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

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 Wohler 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.