6. On Brittle Fracture Initiation (First Report-Deep Notch Test)

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Change of a slow crack or a fatigue crack which propagated from a weld defect or the stress concentrated zone, into a fast crack such as a brittle crack at a crack length provides a significant meaning from a viewpoint of safety for welded structures. In general, a notched tensile test specimen for mild steel fractures after the general yielding occurred on the notch section. Therefore, the superpositions of additional stresses such as the impact stress due to wedge impact, the welding residual stress and the concentrated stress due to structural discontinuities, or metallurgical change on the tip of notch are needed for the brittle fracture initiation at low stress levels below yield point. In this paper, under the consideration that the Griffith-Orowan energy condition for brittle crack initiation at low stress level can be satisfied when the stress field has been kept in elastic condition, the plastically deformed zone around the tip of notch has been great]y restricted by increased yield point. Further, since the longer notch results in the lower fracture stress, the deep notch test for high strength steel has been conducted at low temperatures, and the low stress fracture without any additional stress or metallurgical embrittlement has been successfully realized. Next, the ESSO test with temperature gradient has been conducted to evaluate the plastic surface energies for brittle crack propagation and arrest, respective]y, in order to compare with that for crack initiation evaluated from deep notch test. The following conclusions have been developed: (1) The Griffith-Orowan energy condition governed the brittle fracture initiation when the general yielding did not occur. (2) The fracture stress-temperature-critical crack length relationship, at which a slow crack changed into a brittle crack, could be obtained by using at least two or three specimens. (3) The plastic surface energies for brittle crack propagation and arrest were less than an order of 1/10 of that for brittle crack initiation at a temperature. (4) The temperature at which the plastic surface energy for brittle crack initiation was about 100〜120℃ lower than that for crack arrest at a stress level for the 80 kg/mm^2 high strength steel tested.
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6. On Brittle Fracture Initiation (First Report-Deep Notch Test)

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