Fatigue performance of Q370qE steel non-coplanar butt-welded joints with inclined principal stress

This study investigates the fatigue performance of non-coplanar butt-welded (NCBW) joints subjected to inclined principal stresses in the cable–girder anchorage zone of cable-stayed bridges through experimental and finite element analyses. Hot spot stresses were measured for NCBW joints with dihedral angles γ ranging from 0° to 8° and angles α between the principal stress direction and the weld axis ranging from 30° to 70°, while crack initiation and propagation were systematically tracked. The results show that stress concentrations occur at the weld toes on the concave side of the non-coplanar joints, with the maximum stress concentration factor increasing with both γ and α and reaching 1.75. Between one and ten cracks initiated in individual specimens, and all cracks propagated approximately perpendicular to the principal stress direction. Three-dimensional scanning and finite element analyses of the weld ripple morphology reveal that pronounced stress concentrations develop at the inter-ripple grooves when they are oriented perpendicular to the principal stress direction. The inter-ripple grooves and adjacent weld toes form elongated notch regions that strongly promote crack initiation. A stress-intensity-factor-like parameter is proposed to evaluate crack initiation locations in NCBW joints subjected to inclined principal stresses. This parameter provides 17% greater explanatory capability for the experimentally observed crack initiation behavior than the conventional hot spot stress criterion. The nominal stress S-N curves specified in current design standards do not provide conservative fatigue life estimates for all specimens tested in this study, whereas the standard hot spot stress S-N curve and master S-N curve family provide conservative estimates for all specimens. Finally, dedicated S-N curves are established for 16-mm-thick NCBW joints subjected to inclined principal stresses at a stress ratio of 0.1. The corresponding design fatigue strengths are 62.5 MPa, 119 MPa, and 185 MPa in terms of nominal stress, hot spot stress, and equivalent structural stress, respectively.