Abstract:

In this paper we obtain the optimum time for changing the stress level in 3-step, step stress accelerated life testing under the cumulative exposure model. The lifetimes of test units are assumed to follow a two-parameter Lomax distribution. The scale parameter of the Lomax failure time distribution at constant stress level is assumed to be a log-linear function of the stress level. We also assume that there may exist a quadratic relationship between the log mean failure time and stress. As an extension of the results for the linear model, the optimum plan for a quadratic model is also proposed. We derive the optimum test plans by minimizing the asymptotic variance of the maximum likelihood estimator of the mean life at a design (use) stress.