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No. of bitstreams: 2 Dissertação - Letícia Nunes de Almeida - 2016.pdf: 4956536 bytes, checksum: ff3b7edcf1cb8b45c40f2292cb032676 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2016-02-25 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The pre-heating and post-curing can improve the mechanical properties of composites, even though there be no studies of fiber reinforced composites. The aim of this study was to evaluate the effect of pre-heating and post-curing autoclave and microwave in flexural strength (FS), diametral tensile strength (DTS), knoop microhardness (KHN) and degree of conversion (DC) of a experimental fiber- reinfoced composite. The experimental material was prepared with 30% glass fibers (3 mm), 22.5% of the resin matrix (40/60 Bis-GMA / TEGDMA) and 47.5% barium silicate particles. Six experimental groups were created by the interaction between the factors under study: heating, on two levels (without heating and heating at 60°C) and post-curing in 3 levels (conventional curing without post-curing, autoclave (120°C for 15 minutes) and microwaves (540 W for 5 minutes) The groups were: F - curing at 1500 mW/cm2 for 40 seconds; F + M - curing and post- polymerization in microwave; F + A - curing and post-curing in an autoclave , AQ + F - the composite heating prior to curing, AQ + F + M - heating prior to curing and post-curing in microwave;. AQ + F + A - heating prior to curing and post-curing and autoclave heating was conducted digital oven for 5 minutes at 60°C. Ten samples of the RF dimensions 25 x 2 x 2 mm and DTS in dimensions of 3 x 6 mm were tested in a universal testing machine Instron 5965, 0.5 mm/min. the KHN test was performed on samples of 3 x 6 mm with a load of 50 g for 30 sec, totaling 50 indentations per group. GC was obtained by Spectroscopy Fourier Transform Infrared (FTIR) on 5 samples. Data were analyzed by a factorial 2x3 and general linear model ANOVA and Tukey tests (α = 0.05). Factor analysis showed significant interaction between the factors just for RTD (p = 0.0001); preheating was significant factor for RF (p = 0.0001), RTD (p = 0.020) and KHN (p = 0.0001); post-curing factor for KHN was significant (p = 0.0001). ANOVA and Tukey tests showed statistically significant differences between groups for DTS (p = 0.001: AQ + F ≥ AQ + F + M = F + A = AQ + F + A = F + M ≥ F), FS (p = 0.016: AQ + F + M ≥ AQ + F + A + F = AQ = AQ + A + M ≥ F ≥ F) and KHN (p = 0.0001: AQ + F + M ≥ AQ + A + F = F = F + A + M ≥ F ≥ M + AQ). GC results showed no statistically significant difference. Through the Pearson correlation coefficient was observed significant positive correlation between the GC and RTD (r = 0.473, p = 0.008) and between DTS and FS (r = 0.263, p = 0.042). The pre-heating and post- polymerization were shown to be favorable to promote better mechanical properties of fiber reinforced composite by studied, specific for each property being analyzed. / O aquecimento prévio à polimerização e a pós-polimerização podem melhorar propriedades mecânicas em compósitos, embora inexistam estudos em compósitos reforçados com fibra. O objetivo deste estudo foi avaliar o efeito do pré- aquecimento e da pós-polimerização em autoclave e microondas na resistência flexural (RF), resistência à tração diametral (RTD), microdureza knoop (KHN) e grau de conversão (GC) de um compósito experimental reforçado por fibra de vidro. O material experimental foi confeccionado com 30% de fibras de vidro (3 mm), 22,5% de matriz resinosa (40/60 Bis-GMA/TEGDMA) e 47,5% de partículas de silicato de bário. Seis grupos experimentais foram criados pela interação entre os fatores em estudo: aquecimento, em dois níveis (controle sem aquecimento e aquecimento a 60oC) e pós-polimerização, em 3 níveis (fotopolimerização convencional sem pós- polimerização, autoclave (120oC por 15 minutos) e microondas (540 W por 5 minutos). Os grupos foram: F - fotopolimerização à 1500 mW/cm2 por 40 segundos; F+M - fotopolimerização e pós- polimerização em microondas; F+A - fotopolimerizacão e pós-polimerização em autoclave; AQ+F - aquecimento do compósito previamente à fotopolimerização; AQ+F+M - aquecimento previamente à fotopolimerização e pós-polimerização em microondas; AQ+F+A - aquecimento previamente à fotopolimerização e pós-polimerização e autoclave. O aquecimento foi realizado em estufa digital por 5 minutos a 60oC. Dez amostras de RF nas dimensões de 25 x 2 x 2 mm e de RTD nas dimensões de 3 x 6 mm foram testadas em máquina de ensaio universal Instron 5965, a 0,5mm/min. O teste de KHN foi realizado em amostras de 3 x 6 mm com carga de 50 g por 30 s, totalizando 50 indentações por grupo. O GC foi obtido através de Espectroscopia de Infravermelho por Transformada de Fourrier (FTIR) em cinco amostras. Os dados foram analisados por um modelo linear geral fatorial 2x3 e testes de ANOVA e Tukey (α=0,05). A análise fatorial mostrou interação significativa entre os fatores apenas para RTD (p=0,0001); o fator pré-aquecimento foi significante para RF (p=0,0001), RTD (p=0,020) e KHN (p=0,0001); o fator pós-polimerização foi significante para KHN (p=0,0001). Os testes de ANOVA e Tukey mostraram diferença estatística entre os grupos para RTD (p = 0,001: AQ+F ≥ AQ+F+M = F+A = AQ+F+A = F+M ≥ F), RF (p = 0,016: AQ+F+M ≥ AQ+F+A = AQ+F = AQ+A ≥ F+M ≥ F) e KHN (p = 0,0001: AQ+F+M ≥ AQ+F+A = F+A= F+M ≥ AQ+F ≥ F). Os resultados de GC não apresentaram diferença estatisticamente significante. Através do coeficiente de correlação de Pearson foi possível observar correlação positiva significante entre o GC e RTD (r = 0,473, p = 0,008) e entre RTD e RF (r = 0,263, p = 0,042) . O pré- aquecimento e a pós-polimerização mostraram-se favoráveis para promover melhores propriedades mecânicas do compósito reforçado por fibra estudado, sendo específicos para cada propriedade analisada.

Os objetivos deste estudo foram: 1) avaliar a resistência à flexão em três pontos de um compósito para uso direto (Filtek Z350 XT, 3M ESPE) e um para uso indireto (Signum, Heraeus, Kulzer) reforçados por uma ou duas camadas de fibras de polietileno (Ribbond -THM, Ribbond®) ou de vidro (Interlig, Ângelus) tratados termicamente (170°C por 10 minutos), comparados com os grupos controle (não reforçados por fibras e/ou não tratados termicamente; 2) avaliar a sorção, a solubilidade e a estabilidade de cor dos compósitos reforçados, após armazenamento em água destilada à 37°C por 14 dias. A estabilidade de cor foi avaliada com auxílio de um espectrofotômetro de contato dental (Vita EasyShade, Vident, CA, USA). Para o ensaio de resistência à flexão, foram confeccionados espécimes retangulares com dimensões de 12 x 2 x 2mm (n=10), com os seguintes fatores de variação: a) compósito (para uso direto ou indireto); b) tipo e número de camadas de fibras (vidro ou polietileno/uma ou duas camadas); c) submetidos ou não a tratamento térmico. O tratamento térmico foi realizado 24 horas após fotoativação em estufa à temperatura de 170°C por 10 minutos. O ensaio foi realizado 24 horas após fotoativação ou tratamento térmico. Para avaliação de sorção/solubilidade e estabilidade de cor, foram confeccionados espécimes em forma de disco com dimensões de 15 x 2mm (n=5), em que foram analisados os seguinte fatores: a) compósito (para uso direto ou indireto); b) tipo de fibra (vidro ou polietileno); c) número de camadas de fibras (uma ou duas). Foi realizada análise dos parâmetros de cor antes e após imersão em água deionizada por 14 dias. Os resultados foram analisados por ANOVA e teste de contraste de Tukey, com nível de significância de 5% e revelaram que a fibra de vidro, quando utilizada em duas camadas, propiciou os maiores valores de resistência à flexão para os dois compósitos testados (165,4 MPa Z350XT e 208,7MPa Signum ). O tratamento térmico não apresentou significância estatística quanto à resistência à flexão do compósito direto. Para o compósito para uso indireto (Signum ) foi encontrada diferença estatisticamente significante para o fator tratamento térmico, que indicou valores de resistência à flexão inferiores para os