A new method to fabricate continuous graphite fiber reinforced aluminum alloy composites has been developed and the tensile properties of the composites have been investigated. Composites with 601, 201 and 7178 alloy matrix containing up to 19 volume per cent of Thornel 50 graphite fiber were studied. These composites showed lower tensile strength values than the expected values from the "rule of mixture". A theoretical model is discussed in order to understand the tensile properties of these composites. In this mechanism, graphite fibers are thought to be broken continuously one after another at maximum loading point of ultimate tensile strength during the tensile test. A further attempt has been made to improve the tensile strength of these composites, based on the above theoretical work. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

La transmission acoustique de plaques et de coques est un problème présent dans de nombreuses industries, de l'automobile à l'aéronautique en passant par le génie civil. Il s'agit alors, en présence d'une source externe, de limiter le niveau de bruit à l'intérieur d'une cavité enclose par une structure vibrante, qu'il s'agisse d'une voiture, d'un avion ou d'un immeuble.Les matériaux composites sont de plus en plus largement utilisés par toutes les industries du transport, du fait de leur faible masse rapportée à leur grande rigidité. Ces caractéristiques, utiles pour réduire la consommation énergétique des véhicules, sont cependant un handicap lorsqu'il s'agit de limiter le niveau de bruit intérieur. Il est alors nécessaire d'ajouter des protections acoustiques. On s'intéresse dans ce travail à la transmission d'un bruit produit à l'extérieur de la cavité, et à des solutions antibruit passives, utilisant des matériaux poreux absorbants. Ceux-ci, mousses ou laines minérales, présentent en général de bonnes performances acoustiques pour des fréquences élevées, mais nettement mois intéressantes en basses fréquences. Ce travail s'inscrit dans le cadre d'un projet international ayant pour objectif d'étudier la réduction de bruit à travers des structures composites incluant des protections acoustiques passives. Deux axes y sont privilégiés, la modélisation et l'analyse de sensibilité. En effet, les matériaux poreux et composites présentent souvent de grandes incertitudes, dues au procédé de fabrication, à la difficulté de mesure, ou encore simplement à des choix de conception à faire. On cherche donc à s'assurer de la robustesse des solutions étudiées.Cette thèse est organisée en trois parties. La première traite de la modélisation des structures composites et des traitements poroélastiques. On s'intéresse surtout à deux types de structures, les plaques et les cylindres, pouvant dans les deux cas inclure un grand nombre de couches et de matériaux différents. La deuxième partie traite des méthodes d'analyse de sensibilité, et d'applications dans le cas de la transmission acoustique à travers des structures composites. On s'intéressera enfin dans la troisième partie à l'effet du traitement poroélastique, à travers des études numériques et expérimentales. / Acoustic transmission through plates and shells is a problem that appear in many applications, for example in the automotive or aerospace industries, or in civil engineering. The idea is to reduce the noise level inside a caivty enclosed by a vibrating structure, which may be a car, a plane or a building. Composite materials are widely used in the transportation industries due to their light weight and high mechanic resistance, but these features tend to increase the acoustic transparency of the structures, thus making it necessary to add acoustic protections for reducing noise in the enclosed cavity. The most frequently used acoustic protections take the form of layers of poroelastic materials, which are very efficient noise absorbers in high frequencies, but perform less efficiently in lower frequency ranges.This thesis is part of an international project aimed at improving the noise reduction performance of composite structures through passive sound packages. This implies the development of reduced models of acoustic transmission through such structures, and optimization studies on these models. Besides, the robustness of a solution with respect to uncertainties in the model have to be ensured. This thesis is organised in three parts, dealing with numerical modelling, sensitivity analysis for acoustic transmission and the effect of a porous treatment on sound transmission.

