Rasmus Klitkou

Fading memory of loading history in polypropylene and a polypropylene/clay nanocomposite

Experimental data are reported for neat polypropylene and a polypropylene/clay nanocomposite in multistep uniaxial cyclic tensile tests at room temperature (oscillations between maximum strains and the zero minimum stress, with the maximum strains increasing monotonically with number of cycles). The fading memory of deformation history is observed: when two test specimens are subjected to loading programs that differ during the first n − 1 cycles and coincide afterwards, their stress–strain diagrams become identical starting from the nth cycle. Constitutive equations are developed in the cyclic viscoelasticity and viscoplasticity of nanocomposites with semicrystalline matrices, and the adjustable parameters of the stress–strain relations are determined by fitting them to experimental data. The ability of the model to predict the fading memory phenomenon is confirmed by numerical simulations.

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

Abstract 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.

CYCLIC VISCOPLASTICITY OF SEMICRYSTALLINE POLYMERS WITH FINITE STRAINS: OBSERVATIONS AND CONSTITUTIVE MODELING

Observations are reported on low density polyethylene and isotactic polypropylene in uniaxial tensile cyclic tests (oscillations between maximum elongation ratios kmax and the zero minimum stress σmin with kmax increasing monotonically with number of cycles). Constitutive equations are developed in cyclic viscoplasticity of semicrystalline polymers with finite strains. Adjustable parameters in the stress–strain relations are found by fitting the experimental data. Ability of the model to describe the characteristic features of the mechanical behavior and to predict the viscoplastic response in cyclic tests is confirmed by comparison of the results of numerical simulation with observations in additional tests.