Porous ceramic-metal composites, or cermets, such as nickel zirconia (Ni-YSZ), are widely used as the anode material in solid oxide fuel cells (SOFC). These materials need to enable electrochemical reactions and provide the mechanical support for the layered cell structure. Thus, for the anode supported planar cells, the thermomechanical behavior of the porous cermet directly affects the reliability of the cell. Porous cermets can be viewed as three-phase composites with a random heterogeneous microstructure. While random in nature, the effective properties and overall behavior of such composites can still be linked to specific stochastic functions that describe the microstructure. The main objective of this research was to develop the relationship between the thermomechanical behavior of porous cermets and their random microstructure. The research consists of three components. First, a stochastic reconstruction scheme was developed for the three-phase composite. From this multiple realizations with identical statistical descriptors were constructed for analysis. Secondly, a finite element model was implemented to obtain the effective properties of interest including thermal expansion coefficient, thermal conductivity, and elastic modulus. Lastly, nonlinear material behaviors were investigated, such as damage, plasticity, and creep behavior. It was shown that the computational model linked the statistical features of the microstructure to its overall properties and behavior. Such a predictive computational tool will enable the design of SOFCs with higher reliability and lower costs.

The objective of this work is to understand the structure-property relationships in a pseudoelastic composition of polycrystalline NiTiPt (Ti-42.7 at% Ni-7.5 at% Pt). Structural characterization of the alloy includes grain size determination and texture analysis while the thermo-mechanical properties are explored using tensile testing. Variation in heat treatment is used as a vehicle to modify microstructure. The results are compared to experiments on Ni-rich NiTi alloy wires (Ti-51.0 at% Ni), which are in commercial use in various biomedical applications. With regards to microstructure, both alloys exhibit a <111> fiber texture along the wire drawing axis, however the NiTiPt alloy's grain size is smaller than that of the Ni-rich NiTi wires, while the latter materials contain second phase precipitates. Given the nanometer scale grain size in NiTiPt and the dispersed, nanometer scale precipitate size in NiTi, the overall strength and ductility of the alloys are essentially identical when given appropriate heat treatments. Property differences include a much smaller stress hysteresis and smaller temperature dependence of the transformation stress for NiTiPt alloys compared to NiTi alloys. Potential benefits and implications for use in vascular stent applications are discussed.

Les composites à matrice polymère sont de plus en plus utilisés dans le secteur automobile. Afin de remplir les conditions exigeantes du cahier des charges vis-à-vis des conditions de mise en service, les pièces en composite doivent maintenir leur forme géométrique sous des conditions thermo-mécaniques parfois extrêmes. Par exemple, un assemblage de hayon composite est soumis à des contraintes mécaniques élevées associées à des variations de température importantes lors des essais de validation du cahier des charges. Les enjeux de la thèse sont axés sur l’aide à la conception dans le domaine quasi-statique de pièces industrielles injectées en thermoplastique renforcé de fibres discontinues. L’amélioration des outils numériques doit permettre un dimensionnement virtuel optimal de ces pièces en anticipant les variations rencontrées en service et les distorsions résiduelles résultantes de chargements thermo-mécaniques. Cette démarche s’appuie sur la connaissance du comportement thermo-mécanique du matériau de l’étude, celui du renfort de hayon, un polypropylène renforcé à 40% en masse de fibres de verre discontinues, et sur la modélisation du comportement de ce type de matériau. / Discontinuous fibers reinforced thermoplastic materials have been widely used for several years in the automotive industry. These parts must resist demanding service life conditions and must meet thermo- mechanical specifications. Indeed, structural automotive spare parts have to endure high