Orientador: Maria Helena Robert / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-20T13:19:51Z (GMT). No. of bitstreams: 1 BahrJunior_Robin_M.pdf: 7586609 bytes, checksum: 13070a11efc37db13eaf8c9c45ac912e (MD5) Previous issue date: 2012 / Resumo: Este trabalho teve como objetivo estudar o processo de produção de compósitos de baixa densidade por tixoinfiltração a partir de lingotes refinados pelo processo de incentivo à nucleação. A liga metálica AA7075 foi submetida a uma análise de tixohabilidade, na qual foi observada sua adequação ao processo e definidas as temperaturas de refino e de infiltração. Um dos focos do trabalho foi a investigação dos parâmetros "material do agente promovedor de nucleação", "profundidade de inserção desse agente no banho" e "temperatura de inserção". Para complementação do estudo, a molhabilidade da liga fundida sob o material do agente resfriador foi medida para a investigação da compatibilidade entre eles. A liga refinada foi então infiltrada em agregados de argila expandida, processo este em que foram variados os parâmetros de recalque. Após a análise da microestrutura, o comportamento térmico do compósito de baixa densidade foi medido através de ensaio de dilatometria, e a absorção de energia de impacto foi determinada em ensaio de compressão dinâmica em uma torre de Drop-Test. O comportamento de todos os materiais se mostrou próximo ao comportamento típico de materiais celulares, com grande platô de deformação plástica. No entanto as amostras apresentaram valores elevados de tensão, quando comparados aos materiais celulares existentes no mercado / Abstract: The main goal of this work was to study a production process of low density composites by means of thixoinfiltration from ingots refined by the process of nucleation promotion. The aluminum alloy AA7075 was subjected to a tixoability analysis, in which its suitability to the process was observed and its refining and infiltration temperatures were defined. One focus of the work was the investigation of the process parameters "nucleation agent material", "depth of the agent immersion into the liquid alloy" and "immersion temperature". To better understand the involved nucleation phenomena's, the contact angle between agent material and the liquid alloy was measured and their compatibility was discussed. The alloy was infiltrated amongst expanded clay particles and the backload was varied from one infiltration to another. After the analysis of the microstructure, the thermal behavior of the composite was observed by dilatometry, and the energy absorption (dynamic compression test) was measured by using a Drop-Test Tower. All samples have shown behavior similar to cellular materials under impact tests, with wide plateaus of plastic deformation, however with high values of compression stress / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

The work in this dissertation deals with the global structural response and local damage effects of a simply supported natural fibre composite sandwich beam subjected to a four-point bend. For the global structural response, we are investigating the flexural behaviour of the composite sandwich beam. We begin by using the principle of virtual work to derive the linear and nonlinear Timoshenko beam theory. Based on these theories, we then proceed to develop the respective finite element models and then implement the numerical algorithm in MATLAB. Comparing the numerical results with experimental results from the CSIR, the numerical model correctly and qualitatively recovers the underlying mechanics with some noted deviances which are explained at the end. The local damage effect of interest is delamination and we begin by reviewing delamination theory with more emphasis on the cohesive zone model. The cohesive zone model relates the traction at the interface to the relative displacement of the interface thereby creating a material model of the interface. We then carry out a cohesive zone model delamination case study in MSC.Marc and MSC.Mentat software packages. The delamination modelling is carried out purely as a numerical study as there are no experimental results to validate the numerical results.