grupos tratados termicamente. O compósito para uso direto apresentou valor de sorção superior (33,6/cm3) ao do compósito para uso indireto (19,1/cm3). Para solubilidade, foi encontrada interação para os fatores compósito e tipo de fibra, indicando maiores valores para o compósito para uso direto associado à fibra de vidro. A análise de alteração de cor demonstrou maior valor de E para a fibra de polietileno (E =1,5) quando comparado à fibra de vidro (E=1,0). Concluiu-se que: 1) a adição de fibras propicia aumento dos valores de resistência à flexão de compósitos para uso direto e indireto, o aumento da resistência foi observado quando do uso de duas camadas de fibras; 2) o tratamento térmico à 170°C por 10 minutos não indicou melhora nas propriedades mecânicas dos compósitos reforçados; 3) adicionar fibras aos compósitos não aumentou os valores de sorção/solubilidade quando imersos em água; 4) Imersão em água não produziu alterações de cor relevantes para os compósitos reforçados com fibras se comparados aos sem fibras. / The aims of this study were: 1) to evaluate the flexural strength of one composite for direct use (Filtek Z350 XT, 3M ESPE) and one for indirect use (Signum, Heraeus, Kulzer) as a function of the reinforcement by one or two layers of polyethylene (THM-Ribbond, Ribbond ®) or glass fibers (Interlig, Angelus) submitted to heat treatment (170°C for 10 minutes) compared with control groups (not reinforced by fibers and/or not heat-treated; 2) evaluate water sorption, solubility and color stability of the reinforced composites, after storage in distilled water at 37°C for 14 days. Color stability was evaluated using a spectrophotometer (Vita Easyshade, Vident, CA, USA). For three point flexural bending test, rectangular specimens were prepared with dimensions of 12 x 2 x 2 mm (n=10), according to the following variation factors: a) composite (for direct or indirect use); b) type of fibers and number of layers (glass or polyethylene/one or two layers; c) subjected or not to heat treatment. The heat treatment was performed 24 hours after curing, in a furnace, at 170 ° C for 10 minutes. Tests were performed 24 hours after curing or heat treatment. To evaluate the water sorption/solubility and color stability, disc-shaped specimens were prepared with dimensions of 15 x 2 mm (n=5) according to the following variation factors: a) composite (for direct or indirect uses); b) fiber type (glass or polyethylene); c) number of fiber layers (one or two). Color parameters were analyze before and after immersion in deionized water for 14 days. The results were analyzed by ANOVA and Tukeys test with significance level of 5%, and indicated that the glass fiber when used in two layers, showed the highest flexural strength for the two tested composites (165.4 MPa - Z350XT and 208.7 MPa - Signum ). The heat treatment did not significantly affect the flexural strength of the direct composite. For the composite for indirect use (Signum ), a statistical significance for the factor heat treatment was found, indicating lower values of flexural strength for heat-treated groups. The composite for direct use showed higher water sorption value (33.6 /cm3) when compared to the composite for indirect use (19.1 /cm3). For solubility, a significant interaction was found for composite and fiber type, indicating higher values for direct composite and glass fiber. Color stability analysis showed higher color difference value for polyethylene fiber (E =1.5) when compared to glass fiber (E=1.0). It was concluded that: a) adding fibers increased the flexural strength values of the composites for direct or indirect use, the increase in strength was more pronounced when using two fiber layers; 2) heat treatment at 170 ° C for 10 minutes showed no improvement of the mechanical properties of fiber reinforced composites; 3) adding fibers to the composite did not increase the sorption/solubility after water immersion, 4) Immersion in water did not change the color of the fiber reinforced composites when compared with those without fibers.

