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The physics of localized plastic deformation and nano-processes in plastic zone ahead of crack tip

#G-2234


Physical aspects of localized plastic deformation and nanostructural changes in plastic zone ahead of microcrack tip and the reasons of nanocrack initiation after LCF in chromium austenitic steels used in power machine building

Tech Area / Field

  • ENV-EHS/Environmental Health and Safety/Environment
  • ENV-MRA/Modelling and Risk Assessment/Environment
  • FIR-DEC/Decommissioning/Fission Reactors
  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors
  • MAT-ALL/High Performance Metals and Alloys/Materials
  • PHY-SSP/Solid State Physics/Physics
  • PHY-STM/Structural Mechanics/Physics

Status
3 Approved without Funding

Registration date
23.12.2015

Leading Institute
Georgian Technical University, Georgia, Tbilisi

Collaborators

  • Korea Advanced Institute of Science and Technology, Korea, Taejon\nThe University of Texas at Arlington, USA, TX, Arlington\nUniversity of Missouri, USA, MO, Columbia

Project summary

There are few industrial sectors where safety issues have the significance they have in the nuclear sector. Notwithstanding the recent advances in structural materials that have enabled the development of improved products which have a positive impact on economic competitiveness, national security, and quality of life, still, there is a big demand in high-quality structural steels and alloys used in nuclear power plants (NPP), turbo generators, fuel elements and other branches of industry. However, because of fatigue, dynamic deformations, thermo-cycling, corrosion, exposure to radiation etc, fracture and degradation of critical parts take place during operating, that in some cases may result in catastrophes (Chernobil, Fukusima). Consequently, degradation and fracture processes in steels are one of the major problems, solution to which ensures normal and safe functioning of all equipment and structures. Therefore, because of the above safety problems, the main effort should be made in the direction of comprehensive and thorough study of fracture physics. Recently, most of the researchers suppose that the points of localized plastic deformation in component blanks are the potential places of crack nucleation. Just in such points a local shear-like transformation of crystal lattice occurs, and nano- or macrocrack form with highest probability. Currently, there is a lack in the researches dealing with the physics of nano- and macrocrack nucleation, regions of their initiation and subsequent growth. The reason of the latter is that the study of the above processes at atomic level in bulk material is practically impossible. However, as to the same processes in thin samples (e.g. thin films prepared for TEM study) some of the researchers doubt the validity of extrapolation of the results obtained for thin films to bulk samples. On the other hand, there is a plenty of papers dealing with the computer simulation of first nanocrack nucleation where a number of atoms, direction of external force etc are taken as an entry conditions. Moreover, a quantum approach to fatigue fracture is also developed. To our strong belief, for confirmation of the results of simulation, it is necessary to study: fine structure of the steel, regions and mechanisms of nanocrack nucleation, direction of their growth and propagation and influence of inclusions and boundaries (low angle as well as high angle boundaries, subgrain boundaries, twins, polygons etc) in polycrystalline materials. Thorough study of the latter is possible at the edges of microcracks or ahead of their tips. The answers to these questions (accurate experimental results) may be obtained through the study of: 1.The regions of localized plastic deformation and 2. Plastic zone ahead of crack tip. This is the way of getting information about changes in structure of the material at nano- and micro-level. Anyway, the changes in internal structure during deformation take place at dislocation level, being a nano-scale defect. These nano-defects and nano-structure form the microstructure of the material, so that the gradient of microstructure and the formed defects are responsible for final properties of a bulk material. Thus, the results obtained for thin films may be used for bulk samples (compare, for simulation of vacancy formation and then nanocrack nucleation, a definite number of atoms and other entry conditions are specified which are close to the conditions existing in thin films). Consequently, the comparison of the experimental results obtained for thin films with those of simulation, provides making of valuable corrections in entry conditions of computer programs and in the design of new advanced steels. Therefore, solution to the above tasks is important and very timely. Furthermore, the obtained results may be used in fundamental researches dealing with the physics of crack nucleation and growth of nano- and microcracks. The results may also be applied to simulation of: 1. New advanced steels with the improved properties, 2. Prediction of premature fracture (fatigue, toughness and brittle) and thereby catastrophes. The promising approach to the prediction of premature fracture and increase in crack resistance of structural steels and other nuclear materials is based on local analysis of the above types of fracture, and is currently intensely developing. Many FP7 and GEN-III projects were dedicated to this problem, and are now included in HORIZON 2020 and GEN-IV. The results of the proposed project may also be applied to the international projects such as EUMAT, FEMES, EMRS, METAMORPHOSE VI etc.

However, here should be noted that in the above projects, dealing with the simulation, the following factors are not taken into consideration yet: anisotropy of microstructure of the material, reasons of localized microplastic deformation, plastic zone ahead of crack tip and the processes occurring in it, points of concentration of elastic internal stresses, regions of nucleation of nano- and microcracks, the reasons and physical mechanism of their formation.

To our opinion, solution to these questions will facilitate application of the results to simulation of fracture of steels with FCC structure. The latter will make it possible to determine the effects of deformation (fatigue and dynamic) and temperature on fracture in order to find out the remedies of its prevention, including fracture caused by corrosion under stress.

Because of the importance of the above problem, many leading industrial companies are engaged in it. Thus INESCO will participate in the INCEFA-PLUS project within the HORIZON 2020 Program of the European Union. The overall objective of the project is to develop new guidelines for the assessment of the environmental fatigue damage of nuclear components and determine the safety conditions of the equipment. In this sense, the result will be the ability of performing more accurate environmental fatigue assessments and obtaining safer operating conditions in both old and new generation nuclear plants.

Early Stage Researcher Position in European Research Network is dealing with the same problems within the Innovative Training Network (ITN) AEOLUS4FUTURE. Fatigue design of complex substructure components (e.g. jacket w. tubular joints) is necessary by use of local concepts to gain a deeper understanding of the damage mechanism and to extend lifetime of critical parts. Materials research topics for the 2-nd Work Program of Horizon 2020 (2016-2017), The European Competence Centre – Joining and integration issues of innovative-to-traditional materials, including the new technologies (innovation) risk management, sustainability, safety and life-cycle analysis (LCA), also intend to solve the problems proposed in this project. The Topic 14 of the program HORIZON 2020, (2016-17) envisages a development of innovative lightweight, high performance metal-based structural materials with tailored microstructure based on modeling and failure mechanism simulation. For the development of such materials, using modeling, it is necessary first of all, to have the answers to the above questions, which we propose to investigate in this project. To better understand the industrial and political context, it is necessary to consider the recent nuclear fission technologies to which the results of the proposed project will considerably contribute: GEN II (NPP construction 1970-2000): safety and reliability of nuclear facilities and energy independence (in order to ensure security of supply worldwide); GEN III (construction 2000-2040): continuous improvement of safety and reliability, and increased industrial competitiveness in a growing energy market; GEN IV (tomorrow, construction from 2040) for increased sustainability (optimal utilization of natural resources and waste minimization).


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