Experimental analysis of static and fatigue performance of hybrid double-lap joints in thin primary metallic aircraft structures

This study investigates and compares the static and fatigue performance of double-lap joint repairs applied to primary thin aircraft structures assembled using traditional fastening, pure bonding, and hybrid methods. Baseline configurations, representing optimally assembled joints with inherent designed strength, were compared to configurations with simulated defects, designed to replicate practical flaws in the bond region that may remain undetected by current non-destructive inspection techniques. Specimens were subjected to static and fatigue tests under controlled loading conditions, with strain gauges and optical camera recordings to provide real-time monitoring of strain distribution and crack growth. Hybrid joints, combining adhesive bonding and mechanical fastening, exhibited superior strength and stiffness compared to mechanically fastened joints, and comparable performance to purely bonded joints under optimal conditions. In the studied defective scenarios, hybrid joints restored up to four times the strength of their purely bonded counterparts. Fatigue tests demonstrated that hybrid joints outperformed mechanically fastened joints, achieving twelve times the fatigue resistance and, exhibited a 10% improvement over purely bonded configurations when normalised to a common strain level. This advantage was particularly evident in scenarios involving undetectable bondline defects that may arise in practice, where hybrid joints effectively safeguarded purely bonded configurations from abrupt failures and significantly improved their fatigue resistance. Visual inspections and strain measurements further confirmed that the inclusion of rivets played a crucial role in suppressing Mode I opening and peeling stresses, thereby arresting crack growth and enhancing joint durability. These findings highlight the potential of hybrid joints to improve the durability and safety of thin aircraft structures, offering significant cost savings and enhanced operational availability. This study presents part of an assessment programme to certify bonded repairs on thin primary metallic aircraft structures.