Multi-factor ageing models-origin and similarities

This paper presents a critical assessment of principal ageing models for combined thermal and electrical stress. The models of Simoni, Ramu, Fallou, and Crine are commonly referred to in the literature, each model representing different semi-empirical or (quasi-) physical approaches. The models are compared to emphasise differences and common features. It is shown that Ramu´s model and Fallou´s model can be represented by Simoni´s model. Hence special importance should be attached to Simoni´s model and Crine´s model. Both models are examined from the point of view of their common thermodynamical origin. Simoni adjusts a thermodynamic expression to fit with the well-known relations for single thermal and single electrical ageing. In this way Simoni obtains an expression for time-to-failure or life at combined thermal and electrical stress which is the product of the single stress lives and a correction term accounting for the synergy effects. Crine applies thermodynamics to describe the complete ageing process as a single crossing of an energy barrier which is deformed by an electrical field. The different interpretation of the multi-factor ageing process presented by Simoni´s model and Crine´s model is discussed in the paper