Comportamiento mecánico de nuevas aleaciones de wolframio en función de la temperatura

La presente memoria de tesis tiene como objetivo principal la caracterización mecánica en función de la temperatura de nueve aleaciones de wolframio con contenidos diferentes en titanio, vanadio, itria y lantana. Las aleaciones estudiadas son las siguientes: W-0.5%Y2O3, W-2%Ti, W-2% Ti-0.5% Y2O3, W-4% Ti-0.5% Y2O3, W-2%V, W- 2%Vmix, W-4%V, W-1%La2O3 and W-4%V-1%La2O3. Todos ellos, además del wolframio puro se fabrican mediante compresión isostática en caliente (HIP) y son suministradas por la Universidad Carlos III de Madrid. La investigación se desarrolla a través de un estudio sistemático basado en ensayos físicos y mecánicos, así como el análisis post mortem de las muestras ensayadas. Para realizar dicha caracterización mecánica se aplican diferentes ensayos mecánicos, la mayoría de ellos realizados en el intervalo de temperatura de 25 a 1000 º C. Los ensayos de caracterización que se llevan a cabo son: • Densidad • Dureza Vicker • Módulo de elasticidad y su evolución con la temperatura • Límite elástico o resistencia a la flexión máxima, y su evolución con la temperatura • Resistencia a la fractura y su comportamiento con la temperatura. • Análisis microestructural • Análisis fractográfico • Análisis de la relación microestructura-comportamiento macroscópico. El estudio comienza con una introducción acerca de los sistemas en los que estos materiales son candidatos para su aplicación, para comprender las condiciones a las que los materiales serán expuestos. En este caso, el componente que determina las condiciones es el Divertor del reactor de energía de fusión por confinamiento magnético. Parece obvio que su uso en los componentes del reactor de fusión, más exactamente como materiales de cara al plasma (Plasma Facing Components o PFC), hace que estas aleaciones trabajen bajo condiciones de irradiación de neutrones. Además, el hecho de que sean materiales nuevos hace necesario un estudio previo de las características básicas que garantice los requisitos mínimos antes de realizar un estudio más complejo. Esto constituye la principal motivación de la presente investigación. La actual crisis energética ha llevado a aunar esfuerzos en el desarrollo de nuevos materiales, técnicas y dispositivos para la aplicación en la industria de la energía nuclear. El desarrollo de las técnicas de producción de aleaciones de wolframio, con un punto de fusión muy alto, requiere el uso de precursores de sinterizado para lograr densificaciones más altas y por lo tanto mejores propiedades mecánicas. Este es el propósito de la adición de titanio y vanadio en estas aleaciones. Sin embargo, uno de los principales problemas de la utilización de wolframio como material estructural es su alta temperatura de transición dúctil-frágil. Esta temperatura es característica de materiales metálicos con estructura cúbica centrada en el cuerpo y depende de varios factores metalúrgicos. El proceso de recristalización aumenta esta temperatura de transición. Los PFC tienen temperaturas muy altas de servicio, lo que facilita la recristalización del metal. Con el fin de retrasar este proceso, se dispersan partículas insolubles en el material permitiendo temperaturas de servicio más altas. Hasta ahora se ha utilizado óxidos de torio, lantano e itrio como partículas dispersas. Para entender cómo los contenidos en algunos elementos y partículas de óxido afectan a las propiedades de wolframio se estudian las aleaciones binarias de wolframio en comparación con el wolframio puro. A su vez estas aleaciones binarias se utilizan como material de referencia para entender el comportamiento de las aleaciones ternarias. Dada la estrecha relación entre las propiedades del material, la estructura y proceso de fabricación, el estudio se completa con un análisis fractográfico y micrográfico. El análisis fractográfico puede mostrar los mecanismos que están implicados en el proceso de fractura del material. Por otro lado, el estudio micrográfico ayudará a entender este comportamiento a través de la identificación de las posibles fases presentes. La medida del tamaño de grano es una parte de la caracterización microestructural. En esta investigación, la medida del tamaño de grano se llevó a cabo por ataque químico selectivo para revelar el límite de grano en las muestras preparadas. Posteriormente las micrografías fueron sometidas a tratamiento y análisis de imágenes. El documento termina con una discusión de los resultados y la compilación de las conclusiones más importantes que se alcanzan después del estudio. Actualmente, el desarrollo de nuevos materiales para aplicación en los componentes de cara al plasma continúa. El estudio de estos materiales ayudará a completar una base de datos de características que permita hacer una selección de ellos más fiable. The main goal of this dissertation is the mechanical characterization as a function of temperature of nine tungsten alloys containing different amounts of titanium, vanadium and yttrium and lanthanum oxide. The alloys under study were the following ones: W-0.5%Y2O3, W-2%Ti, W-2% Ti-0.5% Y2O3, W-4% Ti-0.5% Y2O3, W-2%V, W- 2%Vmix, W-4%V, W-1%La2O3 and W-4%V-1%La2O3. All of them, besides pure tungsten, were manufactured using a Hot Isostatic Pressing (HIP) process and they were supplied by the Universidad Carlos III de Madrid. The research was carried out through a systematic study based on physical and mechanical tests as well as the post mortem analysis of tested samples. Diverse mechanical tests were applied to perform this characterization; most of them were conducted at temperatures in the range 25-1000 ºC. The following characterization tests were performed: • Density • Vickers hardness • Elastic modulus • Yield strength or ultimate bending strength, and their evolution with temperature • Fracture toughness and its temperature behavior • Microstructural analysis • Fractographical analysis • Microstructure-macroscopic relationship analysis This study begins with an introduction regarding the systems where these materials could be applied, in order to establish and understand their service conditions. In this case, the component that defines the conditions is the Divertor of magnetic-confinement fusion reactors. It seems obvious that their use as fusion reactor components, more exactly as plasma facing components (PFCs), makes these alloys work under conditions of neutron irradiation. In addition to this, the fact that they are novel materials demands a preliminary study of the basic characteristics which will guarantee their minimum requirements prior to a more complex study. This constitutes the motivation of the present research. The current energy crisis has driven to join forces so as to develop new materials, techniques and devices for their application in the nuclear energy industry. The development of production techniques for tungsten-based alloys, with a very high melting point, requires the use of precursors for sintering to achieve higher densifications and, accordingly, better mechanical properties. This is the purpose of the addition of titanium and vanadium to these alloys. Nevertheless, one of the main problems of using tungsten as structural material is its high ductile-brittle transition temperature. This temperature is characteristic of metallic materials with body centered cubic structure and depends on several metallurgical factors. The recrystallization process increases their transition temperature. Since PFCs have a very high service temperature, this facilitates the metal recrystallization. In order to inhibit this process, insoluble particles are dispersed in the material allowing higher service temperatures. So far, oxides of thorium, lanthanum and yttrium have been used as dispersed particles. Tungsten binary alloys are studied in comparison with pure tungsten to understand how the contents of some elements and oxide particles affect tungsten properties. In turn, these binary alloys are used as reference materials to understand the behavior of ternary alloys. Given the close relationship between the material properties, structure and manufacturing process, this research is completed with a fractographical and micrographic analysis. The fractographical analysis is aimed to show the mechanisms that are involved in the process of the material fracture. Besides, the micrographic study will help to understand this behavior through the identification of present phases. The grain size measurement is a crucial part of the microstructural characterization. In this work, the measurement of grain size was carried out by chemical selective etching to reveal the boundary grain on prepared samples. Afterwards, micrographs were subjected to both treatment and image analysis. The dissertation ends with a discussion of results and the compilation of the most important conclusions reached through this work. The development of new materials for plasma facing components application is still under study. The analysis of these materials will help to complete a database of the features that will allow a more reliable materials selection.