Tensile properties of polyethylene fibres are shown to be very good in comparison to properties of other advanced composite reinforcing fibres. Nevertheless, the use of polyethylene fibres in polymeric matrix composites suffers due to a poor fibre to resin adhesion. However, its ballistic properties are excellent because of the poor adhesion and also fibre ductility. Applications involving structural use of polyethylene fibres are limited by, among others, the low compressive and shear strengths. These are affected strongly by the degree of adhesion. Improvements in bonding are expected to result in greater commercial appeal for the fibres as the property limitations are reduced. Ultra Violet radiation has been shown previously, in laboratory scale batch studies, to induce graft co-polymerization of monomers to polyethylene films. Improvements in wettability and adhesion result when the grafted polymer is compatible to the bonding medium. In this study the technique was adapted to bench scale, continuous fibre treatment, whereby the monomer was surface grafted to the polyethylene substrate. Acrylic acid monomer was used for this due to its relative safety, small molecular size, and high reactivity. Reaction initiation was provided by use of a benzophenone photosensitizer due to the stability of polyethylene to UV radiation. The reaction was performed by pre-coating the fibres with reactants, then exposure to UV radiation. Results of the continuous process for fibre treatment indicate that the monomer concentration and temperature of the preliminary soakings are key variables. Adhesion improvement was measured by single fibre pullout tests and interlaminar shear strength (ILSS) tests. Of these, the ILSS appeared to be more sensitive for judging small improvements. Tensile tests were used to judge property deterioration due to treatment, and flexural property tests gave a preliminary indication of material behavior. The ILSS showed marked improvement from 1.5 ksi for untreated material to over 5.2 ksi for the better treatments. A competing treatment, plasma, shows ILSS values around 3 ksi. The flexural test indicated that failure of UV-grafted polyethylene was in tension, whereas failure of plasma and untreated material was in compression. The study has proven successful in improving the adhesion of polyethylene fibres to an epoxy resin matrix. Commercial viability is currently being developed through decreased process residence times and irradiation exposures. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Master of Engineering

Graphite and graphite composite materials are of great importance in various applications; however, they have been widely used in nuclear applications. Primarily in nuclear applications such, as a moderator where its primary aim is to stop the fast neutrons to thermal neutron. The composite graphite (HTR-10) has potential applications as a moderator and other applications including in aerospace field. Structurally the composite shows stable hexagonal form of graphite and no traces of the unstable Rhombohedral patterns. Thermal conductivity indicates the same trends observed and known for nuclear graded graphite. The composite was made as a mixture of 64 wt% of natural graphite, 16 wt% of synthetic graphite binded together by 20 wt% of phenolic resin. The resinated graphite powder was uni-axially pressed by 19.5 MPa to form a disc shaped specimen. The disc was then cut and annealed to 1800 °C. The composite was further cut into two directions (parallel and perpendicular) to the pressing direction. For characterization the samples were cut into 2.5 x 2.5 x 10 mm3. There were exposed to proton irradiation for 3 and 4.5 hrs respectively and characterized both structurally and thermally. Through the study what we have observed was that as the composite is exposed to proton irradiation there is an improvement structurally. Thus, the D peak in the Raman spectroscopy has decreased substantially with the irradiated samples. XRD has indicated that there is no un-stable Rhombohedral phase pattern in both the pristine and the irradiated samples. However this was further confirmed with that thermal conductivity is also increasing with irradiation exposure. This is anomalous to irradiated graphite in which defects are supposedly induced. Looking into the electrical resistivity we have noted that pristine samples have higher resistivity as compared to the irradiated samples. Seebeck coefficient indicates that there is some form of structural perfection and the samples have a phonon drag dip at the known graphite temperature of 35 K. This has shown us there are no impurities induced by irradiation of the samples. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Physics / Unrestricted

No

The most promising strategy for reducing the cost of manufacturing polymer matrix composites while improving their reliability is the use of sensors during processing to permit control of the cure cycle based on measurements of the material's internal state. While sensors have been demonstrated that infer the material state indirectly through measurements of acoustic impedance, electrical impedance, or refractive index, sensors that directly measure parameters critical to composite manufacturing, such as resin rheology and resin hydrostatic pressure, would improve characterization of thermoset resins during cure. Here we describe the development of a multifunctional fiber optic sensor that may be embedded in a composite part during lay-up to monitor the state of the polymer matrix during processing. This sensor will output quantitative data which will indicate the viscoelasticity of the thermoset matrix resin. The same sensor will additionally function as a strain sensor following fabrication, capable of monitoring residual strains due to manufacturing or in-service internal strains. / Ph. D.

