Assessment of expected damage on buildings subjected to Lorca earthquake through an energy-based seismic index method and nonlinear dynamic response analyses

The city of Lorca (Spain) was hit on May 11th, 2011, by two consecutive earth-quakes of magnitudes 4.6 and 5.2 Mw, causing casualties and important damage in buildings. Many of the damaged structures were reinforced concrete frames with wide beams. This study quantifies the expected level of damage on this structural type in the case of the Lorca earth-quake by means of a seismic index Iv that compares the energy input by the earthquake with the energy absorption/dissipation capacity of the structure. The prototype frames investigated represent structures designed in two time periods (1994–2002 and 2003–2008), in which the applicable codes were different. The influence of the masonry infill walls and the proneness of the frames to concentrate damage in a given story were further investigated through nonlinear dynamic response analyses. It is found that (1) the seismic index method predicts levels of damage that range from moderate/severe to complete collapse; this prediction is consistent with the observed damage; (2) the presence of masonry infill walls makes the structure very prone to damage concentration and reduces the overall seismic capacity of the building; and (3) a proper hierarchy of strength between beams and columns that guarantees the formation of a strong column-weak beam mechanism (as prescribed by seismic codes), as well as the adoption of counter-measures to avoid the negative interaction between non-structural infill walls and the main frame, would have reduced the level of damage from Iv=1 (collapse) to about Iv=0.5 (moderate/severe damage)

Hydrodynamic stability theory of double ablation front structures in inertial confinement fusion.

The aim of inertial confinement fusion is the production of energy by the fusion of thermonuclear fuel (deuterium-tritium) enclosed in a spherical target due to its implosion. In the direct-drive approach, the energy needed to spark fusion reactions is delivered by the irradiation of laser beams that leads to the ablation of the outer shell of the target (the so-called ablator). As a reaction to this ablation process, the target is accelerated inwards, and, provided that this implosion is sufficiently strong a symmetric, the requirements of temperature and pressure in the center of the target are achieved leading to the ignition of the target (fusion). One of the obstacles capable to prevent appropriate target implosions takes place in the ablation region where any perturbation can grow even causing the ablator shell break, due to the ablative Rayleigh-Taylor instability. The ablative Rayleigh-Taylor instability has been extensively studied throughout the last 40 years in the case where the density/temperature profiles in the ablation region present a single front (the ablation front). Single ablation fronts appear when the ablator material has a low atomic number (deuterium/tritium ice, plastic). In this case, the main mechanism of energy transport from the laser energy absorption region (low density plasma) to the ablation region is the electron thermal conduction. However, recently, the use of materials with a moderate atomic number (silica, doped plastic) as ablators, with the aim of reducing the target pre-heating caused by suprathermal electrons generated by the laser-plasma interaction, has demonstrated an ablation region composed of two ablation fronts. This fact appears due to increasing importance of radiative effects in the energy transport. The linear theory describing the Rayleigh-Taylor instability for single ablation fronts cannot be applied for the stability analysis of double ablation front structures. Therefore, the aim of this thesis is to develop, for the first time, a linear stability theory for this type of hydrodynamic structures.

Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies

Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (108–109 W cm−2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.

