FFOP prediction method based on bathtub-shaped failure rate function

Failure free operating period (FFOP) is defined as a period during which no failures resulting in a loss of system functionality occur. First, the concept of FFOP and the difference between FFOP and mean time between failures (MTBF) are introduced. Three hypotheses are held as the preconditions of the presented methodology. First, the fault event is a Possion process. Second, the failure rate function is shaped like a bathtub curve. Third, constant interval scheduled maintenance is permitted during which the product must maintain trouble-free function. A modified Weibull distribution function is used to model the bathtub-shaped failure rate function. Furthermore, based on the probability and stochastic processes theory, FFOP prediction algorithm and procedure are developed, whose accuracy is verified through simulation. Finally, an actuator is selected as the sample case to validate the feasibility of the proposed method. The result shows the FFOP is correlated with maintenance free operating period (MFOP) and the predefined confidence coefficient. Shorter MFOP will deliver both higher maintenance and operating costs and longer FFOP. In engineering practice, a tradeoff between FFOP and maintenance and operation cost should be considered.