Investigating the Mechanical Properties and Failure Behavior of Groups of Resistance Spot Welds under Tensile-Bending Loading Conditions

The mechanical properties and failure behavior of groups of resistance spot welds made from two grades of third-generation advanced high strength steels (3G-980 and 3G-1180) were investigated using two partially joined hat channel rails, which were separated under tensile-bending loading conditions. The component tests revealed 2.3% higher strength and 13% higher energy absorption for the 3G-980 weld groups compared with the 3G-1180 weld groups. Channels made from 3G-980 showed more severe plastic deformation of the sidewalls and flanges (channel collapse) than those made from 3G-1180. More deformation of the 3G-980 channels increased the time before the initiation of the failure of the first weld by 25 s, on average. The combination of higher plastic work due to channel deformation, delayed joint failure, and ductile failure mode was responsible for the superior mechanical performance of 3G-980 weld groups. A novel testing methodology comprising two half-sectioned spot welds on L-shaped coupons was proposed to observe the strain localization, failure initiation sub-regions, and failure propagation paths under tensile-bending loading conditions similar to the weld group tests. Metallurgical characterizations for understanding the lower susceptibility of 3G-980 FZ to cracking revealed 9.6% finer prior austenite grain size and 10% higher density of geometrically necessary dislocations at the FZ edge of 3G-980 joints as compared with that of 3G-1180. The combination of finer grains and higher dislocation density resulted in the formation of martensitic structures with 13% higher local yield strength than that of the 3G-1180 joints, effectively inhibiting crack propagation through the 3G-980 FZ.