Papers by Author: Sergiy Kotrechko

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Authors: Vladimir Grishchenko, Sergiy Kotrechko, Sergey Mamedov, Oleksandra Zatsarna, Ivo Dlouhý
Abstract: The complexity of calibration procedure for determination of Weibull distribution parameters as well as necessity to perform FEM calculations is one of the shortcomings of conventional versions of Local Approach (LA). This report presents an attempt to ascertain interrelation between local and global characteristics of fracture of pre-cracked and notched specimens of reactor pressure vessel steels, high-strength steels etc. Criterion of cleavage fracture initiation ahead of a crack (notch) is formulated. Dimensionless parameter characterizing the effect of embrittlement by the crack or notch is proposed on the basis of this criterion. Based on the findings of low-temperature tests of pre-cracked or notched specimens of structural steels, it is proved that this parameter may be determined by both local and global characteristics.
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Authors: Sergiy Kotrechko, V. Gryshchenko, Vladislav Kozák, Ivo Dlouhý
Abstract: The contribution is focused on a new methodology description for determination of threshold stress, as the third parameter in Beremin local approach to cleavage fracture that is using three-parameter Weibull statistics. Nature of the methodology lies in tensile testing of rounded notched specimens at liquid nitrogen temperature and corresponding calculations. Reactor pressure vessel steel was chosen as an example for the illustration.
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Authors: Sergiy Kotrechko, Nataliya Stetsenko, Eugene Kolyvoshko
Abstract: Thermo-fluctuation model of failure of one-dimensional nanocrystals is offered for wide range of change in temperature, time and applied loading. Unlike the classic models, which are based on the theory of reaction rates, in present model the failure initiation is related to the instability of atomic interaction. It enables to account for the effect of both temperature and applied loading on the lifetime of crystal within a framework of the unified approach. The model proposed was employed to predict the temperature dependence of both strength and lifetime of carbyne nanoconductor (CNC). Obtained temperature dependences agree well with the MD-simulation findings. Within the framework of the above model, CNC lifetime was estimated. It is exhibited that for temperatures not higher than 600K, CNC have a long-lasting lifetime, so, they can be used in nanodevices.
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Authors: Sergiy Kotrechko, O. Filatov, O. Ovsjannikov
Abstract: Atomic mechanisms of the beginning of plastic deformation and failure initiation in nanoparticles of b.c.c. transition metals are presented in this report. It is shown that strength level of nanoparticles of b.c.c. transition metals is pre-determined by the lattice instability within the local region of the crystal. At uniaxial tension even at low temperatures perfect crystal becomes unstable to shear („orthorhombic“ path), i.e. local shear instability is the main mechanism of stress relaxation in nanoparticles of b.c.c. metals. Specific features of local instability of nanoparticle under hydrostatic tension are considered. A model of the temperature dependence of strength is offered. It is shown that nanoparticle strength decreases as square root function of temperature with temperature growth. Just this is essential difference of the temperature dependence of nanoparticle strength from the same for “ordinary” single- and polycrystals.
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Authors: Sergiy Kotrechko, Igor Mikhailovskij, Tatiana Mazilova, Oleksandr Ovsjannikov
Abstract: A concept of atomic mechanisms governing strength of nanosized defect-free crystals is presented. It is exhibited that these mechanisms consist in local instability of the lattice. Two main reasons for localization of instability in three-dimension (3D) crystals are analyzed, namely, (i) fluctuation of local stresses induced by thermal vibrations of atoms, and (ii) non-uniform distribution of local stresses caused by a surface tension. Based on this conception, explanations of both the temperature dependence of strength of 3D nanocrystals and scale effect are given. Ideas on the reasons for and regularities of change in strength at transition from 3D to 2D (graphene) and 1D (monatomic chain) crystals are represented. It is shown that dimensionality of crystal is one of the main factors governing strength of defect-free crystals. Experimental values of the strength of carbon monatomic chains are given, which times exceeds the strength of graphene and is the highest attainable level of strength in the world.
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Authors: Sergiy Kotrechko, Sergii Mamedov, Ivo Dlouhy, Vladislav Kozák
Abstract: Possibility of use of Local Approach (LA) to prediction of the effect of neutron irradiation on the fracture toughness of pressure vessel steel is discussed. The fundamental of new version of LA to fracture is briefly stated. Specific feature of this version of LA is that Weibull distribution is not used for description of distribution function of fracture probability. Probability of fracture is estimated by modeling of regularities of the crack nucleus formation and instability in polycrystal. Findings on simulation of fracture of reactor pressure vessel steel 2Cr-Mo-V in initial and irradiated states are presented.
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