Multiscale modeling of the plastic behaviour in single crystal tungsten: from atomistic to crystal plasticity simulations

En una planta de fusión, los materiales en contacto con el plasma así como los materiales de primera pared experimentan condiciones particularmente hostiles al estar expuestos a altos flujos de partículas, neutrones y grandes cargas térmicas. Como consecuencia de estas diferentes y complejas condiciones de trabajo, el estudio, desarrollo y diseño de estos materiales es uno de los más importantes retos que ha surgido en los últimos años para la comunidad científica en el campo de los materiales y la energía. Debido a su baja tasa de erosión, alta resistencia al sputtering, alta conductividad térmica, muy alto punto de fusión y baja retención de tritio, el tungsteno (wolframio) es un importante candidato como material de primera pared y como posible material estructural avanzado en fusión por confinamiento magnético e inercial. Sin embargo, el tiempo de vida del tungsteno viene controlado por diversos factores como son su respuesta termo-mecánica en la superficie, la posibilidad de fusión y el fallo por acumulación de helio. Es por ello que el tiempo de vida limitado por la respuesta mecánica del tungsteno (W), y en particular su fragilidad, sean dos importantes aspectos que tienes que ser investigados. El comportamiento plástico en materiales refractarios con estructura cristalina cúbica centrada en las caras (bcc) como el tungsteno está gobernado por las dislocaciones de tipo tornillo a escala atómica y por conjuntos e interacciones de dislocaciones a escalas más grandes. El modelado de este complejo comportamiento requiere la aplicación de métodos capaces de resolver de forma rigurosa cada una de las escalas. El trabajo que se presenta en esta tesis propone un modelado multiescala que es capaz de dar respuestas ingenieriles a las solicitudes técnicas del tungsteno, y que a su vez está apoyado por la rigurosa física subyacente a extensas simulaciones atomísticas. En primer lugar, las propiedades estáticas y dinámicas de las dislocaciones de tipo tornillo en cinco potenciales interatómicos de tungsteno son comparadas, determinando cuáles de ellos garantizan una mayor fidelidad física y eficiencia computacional. Las grandes tasas de deformación asociadas a las técnicas de dinámica molecular hacen que las funciones de movilidad de las dislocaciones obtenidas no puedan ser utilizadas en los siguientes pasos del modelado multiescala. En este trabajo, proponemos dos métodos alternativos para obtener las funciones de movilidad de las dislocaciones: un modelo Monte Cario cinético y expresiones analíticas. El conjunto de parámetros necesarios para formular el modelo de Monte Cario cinético y la ley de movilidad analítica son calculados atomísticamente. Estos parámetros incluyen, pero no se limitan a: la determinación de las entalpias y energías de formación de las parejas de escalones que forman las dislocaciones, la parametrización de los efectos de no Schmid característicos en materiales bcc,etc. Conociendo la ley de movilidad de las dislocaciones en función del esfuerzo aplicado y la temperatura, se introduce esta relación como ecuación de flujo dentro de un modelo de plasticidad cristalina. La predicción del modelo sobre la dependencia del límite de fluencia con la temperatura es validada experimentalmente con ensayos uniaxiales en tungsteno monocristalino. A continuación, se calcula el límite de fluencia al aplicar ensayos uniaxiales de tensión para un conjunto de orientaciones cristalográticas dentro del triángulo estándar variando la tasa de deformación y la temperatura de los ensayos. Finalmente, y con el objetivo de ser capaces de predecir una respuesta más dúctil del tungsteno para una variedad de estados de carga, se realizan ensayos biaxiales de tensión sobre algunas de las orientaciones cristalográficas ya estudiadas en función de la temperatura.-------------------------------------------------------------------------ABSTRACT ----------------------------------------------------------Tungsten and tungsten alloys are being considered as leading candidates for structural and functional materials in future fusion energy devices. The most attractive properties of tungsten for the design of magnetic and inertial fusion energy reactors are its high melting point, high thermal conductivity, low sputtering yield and low longterm disposal radioactive footprint. However, tungsten also presents a very low fracture toughness, mostly associated with inter-granular failure and bulk plasticity, that limits its applications. As a result of these various and complex conditions of work, the study, development and design of these materials is one of the most important challenges that have emerged in recent years to the scientific community in the field of materials for energy applications. The plastic behavior of body-centered cubic (bcc) refractory metals like tungsten is governed by the kink-pair mediated thermally activated motion of h¿ (\1 11)i screw dislocations on the atomistic scale and by ensembles and interactions of dislocations at larger scales. Modeling this complex behavior requires the application of methods capable of resolving rigorously each relevant scale. The work presented in this thesis proposes a multiscale model approach that gives engineering-level responses to the technical specifications required for the use of tungsten in fusion energy reactors, and it is also supported by the rigorous underlying physics of extensive atomistic simulations. First, the static and dynamic properties of screw dislocations in five interatomic potentials for tungsten are compared, determining which of these ensure greater physical fidelity and computational efficiency. The large strain rates associated with molecular dynamics techniques make the dislocation mobility functions obtained not suitable to be used in the next steps of the multiscale model. Therefore, it is necessary to employ mobility laws obtained from a different method. In this work, we suggest two alternative methods to get the dislocation mobility functions: a kinetic Monte Carlo model and analytical expressions. The set of parameters needed to formulate the kinetic Monte Carlo model and the analytical mobility law are calculated atomistically. These parameters include, but are not limited to: enthalpy and energy barriers of kink-pairs as a function of the stress, width of the kink-pairs, non-Schmid effects ( both twinning-antitwinning asymmetry and non-glide stresses), etc. The function relating dislocation velocity with applied stress and temperature is used as the main source of constitutive information into a dislocation-based crystal plasticity framework. We validate the dependence of the yield strength with the temperature predicted by the model against existing experimental data of tensile tests in singlecrystal tungsten, with excellent agreement between the simulations and the measured data. We then extend the model to a number of crystallographic orientations uniformly distributed in the standard triangle and study the effects of temperature and strain rate. Finally, we perform biaxial tensile tests and provide the yield surface as a function of the temperature for some of the crystallographic orientations explored in the uniaxial tensile tests.

