Recently, weight saving of car bodies have been required due to the enhancement of regulations for emissions of CO2, and to improve collision safety, high tensile steel is widely used on car bodies. However, joint strength may decrease on high tensile steel. In addition, joint strength depends on fracture mode. Therefore, it is necessary to predict crack propagation direction and investigate the factors which influence the crack propagation direction. In this research, the author has proposed ductile crack analysis method based on continuum damage mechanics. In ductile crack propagation analysis, various potential models have been proposed as damage term. However, in such models, it is difficult to obtain convergence on the calculation of nonlinear constitutive relation. Therefore, in this study, a simplified ductile fracture evaluation method, which is based on the elastic-plastic analysis considering large deformation and strain, was proposed. The proposed method was applied to the analysis of cross tension test on spot weld joints. As a result, it was found that crack propagates along the outer periphery of the weld nugget on larger nugget diameter. These fracture modes are called plug fracture and higher cross tensile strength can be obtained in plug fracture. However, in smaller nugget diameter, crack propagates across the internal nugget. It is called interface fracture and joint strength decreases in this fracture mode. These tendencies agree with the results obtained by experiments. Therefore, in this study, it can be said that crack propagation direction can be predicted on cross tension test on spot weld joints by using the proposed method.

In this study, the authors developed a three-dimensional coupling analysis method for spot welding by using Idealized Explicit FEM, which can analyze large-scale welding residual stress and deformation problems in practical computing time. This proposed method was applied to a spot welding problem to show its analysis accuracy. It was found that the nugget sizes obtained by experimental measurements and the method are in good agreement. In addition, to investigate the effect of shunt current on the nugget size in multiple-spot welding, the method was applied to three-dimensional spot welding with four points of welding. As a result, it was shown that the influence of shunt current in multiple-spot welding can be evaluated by using the proposed method in realistic computing time.