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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Estudo da formação da fase cristalina beta nos compósitos de polipropileno contendo anidrido maléico e carbono de cálcio. / Analysis of crystaline phase fraction and mechanical properties of Polypropylene / Calcium carbonate / PP-g-MA.

Rogério Massanori Sakahara 29 October 2012 (has links)
Este trabalho estuda a influência do carbonato de cálcio (CaCO3) nas propriedades mecânicas e na formação da fase cristalina beta do polipropileno (PP). Com o intuito de produzir amostras para o estudo, foi feita uma análise preliminar sobre o enxerto do anidrido maléico no polipropileno, porque este material graftizado (PP-g-MA) contribui significativamente em blendas e compósitos ao melhorar a adesão superficial entre o PP e o CaCO3. Foram estudados dois métodos de obtenção deste produto (PP-g-MA) utilizando-se peróxido orgânico e os produtos obtidos foram caracterizados e comparados. Apesar dos resultados das análises feitas por calorimetria diferencial exploratória (DSC), análise termogravimétrica (TGA), microscopia eletrônica de varredura (MEV) e espectroscopia de energia dispersiva (EDS) indicarem importantes diferenças entre os dois métodos, a análise por espectroscopia no infravermelho (FTIR) trouxe conclusões sobre a eficácia dos métodos de graftização. Duas séries de compósitos a base de PP contendo CaCO3 foram produzidos por mistura intensiva em fusão (misturador Drais), uma contendo PP-g-MA e a outra sem. Quatro tipos de CaCO3 foram utilizados, diâmetros de 0,9 µm, 2,5 µm e 3 µm, sendo que o CaCO3 0,9 µm apresentou-se com superfície tratada e não-tratada. A concentração de CaCO3 foi mantida em 5% e a de PP-g-MA em 5% quando presente. Os compósitos foram submetidos a testes de resistência à tração, módulo na flexão e resistência ao impacto em duas temperaturas. As amostras contendo menores tamanhos de partículas de CaCO3 e PP-g-MA apresentaram melhora sinérgica na resistência mecânica, em que aumentos da resistência a impacto e da resistência a flexão foram observados. A análise da fase cristalina beta nestas amostras foi feita utilizando-se DSC e difratometria de raios-x. Também foi analisada a influência da adesão superficial entre a carga e a matriz de PP, quanto maior a adesão superficial e menor o tamanho de partícula do CaCO3, maior a formação da fase cristalina beta, o que contribuiu para a sinergia entre todas as propriedades mecânicas avaliadas neste trabalho. / This study aimed at improving the comprehension of the influence of calcium carbonate (CaCO3) in the formation of the beta crystalline phase of polypropylene (PP), as well as the changes in the mechanical properties of this polymer. A preliminary analysis of the grafting of the maleic anhydride in the polypropylene was carried out in order to produce specimens for the study, owing to the fact that this grafted polypropylene (PP-g-MA) contributes substantially to change the polarity of the polymer and therefore, enhance the superficial adhesion between PP and CaCO3. Two grafting methods using organic peroxide were studied. The grafted copolymers were analyzed by DSC, TGA, SEM, EDS, and FTIR. Two series of PP composites containing CaCO3 were produced by intensive melt mixing (Drais mixer), one of them having MA-g-PP. Four types of CaCO3 were used, which diameters were 0.9 µm, 2.5 µm and 3 µm, though the CaCO3 0.9 µm was surface-treated and non-treated. The concentration of CaCO3 was maintained at 5% and PP-g-MA at 5 % also, when present. The composites were tested for tensile strength, flexural modulus and impact strength (at two temperatures). Samples containing smaller particle sized CaCO3 and PP-g-MA showed synergistic improvement in the mechanical strength, and increases in the impact resistance and flexural strength were observed. Analysis of the beta crystal phase in these samples was performed using DSC and x-ray diffractometry. The influence of superficial adhesion between CaCO3 and PP was also analyzed, higher concentration of the beta crystalline phase was observed for better surface adhesion and smaller CaCO3 particle size, which contributed to the synergy between all the mechanical properties evaluated in this work.
12

Polymer-Based Photoactive Surface for the Efficient Immobilization of Nanoparticles, Polymers, Graphene and Carbohydrates