Plastic Furniture: las sillas-mundo

Investigación sobre las herramientas arquitectónicas utilizadas por Charles y Ray Eames a través del estudio de la familia de mobiliario que ha recibido el nombre de Plastic Furniture

Modelos de elementos finitos explícitos para explosiones en estructuras reticuladas de hormigón armado : aplicaciones al estudio del colapso de edificios

La frecuencia con la que se producen explosiones sobre edificios, ya sean accidentales o intencionadas, es reducida, pero sus efectos pueden ser catastróficos. Es deseable poder predecir de forma suficientemente precisa las consecuencias de estas acciones dinámicas sobre edificaciones civiles, entre las cuales las estructuras reticuladas de hormigón armado son una tipología habitual. En esta tesis doctoral se exploran distintas opciones prácticas para el modelado y cálculo numérico por ordenador de estructuras de hormigón armado sometidas a explosiones. Se emplean modelos numéricos de elementos finitos con integración explícita en el tiempo, que demuestran su capacidad efectiva para simular los fenómenos físicos y estructurales de dinámica rápida y altamente no lineales que suceden, pudiendo predecir los daños ocasionados tanto por la propia explosión como por el posible colapso progresivo de la estructura. El trabajo se ha llevado a cabo empleando el código comercial de elementos finitos LS-DYNA (Hallquist, 2006), desarrollando en el mismo distintos tipos de modelos de cálculo que se pueden clasificar en dos tipos principales: 1) modelos basados en elementos finitos de continuo, en los que se discretiza directamente el medio continuo mediante grados de libertad nodales de desplazamientos; 2) modelos basados en elementos finitos estructurales, mediante vigas y láminas, que incluyen hipótesis cinemáticas para elementos lineales o superficiales. Estos modelos se desarrollan y discuten a varios niveles distintos: 1) a nivel del comportamiento de los materiales, 2) a nivel de la respuesta de elementos estructurales tales como columnas, vigas o losas, y 3) a nivel de la respuesta de edificios completos o de partes significativas de los mismos. Se desarrollan modelos de elementos finitos de continuo 3D muy detallados que modelizan el hormigón en masa y el acero de armado de forma segregada. El hormigón se representa con un modelo constitutivo del hormigón CSCM (Murray et al., 2007), que tiene un comportamiento inelástico, con diferente respuesta a tracción y compresión, endurecimiento, daño por fisuración y compresión, y rotura. El acero se representa con un modelo constitutivo elastoplástico bilineal con rotura. Se modeliza la geometría precisa del hormigón mediante elementos finitos de continuo 3D y cada una de las barras de armado mediante elementos finitos tipo viga, con su posición exacta dentro de la masa de hormigón. La malla del modelo se construye mediante la superposición de los elementos de continuo de hormigón y los elementos tipo viga de las armaduras segregadas, que son obligadas a seguir la deformación del sólido en cada punto mediante un algoritmo de penalización, simulando así el comportamiento del hormigón armado. En este trabajo se denominarán a estos modelos simplificadamente como modelos de EF de continuo. Con estos modelos de EF de continuo se analiza la respuesta estructural de elementos constructivos (columnas, losas y pórticos) frente a acciones explosivas. Asimismo se han comparado con resultados experimentales, de ensayos sobre vigas y losas con distintas cargas de explosivo, verificándose una coincidencia aceptable y permitiendo una calibración de los parámetros de cálculo. Sin embargo estos modelos tan detallados no son recomendables para analizar edificios completos, ya que el elevado número de elementos finitos que serían necesarios eleva su coste computacional hasta hacerlos inviables para los recursos de cálculo actuales. Adicionalmente, se desarrollan modelos de elementos finitos estructurales (vigas y láminas) que, con un coste computacional reducido, son capaces de reproducir el comportamiento global de la estructura con una precisión similar. Se modelizan igualmente el hormigón en masa y el acero de armado de forma segregada. El hormigón se representa con el modelo constitutivo del hormigón EC2 (Hallquist et al., 2013), que también presenta un comportamiento inelástico, con diferente respuesta a tracción y compresión, endurecimiento, daño por fisuración y compresión, y rotura, y se usa en elementos finitos tipo lámina. El acero se representa de nuevo con un modelo constitutivo elastoplástico bilineal con rotura, usando elementos finitos tipo viga. Se modeliza una geometría equivalente del hormigón y del armado, y se tiene en cuenta la posición relativa del acero dentro de la masa de hormigón. Las mallas de ambos se unen mediante nodos comunes, produciendo una respuesta conjunta. En este trabajo se denominarán a estos modelos simplificadamente como modelos de EF estructurales. Con estos modelos de EF estructurales se simulan los mismos elementos constructivos que con los modelos de EF de continuo, y comparando sus respuestas estructurales frente a explosión se realiza la calibración de los primeros, de forma que se obtiene un comportamiento estructural similar con un coste computacional reducido. Se comprueba que estos mismos modelos, tanto los modelos de EF de continuo como los modelos de EF estructurales, son precisos también para el análisis del fenómeno de colapso progresivo en una estructura, y que se pueden utilizar para el estudio simultáneo de los daños de una explosión y el posterior colapso. Para ello se incluyen formulaciones que permiten considerar las fuerzas debidas al peso propio, sobrecargas y los contactos de unas partes de la estructura sobre otras. Se validan ambos modelos con un ensayo a escala real en el que un módulo con seis columnas y dos plantas colapsa al eliminar una de sus columnas. El