Composites from a forest biorefinery byproduct and agrofibers: Lignosulfonate-phenolic type matrices reinforced with sisal fibers

The replacement of phenol with sodium lignosulfonate and formaldehyde with glutaraldehyde in the preparation of resins resulted in a new resol-type phenolic resin, sodium lignosulfonate-glutaraldehyde resin, in addition to sodium lignosulfonate-formaldehyde and phenol-formaldehyde resins. These resins were then used to prepare thermosets and composites reinforced with sisal fibers. Different techniques were used to characterize raw materials and/or thermosets and composites, including inverse gas chromatography, thermogravimetric analysis, and mechanical impact and flexural tests. The substitution of phenol by sodium lignosulfonate in the formulation of the composite matrices increased the impact strength of the respective composites from approximately 400 Jm(-1) to 800 J m(-1) and 1000 J m(-1), showing a considerable enhancement from the replacement of phenol with sodium lignosulfonate. The wettability of the sisal fibers increased when the resins were prepared from sodium lignosulfonate, generating composites in which the adhesion at the fiber-matrix interface was stronger and favored the transference of load from the matrix to the fiber during impact. Results suggested that the composites experienced a different mechanism of load transfer from the matrix to the fiber when a bending load was applied, compared to that experienced during impact. The thermogravimetric analysis results demonstrated that the thermal stability of the composites was not affected by the use of sodium lignosulfonate as a phenolic-type reagent during the preparation of the matrices.

Active fibre-reinforced composites with embedded shape memory alloys

This dissertation concerns active fibre-reinforced composites with embedded shape memory alloy wires. The structural application of active materials allows to develop adaptive structures which actively respond to changes in the environment, such as morphing structures, self-healing structures and power harvesting devices. In particular, shape memory alloy actuators integrated within a composite actively control the structural shape or stiffness, thus influencing the composite static and dynamic properties. Envisaged applications include, among others, the prevention of thermal buckling of the outer skin of air vehicles, shape changes in panels for improved aerodynamic characteristics and the deployment of large space structures. The study and design of active composites is a complex and multidisciplinary topic, requiring in-depth understanding of both the coupled behaviour of active materials and the interaction between the different composite constituents. Both fibre-reinforced composites and shape memory alloys are extremely active research topics, whose modelling and experimental characterisation still present a number of open problems. Thus, while this dissertation focuses on active composites, some of the research results presented here can be usefully applied to traditional fibre-reinforced composites or other shape memory alloy applications. The dissertation is composed of four chapters. In the first chapter, active fibre-reinforced composites are introduced by giving an overview of the most common choices available for the reinforcement, matrix and production process, together with a brief introduction and classification of active materials. The second chapter presents a number of original contributions regarding the modelling of fibre-reinforced composites. Different two-dimensional laminate theories are derived from a parent three-dimensional theory, introducing a procedure for the a posteriori reconstruction of transverse stresses along the laminate thickness. Accurate through the thickness stresses are crucial for the composite modelling as they are responsible for some common failure mechanisms. A new finite element based on the First-order Shear Deformation Theory and a hybrid stress approach is proposed for the numerical solution of the two-dimensional laminate problem. The element is simple and computationally efficient. The transverse stresses through the laminate thickness are reconstructed starting from a general finite element solution. A two stages procedure is devised, based on Recovery by Compatibility in Patches and three-dimensional equilibrium. Finally, the determination of the elastic parameters of laminated structures via numerical-experimental Bayesian techniques is investigated. Two different estimators are analysed and compared, leading to the definition of an alternative procedure to improve convergence of the estimation process. The third chapter focuses on shape memory alloys, describing their properties and applications. A number of constitutive models proposed in the literature, both one-dimensional and three-dimensional, are critically discussed and compared, underlining their potential and limitations, which are mainly related to the definition of the phase diagram and the choice of internal variables. Some new experimental results on shape memory alloy material characterisation are also presented. These experimental observations display some features of the shape memory alloy behaviour which are generally not included in the current models, thus some ideas are proposed for the development of a new constitutive model. The fourth chapter, finally, focuses on active composite plates with embedded shape memory alloy wires. A number of di®erent approaches can be used to predict the behaviour of such structures, each model presenting different advantages and drawbacks related to complexity and versatility. A simple model able to describe both shape and stiffness control configurations within the same context is proposed and implemented. The model is then validated considering the shape control configuration, which is the most sensitive to model parameters. The experimental work is divided in two parts. In the first part, an active composite is built by gluing prestrained shape memory alloy wires on a carbon fibre laminate strip. This structure is relatively simple to build, however it is useful in order to experimentally demonstrate the feasibility of the concept proposed in the first part of the chapter. In the second part, the making of a fibre-reinforced composite with embedded shape memory alloy wires is investigated, considering different possible choices of materials and manufacturing processes. Although a number of technological issues still need to be faced, the experimental results allow to demonstrate the mechanism of shape control via embedded shape memory alloy wires, while showing a good agreement with the proposed model predictions.