Influence of high aluminium content on the mechanical properties of directionally solidified multicrystalline silicon

The purpose of this research is the mechanical characterisation of multicrystalline silicon crystallised from silicon feedstock with a high content of aluminium for photovoltaic applications. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within the multicrystalline silicon block to quantify the impact of the segregation of impurities on these mechanical properties. Aluminium segregated to the top of the block and caused extensive micro-cracking of the silicon matrix due to the thermal mismatch between silicon and the aluminium inclusions. Silicon nitride inclusions reduced the fracture toughness and caused failure by radial cracking in its surroundings due to its thermal mismatch with silicon. However, silicon carbide increased the fracture toughness and elastic modulus of silicon.

Effect of layer thickness on the high temperature mechanical properties of Al/SiC nanolaminates

Composite laminates on the nanoscale have shown superior hardness and toughness, but little is known about their high temperature behavior. The mechanical properties (elastic modulus and hardness) were measured as a function of temperature by means of nanoindentation in Al/SiC nanolaminates, a model metal–ceramic nanolaminate fabricated by physical vapor deposition. The influence of the Al and SiC volume fraction and layer thicknesses was determined between room temperature and 150 °C and, the deformation modes were analyzed by transmission electron microscopy, using a focused ion beam to prepare cross-sections through selected indents. It was found that ambient temperature deformation was controlled by the plastic flow of the Al layers, constrained by the SiC, and the elastic bending of the SiC layers. The reduction in hardness with temperature showed evidence of the development of interface-mediated deformation mechanisms, which led to a clear influence of layer thickness on the hardness.

Mechanical behavior and failure micromechanisms of hybrid 3D woven composites in tension

The deformation and failure micromechanisms of a hybrid 3D woven composite were studied in tension. Plain and open-hole composite coupons were tested in tension until failure in the fill and warp directions, as well as fiber tows extracted from the dry fabric and impregnated with the matrix. The macroscopic evolution of damage in the composite coupons was assessed by means of periodic unloading–reloading (to obtain the elastic modulus and the residual strain), whereas the microscopic mechanism were established by means of X-ray computed microtomography. To this end, specimens were periodically removed from the mechanical testing machine and infiltrated with ZnI-containing liquid to assess the main damage modes as a function of the applied strain. The experimental observations and the predictions of an isostrain model were used to understand the key factors controlling the elastic modulus, strength and notch sensitivity of hybrid 3D woven composites in tension. It was found that the full contribution of the glass fibers to the composite strength was not employed, due to the premature fracture of the carbon fibers, but their presence increased the fracture strain and the energy dissipated during fracture. Thus, hybridization of the 3D woven composite led to a notch-insensitive behavior as demonstrated by open-hole tests

Mechanical characterization of GdBCO/Ag and YBCO single grains fabricated by top-seeded melt growth at 77 and 300 K

YBaCuO and GdBaCuO + 15 wt% Ag large, single-grain, bulk superconductors have been fabricated via the top-seeded, melt-growth (TSMG) process using a generic NdBCO seed. The mechanical behavior of both materials has been investigated by means of three-point bending (TPB) and transversal tensile tests at 77 and 300 K. The strength, fracture toughness and hardness of the samples were studied for two directions of applied load to obtain comprehensive information about the effect of microstructural anisotropy on the macroscopic and microscopic mechanical properties of these technologically important materials. Splitting (Brazilian) tests were carried out on as-melt-processed cylindrical samples following a standard oxygenation process and with the load applied parallel to the growth-facet lines characteristic of the TSMG process. In addition, the elastic modulus of each material was measured by three different techniques and related to the microstructure of each sample using optical microscopy. The results show that both the mechanical properties and the elastic modulus of both YBCO and GdBCP/Ag are improved at 77 K. However, the GdBCO/Ag samples are less anisotropic and exhibit better mechanical behavior due to the presence of silver particles in the bulk, superconducting matrix. The splitting tensile strength was determined at 77 K and both materials were found to exhibit similar behavior, independently of their differences in microstructure.

