New fracture criterion for spot welds of advanced high-strength steels

Resistance spot welds of advanced high strength steel (AHSSs) are susceptible to various fracture modes associated with distinct fracture paths leading to different loading capacity. To better predict the potential fracture modes and loading capacity of AHSS welds, a new force-based fracture criterion was developed based on the material strength of different regions in the welds together with identification of potential fracture paths. The established fracture criterion is then implemented into finite element (FE) models with the nugget simplification as a beam element in LS-Dyna. To be specific, the dissimilar spot weld stack-ups of Gen3 1180 steel and LCE 1000 steel sheets are investigated. The proposed model is validated by tests of coach peel, tensile shear, and KSII with different loading orientations. The model also predicts the transition of the fracture modes well, particularly when altering the welding procedure to vary the nugget size or microstructure of the heat affected zone (HAZ). Further, it is revealed that using the average material properties across the entire HAZ instead of the distinct HAZ regions, the model still accurately predicts the fracture modes and load capacity of the spot welds.