近年來因高科技產品愈來愈普及，一旦在使用中突然發生故障所導致的損失也愈來愈嚴重，因此適當的對產品進行維護與修理，以降低可能帶來的損害，日益受到重視。本論文提出兩種廣義的預防維修置換模型加以分析。
其一是探討廣義的逐次預防維修策略，假設系統因衝擊而導致失效。失效的處理分為小修理及維修，視嚴重情況而定。當經過多個維修週期(包含預防維修)後，系統即進行置換。
另一則是探討廣義的k-out-of-n:F系統置換策略，假設系統由n個元件所組成，系統因衝擊而導致元件失效。若失效的元件數目小於k個，則進行小修理，當同時壞掉k個以上元件時即進行置換。另外有考慮預防置換時間T。以上兩個策略分別都考慮到失效的嚴重程度並非與時間相關，而是與失效次數相關。同時也都考慮較一般化的成本結構。兩者都有推導出相關的長期每單位時間成本函數，也都有探討最佳解存在時的條件。本研究可作為類似系統或產品在訂定維修置換策略時的參考。

Since the high technology product is more popular recent year, the damage conducted by the failure of a product in use is more serious. To study the maintenance and replacement policies in order to reduce the operating cost and the risk of a catastrophic breakdown is more important. In this research we study two generalized maintenance and replacement policies.
The first is a generalized sequential preventive maintenance policy of a system subject to shocks. Shocks can make a system breakdown. Upon the arrival of a shock, the system is maintained or repaired minimally depending on the order of severity. After several maintained periods (including preventive maintenance), the system is replaced.
The second is to determine an optimal replacement period for a k-out-of-n:F system subject to shocks. A k-out-of-n:F system consists of n components and fails if at least k of the n components fail. If a shock makes the system''s failure components number less than k, the system is minimal repaired. Otherwise the system is replaced. Furthermore, we consider the preventive replacement age T. The above two policies all consider the order of severity is depending on the number of failure. Moreover, both consider more generalized cost structure, derive the long run average cost functions and study the conditions for optimal solution. This research can be a reference for setting maintenance and replacement policies to analogous product and system.