Hot spot stress and fatigue behavior of bird-beak SHS X-joints subjected to brace in-plane bending

This paper deals with the stress concentrations and fatigue behaviour of X-shape bird-beak joints under brace in-plane bending. Static load was first implemented to record the strain distribution along the hot lines, and then stress concentration factors (SCFs) were obtained through quadratic extrapolation method. Numerical analysis was then developed to further determine key parameters affecting SCFs and to propose corresponding SCF design formulas. During the subsequent high-cycle fatigue test, the fatigue cracking process including crack initiation, crack propagation, and fatigue lives, as well as the rigidity degradation were captured. Results show that crack initiation always occurred at the crown area where the SCFs are proved to be the greatest. The through-thickness lives may be utilized to conservatively represent the fatigue design lives of SHS bird-beak X-joints. Bird-beak SHS X-joints demonstrated better fatigue behaviors than those for traditional joints if the same service load is applied.