Ballistic performance of nanocrystalline and nanotwinned ultrafine crystal steel

Because of their remarkable mechanical properties, nanocrystalline metals have been the focus of much research in recent years. Refining their grain size to the nanometer range (<100 nm) effectively reduces their dislocation mobility, thus achieving very high yield strength and surface hardness—as predicted by the Hall–Petch relation—as well as higher strain-rate sensitivity. Recent works have additionally suggested that nanocrystalline metals exhibit an even higher compressive strength under shock loading. However, the increase in strength of these materials is generally accompanied by an important reduction in ductility. As an alternative, efforts have been focused on ultrafine crystals, i.e. polycrystals with a grain size in the range of 500 nm to 1 μm, in which “growth twins” (twins introduced inside the grain before deformation) act as barriers against dislocation movement, thus increasing the strength in a similar way as nanocrystals but without significant loss of ductility. Due to their outstanding mechanical properties, both nanocrystalline and nanotwinned ultrafine crystalline steels appear to be relevant candidates for ballistic protection. The aim of the present work is to compare their ballistic performance against coarse-grained steel, as well as to identify the effect of the hybridization with a carbon fiber–epoxy composite layer. Hybridization is proposed as a way to improve the nanocrystalline brittle properties in a similar way as is done with ceramics in other protection systems. The experimental campaign is finally complemented by numerical simulations to help identify some of the intrinsic deformation mechanisms not observable experimentally. As a conclusion, nanocrystalline and nanotwinned ultrafine crystals show a lower energy absorption than coarse-grained steel under ballistic loading, but under equal impact conditions with no penetration, deformation in the impact direction is smaller by nearly 40%. This a priori surprising difference in the energy absorption is rationalized by the more important local contribution of the deviatoric stress vs. volumetric stress under impact than under uniaxial deformation. Ultimately, the deformation advantage could be exploited in the future for personal protection systems where a small deformation under impact is of key importance.

A shake table test of typical mediterranean reinforced concrete structures

This research investigates the ultimate earthquake resistance of typical RC moment resisting frames designed accordingly to current standards, in terms of ultimate energy absorption/dissipation capacity. Shake table test of a 2/5 scale model, under several intensities of ground motion, are carried out. The loading effect of the earthquake is expressed as the total energy that the quake inputs to the structure, and the seismic resistance is interpreted as the amount of energy that the structure dissipates in terms of cumulative inelastic strain energy.

X-ray microtomography analysis of the damage micromechanisms in 3D woven composites under low-velocity impact

3D woven composites reinforced with either S2 glass, carbon or a hybrid combination of both and containing either polyethylene or carbon z-yarns were tested under low-velocity impact. Different impact energies (in the range of 21–316 J) were used and the mechanical response (in terms of the impact strength and energy dissipated) was compared with that measured in high-performance, albeit standard, 2D laminates. It was found that the impact strength in both 2D and 3D materials was mainly dependent on the in-plane fiber fracture. Conversely, the energy absorption capability was primarily influenced by the presence of z-yarns, having the 3D composites dissipated over twice the energy than the 2D laminates, irrespective of their individual characteristics (fiber type, compaction degree, porosity, etc.). X-ray microtomography revealed that this improvement was due to the z-yarns, which delayed delamination and maintained the structural integrity of the laminate, promoting energy dissipation by tow splitting, intensive fiber breakage under the tup and formation of a plug by out-of-plane shear.

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).

Demonstration of the operation principles of intermediate band solar cells at room temperature

In this work we report, for the first time at room temperature, experimental results that prove, simultaneously in the same device, the two main physical principles involved in the operation of intermediate band solar cells: (1) the production of sub-bandgap photocurrent by two optical transitions through the intermediate band; (2) the generation of an output voltage which is not limited by the photon energy absorption threshold. These principles, which had always required cryogenic temperatures to be evidenced all together, are now demonstrated at room temperature on an intermediate band solar cell based on InAs quantum dots with Al0.3Ga0.7As barriers.

Evaluación teórico experimental de la influencia de espumas de relleno metálicas como elementos de absorción de energía en perfiles tubulares de pequeño espesor. Su aplicación a vehículos para el transporte colectivo de personas

