A general multi-stress life model for insulating materials with or without evidence for thresholds

A general life model, valid for any type of insulating material, with or without evidence of an electrical threshold at room temperature, is proposed. The definition of threshold is changed; the behavior of materials is classified as threshold or no-threshold according to the value of applied stresses. The electrical gradient-temperature graph is examined, and the relationships between electrical and thermal thresholds are shown. The various life models for electrical and thermal stress used by researchers are examined and discussed. The combined-stress life model for threshold materials is improved by adding to the denominator a cross term which gives rise to a threshold line which is curved instead of straight. The application of the model is outlined, and the results are compared with experimental data obtained for several insulating materials