Abstract: The state of most systems after maintenance is somewhere between as good as new and as bad as old. Based on the imperfect maintenance theory, a new improvement factor considering the equipment working age and the maintenance cost is introduced. In order to make the policy more suitable in operation, the concept of phasic sequential preventive maintenance is put forward. By minimizing the cost rate of the maintenance in the life cycle of the system, two sequential preventive maintenance models are proposed, the general sequential one and the phasic sequential one. The optimal preventive maintenance intervals and the number of preventive maintenance activities are obtained from the model. The numerical results show that the phasic sequential preventive maintenance policy is more operable in the actual maintenance activities and more appropriate to the properties of the deteriorating systems. Compared with the general model, the phasic one achieves a better economical efficiency and operability at the same maintenance cost.

Key words: Deteriorating system, Effective age, Imperfect maintenance, Improvement factor, Phasic sequential preventive maintenance