Model to predict the viscoelastic response of a semi-crystalline polymer under complex cyclic mechanical loading and unloading conditions

To predict the behaviour of a semi-crystalline polymer under complex cyclic mechanical loading and unloading conditions, a non-linear rheological model, previously developed to predict the viscoelastic behaviour under uniaxial monotonous loading and unloading conditions, was used as a starting point and generalised in three-dimensions (3D). The initial model was created on the assumption that the amorphous phase of the semi-crystalline polymer follows the particular reversible mechanism of viscoelastic deformations. This assumption allowed to model the mechanical behaviour of the semi-crystalline polymer under loading and unloading conditions without changing the parameters at the point when the loading was reversed. The parameters of the 3D generalised model under tension, torsion (shearing) and compression conditions were identified from the fitting of the respective uniaxial mechanical tests. It was demonstrated that the 3D model agreed with the experimental data obtained under complex proportional and non-proportional mechanical loading. It was also demonstrated that the model is able to reproduce more then one loop of the cyclic loading.