Los accidentes con implicación de autocares en los que se producen vuelcos ponen de manifiesto la especial agresividad de los mismos, como lo confirman las estadísticas. Como medida para mejorar la seguridad de los Vehículos de Grandes Dimensiones para el Transporte de Pasajeros (V.G.D.T.P.) frente a vuelco fue aprobado por las Naciones Unidas el Reglamento Nº 66 de Ginebra. Este reglamento establece los requisitos mínimos que las estructuras de los vehículos de grandes dimensiones deben cumplir con respecto a vuelco. El reglamento 66 ha supuesto un paso adelante muy importante en relación con la seguridad de los autocares, puesto que especifica por primera vez requerimientos estructurales a este tipo de vehículos, y en general ha supuesto una mejora del vehículo . Por otro lado, a consecuencia de la obligatoriedad de instalación de cinturones de seguridad, existe una unión entre pasajeros y vehículo, pero como no se trata de una unión rígida, hay que contemplar el porcentaje de la masa de los ocupantes que influye en la absorción de energía de la estructura. Además la retención de los ocupantes con cinturones de seguridad influye en la energía a absorber por la estructura del vehículo en dos aspectos, por un lado aumenta la masa del vehículo y en el otro se incrementa la altura el centro de gravedad. Esta situación a conducido a elaborar por parte de las Naciones Unidas la revisión 01 del Reglamento 66, en el que se considera que el 50 % de la masa total de los pasajeros posee una unión rígida con la estructura del vehículo, y por lo tanto debe ser tenida en cuenta si el vehículo posee sistemas de retención. En la situación actual, con limitaciones de peso del vehículo y peso por eje, los elementos de confort, seguridad y espacio para maleteros contribuyen a aumentar el peso del vehículo. Esto unido a la dificultad de introducción de cambios radicales en la concepción actual de fabricación de este tipo de vehículos por suponer unas pérdidas importantes para los fabricantes existentes, tanto en su conocimiento del producto como en su metodología de proceso, conlleva la necesidad cada vez más agobiante de analizar y evaluar otras alternativas estructurales que sin suponer grandes revoluciones a los productos actualmente en fabricación los complementen permitiendo adaptarse a los nuevos requerimientos en seguridad. Recientes desarrollos en la relación costo-beneficio de los procesos para la producción de materiales celulares metálicos de baja densidad, tales como las espumas metálicas, los posiciona como una alternativa de especial interés para la aplicación como elementos de absorción de energía para reforzar estructuras. El relleno con espumas metálicas puede ser más eficiente en términos de optimización de peso comparado con el aumento de espesor de los perfiles estructurales, dado que la absorción de energía se produce en una fracción relativamente pequeña de los perfiles, en las denominadas rótulas plásticas. La aplicación de espumas de relleno metálicas en estructuras de vehículos se está empezando a emplear en determinadas zonas de los vehículos de turismo, siendo totalmente novedosa cualquier intento de aplicación en estructuras de autobuses y autocares. Conforme a lo expuesto, y con el objeto de resolver estos problemas, se ha elaborado el presente trabajo de tesis doctoral, cuyos objetivos son: -Desarrollar un modelo matemático, que permita simular el ensayo de vuelco, considerando la influencia de los ocupantes retenidos con cinturones de seguridad para evaluar su influencia en la absorción de energía de la estructura. -Validar el modelo matemático de vuelco de la estructura mediante ensayos de secciones representativas de la estructura del vehículo y mediante el ensayo de un vehículo completo. -Realizar un estudio de las propiedades de las espumas metálicas que permitan incorporarlas como elemento de absorción de energía en el relleno de componentes de la superestructura de autobuses y autocares. -Desarrollar un modelo matemático para evaluar el aporte del relleno de espuma metálica en la absorción de energía ante solicitaciones por flexión estática y dinámica en componentes de la superestructura de autobuses o autocares. -Realizar un programa de ensayos a flexión estáticos y dinámicos para validar el modelo matemático del aporte del relleno de espuma metálica sobre componentes de la superestructura de autobuses y autocares. . -Incorporar al modelo matemático de vuelco de la estructura, los resultados obtenidos sobre componentes con relleno de espuma metálica, para evaluar