Using equivalent failure rates to assess the unavailability of an ageing system

In this paper, the authors are interested in the calculation of some dependability measures for a system that is subjected to ageing. This system can be in two different states: the perfect operating state; and the failure state. First, the problem of the modeling of aging is discussed. Then, they study a general model and derive formulae for the unavailability and the mean number of failures over a period. For a highly available system, these formulae may be used to reduce the number of simulations necessary to assess these dependability measures. Ageing models are often really complex, and may lead to some practical problems, such as the number of parameters to be estimated. The results of this article justify the use of a simplified ageing model, involving only a few parameters. Moreover, the computation stage is much easier in this model, providing an alternative to Monte Carlo simulation. In the final section, the authors apply this simplification method to a problem of preventive maintenance optimization.