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Mechanical response during dynamic loading of layers prepared by cold spray technology
Šprincl, Jan ; Válka, Libor (referee) ; Řehořek, Lukáš (advisor)
Cold spray is a relatively modern technology that falls into the additive category. It is a coating method that doesn´t cause the input material to melt. This brings many advantages, including the minimization of changes to the original structure, the absence of a heat-affected zone, and the possibility of combining different types of materials. This work mainly focuses on the deposition of Ti6Al4V titanium powder, along with other materials to supplement or modify the spray, improve its efficiency, and enhance its mechanical properties. Ballistic tests, three-point bending tests, indentation methods, optical analysis using light and electron microscopy, X-ray diffraction, differential scanning calorimetry, and the influence of thermal treatment on the structure were used to evaluate the properties. Thermal treatment led to the formation of completely new phases in the structure, resulting in a significant increase in the coating's hardness along with a reduction in its porosity. Highly hard phases based on TiC and Ti5Si3 were formed in a tough matrix composed of varying ratios of Ni-Ti and associated elements – V, Al, Si. Finally, proposals for potential future development of this material are described. Preconditions for specific improvements in the methods and procedures used in this work are laid out, with a very promising outlook for the future of this material and its potential implementation in real ballistic protection, typically in the military industry.
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Elastic characteristics of coatings
Olivo, Tomas ; Řehořek, Lukáš (referee) ; Válka, Libor (advisor)
The thesis focuses on the determination of Young's modulus of elasticity in tension of coatings with the inclusion of the effect of porosity using the impulse excitation method (IEM). In the theoretical part, the basics of the impulse excitation method and nanoindentation are discussed. Subsequently, the types of composite materials are presented, followed by a description of the Cold Spray technology used to produce the coatings presented in this work. In the next section, the composite models used to evaluate the measured data are defined. The experimental part is devoted to the description of the preparation of the test specimens, i.e., the substrate-coating system, together with the nanoindentation and pulsed excitation test procedure. Based on these tests, the tensile elastic moduli of the deposited coatings were evaluated using four composite models and compared with the results from the nanoindentation test. The most appropriate of these models was found to be the one based on the rule of mixtures with the inclusion of the nonlinear effect of porosity.
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Small Punch test brittle materials
Saňka, Martin ; Válka, Libor (referee) ; Zapletal, Josef (advisor)
Bachelor’s thesis is focused on description of methods for material testing of miniature samples called „Small Punch Test“. Thesis describes the basic test procedures, interpretation of their results and sample removal techniques. Recognized methods of correlating test results with parameters obtained from conventional material tests and after test sample evaluation were described in the thesis. Furthermore, the thesis deals with the evaluation of mechanical properties of brittle materials using the „Small Punch Test“, where a statistical appoach to obtaining their characteristic strength and specifying the procedure for determining the fracture toughness was described.
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Point Pleasant bridge collapse - casual analysis of the accident
Jagoš, Jiří ; Němec, Karel (referee) ; Válka, Libor (advisor)
The bachelor thesis is focused on the analysis of the causes of the accident of the Silver Bridge, which occurred in 1967, forty years since the introduction of the bridge to the traffic. In the work are discussed in detail all possible causes of the collapse of the bridge on the basis information and facts mentioned both directly in the inquiry reports of committees, as well as in documents and literary sources, dealing with material analysis of the structural nodes of the wrecked bridge. The work carried out critical analysis of the available and relevant information about the crash. The bachelor thesis outlines also the possible causes of the fall and collapse of the bridge from structural point of view and from terms of the material.
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Mechanical properties of magnesium alloy AZ91E prepared by ECAP method
Darida, Jiří ; Válka, Libor (referee) ; Pantělejev, Libor (advisor)
This thesis deals with mechanical and fatigue properties of magnesium alloy AZ91 processed by EX-ECAP method. This method involves the application of extrusion followed by equal-channel angular pressing. To obtain basic mechanical characteristics, the tensile test were carried out at room and elevated temperatures. Further the fatigue tests were performed and obtained data were used to compile S-N curve. The work also includes metallographics analysis of microstructure and fractographic analysis of fracture surfaces of tensile and fatigue test specimens.
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Determination of mechanical properties of light metals and alloys and composites via small punch test
Langer, Jiří ; Dobeš, Ferdinand (referee) ; Válka, Libor (advisor)
The aim of diploma thesis is estimate mechanical properties (yield strength, maximum strength and elongation) of light alloys by means of SPT. For the experiments were selected aluminium alloys (Al 2024, Al 6082 T6, Al 7020 and NASA 398) magnesium alloys (MgZnMn, AZ31, AZ61) and composites (AZ91 + 20 % saffilu a Al + Al4C3). Theoretical part of this thesis is focused on analysis of conversion formulas, which were made from SPT data and conventional testing. Experimental part is dedicated to evaluation of experimental data and critical analysis validity of conversion formulas. In this part of thesis is discused the problematics of reproducibility methodology of SPT.
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The use of small punch tests for materials with a higher level of structural inhomogeneity
Gordiak, Michal ; Řehořek, Lukáš (referee) ; Válka, Libor (advisor)
Master's thesis deals with evaluating applicability of correlation relationships between material characteristics determined by Small Punch Test and standard tensile test for material AlSi7Mg0,6 manufactured by casting and technology SLM. Results of Small Punch Tests are correlated with yield strength, tensile strength, elongation, and Young's modulus of elasticity. For each material characteristic various correlation methods are compared, while for each method corresponding coefficients are determined. Consequently, the applicability of individual methods is evaluated by substituting coefficients determined by various studies. Primarily analyzed are correlation methods for which future normalization is expected. The results of master's thesis show that structural inhomogeneity caused by SLM process does not result in high inaccuracies in determining material characteristics. Larger impact on material characteristics has high porosity, which was identified in cast material and led to significant deviations in evaluating tensile strength and elongation.
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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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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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