Análisis de metodologías para la medida de la resistencia a tracción por spalling

En el presente trabajo se presenta un estudio teórico y experimental de la determinación de la resistencia a tracción de materiales frágiles a partir de la técnica experimental de spalling. Se utilizan diferentes metodologías propuestas por varios autores para determinar la resistencia a tracción por spalling y se lleva a cabo un estudio mediante simulaciones numéricas de las diversas variables que influyen en este tipo de ensayos. Además, se realiza una campaña experimental de ensayos a una alúmina del 99,5% de pureza cuyos resultados son utilizados para la determinación de la resistencia a tracción de este material a partir de tres métodos diferentes propuestos por varios autores. Se expone el estudio y comparación de los resultados experimentales obtenidos de resistencia a tracción de la alúmina empleando técnicas de fotografía a alta velocidad y un sistema de correlación digital de imágenes. Los resultados muestran que la resistencia a tracción obtenida difiere en función de las diferentes metodologías propuestas.A theoretical and experimental study of the tensile strength of brittle materials using the experimental procedure of spalling of long bars is presented in this article. Different methodologies proposed by several authors are used to obtain the tensile strength of Al2O3 monolithic ceramic. The hypotheses needed for the experimental set-up are also checked, and the requirements of the set-up and the variables are also studied by means of numerical simulations. An experimental campaign has been carried out including high speed video and a digital image correlation system to obtain the tensile strength of alumina. Finally, a comparison of the test results provided by three different methods proposed by different authors are presented. The tensile strength obtained from three different methods on the same specimens provides different results.

Towards the development of poly (phenylene sulphide) based nanocomposites with enhanced mechanical, electrical and tribological properties

Novel poly(phenylene sulphide) (PPS) nanocomposites reinforced with an aminated derivative (PPS-NH2) covalently attached to acid-treated single-walled carbon nanotubes (SWCNTs) were prepared via simple melt-blending technique. Their morphology, viscoelastic behaviour, electrical conductivity, mechanical and tribological properties were investigated. Scanning electron microscopy revealed that the grafting process was effective in uniformly dispersing the SWCNTs within the matrix. The storage and loss moduli as a function of frequency increased with the SWCNT content, tending to a plateau in the low-frequency regime. The electrical conductivity of the nanocomposites was considerably enhanced in the range 0.1?0.5 wt% SWCNTs; electrical and rheological percolation thresholds occurred at similar nanotube concentrations. Mechanical tests demonstrated that with only 1.0 wt% SWCNTs the Young's modulus and tensile strength of the matrix improved by 51 and 37%, respectively, without decrement in toughness, ascribed to a very efficient load transfer. A moderate decrease in the friction coefficient and a 75% reduction in wear rate were found for the abovementioned nanotube loading, indicating that PPS-NH2-g-SWCNTs are good tribological additives for thermoplastic polymers. Based on the promising results obtained in this work, it is expected that these nanofillers will be used to develop high-performance thermoplastic/CNT nanocomposites for structural applications.

Laser shock processing: an emerging technique for the improvement of fatigue life and surface properties of high reliability metallic components

•Introduction •Process Experimental Setup •Experimental Procedure •Experimental Results for Al2024 - T351, Ti6Al4V and AISI 316L - Surface Roughness and Compactation - Residual stresses - Tensile Strength - Fatigue Life •Discussion and Outlook - Prospects for technological applications of LSP

Application of high-intensity short-pulse lasers to the mechanical and surface properties improvement of high reliability metallic components by shock processing

Outline: • Introduction • Process Experimental Setup • Experimental Procedure • Experimental Results for Al2024-T351 and Ti6Al4V - Residual stresses - Tensile Strength - Fatigue Life • Discussion and Outlook - Prospects for technological applications of LSP

Desarrollo experimental de un elemento constructivo de envolvente, no estructural, a partir de una matriz de plástico reciclado, reforzado con fibras naturales y/o sintéticas

