Novel repair of bolted composite joints using 3D printed continuous fibre patches with custom fibre paths

Extending the service life of composite structures often involves various repair techniques, particularly for thermoset composites, which require specialised approaches. Given the rising significance of bolted composite joints in assembling composite structures, evaluating their repairability has become increasingly important. This study presents a novel approach for repairing deformed bolt holes in mechanically fastened thermoset composite plates, which are commonly considered as non-reusable under service conditions. The approach involves utilizing 3D printing techniques to custom-fabricate bespoke continuous carbon fibre patches, with specifically tailored shapes, to restore bolt holes in thermoset material systems to their original dimensions and functionality. Two repair configurations were proposed to investigate the enhancement of mechanical performance. This customized solution not only recovers mechanical properties to a certain degree but also significantly enhances its resistance to initial damage, specifically increasing the initial strength by up to 60.79% and the initial fracture energy absorption by up to 205.01%, compared to the original specimen. A multi-scale finite element (FE) model was applied to illustrate post-repair failure mechanisms, incorporating the LaRC05 criterion for predicting intralaminar failure and a cohesive model for simulating interlaminar failure. Furthermore, comparative analysis through mechanical tests, X-ray micro-computed tomography (micro-CT) characterisation and finite element (FE) modelling demonstrates that the continuous fibre repair patch, designed based on finite element analysis, significantly outperforms simpler, geometrically based paths in overall repair efficacy. This improvement is achieved by strategically designing the fibre to endure varying stress conditions across different regions, resulting in an additional recovery of initial peak strength, ultimate strength, initial fracture energy and ultimate fracture energy absorption by 32.69%, 11.11%, 130.59% and 25.09% respectively, compared to simpler, geometrically based paths.