el aporte en la absorción de energía. -Validar el modelo de vuelco de la estructura del autobús o autocar con relleno de espuma metálica, mediante ensayos de secciones de carrocería. ABSTRACT Accidents involving buses in which rollovers occur reveal the special aggressiveness thereof, as the statistics prove. As a measure to improve the safety of large vehicles for the transport of passengers to rollover, Regulation 66 of Geneva was approved by the United Nations. This regulation establishes the minimum requirements that structures of large vehicles must comply with respect to rollovers. The regulation 66 has been a major step forward in relation to the safety of coaches, since it specifies structural requirements to such vehicles and has been an improvement for the vehicle. In turn, as a result of compulsory installation of safety belts, there is contact between passengers and vehicle, but as it is not a rigid connection we must contemplate the percentage of the mass of the occupants that impacts on the energy absorption of the structure. Thus, the passengers’ restraining modifies the energy to absorb by the vehicle in two different aspects: On the one hand, it increases the vehicle weight and on the other the height of the center of gravity. This circumstance has taken the United Nations to elaborate Revision 01 of Regulation 66, in which it is considered that the 50 percent of passengers’ mass has a rigid joint together with the vehicle structure and, therefore, the passengers’ mass mentioned above should be highly considered if the vehicle has seat belts. In the present situation, in which limitations in vehicle weight and weight in axles are stricter, elements of comfort, safety and space for baggage are contributing to increase the weight of the vehicle. This coupled with the difficulty of introducing radical changes in the current conception of manufacturing such vehicles pose significant losses for existing manufacturers, both in product knowledge and process methodology, entails the overwhelming need to analyze and evaluate other structural alternatives without assuming relevant modifications on the products manufactured currently allowing them to adapt to the new safety requirements. Recent developments in cost-benefit processes for the production of metallic foams of low density, such as metal foams, place them as an alternative of special interest to be used as energy absorbers to strengthen structures. The filling with metal foams can be more efficient in terms of weight optimization compared with increasing thickness of the structural beams, since the energy absorption occurs in a relatively small fraction of the beams, called plastic hinges. The application of metal filling foams in vehicle structures is beginning to be used in certain areas of passenger cars, being an innovative opportunity in structures for application in buses and coaches. According to the mentioned before, and in order to come forward with a solution, this doctoral thesis has been prepared and its objectives are: - Develop a mathematical model to simulate the rollover test, considering the influence of the occupants held with seat belts to assess their influence on energy absorption structure. - Validate the mathematical model of the structure rollover by testing representative sections of the vehicle structure and by testing a complete vehicle. - Conduct a study of the properties of metal foams as possible incorporation of energy absorbing element in the filler component of the superstructure of buses and coaches. - Elaborate a mathematical model to assess the contribution of the metal foam filling in absorbing energy for static and dynamic bending loads on the components of buses or coaches superstructure. - Conduct a static and dynamic bending test program to validate the mathematical model of contribution of metal foam filling on components of the superstructure of buses and coaches bending. - To incorporate into the mathematical model of structure rollover, the results obtained on components filled with metal foam, to evaluate the contribution to the energy absorption. - Validate the rollover model structure of the bus or coach filled with metal foam through tests of bay sections. The objectives in this thesis have been achieved successfully. The contribution calculation model with metal foam filling in the vehicle structure has revealed that the filling with metal foam is more efficient than increasing thickness of the beams, as demonstrated in the experimental validation of bay sections.

