Effect of Patch and Substrate Material on the Fatigue Life and Failure Modes of Adhesively Bonded Composite Patch Repairs

ABSTRACT Repeated loading can lead to crack initiation and progressive damage in structural components. Adhesively bonded composite patch repair has emerged as an effective technique to reduce crack growth without introducing additional stress concentrations. However, the combined influence of patch and substrate material selection on fatigue performance and failure mechanisms remains insufficiently characterized across aluminum and steel substrates. This study investigates the fatigue performance of adhesively bonded composite patch repairs across various substrate and patch configurations. Aluminum 6061 and A36 steel serve as substrate materials, while glass‐fiber/epoxy and carbon‐fiber/epoxy composites are used as patch materials. Different patch and substrate thicknesses are utilized to assess their effects on fatigue behavior. All repaired configurations are subjected to cyclic loading, and the resulting fatigue behavior is interpreted through failure mode observations and substrate‐to‐patch rigidity ratio analysis. The experimental results were further interpreted using substrate‐to‐patch rigidity ratio analysis together with crack initiation and crack propagation analyses, providing additional insight into the observed fatigue behavior. Critically, the findings reveal that the most effective patch material depends on the substrate: glass‐fiber patches provide superior fatigue life improvement for aluminum substrates, whereas carbon‐fiber patches provide greater improvement for steel substrates. This trend has not been previously demonstrated through systematic experimental comparisons across different substrate materials. Consequently, this study provides experimentally validated guidance for selecting composite patch materials based on the substrate material, with direct implications for structural repair design in multi‐material engineering applications.