Fatigue damage monitoring of hybrid step-lap joints in thick primary metallic aircraft structures using distributed optical fibre sensors

Hybrid joint configurations, which combine adhesive bonding with mechanical fastening, enhance joint strength in primary aircraft structures by leveraging the strengths of both methods. However, they also inherit the quality control challenges of adhesive bonding, particularly in ensuring bond integrity over time. Current non-destructive inspection techniques are limited in reliably detecting weak bonds in hybrid and bonded joints under real-world conditions. This study investigates the behaviour of hybrid and bonded joints with simulated bondline defects that may occur in practice. Distributed optical fibre sensors (DOFS) are employed to provide real-time strain monitoring while capturing disbond initiation and propagation across the overlap region. Findings show that fasteners in hybrid joints delayed crack initiation and slowed disbond growth by reducing Mode I opening and peeling stresses, leading to gradual strain increases and preserving residual strength. Thus, offering a window for maintenance before complete failure. In contrast, bonded joints with similar defects exhibited abrupt failure due to adhesive brittleness. Finite element analysis of the bonded configuration with a degraded adhesive was used to qualitatively validate experimental strain trends. Statistical analyses of the numerically simulated strains along the fibre and joint, including Root Mean Square Error (RMSE), Pearson correlation coefficient, and Maximum Absolute Error (MAE), were conducted to evaluate DOFS reliability. Results indicated a strong correlation between the two, though measurement precision slightly declined for larger disbonds. This study highlights how hybrid joints can enhance the structural durability of bonded step-lap repairs in thick metallic structures when concealed defects are present in the bondline, while establishing DOFS as a robust tool for real-time monitoring and proactive maintenance. This research is part of a certification assessment program for hybrid joining repairs in primary metallic aircraft structures.