Design of accelerated life tests under cyclic-stress loading

Abstract

Accelerated life tests (ALTs) have been frequently used in industry to evaluate the reliability of products within an affordable amount of time and cost. Before conducting an ALT, such factors as stress conditions, total number of test units, and proportions of units allocated to the chosen stress conditions should be carefully determined in advance to efficiently estimate the reliability-related quantities at the use condition. Most of the previous works on designing ALT plans are concerned with the case of constant-, step- or progressive-stress loading, which may not be adequate for products that operate under cyclic-stress loading at the use condition. For such products, it is desirable to adopt cyclic-stress loading also at the accelerated condition to induce the same failure mechanisms that occur at the use condition. In this thesis, optimal and compromise ALT plans are developed when cyclic-stress loading is employed. The lifetimes of test units are assumed to follow either a lognormal (Chapter III) or a Weibull (Chapter IV) distribution. For both cases, Type-I censoring as well as complete observation is assumed. As an optimization criterion, the asymptotic variance of the maximum likelihood estimator of the q-th quantile of the lifetime distribution at the use condition is adopted. In particular, the common floor level of, and the proportions of units allocated to two (for optimal plans) or three (for compromise plans) cyclic-stress conditions are determined and tabulated for various combinations of model parameter values, and the influences of these parameters on the optimal and compromise ALT plans are identified. In addition, a method for determining the sample size is developed under the constraint on the precision of the estimator of q-th quantile of the lifetime distribution at the use condition. Furthermore, the optimal and compromise plans are compared in terms of statistical efficiency. The computational results indicate that the proposed comp...