Constitutive viscoplastic behavior of amorphous PET during plane-strain tensile stretching

Plates of polyethylene terephtalate (PET) mere prepared by an injection-com pression process such that the initial microstructure was almost completely amorphous. Specimens machined from these plates were subjected to plane-st rain stretching experiments by means of an original video-controlled testin g system (VideoTraction(TM)) that gives access directly to the intrinsic st ress-strain behavior at constant strain rate above the glass transition tem perature. True strain was controlled from the current distortion of an arra y of ink dots printed initially onto the samples. Drawing was performed at 5 x 10(-3), 1 x 10(-3) and 5 x 10(-4) s(-1). The stretching behavior has re vealed a marked strain hardening, which increases drastically at large stra in. Furthermore, the influence of temperature on the strain rate sensitivit y coefficient was determined in the glass transition by means of a special technique based on mechanical spectroscopy. The true stress-true strain con stitutive behavior of PET thus characterized was analyzed in terms of a the oretical model using viscoplasticity and finite chain entropic hyperelastic ity. Parameters of this model, especially those describing the ultimate str etching response, are correlated to the strain-induced crystallization of t he PET samples upon stretching, which was assessed by DSC measurements.