Choice of the best time scale for system reliability analysis

Several time scales (total operation time, the number of high and load operation cycles, the number of low load operation cycles, their linear combinations, etc.) are compared. Examples of test and field data are analyzed and it is shown that it is possible to find a linear combination of different scales which guarantees a considerable decrease in the coefficient of variation (c.v.) of the system lifetime. It means that, using this ‘optimal’ scale, an accurate reliability prediction can be done for a wide spectrum of loading conditions. A formal model for selecting an optimal (‘load invariant’) time scale is described. In this scale the ∈-quantiles of lifetime under various loadings are maximally ‘compressed’ in terms of their c.v. ‘Artificial’ time scales based on damage counters are suggested and some aspects of their use are discussed.