Yuwen, Jing 01 January 2011 (has links)
This thesis focuses on developing a new photocoupling surface, base on polyallyamine (PAAm), to increase the efficiency of the photocoupling agent perfluorophenyl azide (PFPA) in the immobilization of nanoparticles, carbohydrates and graphene. Extensive studies have been carried out in our lab on the covalent immobilization of polymers and graphene using PFPA-functionalized surfaces. Here we show that PAAm-based PFPA surface can be used to efficiently immobilize not only graphene and polymers but also nanomaterials and small molecules. This was accomplished by first silanizing silicon wafers with PFPA-silane followed by attaching a thin film of PAAm by UV radiation. Treating the PAAm surface with N-hydroxysuccinimide-derivatized PFPA (PFPA-NHS) yielded the PAAm-PFPA surface. The functionalized surfaces were characterized by ellipsometry (layer thickness), contact angle (surface tension), and ATR-FTIR. The PAAm surface was further characterized by determining the density of amino groups on the surface. The PAAm-PFPA surfaces were subsequently used to covalently immobilize polymers, nanomaterials, carbohydrates and graphene by a simple procedure of coating the molecules or materials on the PAAm-PFPA surface followed by UV irradiation. The resulting surfaces were characterized using ellipsometry, AFM, optical microscopy. The attached carbohydrates were further evaluated using lectins, i.e., carbohydrate-binding proteins.
13

Micromachined flow sensors for velocity and pressure measurement

Song, Chao 27 August 2014 (has links)
This research focuses on developing sensors for properties of aerodynamic interest (i.e., flow and pressure) based on low-cost polymeric materials and simple fabrication processes. Such sensors can be fabricated in large arrays, covering the surface of airfoils typically used in unmanned vehicles, allowing for the detection of flow separation. This in turn potentially enables, through the use of closed-loop control, an expansion of the flight envelope of these vehicles. A key advance is compensation for the typically inferior performance of these low cost materials through both careful design as well as new readout methods that reduce drift, namely a readout methodology based on aeroelastic flutter. An all-polymer micromachined piezoresistive flow sensor is fabricated, based on a flexible polyimide substrate and an elastomeric piezoresistive composite material. The flow sensor comprises a cantilever that is extended into the embedding flow; flow-induced stress on the cantilever is sensed through the piezoresistive composite material. Increasing the sensitivity of the sensor is achieved by either utilizing a long single-cantilever beam or using a dual-cantilever beam supporting a flap extending into the flow. In the latter case, the sensor demonstrates increased sensitivity with a reduced cantilever length. The increase in sensitivity helps to reduce sensor drift, which in turn is further reduced by a new measurement method, the vibration amplitude measurement method. In this drift reduction measurement method, the flow-induced vibration amplitude of the sensor structure (i.e., the amplitude of the aeroelastic flutter induced by the flow), instead of the absolute value of cantilever deflection, is measured in order to find the flow rate. Measurement of this relative resistance change instead of the absolute resistance in the piezoresistor rejects common-mode drift and greatly reduces overall drift. Experimental results verify the expected drift reduction. Sensor drift is also reduced when the elastomeric piezoresistive material is replaced by a Pt thin film piezoresistor. Development of pressure sensors based on polymers proceeds by encapsulating a reference cavity within a multilayer polymer structure and forming capacitor plates on the polymeric membranes encapsulating the cavity. Measuring the capacitance change induced by changes in the embedding pressure (which cause changes in the positions of the bounding polymeric membranes) enables calculation of the pressure. The use of polymeric membranes requires understanding the leakage rate of gas into the reference cavity, which is a source of pressure drift. Developing a polymer-based pressure sensor that solves the problem of sensor drift as a result of gas permeation entails the fabrication of a silicon pressure reference cavity embedded in the polymer substrate, which results in a more hermetic and lower drift sensor while preserving the flexibility of the embedding polymer. Both wired and wireless versions of pressure and flow sensors of these types were developed and characterized. Further, the sensors were characterized on airfoils and their performance in a wind tunnel was determined.
14

