A Log-Logistic Step-Stress Model

This paper presents an optimum plan for simple (two stresses) time-step-stress accelerated life tests under the log-logistic model considered appropriate for insulating materials. A log-logistic life distribution with a median that is a log-linear function of stress, and a cumulative exposure model are assumed. The optimum test plan is obtained by minimizing the asymptotic variance of the maximum likelihood estimator (MLE) of the median life at a design stress. The confidence intervals and hypotheses tests about model parameters have also been discussed.