Abstract

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.

Highlights

  • Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current CFRP aircraft fuselage structures. Traditional joint topologies such as single overlap joints (SLJ) induce high peel stresses into the composite adherends thickness direction, resulting in sudden failure and low joint strength when compared to metal adherends [1,2]

  • This study aims to explore the effect of a multi-stacked finger joint topology in comparison with a conventional single overlap joint topol­ ogy on the tensile strength of composite bonded joints

  • 8 different topology configurations are studied under quasi-static tensile loading

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Summary

Introduction

Adhesive bonding is one of the most suitable joining technologies in terms of weight and mechanical performance for current CFRP aircraft fuselage structures. Traditional joint topologies such as single overlap joints (SLJ) induce high peel stresses into the composite adherends thickness direction, resulting in sudden failure and low joint strength when compared to metal adherends [1,2]. Current safety-critical bonded joints are always used in combination with redundant fasteners. This practice jeopardizes the weight-efficiency of full-scale composite structures, where joints are essential. Compared to the traditional SLJ-design, mainly chosen for its easi­ ness of manufacturing [3], finger joints (FJ) could be a promising alternative to increase joint strength due to a more gradual load transfer to the composite adherends as they lead to lower peel stresses [1,3].

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