Detailed damage mechanisms assessment in composite materials by means of X-ray tomography

This research focused on the evaluation of damage formation on ��45�� carbon fiber laminates subjected to tensile tests. The damage was evaluated by means of X-ray tomography. A high density of cracks developed during the plateau of the stress-strain curve and were qualitatively analyzed, showing that the inner plies eventually developed a higher crack concentration than the outer plies. Delamination started to occur in the outermost ply interface when the slope after the plateau of the stress-strain curve began to increase.

A level set approach for the analysis of flow and compaction during resin infusion in composite materials

Fluid flow and fabric compaction during vacuum assisted resin infusion (VARI) of composite materials was simulated using a level set-based approach. Fluid infusion through the fiber preform was modeled using Darcy���s equations for the fluid flow through a porous media. The stress partition between the fluid and the fiber bed was included by means of Terzaghi���s effective stress theory. Tracking the fluid front during infusion was introduced by means of the level set method. The resulting partial differential equations for the fluid infusion and the evolution of flow front were discretized and solved approximately using the finite differences method with a uniform grid discretization of the spatial domain. The model results were validated against uniaxial VARI experiments through an [0]8 E-glass plain woven preform. The physical parameters of the model were also independently measured. The model results (in terms of the fabric thickness, pressure and fluid front evolution during filling) were in good agreement with the numerical simulations, showing the potential of the level set method to simulate resin infusion

Study of the increase in bending stiffness on timber beams reinforced with composite materials

It is common to find structures that need to be reinforced due to deterioration or because the function of the building changes. The economic cost involved in these forms of interventions is considerable. Therefore, it is interesting to progress in the existing strengthening techniques and the study of new reinforcement systems. This paper analyses the behaviour of timber beams reinforced with carbon and basalt fiber composite materials. The main objective of this study is to test the stiffness increase produced by the carbon and basalt FRP on reinforced beams. The results show the stiffness increase produced by the different types of reinforcement.

Application of PZT and FBG impedance sensors for structural health monitoring of reinforced concrete beams strengthened with FRP composite materials

Esta Tesis tiene como objetivo principal el desarrollo de m��todos de identificaci��n del da��o que sean robustos y fiables, enfocados a sistemas estructurales experimentales, fundamentalmente a las estructuras de hormig��n armado reforzadas externamente con bandas fibras de pol��meros reforzados (FRP). El modo de fallo de este tipo de sistema estructural es cr��tico, pues generalmente es debido a un despegue repentino y fr��gil de la banda del refuerzo FRP originado en grietas intermedias causadas por la flexi��n. La detecci��n de este despegue en su fase inicial es fundamental para prevenir fallos futuros, que pueden ser catastr��ficos. Inicialmente, se lleva a cabo una revisi��n del m��todo de la Impedancia Electro-Mec��nica (EMI), de cara a exponer sus capacidades para la detecci��n de da��o. Una vez la tecnolog��a apropiada es seleccionada, lo que incluye un analizador de impedancias as�� como novedosos sensores PZT para monitorizaci��n inteligente, se ha dise��ado un procedimiento autom��tico basado en los registros de impedancias de distintas estructuras de laboratorio. Bas��ndonos en el hecho de que las mediciones de impedancias son posibles gracias a una colocaci��n adecuada de una red de sensores PZT, la estimaci��n de la presencia de da��o se realiza analizando los resultados de distintos indicadores de da��o obtenidos de la literatura. Para que este proceso sea autom��tico y que no sean necesarios conocimientos previos sobre el m��todo EMI para realizar un experimento, se ha dise��ado e implementado un Interfaz Gr��fico de Usuario, transformando la medici��n de impedancias en un proceso f��cil e intuitivo. Se eval��a entonces el da��o a trav��s de los correspondientes ��ndices de da��o, intentando estimar no s��lo su severidad, sino tambi��n su localizaci��n aproximada. El desarrollo de estos experimentos en cualquier estructura genera grandes cantidades de datos que han de ser procesados, y algunas veces los ��ndices de da��o no son suficientes para una evaluaci��n completa de la integridad de una estructura. En la mayor��a de los casos se pueden encontrar patrones de da��o en los datos, pero no se tiene informaci��n a priori del estado de la estructura. En este punto, se ha hecho una importante investigaci��n en t��cnicas de reconocimiento de patrones particularmente en aprendizaje no supervisado, encontrando aplicaciones interesantes en el campo de la medicina. De ah�� surge una idea creativa e innovadora: detectar y seguir la evoluci��n del da��o en distintas estructuras como si se tratase de un c��ncer propag��ndose por el cuerpo humano. En ese sentido, las lecturas de impedancias se emplean como informaci��n intr��nseca de la salud de la propia estructura, de forma que se pueden aplicar las mismas t��cnicas que las empleadas en la investigaci��n del c��ncer. En este caso, se ha aplicado un algoritmo de clasificaci��n jer��rquica dado que ilustra adem��s la clasificaci��n de los datos de forma gr��fica, incluyendo informaci��n cualitativa y cuantitativa sobre el da��o. Se ha investigado la efectividad de este procedimiento a trav��s de tres estructuras de laboratorio, como son una viga de aluminio, una uni��n atornillada de aluminio y un bloque de hormig��n reforzado con FRP. La primera ayuda a mostrar la efectividad del m��todo en sencillos escenarios de da��o simple y m��ltiple, de forma que las conclusiones extra��das se aplican sobre los otros dos, dise��ados para simular condiciones de despegue en distintas estructuras. Demostrada la efectividad del m��todo de clasificaci��n jer��rquica de lecturas de impedancias, se aplica el procedimiento sobre las estructuras de hormig��n armado reforzadas con bandas de FRP objeto de esta tesis, detectando y clasificando cada estado de da��o. Finalmente, y como alternativa al anterior procedimiento, se propone un m��todo para la monitorizaci��n continua de la interfase FRP-Hormig��n, a trav��s de una red de sensores FBG permanentemente instalados en dicha interfase. De esta forma, se obtienen medidas de deformaci��n de la interfase en condiciones de carga continua, para ser implementadas en un modelo de optimizaci��n multiobjetivo, cuya soluci��n se haya por medio de una expansi��n multiobjetivo del m��todo Particle Swarm Optimization (PSO). La fiabilidad de este ��ltimo m��todo de detecci��n se investiga a trav��s de sendos ejemplos tanto num��ricos como experimentales. ABSTRACT This thesis aims to develop robust and reliable damage identification methods focused on experimental structural systems, in particular Reinforced Concrete (RC) structures externally strengthened with Fiber Reinforced Polymers (FRP) strips. The failure mode of this type of structural system is critical, since it is usually due to sudden and brittle debonding of the FRP reinforcement originating from intermediate flexural cracks. Detection of the debonding in its initial stage is essential thus to prevent future failure, which might be catastrophic. Initially, a revision of the Electro-Mechanical Impedance (EMI) method is carried out, in order to expose its capabilities for local damage detection. Once the appropriate technology is selected, which includes impedance analyzer as well as novel PZT sensors for smart monitoring, an automated procedure has been design based on the impedance signatures of several lab-scale structures. On the basis that capturing impedance measurements is possible thanks to an adequately deployed PZT sensor network, the estimation of damage presence is done by analyzing the results of different damage indices obtained from the literature. In order to make this process automatic so that it is not necessary a priori knowledge of the EMI method to carry out an experimental test, a Graphical User Interface has been designed, turning the impedance measurements into an easy and intuitive procedure. Damage is then assessed through the analysis of the corresponding damage indices, trying to estimate not only the damage severity, but also its approximate location. The development of these tests on any kind of structure generates large amounts of data to be processed, and sometimes the information provided by damage indices is not enough to achieve a complete analysis of the structural health condition. In most of the cases, some damage patterns can be found in the data, but none a priori knowledge of the health condition is given for any structure. At this point, an important research on pattern recognition techniques has been carried out, particularly on unsupervised learning techniques, finding interesting applications in the medicine field. From this investigation, a creative and innovative idea arose: to detect and track the evolution of damage in different structures, as if it were a cancer propagating through a human body. In that sense, the impedance signatures are used to give intrinsic information of the health condition of the structure, so that the same clustering algorithms applied in the cancer research can be applied to the problem addressed in this dissertation. Hierarchical clustering is then applied since it also provides a graphical display of the clustered data, including quantitative and qualitative information about damage. The performance of this approach is firstly investigated using three lab-scale structures, such as a simple aluminium beam, a bolt-jointed aluminium beam and an FRP-strengthened concrete specimen. The first one shows the performance of the method on simple single and multiple damage scenarios, so that the first conclusions can be extracted and applied to the other two experimental tests, which are designed to simulate a debonding condition on different structures. Once the performance of the impedance-based hierarchical clustering method is proven to be successful, it is then applied to the structural system studied in this dissertation, the RC structures externally strengthened with FRP strips, where the debonding failure in the interface between the FRP and the concrete is successfully detected and classified, proving thus the feasibility of this method. Finally, as an alternative to the previous approach, a continuous monitoring procedure of the FRP-Concrete interface is proposed, based on an FBGsensors Network permanently deployed within that interface. In this way, strain measurements can be obtained under controlled loading conditions, and then they are used in order to implement a multi-objective model updating method solved by a multi-objective expansion of the Particle Swarm Optimization (PSO) method. The feasibility of this last proposal is investigated and successfully proven on both numerical and experimental RC beams strengthened with FRP.