temperatures, like a few tens of degrees Celsius, for a long duration, at least a few hours. As an example, a structural part of tailgate is subject to high mechanical loading, associated to strong temperature variations, during the validation test, regarding specifications. The purpose of this work is to improve the design of complex industrial parts, like the tailgate in quasi-static domain, by relying on numerical simulation. One of the challenges related to the use of such material, is to have a reliable virtual design of industrial parts by predicting the geometrical variations during service life conditions, and residual strain. Therefore, it is necessary to characterize and to model the thermo-mechanical behavior of the tailgate material, a polypropylene matrix reinforced with discontinuous glass fibers, with a given mass fraction of 40%

In last decades, there has been a relevant growth in the application of composite materials reinforced by fibers in the several industries, especially the aeronautical, automotive, construction and offshore structures segments. How the composites exhibit relationships stiffness/weight and resistance/high weight, and other interesting features compared with other materials, they have been increasingly used to replace conventional materials. The present work has as objective to develop a study about laminated polymeric composite tubes subjected to thermomechanical loads. The laminae are composed of polymeric matrix involving unidirectional long fiber reinforcement. The walls of the studied tubes present various lamination schemes and different constituent materials. The analyses are set by analytical formulations that permits obtaining the displacements, stresses and strains, as well as verification failures in the laminae. Different empirical and semi-empirical analytical models are used for failure analyses. Effects of thermomechanical loads, such as internal pressure, bending, and temperature changes are considered. The analyzed cases consist of tubes with walls comprised of several layers with different directions of fibers, some including laminae sand impregnated with resin. The results are also compared with other available in the literature. It is verified that the Tsai-Wu criterion provides superior results to the other two failure models used in the work, especially for angles greater than 50°. Moreover, this criterion obtained an inconsistent result in the analysis of the temperature effects for the laminae [+55°/−55°]2. / Fundação de Amparo a Pesquisa do Estado de Alagoas / Nas últimas décadas, houve um grande crescimento na aplicação de materiais compósitos reforçados por fibras em diversos setores, com destaque para os segmentos aeronáutico, automotivo, construção civil e de estruturas offshore. Como os referidos compósitos apresentam relações rigidez/peso e resistência/peso elevadas, além de outras características interessantes, em comparação com outros materiais, eles têm sido cada vez mais utilizados em substituição aos materiais convencionais. O presente trabalho tem como objetivo o desenvolvimento de um estudo sobre tubos de materiais compósitos poliméricos laminados submetidos a cargas termomecânicas. As lâminas são constituídas por uma matriz polimérica envolvendo fibras unidirecionais longas de reforço. As paredes dos tubos estudadas apresentam diferentes esquemas de laminação, podendo ser constituídas por diferentes materiais. As análises são feitas através de formulações analíticas que permitem a obtenção dos deslocamentos, tensões e deformações, assim como a verificação de falhas nas lâminas. Para a análise de falhas são empregados diferentes modelos analíticos empíricos e semiempíricos. Efeitos de cargas termomecânicas, tais como pressão interna, flexão e de variação de temperatura são considerados. Os casos analisados consistem de tubos com paredes constituídas por várias camadas com diferentes direções de fibras, alguns incluindo lâminas de areia impregnada com resina. Os resultados encontrados também são comparados com outros disponíveis na literatura. Verifica-se que o critério de Tsai-Wu fornece resultados superiores aos dos outros dois modelos de falha utilizados no trabalho, principalmente para ângulos superiores a 50°. Além disso, esse critério obteve resultado inconsistente na análise dos efeitos de temperatura para o laminado [+55°/−55°]2.