Microstructural and mechanical characterization of tungsten based materials for fusion reactors

El wolframio (W) y sus aleaciones se consideran los mejores candidatos para la construcción del divertor en la nueva generación de reactores de fusión nuclear. Este componente va a recibir las cargas térmicas más elevadas durante el funcionamiento del reactor ya que estará en contacto directo con el plasma. En los últimos años, después de un profundo análisis y siguiendo una estrategia de reducción de costes, la Organización de ITER tomó la decisión de construir el divertor integramente de wolframio desde el principio. Por ello, el wolframio no sólo actuará como material en contacto con el plasma (PFM), sino que también tendría aplicaciones estructurales. El wolframio, debido a sus excelentes propiedades termo-físicas, cumple todos los requerimientos para ser utilizado como PFM, sin embargo, su inherente fragilidad pone en peligro su uso estructural. Por tanto, uno de los principales objetivos de esta tesis es encontrar una aleación de wolframio con menor fragilidad. Durante éste trabajo, se realizó la caracterización microstructural y mecánica de diferentes materiales basados en wolframio. Sin embargo, ésta tarea es un reto debido a la pequeña cantidad de material suministrado, su reducido tamaño de grano y fragilidad. Por ello, para una correcta medida de todas las propiedades físicas y mecánicas se utilizaron diversas técnicas experimentales. Algunas de ellas se emplean habitualmente como la nanoindentación o los ensayos de flexión en tres puntos (TPB). Sin embargo, otras fueron especificamente desarrolladas e implementadas durante el desarrollo de esta tesis como es el caso de la medida real de la tenacidad de fractura en los materiales masivos, o de las medidas in situ de la tenacidad de fractura en las láminas delgadas de wolframio. Diversas composiciones de aleaciones de wolframio masivas (W-1% Y2O3, W-2% V-0.5% Y2O3, W-4% V-0.5% Y2O3, W-2% Ti-1% La2O3 y W-4% Ti-1% La2O3) se han estudiado y comparado con un wolframio puro producido en las mismas condiciones. Estas aleaciones, producidas por ruta pulvimetalúrgica de aleado mecánico (MA) y compactación isostática en caliente (HIP), fueron microstructural y mecánicamente caracterizadas desde 77 hasta 1473 K en aire y en alto vacío. Entre otras propiedades físicas y mecánicas se midieron la dureza, el módulo elástico, la resistencia a flexión y la tenacidad de fractura para todas las aleaciones. Finalmente se analizaron las superficies de fractura después de los ensayos de TPB para relacionar los micromecanismos de fallo con el comportamiento macroscópico a rotura. Los resultados obtenidos mostraron un comportamiento mecánico frágil en casi todo el intervalo de temperaturas y para casi todas las aleaciones sin mejoría de la temperatura de transición dúctil-frágil (DBTT). Con el fin de encontrar un material base wolframio con una DBTT más baja se realizó también un estudio, aún preliminar, de láminas delgadas de wolframio puro y wolframio dopado con 0.005wt.% potasio (K). Éstas láminas fueron fabricadas industrialmente mediante sinterizado y laminación en caliente y en frío y se sometieron posteriormente a un tratamiento térmico de recocido desde 1073 hasta 2673 K. Se ha analizado la evolución de su microestructura y las propiedades mecánicas al aumentar la temperatura de recocido. Los resultados mostraron la estabilización de los granos de wolframio con el incremento de la temperatura de recocido en las láminas delgadas de wolframio dopado con potasio. Sin embargo, es necesario realizar estudios adicionales para entender mejor la microstructura y algunas propiedades mecánicas de estos materiales, como la tenacidad de fractura. Tungsten (W) and tungsten-based alloys are considered to be the best candidate materials for fabricating the divertor in the next-generation nuclear fusion reactors. This component will experience the highest thermal loads during the operation of a reactor since it directly faces the plasma. In recent years, after thorough analysis that followed a strategy of cost reduction, the ITER Organization decided to built a full-tunsgten divertor before the first nuclear campaigns. Therefore, tungsten will be used not only as a plasma-facing material (PFM) but also in structural applications. Tungsten, due to its the excellent thermo-physical properties fulfils the requirements of a PFM, however, its use in structural applications is compromised due to its inherent brittleness. One of the objectives of this phD thesis is therefore, to find a material with improved brittleness behaviour. The microstructural and mechanical characterisation of different tunsgten-based materials was performed. However, this is a challenging task because of the reduced laboratory-scale size of the specimens provided, their _ne microstructure and their brittleness. Consequently, many techniques are required to ensure an accurate measurement of all the mechanical and physical properties. Some of the applied methods have been widely used such as nanoindentation or three-point bending (TPB) tests. However, other methods were specifically developed and implemented during this work such as the measurement of the real fracture toughness of bulk-tunsgten alloys or the in situ fracture toughness measurements of very thin tungsten foils. Bulk-tunsgten materials with different compositions (W-1% Y2O3, W-2% V- 0.5% Y2O3, W-4% V-0.5% Y2O3, W-2% Ti-1% La2O3 and W-4% Ti-1% La2O3) were studied and compared with pure tungsten processed under the same conditions. These alloys, produced by a powder metallurgical route of mechanical alloying (MA) and hot isostatic pressing (HIP), were microstructural and mechanically characterised from 77 to 1473 K in air and under high vacuum conditions. Hardness, elastic modulus, flexural strength and fracture toughness for all of the alloys were measured in addition to other physical and mechanical properties. Finally, the fracture surfaces after the TPB tests were analysed to correlate the micromechanisms of failure with the macroscopic behaviour. The results reveal brittle mechanical behaviour in almost the entire temperature range for the alloys and micromechanisms of failure with no improvement in the ductile-brittle transition temperature (DBTT). To continue the search of a tungsten material with lowered DBTT, a preliminary study of pure tunsgten and 0.005 wt.% potassium (K)-doped tungsten foils was also performed. These foils were industrially produced by sintering and hot and cold rolling. After that, they were annealed from 1073 to 2673 K to analyse the evolution of the microstructural and mechanical properties with increasing annealing temperature. The results revealed the stabilisation of the tungsten grains with increasing annealing temperature in the potassium-doped tungsten foil. However, additional studies need to be performed to gain a better understanding of the microstructure and mechanical properties of these materials such as fracture toughness.

