A General Multi-stress Life Model for Insulating Materials with or without Evidence for Threshholds

In this paper a general life model, valid for any type of insulating materials, with or without evidence of an electrical threshold at room temperature, is proposed. First of all, the old definition of threshold or no-threshold materials is changed and the behavior of materials is considered, as a threshold or as a no-threshold material, according to the value of applied stresses. The electrical gradient-temperature graph is examined, showing the relationships between electrical and thermal thresholds. The various life models for electrical and thermal stress used by researchers are examined and discussed; the old 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. Since for no-threshold materials the denominator should be equal to 1, this is raised to an exponent β, a function of the ratios between the applied stresses and thresholds, which becomes 0 when the stresses are higher than the thresholds (no-threshold behavior) and is, instead, > 0 if at least one of the stresses is lower than the corresponding threshold (threshold behavior). In such a way, the improved model assumes general validity. Application of the model is outlined and compared with experimental data obtained for several insulating materials. © 1993 IEEE