The severity of local undercut on fatigue life of welded joints

• Elasto-plastic FEA of welded joints with different steel grades and local undercuts. • Novel local undercut severity index for fatigue crack initiation life. • An SCF formulation for welds, considering local undercut and weld geometries. • Existing formulas overestimate SCF for welds with undercuts, ignoring 3D geometry. • Undercut affects fatigue severity based on weld bead, materials, and stress range. Fatigue cracks most commonly initiate from local weld imperfections, such as undercuts, that have a significant impact on the fatigue life of the structure. The influence of local undercut on fatigue has been traditionally classified on the level of workmanship for quality control purposes, and a systematic quantified approach to determine the influence of local undercut on fatigue strength is needed. In this paper, the influence of the 3D undercut geometry on the fatigue strength of welded joints is investigated to quantify the severity of different undercut sizes and shapes in butt welded joints. Systematic elastic and elasto-plastic analyses are performed on butt-welded joints with different steel grades and local undercut geometries under multiple stress ranges. As a result, a semi-analytical formulation is proposed to estimate the elastic stress concentration factor for butt welds with an undercut, considering the 3D undercut geometry as well as the global weld geometry. Furthermore, a novel undercut severity index for fatigue analysis is introduced and validated, considering the variation of the local undercut 3D shape, weld bead geometry, stress ranges, and materials.