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

Resistance spot weld failure criteria in computer-aided engineering (CAE) models are based upon a critical force that is interpolated from test data obtained in various loading conditions including different proportions of shear-tensile forces and tensile force-bending moments. The decomposition of the critical load into its respective shear, tensile, and bending moment components is influenced by the rigid body motion from their corresponding mechanical tests. Continuous tracking of the weld orientation and the deformed coupons is required for the most accurate characterization of critical failure load components at the onset of failure. This study systematically evaluates the available empirical failure prediction models for resistance spot welds with optimal and suboptimal fusion zone diameters of two grades of third generation advanced high strength steel. The experimental failure loci of the investigated spot welds were developed within shear-tensile and tensile-bending moment dominated conditions using different loading orientations of the KS-II tests and various geometries of the coach peel coupons, respectively. All mechanical tests were accompanied by full-field digital image correlation (DIC) techniques to track the rigid body rotation of coupons during the tests. New analysis methodologies using DIC were developed to properly determine the shear, normal, and bending moments acting on the spot welded joints. Results highlighted the expansion of the failure loci within combined shear-tensile load mixites due to changes in the operative failure mode of the joints. The available failure models were found incapable of predicting this experimentally observed failure behavior and suitable alternative spot weld failure functional forms were proposed. Results from coach peel tests revealed that the available RSW failure functional forms overestimated the area under tensile-bending failure contours between 41% and 120%.