No abstract

Synopsis :The authors proposed a three dimensional model to explain microscopic behavior of cleavage crack propagation in steel, which was validated by experimental results, in the previous paper. Purpose of this paper is to explore further into microscopic cleavage crack propagation behavior in steels having anisotropy using the proposed model. Charpy impact testing and small size crack arrest testing were conducted for observing anisotropy of cleavage fracture toughness and it was confirmed that R direction has lower toughness than LT and TL directions. And simulation was also conducted using the data of crystal orientation, which was derived from Electron Back Scattering Diffraction analysis. The comparison between the experiment and the simulation showed good agreement and the lowest toughness in the R direction was explained by a concentration of {100} planes in that direction.

No abstract

No abstract

The present authors propose a cleavage fracture initiation model for bainite steels. The authors considered three stages of fracture initiation in the model: stage I; micro crack initiation in martensite-austenite constituent (MA) in low carbon bainite, stage II; propagation of the micro crack into low carbon bainite and stage III; propagation of the cleavage crack across grain boundary. Stage I is described as probabilistic event; cracking probability is formulated based on the experimental results. Stage II and Stage III are formulated by the fracture stress theory. In this model, multiple volume elements are defined around a notch tip and microstructure is arranged for each volume element. In each time step, Stage I, II and III are judged with stress and strain at each volume element obtained by finite element method. The authors assume that cleavage fracture is initiated when the conditions of these three stages are simultaneously satisfied in any one of the volume elements. The present model is validated by comparison between simulation results and experimental results of notched three point bend tests. The simulation results and the experimental results show good agreement with regard to fracture toughness and fracture initiation points.