The poor fatigue performance of fillet welded terminations was studied. A review of the literature, an experimental study of mild-steel weldments with longitudinal attachments, and a companion FEM analytical study all indicated that the poor fatigue performance of weldments with longitudinal attachments is due to the combination of a very high 3-D stress concentration, very high tensile residual stresses, and the existence of serious weld-toe stress concentrators such as small weld toe radii or defects such as undercut or cold lap. Cold-lap defects were found to occur at the weld toes when gravitational and/or short circuit transfer gas-metal-arc-welding (GMAW) was used; whereas, the higher heat-input, spray transfer GMAW was found to eliminate these weld toe defects.
A specially designed stress-concentration-reducing part termed “stress diffuser” was developed which was incorporated in the wrap-around welds at the ends of the longitudinal attachments. For longitudinal attachments with a fatigue life of 2E+06 cycles, increases of 360% in fatigue life and 32% in fatigue strength were obtained with the use of the stress diffuser. The stress diffuser increased the fatigue strength of longitudinal attachments to equal that of transverse attachments. The use of a stress diffuser in a beam-to-column connection was found to eliminate the high stress concentration at the weld access hole and to reduce the maximum principal stresses in the region of the intersection of the bottom beam flange and column flange by 31%.
Fatigue life predictions made using the FEM results of this study were in good agreement with the experimental results. Cold-lap defects were predicted to eliminate the fatigue crack initiation life but to affect the fatigue crack propagation life very little.
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