Life estimation using damage equalization method and step-stress breakdown tests

Abstract

Conventional constant-stress accelerated ageing tests are time consuming, cumbersome and suffer from the disadvantage that some samples may not fail even after a long time. This necessitates censoring the data for life estimation, increasing the complexity of data analysis and leaving some amount of uncertainty due to censoring. Step-stress breakdown tests for life estimation were already proposed in the literature. Complex Weibull statistics were resorted to for life estimation. In this paper the authors present a different data analysis procedure i.e. damage equalization method for estimating the life of power equipment. Inverse power law model is used in conjunction with Miner's cumulative damage theory. Also, the procedure suggested is much simpler than the previous procedures, which involve solving a complex maximum likelihood function for which non-linear techniques like Newton-Raphson method is required. The approach is further compared with existing techniques of life estimation and found to be matching well. Life estimation of cables supplied by a local manufacturer for dc voltage applications has been performed. Several types of breakdown experiments were conducted for cables, with different step-size of step-stress accelerated ageing tests, in authors' laboratory, at two distinct temperatures. In addition, tests were also conducted on low density polyethylene samples.