coste computacional del modelo de EF de continuo para la simulación de este ensayo es mucho mayor que el del modelo de EF estructurales, lo cual hace inviable su aplicación en edificios completos, mientras que el modelo de EF estructurales presenta una respuesta global suficientemente precisa con un coste asumible. Por último se utilizan los modelos de EF estructurales para analizar explosiones sobre edificios de varias plantas, y se simulan dos escenarios con cargas explosivas para un edificio completo, con un coste computacional moderado. The frequency of explosions on buildings whether they are intended or accidental is small, but they can have catastrophic effects. Being able to predict in a accurate enough manner the consequences of these dynamic actions on civil buildings, among which frame-type reinforced concrete buildings are a frequent typology is desirable. In this doctoral thesis different practical options for the modeling and computer assisted numerical calculation of reinforced concrete structures submitted to explosions are explored. Numerical finite elements models with explicit time-based integration are employed, demonstrating their effective capacity in the simulation of the occurring fast dynamic and highly nonlinear physical and structural phenomena, allowing to predict the damage caused by the explosion itself as well as by the possible progressive collapse of the structure. The work has been carried out with the commercial finite elements code LS-DYNA (Hallquist, 2006), developing several types of calculation model classified in two main types: 1) Models based in continuum finite elements in which the continuous medium is discretized directly by means of nodal displacement degrees of freedom; 2) Models based on structural finite elements, with beams and shells, including kinematic hypothesis for linear and superficial elements. These models are developed and discussed at different levels: 1) material behaviour, 2) response of structural elements such as columns, beams and slabs, and 3) response of complete buildings or significative parts of them. Very detailed 3D continuum finite element models are developed, modeling mass concrete and reinforcement steel in a segregated manner. Concrete is represented with a constitutive concrete model CSCM (Murray et al., 2007), that has an inelastic behaviour, with different tension and compression response, hardening, cracking and compression damage and failure. The steel is represented with an elastic-plastic bilinear model with failure. The actual geometry of the concrete is modeled with 3D continuum finite elements and every and each of the reinforcing bars with beam-type finite elements, with their exact position in the concrete mass. The mesh of the model is generated by the superposition of the concrete continuum elements and the beam-type elements of the segregated reinforcement, which are made to follow the deformation of the solid in each point by means of a penalty algorithm, reproducing the behaviour of reinforced concrete. In this work these models will be called continuum FE models as a simplification. With these continuum FE models the response of construction elements (columns, slabs and frames) under explosive actions are analysed. They have also been compared with experimental results of tests on beams and slabs with various explosive charges, verifying an acceptable coincidence and allowing a calibration of the calculation parameters. These detailed models are however not advised for the analysis of complete buildings, as the high number of finite elements necessary raises its computational cost, making them unreliable for the current calculation resources. In addition to that, structural finite elements (beams and shells) models are developed, which, while having a reduced computational cost, are able to reproduce the global behaviour of the structure with a similar accuracy. Mass concrete and reinforcing steel are also modeled segregated. Concrete is represented with the concrete constitutive model EC2 (Hallquist et al., 2013), which also presents an inelastic behaviour, with a different tension and compression response, hardening, compression and cracking damage and failure, and is used in shell-type finite elements. Steel is represented once again with an elastic-plastic bilineal with failure constitutive model, using beam-type finite elements. An equivalent geometry of the concrete and the steel is modeled, considering the relative position of the steel inside the concrete mass. The meshes of both sets of elements are bound with common nodes, therefore producing a joint response. These models will be called structural FE models as a simplification. With these structural FE models the same construction elements as with the continuum FE models are simulated, and by comparing their response under explosive actions a calibration of the former is carried out, resulting in a similar response with a reduced computational cost. It is verified that both the continuum FE models and the structural FE models are also accurate for the analysis of the phenomenon of progressive collapse of a structure, and that they can be employed for the simultaneous study of an explosion damage and the resulting collapse. Both models are validated with an experimental full-scale test in which a six column, two floors module collapses after the removal of one of its columns. The computational cost of the continuum FE model for the simulation of this test is a lot higher than that of the structural FE model, making it non-viable for its application to full buildings, while the structural FE model presents a global response accurate enough with an admissible cost. Finally, structural FE models are used to analyze explosions on several story buildings, and two scenarios are simulated with explosive charges for a full building, with a moderate computational cost.

