TEMPERATURE, STRAIN-RATE, AND STRAIN STATE DEPENDENCE OF THE EVOLUTION IN MECHANICAL-BEHAVIOR AND STRUCTURE OF POLY(ETHYLENE-TEREPHTHALATE)WITH FINITE STRAIN DEFORMATION

Mechanical tests and differential scanning calorimetry (d.s.c.) analys is that characterize the effects of temperature, strain rate and strai n state on the finite deformation and occurrence of strain-induced cry stallization of initially nearly amorphous poly(ethylene terephthalate ) (PET) are presented. Uniaxial compression in the glassy (25-60 degre es C) and glass transition (T-g) regime (60-76 degrees C), over a wide range of strain rates (0.005-0.5 s(-1)), shows a decrease in the yiel d 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 lo garithmic strain of -1.5 show a decrease in the cold crystallization t emperature with an increase in deformation temperature and imparted st rain and no change in the cold crystallization exotherm and crystallin ity from their pre-deformation values. It follows that uniaxial compre ssion below and through the T-g region induces network orientation wit hout strain-induced crystallization (SIC). However, uniaxial compressi on in the rubbery regime at 80 degrees C, 0.5 s(-1) and imparted logar ithmic strains up to -1.75 show a distinctively larger strain hardenin g 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 h ardening behaviour may be the result of SIC. The plane strain deformat ion in the glassy and T-g regions is characterized by an apparent incr ease in the yield stress and a larger strain hardening behaviour than that observed in uniaxial compression. D.s.c. analysis on the plane st rain specimens shows the evolution of both molecular orientation and c rystallization 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 PE T after deformation as it cools from the test temperature. (C) 1997 El sevier Science Ltd.