Pasteurized skim milk was acidified using glucono-δ-lactone (GDL) at 10, 20, 30, and 40°C or with 1.2% freeze-dried yogurt starter culture at 40°C. Milk coagulation was followed by measuring turbidity, curd firmness, particle size, and casein micelle microstructural changes using transmission electron microscopy . The pH of milk was gradually lowered during acidification with GDL or starter culture. Acidification rate showed greater influence on turbidity change at 10°C than at 20, 30, or 40°C. Average casein micelle size increased with decreasing temperature. The patterns of average micelle size versus pH were not affected by temperature. No great variation of average micelle size was observed above pH 5.2. Below pH 5.0 the size increased exponentially as the milk gelled. Acidification rate did not influence average micelle size at 10°C. Acidification rate, types of acidifying agents, and temperature had no effect on the Formagraph gelation pH and the rate at which curd firmness developed. Casein micelles became less compact and less distinct with decreasing temperature before acidification. As pH was lowered, protein was dissociated from and then reassociated with casein micelles. Acidification rate had no effect on microstructure change of casein micelles at 10°C.

Les nouvelles technologies mécatroniques permettent de réduire fortement la consommation d'énergie et les émissions des véhicules individuels, en introduisant des ruptures indispensables pour une chaîne de traction électrifiée complémentaire ou alternative aux moteurs thermiques. Les assemblages en électronique de puissance utilisés dans les systèmes alterno-démarreurs emploient des alliages de brasure dont il s'agit de trouver des substituants, sans plomb, en accord avec les normes internationales. Cette thèse contribue à la caractérisation métallurgique et mécanique de deux joints brasés sans plomb innovants riches en étain. Ces joints sont produits industriellement par un procédé de brasage laser qui leur confère une microstructure de solidification très hétérogène, peu reproductible, multiphasée et qui présente un grand nombre de défauts. L'objectif de cette thèse est d'apporter une meilleure compréhension à la tenue mécanique de ces joints brasés au cours du vieillissement thermomécanique des assemblages. Les sollicitations thermiques engendrent des contraintes et des déformations plastiques à cause de la dilatation différentielle qui existe entre les différentes couches des matériaux brasés. Des lois de comportement isotropes ont été identifiées à partir d'une base expérimentale d'essais de traction sur des matériaux massifs. Ces lois, utilisées dans des simulations aux éléments finis, ont permis d'évaluer l'effet négatif du défaut de porosité inhérent au procédé de brasage. Des essais de vieillissement couplés à des observations de l'évolution de la microstructure ont permis de montrer l'influence de l'orientation des grains d'étain sur l'amorçage de fissure. Nous n'avons pas pu proposer de volume élémentaire représentatif du fait de la complexité de la structure. Une méthode inverse a été mise en oeuvre en parallèle de la conception d'un banc d'essai de flexion in-situ sous profilomètre afin de placer les premières briques permettant la caractérisation mécanique de joints brasés industriels. / The new mechatronic technologies can significantly reduce the energy consumption and gas emissions of personal cars, by introducing rupture innovations in electrified powertrains complementarily or alternatively to combustion engines. The power electronics assemblies used in starter-alternator systems use solder joints which need to be substituted with lead-free solder in agreements with international standards. This thesis contributes to the metallurgical and mechanical characterization of two tin-based lead-free solder joints. These joints are produced industrially with a Die Laser Soldering process which leads to heterogeneous solidification microstructures, poorly reproducible, multiphased, and with defects. The objective of this thesis is to provide a better understanding of the solder joints lifetime during thermomechanical aging. Thermal aging generates stresses and plastic deformation due to the mismatch in the coefficients of thermal expansion between the different layers of the assemblies. Isotropic constitutive laws were identified from an experimental database of tensile tests on bulk specimens. Those constitutive laws were used in finite element simulations in order to assess the negative effect of the solder joint porosity, inherent flaw traced back to the soldering process. Aging tests coupled with observations of the microstructure evolution have shown the influence of tin grains orientation on crack initiation. The heterogeneity of the microstructure prevents us from proposing a representative volume element of the materials. An inverse method has been implemented in parallel with the development of an in situ bending test bench under a profilometer in order to build the first steps for the mechanical characterization of industrial solder joints.

