Temperature, strain rate, and strain state dependence of the evolution in mechanical behaviour and structure of poly(ethylene terephthalate) with finite strain deformation

Abstract

Mechanical tests and differential scanning calorimetry (d.s.c.) analysis that characterize the effects of temperature, strain rate and strain state on the finite deformation and occurrence of strain-induced crystallization of initially nearly amorphous poly(ethylene terephthalate) (PET) are presented. Uniaxial compression in the glassy (25–60°C) and glass transition ( T g) regime (60–76°C), over a wide range of strain rates (0.005–0.5 s −1), shows a decrease in the yield stress and flow stress and a small decrease in the strain hardening modulus, with an increase in temperature and a decrease in strain rate. Post-deformation thermograms on specimens deformed to an imparted logarithmic strain of −1.5 show a decrease in the cold crystallization temperature with an increase in deformation temperature and imparted strain and no change in the cold crystallization exotherm and crystallinity from their pre-deformation values. It follows that uniaxial compression below and through the T g region induces network orientation without strain-induced crystallization (SIC). However, uniaxial compression in the rubbery regime at 80°C, 0.5 s −1 and imparted logarithmic strains up to −1.75 show a distinctively larger strain hardening from that observed at 0.005 s −1. D.s.c. analysis on the specimens deformed at the rapid rate condition shows that the different strain hardening behaviour may be the result of SIC. The plane strain deformation in the glassy and T g regions is characterized by an apparent increase in the yield stress and a larger strain hardening behaviour than that observed in uniaxial compression. D.s.c. analysis on the plane strain specimens shows the evolution of both molecular orientation and crystallization at all temperatures which are expected to contribute to the strain hardening. As the temperature in the transition region, it is not clear how much of the end crystallinity, of the order of 41%, is the result of SIC during straining or annealing of the stretched PET after deformation as it cools from the test temperature.