Novel analytical approach for evaluating the mechanical properties of friction stir spot joints through constitutive modeling

•Complex structural stresses of spot welds during tensile shear test were estimated.•For the first time stress-strain curve and work hardening rate were estimated.•Work hardening of TWIP steel joints showed plateau due to occurrence of TWIPing. Proper assessment of mechanical properties for friction stir spot welds is controversial due to complex stress conditions within the weld nugget. Because of that, mechanical testing results are expressed in terms of load displacement curves; therefore, interpretation of the plastic behavior by stress-strain curve is theoretically impossible. In the present work, stress intensity factors and complex structural stresses of spot welds during tensile shear test were estimated through analytical simulation based on fracture mechanics methods for elastic and plastic regions. Subsequently, the stress-strain curve and the corresponding work hardening rate were estimated and interpreted with acceptable precision. The experiments were performed using a high manganese twinning induced plasticity steel which is one of the commonly used steel grades in automotive industry. The best combination of tensile-shear strength and extension was found for the joints welded at rotation speed of 1600 rpm where stress intensity factor of mode I contribution was the highest. The occurrence of twinning induced plasticity in a higher extent was the main reason for an increase in extension and work hardening of this sample.