Experimental and modeling investigation of the failure resistance of Advanced High Strength Steels spot welds

Damage of Advanced High Strength Steel resistance spot welds is investigated in Cross Tension by means of coupled microtomography, metallography and fractography. Three main failure mechanisms and failure zones are identified: (i) strain localization in the base metal/sub-critical Heat Affected Zone (HAZ), (ii) ductile shear around the weld and (iii) semi-brittle fracture in the weld nugget. A finite element model is developed in order to illustrate how the mechanisms compete and lead to a given failure type. The local constitutive behavior is obtained from tensile tests on simulated HAZ microstructures. The model enables capturing the main trends in the transition between failure types as a function of weld geometry as well as a reliable estimation of the load bearing capacity.