Fracture Toughness Prediction Model for Low-carbon Bainite Steels Containing Inter-lath Martensite

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.