System repairs: When to perform and what to do?

In this paper an indicator is used that reflects the 'state' of a system as a function of the ages of the (groups of) subsystems of which it consists. The contribution of the ages of the subsystems to the state of the system is defined by their weights. The indicator can be interpreted as the virtual age of the system, and can therefore be used to define age-reduction factors of different types of repair in a virtual age or age-reduction process. The state indicator is used as the time scale in a proportional intensity model. In this way, the joint impact of different repair strategies and covariates on the system failure intensity can be evaluated. This relationship is then used to address the question of which subsystems to replace whenever a system comes in for repair and when to set the preventive inspection/repair interval, in order to minimize the expected costs per unit time until the next inspection and/or repair. A numerical and a practical example are given