Condition-based maintenance for continuously monitored degrading systems with multiple failure modes

This article develops an optimum Condition-Based Maintenance (CBM) policy for continuously monitored degrading systems with multiple failure modes. The degradation of system state is described by a stochastic process, and a maintenance alarm is used to signal when the degradation reaches a threshold level. Unlike existing CBM models, this article considers multiple sudden failures that can occur during a system's degradation. The failure rate corresponding to each failure mode is influenced by either the age of the system, the state degradation of the system, or both. A joint model is constructed for the statistically dependent time-to-maintenance due to system degradation and time-to-failure of different failure modes. This model is then utilized to obtain the optimum maintenance threshold level that maximizes the system's limiting availability over its life cycle or minimizes the long-run cost per unit time. A numerical example, using real-life data from a reliability test of communication systems, is provided to demonstrate the application of the proposed approach.