The effects of countersink depth on fatigue performance of CFRP joint

Abstract Countersink bolts are extensively utilized in aircraft connection structures due to their excellent connectivity performance. However, the countersink depth error that occurs during the manufacturing process of CFRP components is challenging to rectify. Consequently, it is imperative to investigate the impact of countersink depth error on the connection performance of the structure. This study conducted fatigue experiments on the CFRP single-lap joints with different countersink sizes, the failure of the countersink bolts and the structural components were observed. A mechanical model of the contact between the bolt cone surface and the hole wall was established to explain the failure behavior of bolts under different countersink depths. The results indicate that the fatigue life and structural stiffness of CFRP components increase with an increase in the depth error of the countersink hole. When the bolt head is recessed into the connecting plate, the stiffness of the specimen increases by approximately 19% compared to the scenario where the bolt head protrudes from the connecting plate. At the same time, it is more prone to bolt failure when the bolt head protrudes from the connecting plate. As the depth of the countersink hole increases, the extrusion stress gradually decreases, and when the bolt head makes full contact with the hole wall, the extrusion stress reduces to 421.03MPa. As the depth of the countersunk hole increases, the bending moment at the conical surface of the bolt head exhibits a trend of initially increasing and then decreasing, reaching its peak when the bolt head is flush with the upper surface of the connecting plate.