Esta Tesis trata sobre el diseño y desarrollo de un material constructivo de fachada (tras ventilada), empleando plástico reciclado (granza de caucho, de neumáticos fuera de uso) para su elaboración. El uso de materiales reciclados para la elaboración de nuevos materiales constructivos, es a día de hoy, un valor agregado que contribuye tanto a la disminución de desechos tóxicos, como a la fabricación de productos de alta calidad. La investigación partió de la necesidad de comprender qué es un plástico, cómo son producidos, cuáles son los factores que permitían su reciclaje y qué propiedades podrían ser aprovechadas para desarrollar un nuevo material constructivo. En el estado del arte, fueron analizados los aspectos del plástico relacionados a su composición, propiedades, tipologías, producción, consumo, legislación europea y española, reciclaje y valorización energética. Para analizar más profundamente los materiales desarrollados a partir de plásticos reciclados, desde textiles hasta elementos constructivos. Con el conocimiento adquirido mediante este análisis previo, se diseñó una metodología de experimentación, utilizando caucho reciclado y derivados del yeso como agregados, en una matriz de resinas poliméricas reforzada con fibras naturales y sintéticas. Los resultados obtenidos en los ensayos físicos y térmicos, con los elementos producidos, demostraron que el material tiene una excelente resistencia a tensión así como una baja conductividad térmica. Esta investigación, servirá como precedente para el desarrollo de nuevos materiales y sistemas constructivos, utilizando agregados de plástico reciclado, en los procesos de fabricación. Ya que ha comprobado el enorme potencial que ofrecen, creando nuevos materiales, y contribuyendo a reducir la contaminación medio ambiental. "La mayor recompensa de nuestro trabajo no es lo que nos pagan por él, sino aquello en lo que nos convierte". John Ruskin Material compuesto (Composite) de caucho reciclado, fibras y resinas poliméricas. ABSTRACT This thesis deals with the design and development of a new facade construction material using recycled plastic (rubber pellets from used tires) for processing. The use of recycled materials for the development of new building materials, today is an added value which contributes both to the reduction of toxic waste, as well as the processing of products of good quality. The research derives from the need to understand what a plastic is, how they are produced, what the factors that allowed recycling are and what properties can be exploited to develop a new building material. In the prior art, were analyzed plastic aspects related to its composition, properties, typologies, production, consumption, European and Spanish legislation, recycling and energy recovery. To further analyze the materials developed from recycled plastics, from textiles to construction elements. With the knowledge gained from this previous analysis, we designed an experimental approach using recycled rubber and plaster derivatives as aggregates in a polymeric resin matrix reinforced with natural and synthetic fibers. The results obtained in physical and thermal testing, with the elements produced, showed that the material has excellent tensile strength and a low thermal conductivity. This research will serve as a precedent for the development of new materials and building systems, using recycled plastic aggregates in the manufacturing processes. Since it was found the enormous potential, creating new materials, and helping reduce environmental pollution. "The greatest reward of our work is not what we get paid for it, but what they make us."

Enhancing the thermomechanical behaviour of poly(phenylene sulphide) based composites via incorporation of covalently grafted carbon nanotubes

The thermal and thermomechanical properties of poly(phenylene sulphide) (PPS) based nanocomposites incorporating a polymer derivative covalently anchored onto single-walled carbon nanotubes (SWCNTs) were investigated. The grafted fillers acted as nucleating agents, increasing the crystallization temperature and degree of crystallinity of the matrix. They also enhanced its thermal stability, flame retardancy, glass transition (Tg) and heat deflection temperatures while reduced the coefficient of thermal expansion at temperatures below Tg. A strong rise in the thermal conductivity, Young?s modulus and tensile strength was found with increasing filler loading both in the glassy and rubbery states. All these outstanding improvements are ascribed to strong matrix-filler interfacial interactions combined with a compatibilization effect that results in very homogeneous SWCNT dispersion. The results herein offer useful insights towards the development of engineering thermoplastic/CNT nanocomposites for high-temperature applications.

