Design and evaluation of a novel variable-length stepped scarf repair technique using a cohesive damage model

The advantages of Carbon Fibre Reinforced Polymers (CFRPs) are well established, but repairing CFRP components remains difficult and costly, posing challenges for industries like aerospace. This paper explores the design, modelling, inspection, and testing of a Variable Length Stepped Scarf (VLSS) repair scheme for highly loaded composite structures. A fully nonlinear 2D Finite Element Model (FEM) is used to design the VLSS repair, predict failure loads and modes, and model adhesive cohesion and delamination. The model incorporates a validated progressive damage model, general contact, and both force and geometric nonlinearities. Two manufacturing techniques involving hard repair patches and glass beads to maintain a constant bond line are employed. A 3D FEM validated against repaired composite coupons under uniaxial tension shows excellent agreement with experimental data. The static strength repair efficiency is approximately 80 % of a pristine sample, with failure displacements at 87 %, and Hooke's stiffness at 102 % of pristine laminates. Cohesive failure at adhesive overlap edges is identified as the cause of stiffness degradation, confirming experimental observations. This study contributes to both composite repair modelling and repair design optimisation.