Abstract The stress ratio is commonly known to have an effect on the fatigue delamination growth (FDG) behavior of composite laminates. The crack closure phenomenon during delamination growth may change the value of stress ratio. Whether the stress ratio effect on the FDG behavior is resulted by the crack closure or not is investigated in this study and experimental evidences are presented in order to obtain a reliable conclusion. Using the quasi-static unloading approach, the crack closure loads during fatigue tests are firstly determined. The effect of crack closure on the mode I FDG in multidirectional composite laminates, which are widely used in engineering field, was experimentally studied. Test results show that the crack closure has an obvious effect on the minimum fatigue load and a linear relationship exists between the crack closure load and the delamination length. However, the influence of crack closure on the FDG rate is negligible. Further discussions illustrate that the crack closure does not fully explain the stress ratio effect in composite multidirectional laminates with large-scale bridging fibers.

中文翻译：

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大尺度纤维桥接复合多向 DCB 层压板疲劳分层中的裂纹闭合

摘要 众所周知，应力比对复合材料层压板的疲劳分层增长 (FDG) 行为有影响。分层增长过程中的裂纹闭合现象可能会改变应力比值。本研究研究了应力比对 FDG 行为的影响是否由裂纹闭合引起，并提供实验证据以获得可靠的结论。使用准静态卸载方法，首先确定疲劳试验期间的裂纹闭合载荷。实验研究了裂缝闭合对工程领域广泛应用的多向复合材料层合板中I型FDG的影响。试验结果表明，裂纹闭合对最小疲劳载荷有明显影响，裂纹闭合载荷与分层长度呈线性关系。然而，裂纹闭合对 FDG 速率的影响可以忽略不计。进一步的讨论表明，裂纹闭合并不能完全解释具有大规模桥接纤维的复合多向层压板中的应力比效应。