Fatigue characteristics of jagged steel pin-reinforced composite hat-joints under pull-off loading

This study focuses on the reinforcement effect provided by surface-treated jagged pins on the pull-off fatigue strength of composite hat-joint structures intended for the joining area of stiffened sections in aircraft composite fuselages under at room temperature. The study utilizes pin diameters of 0.3 mm, 0.5 mm, and 0.7 mm, along with pin-areal-densities of 0.5% and 2%, as the test parameters. The investigation involved subjecting the joints to cyclic pull-off loading. Additionally, to evaluate the reinforcing impact of jagged pins, a comparison was made with a hat-joint reinforced by a standard, untreated pin. The results revealed a significant enhancement in pull-off fatigue strength compared to non-reinforced hat-joints. At a pin diameter of 0.3 mm and a pin-areal-density of 2%, the maximum improvement observed was 141%. Furthermore, the pull-off fatigue strength achieved by the jagged pins was 52% higher than that of regular pins. These test outcomes lead to the conclusion that the implementation of jagged pin reinforcement can substantially improve the pull-off fatigue strength of composite hat-joint structures.