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Utilization of finite fracture mechanics for correction of fracture toughness measurement on samples with various notch root geometry
Dohnal, Petr ; Majer, Zdeněk (referee) ; Ševeček, Oldřich (advisor)
The work deals with the use of numerical simulations and the theory of finite fracture mechanics for the analysis of conditions for the formation of a crack in a ceramic test sample with a notch (loaded by four-point bending) and possible ways of correcting the fracture toughness determined experimentally on samples with different notch root radius. For this purpose, a parametric calculation model was created to simulate the fracture toughness test by four-point bending. In the first step, the calculation of the critical force at which a crack occurs in a test specimen with a rounded notch and its subsequent unstable propagation through the remaining cross-section of the specimen was debugged. For this purpose, the finite element method (FEM) and the coupled stress-energy criterion were used. In the next step, the procedure of calculation of fracture forces was also applied to a set of experimental samples in order to compare the numerical prediction with experimentally measured values. Two methods of correction of the fracture toughness determined experimentally are also proposed in the thesis. One uses for a correction of the measured values numerical calculation using the coupled stress-energy criterion. In the second case, a correction function was derived using numerical simulation, which no longer needs an FEM calculation for its future use. Correction of the measured fracture toughness using the function proved to be better but not completely effective for the entire range of investigated notch root radii. When comparing with other correction functions found in the literature, it can be stated that the correction procedure proposed in the work provides a better agreement with the real value of the fracture toughness of the given material up to a certain value of the notch root radius.
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Refractory high entropy alloys - preparation and properties
Urbancová, Kateřina ; Jan, Vít (referee) ; Dlouhý, Ivo (advisor)
Tato diplomová práce zkoumá mikrostrukturu a mechanické vlastnosti dvou vysokotavitelných vysokoteplotních slitin (RHEA), MoNbTaVW a HfNbTaTiZr, připravených mechanickým legováním a slinováním (SPS). Teoretická část shrnuje hlavní koncepty vysokoentropických slitin a zaměřuje se především na RHEA, jejich metody přípravy, mikrostrukturu, mechanismy zpevnění a mechanické vlastnosti. V experimentální části termodynamické předpovědi a rovnovážné diagramy potvrdily stabilitu jednofázových tuhých roztoků u obou slitin při nepřítomnosti jiných příměsí. Byla provedena analýza procesu mechanického legování v závislosti na době mletí pomocí zkoumání prášku elektronovou disperzní spektrometrií spolu s rentgenovou difrakcí. Tato analýza ukázala, že MoNbTaVW vytvořil homogenní pevný roztok po 20 hodinách, zatímco HfNbTaTiZr vyžadoval delší dobu mletí. Připravené slitiny vykazovaly vyšší tvrdost, než se uvádí v literatuře, což bylo způsobeno modifikací jejich struktury dalšími fázemi, které vznikly během slinování. Měření lomové houževnatosti naznačila, že MoNbTaVW má vyšší houževnatost ve srovnání s HfNbTaTiZr, přičemž hodnoty se zvyšovaly s délkou mletí. Zavedení železa do MoNbTaVW významně ovlivnilo mikrostrukturu a mechanické vlastnosti. Vedlo to ke složitějšímu fázovému složení a zvýšené tvrdosti, která dosáhla maximální hodnoty 1077,8 HV v kombinaci s lomovou houževnatostí 4,3±0,3 MPa·m0,5. Tato slitina vykazuje rovnováhu mezi vysokou tvrdostí a střední lomovou houževnatostí, což dokazuje její potenciál pro aplikace vyžadující odolnost proti opotřebení. Tato práce zdůrazňuje důležitost vzájemného působení složení slitiny, délky mletí a podmínek spékání při úpravě mikrostruktury a vlastností RHEA pro pokročilé aplikace.
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Hybrid Composites Combining Short Deformable Fibers and Particulate Fillers in Polymeric Matrix
Smrčková, Markéta ; Pavelka,, Vladimír (referee) ; Poláček, Petr (advisor)
In this work, the effect of adding short deformable polyvinylalcohol fibers into dimethacrylate resins on mechanical properties of composite materials was investigated. Furthermore, micro- or nano-fillers were added into the matrix. The effect of filler/short fibers combination on mechanical properties of so-called hybrid composites was studied. Composites were characterized by differential compensation photocalorimetry (DPC), termogravimetric (TGA) and dynamic mechanical (DMA) analysis. The fracture surfaces were examined using scanning electron (SEM) and confocal laser (CLSM) microscopy. Elastic modulus and strength, the critical value of stress intensity factor and strain energy release rate and the viscoelastic properties of composite materials were measured. The mechanical properties of composite materials are affected not only by the type of filler/reinforcement, but also by properties of the matrix. These properties also depend on the way of network formation during photo-initiated polymerization. For this reason, heat of polymerization, degree of conversion and the maximum polymerization rate of mixture of dimethacrylate monomers were determined.