Determining the elastic-plastic properties of metallic materials through instrumented indentation

En los últimos años, y asociado al desarrollo de la tecnología MEMS, la técnica de indentación instrumentada se ha convertido en un método de ensayo no destructivo ampliamente utilizado para hallar las características elástico-plásticas de recubrimientos y capas delgadas, desde la escala macroscópica a la microscópica. Sin embargo, debido al complejo mecanismo de contacto debajo de la indentación, es urgente proponer un método más simple y conveniente para obtener unos resultados comparables con otras mediciones tradicionales. En este estudio, el objetivo es mejorar el procedimiento analítico para extraer las propiedades elástico-plásticas del material mediante la técnica de indentación instrumentada. La primera parte se centra en la metodología llevada a cabo para medir las propiedades elásticas de los materiales elásticos, presentándose una nueva metodología de indentación, basada en la evolución de la rigidez de contacto y en la curva fuerza-desplazamiento del ensayo de indentación. El método propuesto permite discriminar los valores de indentación experimental que pudieran estar afectados por el redondeo de la punta del indentador. Además, esta técnica parece ser robusta y permite obtener valores fiables del modulo elástico. La segunda parte se centra en el proceso analítico para determinar la curva tensión-deformación a partir del ensayo de indentación, empleando un indentador esférico. Para poder asemejar la curva tension-deformación de indentación con la que se obtendría de un ensayo de tracción, Tabor determinó empíricamente un factor de constricción de la tensión () y un factor de constricción de la deformación (). Sin embargo, la elección del valor de y  necesitan una derivación analítica. Se describió analíticamente una nueva visión de la relación entre los factores de constricción de tensión y la deformación basado en la deducción de la ecuación de Tabor. Un modelo de elementos finitos y un diseño experimental se realizan para evaluar estos factores de constricción. A partir de los resultados obtenidos, las curvas tension-deformación extraidas de los ensayos de indentación esférica, afectadas por los correspondientes factores de constricción de tension y deformación, se ajustaron a la curva nominal tensión-deformación obtenida de ensayos de tracción convencionales. En la última parte, se estudian las propiedades del revestimiento de cermet Inconel 625-Cr3C2 que es depositado en el medio de una aleación de acero mediante un láser. Las propiedades mecánicas de la matriz de cermet son estudiadas mediante la técnica de indentación instrumentada, haciendo uso de las metodologías propuestas en el presente trabajo. In recent years, along with the development of MEMS technology, instrumented indentation, as one type of a non-destructive measurement technique, is widely used to characterize the elastic and plastic properties of metallic materials from the macro to the micro scale. However, due to the complex contact mechanisms under the indentation tip, it is necessary to propose a more convenient and simple method of instrumented indention to obtain comparable results from other conventional measurements. In this study, the aim is to improve the analytical procedure for extracting the elastic plastic properties of metallic materials by instrumented indentation. The first part focuses on the methodology for measuring the elastic properties of metallic materials. An alternative instrumented indentation methodology is presented. Based on the evolution of the contact stiffness and indentation load versus the depth of penetration, the possibility of obtaining the actual elastic modulus of an elastic-plastic bulk material through instrumented sharp indentation tests has been explored. The proposed methodology allows correcting the effect of the rounding of the indenter tip on the experimental indentation data. Additionally, this technique does not seem too sensitive to the pile-up phenomenon and allows obtaining convincing values of the elastic modulus. In the second part, an analytical procedure is proposed to determine the representative stress-strain curve from the spherical indentation. Tabor has determined the stress constraint factor (stress CF), and strain constraint factor (strain CF), empirically but the choice of a value for and is debatable and lacks analytical derivation. A new insight into the relationship between stress and strain constraint factors is analytically described based on the formulation of Tabor’s equation. Finite element model and experimental tests have been carried out to evaluate these constraint factors. From the results, representative stress-strain curves using the proposed strain constraint factor fit better with the nominal stress-strain curve than those using Tabor’s constraint factors. In the last part, the mechanical properties of an Inconel 625-Cr3C2 cermet coating which is deposited onto a medium alloy steel by laser cladding has been studied. The elastic and plastic mechanical properties of the cermet matrix are studied using depth-sensing indentation (DSI) on the micro scale.

