Abstract
The conventional micromechanical approaches today are still not able to properly predict the ductile-to-brittle transition (DBT) of steels because of their inability to consider the co-operating ductile fracture and cleavage mechanisms in the transition region, and simultaneously to incorporate the inherent complexity of microstructures. In this study, a complete methodology with coupled cellular automata finite element method (CAFE) and multi-barrier microcrack propagation models is presented to advance the prediction of DBT. The methodology contains three key elements: (i) a multiscale CAFE modelling approach to realize the competition between ductile damage and cleavage fracture and embrace the probabilistic nature of microstructures, (ii) a continuum approach to estimate the effective surface energy for a microcrack to penetrate over particle/matrix interface, and (iii) a method to calculate the effective surface energy for the microcrack to propagate across grain boundaries. The prediction of DBT therefore needs only (1) the stress-strain curves tested at different temperatures, (2) the activation energy for DBT, (3) the ratio between the size of cleavage facets and cleavage-initiating defects, and (4) key statistical distributions of the given microstructures. The proposed methodology can accurately reproduce the experimental DBT curve of a low-carbon ultrahigh-strength steel.
Highlights
Comprehending the factors affecting the ductile-to-brittle transition (DBT) of structural materials, e.g. steels, is of paramount importance to structural integrity assessment
The competition and interaction between ductile damage and cleavage in the transition re gion are absent. It is difficult for finite element method to process two types of fractures within a single mesh since ductile fracture and cleavage occur within two independent material length scales, i.e. the ligament between dominant voids and the cleavage fracture unit, respectively
It contains three key elements: (i) the competition between ductile damage and cleavage processed by the cellular automata finite element method (CAFE) method, where the probabilistic nature of microstructures is incorporated; (ii) a physically-based variable γpm that describes the second stage of cleavage propagation is estimated by using a continuum model based on the shielding effect of dislocation mobility at crack tip; (iii) a correlation between γmm and γpm established based on the likeli hood between particle-size controlled and grain-size controlled cleavage fracture in transition region to depict the occurrence of the third stage of cleavage propagation
Summary
Comprehending the factors affecting the ductile-to-brittle transition (DBT) of structural materials, e.g. steels, is of paramount importance to structural integrity assessment. The effective surface energy γpm is usually regarded as temperatureindependent (Kawata et al, 2018) and can be measured from the rela tionship between local fracture stress and the reciprocal square root of particle size in terms of the Griffith theory (Bowen et al, 1986; San Martin and Rodriguez-Ibabe, 1999). It varies normally in a range of 7–20J/m2 for steels, depending on the microstructure and method of measurement (Kawata et al, 2018). Characterization of microstructure, mechanical tests, and frac tographic analysis of the low-carbon ultrahigh-strength steel are performed to capture the essential parameters for the prediction of DBT
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