許多學者以不同的角度去分析複材複雜的疲勞累積行為，如單純以疲勞週期數，也有以材料性質的改變(如勁度、強度，..)為量測方法。且由Clark, et al.提出指數函數可修正乘冪函數的剪力模數遞減模式，且成功預測三明治複材疲勞行為，吾人延伸此觀念修正Yang的勁度遞減模式，用以預測Graphite/Epoxy複合層板的殘餘勁度與疲勞壽命，以驗證模式的可行性。
　　複材受疲勞作用力，尤其是二元應力階後之疲勞行為及如何準確預測其疲勞累積損傷，一直是十分重要的課題。而要探討複材受二元應力階的疲勞行為，受力順序效應對複材的影響，就必須確實了解。本論文將以殘餘勁度遞減模式建立二元應力階的疲勞理論模式，用以預測複合材料受等振幅疲勞外力後的殘餘勁度降低情形。透過等效損傷效應，導出殘餘勁度遞減模式的受力順序理論，配合破壞應變準則，建立疲勞壽命損傷模式，並與不同的累積損傷模式比較複材受二元應力階的累積損傷行為。

　　Many scholars analyzed the complicated cumulative fatigue composite behaviors by different angles, such as simply in terms of fatigue cycles, or in terms of the change of material natures (stiffness, strength…). Clark, et al. modified the power-law shear modulus degradation model and suggested the exponential function can be effectively used instead of power function, and succeed in predicting the fatigue behaviors of sandwich composite. We extend this concept to modify Yang’s residual stiffness degradation model, then prove the validity of predict the residual stiffness and fatigue life of composite.

　　Composite laminates subject dual-step loading and how to predict the fatigue behaviors accurately, it has been an important subject all the time. However, as long as the effects of loading sequence of composite subjected to fatigue loadings is accounted, the aforementioned fatigue models can predict the fatigue behavior of composite under two-step loading definitely and completely. In this research, a mathematical model of composite laminates under dual stress levels established based on residual stiffness degradation. It is, then, used to investigate the loading sequence effect of Graphite/Epoxy composite laminates under fatigue loadings. Though the equivalent damage model, we can derived the loading sequence theory of residual stiffness degradation model, then co-operate with failure strain criterion, established the fatigue life damage model, and comparing experimental test results in three cumulative damage models.

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