The goal of this study is to explore new topologies for adhesively bonded composite overlap joints in order to improve their strength under tensile loading. Multiple stacked overlaps, also referred as finger joints, are compared with single overlap topologies. The quasi-static tensile behaviour of single lap joints with two overlap lengths 12.7 mm and 25.4 mm are compared to finger joints with 1 and 2 stacked overlaps through thickness with constant 12.7 mm overlap length. Two composite adherend stacking sequences are tested for each topology [0/90]4s and [90/0]4s. A non-linear FE-analysis is performed to analyse the shear and peel stresses along the adhesive bond line. A difference in peak shear and peel stress, at the tip of the bonded region could be observed: (i) for 1 finger, the peak peel stress is higher than in the single lap joint configurations because the beneficial effect of avoiding eccentricity in the finger joint is outperformed by the detrimental effect of reducing to half the adherend stiffness at the overlap; (ii) for 2 fingers, the stress field changes significantly leads to a 23% decrease in peak shear and 33% in peak peel stress, compared to the single lap joint topologies. In addition, experimental lap shear tests are performed and monitored using acoustic emission technique, to follow the damage events. Different trends at damage initation and at maximum load are believed to result from how the damage propagates inside the joint. A topology with 2 fingers and layup [90/0]4s, which fails entirely inside the adherend, provides the lowest peak shear and peel stress and the highest load at damage initiation. It is however outperformed in maximum load by a single lap joint topology with layup [0/90]4s, with mostly cohesive failure. It is further found that, unlike in single overlap topologies, the most dominant stress component for damage initiation inside the finger joints is the in-plane tensile stress, at the butt joint resin pockets, rather than peel stresses at the overlap region. Lastly, if weight efficiency is the main requirement, a finger joint design can effectively replace a single overlap joint design. However, for absolute maximum joint strength, the single overlap joint is a better choice than the finger joint.

中文翻译：

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搭接拓扑结构对复合材料粘接接头抗拉强度的影响：单一搭接与搭接堆叠

本研究的目的是探索粘合复合材料搭接接头的新拓扑结构，以提高其在拉伸载荷下的强度。多个堆叠重叠，也称为指状关节，与单个重叠拓扑进行比较。将具有两个重叠长度 12.7 毫米和 25.4 毫米的单搭接接头的准静态拉伸行为与具有 1 和 2 次堆叠重叠厚度且重叠长度恒定为 12.7 毫米的指状接头进行比较。对每个拓扑 [0/90]4s 和 [90/0]4s 测试两个复合粘附体堆叠序列。执行非线性有限元分析以分析沿粘合剂粘合线的剪切和剥离应力。在粘合区域的尖端可以观察到峰值剪切应力和剥离应力的差异：（i）对于 1 个手指，峰值剥离应力高于单搭接接头配置，因为避免手指接头偏心的有益效果优于将重叠处被粘物刚度降低一半的不利影响；(ii) 对于 2 个手指，与单搭接接头拓扑相比，应力场变化显着导致峰值剪切力降低 23%，峰值剥离应力降低 33%。此外，使用声发射技术执行和监测实验性搭接剪切测试，以跟踪损坏事件。损伤开始和最大负载时的不同趋势被认为是由损伤在关节内传播的方式引起的。具有 2 个手指和上篮 [90/0]4s 的拓扑结构，在被粘物内部完全失效，在损伤开始时提供最低的峰值剪切和剥离应力以及最高的载荷。然而，它在最大负载方面的表现优于具有叠层 [0/90]4s 的单搭接拓扑结构，主要是内聚破坏。进一步发现，与单重叠拓扑不同，指关节内部损伤起始的最主要应力分量是对接接头树脂袋处的平面内拉伸应力，而不是重叠区域的剥离应力。最后，如果重量效率是主要要求，则手指关节设计可以有效地取代单个重叠关节设计。然而，对于绝对最大关节强度，单重叠关节是比手指关节更好的选择。进一步发现，与单重叠拓扑不同，指关节内部损伤起始的最主要应力分量是对接接头树脂袋处的平面内拉伸应力，而不是重叠区域的剥离应力。最后，如果重量效率是主要要求，则手指关节设计可以有效地取代单个重叠关节设计。然而，对于绝对最大关节强度，单重叠关节是比手指关节更好的选择。进一步发现，与单重叠拓扑不同，指关节内部损伤起始的最主要应力分量是对接接头树脂袋处的平面内拉伸应力，而不是重叠区域的剥离应力。最后，如果重量效率是主要要求，则手指关节设计可以有效地取代单个重叠关节设计。然而，对于绝对最大关节强度，单重叠关节是比手指关节更好的选择。指接设计可有效替代单一重叠接设计。然而，对于绝对最大关节强度，单重叠关节是比手指关节更好的选择。指接设计可有效替代单一重叠接设计。然而，对于绝对最大关节强度，单重叠关节是比手指关节更好的选择。