Critical factors influencing fatigue behavior of high-strength dissimilar welded joints with thickness steps

Abstract This paper investigates the primary factors influencing the fatigue behavior of butt-welded joints connecting normal- and high-strength steels of varying thicknesses. The research focused on dissimilar joints composed of 20 mm S690QL and 15 mm AH36, with similar joints (20 mm S690QL and 15 mm S690QL) included for comparison. Comprehensive experimental measurements of fatigue strength, misalignment, local weld geometry, and local material hardness were carried out for both joint types featuring thickness steps. Fatigue tests and fracture surface analysis were conducted to identify failure locations for crack initiation. Fatigue strength assessments using the structural stress and effective notch stress approaches were carried out to evaluate the influence of geometry-dependent stress concentration on fatigue failure behavior. The experimental results indicate that the failure locations varied depending on the specimen type. For the dissimilar joints, the locations were identified at the weld toe, specifically at the boundary between the heat-affected zone and weld metal in S690 side, which exhibited lower microhardness values. A comparison of local stress ranges, accounting for the effects of misalignments and local weld geometries, showed higher stress ranges at the failure locations. The scatter of fatigue test results was reduced significantly when the local stress ranges were considered. Consequently, the study confirms that the interaction of local material properties and stress concentrations from global and local weld geometries influences the fatigue failure behavior of high-strength dissimilar welded joints with thickness steps.