Comparison of push-in and push-out tests for measuring interfacial shear strength in nano-reinforced composite materials

The influence of the carbon nanotubes (CNTs) content on the fiber/matrix interfacial shear strength (IFSS) in glass/fiber epoxy composites was measured by means of push-in and push-out tests. Both experimental methodologies provided equivalent values of the IFSS for each material. It was found that the dispersion of CNTs increased in IFSS by 19% in average with respect to the composite without CNTs. This improvement was reached with 0.3 wt.% of CNTs and increasing the CNT content up to 0.8 wt.% did not improve the interface strength.

Mejora del comportamiento t��rmico de los morteros de cal aditivados y su empleo en la rehabilitaci��n de inmuebles = Improvement of thermal behaviour of additived lime mortars and their use in rehabilitation of buildings

La envolvente de la edificaci��n es la responsable de equilibrar el intercambio energ��tico entre el interior y el exterior, por lo tanto cualquier actuaci��n encaminada a la reducci��n del consumo energ��tico ha de establecer, como uno de sus objetivos prioritarios, la mejora del comportamiento de la misma. Las edificaciones anteriores a 1940 constituyen la mayor parte de las existentes en ��reas rurales y centros urbanos. En ellas, la repercusi��n de la fachada sobre las transmitancias globales pone de manifiesto la necesidad de intervenci��n. Sin embargo, su elevada inercia t��rmica y los importantes saltos t��rmicos caracter��sticos de gran parte de Espa��a plantean la importancia de que aqu��lla se efect��e por el exterior. A tal respecto, la falta de disponibilidad de espesor suficiente para implantar sistemas tipo SATE deriva en que, frecuentemente, la ��nica soluci��n viable sea la de aislar por el interior perdiendo con ello la capacidad de acumulaci��n t��rmica del muro y con el asociado riesgo de condensaciones. La amplia tradici��n en el empleo de revestimientos, especialmente en base de cal, permiten que ��stos sean utilizados no s��lo como elemento est��tico o de protecci��n de la obra de f��brica antigua sino tambi��n para la mejora del comportamiento t��rmico del soporte, si se aprovecha el mecanismo de transmisi��n t��rmica por radiaci��n. ��ste es el objetivo de la presente Tesis Doctoral en la que se estudia la modificaci��n de las propiedades radiantes de los morteros de revestimiento para la mejora de la eficiencia energ��tica de las construcciones hist��ricas, principalmente las constituidas por muros monol��ticos, aunque podr��a ser de aplicaci��n a otro tipo de construcciones compuestas por diversas capas. Como punto de partida, se estudi�� y revis�� la documentaci��n disponible sobre las investigaciones de las tres ��reas cient��fico-tecnol��gicas que convergen en la Tesis Doctoral: rehabilitaci��n, material y comportamiento t��rmico, a partir de lo cual se comprob�� la inexistencia de estudios similares al objeto de la presente Tesis Doctoral. Complementariamente, se analizaron los revestimientos en lo concerniente a los materiales que los constituyen, la composici��n de las mezclas y caracter��sticas de cada una de las capas as�� como al enfoque que, desde el punto de vista t��rmico, se estimaba m��s adecuado para la obtenci��n de los objetivos propuestos. Bas��ndonos en dichos an��lisis, se preseleccionaron ochenta materiales que fueron ensayados en t��rminos de reflectancia y emisividad para elegir cuatro con los que se continu�� la investigaci��n. ��stos, junto con la cal elegida para la investigaci��n y el ��rido marm��reo caracter��stico de la ��ltima capa de revestimiento, fueron caracterizados t��rmicamente, de forma pormenorizada, as�� como qu��mica y f��sicamente. Los fundamentos te��ricos y los estudios preliminares desarrollados con distintos materiales, en estado fresco y endurecido, fueron empleados en la dosificaci��n de componentes de las mezclas, en dos proporciones distintas, para el estudio del efecto del agregado. ��stas se ensayaron en estado fresco, para comprobar su adecuaci��n de puesta en obra y prever su VI adherencia al soporte, as�� como en estado endurecido a 28 y 90 d��as de curado, para conocer las propiedades que permitieran prever su compatibilidad con aqu��l y estimar el alcance de la reducci��n de transferencias t��rmicas lograda. Adem��s, se estudiaron las caracter��sticas generales de las mezclas que sirvieron para establecer correlaciones entre distintas propiedades y entender los resultados mec��nicos, f��sicos (comportamiento frente al agua) y energ��ticos. Del estudio conjunto de las distintas propiedades analizadas se propusieron dos mezclas, una blanca y otra coloreada, cuyas caracter��sticas permiten garantizar la compatibilidad con la obra de f��brica antigua, desde el punto de vista f��sico y mec��nico, y preservar la autenticidad de los revestimientos, en cuanto a la t��cnica de aplicaci��n tradicional en sistemas multicapa. El comportamiento t��rmico de las mismas, sobre una obra de f��brica de 40 cm de espesor, se estim��, en estado estacionario y pseudo-transitorio, demostr��ndose reducciones del flujo t��rmico entre 16-48%, en condiciones de verano, y entre el 6-11%, en invierno, en funci��n de la coloraci��n y de la rugosidad de la superficie, en comparaci��n con el empleo de la mezcla tradicional. Por lo que, se constata la viabilidad de los materiales compuestos propuestos y su adecuaci��n al objetivo de la investigaci��n. VII ABSTRACT The envelope is responsible for balancing the energy exchange between the inside and outside in buildings. For this reason, any action aimed at reducing energy consumption must establish, as one of its key priorities, its improvement. In rural areas and urban centers, most of the constructions were built before 1940. In them, the impact of the fa��ade on the global transmittance highlights the need for intervention. However, its high thermal inertia and fluctuation of temperatures in the majority of Spain bring up that it should be placed outside the insulation. In this regard, the lack of availability of enough thickness to implement systems such as ETICS results in that often the only viable solution is to isolate the interior, losing thereby the wall���s heat storage capacity with the associated risk of condensation. The tradition in the use of renders, especially lime-based, allows us to use them not only as an aesthetic element or to protect the ancient masonry, but also for improved thermal performance of the support by taking advantage of the heat transfer mechanism by radiation. This is the aim of this Doctoral Thesis in which the modification of the radiative properties of lime mortars for renders to improve the energy efficiency of historic buildings, mainly composed of monolithic walls, is studied, although it could be applied to other structures composed of several layers. As a starting point, the available literature in the three scientific-technological areas that converge at the Doctoral Thesis: rehabilitation, material and thermal behaviour, was reviewed, and confirmed the absence of researches similar to this Doctoral Thesis. Additionally, the renders were studied with regard to the materials that constitute them, the composition of the mixtures and the characteristics of each layer, as well as to the approach which, from a thermal point of view, was deemed the most suitable for achieving the objectives sets. Based on thre aforementioned analysis, eighty materials tested in terms of reflectance and emissivity were pre-selected, to choose four with which the research was continued. Common marble sand, used in the last layer of the renders, together with the appointed materials and hydrated lime were characterized thermally, in detail, as well as chemically and physically. The theoretical bases and preliminary studies with different materials, in fresh and hardened state, were used in the dosage of the composition of the mixtures. In order to study their effect they were used in two different proportions, that is, ten mixtures in total. These were tested in their fresh state to evaluate their setting-up suitability and foresee their adhesion to the support, as well as in their hardened state, at 28 and 90 days of curing, to establish the properties which enabled us to anticipate their compatibility with the old masonry walls and estimate the scope of the reduction of heat transfers achieved. In addition, the general characteristics of the mixtures used to establish correlations and to understand the mechanical, physical and energy results were studied. Two mixtures, one white and one colored, were proposed as the result of the different properties analysed, whose characteristics allow the guarantee of mechanical and physical compatibility VIII with the old masonry and preserve the authenticity of the renders. The thermal behavior of both, applied on a masonry wall 40 cm thick, was estimated at a steady and pseudo-transient state, with heat flow reductions between 16-48% during summertime and 6-11% during wintertime, depending on the color and surface roughness, compared to the use of the traditional mixture. So, the viability of the proposed composite materials and their fitness to the aim of the research are established.