Relationship between residual stresses and mechanical behavior in ceramic composites

En este trabajo, materiales de tipo alúmina/Y-TZP (ZrO2 tetragonal, estabilizada con 3 mol. % Y2O3), como sistema cerámico popular por sus mejoradas propiedades mecánicas en comparación con las cerámicas de alúmina puras, han sido estudiados en términos de propiedades mecánicas y tensiones residuales. El novedoso método de colado en cinta, consistente en el apilamiento de cintas de cerámica verde a temperatura ambiente y el uso de bajas presiones, se ha escogido para la presente investigación con el fin de poder aprovechar al máximo el futuro desarrollo de materiales laminados de alúmina-óxido de circonio. Se han determinado las propiedades de los materiales obtenidos por este nuevo método de procesamiento comparándolas con las de los materiales obtenidos por “slip casting”, con el fin de analizar si el método propuesto afecta a la microestructura y, por tanto, a las propiedades mecánicas y tensiones residuales propias de estos materiales. Para analizar la idoneidad del proceso de fabricación, utilizado para evitar la presencia de discontinuidades en las intercaras entre las láminas así como otros fenómenos que puedan interferir con las propiedades mecánicas, se estudiaron materiales cerámicos con la misma composición en cintas. Por otra parte también se analizó el efecto de la adición de óxido de circonio sobre la aparición de tensiónes residuales en cerámicas Al2O3/Y-TZP, teniendo en cuenta su notable influencia sobre las propiedades microestructurales y mecánicas de los materiales, así como el requisito de co-sinterización de capas con diferentes materiales compuestos en materiales laminados. La caracterización del material incluye la determinación de la densidad, el análisis de la microestructura, la obtención de las propiedades mecánicas (módulo de elasticidad, dureza, resistencia a la flexión y tenacidad de fractura) así como de las tensiones residuales. En combinación con otros métodos de medida tradicionales, la nanoindentación también se empleó como una técnica adicional para la medida del módulo de elasticidad y de la dureza. Por otro lado, diferentes técnicas de difracción con neutrones, tanto las basadas en longitud de onda constante (CW) como en tiempo de vuelo (TOF), han sido empleadas para la medición fiable de la deformación residual a través del grosor en muestras a granel. Las tensiones residuales fueron determinadas con elevada precisión, aplicando además métodos de análisis apropiados, como por ejemplo el refinamiento de Rietveld. Las diferentes fases en cerámicas sinterizadas, especialmente las de zirconia, se examinaron con detalle mediante el análisis de Rietveld, teniendo en cuenta el complicado polimorfismo del Óxido de Zirconio (ZrO2) así como las posibles transformaciones de fase durante el proceso de fabricación. Los efectos del contenido de Y-TZP en combinación con el nuevo método de procesamiento sobre la microestructura, el rendimiento mecánico y las tensiones residuales de los materiales estudiados (Al2O3/Y-TZP) se resumen en el presente trabajo. Finalmente, los mecanismos de endurecimiento, especialmente los relacionados con las tensiones residuales, son igualmente discutidos. In present work, Alumina/Y-TZP (tetragonal ZrO2 stabilized with 3 mol% Y2O3) materials, as an popular ceramic system with improved mechanical properties compared with the pure alumina ceramics, have been studied in terms of mechanical properties and residual stresses. The novel tape casting method, which involved the stacking of green ceramics tapes at room temperature and using low pressures, is selected for manufacturing and investigation, in order to take full advantage of the future development of alumina-zirconia laminated materials. Features of materials obtained by the new processing method are determined and compared with those of materials obtained by conventional slip casting in a plaster mold, in order to study whether the proposed method of processing affects microstructure and thereby the mechanical properties and residual stresses characteristics of materials. To analyse the adequacy of the manufacturing process used to avoid the presence of discontinuities at the interfaces between the sheets and other phenomena that interfere with the mechanical properties, ceramic materials with the same composition in tapes were investigated. Moreover, the effect of addition of zirconia on residual stress development of Al2O3/Y-TZP ceramics were taken into investigations, considering its significantly influence on the microstructure and mechanical properties of materials as well as the requirement of co-sintering of layers with different composites in laminated materials. The characterization includes density, microstructure, mechanical properties (elastic modulus, hardness, flexure strength and fracture toughness) and residual stresses. Except of the traditional measurement methods, nanoindentation technique was also used as an additional measurement of the elastic modulus and hardness. Neutron diffraction, both the constant-wavelength (CW) and time-of-flight (TOF) neutron diffraction techniques, has been used for reliable through-thickness residual strain measurement in bulk samples. Residual stresses were precisely determined combined with appropriate analysis methods, e.g. the Rietveld refinement. The phase compositions in sintered ceramics especially the ones of zirconia were accurately examined by Rietveld analysis, considering the complex polymorph of ZrO2 and the possible phase transformation during manufacturing process. Effects of Y-TZP content and the new processing method on the microstructure, mechanical performance and residual stresses were finally summarized in present studied Al2O3/Y-TZP materials. The toughening mechanisms, especially the residual stresses related toughening, were theoretically discussed.

