Accounting for Crack Propagation in a Model to Predict Fracture Toughness in Ferritic Steels

A program was undertaken to develop a predictive model of the scatter in toughness of a structural steel across the wide temperature range of ductile-to-brittle transition based on physical understanding of deformation and fracture behaviors. The initial model was focused on microcrack initiation and includes criteria to describe the propagation of particle-sized microcracks into the surrounding ferritic matrix. Parametric studies using this model found that the temperature dependence for fracture toughness, derived from microcrack, was insufficient to describe the temperature dependence observed in measured K_Jc toughness values. A microcrack propagation model was developed to account for additional barriers associated with transgranular crack propagation to failure. This propagation model accounts for the temperature dependence of crack propagation across grain boundaries and is therefore expected to increase the degree of temperature dependence. This paper summarizes the initiation model and discusses the approach taken in developing a microcrack propagation model component and discusses preliminary results from a Monte Carlo simulation.