Estudio de la redistribuci��n tensional en la interfase frp-hormig��n

El uso de materiales compuestos para el refuerzo, reparaci��n y rehabilitaci��n de estructuras de hormig��n se ha convertido en una t��cnica muy utilizada en la ��ltima d��cada. Con independencia de la t��cnica del refuerzo, uno de los principales condicionantes del dise��o es el fallo de la adherencia entre el hormig��n y el material compuesto, atribuida generalmente a las tensiones en la interfaz de estos materiales. Las propiedades mec��nicas del hormig��n y de los materiales compuestos son muy distintas. Los materiales compuestos com��nmente utilizados en ingenier��a civil poseen alta resistencia a tracci��n y tienen un comportamiento el��stico y lineal hasta la rotura, lo cual, en contraste con el ampliamente conocido comportamiento del hormig��n, genera una clara incompatibilidad para soportar esfuerzos de forma conjunta. Esta incompatibilidad conduce a fallos relacionados con el despegue del material compuesto del sustrato de hormig��n. En vigas de hormig��n reforzadas a flexi��n o a cortante, el despegue del material compuesto es un fen��meno que frecuentemente condiciona la capacidad portante del elemento. Existen dos zonas potenciales de iniciaci��n del despegue: los extremos y la zona entre fisuras de flexi��n o de flexi��n-cortante. En el primer caso, la experiencia a trav��s de los ��ltimos a��os ha demostrado que se puede evitar prolongando el refuerzo hasta los apoyos o mediante el empleo de alg��n sistema de anclaje. Sin embargo, las recomendaciones para evitar el segundo caso de despegue a��n se encuentran lejos de poder prever el fallo de forma eficiente. La necesidad de medir la adherencia experimentalmente de materiales FRP adheridos al hormig��n ha dado lugar a desarrollar diversos m��todos por la comunidad de investigadores. De estas campa��as experimentales surgieron modelos para el pron��stico de la resistencia de adherencia, longitud efectiva y relaci��n tensi��n-deslizamiento. En la presente tesis se propone un ensayo de beam-test, similar al utilizado para medir la adherencia de barras de acero, para determinar las caracter��sticas de adherencia del FRP al variar la resistencia del hormig��n y el espesor del adhesivo. A la vista de los resultados, se considera que este ensayo puede ser utilizado para investigar diferentes tipos de adhesivos y otros m��todos de aplicaci��n, dado que representa con mayor realidad el comportamiento en vigas reforzadas. Los resultados experimentales se trasladan a la comprobaci��n del fallo por despegue en la regi��n de fisuras de flexi��n o flexi��n cortante en vigas de hormig��n presentando buena concordancia. Los resultados condujeron a la propuesta de que la limitaci��n de la deformaci��n constituye una alternativa simple y eficiente para prever el citado modo de fallo. Con base en las vigas analizadas, se propone una nueva expresi��n para el c��lculo de la limitaci��n de la deformaci��n del laminado y se lleva a cabo una comparaci��n entre los modelos existentes mediante un an��lisis estad��stico para evaluar su precisi��n. Abstract The use of composite materials for strengthening, repairing or rehabilitating concrete structures has become more and more popular in the last ten years. Irrespective of the type of strengthening used, design is conditioned, among others, by concrete-composite bond failure, normally attributed to stresses at the interface between these two materials. The mechanical properties of concrete and composite materials are very different. Composite materials commonly used in civil engineering possess high tensile strength (both static and long term) and they are linear elastic to failure, which, in contrast to the widely known behavior of concrete, there is a clear incompatibility which leads to bond-related failures. Bond failure in the composite material in bending- or shear-strengthened beams often controls bearing capacity of the strengthened member. Debonding failure of RC beams strengthened in bending by externally-bonded composite laminates takes place either, at the end (plate end debonding) or at flexure or flexure-shear cracks (intermediate crack debonding). In the first case, the experience over the past years has shown that this can be avoided by extending laminates up to the supports or by using an anchoring system. However, recommendations for the second case are still considered far from predicting failure efficiently. The need to experimentally measure FRP bonding to concrete has induced the scientific community to develop test methods for that purpose. Experimental campaigns, in turn, have given rise to models for predicting bond strength, effective length and the stress-slip relationship. The beam-type test proposed and used in this thesis to determine the bonding characteristics of FRP at varying concrete strengths and adhesive thicknesses was similar to the test used for measuring steel reinforcement to concrete bonding conditions. In light of the findings, this test was deemed to be usable to study different types of adhesives and application methods, since it reflects the behavior of FRP in strengthened beams more accurately than the procedures presently in place. Experimental results are transferred to the verification of peeling-off at flexure or flexure-shear cracks, presenting a good general agreement. Findings led to the conclusion that the strain limitation of laminate produces accurate predictions of intermediate crack debonding. A new model for strain limitation is proposed. Finally, a comprehensive evaluation based on a statistical analysis among existing models is carried out in order to assess their accuracy.