Foamed Eva-bitumen Blends And Composites

Cankaya, Burhan Fuat 01 January 2009 (has links) (PDF)
The thermal conductivities of foamed polymer based materials are much lower thermal conductivity values than unfoamed polymeric materials. Especially, thermal conductivity values of foamed polymers with closed-cell structure decreases to 0.03 W/m.K. The reinforcement of foamed polymeric materials by mixing with bitumen lowers the raw material cost. The main objective of this study is to make a new thermal insulation material with low thermal conductance. In this study, the effects of concentration of calcium carbonate as inorganic filler and the effects of cross-linking on the properties foamed and unfoamed ethylene-vinyl acetate (EVA) copolymer based bituminous blends and composites were investigated. Applications such as thermal, mechanical characteristics of foamed and unfoamed EVA based bituminous composites were investigated. Foamed EVA based bituminous composites were prepared by using Brabender Plastic Coder, PLV 151. Mixing was made at 120 &ordm / C at 60 rpm for 15 minutes. The prepared blends were molded by a technique called Hand Lay-up Self-expanding Batch Molding (HLUSEBM) which was firstly applied by our group. The molding temperature was 170 &ordm / C at which chemical blowing agent and cross-linking agent decomposes. According to test results, at moderate chemical blowing agent and EVA content, the best closed-cell structure with high porosity and low thermal conductivity values were obtained. The compressive properties of foamed polymer based bituminous composites (FPBBCs) increase with increasing CBA and EVA content. With increasing calcium carbonate and EVA concentration, the porosity of FPBBCs increases but thermal conductivity of them decreases. On the other hand, with increasing filler content but with decreasing EVA concentration elastic modulus of FPBBCs increases but elastic recovery decreases.
15

Bioactive Surface Design Based On Conducting Polymers And Applications To Biosensors

Ekiz, Fulya 01 June 2012 (has links) (PDF)
ABSTRACT BIOACTIVE SURFACE DESIGN BASED ON CONDUCTING POLYMERS AND APPLICATIONS TO BIOSENSORS Ekiz, Fulya M. Sc., Department of Biotechnology Supervisor: Prof. Dr. Levent Toppare Co-Supervisor: Prof. Dr. Suna Timur June 2012, 88 pages An underlying idea of joining the recognition features of biological macromolecules to the sensitivity of electrochemical devices has brought the concept of biosensors as remarkable analytical tools for monitoring desired analytes in different technological areas. Over other methods, biosensors have some advantages including high selectivity, sensitivity, simplicity and this leads to solutions for some problems met in the measurement of some analytes. In this context, conducting polymers are excellent alternatives with their biocompatibility and ease of applicability for an efficient immobilization of biomolecules in preparing biosensors. Using several materials and arranging the surface properties of the electrodes, more efficient and seminal designs can be achieved. In this thesis, it is aimed to create new direct biosensors systems for the detection of several analytes such as glucose and pesticides thought to be harmful to the environment. Recently synthesized conducting polymers (polyTBT) / (poly(2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[ d][1,2,3]triazole) and (poly(TBT 6 -NH2 ) / poly(6-(4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazol-2-yl)hexan-1-amine) were utilized as a matrices for biomolecule immobilization. After successful electrochemical deposition the polymers on the graphite electrode surfaces, immobilization of glucose oxidase (GOx) and choline oxidase (ChO) were carried out. Amperometric measurements were recorded by monitoring oxygen consumption in the presence of substrates at -0.7 V. The optimized biosensors showed a very good linearity with rapid response times and low detection limits (LOD) to glucose and choline. Also, kinetic parameters, operational and storage stabilities were determined. Finally, designed biosensor systems were applied for glucose and pesticide detection in different media.
16

Polymer-based treatments to control runoff, leachate and erosion from engineered slopes at Simfer Mine, Guinea, Africa