Modelización de la respuesta de elementos estructurales lineales de vidrio plano : vigas y pilares pretensados

El vidrio se trata de un material muy apreciado en la arquitectura debido a la transparencia, característica que pocos materiales tienen. Pero, también es un material frágil, con una rotura inmediata cuando alcanza su límite elástico, sin disponer de un período plástico, que advierta de su futura rotura y permita un margen de seguridad. Por ambas razones, el vidrio se ha utilizado en arquitectura como elemento de plementería o relleno, desde tiempos antiguos, pero no como elemento estructural o portante, pese a que es un material interesante para los arquitectos para ese uso, por su característica de transparencia, ya que conseguiría la desmaterialización visual de la estructura, logrando espacios más ligeros y livianos. En cambio, si se tienen en cuenta las propiedades mecánicas del material se puede comprobar que dispone de unas características apropiadas para su uso estructural, ya que su Módulo elástico es similar al del aluminio, elemento muy utilizado en la arquitectura principalmente en las fachadas desde los últimos años, y su resistencia a compresión es muy superior incluso al hormigón armado; aunque su principal problema es su resistencia a tracción que es muy inferior a su resistencia a compresión, lo que penaliza su resistencia a flexión. En la actualidad se empieza a utilizar el vidrio como elemento portante o estructural, pero debido a su peor resistencia a flexión, se utilizan con grandes dimensiones que, a pesar de su transparencia, tienen una gran presencia. Por ello, la presente investigación pretende conseguir una reducción de las secciones de estos elementos estructurales de vidrio. Entonces, para el desarrollo de la investigación es necesario responder a una serie de preguntas fundamentales, cuyas respuestas serán el cuerpo de la investigación: 1. ¿Cuál es la finalidad de la investigación? El objetivo de esta investigación es la optimización de elementos estructurales de vidrio para su utilización en arquitectura. 2. ¿Cómo se va a realizar esa optimización? ¿Qué sistemas se van a utilizar? El sistema para realizar la optimización será la pretensión de los elementos estructurales de vidrio 3. ¿Por qué se va a utilizar la precompresión? Porque el vidrio tiene un buen comportamiento a compresión y un mal comportamiento a tracción lo que penaliza su utilización a flexión. Por medio de la precompresión se puede incrementar esta resistencia a tracción, ya que los primeros esfuerzos reducirán la compresión inicial hasta comenzar a funcionar a tracción, y por tanto aumentará su capacidad de carga. 4. ¿Con qué medios se va a comprobar y justificar ese comportamiento? Mediante simulaciones informáticas con programas de elementos finitos. 5. ¿Por qué se utilizará este método? Porque es una herramienta que arroja ventajas sobre otros métodos como los experimentales, debido a su fiabilidad, economía, rapidez y facilidad para establecer distintos casos. 6. ¿Cómo se garantiza su fiabilidad? Mediante el contraste de resultados obtenidos con ensayos físicos realizados, garantizando de ésta manera el buen comportamiento de los programas utilizados. El presente estudio tratará de responder a todas estas preguntas, para concluir y conseguir elementos estructurales de vidrio con secciones más reducidas gracias a la introducción de la precompresión, todo ello a través de las simulaciones informáticas por medio de elementos finitos. Dentro de estas simulaciones, también se realizarán comprobaciones y comparaciones entre distintas tipologías de programas para comprobar y contrastar los resultados obtenidos, intentando analizar cuál de ellos es el más idóneo para la simulación de elementos estructurales de vidrio. ABSTRACT Glass is a material very appreciated in architecture due to its transparency, feature that just a few materials share. But it is also a brittle material with an immediate breakage when it reaches its elastic limit, without having a plastic period that provides warning of future breakage allowing a safety period. For both reasons, glass has been used in architecture as infill panels, from old times. However, it has never been used as a structural or load‐bearing element, although it is an interesting material for architects for that use: because of its transparency, structural glass makes possible the visual dematerialization of the structure, achieving lighter spaces. However, taking into account the mechanical properties of the material, it is possible to check that it has appropriate conditions for structural use: its elastic modulus is similar to that of aluminium, element widely used in architecture, especially in facades from recent years; and its compressive strength is much higher than even the one of concrete. However, its main problem consists in its tensile strength that is much lower than its compressive strength, penalizing its resistance to bending. Nowadays glass is starting to be used as a bearing or structural element, but due to its worse bending strength, elements with large dimensions must be used, with a large presence despite its transparency. Therefore this research aims to get smaller sections of these structural glass elements. For the development of this thesis, it is necessary to answer a number of fundamental questions. The answers will be the core of this work: 1. What is the purpose of the investigation? The objective of this research is the optimization of structural glass elements for its use in architecture. 2. How are you going to perform this optimization? What systems will be implemented? The system for optimization is the pre‐stress of the structural elements of glass 3. Why are you going to use the pre‐compression? Because glass has a good resistance to compression and a poor tensile behaviour, which penalizes its use in bending elements. Through the pre‐compression it is possible to increase this tensile strength, due to the initial tensile efforts reducing the pre‐stress and increasing its load capacity. 4. What are the means that you will use in order to verify and justify this behaviour? The means are based on computer simulations with finite element programs (FEM) 5. Why do you use this method? Because it is a tool which gives advantages over other methods such as experimental: its reliability, economy, quick and easy to set different cases. 6. How the reliability is guaranteed? It’s guaranteed comparing the results of the simulation with the performed physical tests, ensuring the good performance of the software. This thesis will attempt to answer all these questions, to obtain glass structural elements with smaller sections thanks to the introduction of the pre‐compression, all through computer simulations using finite elements methods. In these simulations, tests and comparisons between different types of programs will also be implemented, in order to test and compare the obtained results, trying to analyse which one is the most suitable for the simulation of structural glass elements.

How do Impurity Inclusions Influence the Mechanical Properties of Multicrystalline Silicon?

