Study on the estimation method for the mean stress effect on the fatigue strength of welded joints with various failure modes and joint types

In compact box-shaped steel structures, partial penetration welds are frequently selected as the welding technique, and root fatigue failure might manifest in these joints. In order to ensure the structural integrity of steel structures, it is necessary to develop an assessment approach for evaluating the efficacy of post-weld heat treatment (stress relief) in enhancing the fatigue strength of root-failed welded joints. In this study, bending fatigue experiments employing stress ratios of R = 0 and − 1 have been carried out on as-welded and stress-relieved welded joint specimens. The test objects include root-failed plug weld specimens, as well as toe-failed out-of-plane gusset weld joint and T-joint specimens. The welding residual stresses near the root notch and weld toe are measured by X-ray diffraction technique. The assessment of the mean stress effect on fatigue strength has been examined through the utilization of the modified MIL-HandBook-5D equivalent stress range. The equivalent stress range is evaluated by using two fatigue assessment stresses: structural stress and elastic–plastic local stress. It has been confirmed that all fatigue test results, irrespective of the failure mode or the joint type, whether from the as-welded or stress-relieved specimens, can be closely approximated using a single S–N curve with either definition of the equivalent stress. This outcome indicates the accomplishment of assessing the mean stress effect on the fatigue strength of welded joints with various failure modes and joint types.