Fatigue performance of resistance spot welds in three sheet stack-ups

This paper describes the fatigue characteristics of spot welds for three equal thickness sheet stack-ups of a Dual Phase (DP600) welded to itself and a Mild steel welded to itself under tensile shear loading. The experiments were designed to investigate the effects of electrode tip geometries, surface indentation levels, and base metal strengths on fatigue life of the tensile shear spot welds. The electrode tip geometries were B-nose for DP600 and Mild steel and E-nose for DP600. Surface indentation levels were 10%, 30% and 50% of the sheet thickness. To obtain the goal, geometric factors were precisely controlled, especially the diameters of spot welds. Welding schedules varied to obtain proper surface indentation levels while keeping the same weld sizes for the different materials and electrode geometries. The fatigue test results showed scatter in the plot of the maximum applied loads versus cycles to failure of spot welds due to weld sizes and base metal strengths. Thus, a normalized structural stress parameter was proposed and correlated well to the fatigue test results.