The purpose of this research is to characterise the mechanical properties of multicrystalline silicon for photovoltaic applications that was crystallised from silicon feedstock with a high content of several types of impurities. The mechanical strength, fracture toughness and elastic modulus were measured at different positions within a multicrystalline silicon block to quantify the effect of impurity segregation on these mechanical properties. The microstructure and fracture surfaces of the samples was exhaustively analysed with a scanning electron microscope in order to correlate the values of mechanical properties with material microstructure. Fracture stresses values were treated statistically via the Weibull statistics. The results of this research show that metals segregate to the top of the block, produce moderate microcracking and introduce high thermal stresses. Silicon oxide is produced at the bottom part of the silicon block, and its presence significantly reduces the mechanical strength and fracture toughness of multicrystalline silicon due to both thermal and elastic mismatch between silicon and the silicon oxide inclusions. Silicon carbide inclusions from the upper parts of the block increase the fracture toughness and elastic modulus of multicrystalline silicon. Additionally, the mechanical strength of multicrystalline silicon can increase when the radius of the silicon carbide inclusions is smaller than ~10 µm. The most damaging type of impurity inclusion for the multicrystalline silicon block studied in this work was amorphous silicon oxide. The oriented precipitation of silicon oxide at grain and twin boundaries eases the formation of radial cracks between inclusions and decreases significatively the mechanical strength of multicrystalline silicon. The second most influencing type of impurity inclusions were metals like aluminium and copper, that cause spontaneous microcracking in their surroundings after the crystallisation process, therefore reducing the mechanical response of multicrystalline silicon. Therefore, solar cell producers should pay attention to the content of metals and oxygen within the silicon feedstock in order to produce solar cells with reliable mechanical properties.

Topographical and mechanical characterization of living eukaryotic cells on opaque substrates: development of a general procedure and its application to the study of non-adherent lymphocytes

The mechanical behavior of living murine T-lymphocytes was assessed by atomic force microscopy (AFM). A robust experimental procedure was developed to overcome some features of lymphocytes, in particular their spherical shape and non-adherent character. The procedure included the immobilization of the lymphocytes on amine-functionalized substrates, the use of hydrodynamic effects on the deflection of the AFM cantilever to monitor the approaching, and the use of the jumping mode for obtaining the images. Indentation curves were analyzed according to Hertz's model for contact mechanics. The calculated values of the elastic modulus are consistent both when considering the results obtained from a single lymphocyte and when comparing the curves recorded from cells of different specimens

The elasticity and failure of fluid-filled cellular solids: Theory and experiment

We extend and apply theories of filled foam elasticity and failure to recently available data on foods. The predictions of elastic modulus and failure mode dependence on internal pressure and on wall integrity are borne out by photographic evidence of distortion and failure under compressive loading and under the localized stress applied by a knife blade, and by mechanical data on vegetables differing only in their turgor pressure. We calculate the dry modulus of plate-like cellular solids and the cross over between dry-like and fully fluid-filled elastic response. The bulk elastic properties of limp and aging cellular solids are calculated for model systems and compared with our mechanical data, which also show two regimes of response. The mechanics of an aged, limp beam is calculated, thus offering a practical procedure for comparing experiment and theory. This investigation also thereby offers explanations of the connection between turgor pressure and crispness and limpness of cellular materials.

Study of interaction between Si(O,C) nanowires and the biological system

Nanomedicine is a new branch of medicine, based on the potentiality and intrinsic properties of nanomaterials. Indeed, the nanomaterials ( i.e. the materials with nano and under micron size) can be suitable to different applications in biomedicine. The nanostructures can be used by taking advantage of their properties (for example superparamagnetic nanoparticles) or functionalized to deliver the drug in a specific target, thanks the ability to cross biological barriers. The size and the shape of 1D-nanostructures (nanotubes and nanowires) have an important role on the cell fate: their morphology plays a key role on the interaction between nanostructure and the biological system. For this reason the 1D nanostructure are interesting for their ability to mime the biological system. An implantable material or device must therefore integrate with the surrounding extracellular matrix (ECM), a complex network of proteins with structural and signaling properties. Innovative techniques allow the generation of complex surface patterns that can resemble the structure of the ECM, such as 1D nanostructures. NWs based on cubic silicon carbide (3C-SiC), either bare (3C-SiC NWs) or surrounded by an amorphous shell (3C-SiC/SiO2 core/shell NWs), and silicon oxycarbide nanowires (SiOxCy NWs) can meet the chemical, mechanical and electrical requirements for tissue engineering and have a strong potential to pave the way for the development of a novel generation of implantable nano-devices. Silicon oxycarbide shows promising physical and chemical properties as elastic modulus, bending strength and hardness, chemical durability superior to conventional silicate glasses in aggressive environments and high temperature stability up to 1300 °C. Moreover, it can easily be engineered through functionalization and decoration with macro-molecules and nanoparticles. Silicon carbide has been extensively studied for applications in harsh conditions, as chemical environment, high electric field and high and low temperature, owing to its high hardness, high thermal conductivity, chemical inertness and high electron mobility. Also, its cubic polytype (3C) is highly biocompatible and hemocompatible, and some prototypes of biomedical applications and biomedical devices have been already realized starting from 3C-SiC thin films. Cubic SiC-based NWs can be used as a biomimetic biomaterial, providing a robust and novel biocompatible biological interface . We cultured in vitro A549 human lung adenocarcinoma epithelial cells and L929 murine fibroblast cells over core/shell SiC/SiO2, SiOxCy and bare 3C-SiC nanowire platforms, and analysed the cytotoxicity, by indirect and direct contact tests, the cell adhesion, and the cell proliferation. These studies showed that all the nanowires are biocompatible according to ISO 10993 standards. We evaluated the blood compatibility through the interaction of the nanowires with platelet rich plasma. The adhesion and activation of platelets on the nanowire bundles, assessed via SEM imaging and soluble P-selectin quantification, indicated that a higher platelet activation is induced by the core/shell structures compared to the bare ones. Further, platelet activation is higher with 3C-SiC/SiO2 NWs and SiOxCyNWs, which therefore appear suitable in view of possible tissue regeneration. On the contrary, bare 3C-SiC NWs show a lower platelet activation and are therefore promising in view of implantable bioelectronics devices, as cardiovascular implantable devices. The NWs properties are suitable to allow the design of a novel subretinal Micro Device (MD). This devices is based on Si NWs and PEDOT:PSS, though the well know principle of the hybrid ordered bulk heterojunction (OBHJ). The aim is to develop a device based on a well-established photovoltaic technology and to adapt this know-how to the prosthetic field. The hybrid OBHJ allows to form a radial p–n junction on a nanowire/organic structure. In addition, the nanowires increase the light absorption by means of light scattering effects: a nanowires based p-n junction increases the light absorption up to the 80%, as previously demonstrated, overcoming the Shockley-Queisser limit of 30 % of a bulk p-n junction. Another interesting employment of these NWs is to design of a SiC based epicardial-interacting patch based on teflon that include SiC nanowires. . Such contact patch can bridge the electric conduction across the cardiac infarct as nanowires can ‘sense’ the direction of the wavefront propagation on the survival cardiac tissue and transmit it to the downstream surivived regions without discontinuity. The SiC NWs are tested in terms of toxicology, biocompatibility and conductance among cardiomyocytes and myofibroblasts.

