Papers by Author: Tamaz Eterashvili

Paper TitlePage

Authors: Tamaz Eterashvili, T. Dzigrashvili, M. Vardosanidze
Abstract: Austenitic chromium-nickel stainless steel CrNiNb 18-10 was studied using TEM technique. Characterizations of thin films prepared from bulk cylindrical samples after low-cycle fatigue (LCF) tests were conducted. Focus was made on the dislocation clusters, slip bands, defects and microstructure changes taking place in the steel during LCF. It is shown that microcracks occur in slip bands. Stereographic and trace analyses revealed the microcrack propagation directions. Two types of microcracks were observed: wedge-shaped and with parallel sides. The obtained results on possible reasons and mechanisms of microcrack formation in the above places are discussed in line with the theoretical assumptions and the existing literature.
237
Authors: Tamaz Eterashvili, M. Vardosanidze
Abstract: The microcrack tip plastic zone sizes in austenitic steels are measured using REM and interference microscope. It is shown that the plastic zone size varies from 300µm to 350µm. The importance of determining this parameter is discussed. Based on the analysis of the conventional continuum equations of linear-elastic approach a simple formula is derived for calculation of plastic zone size, R=d E/2π σF, establishing relation between the plastic zone radius (R), microcrack width (d), elasticity modulus (E) and the yield strength of the material (σF). The measured values of plastic zone size are in a good agreement with those reported in literature, and calculated by the above formula.
4145
Authors: Tamaz Eterashvili, T. Dzigrashvili, M. Vardosanidze
Abstract: This study deals with the SEM and optical microscopic characterization of fatigue plastic deformation process during fatigue crack initiation to understand where, why and how cracks initiate under conditions of low cycle fatigue. Samples were prepared from the 13Х11Н2В2МФ high-chromium stainless steel used for fusion power applications. The low-cycle tests were conducted at room temperature with the standard V-notched samples prepared from conventional stainless steel. The following characteristics were studied during fatigue tests: 1 macrocrack propagation, 2. interaction between macrocrack and isolated microcracks, 3. interaction between macrocrack and slip bands, 4. interaction between macrocrack and microstructure elements of the steel. The above experiments show that during macrocrack propagation a plastic zone is formed around it, where isolated microcracks and slip bands of 2-3 different directions are observed. Measurement of plastic zone dimensions after different number of cycles of deformation show that plastic zone size increases during the first stage of cyclic deformation (until definite number of cycles are completed), and then remains unchanged. The observations show that main crack is composed of individual micro-components, the lengths of which are in a good correlation with the dimensions of microstructure elements of the steel (former austenite grains, martensite crystals). It was revealed that during growth, as a rule, macrocrack rarely propagates along isolated microcracks and slip bands. Direction of macrocrack propagation changes while passing from one microstructure element to another, so that main direction is the same. No preferable transcrystalline or intercrystalline propagation of macrocrack has been observed in the investigated steel. It is shown that after subsequent fatigue tests, dimensions of the previously created slip bands increase, and additional new slip band are also formed. The sites and frequency of slip bands’ formation in plastic zone are also studied. It was observed that the boundaries and mainly the sites of intersection of martensite crystals are the sites of isolated (rough) microcracks’ formation. The dimensions of slip bands are comparable with those of martensite crystals. The angles between the main crack propagation direction and slip bands varied from 30o to 60o, however, most of the slip bands were oriented at 45o to the main crack. Based on the obtained results a conclusion is made that plastic deformation in samples go inhomogeneously. In plastic zones, along with the heavily deformed areas, almost non-deformed areas are also observed. The speed of fatigue fracture increases with the increase in frequency and amplitude of deformations. Generally, the annealed samples are destructed prematurely in comparison with non-annealed ones of the investigated steel.
385
Authors: Nugzar Khidasheli, Gocha Beradze, Elguja Miminoshvili, Elguja Kutelia, Tamaz Eterashvili
405
Authors: Tamaz Eterashvili, Elguja Kutelia, T. Dzigrashvili, M. Vardosanidze
Abstract: Low cycle fatigue of high-chromium 13Х11Н2В2МФ stainless steel has been studied after cyclic tests at room temperature with the frequency of loading, 0.45Hz and amplitude, ± 1mm. The samples were v-notched with the dimensions x2x50, where =3mm. The peculiarities of fatigue crack propagation and influence of heat treatment, sizes of grains and laths, and disposition of microcrack and microstructure elements of the steel were studied. Next, the main effect on propagation direction is caused by the shape of grains and laths. It turned apparent that main microcrack is composed of individual micro-components with the lengths correlating with the dimensions of grains and martensitic laths. During growth crack propagation direction changes from lath to lath; however, general trend remains unchanged. The results of tests indicate that speed of fatigue failure rises when the frequency and amplitude of loading increases. The work includes x-ray characterization of the steel, statistical distribution curve for angles between the main direction of macrocrack propagation and micro-components, and explanation of micro- and macrocrack propagation alteration is given.
298
Authors: Tamaz Eterashvili, T. Dzigrashvili, M. Vardosanidze
Abstract: The structure of austenitic steel before and after 25% of total number of cycles of low cycle fatigue tests conducted at room temperature is studied using TEM. It is shown that the cyclic deformation of the steel proceeds heterogeneously. The microstructure of the steel is investigated in the area between the deformed and undistorted parts of the samples. The crystallography of the observed twins and the slip bands is specified. The value of local plastic deformation within a micro area of a grain is measured, and the influence of microstructure on crack initiation is discussed.
1487
Authors: Tamaz Eterashvili, T. Dzigrashvili, M. Vardosanidze
Abstract: The structure of austenitic steel before and after 25% of total number of cycles of low cycle fatigue tests conducted at room temperature is studied using TEM. It is shown that the cyclic deformation of the steel proceeds heterogeneously. The microstructure of the steel is investigated in the area between the deformed and undistorted parts of the samples. The crystallography of the observed twins and the slip bands is specified. The value of local plastic deformation within a micro area of a grain is measured, and the influence of microstructure on crack initiation is discussed.
3505
Authors: Tamaz Eterashvili, M. Vardosanidze
73
Authors: Tamaz Eterashvili, T. Dzigrashvili, M. Vardosanidze
Abstract: The microstructure changes, development of micro plastic deformation and formation and distribution of slip bands were studied. It is shown that development of micro deformation during LCF depends on loading conditions (amplitude and number of cycles) and microstructureIt is shown that as non-localized as well as localized micro plastic deformation takes place because of structural inhomogeneity. Supposedly, the localized deformation is related to the sites of internal stress concentration accumulated during the LCF.The effect of microstructure of structural steels on the rate of local cyclic deformation, leading to nucleation and growth of slip bands of fatigue cracks, was studied. The interaction of slip bands with precipitates, grain boundaries and low-angle boundaries were also analyzed.The sites of nucleation of primary and secondary slip bands were identified, and the following aspects were considered: 1. the possibility of microcrack nucleation on (or in) slip bands, 2. The kind of slip bands the slip bands may nucleate in, 3. The potential sites (except the slip bands) and reasons of nanocrack formation are specified.
141
Showing 1 to 10 of 11 Paper Titles