Dissertations / Theses on the topic 'Discontinuous fiber composites'
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Kunc, Vlastimil. "Structure-property relationships in flow formed discontinuous fiber reinforced composites." Diss., Virginia Tech, 2013. http://hdl.handle.net/10919/52934.
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This dissertation presents a new method for obtaining fully anisotropic stiffness tensor for materials containing discontinuous curverd fibers. It is demonstrated that the definition of fiber configuration and configuration averaging allow us to obtain better match with experimental results when compared to theory relying on the assumption of straight fibers. The experimental results are obtained using novel X-ray micro-tomography setup allowing observation of material microstructure under load.Ph. D.
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Dibelka, Jessica Anne. "Mechanics of Hybrid Metal Matrix Composites." Diss., Virginia Tech, 2013. http://hdl.handle.net/10919/50579.
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The appeal of hybrid composites is the ability to create materials with properties which normally do not coexist such as high specific strength, stiffness, and toughness. One possible application for hybrid composites is as backplate materials in layered armor. Fiber reinforced composites have been used as backplate materials due to their potential to absorb more energy than monolithic materials at similar to lower weights through microfragmentation of the fiber, matrix, and fiber-matrix interface. Composite backplates are traditionally constructed from graphite or glass fiber reinforced epoxy composites. However, continuous alumina fiber-reinforced aluminum metal matrix composites (MMCs) have superior specific transverse and specific shear properties than epoxy composites. Unlike the epoxy composites, MMCs have the ability to absorb additional energy through plastic deformation of the metal matrix. Although, these enhanced properties may make continuous alumina reinforced MMCs advantageous for use as backplate materials, they still exhibit a low failure strain and therefore have low toughness. One possible solution to improve their energy absorption capabilities while maintaining the high specific stiffness and strength properties of continuous reinforced MMCs is through hybridization. To increase the strain to failure and energy absorption capability of a continuous alumina reinforced Nextel" MMC, it is laminated with a high failure strain Saffil® discontinuous alumina fiber layer. Uniaxial tensile testing of hybrid composites with varying Nextel" to Saffil® reinforcement ratios resulted in composites with non-catastrophic tensile failures and an increased strain to failure than the single reinforcement Nextel" MMC. The tensile behavior of six hybrid continuous and discontinuous alumina fiber reinforced MMCs are reported, as well as a description of the mechanics behind their unique behavior. Additionally, a study on the effects of fiber damage induced during processing is performed to obtain accurate as-processed fiber properties and improve single reinforced laminate strength predictions. A stochastic damage evolution model is used to predict failure of the continuous Nextel" fabric composite which is then applied to a finite element model to predict the progressive failure of two of the hybrid laminates.Ph. D.
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Lu, Yunkai. "Mechanical Properties of Random Discontinuous Fiber Composites Manufactured from Wetlay Process." Thesis, Virginia Tech, 2002. http://hdl.handle.net/10919/34503.
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The random discontinuous fiber composite has uniform properties in all directions. The wetlay process is an efficient method to manufacture random discontinuous thermoplastic preform sheets that can be molded into random composite plaques in the hot-press. Investigations were done on the molding parameters that included the set-point mold pressure, set-point mold temperature and cooling methods. The fibers used in the study included glass and carbon fiber. Polypropylene (PP) and Polyethylene Terephthalate (PET) were used as the matrix.
Glass/PP and Glass/PET plaques that had fiber volume fractions ranging from 0.05 to 0.50 at an increment of 0.05 were molded. Both tensile and flexural tests were conducted. The test results showed a common pattern, i.e., the modulus and strength of the composite increased with the fiber volume fraction to a maximum and then started to descend. The test results were analyzed to find out the optimal fiber volume fraction that yielded the maximum modulus or strength. Carbon/PET composites plaques were also molded to compare their properties with Glass/PET composite at similar fiber volume fractions. Micrographs were taken of selected specimens to examine the internal structure of the material.