Caracterização geométrica e mecânica de macrofibras poliméricas.

Uma grande diversidade de macrofibras poliméricas para reforço de concreto se encontram disponibilizadas hoje em dia. Por natureza estas fibras apresentam grande diversidade de características e propriedades. Estas variações afetam sua atuação como reforço no concreto. No entanto, não há normas brasileiras sobre o assunto e as metodologias de caracterização de normas estrangeiras apresentam divergências. Algumas normas definem que a caracterização do comportamento mecânico deva ser feita nos fios originais e outras que se devam utilizar métodos definidos para caracterização de materiais metálicos. A norma EN14889-2:2006 apresenta maior abrangência, mas deixa dúvidas quanto à adequação dos critérios de caracterização geométrica das fibras e não define um método de ensaio específico para sua caracterização mecânica. Assim, há a necessidade de estabelecimento de uma metodologia que permita a realização de um programa de controle de qualidade da fibra nas condições de emprego. Esta metodologia também proporcionaria uma forma de caracterização do material para estudos experimentais, o que permitiria maior fundamentação científica desses trabalhos que, frequentemente, fundamentam-se apenas em dados dos fabricantes. Assim, foi desenvolvido um estudo experimental focando a caracterização de duas macrofibras poliméricas disponíveis no mercado brasileiro. Focou-se o estudo na determinação dos parâmetros geométricos e na caracterização mecânica através da determinação da resistência à tração e avaliação do módulo de elasticidade. Na caracterização geométrica foi adotada como referência a norma europeia EN14889-2:2006. As medições do comprimento se efetuaram por dois métodos: o método do paquímetro e o método de análise de imagens digitais, empregando um software para processamento das imagens. Para a medição do diâmetro, além das metodologias mencionadas, foi usado o método da densidade. Conclui-se que o método do paquímetro, com o cuidado de esticar previamente as macrofibras, e o método das imagens digitais podem ser igualmente utilizados para medir o comprimento. Já parar determinar o diâmetro, recomenda-se o método da densidade. Quanto à caracterização mecânica, foi desenvolvida uma metodologia própria a partir de informações obtidas de outros ensaios. Assim, efetuaram-se ensaios de tração direta nas macrofibras coladas em molduras de tecido têxtil. Complementarmente, foi avaliado também o efeito do contato abrasivo das macrofibras com os agregados durante a mistura em betoneira no comportamento mecânico do material. Também se avaliou o efeito do método de determinação da área da seção transversal nos resultados medidos no ensaio de tração da fibra. Conclui-se que o método proposto para o ensaio de tração direta da fibra é viável, especialmente para a determinação da resistência à tração. O valor do módulo de elasticidade, por sua vez, acaba sendo subestimado. A determinação da área da seção da fibra através do método da densidade forneceu também os melhores resultados. Além disso, comprovou-se que o atrito das fibras com o agregado durante a mistura compromete o comportamento mecânico, reduzindo tanto a resistência quanto o módulo de elasticidade. Assim, pode-se afirmar que a metodologia proposta para o controle geométrico e mecânico das macrofibras poliméricas é adequada para a caracterização do material.

Comportamento em desgaste por erosão cavitação, erosão - corrosão e em ensaios de microesclerometria linear instrumentada de um aço inoxidável martensítico AISI 410 nitretado a plasma em baixa temperatura, utilizando a tecnologia de tela ativa.

