Abstract: The technology of accelerated testing (AT) with competing failure modes is the foundation of application extending from products with simple structures to products with complex structures. However, most of the methods proposed in relevant literature are for constant stress AT, and there is scarcely any study concerning methods for step stress AT. This article proposes a model and a method for analyzing a common type of step stress AT with competing failure modes in which both hard failures and soft failures exist. Full consideration is given to the incompleteness of testing data, which may be due to censoring as well as masking that occurs when the failure time is observed, but its corresponding failure mode is not identified. Then maximum likelihood estimation (MLE) based on expectation maximum (EM) algorithm and pseudo-lifetime MLE are proposed for analyzing respectively the hard failure modes and soft failure modes. Finally, case studies are performed to check the validity of the model and the method.

Key words: accelerated testing, statistics, competing failure mode, step stress, incomplete data, expectation maximum algorithm