It was shown previously that a crack in a weld line between high tension steel and mild steel causes mode II type fracture, even under so called mode I type loading, due to the difference of material properties, especially, to the difference of yield stress. In this study, the fracture experiments of the specimen with a crack in a weld line between two different steels were carried out changing the yield stress ratio of two steels from 1.0 to 2.67 (smaller value is taken as a unit). In the experiments, it was observed that fracture mode changes from mode I type to mode II type around yield stress ratio of 1.22 with the increase of its value. Finite element analyses corresponding to the experiments were also carried out taking the variation of yield stress in the weld line and heat affected zone by welding into account and the CED (crack energy density) in an arbitrary direction was evaluated through the results. It is demonstrated that the fracture criterion based on the CED in an arbitrary direction works well to explain the growth initiation behavior of a crack in a weld line between different steels and, moreover, the fracture mode change with the increase of yield stress ratio can also be explained based on the criterion.
