Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Abstract

Both scientists and engineers are very much concerned with the study of ductileto-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.

Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks

“…However, in ferritic steels, it has been shown that the misorientation between the ferritic packets was much more important in martensitic steels (such as 21/4 Cr steel) than in bainitic steels (such as A508 steel). These modifications in misorientation distribution between the effective units for cleavage fracture are accompanied by significant improvement in fracture toughness [56].…”

Predictions of Fracture Toughness for Weld Material When Adopting a Novel Particle Failure Criterion and a Measured Experimental Particle Distribution

“…(20) replaces the exponent of plastic strain in the modified Griffith criterion used previously [2][3][4][5][6][7][8][9][10][11][12][13][14]. As before, it is intended to account for the reduction in crack driving force due to plastic dissipation in the matrix (i.e.…”

“…A different direction of work considers ductile damage prior to cleavage in the DBT regime, employing damage models for the material behaviour coupled with subsequent calculation of failure probability following Beremin [18,19]. This approach has the potential to capture better the fracture behaviour in the DBT, but the models are presently too simplified with no interaction of void growth and micro-cracking at microstructure level [20]. Until advanced coupled models are developed and tested, the wider engineering community would benefit from a simpler uncoupled cleavage fracture model for the DBT region.…”

Two Step Ductile to Brittle Transition Behavior on Ferrite + Pearlite Structure Steel Sheet

“…[12][13][14] DP microstructure is mainly applied for sheet steels that are thinner than full-size Charpy impact specimens (10 mm Although a few studies on the fracture behavior of dual phase (DP) steels have been conducted, the mechanism of impact fracture in DP steels has not yet been clarified. In this study, the ductile to brittle transition behavior in ferrite + pearlite DP steel sheets was evaluated using the Charpy impact test with a sub-size specimen.…”