A plastic analysis of nonprismatic folded plate structures

A computer method for the plastic analysis of folded plate structures is presented. The method considers the specific characteristics of the folded plate structural model using a simplified one-dimensional theory. and it can be applied to the analysis of any type of folded pIates, either prismatic or nonprismatic, with arbitrary cross-section. A simple example is analyzed in order to show the possibilities of the procedure and some results of interest are presented

Límites de ductilidad y de servicio al cálculo en carga última de estructuras de hormigón armado y de fábrica

En este trabajo se aborda una cuestión central en el diseño en carga última de estructuras de hormigón armado y de fábrica: la posibilidad efectiva de que las deformaciones plásticas necesarias para verificar un estado de rotura puedan ser alcanzadas por las regiones de la estructura que deban desarrollar su capacidad última para verificar tal estado. Así, se parte de las decisiones de diseño que mediante mera estática aseguran un equilibrio de la estructura para las cargas últimas que deba resistir, pero determinando directamente el valor de las deformaciones necesarias para llegar a tal estado. Por tanto, no se acude a los teoremas de rotura sin más, sino que se formula el problema desde un punto de vista elastoplástico. Es decir, no se obvia el recorrido que la estructura deba realizar en un proceso de carga incremental monótono, de modo que las regiones no plastificadas contribuyen a coaccionar las libres deformaciones plásticas que, en la teoría de rotura, se suponen. En términos de trabajo y energía, se introduce en el balance del trabajo de las fuerzas externas y en el de la energía de deformación, aquella parte del sistema que no ha plastificado. Establecido así el balance energético como potencial del sistema es cuando la condición de estacionariedad del mismo hace determinados los campos de desplazamientos y, por tanto, el de las deformaciones plásticas también. En definitiva, se trata de un modo de verificar si la ductilidad de los diseños previstos es suficiente, y en qué medida, para verificar el estado de rotura previsto, para unas determinadas cargas impuestas. Dentro del desarrollo teórico del problema, se encuentran ciertas precisiones importantes. Entre ellas, la verificación de que el estado de rotura a que se llega de manera determinada mediante el balance energético elasto-plástico satisface las condiciones de la solución de rotura que los teoremas de carga última predicen, asegurando, por tanto, que la solución determinada -unicidad del problema elásticocoincide con el teorema de unicidad de la carga de rotura, acotando además cuál es el sistema de equilibrio y cuál es la deformada de colapso, aspectos que los teoremas de rotura no pueden asegurar, sino sólo el valor de la carga última a verificar. Otra precisión se basa en la particularidad de los casos en que el sistema presenta una superficie de rotura plana, haciendo infinitas las posibilidades de equilibrio para una misma deformada de colapso determinada, lo que está en la base de, aparentemente, poder plastificar a antojo en vigas y arcos. Desde el planteamiento anterior, se encuentra entonces que existe una condición inherente a cualquier sistema, definidas unas leyes constitutivas internas, que permite al mismo llegar al inicio del estado de rotura sin demandar deformación plástica alguna, produciéndose la plastificación simultánea de todas las regiones que hayan llegado a su solicitación de rotura. En cierto modo, se daría un colapso de apariencia frágil. En tal caso, el sistema conserva plenamente hasta el final su capacidad dúctil y tal estado actúa como representante canónico de cualquier otra solución de equilibrio que con idéntico criterio de diseño interno se prevea para tal estructura. En la medida que el diseño se acerque o aleje de la solución canónica, la demanda de ductilidad del sistema para verificar la carga última será menor o mayor. Las soluciones que se aparten en exceso de la solución canónica, no verificarán el estado de rotura previsto por falta de ductilidad: la demanda de deformación plástica de alguna región plastificada estará más allá de la capacidad de la misma, revelándose una carga de rotura por falta de ductilidad menor que la que se preveía por mero equilibrio. Para la determinación de las deformaciones plásticas de las rótulas, se ha tomado un modelo formulado mediante el Método de los Elementos de Contorno, que proporciona un campo continuo de desplazamientos -y, por ende, de deformaciones y de tensiones- incluso en presencia de fisuras en el contorno. Importante cuestión es que se formula la diferencia, nada desdeñable, de la capacidad de rotación plástica de las secciones de hormigón armado en presencia de cortante y en su ausencia. Para las rótulas de fábrica, la diferencia se establece para las condiciones de la excentricidad -asociadas al valor relativo de la compresión-, donde las diferencias entres las regiones plastificadas con esfuerzo normal relativo alto o bajo son reseñables. Por otro lado, si bien de manera un tanto secundaria, las condiciones de servicio también imponen un límite al diseño previo en carga última deseado. La plastificación lleva asociadas deformaciones considerables, sean locales como globales. Tal cosa impone que, en estado de servicio, si la plastificación de alguna región lleva asociadas fisuraciones excesivas para el ambiente del entorno, la solución sea inviable por ello. Asimismo, las deformaciones de las estructuras suponen un límite severo a las posibilidades de su diseño. Especialmente en edificación, las deformaciones activas son un factor crítico a la hora de decidirse por una u otra solución. Por tanto, al límite que se impone por razón de ductilidad, se debe añadir el que se imponga por razón de las condiciones de servicio. Del modo anterior, considerando las condiciones de ductilidad y de servicio en cada caso, se puede tasar cada decisión de diseño con la previsión de cuáles serán las consecuencias en su estado de carga última y de servicio. Es decir, conocidos los límites, podemos acotar cuáles