On visco-elastic modelling of polyethylene terephthalate behaviour during multiaxial elongations slightly over the glass transition temperature

The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature Tg, the stress–strain curve presents a strain softening effect vs strain rate but a strain hardening effect vs strain under conditions of large deformations. The goal of this work is to propose a visco hyperelastic model to predict the PET behaviour when subjected to large deformations and to manage the identification of the material properties from the experimental data by an original method. To represent the non-linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained through a comparison with the experimental uniaxial and biaxial tests. The influence of the temperature on the viscous part is also modelled and an evaluation of the adiabatic self heating of the specimen is compared to experimental results.