Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta dissertação de Mestrado do programa de Pós-Graduação em Ciência e Tecnologia dos Materiais é apresentado um estudo para a caracterização do concreto reforçado com fibras de polipropileno e de aço pela análise das imagens de microtomografia computadorizada por transmissão de raios X (μCT). Foram produzidos corpos de prova de concreto para determinar a sua resistência mecânica à compressão. As imagens foram obtidas no sistema Skyscan, modelo 1174, reconstruídas e analisadas. Foi possível observar na análise das imagens a estrutura da fibra de aço dispersa na matriz do concreto e quantificá-las pelo programa de análise de imagens Ctan e perceber um ganho na resistência mecânica em relação ao concreto sem fibras. Não foi feito a quantificação das amostras de fibras de polipropileno dispersas na matriz de concreto, mas foi observada a presença de aglomerados dessa fibra que resultaram na perda da resistência mecânica em relação ao concreto sem fibras. / In this dissertation Masters Program Graduate in Science and Technology of Materials is presented a study for characterization of concrete reinforced with polypropylene fibers and steel by image analysis of computed microtomography X ray transmission (μCT). Concrete samples were produced to determine their mechanical resistance to compression. The images were obtained in the SkyScan model 1174, reconstructed and analyzed. It was possible, in image analysis, to observe the structure of steel fiber dispersed in the concrete matrix and quantify them by image analysis program Ctan and observed a gain in strength compared to concrete fiber less. The quantification was not did to samples of polypropylene fibers dispersed in the matrix of concrete, but we observed the presence of clusters fibers resulting in the loss of mechanical strength compared to concrete fiber less.

This thesis deals with peritectic reactions and transformations that occur during the solidification of many alloys. Peritectics are believed to be a major cause of crack-formation in many steels, thus, good knowledge of the mechanisms by which these phenomena occur is essential for preventing such defects. The thesis also handles the behaviour of metals, in particular cast structures, during hot forming. Grain size and microstructure are of most importance in determining the strength, toughness and performance of a steel. For achieving enhanced mechanical and microstructural properties, good understanding of the phenomena occurring during hot forming is required. Peritectic reactions and transformations were studied in Fe-base and steel alloys through differential thermal analysis (DTA) experiments and micrographic investigation of quenched DTA samples. The effect of the ferrite/austenite interface strain during the peritectic reaction on equilibrium conditions was thermodynamically analysed, and the results were related to temperature observations from DTA experiments conducted on Fe-base alloys and low-alloy steels. Massive transformations from ferrite to austenite were observed in the micrographs of a number of quenched low-alloy steel samples and it was proposed that these transformations are uncontrolled by diffusion, and occur in the solid state as a visco-plastic stress relief process. DTA study of an austenitic stainless steel indicated that the alloy can exhibit primary precipitations to either ferrite or austenite. A continuously-cast breakout shell of the steel was analyzed and it was suggested that the observed irregularities in growth were due to alternating precipitations of ferrite and austenite; parts of the shell with higher ratios of primary-precipitated ferrite shrink in volume at the peritectic temperature and experience reduced growths. An experimental method for studying the behaviour of metals during hot forming developed, and hot compression tests were conducted on cast copper and ball-bearing steel samples. Flow stress curves were obtained at varying temperatures and strain rates, and the results showed good agreement with earlier observations reported in literature. Micrographic analysis of quenched samples revealed variations in grain size and a model was fitted to describe the grain size as a function of deformation temperature and strain. Solidification growth during continuous casting of stainless steel and copper was numerically modelled. A varying heat transfer coefficient was proposed to approximate the experimentally measured growth irregularities in the continuously-cast stainless steel breakout shell. Solidification growth of pure copper was also modelled in the Southwire continuous casting process. Temperature measurements from the chill mould were used to approximate the temperature gradient and the heat extraction from the solidifying strand, and the results were used in a two-dimensional model of solidification. / QC 20100803

Disertační práce se zabývá studiem strukturních a mechanických vlastností anorganických materiálů. Cílem je nalezení jednotlivých fází ve zkoumaném materiálu a hlavně lokalizace (mechanicky) nejslabšího místa, jeho ovlivnění a následně výroba materiálu o lepších mechanických vlastnostech. Z důvodu velkého množství použitých metod je základní teorie vložena vždy na začátku příslušné kapitoly. Taktéž z důvodu značného množství výsledků jsou na konci kapitol uvedeny dílčí závěry. Práce je rozdělena na tři části, kdy první se zabývá seznámením s možnostmi modelování mikro-mechanických vlastností a provedením experimentů umožňujících posouzení rozsahu platnosti některého modelu. V druhé části je provedeno shrnutí současných možností indentačních zkoušek pro měření mechanických vlastností strukturních složek betonu a praktické zvládnutí metodiky vhodné k užití pro výzkum materiálů zkoumaných domovským pracovištěm. V třetí části je navržena metoda identifikace nejslabších článků struktury anorganických pojiv a její ověření na konkrétním materiálu zkoumaném na domovském pracovišti. V této dizertační práci jsou použity tyto metody: kalorimetrie, ultrazvukové testování, jednoosá pevnost v tlaku, nanoindentace, korelativní mikroskopie a rastrovací elektronová mikroskopie s energiově disperzním spektrometrem. Dílčími výsledky jsou kompletní charakterizace cementových materiálů, upřesnění stávajících poznatků a nalezení optimálního postupu pro charakterizaci. Hlavním výsledkem je inovativní přístup vedoucí k pozitivnímu ovlivnění materiálu.

No description available.