New thermo-electrical life model based on space-charge trapping

A new thermo-electrical model which relates the time-to-formation of microcavities or crazes in insulating materials to the applied dc electrical and thermal stresses is proposed. The model is based on the Eyring law, but factors are introduced in the expression of the state free energy, which are functions of the electrostatic and electromechanical energy associated with space-charge trapped within the material. The presence of both electrical and thermal thresholds characterizes the model which can, therefore, describe properly typical behaviours of polymeric insulating materials reported in literature. In the paper, the model is applied to the results of multi-stress life tests performed, at different levels of temperature and electric stress, on flat specimens of Polyethylene-Terephthalate (PET). It is shown that the model can satisfactorily describe the behaviour of the experimental data, providing insights into the chemical-physical characteristics of the insulation.