Fatigue tests and design of CFRP-strengthened CHS gap K-joints

This paper presents an experimental study on the fatigue behaviour of circular hollow section (CHS) gap K-joints strengthened by carbon fibre-reinforced polymer (CFRP) sheets (CFRP-CHS K-joint). Eight CFRP-CHS K-joints and three reference un-strengthened CHS gap K-joints were tested under cyclic axial loading. Fatigue failure modes of the CFRP-strengthened and un-strengthened joints were determined and compared. Fatigue lives in three fatigue failure criteria were recorded: crack-initiation fatigue N₂, through-thickness fatigue N₃, and end-of-test fatigue N₄. The comparisons indicate that CFRP-CHS K-joints have a longer fatigue life than un-strengthened joints in the same nominal stress range, regardless of fatigue failure criteria (N₂, N₃, or N₄). The increase in N₃ was approximately 138%, on average. No significant difference in fatigue data in terms of hot spot stress range was found between the CFRP-strengthened and un-strengthened joints. Based on the experimental fatigue data, a modified S_r,hs-N curve based on the CIDECT recommended curve was proposed for CFRP-CHS K-joints.