Evolution of microstructure, macrotexture and mechanical properties of commercially pure Ti during ECAP-conform processing and drawing

Long-length ultrafine-grained (UFG) Ti rods are produced by equal-channel angular pressing via the conform scheme (ECAP-C) at 200 °C, which is followed by drawing at 200 °C. The evolution of microstructure, macrotexture, and mechanical properties (yield strength, ultimate tensile strength, failure stress, uniform elongation, elongation to failure) of pure Ti during this thermo-mechanical processing is studied. Special attention is also paid to the effect of microstructure on the mechanical behavior of the material after macrolocalization of plastic flow. The number of ECAP-C passes varies in the range of 1–10. The microstructure is more refined with increasing number of ECAP-C passes. Formation of homogeneous microstructure with a grain/subgrain size of 200 nm and its saturation after 6 ECAP-C passes are observed. Strength properties increase with increasing number of ECAP passes and saturate after 6 ECAP-C passes to a yield strength of 973 MPa, an ultimate tensile strength of 1035 MPa, and a true failure stress of 1400 MPa (from 625, 750, and 1150 MPa in the as-received condition). The true strain at failure failure decreases after ECAP-C processing. The reduction of area and true strain to failure values do not decrease after ECAP-C processing. The sample after 6 ECAP-C passes is subjected to drawing at 200¯C resulting in reduction of a grain/subgrain size to 150 nm, formation of (10 1¯0) fiber texture with respect to the rod axis, and further increase of the yield strength up to 1190 MPa, the ultimate tensile strength up to 1230 MPa and the true failure stress up to 1600 MPa. It is demonstrated that UFG CP Ti has low resistance to macrolocalization of plastic deformation and high resistance to crack formation after necking.

Identification and dynamics of polyglycine II nanocrystals in Argiope trifasciata flagelliform silk

Spider silks combine a significant number of desirable characteristics in one material, including large tensile strength and strain at breaking, biocompatibility, and the possibility of tailoring their properties. Major ampullate gland silk (MAS) is the most studied silk and their properties are explained by a double lattice of hydrogen bonds and elastomeric protein chains linked to polyalanine β-nanocrystals. However, many basic details regarding the relationship between composition, microstructure and properties in silks are still lacking. Here we show that this relationship can be traced in flagelliform silk (Flag) spun by Argiope trifasciata spiders after identifying a phase consisting of polyglycine II nanocrystals. The presence of this phase is consistent with the dominant presence of the –GGX– and –GPG– motifs in its sequence. In contrast to the passive role assigned to polyalanine nanocrystals in MAS, polyglycine II nanocrystals can undergo growing/collapse processes that contribute to increase toughness and justify the ability of Flag to supercontract.

Fracture of concrete structural members subjected to blast

If reinforced concrete structures are to be safe under extreme impulsive loadings such as explosions, a broad understanding of the fracture mechanics of concrete under such events is needed. Most buildings and infrastructures which are likely to be subjected to terrorist attacks are borne by a reinforced concrete (RC) structure. Up to some years ago, the traditional method used to study the ability of RC structures to withstand explosions consisted on a choice between handmade calculations, affordable but inaccurate and unreliable, and full scale experimental tests involving explosions, expensive and not available for many civil institutions. In this context, during the last years numerical simulations have arisen as the most effective method to analyze structures under such events. However, for accurate numerical simulations, reliable constitutive models are needed. Assuming that failure of concrete elements subjected to blast is primarily governed by the tensile behavior, a constitutive model has been built that accounts only for failure under tension while it behaves as elastic without failure under compression. Failure under tension is based on the Cohesive Crack Model. Moreover, the constitutive model has been used to simulate the experimental structural response of reinforced concrete slabs subjected to blast. The results of the numerical simulations with the aforementioned constitutive model show its ability of representing accurately the structural response of the RC elements under study. The simplicity of the model, which does not account for failure under compression, as already mentioned, confirms that the ability of reinforced concrete structures to withstand blast loads is primarily governed by tensile strength.

