A hybrid age-based maintenance policy for heterogeneous items

In age-based replacement, an item is replaced at failure and also preventively replaced when the item reaches age T . A block replacement policy replaces an item on failure and at times kΔ, k = 1, 2, . . . regardless of the age of the item (Barlow and Proschan, 1965). The presumption here is that an item is located in a socket (in a system) and together the item and socket perform some operational function within the system (Ascher and Feingold, 1984). These models are appropriate for items that possess an increasing failure (hazard) rate function.