Abstract: In this paper, a finite element model of ultra-high strength steel spot-welded lap-shear joint is established by taking the 1.5mm stamped boron steel as example, the geometric dimension and material properties of the heat affected zone are defined, and the effect of the nugget size on the fracture modes of a spot-welded joint is analyzed. The results show that PPF (part interfacial fracture) occurs in the softening zone and fails inside the nugget along the thickness direction of the sheets; PF (pull-out fracture) occurs outside the nugget along the thickness direction of the sheets. The critical nugget sizes of different fracture modes (interfacial fracture, part interfacial fracture and pull-out fracture) are obtained. The study provides technical reference for the evaluation and prediction of the fracture modes of ultra-high strength steel spot-welded joints.

Key words: resistance spot welding, ultra-high strength steel, fracture mode, nugget size