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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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Processing and Properties of 1D and 2D Boron Nitride Nanomaterials Reinforced Glass Composites
Saggar, Richa ; Cihlář, Jaroslav (referee) ; Tatarko, Peter (referee) ; Dlouhý, Ivo (advisor)
Glasses and ceramics offer several unique characteristics over polymers or metals. However, they suffer from a shortcoming due to their brittle nature, falling short in terms of fracture toughness and mechanical strength. The aim of this work is to reinforce borosilicate glass matrix with reinforcements to increase the fracture toughness and strength of the glass. Boron nitride nanomaterials, i.e. nanotubes and nanosheets have been used as possible reinforcements for the borosilicate glass matrix. The tasks of the thesis are many fold which include: 1. Reinforcement of commercially derived and morphologically different (bamboo like and cylinder like) boron nitride nanotubes in borosilicate glass with the concentration of 0 wt%, 2.5 wt% and 5 wt% by ball milling process. Same process was repeated with reinforcing cleaned boron nitride nanotubes (after acid purification) into the borosilicate glass with similar concentrations. 2. Production of boron nitride nanosheets using liquid exfoliation technique to produce high quality and high aspect ratio nanosheets. These boron nitride nanosheets were reinforced in the borosilicate glass matrix with concentrations of 0 wt%, 2.5 wt% and 5 wt% by ball milling process. The samples were consolidated using spark plasma sintering. These composites were studied in details in terms of material analysis like thermo-gravimetric analysis, detailed scanning electron microscopy and transmission electron microscopy for the quality of reinforcements etc.; microstructure analysis which include the detailed study of the composite powder samples, the densities of bulk composite samples etc; mechanical properties which include fracture toughness, flexural strength, micro-hardness, Young’s modulus etc. and; tribological properties like scratch resistance and wear resistance. Cleaning process of boron nitride nanotubes lead to reduction in the Fe content (present in boron nitride nanotubes during their production as a catalyst) by ~54%. This leads to an improvement of ~30% of fracture toughness measured by chevron notch technique for 5 wt% boron nitride nanotubes reinforced borosilicate glass. It also contributed to the improvement of scratch resistance by ~26% for the 5 wt% boron nitride nanotubes reinforced borosilicate glass matrix. On the other hand, boron nitride nanosheets were successfully produced using liquid exfoliation technique with average length was ~0.5 µm and thickness of the nanosheets was between 4-30 layers. It accounted to an improvement of ~45% for both fracture toughness and flexural strength by reinforcing 5 wt% of boron nitride nanosheets. The wear rates reduced by ~3 times while the coefficient of friction was reduced by ~23% for 5 wt% boron nitride nanosheets reinforcements. Resulting improvements in fracture toughness and flexural strength in the composite materials were observed due to high interfacial bonding between the boron nitride nanomaterials and borosilicate glass matrix resulting in efficient load transfer. Several toughening and strengthening mechanisms like crack bridging, crack deflection and significant pull-out were observed in the matrix. It was also observed that the 2D reinforcement served as more promising candidate for reinforcements compared to 1D reinforcements. It was due to several geometrical advantages like high surface area, rougher surface morphology, and better hindrance in two dimensions rather than just one dimension in nanotubes.
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ReReSearch Management System
Dlouhý, Ivo ; Šperka, Svatopluk (referee) ; Otrusina, Lubomír (advisor)
Aim of this thesis is to design and implement database, file archive, webpage change monitoring system and module communication API in ReReSearch project. It also contains analysis of digital libraries and ReReSearch system, explains terms and describes design, implementation and testing of these components.
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Determination of the Mechanical Properties of Weld Materials based on Hardness Test
Stodola, Martin ; Španiel, Miroslav (referee) ; Dlouhý, Ivo (referee) ; Junek, Lubomír (advisor)
This dissertation thesis deals with determination of the mechanical properties of weld materials based on instrumented hardness test. This work was caused by the research project between IAM Brno Ltd and CEZ corp., when the task was to develop a methodology for evaluation of mechanical material properties using NDT methods. For these reasons the development of methodology is reflected especially for use in practice focusing to determining the mechanical properties of the heterogeneous weld joint on the assembled equipment. The mechanical material properties are determined by carrying out non-destructive tests of the instrumented Vickers hardness. The developed methodology for determining of basic mechanical properties is based on the principle of inverse FEM modeling of the instrumented hardness tests, when output of the testing is indentation curve and measured surface imprint after unloading. The basic material mechanical properties are achieved by comparing the outputs of the experimental instrumented hardness tests and the outputs of the modeled hardness test using FEM.
