress) of iPP strongly exceeds that of non-annealed mPP, and annealing of mPP increases this difference. To rationalize these findings, constitutive equations are developed in cyclic viscoelasticity and viscoplasticity of semicrystalline polymers, and adjustable parameters in the stress-strain relations are found by fitting the observations. The ability of the model to describe the observed phenomenon and to predict the mechanical response in multi-cycle tensile tests, with various deformation programs, is demonstrated by numerical simulation.