Abstract
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-III 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-III. 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.
