Fracture response of resistance spot welded dual phase steel sheets: Experiments and modeling

Predicting the failure of spot welds remains one of the most important modeling challenges in automotive engineering. In essence, the structure surrounding a spot weld corresponds to a graded material with severe variations in the plasticity and fracture properties as a function of the distance to the center of the weld nugget. While it is common practice to identify hardness variations around a spot weld, a newly developed micro-tensile testing technique is used to characterize spatial variations in both strain hardening and fracture strains. Furthermore, a new experimental device is presented to subject spot welds to combined tension, shear and bending loads. Following a careful experimental characterization of resistance spot welds connecting two 1.5 mm thick DP600 sheets, a detailed finite element model is built which accounts for the identified property gradients. In addition, a simplified surrogate model for use in conjunction with coarse shell element meshes is also calibrated. Simulations are performed of all structural experiments to assess the range of validity of the detailed and simplified FE models. It is found that the detailed model captures the experimentally observed failure modes including changes in the location of fracture initiation as a function of the loading, while the simplified model is able to predict the spot weld strength with reasonable accuracy.