Planning of step-stress accelerated degradation test based on non-stationary gamma process with random effects

This paper discusses the design problem of the step-stress accelerated degradation test (SSADT) based on the non-stationary gamma process with random effects. The cumulative exposure (CE) model is used to link the degradation paths of the SSADT under different stress levels. The expectation maximization (EM) algorithm is applied to estimate the model parameters. The purpose is to design an optimal experiment plan by minimizing the asymptotic variance of the estimated reliability of the product at a predesigned mission time of interest. Under the budget and boundary constraints, the design variables such as sample size, the measurement frequency at each stress level, and the number of measurements at each stress level are obtained. In the end, an example about the light emitting diode (LED) chip is used to illustrate the proposed model.