Fatigue life assessment of potential weld improvement techniques using 3D fracture analysis

Abstract: This thesis work was done in conjunction with the VINNOVA-funded research and development project LIGHTSTRUCT. It was performed in the department of lightweight structures at KTH. The thesis investigated two potential weld improvement techniques called toe weaving and extended leg. These methods use double pass welding in order to create favorable weld geometries in order to improve the fatigue life. The toe weaving technique produces a wavy weld toe line that should induce a specific crack growth behavior, while the extended leg technique has a straight toe line and will provide a smoother transition between weld and baseplate. The investigation was done by performing 3D fracture analyses of the weld geometries as well as fatigue tests on manually welded cruciform joints. Both simulations and fatigue tests showed a slight fatigue life improvement for both techniques. A high variation in fatigue lives was observed in testing, mostly due to the prevalence of undercut defects. The specific crack growth for the toe weaved specimens could not be observed as in previous studies. The fatigue life enhancement was therefore most likely due to a smoother weld transition between weld and baseplate, reducing the stress concentration factor at the weld toe. The weaving technique is not suitable for welded joints loaded in the longitudinal direction, making its use more limited compared to the extended leg technique. From a production point of view, the toe weaving technique is more time and knowledge demanding than the extended leg technique. It is therefore recommended to further investigate the extended leg technique.