Estudio de la permeabilidad y de la resistencia a tracción en hormigones precomprimidos

Entre los requisitos que deben cumplir las estructuras se debe garantizar que estas posean la durabilidad necesaria para permanecer en servicio a lo largo de todo el periodo de vida útil para el que han sido proyectadas. Para conseguir este objetivo las normativas han ido incorporando prescripciones para el diseño del hormigón, en base a distintas clases de exposición dependiendo del origen y magnitud de la agresividad exterior. En ambientes con una elevada agresividad, una de las comprobaciones que debe cumplir el hormigón es que tenga una permeabilidad inferior a los valores máximos fijados según la clase de exposición, y que en caso de considerar como ensayo de referencia el de penetración de agua, analiza el frente de penetración limitando las profundidades de penetración media y máxima. Adicionalmente a las condiciones de diseño según el tipo de ambiente, principalmente basadas en la dosificación del hormigón en términos de la relación agua/cemento y el mínimo contenido de cemento y el recubrimiento de las armaduras, durante la vida en servicio las estructuras pueden están solicitadas por distintas acciones imprevistas que pueden provocar cambios en la microestructura interna del hormigón que modifican su permeabilidad y resistencia, y por tanto pueden alterar la durabilidad inicialmente prevista. Es conocido el efecto de cansancio del hormigón cuando está solicitado por cargas de compresión mantenidas en el tiempo, provocando bajas en su resistencia debido al incremento de la microfisuración. Dada la relación entre la permeabilidad y la microfisuración del hormigón, es previsible el aumento de la permeabilidad en hormigones que han sido precomprimidos durante un periodo largo de tiempo. Los estudios de la permeabilidad en hormigones previamente comprimidos se han realizado analizando periodos de tiempo de compresión cortos que no permiten evaluar el efecto del cansancio sobre la permeabilidad. La presente tesis doctoral investiga la permeabilidad y resistencia a tracción en hormigones que previamente han sido comprimidos en carga mantenida durante distintos plazos de tiempo, al objeto de conocer su evolución en base al tiempo de precompresión. La investigación se apoya en el estudio de otras dos variables como son el tipo de hormigón de acuerdo a su dosificación según el tipo de ambiente considerando una agresividad baja, media o alta, y el grado de compresión aplicado respecto de su carga última de rotura. En los resultados del plan experimental desarrollado se ha obtenido que la permeabilidad presenta un incremento significante con el tiempo de precompresión, que dependiendo del valor inicial de la permeabilidad que tiene el hormigón puede provocar que hormigones que previamente satisfacen las limitaciones de permeabilidad pasen a incumplirlas, pudiendo afectar a su durabilidad. También se confirma la influencia del tiempo de precompresión sobre la resistencia a tracción obteniendo bajas de resistencia importantes en los casos pésimos ensayados, que deben ser tenidas en consideración en tanto afectan a la capacidad resistente del hormigón como a otros aspectos fundamentales como el anclaje de las armaduras en el hormigón armado y pretensado. One of the requirements that structures must meet is to guarantee their durability to remain in service throughout all the working life period for which they have been designed. To achieve this goal, building standards and codes have included specifications for the design of concrete structures, based on different exposure classes depending on the environmental conditions and their origin and magnitude. In severe aggressive environments, one of the specifications the concrete must meet is to have a permeability lower than the maximum values set for a certain exposure class. If this parameter is referenced to water penetration on specimens, then the average and maximum depths of front penetration are analyzed. In addition to the design conditions depending on the exposure class, which regulate the dosage of concrete in terms of the water/cement ratio, minimum samples that have been pre-compressed for a long period of time. Previous studies on permeability have been carried on pre-compressed concrete elements analyzing short periods of time. However, they have not studied the effects of compression forces on concrete in the long term. This Thesis investigates permeability and tensile strength of concrete samples that have been previously compressed under loads applied for different periods of time. The goal is to understand its evolution based on the time exposed to compression. The research variables also include the type of concrete according to the dosage used - depending on the environmental exposure it will have low, medium or high aggressiveness-, and the amount of compression applied in relation to its failure load. Results of the experimental tests showed that permeability increases significantly over the time of pre-compression. Depending on the initial value of permeability, this change could make the concrete not meet the original permeability restrictions and therefore affect its durability. These investigations also confirmed the influence of time of pre-compression in tensile strength, where some cases showed a significant decrease of resistance. These issues must be taken into consideration as they affect the bearing capacity of the material and other key features such as the anchoring of steel bars in reinforced and pre-stressed concrete. amount of cement content and the minimum concrete cover of the steel bars, during their working life structures may be subject to various unforeseen actions. As a result, the concrete’s internal microstructure might be affected, changing its permeability and resistance, and possibly altering the original specified durability. It is a known fact that when concrete is loaded in compression maintained over a long time, its resistance to compression forces is diminished due to the increase in micro-cracking. Considering the relationship between permeability and microcracking of concrete, an increase in permeability may be expected in concrete

