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

• Published in: Polymer engineering and science 1999-12, Vol.39 (12), p.2366-2376
• Main Authors: Vigny, M., Aubert, A., Hiver, J. M., Aboulfaraj, M., G'Sell, C.
• Format: Article
• Language: English
• Subjects: Applied sciences; Composition; Crystallization; Exact sciences and technology; Mechanical properties; Organic polymers; Physicochemistry of polymers; Polyethylene terephthalate; Polymers; Properties and characterization
• ISSN: 0032-3888; 1548-2634
• DOI: 10.1002/pen.11625

Plates of polyethylene terephtalate (PET) were prepared by an injection‐compression process such that the initial microstructure was almost completely amorphous. Specimens machined from these plates were subjected to plane‐strain stretching experiments by means of an original video‐controlled testing system (VidéoTractionTM) that gives access directly to the intrinsic stress‐strain behavior at constant strain rate above the glass transition temperature. True strain was controlled from the current distortion of an array of ink dots printed initially onto the samples. Drawing was performed at 5 × 10−3, 1 × 10−3 and 5 × 10−4 s−1. The stretching behavior has revealed a marked strain hardening, which increases drastically at large strain. Furthermore, the influence of temperature on the strain rate sensitivity coefficient was determined in the glass transition by means of a special technique based on mechanical spectroscopy. The true stress‐true strain constitutive behavior of PET thus characterized was analyzed in terms of a theoretical model using viscoplasticity and finite chain entropic hyperelasticity. Parameters of this model, especially those describing the ultimate stretching response, are correlated to the strain‐induced crystallization of the PET samples upon stretching, which was assessed by DSC measurements.