Eutectoid steel strips are designed for the production of parts for intensive use such as clutches, seat slides, and springs as they exhibit<p>excellent strength levels and wear resistance. These properties arise from the unique morphology of lamellar pearlite which can be considered<p>as a self-laminated nanoscale composite. However, a spheroidization annealing step is nowadays necessary to improve the cold forming properties before further cold rolling steps.<p>This thesis is aimed at improving the tensile ductility of the hot rolled products of eutectoid composition in order to eliminate the intermediate<p>annealing step. Two strategies are proposed.<p>The first is to transpose the concept of controlled rolling developed for HSLA to<p>eutectoid steels. Through a strict adjustment of the austenite processing and of the cooling strategy, it is possible to improve the ductility<p>of the final lamellar microstructure. The way the processing parameters influence the hot deformation of austenite, the eutectoid transformation and of the subsequent spheroidization annealing is deeply<p>investigated. It is found that refinement and pancaking of austenite<p>is beneficial as it reduces the pearlite block size improving the total<p>tensile elongation. Accelerated cooling is of paramount importance to<p>achieve fine Interlamellar spacing (ILS), which lead to high strength<p>levels and accelerate spheroidization during subsequent annealing.<p>The second approach involves intercritical or warm deformation. Warm processing of eutectoid steels is first explored by torsion testing<p>and then up-scaled to a pilot rolling-line. The interactions between thermomechanical parameters, rolling forces generated and microstructural<p>evolution are carefully scrutinized. During concurrent hot deformation, spheroidization of cementite takes place almost instantaneously<p>in both torsion and rolling. The restoration processes occurring in the ferrite matrix depends on the strain path and the strain rates. Low strain rates (0,1 s−1) and simple shear promotes the formation of a recrystallized-like HABs network of about 3μm in size.<p>Plane strain compression and high strain rates (10 s−1) leads to the formation of a typical recovered dislocation substructure (LABs) of 1μm in size. During annealing, no recrystallization occurs and the LABs substructure remains stable. This substructure influences drammatically the mechanical properties: the strength is very high and the work-hardening behavior is poor due to high recovery rate in the region close to the LABs. However, due to the presence of spheroidized<p>cementite particles the ductility of warm rolled eutectoid steels is higher than that of ultra fine grained low carbon steels. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

No description available.

The thermomechanical properties of monolayer graphene and the interfacial interactions between graphene and an SiO₂ substrate are investigated in this dissertation using a multiscale approach. The temperature dependent mechanical behavior of graphene with thermal fluctuations is studied by statistical mechanics analysis under harmonic approximation, which is then compared to molecular dynamics simulations. It is found that the amplitude of thermal fluctuation depends nonlinearly on the graphene size due to anharmonic interactions between bending and stretching modes, but a small positive pre-strain could suppress fluctuation amplitude considerably and results in very different scaling behavior. The thermal expansion of graphene depends on two competing effects: positive expansion due to in-plane modes and negative expansion due to out-of-plane fluctuations. The in-plane stress-strain relation of graphene becomes nonlinear even at infinitesimal strain due to the entropic contribution. Consequently, the modulus of graphene depends on strain non-monotonically, with strain stiffening followed by intrinsic softening. Moreover, it is found that the thermomechnical behavior of graphene is dependent on its interactions with environment such as supporting substrate. The interfacial interactions between graphene and SiO₂ substrate is investigated in terms of three perspectives. Firstly, the interaction mechanisms between graphene and SiO₂ substrate are studied by density functional theory (DFT). The dispersion interaction is found to be the predominant mechanism, and the interaction strength is strongly influenced by changes of SiO₂ surface structures due to surface reactions with water. The adhesion energy is reduced when the reconstructed SiO₂ surface is hydroxylated, and further reduced when covered by a monolayer of adsorbed water molecules. Next, we study the interfacial interactions between graphene and a wet substrate that is covered by a liquid-like water film. During the separation of graphene from the wet substrate, MD simulations show evolution of the water from a continuous film to discrete islands. The water bridging effects are further described by continuum models. Finally, a continuum model is developed to predict how the surface roughness may affect the adhesion between graphene membranes and their substrate. / text

