A theoretical, multi-scale model has been developed to predict the fracture toughness of ferritic steels in the ductile-to-brittle fracture mode transition temperature region. The new model is being implemented into the DISlocation-based FRACture (DISFRAC) computer code at the Oak Ridge National Laboratory (ORNL) and will permit fracture safety assessments of ferritic structures with only tensile properties and microstructural information (grain and carbide size) required as input. The theoretical basis of this model provides a means of predicting fracture behavior outside of the ranges of data currently used in deriving empirically-based models and should provide a means of improving the understanding of fracture behavior in the fracture mode transition region. Dislocation distribution equations, derived from dislocation theory developed by Yokobori et al., are combined with modified boundary layer solutions to account for the stress state local to various microstructural features believed to control fracture behavior. Terms are included to account for microcrack initiation in brittle grain boundary particles, propagation of the microcrack into the first ferrite grain and then through subsequent grain boundaries accounting for local tilt and twist grain misorientation across boundaries. This paper summarizes the DISFRAC model and provides the results of a study performed to investigate the role of grain size in microcrack initiation, propagation and the resulting prediction of fracture toughness.

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