Mechanical behaviour of laminated functionally graded fibre-reinforced self-compacting cementitious composites

El hormigón autocompactante (HAC) es una nueva tipología de hormigón o material compuesto base cemento que se caracteriza por ser capaz de fluir en el interior del encofrado o molde, llenándolo de forma natural, pasando entre las barras de armadura y consolidándose únicamente bajo la acción de su peso propio, sin ayuda de medios de compactación externos, y sin que se produzca segregación de sus componentes. Debido a sus propiedades frescas (capacidad de relleno, capacidad de paso, y resistencia a la segregación), el HAC contribuye de forma significativa a mejorar la calidad de las estructuras así como a abrir nuevos campos de aplicación del hormigón. Por otra parte, la utilidad del hormigón reforzado con fibras de acero (HRFA) es hoy en día incuestionable debido a la mejora significativa de sus propiedades mecánicas tales como resistencia a tracción, tenacidad, resistencia al impacto o su capacidad para absorber energía. Comparado con el HRFA, el hormigón autocompactante reforzado con fibras de acero (HACRFA) presenta como ventaja una mayor fluidez y cohesión ofreciendo, además de unas buenas propiedades mecánicas, importantes ventajas en relación con su puesta en obra. El objetivo global de esta tesis doctoral es el desarrollo de nuevas soluciones estructurales utilizando materiales compuestos base cemento autocompactantes reforzados con fibras de acero. La tesis presenta una nueva forma de resolver el problema basándose en el concepto de los materiales gradiente funcionales (MGF) o materiales con función gradiente (MFG) con el fin de distribuir de forma eficiente las fibras en la sección estructural. Para ello, parte del HAC se sustituye por HACRFA formando capas que presentan una transición gradual entre las mismas con el fin de obtener secciones robustas y exentas de tensiones entre capas con el fin de aplicar el concepto “MGF-laminados” a elementos estructurales tales como vigas, columnas, losas, etc. El proceso incluye asimismo el propio método de fabricación que, basado en la tecnología HAC, permite el desarrollo de interfases delgadas y robustas entre capas (1-3 mm) gracias a las propiedades reológicas del material. Para alcanzar dichos objetivos se ha llevado a cabo un amplio programa experimental cuyas etapas principales son las siguientes: • Definir y desarrollar un método de diseño que permita caracterizar de forma adecuada las propiedades mecánicas de la “interfase”. Esta primera fase experimental incluye: o las consideraciones generales del propio método de fabricación basado en el concepto de fabricación de materiales gradiente funcionales denominado “reología y gravedad”, o las consideraciones específicas del método de caracterización, o la caracterización de la “interfase”. • Estudiar el comportamiento mecánico sobre elementos estructurales, utilizando distintas configuraciones de MGF-laminado frente a acciones tanto estáticas como dinámicas con el fin de comprobar la viabilidad del material para ser usado en elementos estructurales tales como vigas, placas, pilares, etc. Los resultados indican la viabilidad de la metodología de fabricación adoptada, así como, las ventajas tanto estructurales como en reducción de costes de las soluciones laminadas propuestas. Es importante destacar la mejora en términos de resistencia a flexión, compresión o impacto del hormigón autocompactante gradiente funcional en comparación con soluciones de HACRFA monolíticos inclusos con un volumen neto de fibras (Vf) doble o superior. Self-compacting concrete (SCC) is an important advance in the concrete technology in the last