Residual Stresses and Strains in Cross-linked Polyethylene Power Cable Insulation

Sammanfattning: The subject of this thesis is modeling of the manufacturing process of high voltage power cables with the aim of predicting residual stresses and strains in the cable insulation. The studied material is cross-linked polyethylene (XLPE), which at room temperature is a semi-crystalline viscoelastic solid. Above the crystallization temperature the material exhibits a rubber type behavior due to the crosslinks.An extensive set of uniaxial tensile relaxation tests were used for the mechanical characterization of XLPE. These experiments were complemented by pressure volume temperature experiments as well as density and crystallinity measurements. Based on the experiments, initially a linear and later a non-linear viscoelastic power law model was formulated, incorporating temperature and crystallinity dependence. The non-linear model is based on the Schapery formulation. Evaluations of the model were performed with additional uniaxial experiments. These included comparisons between predicted stress responses and measured values during relaxation tests with transient temperature histories, during two step relaxation experiments and during uniaxial tests with constant strain rate loading.The initial modeling work focused on the prediction of residual stresses which develop during the cooling stage of the manufacturing process. As the constitutive model incorporates temperature and crystallinity dependence, the mechanical problem is coupled to the heat transfer and crystallization problems. Calculations were performed for a vertical manufacturing line. The effects of a viscoelastic material model are illustrated by a comparison to a stress state predicted by a thermo-elastic material model.A final study concerns the modeling of the residual strains in the insulation. It was found that strains originating at cross-linking of the molecules play a signifi- cant role for buildup of residual strains. Calculations are performed for the same vertical process line as before. Good agreement was found between predicted and experimentally obtained residual strains. Based on the residual strain state an estimate is made for the upper limit of shrink-back of the cable insulation.