Modelling the initiation of cleavage fracture of ferritic steels

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-ICd. For microcracks in the size range 0.1-10 mum, the applied stress at fracture, sigma(F), is found to be independent of the yield stress (in the range 200-1600 MPa) and therefore of temperature. The dependence of sigma(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 sigma(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. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.