The fatigue-induced stress redistribution in three-dimensional (3D) braided composites, even when subjected to externally constant-amplitude fatigue loads, results in locally-variable-amplitude fatigue loading, significantly impacting their fatigue performance degradation. This paper is aimed to propose a novel fatigue damage analysis method to predict the fatigue behavior of 3D braided composites subjected to locally-variable-amplitude loading. To this end, the continuous damage method, residual strength, residual stiffness, and fatigue life models are combined to characterize the properties degradation of 3D braided composites with cycling. Once stress redistribution occurs, the fatigue properties degrade based on the new stress level, and the equivalent cycle number is recalculated accordingly. As the material undergoes different cycle blocks, damage accumulates, ultimately leading to failure when the overall stiffness of the material degrades to 80%. Traditional fatigue models are also employed to predict the fatigue behavior of representative volume elements for comparison. The results predicted by the present model shows good agreement with experiments. Furthermore, the influence of stress levels and architectures of 3D braided composites on fatigue performance is systematically discussed.

中文翻译：

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局部变幅载荷下三维编织复合材料的新型渐进疲劳损伤模型

三维 (3D) 编织复合材料中疲劳引起的应力重新分布，即使受到外部恒幅疲劳载荷，也会导致局部变幅疲劳载荷，从而显着影响其疲劳性能下降。本文旨在提出一种新颖的疲劳损伤分析方法来预测局部变幅载荷下 3D 编织复合材料的疲劳行为。为此，结合连续损伤法、残余强度、残余刚度和疲劳寿命模型来表征3D编织复合材料随循环的性能退化。一旦发生应力重新分布，疲劳性能就会根据新的应力水平而降低，并相应地重新计算等效循环数。当材料经历不同的循环块时，损伤会累积，最终在材料的整体刚度降低至 80% 时导致失效。传统的疲劳模型也用于预测代表性体积单元的疲劳行为以进行比较。本模型的预测结果与实验吻合良好。此外，系统地讨论了 3D 编织复合材料的应力水平和结构对疲劳性能的影响。