Reliability assessment of systems operating in variable conditions

The performance and reliability of a system are strongly influenced by its operating conditions. However, existing reliability models are unable to calculate the reliability of a system operating in variable conditions. The paper presents a new approach to this problem and discusses two typical cases: one item operating sequentially under different conditions, where the transition points are deterministic; a system consisting of two parallel subsystems sharing a load, where the failure rate of each subsystem is dependent on the load it shares. We show that if the underlying failure time distribution of the item (or subsystem) can be represented by the accelerated failure time model (AFTM), the system reliability at any time can be obtained easily by using the formulas derived in the paper