Mechanical performance of hybrid step-lap joints in thick primary metallic aircraft structures

This study investigates the static and fatigue performance of bonded, mechanically fastened, and hybrid step-lap metallic joints in primary thick aircraft structures, focusing on both baseline configurations, representing optimally assembled joints with designed inherent strength, and those assembled with predefined bondline defects to simulate practical imperfections, that can go undetected by current non-destructive inspection techniques. The results demonstrate that hybrid joints, which integrate fasteners with adhesive bonding, significantly enhance static and fatigue failure resistance compared to traditional methods. While purely bonded joints nearly restore original stiffness, they remain prone to abrupt failure, particularly in the presence of undetectable bondline defects. In contrast, the hybrid joints tested in this study extended the fatigue life of the structure to more than nine times that of mechanically fastened joints, surpassing the aircraft’s service life by over twofold. The inclusion of fasteners effectively arrested crack propagation, preventing catastrophic failure and improving overall durability. Visual inspections, strain gauges, and optical monitoring confirmed the bolts’ role in reducing Mode I opening and peeling stresses. These findings underscore the potential of hybrid joints to enhance the durability and safety of thick aircraft structures, leading to significant cost savings by reducing the frequency of repairs and downtime.