Composite materials show heterogeneity at different length scales. hence concurrent multiscale analysis is the only reliable method to analyze them. But unfortunately there is no concurrent multi-scale strategy that is efficient, and accurate while addressing all kinds of problems. This lack of reliability is partly because there is no micro-mechanical model which inherently keeps all relevent global information with it. This thesis tries to fill this gap. The presented micro-mechanical model not only homogenizes the micro-structure but also keeps the global information with it. Most of the micro-mechanical models in the literature extract the Representative Volume Element (RVE) from the continuum for analysis which results in loss of information and accuracy. In the present approach also, the RVE has been extracted from the continuum but with the major difference that all the macro/meso-scopic parameters are accounted for. Five macro/meso-scopic one dimensional parameters have been defined which completely define the effect of continuum. 11 for one dimensional stretch, _1 for torsion, __ (_ = 2, 3) for bending and _33 for uniform pressurization due to the presence of the continuum. Further, the above macro/meso-scopic parameters are proven, by the asymptotic, theory to be constant at a cross section but vary, in general, over the length of the fiber. Hence, the analysis is valid for any location and is not restricted to any local domain. Three major problems have been addressed: • Homogenization and analysis of RVE without any defects • Homogenization and analysis of RVE with fiber-matrix de-bonding • Homogenization and analysis of RVE with radial matrix cracking. Variational Asymptotic Method (VAM) has been used to solve the above mentioned problems analytically. The results have been compared against standard results in the literature and against 3D FEA. At the end, results for “Radial deformation due to torsion” problem will be presented which was solved “accidentally.”

<p>Naturally-occurring biological materials with stiff mineralized reinforcement embedded in a ductile matrix are commonly known to achieve excellent balance between stiffness, strength and ductility. Interestingly, nature offers a broad diversity of architectural motifs, exemplify the multitude of ways in which exceptional mechanical properties can be achieved. Such diversity is the source of bio-inspiration and its translation to synthetic material systems. In particular, the helicoid and the “brick and mortar” architectured materials are two key architectural motifs we are going to study and to synthesize new bio-inspired materials. </p> <p>Due to geometry mismatch(misorientation) and incompatibilities of mechanical properties between fiber and matrix materials, it is acknowledged that misoriented stiff fibers would rotate in compliant matrix beneath uniaxial deformation. However, the role of fiber reorientation inside the flexible matrix of helicoid composites on their mechanical behaviors have not yet been extensively investigated. In the present project, fiber reorientation values of single misoriented laminae, mono-balanced laminates and helicoid architectures under uniaxial tensile are calculated and compared. In the present work, we introduce a Discontinuous Fiber Helicoid (DFH) composite inspired by both the helicoid microstructure in the cuticle of mantis shrimp and the nacreous architecture of the red abalone shell. We employ 3D printed specimens, analytical models and finite element models to analyze and quantify in-plane fiber reorientation in helicoid architectures with different geometrical features. We also introduce additional architectures, i.e., single unidirectional lamina and mono-balanced architectures, for comparison purposes. Compared with associated mono-balanced architectures, helicoid architectures exhibit less fiber reorientation values and lower values of strain stiffening. The explanation for this difference is addressed in terms of the measured in-plane deformation, due to uniaxial tensile of the laminae, correlated to lamina misorientation with respect to the loading direction and lay-up sequence.</p> <p>In addition to fiber, rod-like, reinforced laminate, platelet reinforced composite materials, “brick and mortar” architectures, are going to be discussed as well, since it can provide in-plane isotropic behavior on elastic modulus that helicoid architecture can offer as well, but with different geometries of reinforcement. Previous “brick and mortar” models available in the literature have provided insightful information on how these structures promote certain mechanisms that lead to significant improvement in toughness without sacrificing strength. In this work, we present a detailed comparative analysis that looks at the three-dimensional geometries of the platelet-like and rod-like structures. However, most of these previous analyses have been focused on two-dimensional representations. We 3D print and test rod-like and tablet-like architectures and analyze the results employing a computational and analytical micromechanical model under a dimensional analysis framework. In particular, we focus on the stiffness, strength and toughness of the resulting structures. It is revealed that besides volume fraction and aspect ratio of reinforcement, the effective shear and tension area in the matrix governs the mechanical behavior as well. In turns, this leads to the conclusion that rod-like microstructures exhibit better performance than tablet-like microstructures when the architecture is subjected to uniaxial load. However, rod-like microstructures tend to be much weaker and brittle in the transverse direction. On the other hand, tablet-like architectures tend to be a much better choice for situations where biaxial load is expected.