Production of banana fiber yarns for technical textile reinforced composites

[EN]Natural fibers have been used as an alternative to synthetic ones for their greener character; banana fibers have the advantage of coming from an agricultural residue. Fibers have been extracted by mechanical means from banana tree pseudostems, as a strategy to valorize banana crops residues. To increase the mechanical properties of the composite, technical textiles can be used as reinforcement, instead of short fibers. To do so, fibers must be spun and woven. The aim of this paper is to show the viability of using banana fibers to obtain a yarn suitable to be woven, after an enzymatic treatment, which is more environmentally friendly.

Curing, defects and mechanical performance of fiber-reinforced composites

Tradicionalmente, la fabricación de materiales compuestos de altas prestaciones se lleva a cabo en autoclave mediante la consolidación de preimpregnados a través de la aplicación simultánea de altas presiones y temperatura. Las elevadas presiones empleadas en autoclave reducen la porosidad de los componentes garantizando unas buenas propiedades mecánicas. Sin embargo, este sistema de fabricación conlleva tiempos de producción largos y grandes inversiones en equipamiento lo que restringe su aplicación a otros sectores alejados del sector aeronáutico. Este hecho ha generado una creciente demanda de sistemas de fabricación alternativos al autoclave. Aunque estos sistemas son capaces de reducir los tiempos de producción y el gasto energético, por lo general, dan lugar a materiales con menores prestaciones mecánicas debido a que se reduce la compactación del material al aplicar presiones mas bajas y, por tanto, la fracción volumétrica de fibras, y disminuye el control de la porosidad durante el proceso. Los modelos numéricos existentes permiten conocer los fundamentos de los mecanismos de crecimiento de poros durante la fabricación de materiales compuestos de matriz polimérica mediante autoclave. Dichos modelos analizan el comportamiento de pequeños poros esféricos embebidos en una resina viscosa. Su validez no ha sido probada, sin embargo, para la morfología típica observada en materiales compuestos fabricados fuera de autoclave, consistente en poros cilíndricos y alargados embebidos en resina y rodeados de fibras continuas. Por otro lado, aunque existe una clara evidencia experimental del efecto pernicioso de la porosidad en las prestaciones mecánicas de los materiales compuestos, no existe información detallada sobre la influencia de las condiciones de procesado en la forma, fracción volumétrica y distribución espacial de los poros en los materiales compuestos. Las técnicas de análisis convencionales para la caracterización microestructural de los materiales compuestos proporcionan información en dos dimensiones (2D) (microscopía óptica y electrónica, radiografía de rayos X, ultrasonidos, emisión acústica) y sólo algunas son adecuadas para el análisis de la porosidad. En esta tesis, se ha analizado el efecto de ciclo de curado en el desarrollo de los poros durante la consolidación de preimpregnados Hexply AS4/8552 a bajas presiones mediante moldeo por compresión, en paneles unidireccionales y multiaxiales utilizando tres ciclos de curado diferentes. Dichos ciclos fueron cuidadosamente diseñados de acuerdo a la caracterización térmica y reológica de los preimpregnados. La fracción volumétrica de poros, su forma y distribución espacial se analizaron en detalle mediante tomografía de rayos X. Esta técnica no destructiva ha demostrado su capacidad para analizar la microestructura de materiales compuestos. Se observó, que la porosidad depende en gran medida de la evolución de la viscosidad dinámica a lo largo del ciclo y que la mayoría de la porosidad inicial procedía del aire atrapado durante el apilamiento de las láminas de preimpregnado. En el caso de los laminados multiaxiales, la porosidad también se vio afectada por la secuencia de apilamiento. En general, los poros tenían forma cilíndrica y se estaban orientados en la dirección de las fibras. Además, la proyección de la población de poros a lo largo de la dirección de la fibra reveló la existencia de una estructura celular de un diámetro aproximado de 1 mm. Las paredes de las celdas correspondían con regiones con mayor densidad de fibra mientras que los poros se concentraban en el interior de las celdas. Esta distribución de la porosidad es el resultado de una consolidación no homogenea. Toda esta información es crítica a la hora de optimizar las condiciones de procesado y proporcionar datos de partida para desarrollar herramientas de simulación de los procesos de fabricación de materiales compuestos fuera de autoclave. Adicionalmente, se determinaron ciertas propiedades mecánicas dependientes de la matriz termoestable con objeto de establecer la relación entre condiciones de procesado y las prestaciones mecánicas. En el caso de los laminados unidireccionales, la resistencia interlaminar depende de la porosidad para fracciones volumétricas de poros superiores 1%. Las mismas tendencias se observaron en el caso de GIIc mientras GIc no se vio afectada por la porosidad. En el caso de los laminados multiaxiales se evaluó la influencia de la porosidad en la resistencia a compresión, la resistencia a impacto a baja velocidad y la resistencia a copresión después de impacto. La resistencia a compresión se redujo con el contenido en poros, pero éste no influyó significativamente en la resistencia a compresión despues de impacto