Národní úložiště šedé literatury Nalezeno 160 záznamů.  1 - 10dalšíkonec  přejít na záznam: Hledání trvalo 0.01 vteřin. 
Metal Matrix Composites Prepared by Powder Metallurgy Route
Moravčíková de Almeida Gouvea, Larissa ; Novák, Pavel (oponent) ; Hadraba, Hynek (oponent) ; Dlouhý, Ivo (vedoucí práce)
The development of new materials for applications in extreme environments is currently of a great importance in modern engineering technologies. Hence, the industries' requirement for enhanced structural performance of materials is constantly increasing. In the light of that, this study aims to evaluate promising compositions of high-entropy alloys for critical applications produced by powder metallurgy through a combination of mechanical alloying and solid state sintering. For comparative purposes, the selected compositions were produced by melting routes in liquid state as well, such as vacuum induction melting and subsequent casting or vacuum arc-melting. The powder metallurgy route enables a consequential development of metal matrix composites (MMC) via the manufacturing of oxide dispersed strengthened HEAs. This is possible due to inherent in-situ reactions during the process. In case of melting route fabrication, metallic materials with great differences in structures and related properties are manufactured, compared to those produced by powder routes. The produced MMCs and their melted counterparts are thoroughly studied. A comprehensive evaluation of the influence of the different processing methods, especially on the materials’ microstructural features and their mechanical properties is undertaken, including the effect of heat treatments on the phase transformations and stability of the materials.
Nástroj pro kontrolu správnosti návrhových diagramů v UML
Dlouhý, Ivo ; Bartík, Vladimír (oponent) ; Rychlý, Marek (vedoucí práce)
Cílem diplomové práce je vytvořit nástroj pro kontrolu správnosti návrhových diagramů v UML zvláště diagramu tříd. Práce popisuje jazyk UML a souvisejících standardy, definuje problém správnosti UML a vysvětluje přístup kontroly správnosti UML pomocí databáze vzorů nesprávnosti. Dále navrhuje technologii QVT vhodnou pro implementaci vzorů pro kontrolu správnosti.Problém je rozdělen na více částí, mezi které patří sdílená databáze vzorů chyb v UML spravovatelná přes webové rozhraní, samostatný nástroj pro použití z příkazové řádky a zásuvný modul pro UML návrhový software Visual Paradigm. Všechny navržené části jsou navrženy, implementovány, otestovány a vyhodnoceny. Důraz je kladen na otevřenost a rozšiřitelnost nástroje.
Geopolymers Incorporating Wastes and Composites Processing
Taveri, Gianmarco ; Perná,, Ivana (oponent) ; Pouchlý, Václav (oponent) ; Dlouhý, Ivo (vedoucí práce)
Buildings construction and realization of public infrastructures have always been a primary need in the human society, developing low cost and user-friendly materials which also encounter safety and durability requirements. Portland cement is the most used material in construction industry from the industrial revolution up to date, but the raising concerns related to the climate change are pushing the governments worldwide to replace it with more eco-friendly and greener materials. Geopolymers are considered to be best alternatives to Portland cement in construction industry, but issues related to cost and mechanical properties are still hindering the commercialization of this material. Geopolymer incorporating wastes is one of the solutions. Fly ash, a thermal power plant by-product, and borosilicate glass, a recycled glass from pharmaceutical vials, are suitable candidates in geopolymers activation. NMR and FTIR spectroscopies demonstrated that borates from borosilicate glass are active compounds in geopolymerization, substituting the alumina is its role, composing a B-Al-Si network never observed before. Various fly ash and borosilicate glass weight contents were studied in terms of mechanical properties (compression test, 3-point bending test). It was found that fly ash 55 wt.% and borosilicate 45 wt.% composition activated in 13 M NaOH solution holds the best compressive and flexural strength (45 and 4 MPa respectively), 25% stronger than similar counterparts found in literature. Cellulose fibres in different weight contents were dispersed into the geopolymeric paste to produce geopolymer composites, with the aim to render the material more suitable for structural applications. 3-point bending test showed an improvement of the flexural strength of about 165% (12 MPa), while the chevron notch method displayed a fracture toughness of 0.7 MPam1/2, in line with the results of geopolymer composites found in literature. In this thesis work, fly ash was also successfully densified in 3 M NaOH solution and distilled water through a new method based on hydraulic pressure, called hydro-pressure sintering. This innovative technology involves a drastic reduction of NaOH utilization in geopolymerization, rendering the material more eco-friendly. XRD spectroscopy conducted on produced samples revealed a higher formation of crystals, most likely induced by the application of hydraulic pressure (450 MPa).
Struktura a vlastnosti tepelných bariér typu YSZ nanesených na krycí vrstvy CoNiCrAlY přetavené elektronovým paprskem
Slavíková, Barbora ; Jan, Vít (oponent) ; Dlouhý, Ivo (vedoucí práce)
Diplomová práce se zabývá charakterizací struktury a vlastností tepelných bariér YSZ nanesených pomocí technologie hybridního plazmatického systému na vazebné povlaky CoNiCrAlY modifikované pomocí elektronového paprsku a vakuového žíhání. Depozice vazebných povlaků byla provedena za použití technologie vysokorychlostního nástřiku plamenem a studené kinetické depozice. V rámci experimentálního vyhodnocení byla analyzována mikrostruktura a chemické složení krycích keramických nástřiků nanesených ve formě prášku a suspenze. Stejně tak byly vyhodnoceny i vazebné povlaky ve stavu po přetavení za použití dvou parametrů elektronového paprsku. Dále byly sledovány změny v mikrostruktuře a chemickém složení přetavených nástřiků po žíhání. Následně byly vyhodnoceny mikromechanické vlastnosti keramických a vazebných povlaků. Keramické povlaky nanesené v prášku vykazovaly strukturu tvořenou splaty, zatímco povlaky nanesené ve formě suspenze měly jemnou strukturu tvořenou kolumnárními zrny. Struktura přetavených vazebných nástřiků byla tvořena dendrity. Žíhání pak mělo za následek zhrubnutí jednotlivých fází a došlo ke změnám v chemickém složení způsobené difúzí prvků.