Multiscale analysis of the mechanical behavior of needle-punched nonwoven fabrics

Los fieltros son una familia de materiales textiles constituidos por una red desordenada de fibras conectadas por medio de enlaces térmicos, químicos o mecánicos. Presentan menor rigidez y resistencia (al igual que un menor coste de procesado) que sus homólogos tejidos, pero mayor deformabilidad y capacidad de absorción de energía. Los fieltros se emplean en diversas aplicaciones en ingeniería tales como aislamiento térmico, geotextiles, láminas ignífugas, filtración y absorción de agua, impacto balístico, etc. En particular, los fieltros punzonados fabricados con fibras de alta resistencia presentan una excelente resistencia frente a impacto balístico, ofreciendo las mismas prestaciones que los materiales tejidos con un tercio de la densidad areal. Sin embargo, se sabe muy poco acerca de los mecanismos de deformación y fallo a nivel microscópico, ni sobre como influyen en las propiedades mecánicas del material. Esta carencia de conocimiento dificulta la optimización del comportamiento mecánico de estos materiales y también limita el desarrollo de modelos constitutivos basados en mecanismos físicos, que puedan ser útiles en el diseño de componentes estructurales. En esta tesis doctoral se ha llevado a cabo un estudio minucioso con el fin de determinar los mecanismos de deformación y las propiedades mecánicas de fieltros punzonados fabricados con fibras de polietileno de ultra alto peso molecular. Los procesos de deformación y disipación de energía se han caracterizado en detalle por medio de una combinación de técnicas experimentales (ensayos mecánicos macroscópicos a velocidades de deformación cuasi-estáticas y dinámicas, impacto balístico, ensayos de extracción de una o múltiples fibras, microscopía óptica, tomografía computarizada de rayos X y difracción de rayos X de gran ángulo) que proporcionan información de los mecanismos dominantes a distintas escalas. Los ensayos mecánicos macroscópicos muestran que el fieltro presenta una resistencia y ductilidad excepcionales. El estado inicial de las fibras es curvado, y la carga se transmite por el fieltro a través de una red aleatoria e isótropa de nudos creada por el proceso de punzonamiento, resultando en la formación de una red activa de fibra. La rotación y el estirado de las fibras activas es seguido por el deslizamiento y extracción de la fibra de los puntos de anclaje mecánico. La mayor parte de la resistencia y la energía disipada es proporcionada por la extracción de las fibras activas de los nudos, y la fractura final tiene lugar como consecuencia del desenredo total de la red en una sección dada donde la deformación macroscópica se localiza. No obstante, aunque la distribución inicial de la orientación de las fibras es isótropa, las propiedades mecánicas resultantes (en términos de rigidez, resistencia y energía absorbida) son muy anisótropas. Los ensayos de extracción de múltiples fibras en diferentes orientaciones muestran que la estructura de los nudos conecta más fibras en la dirección transversal en comparación con la dirección de la máquina. La mejor interconectividad de las fibras a lo largo de la dirección transversal da lugar a una esqueleto activo de fibras más denso, mejorando las propiedades mecánicas. En términos de afinidad, los fieltros deformados a lo largo de la dirección transversal exhiben deformación afín (la deformación macroscópica transfiere directamente a las fibras por el material circundante), mientras que el fieltro deformado a lo largo de la dirección de la máquina presenta deformación no afín, y la mayor parte de la deformación macroscópica no es transmitida a las fibras. A partir de estas observaciones experimentales, se ha desarrollado un modelo constitutivo para fieltros punzonados confinados por enlaces mecánicos. El modelo considera los efectos de la deformación no afín, la conectividad anisótropa inducida durante el punzonamiento, la curvatura y re-orientación de la fibra, así como el desenredo y extracción de la fibra de los nudos. El modelo proporciona la respuesta de un mesodominio del material correspondiente al volumen asociado a un elemento finito, y se divide en dos bloques. El primer bloque representa el comportamiento de la red y establece la relación entre el gradiente de deformación macroscópico y la respuesta microscópica, obtenido a partir de la integración de la respuesta de las fibras en el mesodominio. El segundo bloque describe el comportamiento de la fibra, teniendo en cuenta las características de la deformación de cada familia de fibras en el mesodominio, incluyendo deformación no afín, estiramiento, deslizamiento y extracción. En la medida de lo posible, se ha asignado un significado físico claro a los parámetros del modelo, por lo que se pueden identificar por medio de ensayos independientes. Las simulaciones numéricas basadas en el modelo se adecúan a los resultados experimentales de ensayos cuasi-estáticos y balísticos desde el punto de vista de la respuesta mecánica macroscópica y de los micromecanismos de deformación. Además, suministran información adicional sobre la