son los diseños a priori que podrán satisfacer seguro las condiciones de ductilidad y de servicio previstas, y en qué medida. Y, en caso de no poderse satisfacer, qué correcciones debieran realizarse sobre el diseño previo para poderlas cumplir. Por último, de las consecuencias que se extraen de lo estudiado, se proponen ciertas líneas de estudio y de experimentación para poder llegar a completar o expandir de manera práctica los resultados obtenidos. ABSTRACT This work deals with a main issue for the ultimate load design in reinforced concrete and masonry structures: the actual possibility that needed yield strains to reach a ultimate state could be reached by yielded regions on the structure that should develop their ultimate capacity to fulfill such a state. Thus, some statically determined design decisions are posed as a start for prescribed ultimate loads to be counteracted, but finding out the determined value of the strains needed to reach the ultimate load state. Therefore, ultimate load theorems are not taken as they are, but a full elasto-plastic formulation point of view is used. As a result, the path the structure must develop in a monotonus increasing loading procedure is not neglected, leading to the fact that non yielded regions will restrict the supposed totally free yield strains under a pure ultimate load theory. In work and energy terms, in the overall account of external forces work and internal strain energy, those domains in the body not reaching their ultimate state are considered. Once thus established the energy balance of the system as its potential, by imposing on it the stationary condition, both displacements and yield strains appear as determined values. Consequently, what proposed is a means for verifying whether the ductility of prescribed designs is enough and the extent to which they are so, for known imposed loads. On the way for the theoretical development of the proposal, some important aspects have been found. Among these, the verification that the conditions for the ultimate state reached under the elastoplastic energy balance fulfills the conditions prescribed for the ultimate load state predicted through the ultimate load theorems, assuring, therefore, that the determinate solution -unicity of the elastic problemcoincides with the unicity ultimate load theorem, determining as well which equilibrium system and which collapse shape are linked to it, being these two last aspects unaffordable by the ultimate load theorems, that make sure only which is the value of the ultimate load leading to collapse. Another aspect is based on the particular case in which the yield surface of the system is flat -i.e. expressed under a linear expression-, turning out infinite the equilibrium possibilities for one determined collapse shape, which is the basis of, apparently, deciding at own free will the yield distribution in beams and arches. From the foresaid approach, is then found that there is an inherent condition in any system, once defined internal constitutive laws, which allows it arrive at the beginning of the ultimate state or collapse without any yield strain demand, reaching the collapse simultaneously for all regions that have come to their ultimate strength. In a certain way, it would appear to be a fragile collapse. In such a case case, the system fully keeps until the end its ductility, and such a state acts as a canonical representative of any other statically determined solution having the same internal design criteria that could be posed for the that same structure. The extent to which a design is closer to or farther from the canonical solution, the ductility demand of the system to verify the ultimate load will be higher or lower. The solutions being far in excess from the canonical solution, will not verify the ultimate state due to lack of ductility: the demand for yield strains of any yielded region will be beyond its capacity, and a shortcoming ultimate load by lack of ductility will appear, lower than the expected by mere equilibrium. For determining the yield strains of plastic hinges, a Boundary Element Method based model has been used, leading to a continuous displacement field -therefore, for strains and stresses as well- even if cracks on the boundary are present. An important aspect is that a remarkable difference is found in the rotation capacity between plastic hinges in reinforced concrete with or without shear. For masonry hinges, such difference appears when dealing with the eccentricity of axial forces -related to their relative value of compression- on the section, where differences between yield regions under high or low relative compressions are remarkable. On the other hand, although in a certain secondary manner, serviceability conditions impose limits to the previous ultimate load stated wanted too. Yield means always big strains and deformations, locally and globally. Such a thing imposes, for serviceability states, that if a yielded region is associated with too large cracking for the environmental conditions, the predicted design will be unsuitable due to this. Furthermore, displacements must be restricted under certain severe limits that restrain the possibilities for a free design. Especially in building structures, active displacements are a critical factor when chosing one or another solution. Then, to the limits due to ductility reasons, other limits dealing with serviceability conditions shoud be added. In the foresaid way, both considering ductility and serviceability conditions in every case, the results for ultimate load and serviceability to which every design decision will lead can be bounded. This means that, once the limits are known, it is possible to bound which a priori designs will fulfill for sure the prescribed ductility and serviceability conditions, and the extent to wich they will be fulfilled, And, in case they were not, which corrections must be performed in the previous design so that it will. Finally, from the consequences derived through what studied, several study and experimental fields are proposed, in order to achieve a completeness and practical expansion of the obtained results.

