Investigation of the Tensile Shear fracture of Advanced High Strength Steel spot welds

Damage of Advanced High Strength Steel resistance spot welds is investigated in Tensile Shear by means of coupled microtomography, metallography and fractography. Two main failure mechanisms and failure zones are identified: (i) strain localization in the base metal/subcritical Heat Affected Zone (HAZ), (ii) ductile shear at the interface in the weld nugget. A finite element model is developed in order to illustrate these competing failure mechanisms leading to distinct macroscopic failure types. The local constitutive behavior is obtained from tensile tests on simulated HAZ microstructures and an estimation of the Rice integral (J) at the notch tip is provided for the assessment of interfacial failure. The model enables capturing the main trends in failure type transition as a function of weld geometry as well as a reliable estimation of the load bearing capacity.