On Viscoelastic Viscoelastic Viscoelastic Modelling of Poly Ethylene 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 main goal of this work is to propose a viscoelastic model to predict the PET behaviour when subjected to large deformations and to determine the material properties from the experimental data. 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 trough a comparison with the experimental uniaxial and biaxial tests.
Constitutive model for the finite deformation stress–strain behavior of poly(ethylene terephthalate) above the glass transition
Abstract A constitutive model for the finite deformation stress–strain behavior of poly(ethylene terephthalate) (PET) at temperatures above the glass transition temperature is presented. In this temperature regime, the behavior of PET is strongly dependent on strain rate and temperature; PET also experiences strain-induced crystallization at these temperatures. The constitutive model accounts for the rate and temperature dependence of the stress–strain behavior by modeling the competition between molecular orientation processes and molecular relaxation processes. The model is fully three-dimensional and is shown to be in good agreement with experimental data over a wide range in strain rates and temperatures as well as under both uniaxial compression and plane strain compression loading conditions.
Molecular description of constant-load stretching of amorphous poly(ethylene terephthalate) films
Abstract This paper is devoted to a molecular description of the deformation of amorphous isotropic poly(ethylene terephthalate) (PET) films at temperatures slightly above the glass transition temperature under constant load. The deformation is qualitatively described by chain relaxation phenomena occurring before stress-induced crystallization, which are followed by the equilibration of a rubber-like network. The junction points include both trapped entanglements and crystalline units. The structure of this network is characterized by the number of segments between crosslinks. This parameter is calculated by comparing the predictions of the rubber elasticity theory (without Gaussian approximation) with the experimentally observed draw ratios under given conditions of temperatures and loads. It is shown that light loads induce soft networks leading to high draw ratios. The predictions of the molecular orientation derived from this treatment are in good agreement with birefringence data on a large variety of samples.