Fatigue crack growth in notched 2024-T3 aluminum plate repaired with composite patch

Aircraft are subject to fatigue damage during navigation, which can compromise structural integrity over time. The adhesively bonded composite patch repair technique has emerged as an effective solution for military aircraft and has recently gained traction in the commercial aviation industry. This study investigates the impact of composite patch repairs, using carbon/epoxy patches, on fatigue crack propagation (FCP) in 2024-T3 aluminum alloy. The performance of double-patch repairs on notched aluminum panels was evaluated and compared to single-patch repairs. Results demonstrate that double-sided patch repairs nearly doubled the fatigue life improvement compared to single-sided repairs, significantly outperforming unpatched specimens. Patch repairs enhanced fatigue life by reducing crack growth rates by up to 50% and increasing cycles to failure by 2–3 times, depending on repair configuration. Furthermore, parametric studies revealed that a stiffness ratio of S≈1 (optimized via Rose’s equation) maximized load transfer efficiency, while higher stress ratios (R = 0.4) extended fatigue life more effectively than lower ratios (R = 0.1). Smaller notch diameters (e.g., D = 10 mm) further prolonged service life by mitigating stress concentrations. Fatigue life predictions using the NASGRO equation corroborated experimental trends, confirming the durability of patch repairs under variable stress conditions.