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Low temperature tests and fracture toughness of selected ceramic materials
Beck, Branislav ; Drdlík, Daniel (referee) ; Chlup, Zdeněk (advisor)
Diploma thesis deals with the determination of fracture toughness values of sintered silicon carbide and casted basalt using both the Single Edge V-Notch Beam (SEVNB) and the Chevron Notched Beam (CNB) methods at room and cryogenic temperatures. The first part of this thesis offers general overview and categorization of ceramic materials. Fracture mechanics of the ceramic materials and available fracture toughness testing methods are discussed in following chapters. The last section in the theoretical part is aimed to the microstructure and properties of the silicon carbide. Experimental part of the work summarizes methods used for characterization of selected materials from the microstructural and fracture point of view. Preparation of samples for microstructural observation and samples for fracture toughness determination are described in details. The results of the fracture toughness data for sintered silicon carbide and cast-basalt obtained at room temperature and at -100 °C are displayed. Consideration of possible measurements errors and application of various fracture toughness testing methods are discussed in the next chapter of this work. The difference in the fracture toughness values with the decrease of testing temperature was found. The difference depends on both, the used method of fracture toughness determination and the material used for the tests. The most important results gained during the experimental work are summarized in the chapter “Conclusions”.
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Fracture behaviour of heat affected zone near heterogeneous weld joint
Jelínek, Vladimír ; Válka, Libor (referee) ; Juliš, Martin (advisor)
The study deals with mechanical characteristic of heterogeneous weld joint. The low carbon steel 22K and austenitic steel have been used as a experimetnal materials. The low carbon steel has special properties in basic condition. The sample has been evaluated in basic condition and after annealation. There have been evaluated impact of annealation proces to structure and mechanical charakteristics in experimental part of study. The particularised analysis of microstructure has been done by pictorial analysis. Mechanical characteristics has been examinated in both conditions by 2 different methods – measurment of microhardness according to Vickers and test of fracture toughness on compact tension specimens.
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Optimisation of fibre-matrix interface in ceramic matrix composite
Halasová, Martina ; Černý,, Martin (referee) ; Chlup, Zdeněk (advisor)
This thesis is concerned to the study of behaviour of fibre composites with ceramics matrix. The composite consists of pyrolysed polysiloxane matrix reinforced by ceramic fibre Nextel 720. Main aim of this work is optimisation of fibre matrix interface through the selection of suitable precursor of the matrix with respect to temperature stability, sufficient strength and reasonable fracture toughness. Samples of matrices were exposed to the long term heat treatment in the range 1100 – 1500 °C. The mechanical properties as hardness and indentation elastic modulus were determined after heat treatment. Selected precursors of matrices were used for composite fabrication. Elastic modulus and fracture toughness at room and elevated temperatures were studied. Discussion is dedicated to the description of changes in mechanical properties with respect to chemical processes taking place during high temperature exposition. Further, reasons of fracture behaviour of composite materials are discussed, and finally, gained knowledge and outlined possibilities of subsequent development are summarised.
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Short fiber reinforced composites in dental applications
Matysová, Dorota ; Vladimír, Pavelka (referee) ; Poláček, Petr (advisor)
This diploma thesis deals with short-fibre reinforced composites for dental applications. Particulate barium filler, short polyacrylonitryle fibres or short glass fibres were added to the dimethacrylate matrix to study their effect on the composite material properties. Methods used for this study were thermogravimetric analysis (TGA), differential compensation photocalorimetry (DPC), dynamic mechanical analysis (DMA), three point bending, scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM). The matrix curing was defined by heat of polymerisation, degree of conversion and polymerisation rate. Composite materials were characterized by the glass transition temperature, storage moduli at 35 °C and 100 °C, elastic modulus, strength, the critical value of stress intensity factor and the critical value of energy release rate. Fracture surface investigation is also included in this thesis.
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Fracture Toughness of Metallic Materials and its Testing
Kosek, Lukáš ; Zapletal, Josef (referee) ; Němec, Karel (advisor)
The purpose of the work is to introduce the basic of the fracture toughness of metallic materials and its measuring under the conditions of the plane strain. There are shown essential concepts of the linear elastic fracture mechanics, which is used for the description of the material behavior at fracture of the material with a crack. It defines stress intensity factor and describes the main elements influencing the fracture toughness. Furthermore, the work contains the particular testing methods, specimens and testing equipments. In the conclusion it recapitulates the availability and the application of the linear elastic fracture mechanics.
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The fracture behaviour of metallic alloys with low toughness level
Novotný, David ; Svoboda, Jiří (referee) ; Stratil, Luděk (advisor)
The diploma thesis is focused on determining mechanical properties of alloys produced by powder metallurgy methods. Two alloys were studied, namely a highly entropic CrMnFeCoNi alloy and an oxide dispersion-strenthened FeAlO alloy. Both alloys were made in laboratory amount of test material and miniature test specimens were used to measure their mechanical properties. The main goal of the diploma thesis was to develop a crack preparation methodology, to examine and to evaluate the properties of miniature test specimens for materials with low levels of resilience. In the analytical part of the thesis, tensile and deformation properties and fracture toughness of both alloys were measured. For the highly-entropic CrMnFeCoNi alloy, the thesis focused on the effect of the strain rate on the properties of the alloy. For the oxide dispersion-strenthened FeAlO, the effect of temperature in the 23-600 °C temperature range on its mechanical properties was studied.
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Mechanical Properties of Plastics
Pavlíková, Petra ; Nezbedová, Eva (referee) ; Molliková, Eva (advisor)
Diploma paper deals with fracture toughtness of pipe type HDPE. The influence of material structure on its fracture toughness at temperature 0°C and 23°C measured by PSI test was determined. Experimental results were compared with gained results of S4 test and the conclusions about usage of PSI test in screening of impact properties of polymeric materials were formulated.
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