TENSILE PROPERTIES AND STRESS-RELAXATION OF POLYPROPYLENE AT ELEVATED-TEMPERATURES

Samples of isotactic polypropylene(PP) were subjected to stress-relaxa tion experiments after simple tensile tests at four strain rates and a t different levels of temperature. The relaxation moduli were determin ed in the range of temperature between 20 and 80 degrees C with a rela xation period of 1200s duration. The activation energy value of the sh ift factor was determined using the time-temperature superposition pri nciple. The calculated stress-strain curves and stress-relaxation curv es were obtained from constitutive equations based on an overstress th eory in which the temperature dependence of viscosity and the activati on energy were considered. The temperature dependence of viscosity was amenable to an Arrhenius type equation. The quasi-equilibrium stress depends on both the current strain and the temperature. The calculated results were obtained by the proposed constitutive equation and compa red with the data. The proposed constitutive equations based on the ov erstress model explain well the viscoelastic-plastic behavior of PP sa mples.