Title : Mechanical behaviour of carbon fibre stitched composite joints and composite repair
Due to their high strength and stiffness, light weight and good stability and durability, fibre-reinforced polymer composites have been increasingly used in the aerospace, automotive and marine industries. Typically, in manufacturing, large scale composite structures are assembled from a number of sub-components. For this reason, joints have become indispensable in large structures. However, when designing and manufacturing joints, stress concentrations, improper design and manufacturing processes can easily result in the degradation of the mechanical properties of the whole structure. Therefore, there is a need to continually improve the strength of joints. This paper presents a novel stitching method for enhancing the mechanical properties of secondary-bonded CFRP single- and double-lap joints, as well as composite patch repair. In this approach, holes were drilled in the secondary bonded joints and carbon fibre threads were stitched through the joints and the scarf patch repaired. The vacuum resin infusion technique was then used to integrate the threads with the joints and fix the patch to the parent part. The tensile properties of the stitched joints were measured and compared with those offered by traditional bonded (unstitched) joints to investigate the potential of the stitching method. In addition, by varying the hole diameter between 2 and 4 mm, the effect of thread size on the joint load-carrying capacity of the joints was investigated. The experimental results show that by using this stitching method, the ultimate tensile strength of the stitched single- and double-lap joints are respectively 200 % and 130 % greater than the corresponding conventional bonded joints. The finite element models have been developed to predict the mechanical response of the stitched joints, in good agreement with the experimental results. The numerical analysis shows that the stiffness degradation at the end of overlap area is held back by the stitching fibres. Also, an innovative stitch?reinforced scarf patch is developed to reduce the amount of parent material that is removed during the repair. Here, the effects of varying both the hole diameter and the scarf angle on the load carrying capacity of the repaired laminates are studied. The tensile strength, strain distribution and failure mechanisms are investigated using the digital image correlation (DIC) technique. It was found that by introducing a 2.5 mm diameter stitching hole, the ultimate tensile strength of repaired laminates related to three scarf angles is increased by up to 20, 27 and 45% respectively, relative to traditional laminates with an equivalent scarf ratio.