Les propriétés généralement demandées aux armatures pour béton armé doivent répondre à des exigences particulières précisées dans des normes nationales, européennes et internationales. Actuellement, dans la pratique de la production de l'acier à béton, on tend à substituer différentes nuances d'armature par une armature unifiée ayant une valeur de limite d'élasticité minimale de 500 MPa. Le remplacement des ronds à béton de nuance 400MPa par cette armature conduit à une économie de métal de l'ordre de 10-20 % d'après les estimations de différents spécialistes.Il existe, généralement, plusieurs techniques de fabrication des armatures, dont la déformation à chaud suivie d'un refroidissement accéléré sous flux d'eau (traitement thermomécanique ou Thermomechanical Control Process - TMCP), ainsi que la déformation à froid (tréfilage â travers une filière conique et une filière à rouleaux, microlaminage...), qui se heurte cependant â l'impossibilité de satisfaire aux exigences des Normes vis-à-vis des propriétés de résistance (Re > 500MPa) et de plasticité.Le présent travail a pour but le développement d'un procédé combiné de production d'acier à béton profitant des aspects positifs de deux types de déformation à chaud ainsi qu'à froid, sous forme du traitement combiné mécano-thermomécanique (post déformations à froid après le TMCP). et d'étudier la possibilité de production d'aciers à béton de qualité 500 MPa (B(A)500), de petits diamètres, en couronne, répondant aux exigences des Normes modernes. L'objectif scientifique de ce travail est d'étudier les mécanismes microstructuraux qui se produisent lors de la déformation à froid de l'acier préalablement traité thermomécaniquement et leurs conséquences sur l'évolution des propriétés mécaniques.Plusieurs nuances d'acier bas carbone de différents diamètres ont été étudiées. De nombreuses techniques de caractérisation macroscopique et microscopique de la structure de l'acier ont été utilisées: traction monotone, essais de microdureté, microscopie optique, microscopie électronique â balayage (MEB) et en transmission (MET), diffraction des rayons X. spectrométrie mécanique..Les résultats des essais montrent qu'il est possible d'obtenir avec assurance l’armature de qualité B(A)500, possédant des propriétés mécaniques qui dépassent les exigences des nonnes, à l'aide d’une déformation par torsion avec un cisaillement maximal de 22%, pour des aciers à 0.15 -0.2 % C(en poids), faiblement alliés en Mn et Si et traités themomécaniquement. La déformation par torsion provoque une augmentation de dureté en surface mais aussi au cœur de l'armature. Cette augmentation de dureté à cœur peut être amplifiée par le traitement thermique de vieillissement à 100°C correspondant à l'utilisation en Génie Civil. Cet effet, lié aux interactions dislocation-impuretés interstitielles, a été expliqué grâce â la MET et à l'étude du frottement intérieur. / Properties which are required from an armature for the reinforced concrete should satisfy the requirements of National, European and International standards. At the present time, in practice of armature production, there is a tendency of replacement of various classes of reinforcing steels by the unified armature with the minimum value of yield strength of 500 MPa. Replacement of the armature of the 400 MPa class by such reinforcing steel brings the economy of metal of an order of 10-20 % according to the estimations of various experts.In general, a set of production technologies of the armature exists, including hot deformation with the subsequent accelerated cooling in water - Thermomechanical Control Process (TMCP), and also cold deformation (drawing in conic die block, drawing in roller die block, microrolling) which nevertheless face the impossibility to provide the required strength (yield strength > 500MPa) and plastic properties.The technological purpose of the given work is:- development of the combined mechano-thermomechanical processing of armature's production which would combine positive aspects of hot and cold deformations (post deformation processing after TMCP);- examination of possibilty to manufacture the reinforcing steel of the class of 500MPa (B(A)500) of the small diameter, in coils, meeting the requirements of modem standards.The scientific objective of the work is the analysis of change of the microstructure of reinforcing steels after the cold deformation which is preliminary subjected to thermomechanical processing, and, also, the examination of the influence of microstructural effect on change of mechanical properties of steels.In the present study, a significant quantity of grades of low-carbon steels of various diameters has been investigated. Also, different techniques for macroscopical and microscopical characterization of the steel structures have been applied: tensile tests, micro-hardness test, optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray analysis and mechanical spectroscopy.Results of tests show that it is possible to obtain, with assurance, the reinforcing steel of the strength class B(A)500, with a complex of the mechanical properties surpassing the requirements of standards. For this purpose it is proposed to use the deformation by cold torsion with the maximum relative shear of 22 %, after thermomechanical processing, for steels with carbon content of 0,15...0,25% and alloyed with Mn and Si. Deformation by torsion provides an increase of hardness not only on the surface, but also in the core of armature. The effect of hardness increase can be strengthened by means of thermal ageing at 100°C corresponding to the use in Civil Engineering. This effect, related to the dislocations- interstitial impurities interactions, was explained thanks to MET and internal friction effect.