An��lisis de elementos de madera reforzados con materiales compuestos

Gran parte del patrimonio construido cuenta con edificios cuya estructura est�� compuesta por elementos de madera. El volumen econ��mico que supone el mantenimiento y renovaci��n de dicho patrimonio es considerable, por ello, es de especial inter��s el estudio de las diferentes t��cnicas de refuerzo aplicables a este tipo de estructuras. Las estructuras de madera han sido tradicionalmente reforzadas con piezas del mismo material, aumentando la secci��n de los elementos da��ados, o con acero. La aparici��n de los materiales compuestos de pol��meros reforzados con fibras, y su progresiva aplicaci��n en obras de construcci��n, hizo que a principios de la d��cada de los noventa se comenzara a aplicar este material en refuerzos de estructuras de madera (Puente de Sins, 1992). La madera es un material natural con una excelente relaci��n entre sus caracter��sticas mec��nicas y su peso. Con el uso de materiales compuestos como refuerzo ��sta caracter��stica se mantiene. En cuanto a su modelo constitutivo, se admite un comportamiento el��stico lineal a tracci��n paralela a la fibra hasta la rotura, mientras que a compresi��n, se considera un comportamiento lineal el��stico inicial, seguido de un tramo pl��stico. En vigas de madera aserrada sometidas a flexi��n predomina el modo de fallo por tracci��n localiz��ndose la fractura frecuentemente en el canto inferior. Los FRP tienen un comportamiento el��stico lineal a tracci��n hasta la rotura y cuentan con excelentes propiedades mec��nicas en relaci��n a su peso y volumen. Si se refuerza la viga por el canto inferior se aumentar�� su capacidad de absorber tracciones y por tanto, es previsible que se produzca un incremento en la capacidad de carga, as�� como un aumento de ductilidad. En este trabajo se analizan los beneficios que aportan distintos sistemas de refuerzos de materiales compuestos. El objetivo es contribuir al conocimiento de esta t��cnica para la recuperaci��n o aumento de las propiedades resistentes de elementos de madera sometidos a flexi��n. Se ha llevado a cabo un estudio basado en datos obtenidos experimentalmente mediante el ensayo a flexi��n de vigas de madera de pino silvestre reforzadas con materiales compuestos. Las fibras que componen los tejidos utilizados para la ejecuci��n de los refuerzos son de basalto y de carbono. En el caso de los compuestos de fibra de basalto se aplican en distintos gramajes, y los de carbono en tejido unidireccional y bidireccional. Se analiza el comportamiento de las vigas seg��n las variables de refuerzo aplicadas y se comparan con los resultados de vigas ensayadas sin reforzar. Adem��s se comprueba el ajuste del modelo de c��lculo no lineal aplicado para predecir la carga de rotura de cada viga reforzada. Con este trabajo queda demostrado el buen funcionamiento del FRP de fibra de basalto aplicado en el refuerzo de vigas de madera y de los tejidos de carbono bidireccionales con respecto a los unidireccionales. ABSTRACT Many of the buildings of the built heritage include a structure composed by timber elements. The economic volume involved in the maintenance and renewal of this built heritage is considerable, therefore, the study of the different reinforcement techniques applicable to this type of structure is of special interest. The wooden structures have traditionally been reinforced either with steel or with pieces of the same material, increasing the section of the damaged parts. The emergence of polymer composites reinforced with fibers, and their progressive use in construction, started to be applied as reinforcement in timber structures at the beginning of the nineties decade in the 20th century (Sins Bridge, 1992). Wood is a natural material with an excellent ratio between its mechanic characteristics and its weight. This feature is maintained with the use of composites as reinforcement. In terms of its constitutive model, linear elastic behavior parallel to the fiber up to fracture is admitted when subjected to tensile stress, while under compression, an initial linear elastic behavior, followed by a section plasticizing, is considered. In sawn timber beams subjected to bending, the predominant failure is mainly due to tensile stress; and frequently the fracture is located at the beam lower face. The FRP have a linear elastic behavior until fracture occurs, and have excellent mechanical properties in relation to their weight and volume. If the beam is reinforced by its lower face, its capacity to absorb tensile stresses will increase, and therefore, an increase in its carrying capacity is likely to be produced, as well as an increase in ductility. This work analyzes the benefits different reinforcement systems of composite materials provide, with the aim of contributing to the knowledge of this technique for recovering or increasing the strength properties of timber elements subjected to bending loads. It is a study based on data obtained experimentally using bending tests of pine timber beams reinforced with composite materials. Fibers used for the execution of the reinforcement are basalt and carbon. Basalt fiber composites are applied in different grammages, whereas with carbon composites, unidirectional and bidirectional fabrics are used. The behavior of the beams was analyzed regarding the reinforcement variables applied, and the results are compared with those of the tested beams without reinforcement. Furthermore it has been proved adjunting the nonlinear calculation model applied to predict the failure load of each reinforced beam. This work proves the good behavior of fiber reinforce plastic (FRP) with basalt fiber when applied to timber beams, and that of bidirectional carbon fabrics as opposed to the unidirectional ones.

Comportamiento estructural de elementos de f��brica reforzados con materiales compuestos avanzados solicitados a flexi��n y flexocompresi��n