decades. It is a new type of high performance concrete with the ability of flowing under its own weight and without the need of vibrations. Due to its specific fresh or rheological properties, such as filling ability, passing ability and segregation resistance, SCC may contribute to a significant improvement of the quality of concrete structures and open up new field for the application of concrete. On the other hand, the usefulness of steel fibre-reinforced concrete (SFRC) in civil engineering applications is unquestionable. SFRC can improve significantly the hardened mechanical properties such as tensile strength, impact resistance, toughness and energy absorption capacity. Compared to SFRC, self-compacting steel fibre-reinforced concrete (SCSFRC) is a relatively new type of concrete with high flowability and good cohesiveness. SCSFRC offers very attractive economical and technical benefits thanks to SCC rheological properties, which can be further extended, when combined with SFRC for improving their mechanical characteristics. However, for the different concrete structural elements, a single concrete mix is selected without an attempt to adapt the diverse fibre-reinforced concretes to the stress-strain sectional properly. This thesis focused on the development of high performance cement-based structural composites made of SCC with and without steel fibres, and their applications for enhanced mechanical properties in front of different types of load and pattern configurations. It presents a new direction for tackling the mechanical problem. The approach adopted is based on the concept of functionally graded cementitious composite (FGCC) where part of the plain SCC is strategically replaced by SCSFRC in order to obtain laminated functionally graded self-compacting cementitious composites, laminated-FGSCC, in single structural elements as beams, columns, slabs, etc. The approach also involves a most suitable casting method, which uses SCC technology to eliminate the potential sharp interlayer while easily forming a robust and regular reproducible graded interlayer of 1-3 mm by controlling the rheology of the mixes and using gravity at the same time to encourage the use of the powerful concept for designing more performance suitable and cost-efficient structural systems. To reach the challenging aim, a wide experimental programme has been carried out involving two main steps: • The definition and development of a novel methodology designed for the characterization of the main parameter associated to the interface- or laminated-FGSCC solutions: the graded interlayer. Work of this first part includes: o the design considerations of the innovative (in the field of concrete) production method based on “rheology and gravity” for producing FG-SCSFRC or as named in the thesis FGSCC, casting process and elements, o the design of a specific testing methodology, o the characterization of the interface-FGSCC by using the so designed testing methodology. • The characterization of the different medium size FGSCC samples under different static and dynamic loads patterns for exploring their possibilities to be used for structural elements as beams, columns, slabs, etc. The results revealed the efficiency of the manufacturing methodology, which allow creating robust structural sections, as well as the feasibility and cost effectiveness of the proposed FGSCC solutions for different structural uses. It is noticeable to say the improvement in terms of flexural, compressive or impact loads’ responses of the different FGSCC in front of equal strength class SCSFRC bulk elements with at least the double of overall net fibre volume fraction (Vf).