Les besoins en électronique de puissance, de plus en plus exigeants, ont motivé des recherches à l'échelle mondiale sur d'autres matériaux tels que le diamant comme remplaçant du silicium. Nos travaux de recherche sont plus spécifiquement axés sur la définition et la qualification de matériaux polymères capables de garantir l'intégrité des fonctions physiques de modules de puissance en environnement sévère. L'étude concerne la durabilité de candidats polymères à retenir pour le boîtier dont l'objectif est de protéger l'interrupteur de l'environnement extérieur. Suite aux choix des différents polymères étudiés, variables dans leur chimie et leur morphologie (amorphe ou semi-cristallin), un premier objectif scientifique est alors de chercher les relations structures/propriétés permettant de contrôler le procédé de mise en forme des polyimides semi-cristallins et d'en déduire les conditions requises à l'obtention de performances optimisées. Un second objectif a concerné la tenue des différents matériaux sélectionnés en vieillissements isothermes thermo-oxydatifs. / Requirements in power electronics are more and more demanding about materials behavior in their operating conditions. This has motivated global scale researches about other materials replacing silicon such as diamond. This study is specifically focusing on the definition and qualification of polymer materials which could preserve physical functions of power modules in severe environments. This study focuses on the durability of several polymers used for the case. This later allows to protect the chip from external environment. The choice of different studied polymers which are dissimilar in chemistry and morphology (i.e. amorphous or semicrystalline) has been made in this study. Then, a first scientific goal was to search the structures/properties relations leading to the control of the manufacturing process of semicrystalline polyimides. A second goal concerned the mechanical strength evaluation of selected materials after thermo-oxidative isothermal ageing.

The electrical energy consumption in thermomechanical pulping (TMP) is very high, in the range of 2 – 3 MWh/adt depending on process solution and on the product quality specifications for the paper product. Both pulpwood and energy prices have increased rapidly for some time. Due to this, the main focus of the research and development is on ways to reduce the electrical energy consumption in wood chip refining. As a step towards a more energy and cost ‐ effective refining process, Holmen Paper AB has invested in a new mechanical pulping process at its Braviken mill. In this case the primary refining stage consists of high consistency (HC) double disc refiners ‐ RGP68DD (machines with two counter rotating discs). Earlier studies on the refining conditions, such as intensity and temperature, have indicated that it should be possible to improve the energy efficiency in double disc refining while maintaining the functional pulp properties such as tensile index. The main goal of this project was to improve the energy efficiency in double disc chip refining with 150 kWh/adt to corresponding pulp properties as measured on pulp samples after refiner. In order to further improve the basic understanding of what happens to the wood fibre material when changing the process conditions, the morphological and ultrastructural changes of fibres were also studied. This part of the research work was performed in cooperation with the research program; Collaborative Research on the Ultrastructure of Wood Fibres (CRUW). This licentiate project is a part of a large development project where different techniques to improve the energy efficiency has been evaluated by means of mill scale trials at the Holmen Paper Braviken Mill. The high consistency double disc chip refining part of the project was financed by The Swedish Knowledge Foundation, Metso Paper and Holmen Paper, in cooperation with FSCN (Fiber Science &amp; Communication Network) at Mid Sweden University. The trials were made on one of the TMP lines at the Holmen Paper Braviken mill with Norway spruce as raw material. The influence of increased specific      refining energy on pulp properties were studied at different refining temperatures, refining intensity, pulp consistency and production rate. Results from these trials were later validated by means of long term trials. Intensity models and simulations for intensity changes by new segment design were made by Juha‐ Pekka Huhtanen from Tampere University of Technology, Finland. The results show that the specific energy consumption to same tensile index can be improved by means of increasing the