Campbell, Stephanie January 2013 (has links)
It is necessary to understand the erodibility and hydrological response of mine-site slope forming materials (SFMs), because of increasing awareness of the environmental impacts of mining. Steep engineered slopes in high intensity rainfall environments present a serious erosion risk. Temporary surface stabilisers, such as polyacrylamides (PAMs) and polyvinylacrylic latex (PVALs) are potentially cost effective erosion control solutions. In this study PAM and PVAL efficacy to reduce runoff, leachate and erosion was assessed at two application rates, with and without gypsum on SFMs from an iron ore mine in Guinea (West Africa). NSPASS (near-surface photogrammetry assessment of slope forming materials’ surface roughness) is a novel method that integrates digital image capture and GIS. It is shown to detect and quantify surface micro-relief changes of 2-3 mm, not visible to the naked eye. As expected, soil and non-soil SFMs were significantly different in terms of their physical and chemical properties. Phase I of the study investigated the erodibility of ten SFMs, including soil, ore and waste-rock. The results indicate that the hydrological response to rainfall of most SFMs is to generate leachate. Weathered phyllite (PHY-WEA) is the most erodible SFM by both runoff and leachate. Multiple regression analysis demonstrated that magnetic susceptibility, mineralogy and dry aggregate distribution; parameters not commonly assessed in erosion studies, are important in explaining SFM erodibility and hydrological response. Phase II evaluated critically the effectiveness of three commercially available polymer solutions (two PAMs and one PVAL) at reducing runoff, leachate and erosion from four of the most erodible SFMs identified in Phase I. The results indicate that some PAM and PVAL treatments significantly reduce runoff, leachate and erosion. Polymer efficacy is highly dependent on the physical and chemical properties of the SFM, as well as the mechanism of polymer to SFM adsorption. Increasing the application rate of select treatments lowered leachate volumes, runoff and leachate total sediment loads. Contrary to previous studies, gypsum amendments did not significantly improve polymer efficiency. This research has added to our understanding of the erodibility and hydrological response of soil and non-soil SFMs. This is the first study to evaluate critically the efficacy of PVALs in controlling erosion from mine-site SFMs. Future studies should continue to optimise NSPASS performance in monitoring changes in surface micro-relief.
17

On the polymer-based nanocomposites for electrical switching applications

Doddapaneni, Venkatesh January 2017 (has links)
Recent research demonstrated that polymer based nanocomposites (PNCs) have been engineered in order to improve the arc interruption capability of the circuit breakers. PNCs are the combination of nano-sized inorganic nanoparticles (NPs) and polymers, opened up new developments in materials science and engineering applications. Inorganic NPs are selected based on their physical and chemical properties which could make multifunctional PNCs in order to interrupt the electrical arcs effectively. In particular, we presented the PNCs fabricated by using CuO, Fe3O4, ZnO and Au NPs in a poly (methyl methacrylate) (PMMA) matrix via in-situ polymerization method, recently developed method to avoid NPs agglomeration, leading to good spatial distribution in the polymer matrix. Thus, several samples with various wt% of NPs in PMMA matrix have been fabricated. These PNCs have been characterized in detail for the morphology of NPs, interaction between NPs and polymer matrix, and radiative/thermal energy absorption properties. In the next stage, PNCs are tested to determine their arc interruption performance and impact on the electrical arcs of current 1.6 kA generated using a specially designed test set-up. When PNCs interact with the electrical arcs, they generate ablation of chemical species towards core of the electrical arc, resulting in cooling-down the arc due to strong temperature and pressure gradient in the arc quenching domain. This thesis demonstrates for the first time that these engineered PNCs are easily processed, reproducible, and can be used to improve the arc interruption process in electrical switching applications. / Ny forskning har visat att polymerbaserade nanokompositer (PNCs) har utformats för att förbättra strömbrytares förmåga att undvika ljusbågar vid överslag. PNCs är en kombination av nanostora oorganiska nanopartiklar (NP) och polymerer, som har öppnat upp för ny utveckling inom materialvetenskap och tekniska tillämpningar. Oorganiska NP väljs baserat på deras fysikaliska och kemiska egenskaper som kan hjälpa PNCs att motverka elektriska ljusbågar effektivt. I synnerhet, presenterade vi PNCs tillverkade genom användning av CuO, Fe3O4, ZnO och Au NP i en poly (metylmetakrylat) (PMMA)-matris via in situ-polymerisationsmetod, nyligen utvecklad för att undvika NP-agglomerering, vilket leder till god rumslig fördelning i polymermatrisen. Därför har flera prover med olika vikt% av NP i PMMA-matris tillverkats. Dessa PNCs har utvärderats i detalj för NP-morfologi, interaktion mellan NP och polymermatris, och strålnings- och värmeenergiabsorption. I nästa skede testas PNCs för att bestämma deras förmåga att undvika ljusbågar och påverkan på de elektriska ljusbågarna av 1,6 kA strömstyrka, genererade med hjälp av en specialdesignad test-set-up. När PNCs interagerar med de elektriska ljusbågarna, genererar de ablation av kemiska ämnen mot kärnan i den elektriska ljusbågen, vilket resulterar i nedkylning av ljusbågen på grund av starka temperatur- och tryckgradienter i området. Denna avhandling visar för första gången att dessa konstruerade PNCs är lätta att framställa, reproducerbara, och kan användas för att förbättra avbrottsprocessen för ljusbågen i elektriska kopplingstillämpningar. / <p>QC 20170303</p>
18