The folded plate model in structural analysis. Elastic, plastic and dynamic formulations for nonprismatic structures

A specific numerical procedure for the analysis of arbitrary nonprismatic folded plate structures is presented. An elastic model is studied and compared with a harmonic solution for a prismatic structure. An extension to the plastic analysis is developed, and the influence of the structural geometry and loading pattern is analyzed. Nonprismatic practical cases, with arbitrary geometry and loading are shown, as well in the elastic range as in the plastic one. Finally, a dynamic formulation is outlined

Paper-like electronic displays: Large-area rubber-stamped plastic sheets of electronics and microencapsulated electrophoretic inks

Electronic systems that use rugged lightweight plastics potentially offer attractive characteristics (low-cost processing, mechanical flexibility, large area coverage, etc.) that are not easily achieved with established silicon technologies. This paper summarizes work that demonstrates many of these characteristics in a realistic system: organic active matrix backplane circuits (256 transistors) for large (≈5 × 5-inch) mechanically flexible sheets of electronic paper, an emerging type of display. The success of this effort relies on new or improved processing techniques and materials for plastic electronics, including methods for (i) rubber stamping (microcontact printing) high-resolution (≈1 μm) circuits with low levels of defects and good registration over large areas, (ii) achieving low leakage with thin dielectrics deposited onto surfaces with relief, (iii) constructing high-performance organic transistors with bottom contact geometries, (iv) encapsulating these transistors, (v) depositing, in a repeatable way, organic semiconductors with uniform electrical characteristics over large areas, and (vi) low-temperature (≈100°C) annealing to increase the on/off ratios of the transistors and to improve the uniformity of their characteristics. The sophistication and flexibility of the patterning procedures, high level of integration on plastic substrates, large area coverage, and good performance of the transistors are all important features of this work. We successfully integrate these circuits with microencapsulated electrophoretic “inks” to form sheets of electronic paper.

Highly plastic chromosomal organization in Salmonella typhi.

Gene order in the chromosomes of Escherichia coli K-12 and Salmonella typhimurium LT2, and in many other species of Salmonella, is strongly conserved, even though the genera diverged about 160 million years ago. However, partial digestion of chromosomal DNA of Salmonella typhi, the causal organism of typhoid fever, with the endonuclease I-CeuI followed by separation of the DNA fragments by pulsed-field gel electrophoresis showed that the chromosomes of independent wild-type isolates of S. typhi are rearranged due to homologous recombination between the seven rrn genes that code for ribosomal RNA. The order of genes within the I-CeuI fragments is largely conserved, but the order of the fragments on the chromosome is rearranged. Twenty-one different orders of the I-CeuI fragments were detected among the 127 wild-type strains we examined. Duplications and deletions were not found, but transpositions and inversions were common. Transpositions of I-CeuI fragments into sites that do not change their distance from the origin of replication (oriC) are frequently detected among the wild-type strains, but transpositions that move the fragments much further from oriC were rare. This supports the gene dosage hypothesis that genes at different distances from oriC have different gene dosages and, hence, different gene expression, and that during evolution genes become adapted to their specific location; thus, cells with changes in gene location due to transpositions may be less fit. Therefore, gene dosage may be one of the forces that conserves gene order, although its effects seem less strong in S. typhi than in other enteric bacteria. However, both the gene dosage and the genomic balance hypotheses, the latter of which states that the origin (oriC) and terminus (TER) of replication must be separated by 180 degrees C, need further investigation.

