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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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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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Fracture Behaviour of Steels and Their Welds for Power Industry
Al Khaddour, Samer ; Kohout, Jan (referee) ; Válka, Libor (referee) ; Dlouhý, Ivo (advisor)
Práce byla zaměřena na ověření platnosti koncepce master křivky pro hodnocení heterogenních svarových spojů, resp. teplotně stárnutých svarů. Současně bylo cílem disertace vyvinout kvantitativní model pro predikci referenční teploty lokalizující tranzitní oblast na teplotní ose za použití dat získaných z tahové zkoušky, a to za použití metody umělých neuronových sítí. Studie je současně zaměřena na heterogenní svarový spoj připravený tavným svařováním. Je zacílena na hodnocení lomového chování v tranzitní oblasti nejméně odolné části svaru, tj. tepelně ovlivněné zóny ferritické oceli v blízkosti zóny natavení s vysokolegovaným materiálem. Pro predikci referenční teploty master křivky je použita zmíněná metoda neuronových sítí, a to za použití dat z tahových zkoušek a měření tvrdosti. Predikovaná referenční teplota byla ověřována na základě výsledku experimentálních měření. Vytvoření modelu za použití neuronových sítí vyžaduje dostatečné množství dat a není vždy snadno tuto podmínku splnit. V případě sledovaného problému to znamenalo použití dat z dostatečně věrohodných zdrojů (skupiny Křehký lom ÚFM AVČR) a se známou metalurgickou historií. Smysl práce je tak možno spatřovat ve vývoji modelu neuronové sítě, která bude dostatečně přesně predikovat referenční teplotu. Celkově byla pro tyto účely použita data z 29 nízkolegovaných ocelí. Pro účely vývoje byly použity kromě hladkých zkušebních tyčí, rovněž tahové zkoušky s obvodovým vrubem testované při kritické teplotě křehkosti (mez makroplastických deformací) a při teplotě pokojové. Při tvorbě modelu byla postupně v různých kombinacích využita všechna data z uvedených zkoušek. Studie ukázala, že referenční teplota charakterizující tranzitní chování lomové houževnatosti oceli s převažující feritickou strukturou je jedinečným parametrem predikovatelným na základě vybraných charakteristik tahových zkoušek.
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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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Initiation of stable crack propagation of selected building composites
Majtánová, Romana Viktória ; Veselý, Václav (referee) ; Keršner, Zbyněk (advisor)
The purpose of my bachelor’s thesis is study of the effects of fibres in normal-weight concrete and light-weight concrete. Specimens were prepared from concrete with polypropylene fibres FORTA FERRO of three different lengths – 19, 38 and 54 mm – and also reference concrete (without fibres) was prepared. This thesis contains gradual evaluation of fracture tests on specimens from mentioned materials. The used fracture test was a three–point bending fracture test on beams with a central notch. From the measured load–deflection in the middle of span and load–crack mouth opening displacement diagrams were determined values of modulus of elasticity, unstable fracture toughness, initiation part of fracture toughness and also load level for beginning of stable crack propagation composites. For determination was used „double–K“ fracture model. For illustration is described evaluation using the effective crack model.
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Evaluation of Fracture Tests on Selected Building Material Specimens via Double-K Model
Havlíková, Ivana ; Králík,, Juraj (referee) ; Němeček,, Jiří (referee) ; Keršner, Zbyněk (advisor)
The purpose of dissertation is the analysis of the calculation of fracture parameters using Double-K fracture model for quasi-brittle specimens with the stress concentrator loaded by three-point bending or wedge splitting. To calculation of these parameters was used the developed DKFM_BUT software in Microsoft Excel application with using of Visual Basic programming language. Furthermore, the adequate shape functions and compliance functions were introduced for the selected wedge splitting test configurations. Main part of this dissertation is the series of comprehensively implemented and evaluated fracture experiments on specimens from advanced building materials, while the attention was paid to the analysis of experimental data. Finally, the selected results obtained using mentioned software support were presented and discussed.
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An assessment of fracture behaviour of weld joints used in the power industry
Rádsetoulal, Milan ; Šiška, Filip (referee) ; Stratil, Luděk (advisor)
This thesis is focused on assessment of fracture behaviour of heterogeneous welded joint. The goal is to determine fracture behaviour at the interface of ferritic base metal and austenitic weld metal at temperature of 255°C. This kind of weld is widely used at energy industry. Fracture toughness was evaluated using CT specimens with size 0,5T in order to determine the initiation values of stable crack propagation. Fracture behaviour of the weld is correlated with results of metallography and fractography analysis. Both light and scanning electron microscopy is used for fractography analysis. An assessment of fracture behaviour of the interface of the weld joint is first of all connected with a need of suitable location of cycled crack to the interface of metals. In the case of well-prepared crack the lowest values of J-integral are observed. In other cases of slight crack deviation from the notch plane fracture values increase. Due to a large scatter of mechanical properties of heterogeneous weld joints an increased number of test specimens is needed to obtain relevant and conservative fracture toughness values. Correlation of fracture toughness with the results of fractography analysis leads to clarification of the crack trajectory and to description of fracture mechanism.
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