The Design Approaches Of Step-stress Accelerated Degradation Tests Based On Gamma Processes

There are many highly reliable and long life products in the area of aerospace, electronic and mechanic, whose performance characteristics degrade by time. The cumulative degradation of the performance characteristics result in the degradation of products'functions until their failure. It exists a lot of credible and accurate information about the life of products during the degradation of products. It can make up the shortage of tranditional lifetime prediction theory and provide a new reliability test and evaluation technology by modeling the degradation path for the performance characteristics.Due to the rigorous limitation to test time of products, we have to use accelerated degradation test (ADT) to quicken the degrading rate of performance characteristics. Then, we use the degradation data under the high stress levels to predict the product lifetime under the normal conditions. Comparing with constant and progressive stress accelerated degradation test, step stress accelerated degradation test (SSADT) can reduce sample size, save test cost and have strong operability. Therefore, SSADT has wide range of applications. But now, most of the research work are based on Wiener processes. In fact, the degradation pathes of some products have non-negative degradation increments and strictly monotonic characteristics, which are best considered as gamma processes. At the same time, the dagradation of performance characteristics may be affected by more than two stresses, and degradation model may be misspecified because of lacking the knowledge of products'mechanisms. These problems are challenging to optimize the plans of SSADT.This thesis focuses on the highly reliable products with non-negative increments and strictly monotonic degradation path and investigates the issues on the optimal design of SSADT based on gamma processes. The main contents are as follows:(1) The fundamental theory of SSADT design is analyzed, including the thought of SSADT model, the basic framework and key elements of SSADT design.(2) The optimal design approaches of SSADT based on gamma processes are discussed. For the highly reliable products, we investigate the corresponding SSADT model, the optimal models of SSADT design and the algorithms for solving the models under single stress and multiple stresses accelerating scenarios. We also analyze the effects on test plans of the changes of model parameters and stress levels.(3) The approaches for analyzing the effects on plans of SSADT due to misspecification of degradation model are discussed. Particularly, we discuss the problem about misspecifying gamma degradation process as Wiener degradation process in detail. The corresponding optimal design of SSADT and the comparisons of experimental results are given.