Sequential imperfect preventive maintenance models with two categories of failure modes

In this paper, we introduce the concept of two categories of failure modes: maintainable failure modes and nonmaintainable failure modes into the modeling of preventive maintenance (PM) activities. The hybrid model proposed by Lin et al. is used to model the effect of a PM activity on the failure rate function of maintainable failure modes. However, PM does not change the hazard rate function of nonmaintainable failure modes. PM is performed at a sequence of intervals. The objective is to determine the optimal PM schedules to minimize the mean cost rate. Numerical examples for Weibull hazard rates are given.