Periodic-replacement models with threshold levels

The authors suggest five replacement policies where a unit is replaced at periodic times, jT(j=1,2, . . .), and the replacement cost is expensive when some number of events occurring in (0,t) is greater than a threshold level. The usual models for inspection, periodic replacement, block replacement, parallel systems, and cumulative damage can be transformed into replacement models with threshold levels. The mean cost-rate of each model is obtained, using well-known results of reliability theory. The optimum replacement time which minimizes the cost-rate of an inspection model is discussed and shown to exist uniquely