Low density polyethylene (LDPE) and Hytrel (a thermoplastic elastomer) were used as polymeric matrices in polymer + metal composites. The concentration of micrometric (Al, Ag and Ni) as well as nanometric particles (Al and Ag) was varied from 0 to 10 %. Composites were prepared by blending followed by injection molding. The resulting samples were analyzed by scanning electron microscopy (SEM) and focused ion beam (FIB) in order to determine their microstructure. Certain mechanical properties of the composites were also determined. Static and dynamic friction was measured. The scratch resistance of the specimens was determined. A study of the wear mechanisms in the samples was performed. The Al micro- and nanoparticles as well as Ni microparticles are well dispersed throughout the material while Ag micro and nanoparticles tend to form agglomerates. Generally the presence of microcomposites affects negatively the mechanical properties. For the nanoparticles, composites with a higher elastic modulus than that of the neat materials are achievable. For both micro- and nanocomposites it is feasible to lower the friction values with respective to the neat polymers. The addition of metal particles to polymers also improves the scratch resistance of the composites, particularly so for microcomposites. The inclusion of Ag and Ni particles causes an increase in the wear loss volume while Al can reduce the wear for both polymeric matrices.

Sustainability through replacement of non-renewable fibers with renewable fibers is an ecological need. Impact of transportation costs from South-east Asia on the life cycle analysis of the composite is detrimental. Kenaf is an easily grown crop in America. Farm based processing involves placing the harvested crop in rivers and ponds, where retting of the fibers from the plant (separation into fibers) can take 2 weeks or more. The objective of this thesis is to analyze industrially viable processes for generating fibers and examine their synergistic impact on mechanical performance, surface topography and chemistry for functional composites. Comparison has been made with commercial and conventional retting process, including alkali retting, enzymatic retting, retting in river and pond water (retting occurs by natural microbial population) with controlled microbial retting. The resulting kenaf fibers were characterized by dynamic mechanical analysis (DMA), Raman spectroscopy (FT-Raman), Fourier transform infrared spectroscopy (FT-IR), polarized optical microscopy (POM), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM) optical fluorescence microscopy, atomic force microscopy (AFM) and carbohydrate analysis. DMA results showed that pectinase and microbe treated fibers have superior viscoelastic properties compared to alkali retting. XPS, Raman, FT-IR and biochemical analysis indicated that the controlled microbial and pectinase retting was effective in removing pectin, hemicellulose and lignin. SEM, optical microscopy and AFM analysis showed the surface morphology and cross sectional architecture were preserved in pectinase retting. Experimental results showed that enzymatic retting at 48 hours and controlled microbial retting at 72 hours yield uniform and superior quality fibers compared to alkali and natural retting process. Controlled microbial retting is an inexpensive way to produce quality fibers for polymer composite reinforcement.

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Aluminum metal-matrix composites (MMCs) are well positioned to replace steel in numerous manufactured structural components, due to their high strength-to-weight and stiffness ratios. For example, research is currently being conducted in the use of such materials in the construction of tank entry doors, which are currently made of steel and are dangerously heavy for military personnel to lift and close. However, the manufacture of aluminum MMCs is inefficient in many cases due to the loss of material through edge cracking during the hot rolling process which is applied to reduce thick billets of as-cast material to usable sheets. In the current work, mechanical characterization and numerical modeling of as-cast aluminum A359-SiCp-30% is employed to determine the properties of the composite and identify their dependence on strain rate and temperature conditions. Tensile and torsion tests were performed at a variety of strain rates and temperatures. Data obtained from tensile tests were used to calibrate the parameters of a material model for the composite. The material model was implemented in the ANSYS finite element software suite, and simulations were performed to test the ability of the model to capture the mechanical response of the composite under simulated tension and torsion tests. A temperature- and strain rate-dependent damage model extended the constitutive model to capture the dependence of material failure on testing or service conditions. Several trends in the mechanical response were identified through analysis of the dependence of experimentally-obtained material properties on temperature and strain rate. The numerical model was found to adequately capture strain rate and temperature dependence of the stressstrain curves in most cases. Ductility modeling allowed prediction of stress and strain conditions iv which would lead to rupture, as well as identification of areas of a solid model which are most likely to fail under a given set of environmental and load conditions.

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