In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed. / text

FundaÃÃo de Amparo à Pesquisa do Estado do Cearà / Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / In this work a procedure is studied for pattern classification related to different types of data, namely: (1) signals obtained from ultrasonic testing ( pulse-echo technique) and magnetic signals obtained from BarkhÃusen noise in samples of ferritic-pearlitic carbon steel tubes which, due to temperature effects, have shown microstructural changes as consequence of the total or partial transformation of the pearlite into spherodite; (2) images built from TOFD ultrasonic testing and 8 bit digital radiographic images obtained from carbon steel 1020 sheets, with different welding defects. From the data obtained, images have been considered with the defects as lack of fusion, lack of penetration, porosity and images without defect. For this aim, non-conventional mathematical techniques have been used for the preprocessing of the data, namely, the statistical analyses, Hurst analysis (RSA) and detrended fluctuation analysis (DFA), and fractal analyses, box counting analysis (BCA) and minimal cover analysis (MCA). The curves obtained with the initial mathematical treatment, discrete functions of the temporal window width, have been handled with the supervised and non-supervised pattern recognition techniques known as principal component analysis and Karhunen-LoÃve (KL) transformation analysis respectively. With respect to the magnetic signals, the KL classifier has been shown to be very efficient when applied to DFA obtained from the magnetic flux, with a success rate around 94%. On the other hand, for the magnetic noise signals we have not obtained an acceptable success rate independently of the pre-processing used. However, when were considered the curves obtained by concatenating all curves of the pre-processing was obtained a consistent average success rate of 85%. As far as the rate of success of the PCA classifier is concerned, an excellent success of 96% has been reached for concatenated curves of selected data of magnetic noise only. As far as the analyses of the backscattered ultrasonic signals is concerned, it was not possible to classify the different stages of the microstructural degradation by using KL or PCA independently of the pre-processing used. As far as the analyses of the D-scan images are concerned, by applying PCA a rate of success of 81% has been obtained with MCA data, 73% has been obtained by concatenating all curves from the different fractal and statistical analyses and around 85% when concatenating the best individual results (DFA and MCA). On the other hand, considering the KL classifier, high success rates have been verified for the training stage, between 96% and 99%, and a maximum success rate (100%), when concatenating all analyses. With respect to the testing results, the best success rate which has been reached was approximately 77%, when concatenating all the curves obtained from the statistical and fractal pre-processing. For the digitalized radiographic images, relevant individual rates of success (between 70% and 90%) for the training set (consisting of all data) have been obtained for the classifier KL only, and a 100% success rate, when concatenating all the curves obtained from the pre-processing of the images. / Neste trabalho estudou-se uma metodologia de classificaÃÃo de padrÃes relacionados a dois tipos de dados: (1) sinais obtidos atravÃs dos ensaios ultrassÃnicos (tÃcnica pulso-eco) e sinais magnÃticos obtidos atravÃs de ruÃdo Barkhausen realizados em amostras de tubos de aÃo carbono ferrÃtico-perlÃtico que devido aos efeitos da temperatura de trabalho apresentaram mudanÃas microestruturais decorrentes da transformaÃÃo parcial ou total da perlita em esferoiditas; e (2) imagens construÃdas a partir de ensaios ultrassÃnicos (tÃcnica TOFD) e imagens radiogrÃficas digitais de chapas de aÃo carbono 1020 soldadas, obtidas com resoluÃÃo de 