Analyse et modélisation des mécanismes d'endommagement et de déformation en fatigue multiaxiale de matériaux composites : polyamide renforcé par des fibres courtes / Analysis and modeling of the damage and deformation mechanisms under multiaxial fatigue of thermoplastic composites : polyamide reinforced by short fibers

Despringre, Nicolas 17 December 2015 (has links)
Le présent travail de thèse se consacre au développement d'un nouveau modèle micromécanique pour les composites en thermoplastique renforcé par des fibres de verre courtes. L'objectif est notamment la modélisation du comportement visco-endommageable en fatigue du PA66-GF30. Ce matériau, particulièrement utilisé dans l'industrie automobile, est sujet à une microstructure spécifique issue du procédé de moulage par injection. L'approche multi-échelles développée consiste en une méthode de Mori-Tanaka modifiée, appliquée à des renforts avec enrobage et prenant en compte l'évolution de l'endommagement à l'échelle microscopique. La description des mécanismes d'endommagement se base sur une investigation expérimentale poussée préalablement menée au sein de l'équipe. Des scénarios d'endommagement ont été proposés et incluent trois processus locaux différents : la décohésion de l'interface, la microfissuration de la matrice et les ruptures de fibres. Ceux-ci sont spécialement affectés par la microstructure. L'approche développée intègre ces cinétiques d'endommagement ainsi que la viscoélasticité non-linéaire de la matrice et la distribution d'orientation des inclusions due au procédé de fabrication. Chaque mécanisme d'endommagement est modélisé par une loi d'évolution basée sur les contraintes locales calculées à l'échelle microscopique. La loi constitutive finale, à l'échelle du volume élémentaire représentatif, est implémentée dans une bibliothèque scientifique en C++, SMART+, et est conçue pour être compatible avec une analyse de structures par éléments finis. L'identification du modèle est réalisée par rétro-ingénierie, en tirant profit de résultats expérimentaux multi-échelles, dont notamment des tests in-situ au MEB ainsi qu'une analyse qualitative et quantitative par μCT. / The current work focuses on a new micromechanical high cycle fatigue visco-damage model for short glass fiber reinforced thermoplastic composites, namely: PA66/GF30. This material, extensively used for automotive applications, has a specific microstructure which is induced by the injection process. The multi-scale developed approach is a modified Mori-Tanaka method that includes coated reinforcements and the evolution of micro-scale damage processes. Their description is based on the experimental investigations of damage mechanisms previously performed by the team. Damage chronologies have been proposed involving three different local degradation processes: fiber-matrix interface debonding/coating degradation, matrix microcracking and fiber breakage. Their occurrence strongly depends on the microstructure. The developed model integrates these damage kinetics and accounts for the complex matrix viscoelasticity and the reinforcement orientation distributions induced by the process. Each damage mechanism is introduced through an evolution law involving local stress fields computed at the microscale. The developed constitutive law at the representative volume element scale is implemented into a C++ scientific library, SMART+, and is designed to work with Finite Element Methods. The model identification is performed via reverse engineering, taking advantage of the multiscale experimental results: in-situ SEM tests as well as quantitative and qualitative μCT investigations.
19

Development of a low-cost in-situ material characterization method and experimental studies of smart composite structures / Développement d'une méthode de caractérisation de matériaux in situ à faible coût et études expérimentales de structures composites intelligentes