El refuerzo de estructuras existentes mediante el encolado exterior de l��minas de pol��meros reforzados con fibras (FRP) se ha convertido en la aplicaci��n m��s com��n de los materiales compuestos avanzados en construcci��n. Estos materiales presentan muchas ventajas frente a los materiales convencionales (sin corrosi��n, ligeros, de f��cil aplicaci��n, etc.). Pero a pesar de las numerosas investigaciones realizadas, a��n persisten ciertas dudas sobre algunos aspectos de su comportamiento y las aplicaciones pr��cticas se llevan a cabo s��lo con la ayuda de gu��as, sin que haya una normativa oficial. El objetivo de este trabajo es incrementar el conocimiento sobre esta t��cnica de refuerzo, y m��s concretamente, sobre el refuerzo a flexi��n de estructuras de f��brica. Con frecuencia el elemento reforzado es de hormig��n armado y las l��minas de FRP encoladas al exterior sirven para mejorar su resistencia a flexi��n, cortante o compresi��n (encamisados). Sin embargo su empleo en otros materiales como las estructuras de f��brica resulta muy prometedor. Las f��bricas se caracterizan por soportar muy bien los esfuerzos de compresi��n pero bastante mal los de tracci��n. Adherir l��minas de materiales compuestos puede servir para mejorar la capacidad resistente de elementos de f��brica sometidos a esfuerzos de flexi��n. Pero para ello, debe quedar garantizada una correcta adherencia entre el FRP y la f��brica, especialmente en edificios antiguos cuya superficie puede estar deteriorada por encontrarse a la intemperie o por el propio paso del tiempo. En el cap��tulo II se describen los objetivos fundamentales del trabajo y el m��todo seguido. En el cap��tulo III se hace una amplia revisi��n del estado de conocimiento sobre el tema. En el apartado III.1 se detallan las principales caracter��sticas y propiedades mec��nicas de fibras, matrices y materiales compuestos as�� como sus principales aplicaciones, haciendo especial hincapi�� en aspectos relativos a su durabilidad. En el apartado III.2 se incluye una revisi��n hist��rica de las l��neas de investigaci��n, tanto te��ricas como emp��ricas, publicadas sobre estructuras de hormig��n reforzadas a flexi��n encolando materiales compuestos. El apartado III.3 se centra en el aspecto fundamental de la adherencia refuerzo-soporte. Se hace un repaso a distintos modelos propuestos para prevenir el despegue distinguiendo si ��ste se inicia en la zona de anclaje o si est�� inducido por fisuras en la zona interior del elemento. Se observa falta de consenso en las propuestas. Adem��s en este punto se relatan las campa��as experimentales publicadas acerca de la adherencia entre materiales compuestos y f��bricas. En el apartado III.4 se analizan las particularidades de las estructuras de f��brica. Adem��s, se revisan algunas de las investigaciones relativas a la mejora de su comportamiento a flexi��n mediante l��minas de FRP. El comportamiento mec��nico de muros reforzados solicitados a flexi��n pura (sin compresi��n) ha sido documentado por varios autores, si bien es una situaci��n poco frecuente en f��bricas reales. Ni el comportamiento mec��nico de muros reforzados solicitados a flexocompresi��n ni la incidencia que el nivel de compresi��n soportado por la f��brica tiene sobre la capacidad resistente del elemento reforzado han sido suficientemente tratados. En cuanto a los trabajos te��ricos, las diferentes propuestas se basan en los m��todos utilizados para hormig��n armado y comparten los principios habituales de c��lculo. Sin embargo, presentan diferencias relativas, sobre todo, a tres aspectos: 1) la forma de modelar el comportamiento de la f��brica, 2) el valor de deformaci��n de c��lculo del refuerzo, y 3) el modo de fallo que se considera recomendable buscar con el dise��o. A pesar de ello, el ajuste con la parte experimental de cada trabajo suele ser bueno debido a una enorme disparidad en las variables consideradas. Cada campa��a presenta un modo de fallo caracter��stico y la formulaci��n que se propone resulta apropiada para ��l. Parece necesario desarrollar un m��todo de c��lculo para f��bricas flexocomprimidas reforzadas con FRP que pueda ser utilizado para todos los posibles fallos, tanto atribuibles a la l��mina como a la f��brica. En el apartado III.4 se repasan algunas lesiones habituales en f��bricas solicitadas a flexi��n y se recogen ejemplos de refuerzos con FRP para reparar o prevenir estos da��os. Para mejorar el conocimiento sobre el tema, se llevan a cabo dos peque��as campa��as experimentales realizadas en el Instituto de Ciencias de la Construcci��n Eduardo Torroja. La primera acerca de la adherencia de materiales compuestos encolados a f��bricas deterioradas (apartado IV.1) y la segunda sobre el comportamiento estructural a flexocompresi��n de probetas de f��brica reforzadas con estos materiales (apartado IV.2). En el cap��tulo V se analizan algunos de los modelos de adherencia propuestos para prevenir el despegue del extremo del refuerzo. Se confirma que las predicciones obtenidas con ellos resultan muy dispares. Se recopila una base de datos con los resultados experimentales de campa��as sobre adherencia de FRP a f��bricas extra��das de la literatura y de los resultados propios de la campa��a descrita en el punto IV.1. Esta base de datos permite conocer cual de los m��todos analizados resulta m��s adecuado para dimensionar el anclaje de l��minas de FRP adheridas a f��bricas. En el cap��tulo VI se propone un m��todo para la comprobaci��n en agotamiento de secciones de f��brica reforzadas con materiales compuestos sometidas a esfuerzos combinados de flexi��n y compresi��n. Est�� basado en el procedimiento de c��lculo de la capacidad resistente de secciones de hormig��n armado pero adaptado a las f��bricas reforzadas. Para ello, se utiliza un diagrama de c��lculo tensi��n deformaci��n de la f��brica de tipo bilineal (acorde con el CTE DB SE-F) cuya simplicidad facilita el desarrollo de toda la formulaci��n al tiempo que resulta adecuado para predecir la capacidad resistente a flexi��n tanto para fallos debidos al refuerzo como a la f��brica. Adem��s se limita la deformaci��n de c��lculo del refuerzo teniendo en consideraci��n ciertos aspectos que provocan que la l��mina adherida no pueda desarrollar toda su resistencia, como el desprendimiento inducido por fisuras en el interior del elemento o el deterioro medioambiental. En concreto, se propone un ���coeficiente reductor por adherencia��� que se determina a partir de una base de datos con 68 resultados