8bits, nas quais foram inseridos diversos tipos de defeitos de soldagem. Dos dados gerados, foram estudadas as imagens com os defeitos de falta de fusÃo (FF), falta de penetraÃÃo (FP), porosidade (PO) e uma classe designada como sem defeito (SD). Para tanto, utilizaram-se de tÃcnicas matemÃticas nÃo convencionais no prÃ-processamentos dos dados conhecidas como anÃlises estatÃsticas de Hurst (RSA) e flutuaÃÃo sem tendÃncia (DFA) e as anÃlises fractais de contagem de caixas (BCA) e de mÃnima cobertura (MCA). Em seguida as curvas obtidas desse tratamento matemÃtico inicial, funÃÃes discretas da largura da janela temporal, foram utilizadas na alimentaÃÃo das tÃcnicas de reconhecimento de padrÃes nÃo supervisionada e supervisionada conhecidas, respectivamente, como anÃlise de componentes principais (PCA) e anÃlise da transformaÃÃo de Karhunen-LoÃve (KL). Em relaÃÃo aos estudos dos sinais magnÃticos, o classificador KL mostrou-se eficiente quando aplicado Ãs DFA do fluxo magnÃtico, com uma taxa de sucesso em torno de 94%. Jà para os sinais do ruÃdo magnÃtico nÃo se obteve uma taxa de sucesso aceitÃvel, independente do prÃ-processamento utilizado. Entretanto quando todas as curvas de todas as anÃlises, dos dois tipos de sinais magnÃticos (ruÃdo e fluxo), foram concatenadas, obteve-se uma taxa mÃdia de sucesso consistente de aproximadamente 85%. No tocante Ãs taxas de sucesso do classificador PCA, somente para o ruÃdo magnÃtico e considerando todas as curvas concatenadas para um grupo de dados selecionados, conseguiu-se uma taxa de sucesso de 96%. A respeito das anÃlises dos sinais ultrassÃnicos retroespalhados, tambÃm nÃo foi possÃvel classificar, nem com a KL e nem com a PCA, os diferentes estÃgios de degradaÃÃo microestrutural, independemente do prÃ-processamento utilizado. No tocante Ãs analises das imagens D-scan, obteve-se com a PCA, taxas de sucesso de 81% considerando apenas os dados das MCA, 73% quando as curvas de todas as anÃlises estatÃsticas e fractais foram concatenadas, e em torno de 85%, quando apenas as curvas das melhores anÃlises (DFA e MCA) foram concatenadas. Jà considerando o classificador KL, verificaram-se taxas de sucesso na etapa de treinamento, entre 96% e 99%, e mÃxima taxa de sucesso (100%) no caso dos vetores de todas as anÃlises concatenados. Em relaÃÃo aos resultados dos testes, a melhor taxa de sucesso alcanÃada foi aproximadamente de 77% quando se concatenaram todas as curvas oriundas dos prÃ-processamentos estatÃsticos e fractais. Com respeito Ãs imagens radiogrÃficas digitalizadas somente com o classificador KL (na etapa de treinamento, com 100% dos vetores) obtiveram-se taxas de sucesso individuais entre 70 e 90% de acertos e 100% de sucesso na classificaÃÃo quando se concatenaram as curvas de todos os prÃ-processamentos das imagens.

This thesis work was conducted at Luleå University of Technology in collaboration with Sandvik Rock Tools. The aim of the work was to determine the influence of carbon content on the wear performance in carburized steel in the dry contact interface of threaded connections between drill rods. In order to investigate this, samples of drill rod steel were carburized to three different carbon concentrations and shot peened to replicate the production process of a drill rod. The samples were wear tested by utilizing a twin-disc wear tester with one disc rotating at 100 RPM and the other at 3000 RPM to mimic the operating conditions in the threaded connection between drill rods. The results was evaluated by wear rate, surface topography, hardness as well as optical analysis by light optical microscopy and scanning electron microscopy. The results show that an increased surface carbon content provide a decrease in wear rate and an increase in hardness in the surface layer that undergo microstructural changes due to the frictional heat and contact pressure during wear testing. The primary wear mechanisms were identified as plastic deformation, adhesive scratching and material removal through delamination.