Chen, Xianlong 12 March 2019 (has links)
Les structures composites intégrant des transducteurs piézoélectriques au cœur de la matière sont utilisées pour leur capacité à modifier leurs propriétés mécaniques en fonction de l’environnement, à contrôler leur intégrité structurale et à interagir avec l’homme ou avec d’autres structures.Ce travail se concentre sur les phases de conception préliminaire des structures composites intelligentes. Ces phases ne représentent que 5% du coût total d’un projet, mais conditionnent 80% du coût final du produit. Les principaux problèmes rencontrés lors de ces phases de conception préliminaire portent sur la détermination des propriétés matériau des transducteurs piézoélectriques et des matériaux composites utilisés, de l'influence de l'emplacement des transducteurs dans la structure ainsi que de l’influence du processus de fabrication, de la température et des endommagements sur le comportement final des structures composites intelligentes.Dans le processus de fabrication développé à l’Université de Technologies Belfort-Montbéliard (UTBM), l’élément-clé est un produit semi-fini appelé “soft layer”. Cette couche permet d’intégrer le réseau de transducteurs piézoélectriques au cœur de la structure composite. Le processus de fabrication de la “soft layer” ainsi que celui des structures intelligentes sont abordés dans cette thèse.Afin de trouver des solutions aux problèmes décrits ci-dessus, deux méthodes de caractérisation de composites intelligents ou adaptatifs sont présentées et utilisées : la méthode dite Resonalyser et la méthode du temps de vol. Après des études expérimentales et une comparaison des résultats obtenus, la méthode du temps de vol a été choisie comme méthode principale en raison de son faible coût de mise en œuvre et du fait qu’il s’agit d’une méthode de caractérisation in-situ. De plus, une nouvelle méthode appelée méthode CMB, basée sur la méthode du temps de vol a été développée afin de pouvoir facilement et rapidement extraire les constantes élastiques, en particulier le coefficient de Poisson.Des analyses expérimentales de sensibilité appliquées aux composites adaptatifs ont été effectuées.Premièrement, l’étude de l’influence de l’emplacement des transducteurs démontre qu’il est nécessaire de tenir compte de la position de la “soft layer” dans la modélisation du comportement de produit final. La position de cette couche dans l’épaisseur du produit a une influence notable sur les fréquences propres ainsi que les amplitudes modales de la structure. Cependant, l’ajout de la “soft layer” n’accroît pas le taux d’amortissement de la structure finale; et sa position dans l’épaisseur n’a aucune influence sur ce taux d’amortissement. La propagation des ondes de Lamb à l’intérieur du composite n’est pas impactée par le “soft layer”.Deuxièmement, l’étude de l’impact du processus de la fabrication nous renseigne sur l’influence notable des divers paramètres de réglage du processus de fabrication sur le comportement final de la structure composite intelligente.Troisièmement, l’étude de l’influence de la température sur des structures constituées de différents matériaux composites montre que le module de Young du produit final décroît quand la température augmente. Mais la diminution du module de Young en fonction de la température est différente selon les et les types de matériaux et les directions des fibres, en particulier pour les structures composites unidirectionnelles. De plus, cette étude montre également la sensibilité de la méthode du temps de vol vis-à-vis de la température. Ce dernier point est par ailleurs consolidé par la comparaison avec des résultats obtenus par une méthode de caractérisation ex-situ standard : l'analyse dynamique de la mécanique (DMA).Enfin, l'étude de l'impact des dommages mécaniques fournit une assez bonne référence pour les recherches futures. De cette façon, il est clair qu’une méthode de temps de vol peut être utilisée dans la surveillance de la santé structurale. / The composite structures embedding piezoelectric implants are developed due to their abilities of modifying mechanical properties according to the environment, of keeping their integrity, of interacting with human beings or with other structures.This study is focused on the preliminary design stages of smart composite structures, which represent only 5% of the total costs of a project, whereas 80% of the life cycle cost are set during the preliminary study phases. The top few problems during the preliminary design of smart composite structures are addressed in this work such as the determination of the material properties of the piezoelectric transducers and composite material used, the influence of transducers location, manufacturing process, temperature and damage on the behavior of the smart composite structures.Due to the manufacturing process developed at the Université de Technologie de Belfort-Montbéliard (UTBM), the most important element is a semi-finished product called “soft layer”. This special layer is used to embed the transducers system into the composite structures. The manufacturing process of “soft layer” as well as the smart composite structures are compiled in this report.In order to solve the problems described above, two characterization methods of composite material (Resonalyser method and Time-of-Flight method (T-o-F method)), are introduced and discussed. After experimental studies and comparing the results of these two methods, the T-o-F method is chosen as the main method for the following studies due to the fact that it is a low-cost and in-situ characterization method. Furthermore, a new method based on the T-o-F method is developed to easily and quickly extract the elastic constants, in particular the Poisson’s ratio.Experimental sensitivity analyses applied to the smart composite structures are performed with respect to the problems describes above. First of all, the study of the influence of transducers location demonstrates that the "soft layer” cannot be neglected to model the behavior of the final product. In particular, the through-the-thickness position has an influence on the eigenfrequencies and the modal amplitudes. However, the "soft layer” does not increase the overall damping ratio of the final structures and the through-the-thickness position of the "soft layer” has no influence on the damping ratios. The Lamb wave propagation inside the composite material is not impacted by the "soft layer”. Secondly, the study of the impact of manufacturing process demonstrates that the impact of variability of parameters due to the manufacturing process is very important on the final response of the structure. Thirdly, the study of the influence of temperature on different kinds of smart composite structures proves that when temperature increases, the Young’s modulus of the smart composites decreases. But the attenuation of Young’s modulus according to temperature is different along different fiber directions, especially for the unidirectional composite structures. Furthermore, in this study, the sensitivity of Time-of-Flight method with respect to temperature is well proved by comparing the results with a traditional method like Dynamic-Mechanical Analysis (DMA). Last but not least, the study of the impact of the mechanical damage gives a quite good reference for the future investigations. Along this way, it is possible to use a Time-of-Flight method in Structural Health Monitoring. In addition, some smart composite structures manufactured by the research team are given and their potential applications are discussed.
20