</p> <p>Through varying the geometry of reinforcement and changing the orientation of reinforcement, different architectural motifs can promote in-plane mechanical properties, such as strain stiffening under uniaxial tensile, strength and toughness under biaxial tensile loading. On the other hand, the various out-of-plane orientation of the reinforcement leads to functionally graded effective indentation stiffness. The external layer of nacre shell is composed of calcite prisms with graded orientation from surface to interior. This orientation gradient leads to functionally graded Young’s modulus, which is confirmed to have higher fracture resistance than homogenous materials under mode I fracture loading act.</p> <p>Similar as graded prism orientation in calcite layer of nacre, the helicoid architecture found in nature exhibits gradients on geometrical parameters as well. The pitch distance of helicoid architecture is found to be functionally graded through the thickness of biological materials, including the dactyl club of mantis shrimp and the fish scale of coelacanth. This can be partially explained by the long-term evolution and selection of living organisms to create high performance biological materials from limited physical, chemical and geometrical elements. This naturally “design” procedure can provide us a spectrum of design motifs on architectural materials. </p> <p>In the present work, linear gradient on pitch distance of helicoid architectures, denoted by functionally graded helicoid (FGH), is chose to be the initial pathway to understand the functionality of graded pitch distance, associated with changing pitch angle. Three-point bending on short beam and low-velocity impact tests are employed in FEA to analyze the mechanical properties of composite materials simultaneously. Both static(three-point bending) and dynamic(low-velocity impact) tests reveal that FGH with pitch angle increasing from surface to interior can provide multiple superior properties at the same time, such as peak load and toughness, while the helicoid architectures with constant pitch angle can only provide one competitive property at one time. Specifically, helicoid architectures with smaller pitch angle, such as 15-degree, show higher values on toughness, but less competitive peak load under static three-point bending loading condition, while helicoid architectures with middle pitch angle, larger than or equal to 22.5-degree and smaller than 45-degree, exhibit less value of toughness, but higher peak load. The explanation on this trend and the benefits of FGH is addressed by analyzing the transverse shear stresses distribution through the thickness in FEA, combined with analytical prediction. In low-velocity impact tests, the projected delamination area of helicoid architectures is observed to increase when the pitch angle is decreasing. Besides, laminates with specific pitch angles, such as 45-degree, classical quasi-isotropic laminate, 60-degree, specific angle ply, and 90-degree, cross-ply, are designed to compare with helicoid architectures and FGH.</p>

Nowadays, exceptional advantages of silk fibroin over synthetic and natural polymers have impelled the scientists to application of this biomaterial for tissue engineering purposes. Recently, we showed that embedding natural degummed silk fibers in regenerated Bombyx mori silk-based scaffold significantly increases the mechanical stiffness, while the porosity of the scaffolds remains the same. In the present study, we evaluated degradation rate, biocompatibility and regenerative properties of the regenerated 2% and 4% wt silk-based composite scaffolds with or without embedded natural degummed silk fibers within 90 days in both athymic nude and wild-type C57BL/6 mice through subcutaneous implantation. In all scaffolds, a suitable interconnected porous structure for cell penetration was seen under scanning electron microscopy. Compressive tests revealed a functional relationship between fiber reinforcement and compressive modulus. In addition, the fiber/fibroin composite scaffolds support cell attachment and proliferation. On days 30 to 90 after subcutaneous implantation, the retrieved tissues were examined via gross morphology, histopathology, immunofluorescence staining and reverse transcription-polymerase chain reaction as shown in Figure 1. Results showed that embedding the silk fibers within the matrix enhances the biodegradability of the matrix resulting in replacement of the composite scaffolds with the fresh connective tissue. Fortification of the composites with degummed fibers not only regulates the degradation profile but also increases the mechanical performance of the scaffolds. This report also confirmed that pore size and structure play an important role in the degradation rate. In conclusion, the findings of the present study narrate key role of additional surface area in improving in vitro and in vivo biological properties of the scaffolds and suggest the potential ability of these fabricated composite scaffolds for connective tissue regeneration.

