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The numerical analysis of interactions between twins and precipitates in magnesium alloys
Bogdan, Miloš ; Gröger, Roman (referee) ; Šiška, Filip (advisor)
Hlavním cílem této práce je analýza napěťových polí, indukovaných dvojčatěním ovlivněných přítomností precipitátu před čelem dvojčete. Zvolený případ popisuje tahové dvojče {1012} 101 1 s lamelarními precipitáty v hořčíkové slitině AZ31. Systém je modelován metodou konečnýh prvků jako 2D elastická eliptická inkluze, se dvěmi elastickými precipitáty tyčovitého tvaru, nacházející se před čelem elipsy obklopené plastickou matricí modelovanou pomocí krystalové plasticity. Analýza dvojčat a precipitátů s různou tlouštkou ukazje inhibující účinek precipitátů na růst tloušťky dvojčat. Velikost tohoto účinku se mění s tloušťkou dvojčete v důsledku komplexní interakce mezi dvojčetem precipitátem a indukovanou plastickou zónou.
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High-entropy alloys – bulk alloys and surface treatments
Pišek, David ; Šiška, Filip (referee) ; Hadraba, Hynek (advisor)
Master‘s thesis deals with the preparation and evaluation single-phase high-entropy alloy based on cobalt, chromium, iron, nickel and manganese and its variants strengthened by dispersion of oxidic particles. High-entropy alloy was prepared in powder form by mechanical alloying from the equiatomic proportions of atomic powders. Obtained powder was subsequently compacted by spark plasma sintering. By the method of mechanical alloying were successfully prepared single-phase high-entropy alloy and its variant strengthened by dispersion of nanometric yttria oxides. It has been found that the oxide particles present in the microstructure of high-entropy alloy significantly block mobility of grain boundary and dislocation at elevated temperatures. As a result of this behavior were observed doubling of alloy strength and decreasing of creep rate at 800 °C.
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Modification of HVOF and cold spray deposited coatings via electron beam technology
Vacek, Petr ; Šiška, Filip (referee) ; Čížek, Jan (advisor)
The aim of this thesis was to modify microstructure and coating-substrate interface of CoNiCrAlY coatings deposited by HVOF and cold spray on Inconel 718 substrates. Electron beam remelting and annealing in a protective atmosphere were used to modify the coatings. Microstructure, chemical and phase composition were analyzed. The effect of beam current, transversal velocity and beam defocus on remelted depth was evaluated. As-sprayed microstructure and chemical composition of coatings were analyzed and compared with remelted samples. The effect of annealing of the as-sprayed and remelted samples was evaluated. Remelted layers exhibited dendritic structure. Chemical composition changed only after remelting of interface and part of a substrate. When only the coating was remelted, chemical composition remained the same. Phases coarsened after the annealing. Chemical composition changed after annealing due to the diffusion.
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Fracture toughness testing at high temperature range using miniaturized CT specimens
Holas, Jiří ; Kuběna, Ivo (referee) ; Šiška, Filip (advisor)
This master´s thesis deals with the evaluation of fracture behavior of ODS steel MA956 at high temperature range. This behavior was tested by using miniaturized CT specimens, on which were performed experiments to measure of ductile crack growth resistance curves (J-R curves). The value of the fracture toughness was determined from these J-R curves. Fracture properties were consequently evaluated by using fractographic analysis of the fracture surfaces. Structural properties of material was identified by hardness measurement and analyzed by metallographic methods. Results of the measurements show drop of the fracture toughness with respect to the increasing temperature.
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Strength, fracture and fatigue properties of materials and weld joints from feeder piping of water turbine
Dlouhý, Ivo ; Kruml, Tomáš ; Čupera, Pavel ; Chlup, Zdeněk ; Šiška, Filip
Sets of deformation, strength, fracture mechanical and fatigue properties of base materials and weld joints from feeder piping of water turbine is summarised in the report. There are data from 30 mm thick steel sheet and weld joint of this sheet with the casting part. R-curves have been also measured by loading - unloading method at room temperature and at 0°C. In addition tests of CT specimens in the upper shelf and transition region have been carried out with the aim to specify the lowest (conservative) value of the fracture toughness. Fatigue tests are reported too, mainly based on cyclic response curves and life time curves of the base material and heat affected zone of the weld joint between sheet materials.
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Strenght, fracture and fatigue properties of base materials and weld joint of inflow piping to water turbine (PART I - Weld plate to plate)
Dlouhý, Ivo ; Kruml, Tomáš ; Čupera, Pavel ; Chlup, Zdeněk ; Šiška, Filip
Sets of strength, strain, fracture toughness and fatigue properties are summarised in the report characterising the base material, heat affected zones and weld joint material of steel used in water turbine system. Data obtained from 30 mm sheet materials and its weld joint are given. Tests have been carried out in agreement with recent standards and loading curves for finite element calculations have been also supplied. Crack resistance curves have been also generated by unloading technique at 0 and +20°C. Tests of compact tensions in transition regions enabling to determine reference temperature have been carried out. There are also data from fatigue tests, namely cyclic response and fatigue life curves, for the same materials.
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The Gurson-Tvergaard-Needleman model calibration from sub-size test specimens by size corresponding to the half of Charpy specimen
Stratil, Luděk ; Šiška, Filip ; Hadraba, Hynek ; Dlouhý, Ivo
The paper deals with a utilization of miniaturized tensile specimens for purposes of local approach application from limited amount of test material. The motivation for application of those miniaturized specimens is the utilization of volume of test material corresponding to the half of Charpy specimen. The tensile testing of miniaturized specimens was conducted at room temperature in ductile regime of the Eurofer97 steel followed by detailed fractography and quantitative analysis of micromechanism of damage. The parameters of Gurson-Tvergaard-Needleman micromechanical model of ductile fracture were calibrated. Except for the slight differences in strength and deformation characteristics no other differences between two sizes of tensile specimens were found. The calibrated sets of parameters of GTN model using the same element size were very similar. The application of different calibrated sets of GTN model parameters for simulation of damage in cracked specimen (R-curve) is sensitive to the element size and needs its calibration.
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A calibration of Gurson-Tvergaard- Needleman micromechanical model of ductile fracture
Stratil, Luděk ; Hadraba, Hynek ; Šiška, Filip ; Dlouhý, Ivo
Fracture mechanics characteristics of ductile fracture often depend on geometry and size of specimens. A micromechanical modelling of material damage proposes a tool for treatment of geometry and size effects in fracture mechanics. Application of micromechanical model for certain materiál requires a calibration of model‘s parameters. This contribution presents the calibration procedure of Gurson-Tvergaard-Needleman model of ductile fracture for Eurofer97 steel. The identification of ductile damage of the steel and its tensile properties at various level of stress triaxiality by testing smooth and notched bars were determined in the previous study. The calibration procedure was performed by hybrid method as a combination of FEA simulations of tensile tests and fractography analyses. The Gurson-Tvergaard-Needleman model performance is greatly influenced by true stress-true strain curve of the material.
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