A detailed evaluation of the suitability of the Iosipescu specimen tested in the modified Wyoming fixture is presented. An experimental investigation using conventional strain gage instrumentation and moire interferometry is performed. A finite element analysis of the Iosipescu shear test for unidirectional and cross-ply composites is used to assess the uniformity of the shear stress field in the vicinity of the notch, and demonstrate the effect of the nonuniform stress field upon the strain gage measurements used for the determination of composite shear moduli. From the test results for graphite-epoxy laminates, it is shown that the proximity of the load introduction point to the test section greatly influences the individual gage readings for certain fiber orientations but the effect upon shear modulus measurement is relatively unimportant. A numerical study of the load contact effect shows the sensitivity of some fiber configurations to the specimen/fixture contact mechanism and may account for the variations in the measured shear moduli. A comparison of the strain gage readings from one surface of a specimen with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to eccentric loading which induced twisting and yielded spurious shear stress-strain curves. In the numerical analysis, it is shown that the Iosipescu specimens for different fiber orientations have to be modeled differently in order to closely approximate the true loading conditions. Correction factors are needed to allow for the non uniformity of the strain field and the use of the average shear stress in the shear modulus evaluation. The correction factors, which are determined for the region occupied by the strain gage rosette, are found to be dependent upon the material orthotropic ratio and the finite element models. Based upon the experimental and numerical results, recommendations for improving the reliability and accuracy of the shear modulus values are made, and the implications for shear strength measurement discussed. Further application of the Iosipescu shear test to woven fabric composites is presented. The limitations of the traditional strain gage instrumentation on the satin weave and high tow plain weave fabrics is discussed. Test result of a epoxy based aluminum particulate composite is also presented. A modification of the Iosipescu specimen is proposed and investigated experimentally and numerically. It is shown that the proposed new specimen design provides a more uniform shear stress field in the test section and greatly reduces the normal and shear stress concentrations in the vicinity of the notches. While the fabrication and the material cost of the proposed specimen is tremendously reduced, it is shown the accuracy of the shear modulus measurement is not sacrificed. / Ph. D.

A brief review of necessary fundamentals relative to composite materials and viscoelasticity is provided. Subsequently the accelerated characterization techniques of Time Temperature Superposition and Time Temperature Stress Superposition are treated in detail. An experimental procedure for applying the latter to composites is given along with results obtained on a particular T300/934 graphite/epoxy. The accelerated characterization predictions are found in good agreement with actual long term tests. A postcuring phenomenon is discussed that necessitates thermal conditioning of the specimen prior to testing. A closely related phenomenon of physical aging is described. The effect of each on the glass transition temperature and strength is discussed. Creep rupture results are provided for a variety of geometries and temperatures for T300/934 graphite/epoxy. The results are found to compare reasonably with a modified kinetic rate theory. / Ph. D.

A theoretical model is derived to study the point-wise stress variations which occur in the constituents of a hybrid 3-D short fiber composite subjected to arbitrary homogeneous loading conditions. The model includes the capability to analyze composites containing different types of fibers, different aspect ratios of fibers (as well as continuous fibers), and different fiber orientations. The composite’s stiffness tensor is developed by volume averaging the point-wise stress field in each constituent present in the material system. Validation of the model is accomplished by comparing predicted stiffness properties to experimental data and other accepted models presently available in the literature for PMC’s, MMC’s, and BMC’s. A derivation of a theoretical model describing the resulting point-wise stress redistribution which occurs in the matrix and the fibrous regions caused by fiber-fiber interaction at the ends of finite length fibers or fractured fibers is also presented. This theoretical development includes the significant dependence of stress redistribution on fiber volume fraction, constituent properties, and crack size. Therefore, its use is not limited to polymeric composites but is also applicable to metal matrix and ceramic matrix systems. The model is extended to include one of the first quantitative analyses of variable fiber spacing which occurs in virtually every composite manufactured. A novel fiber discount method is proposed to study multiple fiber fractures which are of extreme importance when attempting to predict tensile strength of fiber dominated composite laminates. A test methodology employing a macro-model composite with embedded strain gauges is presented which can be used to validate (or invalidate) micro-mechanical models currently being developed and used by the scientific community. Results obtained with the embedded resistance gauges and the embedded fiber optic strain sensors (FP-FOSS) are validated with classical test and analytical techniques. These techniques include model composites subjected to thermal effects and mechanical loading sequences. The ability to vary specific physical parameters in the experimental model, such as fiber aspect ratio, fiber volume fraction, interphase/interface, and constituent properties (i.e. model PMC’s and MMC’s), in a systematic fashion enables this technique to study various physical aspects present in actual composite systems. The capability to initiate a fiber fracture at a specific location and load level is demonstrated. It is revealed that significantly different strain concentration exists in PMC composites which contain different fiber volume fractions and crack sizes. By varying fiber spacing between neighbors, a study is initiated on composites containing eccentrically located fibers. These results demonstrate that an asymmetric stress state exists in composites containing variable fiber spacing and fiber fractures. The fact that multiple fiber fracture is achieved in a methodical fashion demonstrates the versatility of the model. These studies show that this experimental technique can model various physical phenomena which occur in actual composite systems. / Ph. D.