<p dir="ltr">The demand and utilization of fiber-reinforced composites are increasing in various sectors, including aerospace, civil engineering, and automotive industries. Non-destructive methods are necessary for monitoring fiber-reinforced composites due to their complex and often visually undetectable failure modes. An emerging method for monitoring composite structures is through the integration of self-sensing capabilities. Self-sensing in nanocomposites can be achieved through nanofiller modifications, which involve introducing an adequate amount of nanofillers into the matrix, such as carbon nanotubes (CNTs) and carbon nanofillers (CNFs). These fillers form an electrically well-connected network that allows the electrical current to travel through conductive pathways. The disruption of connectivity of these pathways, caused by mechanical deformations or damages, results in a change in the overall conductivity of the material, thereby enabling intrinsic self-sensing.</p><p dir="ltr">Currently, the majority of predictive modeling attempts in the field of self-sensing nanocomposites have been dedicated to microscale piezoresistivity. There has been a lack of research conducted on the modeling of strain-induced resistivity changes in macroscale fiber-matrix material systems. As a matter of fact, no analytical macroscale model that addresses the impact of continuous fiber reinforcement in nanocomposites has been presented in the literature. This gap is significant because it is impossible to make meaningful structural condition predictions without models relating observed resistivity changes to the mechanical condition of the composite. Accordingly, this dissertation presents a set of three research contributions. The overall objective of these contributions is to address this knowledge gap by developing and validating an analytical model. In addition to advancing our theoretical understanding, this model provides a practical methodology for predicting the piezoresistive properties of continuous fiber-reinforced composites with integrated nanofillers.</p><p dir="ltr">To bridge the above-mentioned research gap, three scholarly contributions are presented in this dissertation. The first contribution proposes an analytical model that aims to predict the variations in resistivity within a material system comprising a nanofiller-modified polymer and continuous fiber reinforcement, specifically in response to axial strain. The fundamental principle underlying our methodology involves the novel use of the concentric cylindrical assembly (CCA) homogenization technique to model piezoresistivity. The initial step involves the establishment of a domain consisting of concentric cylinders that represent a continuous reinforcing fiber phase wrapped around by a nanofiller-modified matrix phase. Subsequently, the system undergoes homogenization to facilitate the prediction of changes in the axial and transverse resistivity of the concentric cylinder as a consequence of longitudinal deformations. The second contribution investigates the effect of radial deformations on piezoresistivity. Here, we demonstrate yet another novel application of the CCA homogenization technique to determine piezoresistivity. This contribution concludes by presenting closed-form analytical relations that describe changes in axial and transverse resistivity as functions of externally applied radial strain. The third contribution involves computationally analyzing piezoresistivity in fiber-reinforced laminae by using three-dimensional representative volume elements (RVE) with a CNF/epoxy matrix. By comparing the single-fiber-based analytical model with the computational model, we can investigate the impact of interactions between multiple adjacent fibers on the piezoresistive properties of the material. The study revealed that the differences between the single-fiber CCA analytical model and the computational model are quite small, particularly for composites with low- to moderate-fiber volume fractions that undergo relatively minor deformations. This means that the analytical methods herein derived can be used to make accurate predictions without resorting to much more laborious computational methods.</p><p dir="ltr">In summary, the impact of this dissertation work lies in the development of novel analytical closed-form nonlinear piezoresistive relations. These relations relate the electrical conductivity/resistivity changes induced by axial or lateral mechanical deformations in directions parallel and perpendicular to the reinforcing continuous fibers within fiber-reinforced nanocomposites and are validated against in-depth computational analyses. Therefore, these models provide an important and first-ever bridge between simply observing electrical changes in a self-sensing fiber-reinforced composite and relating such observations to the mechanical state of the material.</p>