ya que quedó enmascarada por otros factores como la secuencia de apilamiento o la magnitud del daño generado tras el impacto. Finalmente, el efecto de las condiciones de fabricación en el proceso de compactación mediante moldeo por compresión en laminados unidireccionales fue simulado mediante el método de los elementos finitos en una primera aproximación para simular la fabricación de materiales compuestos fuera de autoclave. Los parámetros del modelo se obtuvieron mediante experimentos térmicos y reológicos del preimpregnado Hexply AS4/8552. Los resultados obtenidos en la predicción de la reducción de espesor durante el proceso de consolidación concordaron razonablemente con los resultados experimentales. Manufacturing of high performance polymer-matrix composites is normally carried out by means of autoclave using prepreg tapes stacked and consolidated under the simultaneous application of pressure and temperature. High autoclave pressures reduce the porosity in the laminate and ensure excellent mechanical properties. However, this manufacturing route is expensive in terms of capital investment and processing time, hindering its application in many industrial sectors. This fact has driven the demand of alternative out-of-autoclave processing routes. These techniques claim to produce composite parts faster and at lower cost but the mechanical performance is also reduced due to the lower fiber content and to the higher porosity. Corrient numerical models are able to simulate the mechanisms of void growth in polymer-matrix composites processed in autoclave. However these models are restricted to small spherical voids surrounded by a viscous resin. Their validity is not proved for long cylindrical voids in a viscous matrix surrounded by aligned fibers, the standard morphology observed in out-of-autoclave composites. In addition, there is an experimental evidence of the detrimental effect of voids on the mechanical performance of composites but, there is detailed information regarding the influence of curing conditions on the actual volume fraction, shape and spatial distribution of voids within the laminate. The standard techniques of microstructural characterization of composites (optical or electron microscopy, X-ray radiography, ultrasonics) provide information in two dimensions and are not always suitable to determine the porosity or void population. Moreover, they can not provide 3D information. The effect of curing cycle on the development of voids during consolidation of AS4/8552 prepregs at low pressure by compression molding was studied in unidirectional and multiaxial panels. They were manufactured using three different curing cycles carefully designed following the rheological and thermal analysis of the raw prepregs. The void volume fraction, shape and spatial distribution were analyzed in detail by means of X-ray computed microtomography, which has demonstrated its potential for analyzing the microstructural features of composites. It was demonstrated that the final void volume fraction depended on the evolution of the dynamic viscosity throughout the cycle. Most of the initial voids were the result of air entrapment and wrinkles created during lay-up. Differences in the final void volume fraction depended on the processing conditions for unidirectional and multiaxial panels. Voids were rod-like shaped and were oriented parallel to the fibers and concentrated in channels along the fiber orientation. X-ray computer tomography analysis of voids along the fiber direction showed a cellular structure with an approximate cell diameter of 1 mm. The cell walls were fiber-rich regions and porosity was localized at the center of the cells. This porosity distribution within the laminate was the result of inhomogeneous consolidation. This information is critical to optimize processing parameters and to provide inputs for virtual testing and virtual processing tools. In addition, the matrix-controlled mechanical properties of the panels were measured in order to establish the relationship between processing conditions and mechanical performance. The interlaminar shear strength (ILSS) and the interlaminar toughness (GIc and GIIc) were selected to evaluate the effect of porosity on the mechanical performance of unidirectional panels. The ILSS was strongly affected by the porosity when the void contents was higher than 1%. The same trends were observed in the case of GIIc while GIc was insensitive to the void volume fraction. Additionally, the mechanical performance of multiaxial panels in compression, low velocity impact and compression after impact (CAI) was measured to address the effect of processing conditions. The compressive strength decreased with porosity and ply-clustering. However, the porosity did not influence the impact resistance and the coompression after impact strength because the effect of porosity was masked by other factors as the damage due to impact or the laminate lay-up. Finally, the effect of the processing conditions on the compaction behavior of unidirectional AS4/8552 panels manufactured by compression moulding was simulated using the finite element method, as a first approximation to more complex and accurate models for out-of autoclave curing and consolidation of composite laminates. The model parameters were obtained from rheological and thermo-mechanical experiments carried out in raw prepreg samples. The predictions of the thickness change during consolidation were in reasonable agreement with the experimental results.