Amostras de um aço inoxidável martensítico AISI 410 temperado e revenido foram nitretadas a plasma em baixa temperatura usando o tratamento de nitretação plasma DC e a nitretação a plasma com tela ativa. Ambos os tratamentos foram realizados a 400 °C, utilizando mistura gasosa de 75 % de nitrogênio e 25 % de hidrogênio durante 20 horas e 400 Pa de pressão. As amostras de aço AISI 410 temperado e revenido foram caracterizadas antes e depois dos tratamentos termoquímicos, usando as técnicas de microscopia óptica, microscopia eletrônica de varredura, medidas de microdureza, difração de raios X e medidas de teor de nitrogênio em função da distância à superfície por espectrometria WDSX de raios X. A resistência à erosão por cavitação do aço AISI 410 nitretado DC e com tela ativa foi avaliada segundo a norma ASTM G32 (1998). Os ensaios de erosão, de erosão - corrosão e de esclerometria linear instrumentada segundo norma ASTM C1624 (2005) somente foram realizados no aço AISI 410 nitretado com tela ativa. Ensaios de nanoindentação instrumentada forma utilizados para medir a dureza (H) e o módulo de elasticidade reduzido (E*) e calcular as relações H/E* e H3/E*2 e a recuperação elástica (We), utilizando o método proposto por Oliver e Pharr. Ambos os tratamentos produziram camadas nitretadas de espessura homogênea constituídas por martensita expandida supersaturada em nitrogênio e nitretos de ferro com durezas superiores a 1200 HV, porém, a nitretação DC produziu maior quantidade de nitretos de ferro do que o tratamento de tela ativa. Os resultados de erosão por cavitação do aço nitretado DC mostraram que a precipitação de nitretos de ferro é prejudicial para a resistência à cavitação já que reduziu drasticamente o período de incubação e aumentou a taxa de perda de massa nos estágios iniciais do ensaio; entretanto, depois da remoção desses nitretos de ferro, a camada nitretada formada somente por martensita expandida resistiu bem ao dano por cavitação. Já no caso do aço nitretado com tela ativa, a resistência à erosão por cavitação aumentou 27 vezes quando comparada com o aço AISI 410 sem nitretar, fato atribuído à pequena fração volumétrica e ao menor tamanho dos nitretos de ferro presente na camada nitretada, às maiores relações H/E* e H3/E*2 e à alta recuperação elástica da martensita expandida. A remoção de massa ocorreu, principalmente, pela formação de crateras e de destacamento de material da superfície dos grãos por fratura frágil sem evidente deformação plástica. As perdas de massa acumulada mostradas pelo aço nitretado foram menores do que aquelas do aço AISI 410 nos ensaios de erosão e de erosão corrosão. O aço nitretado apresentou uma diminuição nas taxas de desgaste em ambos os ensaios de aproximadamente 50 % quando comparadas com o aço AISI 410. O mecanismo de remoção de material foi predominantemente dúctil, mesmo com o grande aumento na dureza. Os resultados de esclerometria linear instrumentada mostraram que a formação de martensita expandida possibilitou uma diminuição considerável do coeficiente de atrito em relação ao observado no caso do aço AISI 410 sem nitretar. O valor de carga crítica de falha foi de 14 N. O mecanismo de falha operante no aço nitretado foi trincamento por tensão.

Obtenção e caracterização de PLA reforçado com nanocelulose.

A celulose é o polímero natural renovável disponível em maior abundância atualmente. Por possuir estrutura semicristalina, é possível extrair seus domínios cristalinos através de procedimentos que ataquem sua fase amorfa, como a hidrólise ácida, obtendo-se assim partículas cristalinas chamadas nanopartículas de celulose (NCs). Estas nanopartículas têm atraído enorme interesse científico, uma vez que possuem propriedades mecânicas, como módulo de elasticidade e resistência à tração, semelhantes a várias cargas inorgânicas utilizadas na fabricação de compósitos. Além disso, possuem dimensões nanométricas, o que contribui para menor adição de carga à matriz polimérica, já que possuem maior área de superfície, quando comparadas às cargas micrométricas. Nanocompósitos formados pela adição destas cargas em matrizes poliméricas podem apresentar propriedades comerciais atraentes, como barreira a gases, melhores propriedades térmicas e baixa densidade, quando comparados aos compósitos tradicionais. Como se trata de uma carga com dimensões nanométricas, obtida de fontes renováveis, uma das principais áreas de interesse para aplicação deste reforço é em biopolímeros biodegradáveis. O poli(ácido lático) (PLA), é um exemplo de biopolímero com propriedades mecânicas, térmicas e de processamento superiores a de outros biopolímeros comerciais. No presente trabalho foram obtidas nanopartículas de celulose (NCs), por meio de hidrólise ácida, utilizando-se três métodos distintos, com o objetivo de estudar o método mais eficiente para a obtenção de NCs adequadas à aplicação em compósitos de PLA. Os Métodos I e II empregam extração das NCs por meio do H2SO4, diferenciando-se apenas pela neutralização, a qual envolve diálise ou neutralização com NaHCO3, respectivamente. No Método III a extração das NCs foi realizada com H3PO4. As NCs foram caracterizadas por diferentes técnicas, como difração de raios X (DRX), análise termogravimétrica (TG), espectroscopia vibracional de absorção no infravermelho (FTIR), microscopia eletrônica de transmissão (MET) e microscopia de força atômica (MFA). Os resultados de caracterização das NCs indicaram que, a partir de todos os métodos utilizados, há formação de nanocristais de celulose (NCCs), entretanto, apenas os NCCs obtidos pelos Métodos II e III apresentaram estabilidade térmica suficiente para serem empregados em compósitos preparados por adição da carga no polímero em estado fundido. A incorporação das NCs em matriz de PLA foi realizada em câmara de mistura, com posterior moldagem por prensagem a quente. Compósitos obtidos por adição de NCs obtidas pelo Método II foram caracterizados por calorimetria exploratória diferencial (DSC), análise termogravimétrica, microscopia óptica, análises reológicas e microscopia eletrônica de varredura (MEV). A adição de NCs, extraídas pelo Método II, em matriz de PLA afetou o processo de cristalização do polímero, o qual apresentou maior grau de cristalinidade. Além disso, a adição de 3% em massa de NCs no PLA foi suficiente para alterar seu comportamento reológico. Os resultados reológicos indicaram que a morfologia do compósito é, predominantemente, composta por uma dispersão homogênea e fina da carga na fase matriz. Micrografias obtidas por MEV corroboram os resultados reológicos, mostrando, predominantemente a presença de partículas de NC em escala nanométrica. Compósitos de PLA com NCs obtidas pelo Método III apresentaram aglomerados de partículas de NC em escala micro e milimétrica, ao longo da fase matriz, e não foram extensivamente caracterizados.