Abstract
A numerical model to quantitatively predict cleavage fracture initiation in ferrite-pearlite steel is proposed. The model is based on microscopic fracture process of three stages; Stage-I: formation of fracture origin in pearlite colony, Stage-II: propagation of the pearlite crack into ferrite matrix, and Stage-III: propagation of the crack across ferrite grain boundary. In the proposed model, fracture conditions are formulated by the probability of pearlite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage-III. Ferrite grains and cementite particles are assigned based on their distributions into each volume element. Applied plastic strain and stress of each volume element are calculated by finite element analysis. Cleavage fracture is assumed to initiate at the time when the fracture conditions of the all stages are satisfied in any one of the volume elements. Cleavage fracture toughness of three point bend test is simulated by the proposed model. The numerical predicted results of fracture toughness show good agreement with experimental ones. The bottleneck process of cleavage fracture is then evaluated by the number of arrested micro-cracks until all of the fracture process is satisfied. Influence of ferrite and pearlite size on cleavage fracture toughness is evaluated. It is shown that steel with finer pearlite colony is tougher, and then the developed model can reproduce the size effect of cleavage fracture toughness. Based on the aforementioned results, the validation and the effectiveness of the proposed model are found out.
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