Probabilistic models for life prediction of insulating materials

A general approach to the investigation of generalized cumulative damage functions that are able to describe the endurance of insulating materials subjected to electrical stress and temperature is developed. In particular, probabilistic life models for insulating materials showing life lines for multiple thermal-electrical stresses which tend towards electrical thresholds are proposed and compared on the basis of materials and systems, i.e. flat specimens of epoxy resin and composite Nomex-Mylar-Nomex, XLPE cable models. The models are obtained starting from the two-parameter Weibull distribution of times to failure, and explaining scale and shape parameters so that time-to-failure percentiles fit life curves showing upward curvature and tendency to infinite life. Having verified the validity of these cumulative damage functions over a wide range of electrical and thermal stresses, and having derived the stress dependence of the model parameters, complete characterization of the multistress endurance as well as estimation of electrical threshold at selected temperatures, including service stresses, is obtained.