experimentales procedentes de publicaciones de varios autores y de los ensayos propios de la campa��a descrita en el punto IV.2. Tambi��n se revisa la formulaci��n propuesta con ayuda de la base de datos. En el cap��tulo VII se estudia la incidencia de las principales variables, como el axil, la deformaci��n de c��lculo del refuerzo o su rigidez, en la capacidad final del elemento. Las conclusiones del trabajo realizado y las posibles l��neas futuras de investigaci��n se exponen en el cap��tulo VIII. ABSTRACT Strengthening of existing structures with externally bonded fiber reinforced polymers (FRP) has become the most common application of advanced composite materials in construction. These materials exhibit many advantages in comparison with traditional ones (corrosion resistance, light weight, easy to apply, etc.). But despite countless researches have been done, there are still doubts about some aspects of their behaviour and applications are carried out only with the help of guidelines, without official regulations. The aim of this work is to improve the knowledge on this retrofitting technique, particularly in regard to flexural strengthening of masonry structures. Reinforced concrete is often the strengthened material and external glued FRP plates are used to improve its flexural, shear or compressive (by wrapping) capacity. However the use of this technique on other materials like masonry structures looks promising. Unreinforced masonry is characterized for being a good material to support compressive stresses but really bad to withstand tensile ones. Glue composite plates can improve the flexural capacity of masonry elements subject to bending. But a proper bond between FRP sheet and masonry must be ensured to do that, especially in old buildings whose surface can be damaged due to being outside or ageing. The main objectives of the work and the methodology carried out are described In Chapter II. An extensive overview of the state of art is done in Chapter III. In Section III.1 physical and mechanical properties of fibers, matrix and composites and their main applications are related. Durability aspects are especially emphasized. Section III.2 includes an historical overview of theoretical and empirical researches on concrete structures strengthened gluing FRP plates to improve their flexural behaviour. Section III.3 focuses on the critical point of bonding between FRP and substrate. Some theoretical models to prevent debonding of FRP laminate are reviewed, it has made a distinction between models for detachment at the end of the plate or debonding in the intermediate zones due to the effects of cracks. It is observed a lack of agreement in the proposals. Some experimental studies on bonding between masonry and FRP are also related in this chapter. The particular characteristics of masonry structures are analyzed in Section III.4. Besides some empirical and theoretical investigations relative to improve their flexural capacity with FRP sheets are reviewed. The mechanical behaviour of strengthened walls subject to pure bending (without compression) has been established by several authors, but this is an unusual situation for real masonry. Neither mechanical behaviour of walls subject to bending and compression nor influence of axial load in the final capacity of the strengthened element are adequately studied. In regard to theoretical studies, the different proposals are based on reinforced concrete analytical methods and share common design principles. However, they present differences, especially, about three aspects: 1) the constitutive law of masonry, 2) the value of ultimate FRP strain and 3) the desirable failure mode that must be looked for. In spite of them, a good agreement between each experimental program and its theoretical study is often exhibited due to enormous disparity in considered test parameters. Each experimental program usually presents a characteristic failure mode and the proposed formulation results appropriate for this one. It seems necessary to develop a method for FRP strengthened walls subject to bending and compression enable for all failure modes (due to FRP or masonry). Some common damages in masonry subject to bending are explained in Section III.4. Examples of FRP strengthening to repair or prevent these damages are also written. Two small experimental programs are carried out in Eduardo Torroja Institute to improve the knowledge on this topic. The first one is concerned about the bond between FRP plates and damaged masonry (section IV.1) and the second one is related to the mechanical behaviour of the strengthened masonry specimens subject to out of plane bending combined with axial force (section IV.2). In the Chapter V some bond models to prevent the debonding at the FRP plate end are checked. It is confirmed that their predictions are so different. A pure-shear test database is compiled with results from the existing literature and others from the experimental program described in section IV.1. This database lets know which of the considered model is more suitable to design anchorage lengths of glued FRP to masonry. In the Chapter VI a method to check unreinforced masonry sections with external FRP strengthening subject to bending and compression to the ultimate limit state is proposed. This method is based on concrete reinforced one, but it is adapted to strengthened masonry. A bilinear constitutive law is used for masonry (according to CTE DB SE-F). Its simplicity helps to develop the model formulation and it has proven to be suitable to predict bending capacity either for FRP failures or masonry crushing. With regard to FRP, the design strain is limited. It is taken into account different aspects which cause the plate can���t reach its ultimate strength, like intermediate FRP debonding induced by opening cracking or environmental damage. A ���bond factor��� is proposed. It is obtained by means of an experimental bending test database that includes 68 results from the existing literature and from the experimental program described in section IV.2. The proposed formulation has also been checked with the help of bending database. The effects of the main parameters, like axial load, FRP design effective strain or FRP stiffness, on the bending capacity of the strengthened element are studied in Chapter VII. Finally, the main conclusions from the work carried out are summarized in Chapter VIII. Future lines of research to be explored are suggested as well.