influencia de las características microstructurales (orientación de la fibra, conectividad de la fibra anisótropa, afinidad, etc) en el comportamiento mecánico de los fieltros punzonados. Nonwoven fabrics are a class of textile material made up of a disordered fiber network linked by either thermal, chemical or mechanical bonds. They present lower stiffness and strength (as well as processing cost) than the woven counterparts but much higher deformability and energy absorption capability and are used in many different engineering applications (including thermal insulation, geotextiles, fireproof layers, filtration and water absorption, ballistic impact, etc). In particular, needle-punched nonwoven fabrics manufactured with high strength fibers present an excellent performance for ballistic protection, providing the same ballistic protection with one third of the areal weight as compared to dry woven fabrics. Nevertheless, very little is known about their deformation and fracture micromechanisms at the microscopic level and how they contribute to the macroscopic mechanical properties. This lack of knowledge hinders the optimization of their mechanical performance and also limits the development of physically-based models of the mechanical behavior that can be used in the design of structural components with these materials. In this thesis, a thorough study was carried out to ascertain the micromechanisms of deformation and the mechanical properties of a needle-punched nonwoven fabric made up by ultra high molecular weight polyethylene fibers. The deformation and energy dissipation processes were characterized in detail by a combination of experimental techniques (macroscopic mechanical tests at quasi-static and high strain rates, ballistic impact, single fiber and multi fiber pull-out tests, optical microscopy, X-ray computed tomography and wide angle X-ray diffraction) that provided information of the dominant mechanisms at different length scales. The macroscopic mechanical tests showed that the nonwoven fabric presented an outstanding strength and energy absorption capacity. It was found that fibers were initially curved and the load was transferred within the fabric through the random and isotropic network of knots created by needlepunching, leading to the formation of an active fiber network. Uncurling and stretching of the active fibers was followed by fiber sliding and pull-out from the entanglement points. Most of the strength and energy dissipation was provided by the extraction of the active fibers from the knots and final fracture occurred by the total disentanglement of the fiber network in a given section at which the macroscopic deformation was localized. However, although the initial fiber orientation distribution was isotropic, the mechanical properties (in terms of stiffness, strength and energy absorption) were highly anisotropic. Pull-out tests of multiple fibers at different orientations showed that structure of the knots connected more fibers in the transverse direction as compared with the machine direction. The better fiber interconnection along the transverse direction led to a denser active fiber skeleton, enhancing the mechanical response. In terms of affinity, fabrics deformed along the transverse direction essentially displayed affine deformation {i.e. the macroscopic strain was directly transferred to the fibers by the surrounding fabric, while fabrics deformed along the machine direction underwent non-affine deformation, and most of the macroscopic strain was not transferred to the fibers. Based on these experimental observations, a constitutive model for the mechanical behavior of the mechanically-entangled nonwoven fiber network was developed. The model accounted for the effects of non-affine deformation, anisotropic connectivity induced by the entanglement points, fiber uncurling and re-orientation as well as fiber disentanglement and pull-out from the knots. The model provided the constitutive response for a mesodomain of the fabric corresponding to the volume associated to a finite element and is divided in two blocks. The first one was the network model which established the relationship between the macroscopic deformation gradient and the microscopic response obtained by integrating the response of the fibers in the mesodomain. The second one was the fiber model, which took into account the deformation features of each set of fibers in the mesodomain, including non-affinity, uncurling, pull-out and disentanglement. As far as possible, a clear physical meaning is given to the model parameters, so they can be identified by means of independent tests. The numerical simulations based on the model were in very good agreement with the experimental results of in-plane and ballistic mechanical response of the fabrics in terms of the macroscopic mechanical response and of the micromechanisms of deformation. In addition, it provided additional information about the influence of the microstructural features (fiber orientation, anisotropic fiber connectivity, affinity) on the mechanical performance of mechanically-entangled nonwoven fabrics.

