Fatigue behavior of bird-beak SHS X-joints under cyclic brace axial forces

This paper presents an experimental investigation on the fatigue behavior of bird-beak X-joints under brace axial force. Two square bird-beak and two diamond bird-beak specimens were fabricated by using 80% partial joint penetration (PJP) welds. Static loading was firstly carried out to measure the hot spot stresses at weld toes, by which stress concentration factors (SCFs) were extrapolated and subsequently the critical locations were identified. A cyclic axial loading was then implemented on the brace end. The fatigue behavior investigated includes crack initiation, crack propagation, failure mode, fatigue life and stiffness degradation. The results show that the SCFs calculated from test data could well predict the location of crack initiation. Furthermore, the through-thickness crack lives (N₃) of all are found to be greater than the fatigue lives derived from the IIW (International Institute of Welding) design S[sbnd]N curves, which indicates that these curves for conventional joints can be conservatively used for estimating the fatigue life of bird-beak SHS X-joints.