一個設備隨著使用年齡或使用率的增加，這個設備的失效頻率就可能會提升，而為了降低設備的失效頻率，就需要採取適當的預防保養措施來減少可能發生的失效次數，藉此減少因為設備失效而產生的各種成本。但是，預防保養的效果通常和設備的使用年齡或使用率有著高度相關性，即是隨著設備的使用年齡或使用率的增加，相同的預防保養效果可能會發生逐漸退化的情況。因此為了分析預防保養效果退化對於成本造成的影響，本文主要提出兩種預防保養效果退化的情況，一種為相同成本下預防保養效果退化，另一種則是相同的預防保養效果但成本增加，以這兩種情況建立數學模型，將期望總成本率最小化做為目標，在週期性預防保養的前提下，來探討各情況下的最佳預防保養次數以及最佳置換策略。此外，本文也透過數值例子來與預防保養效果不退化的情況進行比較，分析退化與不退化之間的成本差異。

Usually, the failure frequency of a device may increase as the age or usage increases. Therefore, appropriate preventive maintenance actions are required to reduce the number of possible failures and to reduce the costs incurred by the device. However, the effect of preventive maintenance is highly correlated with the age or usage of the device. The same preventive maintenance action may have less effect as the age or usage of the device increases. Therefore, in order to analyze the impact of degrading preventive maintenance on the maintenance policy, this paper proposes two situations of degrading preventive maintenance and establishes mathematical model to find to optimal policy such that the expected cost rate is minimized. Under periodical preventive maintenance, we will investigate optimal number of preventive maintenance and replacement policy. Moreover, a comparison of the optimal replacement policies with and without degrading effects is conducted through numerical examples.

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