Polypropylene/glass fiber hierarchical composites incorporating inorganic fullerene-like nanoparticles for advanced technological applications

Novel isotactic polypropylene (iPP)/glass fiber (GF) laminates reinforced with inorganic fullerene-like tungsten disulfide (IF-WS2) nanoparticles as environmentally friendly fillers have been successfully fabricated by simple melt-blending and fiber impregnation in a hot-press without the addition of any compatibilizer. The influence of IF-WS2 concentration on the morphology, viscosity. and thermal and mechanical behavior of the hierarchical composites has been investigated. Results revealed an unprecedented 62 ��C increase in the degradation temperature of iPP/GF upon addition of only 4.0 wt % IF-WS2. The coexistence of both micro- and nanoscale fillers resulted in synergistic effects on enhancing the stiffness, strength, crystallinity, thermal stability, glass transition (Tg) and heat distortion temperature (HDT) of the matrix. The approach used in this work is an efficient, versatile, scalable and economic strategy to improve the mechanical and thermal behavior of GF-reinforced thermoplastics with a view to extend their use in advanced technological applications. This new type of composite materials shows great potential to improve the efficiency and sustainability of many forms of transport.

Microstructure and mechanical properties at high temperature of SiC-matrix by electrophoretic deposition and polymer infiltration and pyrolysis process

The EFDA-ITER programme for materials wants to develop new structural materials for future nuclear magnetic fusion reactors. In this context, special attention must be paid in the development of new composite materials that could support the hard working conditions of the nuclear fusion reactors: high temperature, high stresses, and high radiation.

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 certificaci��n y desarrollo asociados con las tediosas campa��as experimentales, que incluyen un gran n��mero 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 contribuci��n a la aproximaci��n multiescala desarrollada en el Instituto IMDEA Materiales para predecir el comportamiento mec��nico de un laminado de material compuesto dadas las propiedades de la l��mina y la intercara. La mec��nica de da��o continuo (CDM) formula el da��o intralaminar a nivel constitutivo de material. El modelo de da��o intralaminar se combina con elementos cohesivos para representar da��o interlaminar. Se desarroll�� e implement�� un modelo de da��o continuo, y se aplic�� a configuraciones simples de ensayos en laminados: impactos de baja y alta velocidad, ensayos de tracci��n, tests a cortadura. El an��lisis del m��todo y la correlaci��n con experimentos sugiere que los m��todos 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 aproximaci��n discreta de elementos finitos enriqueciendo la cinem��tica para incluir discontinuidades embebidas: el m��todo extendido de los elementos finitos (X-FEM). Se adapt�� X-FEM para un esquema expl��cito de integraci��n temporal. El m��todo es capaz de representar cualitativamente los mecanismos de fallo detallados en laminados. Sin embargo, los resultados muestran inconsistencias en la formulaci��n que producen resultados cuantitativos err��neos. Por ��ltimo, se ha revisado el m��todo tradicional de X-FEM, y se ha desarrollado un nuevo m��todo para superar sus limitaciones: el m��todo cohesivo X-FEM estable. Las propiedades del nuevo m��todo se estudiaron en detalle, y se concluy�� que el m��todo es robusto para implementaci��n en c��digos expl��citos din��micos escalables, resultando una nueva herramienta ��til para la simulaci��n de da��o 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.