Fiber-reinforced composites comprise an important class of lightweight materials which are finding increasing applications in engineering structures including body components of automobiles and aircraft. Traditionally, synthetic fibers made of glass, carbon, etc. along with a polymeric resin have constituted the most common composites. However, due to environmental concern, occupational health safety considerations, higher cost, etc., research has been focused on substituting synthetic fibers, especially glass fibers with safer, economic and biodegradable natural fibers. Due to the ease of availability and affordability in terms of cost, woven jute mats, among a wide variety of natural fiber-based reinforcements, offer a good choice in combination with a suitable resin such as polyester or epoxy for fabrication of composite laminates. In structural applications, joining of parts made of jute fiber-reinforced composites (JFRCs) would be a natural requirement. Alternatives to joining processes for metals such as welding, riveting, etc. are required for composites. A joining process of high potential is adhesive bonding which has the advantages of reducing stress concentration, permitting fastening of dissimilar materials, etc. In the present study, adhesively bonded joints of JFRCs and their mechanical behavior are investigated under quasi-static and cyclic loading conditions. Initially, characterization of substrates is carried out under monotonic loading. This is followed by determination of stress- Strain curves, failure load and mean shear strength of bonded joints as functions of joint curing temperature and overlap length using a two-part structural epoxy adhesive. All tests are carried out according to relevant ASTM standards. It has been observed that higher curing temperatures give rise to only marginally high failure load and mean shear stress at failure compared to curing at room temperature. For a given curing temperature, failure load increases while mean shear strength decreases with respect to overlap length in both types of joints. As fatigue failure is a crucial consideration in design, the behavior of adhesively bonded JFRC joints is studied for the first time under cyclic loading conditions leading to the commonly-used S-N curve for characterization of failure of materials at different loading-unloading cycles. Interestingly, the fatigue strength for infinite life of adhesively bonded JFRC joints turns out to be approximately 30% of the quasi-static strength, a correlation which usually applies to materials in general. The effect of joint overlap length on fatigue life is studied and it is observed that the above relation between fatigue and quasi static strength is retained for different overlap lengths. Additionally, insights are provided into failure modes of joints under different loading conditions and for varying overlap lengths. Various empirical predictors such as exponent, power and hybrid models fitting the S-N curve are obtained and their relative efficacy (in terms of Coefficient of Determination R2, Adjusted-R2, Akaike’s Information Criterion and Residual Sum of Squares) enumerated in prediction of failure load including quasi-static failure load. As numerical simulation is an indispensable tool in designing geometrically complex structures under nonlinear conditions including failure and contact, finite element modeling of JFRC substrates, bulk adhesive and adhesively bonded joints has been investigated using implicit and explicit LS-DYNA solvers. In this context, the effects of various modeling parameters (mesh size and loading rate) and details of constitutive models capable of capturing plasticity and failure in an orthotropic composite and isotropic adhesive are discussed. Mesh size has been found to be an important parameter affecting computed results. Finally, a good correlation within ~(4% - 7%) was found between the predicted and experimental results for JFRC substrates, bulk adhesive and adhesively bonded single lap joints.

