Modeling and Testing of Spot Welds under Dynamic Impact Loading Conditions

Failure behavior of spot welds is investigated under impact loading conditions. Three different impact speeds were selected to test both HSLA steel and mild steel specimens under combined opening and shear loading conditions. A test fixture was designed and used to obtain the failure loads of spot weld specimens of different thicknesses under a range of combined opening and shear loads with different impact speeds. Accelerometers were installed on the fixtures and the specimens for investigation of the inertia effects. Optical micrographs of the cross sections of failed spot welds were obtained to understand the failure processes in both HSLA steel and mild steel specimens under different combined impact loads. The experimental results indicate that the failure mechanisms of spot welds are very similar for both HSLA steel and mild steel specimens with the same sheet thickness. These micrographs show that the sheet thickness can affect the failure mechanisms. For 1.0 mm specimens, the failure occurs near the base metal in a necking/shear failure mode. For 1.5 mm specimens, the failure occurs in the heat affected zone in a shear failure mode. Based on the experimental results, the effects of inertia and separation speeds are investigated. An engineering failure criterion is proposed to characterize the dynamic failure loads of spot welds for engineering applications. The failure load is expressed as a function of the tensile strength of the base metal, the nugget size, the sheet thickness, the separation speed, and empirical coefficients for a given welding schedule.