Estudio teórico-experimental para la mejora de la resistencia estructural de autocares basado en el diseño de uniones y asientos

La presente tesis analiza la mejora de la resistencia estructural ante vuelco de autocares enfocando dos vías de actuación: análisis y propuestas de requisitos reglamentarios a nivel europeo y la generación de herramientas que ayuden al diseño y a la verificación de estos requisitos. Los requisitos reglamentarios de resistencia estructural a vuelco contemplan la superestructura de los vehículos pero no para los asientos y sistemas de retención. La influencia de los pasajeros retenidos es superior a la incluida en reglamentación (Reg. 66.01) debiendo considerarse unida al vehículo un porcentaje de la masa de los pasajeros del 91% para cinturón de tres puntos y del 52% para cinturón subabdominal frente al 50% reglamentario para todos los casos. Se ha determinado la cinemática y dinámica del vuelco normativo en sus diferentes fases, formulando las energías en las fases iniciales (hasta el impacto contra el suelo) y determinando la fase final de deformación a través del análisis secuencial de ensayos de módulos reales. Se han determinado los esfuerzos para los asientos que se dividen en dos fases diferenciadas temporalmente: una primera debida a la deformación estructural y una segunda debida al esfuerzo del pasajero retenido que se produce en sentido opuesto (con una deceleración del pasajero en torno a 3.3 g). Se ha caracterizado a través de ensayos cuasi.estáticos el comportamiento de perfiles a flexión y de las uniones estructurales de las principales zonas del vehículo (piso, ventana y techo) verificándose la validez del comportamiento plástico teórico Kecman.García para perfiles de hasta 4 mm de espesor y caracterizando la resistencia y rigidez en la zona elástica de las uniones en función del tipo de refuerzo, materiales y perfiles (análisis de más de 180 probetas). Se ha definido un método de ensayo cuasi.estático para asientos ante esfuerzos de vuelco, ensayándose 19 butacas y determinándose que son resistentes (salvo las uniones a vehículo con pinzas), que son capaces de absorber hasta más de un 17% de la energía absorbida, aunque algunos necesitan optimización para llegar a contribuir en el mecanismo de deformación estructural. Se han generado modelos simplificados para introducir en los modelos barra.rótula plástica: un modelo combinado unión+rótula plástica (que incluye la zona de rigidez determinada en función del tipo de unión) para la superestructura y un modelo simplificado de muelles no.lineales para los asientos. Igualmente se ha generado la metodología de diseño a través de ensayos virtuales con modelos de detalle de elementos finitos tanto de las uniones como de los asientos. Se ha propuesto una metodología de diseño basada en obtener el “mecanismo óptimo de deformación estructural” (elevando la zona de deformación lateral a nivel de ventana y en pilar o en costilla en techo). Para ello se abren dos vías: diseño de la superestructura (selección de perfiles y generación de uniones resistentes) o combinación con asientos (que en lugar de solo resistir las cargas pueden llegar a modificar el mecanismo de deformación). Se ha propuesto una metodología de verificación alternativa al vuelco de vehículo completo que contempla el cálculo cuasi.estático con modelos simplificados barra.rótula plástica más el ensayo de una sección representativa con asientos y utillajes antropomórficos retenidos que permite validar el diseño de las uniones, determinar el porcentaje de energía que debe absorberse por deformación estructural (factor C) y verificar el propio asiento como sistema de retención. ABSTRACT This research analyzes the improvement of the structural strength of buses and coaches under rollover from two perspectives: regulatory requirements at European level and generation of tools that will help to the design and to the verification of requirements. European Regulations about rollover structural strength includes requirements for the superstructure of the vehicles but not about seats, anchorages and restraint systems. The influence of the retained passengers is higher than the one included currently in the Regulations (Reg. 66.01), being needed to consider a 91% of the passenger mass as rigidly joint to the vehicle (for 3 points’ belt, a 52% for 2 points’ belt) instead of the 50% included in the Regulation. Kinematic and dynamic of the normative rollover has been determined from testing of different sections, formulating the energies of the first phases (up to the first impact with the ground) and determining the last deformation phase through sequential analysis of movements and deformations. The efforts due to rollover over the seats have been established, being divided in two different temporal phases: a first one due to the structural deformation of the vehicle and a second one due to the effort of the restrained passenger being this second one in opposite sense (with a passenger deceleration around 3.3 g). From quasi.static testing, the behavior of the structural tubes under flexural loads, including the principal joints in the vehicle (floor, window and roof), the validity of the theoretical plastic behavior according Kecman.García theories have been verified up to 4 mm of thickness. Strength of the joints as well as the stiffness of the elastic zone has been determined in function of main parameters: type of reinforcement, materials and section of the tubes (more than 180 test specimens). It has been defined a quasi.static testing methodology to characterize the seats and restrain system behavior under rollover, testing 19 double seats and concluding that they are resistant (excepting clamping joints), that they can absorb more than a 17 of the absorbed energy, and that some of them need optimization to contribute in the structural deformation mechanism. It has been generated simplified MEF models, to analyze in a beam.plastic hinge model: a combined model joint+plastic hinge (including the stiffness depending on the type of joint) for the superstructure and a simplified model with non.lineal springs to represent the seats. It has been detailed methodologies for detailed design of joints and seats from virtual testing (MEF models). A design methodology based in the “optimized structural deformation mechanism” (increasing the height of deformation of the lateral up to window level) is proposed. Two possibilities are analyzed: design of the superstructure based on the selection of profiles and design of strength joints (were seats only resist the efforts and contribute in the energy absorption) or combination structure.seats, were seats contributes in the deformation mechanism. An alternative methodology to the rollover of a vehicle that includes the quasi.static calculation with simplified models “beam.joint+plastic hinge” plus the testing of a representative section of the vehicle including seats and anthropomorphic ballast restrained by the safety belts is presented. The test of the section allows validate the design of the joints, determine the percentage of energy to be absorbed by structural deformation (factor C) and verify the seat as a retention system.

