Abstract: A new model of time-variant reliability under uncertain loads is developed after discussing the existing methods for modeling time-variant reliability. The uncertain loading process is studied and characterized by a two-dimension description. The expressions are derived of the reliability and failure rate of components based on the probability distribution of load and strength, with and without strength degeneration. The result shows that even without strength degeneration, the reliability and failure rate of components decrease with time. When strength degenerates, reliability decreases with time more obviously, and the failure rate has the feature of a bathtub curve. On condition that the probability characteristics of load and strength are known, the reliability and failure rate of components at any time can be calculated by the model proposed. When the allowable failure rate is specified, the three periods (namely, early failure period, random failure period， and wearout failure period) can be reasonably divided according to the proposed model. Furthermore, the time for stress screening test and reliable operation life can be determined. Therefore, the model developed in this article is helpful for the life cycle management of components.

Key words: time-variant reliability, models, uncertain load, failure rate, load strength interference