A model for the integrity assessment of ageing repairable systems

The failure rate of mechanical repairable systems that deteriorate with time due to ageing can usually be visualized by a bathtub curve. This study shows that an equation that is valid in other respects for describing creep curves can easily be deduced from functional forms of the failure rate of mechanical repairable systems. Creeping pieces can be considered repairable systems that evolve under an applied load, as combining positive and negative feedback loops. More generally, this can be extended to mechanical repairable systems, the negative feedback loops corresponding to repair and overhaul operations. The equation describing creep curves reflects the ageing of mechanical repairable systems. A critical time at which the system can no longer be restored to full performance, in spite of repair and/or replacement of subparts, can then be predicted. An application example is given using published failure data corresponding to a submarine main-propulsion diesel engine. The proposed model should apply every time that mechanical system ageing is expressed by a bathtub curve