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.

Reverse Vending Machines: A Potential Solution for the Florida Plastic Waste Management Challenge

The waste of plastic beverage bottles creates environmental problems and takes up a large volume of landfill space. The high rate of consumption of plastics in the State of Florida is challenging the disposal capacity of waste authorities. The lack of the reverse vending machines in the State of Florida, including applicable scientific or technical literature represented an opportunity for this research to discuss the applicability of this equipment as a potential solution for the management of the plastic waste in Florida. With this research document, I will propose a recycling system for plastic bottles made with PET based on the implementation of reverse vending machines, stressing the importance of the creation of policies that promote recycling and public participation.

Refinery Nonconventional Feedstocks: Influence of the Coprocessing of Vacuum Gas Oil and Low Density Polyethylene in Fluid Catalytic Cracking Unit on Full Range Gasoline Composition

Gasoline coming from refinery fluid catalytic cracking (FCC) unit is a major contributor to the total commercial grade gasoline pool. The contents of the FCC gasoline are primarily paraffins, naphthenes, olefins, aromatics, and undesirables such as sulfur and sulfur containing compounds in low quantities. The proportions of these components in the FCC gasoline invariable determine its quality as well as the performance of the associated downstream units. The increasing demand for cleaner and lighter fuels significantly influences the need not only for novel processing technologies but also for alternative refinery and petrochemical feedstocks. Current and future clean gasoline requirements include increased isoparaffins contents, reduced olefin contents, reduced aromatics, reduced benzene, and reduced sulfur contents. The present study is aimed at investigating the effect of processing an unconventional refinery feedstock, composed of blend of vacuum gas oil (VGO) and low density polyethylene (LDPE) on FCC full range gasoline yields and compositional spectrum including its paraffins, isoparaffins, olefins, napthenes, and aromatics contents distribution within a range of operating variables of temperature (500–700 °C) and catalyst-feed oil ratio (CFR 5–10) using spent equilibrium FCC Y-zeolite based catalyst in a FCC pilot plant operated at the University of Alicante’s Research Institute of Chemical Process Engineering (RICPE). The coprocessing of the oil-polymer blend led to the production of gasoline with very similar yields and compositions as those obtained from the base oil, albeit, in some cases, the contribution of the feed polymer content as well as the processing variables on the gasoline compositional spectrum were appreciated. Carbon content analysis showed a higher fraction of the C9–C12 compounds at all catalyst rates employed and for both feedstocks. The gasoline’s paraffinicity, olefinicity, and degrees of branching of the paraffins and olefins were also affected in various degrees by the scale of operating severity. In the majority of the cases, the gasoline aromatics tended toward the decrease as the reactor temperature was increased. While the paraffins and iso-paraffins gasoline contents were relatively stable at around 5 % wt, the olefin contents on the other hand generally increased with increase in the FCC reactor temperature.

Seismic behavior of a masonry chimney with severe cracking condition: preliminary study

This paper presents a structural analysis of a masonry chimney built in the 1940s, which is currently being cataloged as local interest heritage. This structure has not served any industrial purpose for the last thirty years. The chimney is located in the town of Agost (Alicante - Spain) and directly exposed to the prevailing winds from the sea, as it is approximately 12 km away from the waterfront and there are not any significant barriers, which could protect the structure against the wind. There are longitudinal cracks and fissures all along the shaft because of the chimney’s geometrical characteristics, the effect of the masonry creep and especially the lack of maintenance. Moreover, there is also a permanent bending deformation in the upper 1/3 of the height due to the wind pressure. A numerical analysis for the static behavior against gravity and wind loads was performed using the structure’s current conditions after a detailed report of its geometry, its construction system and the cracking pattern. Afterwards, the dynamic behavior was studied, i.e. a seismic analysis using both response spectra and accelerograms in order to examine the structural stability. This work shows the pre-monitoring analysis before any experimental testing. Using the current results the future test conditions will be determined (e.g. number of sensors and monitoring point location, excitation systems, etc) prior to a possible structural reinforcement by applying composite material (fiber reinforced polymers).

Realizing the Commercial Potential of Hierarchical Zeolites: New Opportunities in Catalytic Cracking

Many approaches to mesoporous zeolites have been reported. The preparation of mesoporous zeolite Y, as the most widely used zeolite in catalysis, its properties, and its application in fluid catalytic cracking (FCC) and hydrocracking are reviewed. Finally, the scale-up and use of mesostrutured zeolite Y on an industrial scale are described, as the first commercial application of hierarchical zeolites.