Intraply fracture of fiber-reinforced composites: microscopic mechanisms and modeling

The fracture behavior parallel to the fibers of an E-glass/epoxy unidirectional laminate was studied by means of three-point tests on notched beams. Selected tests were carried out within a scanning electron microscope to ascertain the damage and fracture micromechanisms upon loading. The mechanical behavior of the notched beam was simulated within the framework of the embedded cell model, in which the actual composite microstructure was resolved in front of the notch tip. In addition, matrix and interface properties were independently measured in situ using a nanoindentor. The numerical simulations very accurately predicted the macroscopic response of the composite as well as the damage development and crack growth in front of the notch tip, demonstrating the ability of the embedded cell approach to simulate the fracture behavior of heterogeneous materials. Finally, this methodology was exploited to ascertain the influence of matrix and interface properties on the intraply toughness.

Three dimensional (3D) microstructure-based modeling of interfacial decohesion in particle reinforced metal matrix composites

Modeling and prediction of the overall elastic–plastic response and local damage mechanisms in heterogeneous materials, in particular particle reinforced composites, is a very complex problem. Microstructural complexities such as the inhomogeneous spatial distribution of particles, irregular morphology of the particles, and anisotropy in particle orientation after secondary processing, such as extrusion, significantly affect deformation behavior. We have studied the effect of particle/matrix interface debonding in SiC particle reinforced Al alloy matrix composites with (a) actual microstructure consisting of angular SiC particles and (b) idealized ellipsoidal SiC particles. Tensile deformation in SiC particle reinforced Al matrix composites was modeled using actual microstructures reconstructed from serial sectioning approach. Interfacial debonding was modeled using user-defined cohesive zone elements. Modeling with the actual microstructure (versus idealized ellipsoids) has a significant influence on: (a) localized stresses and strains in particle and matrix, and (b) far-field strain at which localized debonding takes place. The angular particles exhibited higher degree of load transfer and are more sensitive to interfacial debonding. Larger decreases in stress are observed in the angular particles, because of the flat surfaces, normal to the loading axis, which bear load. Furthermore, simplification of particle morphology may lead to erroneous results.

