A typical hazard (failure rate) function of an item/a system within its life cycle is shown in Fig. 1, including infant mortality, useful life, and wear-out periods. Usually the infant mortality failures are related to the manufacturing and debugging processes and they have been solved before operating the item/system. Therefore, the item will experience random chance failures within its useful lifetime. Moreover, due to the aging of materials, the item may enter wear-out phase depending on the material strength and applied stresses within long-term operation.