Intermediate band materials for more efficient solar energy use: quantum modelling and experimental realizations

The intermediate band (IB) solar cell (Fig. 1) has been proposed [1] to increase photovoltaic efficiency by a factor above 1.5, based on the absorption of two sub-bandgap photons to promote an electron across the bandgap. To realize this principle, that can be applied also to obtain efficient photocatalysis with sunlight, we proposed in recent years several materials where a metal or heavy element, substituting for an electropositive atom in a known semiconductor that has an appropriate band gap width (around 2 eV), forms inside the gap the partially filled levels needed for this aim

O 1s excitation and ionization processes in the CO2 molecule studied via detection of low-energy fluorescence emission

Oxygen 1s excitation and ionization processes in the CO2 molecule have been studied with dispersed and non-dispersed fluorescence spectroscopy as well as with the vacuum ultraviolet (VUV) photon?photoion coincidence technique. The intensity of the neutral O emission line at 845 nm shows particular sensitivity to core-to-Rydberg excitations and core?valence double excitations, while shape resonances are suppressed. In contrast, the partial fluorescence yield in the wavelength window 300?650 nm and the excitation functions of selected O+ and C+ emission lines in the wavelength range 400?500 nm display all of the absorption features. The relative intensity of ionic emission in the visible range increases towards higher photon energies, which is attributed to O 1s shake-off photoionization. VUV photon?photoion coincidence spectra reveal major contributions from the C+ and O+ ions and a minor contribution from C2+. No conclusive changes in the intensity ratios among the different ions are observed above the O 1s threshold. The line shape of the VUV?O+ coincidence peak in the mass spectrum carries some information on the initial core excitation

Optical and structural properties of InAlN/GaN Bragg reflectors examined by transmission electron microscopy and electron energy loss spectroscopy

Molecular beam epitaxy growth of ten-period lattice-matched InAlN/GaN distributed Bragg reflectors (DBRs) with peak reflectivity centered around 400nm is reported including optical and transmission electron microscopy (TEM) measurements [1]. Good periodicity heterostructures with crack-free surfaces were confirmed, but, also a significant residual optical absorption below the bandgap was measured. The TEM characterization ascribes the origin of this problem to polymorfism and planar defects in the GaN layers and to the existence of an In-rich layer at the InAlN/GaN interfaces. In this work, several TEM based techniques have been combined.

Analytical solution to the one-dimensional non-uniform absorption of solar radiation in uncoated and coated single glass panes

The analytical solution to the one-dimensional absorption–conduction heat transfer problem inside a single glass pane is presented, which correctly takes into account all the relevant physical phenomena: the appearance of multiple reflections, the spectral distribution of solar radiation, the spectral dependence of optical properties, the presence of possible coatings, the non-uniform nature of radiation absorption, and the diffusion of heat by conduction across the glass pane. Additionally to the well established and known direct absorptance αe, the derived solution introduces a new spectral quantity called direct absorptance moment βe, that indicates where in the glass pane is the absorption of radiation actually taking place. The theoretical and numerical comparison of the derived solution with existing approximate thermal models for the absorption–conduction problem reveals that the latter ones work best for low-absorbing uncoated single glass panes, something not necessarily fulfilled by modern glazings.