Application of digital image correlation at the microscale in fiber-reinforced composites

Digital image correlation (DIC) is applied to analyzing the deformation mechanisms under transverse compression in a fiber-reinforced composite. To this end, compression tests in a direction perpendicular to the fibers were carried out inside a scanning electron microscope and secondary electron images obtained at different magnifications during the test. Optimum DIC parameters to resolve the displacement and strain field were computed from numerical simulations of a model composite and they were applied to micrographs obtained at different magnifications (250_, 2000_, and 6000_). It is shown that DIC of low-magnification micrographs was able to capture the long range fluctuations in strain due to the presence of matrix-rich and fiber-rich zones, responsible for the onset of damage. At higher magnification, the strain fields obtained with DIC qualitatively reproduce the non-homogeneous deformation pattern due to the presence of stiff fibers dispersed in a compliant matrix and provide accurate results of the average composite strain. However, comparison with finite element simulations revealed that DIC was not able to accurately capture the average strain in each phase.

High fidelity simulations of failure in fiber-reinforced composites

Los ensayos virtuales de materiales compuestos han aparecido como un nuevo concepto dentro de la industria aeroespacial, y disponen de un vasto potencial para reducir los enormes costes de certificación y desarrollo asociados con las tediosas campañas experimentales, que incluyen un gran número de paneles, subcomponentes y componentes. El objetivo de los ensayos virtuales es sustituir algunos ensayos por simulaciones computacionales con alta fidelidad. Esta tesis es una contribución a la aproximación multiescala desarrollada en el Instituto IMDEA Materiales para predecir el comportamiento mecánico de un laminado de material compuesto dadas las propiedades de la lámina y la intercara. La mecánica de daño continuo (CDM) formula el daño intralaminar a nivel constitutivo de material. El modelo de daño intralaminar se combina con elementos cohesivos para representar daño interlaminar. Se desarrolló e implementó un modelo de daño continuo, y se aplicó a configuraciones simples de ensayos en laminados: impactos de baja y alta velocidad, ensayos de tracción, tests a cortadura. El análisis del método y la correlación con experimentos sugiere que los métodos son razonablemente adecuados para los test de impacto, pero insuficientes para el resto de ensayos. Para superar estas limitaciones de CDM, se ha mejorado la aproximación discreta de elementos finitos enriqueciendo la cinemática para incluir discontinuidades embebidas: el método extendido de los elementos finitos (X-FEM). Se adaptó X-FEM para un esquema explícito de integración temporal. El método es capaz de representar cualitativamente los mecanismos de fallo detallados en laminados. Sin embargo, los resultados muestran inconsistencias en la formulación que producen resultados cuantitativos erróneos. Por último, se ha revisado el método tradicional de X-FEM, y se ha desarrollado un nuevo método para superar sus limitaciones: el método cohesivo X-FEM estable. Las propiedades del nuevo método se estudiaron en detalle, y se concluyó que el método es robusto para implementación en códigos explícitos dinámicos escalables, resultando una nueva herramienta útil para la simulación de daño en composites. Virtual testing of composite materials has emerged as a new concept within the aerospace industry. It presents a very large potential to reduce the large certification costs and the long development times associated with the experimental campaigns, involving the testing of a large number of panels, sub-components and components. The aim of virtual testing is to replace some experimental tests by high-fidelity numerical simulations. This work is a contribution to the multiscale approach developed in Institute IMDEA Materials to predict the mechanical behavior of a composite laminate from the properties of the ply and the interply. Continuum Damage Mechanics (CDM) formulates intraply damage at the the material constitutive level. Intraply CDM is combined with cohesive elements to model interply damage. A CDM model was developed, implemented, and applied to simple mechanical tests of laminates: low and high velocity impact, tension of coupons, and shear deformation. The analysis of the results and the comparison with experiments indicated that the performance was reasonably good for the impact tests, but insuficient in the other cases. To overcome the limitations of CDM, the kinematics of the discrete finite element approximation was enhanced to include mesh embedded discontinuities, the eXtended Finite Element Method (X-FEM). The X-FEM was adapted to an explicit time integration scheme and was able to reproduce qualitatively the physical failure mechanisms in a composite laminate. However, the results revealed an inconsistency in the formulation that leads to erroneous quantitative results. Finally, the traditional X-FEM was reviewed, and a new method was developed to overcome its limitations, the stable cohesive X-FEM. The properties of the new method were studied in detail, and it was demonstrated that the new method was robust and can be implemented in a explicit finite element formulation, providing a new tool for damage simulation in composite materials.