Mechanical behavior of 2G REBCO HTS at 77 and 300 K

El gran esfuerzo realizado durante la última década con el fin de integrar los diferentes materiales superconductores en el campo de los sistemas eléctricos y en otras aplicaciones tecnológicas ha dado lugar a un campo de investigación amplio y prometedor. El comportamiento eléctrico de los Superconductores de Alta Temperatura (SAT) crítica (masivo y cintas) depende de diferentes parámetros desde su fabricación hasta la aplicación final con imanes o cables. Sin embargo, las aplicaciones prácticas de estos materiales están fuertemente vinculadas con su comportamiento mecánico tanto a temperatura ambiente (manipulación durante fabricación o instalación) como a temperaturas criogénicas (condiciones de servicio). En esta tesis se ha estudiado el comportamiento mecánico de materiales masivos y cintas de alta temperatura crítica a 300 y 77 K (utilizando nitrógeno líquido). Se han obtenido la resistencia en flexión, la tenacidad de fractura y la resistencia a tracción a la temperatura de servicio y a 300 K. Adicionalmente, se ha medido la dureza mediante el ensayo Vickers y nanoindentación. El módulo Young se midió mediante tres métodos diferentes: 1) nanoindentación, 2) ensayos de flexión en tres puntos y 3) resonancia vibracional mediante grindosonic. Para cada condición de ensayo, se han analizado detalladamente las superficies de fractura y los micromecanismos de fallo. Las propiedades mecánicas de los materiales se han comparado con el fin de entender la influencia de las técnicas de procesado y de las características microestructurales de los monocristales en su comportamiento mecánico. Se ha estudiado el comportamiento electromecánico de cintas comerciales superconductoras de YBCO mediante ensayos de tracción y fatiga a 77 y 300 K. El campo completo de deformaciones en la superficie del material se ha obtenido utilizando Correlación Digital de Imágenes (DIC, por sus siglas en inglés) a 300 K. Además, se realizaron ensayos de fragmentación in situ dentro de un microscopio electrónico con el fin de estudiar la fractura de la capa superconductora y determinar la resistencia a cortante de la intercara entre el substrato y la capa cerámica. Se ha conseguido ver el proceso de la fragmentación aplicando tensión axial y finalmente, se han implementado simulaciones mediante elementos finitos para reproducir la delaminación y el fenómeno de la fragmentación. Por último, se han preparado uniones soldadas entre las capas de cobre de dos cintas superconductoras. Se ha medido la resistencia eléctrica de las uniones con el fin de evaluar el metal de soldadura y el proceso. Asimismo, se ha llevado a cabo la caracterización mecánica de las uniones mediante ensayos "single lap shear" a 300 y 77 K. El efecto del campo magnético se ha estudiado aplicando campo externo hasta 1 T perpendicular o paralelo a la cinta-unión a la temperatura de servicio (77 K). Finalmente, la distribución de tensiones en cada una de las capas de la cinta se estudió mediante simulaciones de elementos finitos, teniendo en cuenta las capas de la cinta mecánicamente más representativas (Cu-Hastelloy-Cu) que influyen en su comportamiento mecánico. The strong effort that has been made in the last years to integrate the different superconducting materials in the field of electrical power systems and other technological applications led to a wide and promising research field. The electrical behavior of High Temperature Superconducting (HTS) materials (bulk and coated conductors) depends on different parameters since their processing until their final application as magnets or cables. However, practical applications of such materials are strongly related with their mechanical performance at room temperature (handling) as well as at cryogenic temperatures (service conditions). In this thesis, the mechanical behavior of HTS bulk and coated conductors was investigated at 300 and 77 K (by immersion in liquid nitrogen). The flexural strength, the fracture toughness and the tensile strength were obtained at service temperature as well as at 300 K. Furthermore, their hardness was determined by Vickers measurements and nanoindentation and the Young's modulus was measured by three different techniques: 1) nanoindentation, 2) three-point bending tests and 3) vibrational resonance with a grindosonic device. The fracture and deformation micromechanics have been also carefully analyzed for each testing condition. The comparison between the studied materials has been performed in order to understand the influence of the main sintering methods and the microstructural characteristics of the single grains on the macroscopic mechanical behavior. The electromechanical behavior of commercial YBCO coated conductors was studied. The mechanical behavior of the tapes was studied under tensile and fatigue tests at 77 and 300 K. The complete strain field on the surface of the sample was obtained by applying Digital Image Correlation (DIC) at 300 K. Addionally, in situ fragmentation tests inside a Scanning Electron Microscope (SEM) were carried out in order to study the fragmentation of the superconducting layer and determine the interfacial shear strength between substrate and ceramic layer. The fragmentation process upon loading of the YBCO layer has been observed and finally, Finite Element Simulations were employed to reproduce delamination and fragmentation phenomena. Finally, joints between the stabilizing Cu sides of two coated conductors have been prepared. The electrical resistivity of the joints was measured for the purpose of qualifying the soldering material and evaluating the soldering process. Additionally, mechanical characterization under single lap shear tests at 300 and 77 K has been carried out. The effect of the applied magnetic field has been studied by applying external magnetic field up to 1 T perpendicular and parallel to the tape-joint at service temperature (77 K). Finally, finite element simulations were employed to study the distribution of the stresses in earch layer, taking into account the three mechanically relevant layers of the coated conductor (Cu-Hastelloy-Cu) that affect its mechanical behavior

