A strategy to predict the fracture toughness of steels with a banded ferrite-pearlite structure based on the micromechanics of brittle fracture initiation

This paper presents a strategy to predict the fracture toughness of steels with a banded ferrite pearlite structure using a new model based on the micromechanics of brittle fracture initiation. The model requires only the (1) ferrite grain size and pearlite band thickness distributions, (2) the stress strain curve, and (3) the specimen geometry and boundary conditions of the fracture toughness test, without the need for any parameter fittings from the experimental results. The model is based on the multiscale model synthesis approach, consisting of three elemental models: (1) the microstructural spatial distribution, (2) a macroscopic finite element analysis, and (3) the microscopic fracture initiation processes, wherein the respective formulations of the fracture criteria of the three stages are proposed, namely, Stage I: micro crack formation in shear in the pearlite colony; Stage II: the crack entering the adjacent ferrite grain; and Stage III: the propagation of the crack across ferrite grain boundary. The proposed model was validated by comparing it with the experimental results of five kinds of steels with a range of carbon concentrations, ferrite grain sizes, and pearlite band thicknesses. The predicted and experimental results agreed well for all steel samples and temperatures. In addition, the influence of the microstructure on the fracture toughness was discussed using virtual candidate steels containing various carbon concentrations, ferrite grain sizes, and pearlite band thicknesses. The results demonstrate that the proposed model is an effective and powerful tool for quantitatively predicting the fracture toughness of steel with a banded ferrite pearlite microstructure. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.