Time behavior and breakdown of XLPE cable models subjected to multiple stresses

Cross-linked polyethylene (XLPE) cable models obtained by a steam curing process have been investigated. These models were subjected to life tests with electrical, thermal, and multiple thermal-electrical stresses. The tests were performed by means of the progressive-censoring procedure in order to obtain information on both failure time percentiles and aging mechanisms by the study of significant properties related to breakdown. The results of multiple-stress tests are presented, and the behavior of such properties as density, melting enthalpy, and DC electrical conductivity is investigated as a function of aging conditions, and is related to the failure times obtained from each life test. It is shown that changes occur in the material during aging under multiple stresses which are not detected at the same aging times when electrical and thermal stresses are applied singly. Moreover, failure times are strongly reduced when stresses are combined, so that the electrical threshold decreases as temperature grows and seems to disappear at temperatures higher than 100°C. The appearance of heterogeneous regions in the bulk of cable models aged under multiple stresses and the time behavior of the investigated properties suggest that the breakdown is mainly due to localized phenomena, even if bulk degradation processes occur in the insulation