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Mechanical Reinforcement of Bioglass®-Based Scaffolds
Bertolla, Luca ; Prof. Dr.-Ing. habil. Aldo R. Boccaccini (referee) ; Kotoul, Michal (referee) ; Pabst, Willi (referee) ; Dlouhý, Ivo (advisor)
Bioactive glasses exhibit unique characteristics as a material for bone tissue engineering. Unfortunately, their extensive application for the repair of load-bearing bone defects is still limited by low mechanical strength and fracture toughness. The main aim of this work was two-fold: the reinforcement of brittle Bioglass®-based porous scaffolds and the production of bulk Bioglass® samples exhibiting enhanced mechanical properties. For the first task, scaffolds were coated by composite coating constituted by polyvinyl alcohol (PVA) and microfibrillated cellulose (MFC). The addition of PVA/MFC coating led to a 10 fold increase of compressive strength and a 20 fold increase of tensile strength in comparison with non-coated scaffolds. SEM observations of broken struts surfaces proved the reinforcing and toughening mechanism of the composite coating which was ascribed to crack bridging and fracture of cellulose fibrils. The mechanical properties of the coating material were investigated by tensile testing of PVA/MFC stand–alone specimens. The stirring time of the PVA/MFC solution came out as a crucial parameter in order to achieve a more homogeneous dispersion of the fibres and consequently enhanced strength and stiffness. Numerical simulation of a PVA coated Bioglass® strut revealed the infiltration depth of the coating until the crack tip as the most effective criterion for the struts strengthening. Contact angle and linear viscosity measurements of PVA/MFC solutions showed that MFC causes a reduction in contact angle and a drastic increase in viscosity, indicating that a balance between these opposing effects must be achieved. Concerning the production of bulk samples, conventional furnace and spark plasma sintering technique was used. Spark plasma sintering performed without the assistance of mechanical pressure and at heating rates ranging from 100 to 300°C /min led to a material having density close to theoretical one and fracture toughness nearly 4 times higher in comparison with conventional sintering. Fractographic analysis revealed the crack deflection as the main toughening mechanisms acting in the bulk Bioglass®. Time–dependent crack healing process was also observed. The further investigation on the non-equilibrium phases crystallized is required. All obtained results are discussed in detail and general recommendations for scaffolds with enhanced mechanical resistance are served.
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Determination of Fracture Mechanical Characteristics From Sub-Size Specimens
Stratil, Luděk ; Džugan, Jan (referee) ; Haušild, Petr (referee) ; Dlouhý, Ivo (advisor)
The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.
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Processing of Nb-containing TiAl intermetallics and its Physical and Chemical Aspects
Barták, Tomáš ; Dlouhý, Ivo (referee) ; Zemčík, Ladislav (referee) ; Dlouhý, Antonín (advisor)
Prezentovaná práce se zabývá vakuovým indukčním tavením intermetalické slitiny Ti-46Al-7Nb (at. %) v žáruvzdorných kelímcích na bázi Y2O3. Byla provedena série taveb pro teploty přehřátí taveniny 1630, 1680 and 1730 C a při různých dobách výdrže na této teplotě v rozmezí 5 až 30 minut. Ze slitin ztuhlých v tavících kelímcích byly připraveny metalografické výbrusy, které sloužily k hodnocení mikrostruktury a vyhodnocení složení fází. Pro získání těchto dat byly použity metosy elektronové mirkoskopie SEM a EDS. Kvantitativní hodnocení mikrostruktury, zejména obsahu oxidické faze ve slitině, bylo provedeno pomocí software Adaptive Contrast Control (ACC). Analýza obsahu kyslíku ve ztuhlé slitině byla provedena metodou IGF (fúze v inertním plynu). V této práci jsou pochody na rozhraní slitina/oxidický kelímek posuzovány také z termodynamického hlediska a to s použitím aktivit jednotlivých složek v systému. Data prezentovaná v této práci mohou být použita pro nastavení a optimalizaci procesů tavení intermetalik TiAlNb.
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Preparation and properties of Bioglass based foam materials
Nedbalová, Radka ; Hadraba, Hynek (referee) ; Dlouhý, Ivo (advisor)
The work deals with the preparation and mechanical properties of coated Bioglass® 45S5 based foam materials with open porosity. The samples have been fabricated applying the replication method with use of polyurethane foam. Furthermore, these samples were coated in order to increase the strength characteristics and crack resistance. Polyvinylalcohol and PVA with cellulose microfibrils have been used as coating. Besides microstructural parameters of investigated materials using the SEM images strength characteristics in compression and in tension were also quantified.
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Embrittlement of TiAl intermetalics induced by surface oxidation
Fukátková, Pavla ; Jan, Vít (referee) ; Dlouhý, Ivo (advisor)
This work aimed to study the effect of surface oxidation of TiAl intermetallic alloys, its fracture behavior and study the formation and stability of picked coatings that could prevent these phenomena. ?-TiAl alloy (Ti46Al7Nb) was examined by three different coatings (AlCr, AlCrN and AlCrNAg). Firstly, it was carried out short-term exposure to high temperature in an inert atmosphere for all coatings. Secondly, there were experiments of cyclic exposure to high temperature in normal atmosphere and isothermal high-temperature exposure in a normal atmosphere. During high-temperature experiments in normal atmosphere mass gain was measured and it was monitored a surface texture. Finally, the most promising coatings were chosen, and those were evaluated in bending strength.
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