Optimizaci��n de la densidad de energ��a en vigas de material compuesto (PRF) sometidas a flexi��n pura

Las necesidades energ��ticas actuales requieren el desarrollo de tecnolog��as eficaces y eficientes en producci��n, transporte y distribuci��n de energ��a. Estas necesidades han impulsado nuevos desarrollos en el ��mbito energ��tico, entre los cuales se encuentran sistemas de almacenamiento de energ��a. El avance en ingenier��a de materiales permite pensar en la posibilidad de almacenamiento mediante deformaci��n el��stica de vigas. Concretamente se parte de un concepto de mecanismo acumulador de energ��a basado en la deformaci��n el��stica de resortes espirales a torsi��n. Dichos resortes se pueden considerar como elementos vigas sometidos a flexi��n pura y grandes deflexiones. Esta Tesis de centra en el dise��o y optimizaci��n de estos elementos con el fin de maximizar la densidad de energ��a que son capaces de absorber. El proceso de optimizaci��n comienza con la identificaci��n del factor cr��tico del que depende dicho proceso, en este caso de trata de la densidad de energ��a. Dicho factor depende de la geometr��a de la secci��n resistente y del material empleado en su construcci��n. En los ��ltimos a��os ha existido un gran desarrollo de los materiales compuestos de tipo polim��rico reforzados con fibras (PRF). Estos materiales est��n sustituyendo gradualmente a otros materiales, como los metales, debido principalmente a su excelente relaci��n entre propiedades mec��nicas y peso. Por otro lado, analizando las posibles geometr��as para la secci��n resistente, se observ�� que la m��s adecuada es una estructura tipo s��ndwich. Se implementa as�� un procedimiento de dise��o de vigas s��ndwich sometidas a flexi��n pura, con las pieles fabricadas en materiales compuestos tipo PRF y un n��cleo que debe garantizar el bajo peso de la estructura. Se desarrolla as�� un procedimiento sistem��tico que se puede particularizar dependiendo de los par��metros de entrada de la viga, y que tiene en cuenta y analiza la aparici��n de todos los posibles modos de fallo posibles. As�� mismo se desarrollan una serie de mapas o ��bacos de dise��o que permiten seleccionar r��pidamente las dimensiones preliminares de la viga. Finalmente se llevan a cabo ensayos que permiten, por un lado, validar el concepto del mecanismo acumulador de energ��a a trav��s del ensayo de un muelle con secci��n monol��tica, y por otro validar los distintos dise��os de vigas s��ndwich propuestos y mostrar el incremento de la densidad de energ��a con respecto a la alternativa monol��tica. Como l��neas futuras de investigaci��n se plantean la investigaci��n en nuevos materiales, como la utilizaci��n de nanotubos de carbono, y la optimizaci��n del mecanismo de absorci��n de energ��a; optimizando el mecanismo de absorci��n a flexi��n pura e implementando sistemas que permitan acumular energ��a mediante la deformaci��n el��stica debida a esfuerzos de tracci��n-compresi��n. ABSTRACT Energy supply requires the development of effective and efficient technologies for the production, transport and distribution of energy. In recent years, many energy storage systems have been developed. Advances in the field of materials engineering has allowed the development of new concepts as the energy storage by elastic deformation of beams. Particularly, in this Thesis an energy storage device based on the elastic deformation of torsional springs has been studied. These springs can be considered as beam elements subjected to pure bending loads and large deflections. This Thesis is focused on the design and optimization of these beam elements in order to maximize its density of stored energy. The optimization process starts with the identification of the critical factors for the elastic energy storage: the density. This factor depends on the geometry of the cross section of the beam and the materials from which it is made. In the last 20 years, major advances in the field of composite materials have been made, particularly in the field of fiber reinforced polymers (FRP). This type of material is substituting gradually metallic materials to their excellent weight-mechanical properties ratio. In the other side, several possible geometries are analyzed for its use in the cross section of the beam; it was concluded that the best option, for maximum energy density, is using a sandwich beam. A design procedure for sandwich beams with skins made up with FRP composites and a light weight core is developed. This procedure can be particularized for different input parameters and it analyzes all the possible failure modes. Abacus and failure mode maps have been developed in order to simplify the design process. Finally several tested was made. Firstly, a prototype of the energy storage system which uses a monolithic composite beam was tested in order to validate the concept of the energy storage by elastic deformation. After that sandwich beam samples are built and tested, validating the design and showing the increase of energy density with respect to the monolithic beam. As futures research lines the following are proposed: research in new materials, as carbon nanotubes; and the optimization of the energy storage mechanism, that means optimizing the pure bending storage mechanism and developing new ones based on traction-compression mechanisms.

Methodology based on FBG sensors for monitoring of FRP-strengthened beams

Advanced composite materials are increasingly used in the strengthening of reinforced concrete (RC) structures. The use of externally bonded strips made of fibre-reinforced plastics (FRP) as strengthening method has gained widespread acceptance in recent years since it has many advantages over the traditional techniques. However, unfortunately, this strengthening method is often associated with a brittle and sudden failure caused by some form of FRP bond failure, originated at the termination of the FRP material or at intermediate areas in the vicinity of flexural cracks in the RC beam. Up to date, little effort in the early prediction of the debonding in its initial instants even though this effect is not noticeable by simple visual observation. An early detection of this phenomenon might help in taking actions to prevent future catastrophes. Fibre-optic Bragg grating (FBG) sensors are able to measure strains locally with high resolution and accuracy. Furthermore, as their physical size is extremely small compared with other strain measuring components, it enables to be embedded at the concrete-FRP interface for determining the strain distribution without influencing the mechanical properties of the host materials. This paper shows the development of a debonding identification methodology based on strains experimentally measured. For, it a simplified model is implemented to simulate the behaviour of FRP-strengthened reinforced concrete beams. This model is taken as a basis to. develop an model updating procedure able to detect minor debonding at the concrete-FRP interface from experimental strains obtained by using FBG sensors embedded at the interface

Hierarchical and hybrid polymer nanocomposites based on carbon nanotubes and inorganic fullerene type nanoparticles

The influence of singlewalled carbon nanotubes (SWCNT) and inorganic fullerenelike tungsten disulfide nanoparticles (IFWS2) on the morphology and thermal, mechanical and electrical performance of multifunctional fibrereinforced polymer composites has been investigated. Significant improvements were observed in stiffness, strength and toughness in poly (ether ether ketone) (PEEK) / (SWCNT) / glass fibre (GF) laminates when a compatibilizer was used for wrapping the CNTs. Hybrid poly(phenylene sulphide) (PPS)/IFWS2/ carbon fibre (CF) reinforced polymer composites showed improved mechanical and tribological properties attributed to a synergetic effect between the IF nanoparticles and CF.