A general phenomenological life model for insulating materials under combined stresses

In this paper, the general phenomenological life model proposed by the author for insulating materials with both a threshold or non-threshold behaviour, is applied to experimental data pertinent to two insulating materials for power cables. The first material, EPR, was found to be a non-threshold material for stresses close to or larger than the service stress, but extrapolation at low temperature seems to show a tendency toward a threshold. Application of the general model does appear very appropriate for this insulation and a good fit to the data has been found. The case of the second material, an old generation of XLPE, is different and the general model does not fit the data. However, an excellent fit is reached by introducing some improvements to the model, where possible and correct. Further complications seem necessary to conform this model to various practical cases, and make its application more difficult. For practical applications, such as selection of materials and/or design of apparatus, the use of properties and indices obtainable by means of long tests, such as those suggested and carried out by the author´s research group, is simpler and more rapid. However, the usefulness of the model also consists in studying the various processes occurring during combined-stress aging of materials, all of which the model can take into account by simple mathematical expressions. This constitutes a significant advance in approaching the complex physical mechanisms involved in aging