Abstract：Eight specimens were fabricated and tested under cyclic brace in-plane force to investigate the fatigue behavior of bird-beak square hollow section (SHS) K-joints. Static loading was first implemented to obtain the hot spot stresses and stress concentration factors (SCFs) by employing quadratic extrapolation method, from which hot spot stress distributions and critical hot spots were revealed. During the subsequent high-cycle constant amplitude cyclic loading, the fatigue behaviors including fatigue failure mode, crack propagation features, rigidity degradation and characteristic fatigue life were all observed. The results show that the maximum SCFs of square bird-beak overlap K-joints always occur at the brace overlap areas, while the maximum SCFs of other joints commonly appear at the brace crown areas. The fatigue cracks were observed to initiate at the hot spots with the highest SCFs and to propagate through the plate thickness soon after initiation. Significant rigidity degradations happened after the crack run through the plate thickness, while the accumulated degradations were measured to be no greater than 10%. The S-N curves recommended by the codes of IIW and API for conventional SHS joints could also be used to predict the fatigue lives of bird-beak SHS K-joints with common thick walls, however, the applicability of the curves to the bird-beak K-joints composed of thin walls should be further validated.
