へき開破壊執性予測モデルによる破壊靱性試験の再現解析

摘要

The prediction model of cleavage fracture toughness proposed in the authors' previous study is validated by its application to three point bend testing of notched specimens. Experiments are carried out by using steels with various ferrite and cementite sizes. The numerical predicted results of fracture toughness show good agreement with the experimental ones under all the conditions of test temperatures of the all steels. The bottleneck process of cleavage fracture is then evaluated by the number of arrested micro-cracks until all of the fracture process is satisfied. A lager number of the arrested micro-cracks are formed at higher temperature. It means that the bottleneck process is changed from Stage-II to Stage-Ill with increasing temperature. Grain size at fracture initiation site is evaluated. The scatter of the grain size in the case of the steels with coarse ferrite grains is larger than that of the steels with fine grains. It is therefore shown that the developed model can reproduce the size effect of cleavage fracture. Dependence of the fracture stress on temperature was clarified. In particular, lower temperature makes larger scatter of the fracture stress both on Stage-II and Stage-Ill. This tendency is remarkable in the cases of the steel with fine grains. Based on the aforementioned results, the validation and the effectiveness of the proposed model are found out.%鋼のへき開破壊は，降伏や加工硬化現象とは異なり，最弱支配型の現象であり，本質的にばらつきを伴う。従来から，フェライト鋼の執性は結晶粒の微細化により向上するだけでなく，セメンタイトなどの脆い第2相の寸法によっても靭性が変化することは広く知られている。