Modelling and Experimental Investigation of Ductile Crack Initiation in Welded Connections with Bevelled Backing Bars and Inelastic Panel Zones

In pre-qualified welded connections, current design and fabrication rules that were formulated after the 1994 Northridge earthquake necessitate high-notch toughness weld requirements and rigorous treatment in the weld backing bar region, including the backing bar removal after welding of the bottom beam flange-to-column flange groove welded portion. Moreover, inelastic deformations within the beam-to-column web panel zone joint are limited, thereby localizing plastic deformations near the steel beam ends. Recent experiments by the authors suggest that the above two requirements may not be necessary to ensure a ductile connection response. One requirement is the use of a bevelled backing bar detail that although it is not removed after the welding process, it still minimizes the fracture potential at the welded connection. The ultimate failure mode of the welded connection is fracture due to ultra-low cycle fatigue at lateral drift demands exceeding 7%. This paper investigates via additional experiments across scales and fracture-mechanics-informed continuum finite element simulations the onset of ductile crack initiation in welded connections with highly dissipative panel zones. The simulations rely on state-of-the-art micromechanics-based fracture models, calibrated to material-scale experiments on samples with variable triaxiality and Lode angle ranges. The simulation results indicate that fracture due to ultra-low-cycle fatigue in these proposed connections initiates at lateral drift demands exceeding 4%.