An experimental methodology to characterize load-based fracture models of third generation advanced high strength steel resistance spot welds

Failure of resistance spot welds in computer-aided engineering models is based upon criteria that incorporate test data obtained in various loading conditions including different proportions of tensile, shear, and moment loads. The decomposition of the critical load into its respective shear, tensile, and bending moment components is influenced by the rigid body motion during their corresponding mechanical tests. Continuous tracking of the weld orientation and the deformed coupons is required for accurate determination of the load components at the onset of failure. A comprehensive experimental investigation was performed to characterize the critical failure load components in combined loading using various orientations of KS-II tests and a range of coach peel coupon geometries. Mechanical testing was coupled with digital image correlation (DIC) to systematically evaluate empirical force-based failure models for resistance spot welds of two third generation advanced high strength steels with optimal and suboptimal fusion zone diameters. New analysis methodologies using DIC were developed to account for rotation and deformation of the joint in the determination of the shear, normal, and bending moments acting on the spot-welded joints. The coach peel test results for both steels revealed a non-convex experimental fracture locus in bending-tension loading cases. The conventional assumption of a convex failure locus overestimated the critical bending moment strength between 7 and 66%. Results indicated that changes in the operative failure mechanism from pullout/partial-pullout to interfacial can expand the fracture loci within the shear-tensile loading mixities. Improved alternative functional forms for the weld failure models were proposed and contrasted with conventional models that assume convexity.