Brittle-ductile fracture transition on a crack in a weld line between two different steels

The fracture experiment of a crack in a weld line between two different steels (SPC and HT 60) was carried out over the temperature range from -196°C to room temperature. Different from a crack in a material, the fracture mode change from mode I (opening) type to mode II (in-plane shearing) type due to the difference of material properties of two steels was observed with the increase of temperature in addition to brittle-ductile transition behavior. That is, in the range below -150°C, only mode I brittle fracture occurred without blunting of a crack tip. In the range from -150°C to -100°C, although a crack tip a little blunted, unstable mode I brittle fracture occurred. In the range from -100°C to -75°C, a crack showed stable mode II ductile fracture first, then, changed to unstable mode I brittle fracture, and, in the range from -75°C to room temperature, a crack showed stable mode II ductile fracture first, then, changed to stable mode I ductile fracture. The CED (crack energy density) in an arbitrary direction were evaluated through the results of finite element analyses corresponding to the fracture experiment. It was shown that a dramatic increase of fracture resistance by brittle-ductile transition cannot be expected for a crack in a weld line between two different steels because brittle-ductile transition is accompanied by fracture mode change from mode I to mode II.