NEW APPROACH TO STRESS-STRAIN CURVE PREDICTION USING BALL INDENTATION TEST
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
This work is concerned with the method for predicting stress-strain behavior of material using instrumented indentation technique. High strength low alloy steel with different thermal treatment was taken into the analysis. Heat treatment for the steel was performed to obtain different mechanical properties. Assessment of mechanical properties was done by using inverse technique of the finite element analysis. The results were confronted with conventional test parameters and prediction procedure defined such Automated Ball Indentation Technique (ABIT). Comparison of the material curves shows good agreement with tensile test properties which makes this non-destructive method suitable for industrial application.
Method of Threshold Stress Determination for a Local Approach to Cleavage Fracture
Kotrechko, S. ; Gryshchenko, V. ; Kozák, Vladislav ; Dlouhý, Ivo
The contribution is focused on a new methodology description for determination of threshold stress sigma th, as the third parameter in Beremin local approach to cleavage fracture that is using three-parameter Weibull statistics. Nature of the methodology lies in tensile testing of rounded notched specimens at liquid nitrogen temperature and corresponding calculations. Reactor pressure vessel steel was chosen as an example for the illustration.
Prediction of the Traction Separation Law of Ceramics Using Iterative Finite Element Modelling
Kozák, Vladislav ; Chlup, Zdeněk ; Padělek, P. ; Dlouhý, Ivo
Specific silicon nitride ceramics, the influence of the grain size and orientation on the bridging mechanisms was found. In ceramic matrix composites, crack-bridging mechanisms can provide substantial toughness enhancement coupled with the same and/or increased strength. The prediction of the crack propagation through interface elements based on the fracture mechanics approach and cohesive zone model is investigated. From a number of damage concepts the cohesive models seem to be especially attractive for the practical applications. Within the standard finite element package Abaqus a new finite element has been developed; it is written via the UEL (user’s element) procedure. Its shape can be modified according to the experimental data for the set of ceramics and composites. The element seems to be very stable from the numerical point a view. The shape of the traction separation law for four experimental materials is estimated via the iterative procedure based on the FEM modeling and experimentally determined displacement in indentation experiments, J-R curve is predicted and stability of the bridging law is tested.
Subsized Specimens for Fracture Resistance Characterisation Including Transferability Issues
Dlouhý, Ivo ; Stratil, Luděk ; Šiška, Filip
The contribution is focused on characterization of methods enabling to apply small/subsized specimens for fracture resistance characterization. The applied methods are divided into transition region and upper shelf region. The approaches used in the upper shelf region represent at the same time methods applicable for ductile materials without transition. Relating to subsized samples two basic approaches are applicable: (i) miniaturized samples based on common standard ones and (ii) specific specimens/methods, e.g. small punch test etc. The results described in the paper belong to the first group. For interpretation of data generated under low constraint conditions toughness scaling models and master curve approached are commented. In ductile region, either the sample used generate valid toughness characteristics, or, if not, there is no way how to correct measured data except damage quantification through micromechanical models.
Fracture Toughness of Massively Transformed and Subsequently Heat Treated TiAl Intermetallic Compound
Sakurai, K. ; Hasegawa, M. ; Dlouhý, Ivo
The effects of massive transformation and subsequent heat treatments on the microstructure of Ti-46Al-7Nb-0.7Cr-0.2Ni-0.1Si (mol%) intermetallic compounds are studied. Massive transformation occurs at the center region of the specimen by cooling from alpha single phase state. At the surface side of the specimen, alpha phase has remained. Fine convoluted microstructure with alpha 2, gamma phases and lamellar structure has formed by heating at (alpha + gamma) two phase state after massive transformation. Colony size or grain size is about 25 micrometer. Fine fully lamellar structure is obtained after heat treatment of convoluted microstructure at alpha phase for 60 s. Fracture toughness seems to be increasing with the increase in lamellar colony size. However, some massively transformed specimens show lower toughness due to the formation of microdamage present in samples before the test.
Crack Resistance Characterization in TiAl Intermetallics with Enhanced Toughness
Dlouhý, Ivo ; Stratil, Luděk ; Fukutomi, H. ; Hasegawa, M.
The paper is focused on the analysis of the role of lamellar microstructure in fracture performance of model TiAl intermetallic compound. Coarse lamellar colonies and, at the same time, fine lamellar morphology were prepared by compressive deformation at 1553 K (region of stable alpha phase in TiAl equilibrium diagram) followed by controlled cooling to 1473 K (region of alpha+gamma phase) with delay on this temperature and then cooling down. The fracture toughness was evaluated by means of chevron notch technique. In addition, because of enhanced toughness, crack resistance curves were obtained by load - unload technique of pre-racked beams, namely in two directions of crack propagation relative to lamellar structure. Extensive development of shear ligament toughening mechanism was observed in fracture surfaces leading to quite good fracture toughness thanks to the heat treatment applied.

Národní úložiště šedé literatury : Nalezeno 160 záznamů.   1 - 10dalšíkonec  přejít na záznam:
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