Development of numerical model to predict cleavage fracture toughness of ferrite steel

Abstract

A numerical model to quantitatively predict the cleavage fracture initiation in ferrite-cementite steel is proposed. The model is based on the microscopic fracture process of the three stages; Stage-I: formation of fracture origin by cementite cracking, Stage-II: propagation of the cementite crack into ferrite matrix and formation of a cleavage crack, and Stage-III: propagation of the cleavage crack across ferrite grain boundary. Influencing factors on Stage-I is quantified by using tensile testing with circumferential notched round bar specimens. The specimens are made from steels with various ferrite and cementite sizes. Probability of cementite cracking is formulated based on the results of SEM observation and measurements. Notched three point bend interrupted tests with fractography shows the importance of Stage-III because a number of arrested micro-cracks are observed on the fracture surface. In the proposed model, fracture conditions are formulated by the ratio of cementite cracking based on the experimental results on Stage-I and the concept of fracture stress of ferrite matrix on Stage-II and Stage- III. Active zone is divided into finite volume elements. 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 macroscopic 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.