Optimum plans for step-stress accelerated life tests at higher usage rates

The life of some products is measured by usage, e.g., mileage and cycles. In accelerated testing, these products are usually run at elevated stress levels and higher usage rates to shorten test length. The increase in usage rate may prolong or shorten usage to failure. In this paper, a model that relates life to stress and usage rate is described. For the model, this paper develops 3-step step-stress optimum test plans, which choose the optimal step-change times by minimizing the asymptotic variance of the estimate of the log mean life at a design stress and usual usage rate. The sensitivity of the test plans to preestimates is evaluated, and the results show that the plans are robust. This paper also presents the maximum likelihood estimates of model parameters and the mean life at the design stress and usual usage rate, and their confidence intervals. The proposed method is applied to estimate the reliability of a compact solenoid valve.