refining pressure/temperature. The energy efficiency was improved by 80     ‐150 kWh/adt depending on load and the inlet‐ and housing pressure. The largest relative specific energy efficiency improvement was reached at low specific energy consumption levels. Similar fibre surface ultrastructure characteristics are gained by pulps with high pressure/temperature and low specific energy consumption compared to low pressure/temperature and high specific energy consumption pulps. High pressure/temperature and high specific energy consumption resulted in significantly increase in the delamination/internal fibrillation of pulp fibres. The surface ultrastructure of these fibres exhibited exposed S2 layer with long ribbontype fibrillation compared to pulps produced with lower temperature and lower specific energy consumption. When the refiner was operated at high pressure, the tensile index was preserved over the whole plate life. The specific light scattering coefficient increased with increasing pressure/temperature. A reason for this could be increased intensity caused by decreased plate gap. Increased intensity by means of refiner segment design changes resulted in large specific light scattering coefficient increase at similar tensile index, lower shives content, lower average fibre length and lower CSF at same specific energy consumption. The fresh steam consumption was reduced by the increased refiner ressure/temperature. / Den höga elenergiförbrukningen vid produktion av mekanisk massa har ställtkrav på mer forskning för att elenergieffektivisera raffineringsprocessen. Som ettsteg mot en mer energi‐ och kostnadseffektiv raffineringsprocess, har HolmenPaper AB investerat i en ny tillverkning av termomekanisk (TMP) massa vidBravikens pappersbruk. Dubbeldiskraffinörerna i den nya massalinjens primäraraffineringssteget studerades i detta projekt. Det finns goda indikationer på att enminskning av energiförbrukningen är möjlig genom att studerar och optimeraraffineringparametrar såsom intensitet och temperatur. Projektets huvudmål varatt energieffektivisera det primära dubbeldiskraffineringssteget med 150 kWh/adttill motsvarande massaegenskaper, så som dragstyrka, mätt på massa efterraffinör. Tillfälle gavs också till att studera morfologiska förändringar på fibrer föratt ytterligare förstå hur massa och fibrerna påverkas av dubbeldiskraffinering ochförändringar i raffineringssystemet.Detta licentiatprojekt är en del av ett större projekt där olika tekniker för attförbättra energieffektiviteten har utvärderats i industriell skala på Holmen PaperBravikens pappersbruk. Licentiatprojektet är finansierat av KK‐stiftelsen, MetsoPaper och Holmen Paper, i samarbete med Mittuniversitetet.Fullskaleförsök gjordes på en av TMP linjerna vid Bravikens pappersbruk, därgran används som råvara. Studien utfördes på dubbeldiskraffinörerna i detprimära raffineringssteget. Malkurvor, med ökande specifik raffineringsenergi,gjordes vid olika raffineringstemperaturer, intensitet, massakoncentration ochproduktion. Resultat som erhållits från malkurvorna bekräftades med längrestudier på raffinörerna. Intensitetsmodeller och simuleringar utfördes av Juha‐Pekka Huhtanen från Tampere University of Technology.De erhållna resultaten visar på att energiförbrukningen till ett visst dragindexkan minskas genom att öka raffineringstrycket/temperaturen. Medraffineringstryck menas inlopp och hustryck i raffinören. Energibesparingen är iintervallet 80‐150 kWh/adt. Den största förbättringen kan uppnås vid lågaenergiinsatser. Massor producerade med högt tryck och temperatur och lägrespecifik energiförbrukning uppvisar liknande ultrastrukturella ytegenskaper sommassor producerade med lågt tryck och temperatur och hög specifik energi. Högttryck och temperaturer med hög specifik energiinsats gav en signifikant förbättringav delaminering/intern fibrillering av massafibrer. Dessa fibrer uppvisadebildningar av långa band‐liknande fibriller från fibrernas S2 skikt, i jämförelse medmassor tillverkade med lägre tryck och temperatur och lägre specifik energi.5Om raffineringen genomförs vid högt tryck/temperatur bevaras dragindexunder hela segmentlivslängden.Den specifika ljusspridningskoefficienten påverkades positivt av ökat tryck ochtemperatur. En orsak till detta kan vara högre intensitet som orsakas av minskadmalspalt.Ökad intensitet genom förändrad segmentdesign leder till stora ökningar i denspecifika ljusspridningskoefficienten. Samtidigt uppnås samma dragindex, lägrespethalt, lägre genomsnittlig fiberlängd och CSF vid samma specifikaenergiförbrukning.Förbrukningen av färskångan sänktes vid tillämning av högre tryck ochtemperatur i raffinören.