The fatigue performance of DY-type joints in self-stressed concrete-filled steel tubes

Self-stressing steel tube concrete is a novel composite structure created by substituting ordinary concrete with self-stressing expansion concrete within a steel tube. Compared to regular steel tube concrete, self-stressing steel tube concrete structures can overcome the limitation that ordinary steel tube concrete is prone to voids. Many engineering studies have shown that these structures are prone to fatigue damage at the welded joints of steel tubes under repeated loads. Factors such as welding residual stress, welding defects, and stress concentration contribute to this susceptibility, which could potentially lead to catastrophic accidents. In this study, fatigue tests and finite element analysis were conducted on self-stressed concrete-filled steel tube joints of DY-type joints. The fatigue behavior of the joints under alternating loads was investigated through experiments and validated with the finite element analysis results. The impact of concrete's self-stress on joint fatigue performance was examined, including the stress concentration factor (SCF) and geometric parameters such as the ratio of the branch pipe outer diameter to the main pipe outer diameter, the ratio of the main pipe's outer radius to its wall thickness, the ratio of the branch pipe wall thickness to the main pipe, and the correlation between the angle of the main pipe and the branch pipe. The research results show that the node can withstand 2 million alternating loads without failure under a fatigue stress amplitude of 47.2 MPa. Additionally, the fatigue life of the test model under variable amplitude loads is converted into that under constant amplitude loads. The fatigue life of the node indicates that it can withstand 4.16 million cycles under the given fatigue amplitude before fatigue cracks appear. Additionally, variations in geometric parameters can impact the fatigue life of the node, but do not lead to changes in the initiation position of fatigue cracks. Moreover, as self-stress increases, the fatigue performance of the tube node will decrease, although the effect of self-stress on the stress distribution around the weld is minimal.