Overloading effect on the fatigue strength in resistance spot welding joints of a DP980 steel

• Published in: International journal of fatigue 2019-04, Vol.121, p.163-171
• Main Authors: Ordoñez, J.H., Ambriz, R.R., García, C., Plascencia, G., Jaramillo, D.
• Format: Article
• Language: English
• Subjects: Compressive properties; Compressive residual stresses; Cooling curves; Crack initiation; Crack propagation; Cyclic loads; DP980 steel; Dual phase steels; Duplex stainless steels; Fatigue behavior; Fatigue failure; Fatigue life; Fatigue strength; Fatigue tests; Fracture mechanics; Lap joints; Materials fatigue; Metal fatigue; Metal sheets; Microhardness; Overloading; Phase transitions; Residual stress; Resistance spot welding; Spot welds; Stress concentration; Stress propagation; Welded joints
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2018.12.026

•Mechanical behavior of the resistance spot lap joints of a DP980 steel.•Hardness, shear resistance and fatigue life are reported.•A heterogeneous behavior of the hardness, attributed to peak temperatures reached during the welding.•Fatigue improvement of the joints due to residual stress redistribution, delaying the fatigue damage. Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases was observed in the RSW lap joints with respect to the base material, which was attributed to phase transformations during the heating and cooling of the steel. Fatigue Wöhler curves using a fixed load ratio of 0.1 were obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition.