Modelling the initiation of cleavage fracture of ferritic steels

Abstract

The initiation of the cleavage fracture of ferritic steels at cracked grain boundary carbides situated in a plastic zone is modelled, using a 2D discrete dislocation simulation of the plastic zones associated with a microcrack under triaxial loading. Fracture is assumed to occur when the local stress intensity factor for the microcrack equals K IC. For microcracks in the size range 0.1–10 μm, the applied stress at fracture, σ F, is found to be independent of the yield stress (in the range 200–1600 MPa) and therefore of temperature. The dependence of σ F on the crack size deviates from a Griffith type relation. These predictions are consistent with experimental data on steels in the literature. The values of σ F predicted are in good agreement with the experimental values. The results suggest that only a small fraction of carbides are in configurations leading to fracture.