Data underlying the research: On the influence of overlap topology on the tensile strength of composite bonded joints: Single overlap versus overlap stacking

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.

本研究旨在探索胶接复合材料搭接接头（adhesively bonded composite overlap joints）的新型拓扑结构，以提升其在拉伸载荷下的力学强度。将多层堆叠搭接结构（亦称指形接头（finger joints））与单层搭接拓扑结构开展对比研究：针对搭接长度分别为12.7 mm与25.4 mm的单搭接接头，以及搭接长度恒定为12.7 mm、具备1层和2层堆叠搭接的指形接头，对比其准静态拉伸力学性能。

每种拓扑结构均测试了两种复合材料被粘物铺层序列：[0/90]4s与[90/0]4s。本研究开展非线性有限元分析（non-linear FE-analysis），以分析胶接界面处的剪切应力与剥离应力分布。研究发现胶接区域端部存在峰值剪切与剥离应力的差异：(i) 对于1层指形接头，其峰值剥离应力高于单搭接接头构型，这是由于避免偏心带来的有益效应，被搭接区域内被粘物刚度减半的不利影响所抵消；(ii) 相较于单搭接拓扑结构，2层指形接头的应力场发生显著变化，峰值剪切应力降低23%，峰值剥离应力降低33%。

此外，本研究开展搭接剪切试验并采用声发射（acoustic emission）技术进行监测，以追踪接头内部的损伤演化事件。研究认为，损伤起始与最大载荷处的不同演化趋势，源于损伤在接头内部的扩展路径差异。其中，具备2层指形结构且铺层为[90/0]4s的接头，其损伤完全发生在被粘物内部，该构型拥有最低的峰值剪切与剥离应力，以及最高的损伤起始载荷。然而，其最大承载载荷却低于铺层为[0/90]4s的单搭接接头构型——后者多发生内聚破坏（cohesive failure）。

进一步研究发现，与单搭接拓扑结构不同，指形接头内部损伤起始的主导应力分量为对接接头树脂凹坑处的面内拉应力，而非搭接区域的剥离应力。最后，若以重量效率为核心设计需求，指形接头设计可有效替代单搭接接头设计。但若追求接头的绝对最大承载强度，单搭接接头仍为更优选择。