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).

Self-sensing properties of alkali activated blast furnace slag (BFS) composites reinforced with carbon fibers

In recent years, several researchers have shown the good performance of alkali activated slag cement and concretes. Besides their good mechanical properties and durability, this type of cement is a good alternative to Portland cements if sustainability is considered. Moreover, multifunctional cement composites have been developed in the last decades for their functional applications (self-sensing, EMI shielding, self-heating, etc.). In this study, the strain and damage sensing possible application of carbon fiber reinforced alkali activated slag pastes has been evaluated. Cement pastes with 0, 0.29 and 0.58 vol % carbon fiber addition were prepared. Both carbon fiber dosages showed sensing properties. For strain sensing, function gage factors of up to 661 were calculated for compressive cycles. Furthermore, all composites with carbon fibers suffered a sudden increase in their resistivity when internal damages began, prior to any external signal of damage. Hence, this material may be suitable as strain or damage sensor.

Multifunctional cement composites strain and damage sensors applied on reinforced concrete (RC) structural elements

In this research, strain-sensing and damage-sensing functional properties of cement composites have been studied on a conventional reinforced concrete (RC) beam. Carbon nanofiber (CNFCC) and fiber (CFCC) cement composites were used as sensors on a 4 m long RC beam. Different casting conditions (in situ or attached), service location (under tension or compression) and electrical contacts (embedded or superficial) were compared. Both CNFCC and CFCC were suitable as strain sensors in reversible (elastic) sensing condition testing. CNFCC showed higher sensitivities (gage factor up to 191.8), while CFCC only reached gage factors values of 178.9 (tension) or 49.5 (compression). Furthermore, damage-sensing tests were run, increasing the applied load progressively up to the RC beam failure. In these conditions, CNFCC sensors were also strain sensitive, but no damage sensing mechanism was detected for the strain levels achieved during the tests. Hence, these cement composites could act as strain sensors, even for severe damaged structures near to their collapse.

In situ TiB-reinforced Cu-based bulk metallic glass composites

Cu-based bulk metallic glass (BMG) composites containing in situ TiB particles were successfully fabricated. The reinforcing TiB particles with a size of 5-10 mu m are uniformly distributed in the amorphous matrix. The particles have a good bonding to the matrix with a reaction layer. The BMG composites exhibit an obvious ductility with a plastic strain of 2% for the 17.5 vol.% TiB sample due to the suppression of shear band propagation and the generation of multiple shear bands during compressive testing. The hardness of the materials is increased from Hv543 for monolithic BMG to Hv650 for 23.6 vol.% TiB-containing BMG composite. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Nanoindentation measurement of surface residual stresses in particle-reinforced metal matrix composites

The surface residual stresses in SiC particle-reinforced Al matrix composites are measured using a recently developed nanoindentation technique. The tensile biaxial residual stress in Al is found to increase with the particle concentration. The stress magnitudes are in reasonable agreement with those from numerical modeling.

Interfacial effects on fatigue and fracture in discontinuously reinforced metal matrix composites

Interfaces in conventional monolithic alloys exert an important influence on fatigue and fracture behavior. In discontinuously reinforced metal matrix composites (MMCs), the role of interface is even more dominant. The interfacial is higher in MMCs and the interfaces are generally of high energy and chemically unstable. This paper reviews the factors which can affect interfacial strength in discontinuously reinforced MMCs, and the ways in which interfacial strength can be controlled. The effects of interfacial strength on fatigue crack propagation and fracture behavior are then illustrated.