Hormigones de alta resistencia con nano-adiciones y fibras de acero

El presente Trabajo Fin de Máster consistió en determinar la influencia que pueden poseer las nano-adiciones de Sílice y Alúmina y fibras de acero en un Hormigón de Alta Resistencia. Partiendo de una dosificación de Hormigón de Alta Resistencia conocida, que contenía humo de Sílice (10%) y fibras de poliolefina (3kg/m3), se les procedió a sustituir por la incorporación de nano-adiciones de Sílice y Alúmina (7% y 3% respectivamente) y añadiendo fibras de acero en lugar de poliolefina. En el presente trabajo se realizó una campaña experimental de laboratorio, en donde se realizaron tres (3) amasadas de Hormigón de Alta Resistencia con nueve (9) probetas cada una, donde el contenido de nano-adiciones no varió, mientras que el contenido de fibras fue de 20 y 40 kg/m3. Posterior a su realización, se procedió a someter las probetas a ensayos de resistencia a compresión, resistencia a tracción indirecta, resistencia a flexotracción, permeabilidad, módulo de elasticidad y coeficiente de Poisson con el fin de conocer el comportamiento de las amasadas una vez añadidas las nano-adiciones y fibras de acero. Luego de ejecutados los ensayos, se procedió a comparar los resultados entre amasadas y con las del hormigón de referencia. Los resultados muestran que la incorporación de las fibras de acero mejoran las propiedades del Hormigón de Alta Resistencia, sin negatividad. This Master’s Degree Thesis was to determine the influence that steel fibers and nano-additions of Silica and Alumina may possess in a High Strength Concrete mix. Based on a known dosage of High Strength Concrete, which contained Silica fume (10%) and polyolefin fibers (3 kg/m3), they were proceeded to be substituted for the incorporation of nano-additions of Silica and Alumina (7% and 3%, respectively) and by adding steel fibers rather than polyolefin fibers. This thesis carried out an experimental laboratory campaign, in which three (3) mixes of High Strength Concrete had nine (9) specimens each, where the content of nano-additions did not change, while the steel fiber content was 20 and 40 kg/m3. Subsequent to its completion, the specimens were subjected to different tests to determine the compressive strength, tensile strength, flexural strength, permeability, modulus of elasticity and Poisson's ratio in order to know the behavior of the mixes once the nano-additions and steel fibers were added. The results indicate that the steel fibers improve the properties of the High Strength Concrete rather to affect in a negative way

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.

Unsaturated polyester-poly(epsilon-caprolactone) hybrid nanocomposites: Thermal-mechanical properties

This paper reports on the thermal behavior and mechanical properties of nanocomposites based on unsaturated polyester resin (UP) modified with poly(ɛ-caprolactone) (PCL) and reinforced with an organically modified clay (cloisite 30B). To optimize the dispersion of 30B and the mixing of PCL in the UP resin, two different methods were employed to prepare crosslinked UP–PCL-30B hybrid nanocomposites. Besides, two samples of poly(ɛ-caprolactone) of different molecular weight (PCL2: Mn = 2.103g.mol−1 and PCL50: Mn = 5.104g.mol−1) were used at several concentrations (4, 6, 10 wt%). The 30B concentration was 4 wt% in all the nanocomposites. The morphology of the samples was studied by scanning electron microscopy (SEM). The analysis of X-ray patterns reveals that intercalated structures have been found for all ternary nanocomposites, independently of the molecular weight, PCL concentration and the preparation method selected. A slight rise of the glass transition temperature, Tg, is observed in UP/PCL/4%30B ternary nanocomposites regarding to neat UP. The analysis of the tensile properties of the ternary (hybrid) systems indicates that UP/4%PCL2/4%30B nanocomposite improves the tensile strength and elongation at break respect to the neat UP while the Young modulus remains constant