Ce travail porte sur la simulation numérique et la modélisation du comportement thermo-mécanique des matériaux composites renforcés par des fibres. Spécifiquement les matériaux SMC (Sheet Moulding Compound) sont utilisés dans le processus de moulage par compression pour construire des pièces automobiles de haute performance. Ce travail est divisé en quatre chapitres, décrivant tout d’abord un modèle thermo-mécanique entièrement couplé pour les matériaux SMC standards et innovants à haute concentration en fibres (> 25% en volume). Le SMC est traité comme un mélange incompressible de fibre et de résine complété éventuellement par une phase de porosité compressible. Son anisotropie est modélisée au moyen de tenseurs structurels. La cinétique de réaction et de consolidation de la pièce est également modélisée et étudiée. Les données expérimentales mécaniques et thermiques enregistrées sur des échantillons de matériaux SMC sont comparées au modèle et à la solution numérique fournie par ce travail. D’un point de vue numérique, nous utilisons la méthode des domaines immergées o`u chaque phase est distinguée par une fonction distance signée. Nous décrivons le procédé de moulage par compression en proposant une résolution compressible anisotrope unifiée capable de décrire la transition compressible / incompressible du matériau SMC sous déformation. Cela permet de décrire la réponse mécanique du SMC et de prédire localement la consolidation (durcissement) de la pièce le long du cycle thermique. / This work deals with the numerical simulation and modeling of thermomechanical analysis of fiber reinforcedcomposites materials. Specifically for SMC (Sheet Molding Compound) materials that are used in compression molding processes to build automotive high performance parts. The work is divided into fourchapters, firstly describing a fully coupled thermo-mechanical model for standard SMC materials and for innovative SMC with high fiber concentration (> 25% in volume). The SMC is treated as an incompressible mixtureof fibers and paste complemented by a compressible porosity phase. Its anisotropy is modeled by means of structural tensors. Kinetic of reaction and consolidation of the part is also modeled and studied. Mechanicaland thermal experimental data recorded on samples of SMC materials are compared to the model and numerical solution provided in this work. A numerical framework, we use the immersed boundary method and the level set method. We describe the compression molding process by proposing an unified anisotropic compressible resolution able to describe the transition between compressible/ incompressible of SMC materials under deformation. We are able to describe the mechanical response of the SMC and to predict locally the consolidation (curing) of thepart throughout the thermal cycle.

Ce travail propose une optimisation de la conductivité électrique transverse des composites structuraux matrice/fibres de carbone. L'influence de la fonctionnalisation électrique de l'ensimage sur la conductivité des composites est particulièrement étudiée. Des feuillets submicroniques d'argent (AgNpts) ont été élaborés en présence de citrate de sodium (TSC). Leur morphologie plane est particulièrement adaptée à une dispersion dans un ensimage. Les feuillets ont été dispersés dans une matrice modèle. Le seuil de percolation électrique des feuillets est déterminé à 5,9 %. Cette valeur est cohérente avec un facteur de forme modéré, compris entre 12 et 28. L'ensimage fonctionnalisé a ensuite été déposé sur une mèche de fibres de carbone, elle-même imprégnée par une matrice PAEK hautes performances. Avant imprégnation des fibres de carbone, des fils submicroniques d'argent ont été introduits dans la matrice PAEK. Des composites matrice-fils submicroniques d'argent/fibres de carbone avec ensimage conducteur ont été mis en œuvre. Leur conductivité électrique est élevée (7 S.m-1), alors que la fraction volumique en particules d'argent (fils et feuillets) est inférieure à 1 % en volume. / This PhD thesis deals with the optimization of transverse electrical conductivity of Thermoplastic Carbon Fiber Reinforced Polymer. The influence of an electrically conductive sizing has been investigated. Silver nanoplates (AgNpts) have been successfully synthesized by a soft chemical reduction, with trisodiumcitrate (TSC) as surfactant. Silver nanoplates have been dispersed into a model matrix, percolation threshold has been determined near 5.9 % in volume fraction. This value is consistent with their moderate aspect ratio (between 12 and 28). Size and morphology of silver nanoplates are suitable for their dispersion in the sizing. Carbon fiber has been coated with conductive sizing. Carbon fiber will be further impregnated by a PAEK thermoplastic matrix. A higher conductivity level has been achieved by introducing silver nanowires in the PAEK matrix. Structural composites consisting of matrix-silver nanowires / continuous carbon fiber sized with conductive sizing have been elaborated. Their electrical conductivity reached 7 S.m-1 for a total silver volume fraction of 1 %.vol.

The diploma thesis is focused on 3D printing of composites parts. Thesis deals with methods of additive manufacturing and describes principle of selected 3D printing technologies. In addition to additive technologies, the theoretical part presents an overview of composite materials and methods of composite production. Then there are four concepts that lead to 3D printing of continuous fiber reinforced composites components. The final concept is selected according to multi-criteria analysis and then designed. Designed extruder allows 3D printing of composite materials. The principle of the function consists of fiber impregnation by matrix inside the heat chamber and then deposition of composite on printing platform. The extruder also consist of fiber cutting mechanism. The extruder's ability is verified by the experiment.