Lifetime predictions of EPR materials using the Wear-out approach

The Wear-out approach for lifetime prediction, based on cumulative damage concepts, is applied to several ethylene propylene rubber (EPR) cable insulation materials. EPR materials typically follow “induction-time” behavior in which their material properties change very slowly until just before failure, precluding the use of such time-dependent properties to predict failure. In the Wear-out approach, a material that has been aged at its ambient aging temperature T_a or at a low accelerated aging temperature is subsequently aged at a higher “Wear-out” temperature T_w in order to cause the material to reach its “failure” condition. In the simplest case, which involves the same chemical processes underlying degradation at T_a and T_w, a linear relationship is predicted between the time spent at T_a and the time required at T_w to complete the degradation. Data consistent with this expectation are presented for one of the EPR insulation materials. When the degradation chemistry at the two temperatures is different, a linear relationship between the time spent at T_a and the time required at T_w to complete the degradation is not generally expected. Even so, the Wear-out results for a second EPR material, which has evidence of changing chemistry, are reasonably linear and therefore useful from a predictive point-of-view. The Wear-out approach can therefore be used to transform non-predictive time-dependent material property results into predictive lifetime estimates. As a final example, the Wear-out approach is applied to an EPR insulation that had been aged in a nuclear power plant environment (∼51 °C) for times up to 23 years to show its likely viability for the hundreds of years predicted at this aging temperature from accelerated aging tests on EPR insulation materials.