Fatigue tests and design of Q460 CHS-CHS T-joints under axial loading and in-plane bending

Previous research has primarily focused on stress concentration factors (SCFs) and fatigue life of tubular joints made from low-strength steel (nominal yield strength of 235-360 MPa). The fatigue behavior of high-strength tubular joints remains insufficiently studied. In this study, a total of sixteen CHS-CHS T-joints fabricated from Q460 steel were designed, eight of which were subjected to axial load and eight of which were under in-plane bending. Static and fatigue tests were conducted to analyze the distribution of hot spot stress, fatigue crack propagation, and fatigue failure modes. The experimental results showed that the crack initiation consistently occurred at location of maximum hot spot stress for all specimens. The maximum SCFs were concentrated at the saddle (90(degrees)) for specimens under axial loads and around the position of 22.5(degrees) for those under in-plane bending. The modified SCF design formula for in-plane bending was proposed based on regression analysis. Prior to crack initiation, all specimens exhibited a relatively slow stiffness degradation due to cumulative damage from fatigue loading. Through least squares analysis of the test data, the fatigue design S-rhs-N curve for Q460 CHS-CHS Tjoints was developed. The proposed S-rhs-N curve is capable of accurately predicting the fatigue life of Q460 CHSCHS T-joint subject to axial loads and in-plane bending. Comparison with previous research and existing design guidelines shows that under the same stress amplitude level, the fatigue lives of CHS-CHS T-joints subjected to axial loading of Q460 steel are comparable to that of low-strength steel (WFG36Z). Under in-plane bending, the fatigue lives for Q460 steel are higher than that for low-strength steels (C350L0, t < 4 mm).