Simulation of interlaminar and intralaminar damage in polymer-based composites for aeronautical applications under impact loading

González Juan, Emilio Vicente 08 March 2011 (has links)
La aplicación de materiales compuestos de matriz polimérica reforzados mediante fibras largas (FRP, Fiber Reinforced Plastic), está en gradual crecimiento debido a las buenas propiedades específicas y a la flexibilidad en el diseño. Uno de los mayores consumidores es la industria aeroespacial, dado que la aplicación de estos materiales tiene claros beneficios económicos y medioambientales. Cuando los materiales compuestos se aplican en componentes estructurales, se inicia un programa de diseño donde se combinan ensayos reales y técnicas de análisis. El desarrollo de herramientas de análisis fiables que permiten comprender el comportamiento mecánico de la estructura, así como reemplazar muchos, pero no todos, los ensayos reales, es de claro interés. Susceptibilidad al daño debido a cargas de impacto fuera del plano es uno de los aspectos de más importancia que se tienen en cuenta durante el proceso de diseño de estructuras de material compuesto. La falta de conocimiento de los efectos del impacto en estas estructuras es un factor que limita el uso de estos materiales. Por lo tanto, el desarrollo de modelos de ensayo virtual mecánico para analizar la resistencia a impacto de una estructura es de gran interés, pero aún más, la predicción de la resistencia residual después del impacto. En este sentido, el presente trabajo abarca un amplio rango de análisis de eventos de impacto a baja velocidad en placas laminadas de material compuesto, monolíticas, planas, rectangulares, y con secuencias de apilamiento convencionales. Teniendo en cuenta que el principal objetivo del presente trabajo es la predicción de la resistencia residual a compresión, diferentes tareas se llevan a cabo para favorecer el adecuado análisis del problema. Los temas que se desarrollan son: la descripción analítica del impacto, el diseño y la realización de un plan de ensayos experimentales, la formulación e implementación de modelos constitutivos para la descripción del comportamiento del material, y el desarrollo de ensayos virtuales basados en modelos de elementos finitos en los que se usan los modelos constitutivos implementados. / The application of polymer-based composites reinforced by long fibers, called advanced Fiber Reinforced Plastic (FRP), is gradually increasing as a result of their good specific mechanical properties and increased flexibility of design. One of the largest consumers is the aerospace industry, since the application of these materials has clear economic and environmental benefits. When composites are to be used in structural components, a design development program is initiated, where a combination of testing and analysis techniques is typically performed. The development of reliable analysis tools that enable to understand the structure mechanical behavior, as well as to replace most, but not all, the real experimental tests, is of clear interest. Susceptibility to damage from concentrated out-of-plane impact forces is one of the major design concerns of structures made of advanced FRPs used in the aerospace industry. Lack of knowledge of the impact effects on these structures is a factor in limiting the use of composite materials. Therefore, the development of virtual mechanical testing models to analyze the impact damage resistance of a structure is of great interest, but even more, the prediction of the post-impact residual strength. In this sense, the present thesis covers a wide range of analysis of the low-velocity and large mass impact events on monolithic, flat, rectangular, polymer-based laminated composite plates with conventional stacking sequences. Keeping in mind that the main goal of this work is the prediction of the residual compressive strength of an impacted specimen coupon, a set of different tasks are performed in order to provide suitable tools to analyze the problem. Accordingly, the topics which are addressed in this thesis are: the analytical description of the impact, the design and the realization of an experimental test plan, the formulation and implementation of constitutive models for the description of the composite material behavior, and the assessment of the performance of virtual tests based on finite element models where the constitutive models are used.

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