Experimental results are presented on the onset and propagation of local delaminations caused by the interaction between specimen edges and intralaminar cracks in fiber bundles of 90° layers in quasi-isotropic [–45/90/45/0]s CF/EP noncrimp fabric (NCF) laminates subjected to tension-tension fatigue loadings. It is confirmed that the first damage mode is intralaminar cracking in 90° layers, which consists of intrabundle cracks and cracks in the matrix between bundles (often beginning from stitches). This damage mode triggers cracking in off-axis layers and local delaminations in positions where the 90° layer crack meets an adjacent layer. The process of local delamination is significantly enhanced at specimen edges, where the out-of-plane edge stresses contribute to the local delamination. During cyclic loadings, delaminations grow and coalesce along the edge and propagate towards the specimen center. These processes are quantified experimentally at different levels of cyclic load. In a low-stress fatigue, a very high number of cycles is required to detect small edge delaminations, and they stay at the edge. In high-stress cyclic tests, delaminations grow faster inside the composite: about 20% of the interface in the central zone can be delaminated. It is found that the reduction in the axial modulus is proportional to the relative delaminated area, proving that delamination is the major stiffness reduction factor in these laminates.

中文翻译：

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疲劳载荷下准各向同性 CF/EP NCF 复合材料层内裂纹与试样边缘相互作用引起的局部分层

实验结果展示了在准各向同性 [–45/90/45/0]s CF/EP 非卷曲织物中由试样边缘与 90°层纤维束中的层内裂纹相互作用引起的局部分层的发生和传播（ NCF) 层压板承受拉-拉疲劳载荷。经证实，第一种损伤模式是90°层的层内裂纹，它由束内裂纹和束间基体中的裂纹（通常从缝线开始）组成。这种损坏模式会触发离轴层的开裂和 90° 层裂纹与相邻层相遇的位置的局部分层。局部分层过程在试样边缘显着增强，其中平面外边缘应力有助于局部分层。在循环加载过程中，分层沿边缘生长和聚结，并向试样中心传播。这些过程在不同水平的循环负载下通过实验量化。在低应力疲劳中，需要非常多的循环来检测小的边缘分层，并且它们停留在边缘。在高应力循环测试中，复合材料内部的分层增长得更快：中心区域约 20% 的界面可以分层。发现轴向模量的降低与相对分层面积成正比，证明分层是这些层压板的主要刚度降低因素。检测小的边缘分层需要非常多的循环次数，并且它们停留在边缘。在高应力循环测试中，复合材料内部的分层增长得更快：中心区域约 20% 的界面可以分层。发现轴向模量的降低与相对分层面积成正比，证明分层是这些层压板的主要刚度降低因素。检测小的边缘分层需要非常多的循环次数，并且它们停留在边缘。在高应力循环测试中，复合材料内部的分层增长得更快：中心区域约 20% 的界面可以分层。发现轴向模量的降低与相对分层面积成正比，证明分层是这些层压板的主要刚度降低因素。