Brittle-ductile transition and scatter in fracture toughness of ferritic steels (original) (raw)
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Dislocation simulation of brittle-ductile transition in ferritic steels
Metallurgical and Materials Transactions A, 2006
Two dimensional discrete dislocation simulations of crack-tip plasticity of a macrocrackmicrocrack system representing the fracture behavior in Ferritic Steels are presented. The crack tip plastic zones are represented as arrays of discrete dislocations emitted from cracktip sources and equilibrated against the lattice friction. The dislocation arrays modify the elastic field of the crack and the resulting field describes the elasto-plastic crack field. The simulated crack system involves a microcrack in the plastic zone of the macrocrack (elastoplastic stress field). Effects of crack-tip blunting of the macrocrack is included in the simulations; as dislocations are emitted the microcrack is kept at a constant distance from the blunted tip of the macrocrack. The brittle-ductile transition curve is obtained by simulating the fracture toughness at various temperatures. Considering the effects of blunting is found to be critical in predicting the sharp upturn of the brittle-ductile transition curve. The obtained results are compared with existing experimental data and are found to be in reasonable agreement. .
International Journal of Mechanics and Materials in Design, 2007
A dislocation simulation model has been proposed to predict the brittle-ductile transition in ferritic steels in Part I. Here we extend the model to address the problem of inherent scatter in fracture toughness measurements. We carried out a series of Monte Carlo simulations using distributions of microcracks situated on the plane of a main macrocrack. Detailed statistical analysis of the simulation results showed the following: (a) fracture is initiated at one of the microcracks whose size is at the tail of the size distribution function, and (b) the inherent scatter arises from the distribution in the size of the critical microcrack that initiates the fracture and not from the variation of the location of the critical microcrack. Utilizing the weakest-link theory, Weibull analysis shows good agreement with the Weibull modulus values obtained from fracture toughness measurements.
Inorganic Materials, 2018
It has been shown by means of EBSD techique that fracture of ferritic steel in ductile-brittle transition temperature region, along with the formation of previously discribed cleavage microcracks, results in the formation of ductile microcracks. It has also been shown that microstructure of plastic zones under brittle and ductile fracture components produced by the main crack propagation differ significantly. Better developed plastic zone under ductile fracture component protects steel from overstress. The plastic zone under brittle fracture surface, apparently, has a reduced local plasticity. Consequently, the cleavage microcracks formation precedes the fracture process. During the main crack formation such microcracks occur in steel microvolumes located both in front of its tip and in adjacent to its edges microvolumes. Further propagation of the main crack is realized in steel which already contains scattered cavities and reduces to ductile fracture of the connections between them.
Multiscale modeling of the brittle to ductile transition
Journal of Nuclear Materials, 2004
A recently introduced method of crack representation as a distribution of three-dimensional Volterra dislocations is used in conjunction with two-dimensional dislocation dynamics simulations to study the brittle to ductile transition behavior of Ferritic Steels. The crack-tip plasticity zone is represented as an array of discrete dislocations emitted from crack-tip sources. The dislocations shield the crack and result in an increase of the applied stress intensity for fracture from the pure Griffith value. The crack system responsible for fracture in Ferritic Steels is modeled by a macrocrack and a microcrack in its field. Crack-tip plasticity of microcrack is also modeled by arrays of emitted dislocations. The simulations are performed for different friction stresses corresponding to different yield stresses or temperatures. The brittle to ductile transition fracture toughness curve is obtained and compared to experiments.
Competing fracture mechanisms in the brittle-to-ductile transition region of ferritic steels
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
Theoretical models are used to investigate the propagation of fracture in the transition region of ferritic steels. Four mechanisms of fracture are allowed: transgranular brittle and ductile and grain boundary brittle and ductile. As fracture propagates decisions are made at each stage about which of these mechanisms will be operative in the next grain or grain boundary. These decisions are based on the relative energies of the different mechanisms, which are functions of temperature, and the orientation of the stress axis. The simulations, which are two-dimensional, enable the proportions of the four mechanisms to be deduced and hence the overall energy of the fracture surface to be determined. The most striking feature of the results is that there is a much greater scatter of mechanisms and of energies than is found in corresponding low temperature and high temperature simulations. This is consistent with experimental results obtained on ferritic steels.
Discrete dislocation modeling of fracture in plastically anisotropic metals
The intrinsic lattice resistance to dislocation motion, or Peierls stress, depends on the core structure of the dislocation and is one essential feature controlling plastic anisotropy in materials such as HCP Zn, Mg, and Ti. Here, we implement an anisotropic Peierls model as a friction stress within a 2d discrete dislocation (DD) plasticity model and investigate the role of plastic anisotropy on the crack tip stress fields, crack growth, toughening, and micro-cracking. First, tension tests for a pure single crystal with no obstacles to dislocation motion are carried out to capture the general flow behavior in pure HCP-like materials having slip on basal and pyramidal planes. Then Mode-I crack growth in such a single crystal of the HCP material is analyzed using the 2d-DD model. Results show that the fracture toughness scales inversely with the tensile yield stress, largely independent of the plastic anisotropy, so that increasing Peierls stress on the pyramidal planes gives decreasing resistance to crack growth, consistent with recent experiments on Zn. Analyzing the results within the framework of Stress Gradient Plasticity concepts shows that the equilibrium dislocation dipole spacing serves as an internal material length scale for controlling fracture toughness. Furthermore, the fracture toughness of materials with flow stress controlled by a Peierls stress (this work) and of materials with flow stress controlled by dislocation obstacles (prior literature) is unified through the Stress Gradient Plasticity concept. Finally, the DD simulations show that local stress concentrations exist sporadically along the pyramidal plane(s) that emanate from the current crack tip, suggesting an origin for experimentally observed basal-plane microcracking near the tip of large cracks.
Microstructural modeling of crack nucleation and propagation in high strength martensitic steels
International Journal of Solids and Structures, 2014
A dislocation-density based multiple-slip crystalline plasticity formulation, a dislocation-density grain boundary (GB) interaction scheme, and an overlapping fracture method were used to investigate crack nucleation and propagation in martensitic steel with retained austenite for both quasi-static and dynamic loading conditions. The formulation accounts for variant morphologies, orientation relationships, and retained austenite that are uniquely inherent to lath martensitic microstructures. The interrelated effects of dislocation-density evolution ahead of crack front and the variant distribution of martensitic blocks on crack nucleation and propagation are investigated. It is shown that dislocation-density generation ahead of crack front can induce dislocation-density accumulations and plastic deformation that can blunt crack propagation. These predictions indicate that variant distribution of martensitic blocks can be optimized to mitigate and potentially inhibit material failure.
International Journal of Fracture, 2003
A computational approach to the optimization of service properties of two-phase materials (in this case, fracture resistance of tool steels) by varying their microstructure is developed. The main points of the optimization of steels are as follows: (1) numerical simulation of crack initiation and growth in real microstructures of materials with the use of the multiphase finite elements (MPFE) and the element elimination technique (EET), (2) simulation of crack growth in idealized quasi-real microstructures (net-like, band-like and random distributions of the primary carbides in the steels) and (3) the comparison of fracture resistances of different microstructures and (4) the development of recommendations to the improvement of the fracture toughness of steels. The fracture toughness and the fractal dimension of a fracture surface are determined numerically for each microstructure. It is shown that the fracture resistance of the steels with finer microstructures is sufficiently higher than that for coarse microstructures. Three main mechanisms of increasing fracture toughness of steels by varying the carbide distribution are identified: crack deflection by carbide layers perpendicular to the initial crack direction, crack growth along the network of carbides and crack branching caused by damage initiation at random sites.