The paper refers to the evaluation of the unavailability of systems made by repairable binary independent components subjected\r\nto aging phenomena. Exponential, exponential-linear, and Weibull distributions are assumed for the components failure times.\r\nWe assume that components failure rate increases only slightly during the maintenance period, but we recognize the effectiveness\r\nof preventive maintenance only in presence of aging phenomena. Importance measures allow the ranking of the input variables.\r\nWe propose analytical equations that allow the estimation of the first-order Differential Importance Measure (DIM) on the basis\r\nof the Birnbaum measures of components, under the hypothesis of uniform percentage changes of parameters. Without further\r\ninformation than that used for the estimation of ââ?¬Å?DIM for components,ââ?¬Â ââ?¬Å?DIM for parametersââ?¬Â allows considering separately the\r\nimportance of random failures, aging phenomena, and preventive and corrective maintenance. A two-step process is proposed for\r\nthe system improvement, by increasing the components reliability and maintainability performance asmuch as possible (within the\r\napplicable technological limits) and then by optimizing preventive maintenance on them. Some examples taken from the scientific\r\nliterature are solved in order to verify the correctness of the analytical equations and to show their use.
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