Existing micromechanics models that predict the tensile modulus or strength of random fiber composites were examined. Predictions from some of the models were compared with test data.Master of Science
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Suwatnodom, Prechaporn. "3-D micromechanical damage models, fiber pullout models and fracture toughness of discontinuous steel fiber reinforced cementitious composites." Diss., Restricted to subscribing institutions, 2008. http://proquest.umi.com/pqdweb?did=1562125051&sid=1&Fmt=2&clientId=1564&RQT=309&VName=PQD.
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Harper, Lee Thomas. "Discontinuous carbon fibre composites for automotive applications." Thesis, University of Nottingham, 2006. http://eprints.nottingham.ac.uk/10246/.
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Increasingly stringent emissions targets are encouraging vehicle manufacturers to prioritise reduction of vehicle mass. The falling cost of carbon fibre is increasing the viability of lightweight carbon-based body panel systems across a broad range of production volumes. In the present work an automated process has been developed for the manufacture of random fibre preforms at medium volume production levels (30-50,000ppa). This thesis seeks to understand the influence of key microstructural parameters on the mechanical and physical properties of carbon fibre laminates produced by directed fibre preforming. The principal parameters studied are fibre length, tow filament count and laminate thickness. A statistical process simulation has been developed to predict preform density variation and the results are compared with experimental tensile properties. Experimental studies have shown that there is a notable reduction in areal density variation and consequently an increase in tensile properties with shorter fibres (115mm to 6mm) and thicker laminates (1.5mm to 4mm for a constant volume fraction). Shorter lengths improved preform coverage and gave higher tensile strength, whilst thicker laminates reduced the presence of unreinforced areas which cause stress concentrations. Tow filamentisation has been induced by pneumatic means to reduce the mean filament count and maximise the mechanical performance when using inexpensive, 24K bundles. By maximising the level of filamentisation both stiffness and strength can be increased by 20% and 45% respectively. An analytical stiffness model is presented to predict the effect of tow filament count on the in-plane elastic constants. Filament count and out-of-plane fibre orientation distributions are determined from optical microscopy and are incorporated into a multi-level Mori-Tanaka based model. Predictions are within 8% of the experimental data for laminates containing large fibre bundles and 10% for laminates with highly filamentised bundles. An expression for critical bundle length has been developed for more accurate strength prediction, based on the number of filaments within the bundle. Experimental results confirm that the critical tow length is proportional to the tow filament count. Directed fibre preforming has been benchmarked against other competing processes in respect of mechanical properties, weight saving potential and cost. A full-scale demonstrator component has been manufactured using a variety of carbon composite solutions, which can all provide 40 to 50% weight saving for an equivalent bending stiffness to steel and greatly improved dent resistance. Directed fibre preforming has shown great promise for both semi-structural and structural components for medium volume applications, particularly when aligned fibres are introduced. The results from this work can be directly scaled for industrial application to provide a cost effective, lightweight alternative to steel.
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Qian, Connie Cheng. "Structural optimisation of discontinuous carbon fibre composites." Thesis, University of Nottingham, 2014. http://eprints.nottingham.ac.uk/14542/.
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There has been a growing interest in using discontinuous carbon fibre composites for semi-structural applications within the automotive industry. The main advantages of discontinuous fibres are low material costs, low wastage and low touch labour compared with processes using carbon fibre textiles. Directed Carbon Fibre Preforming (DCFP) is an automated process for producing complex 3D preforms for liquid moulding. DCFP offers the potential for producing highly optimised structures, with local control over tow size, fibre length and volume fraction within the component. The execution of this is challenging however, as confidence in the current library of material properties is low and existing structural optimisation packages only consider a very limited number of design variables, which are restricted to more conventional composite materials. This thesis aims to establish a structural design tool to exploit the design freedom offered by the DCFP process. A large number of parameters associated with the fibre architecture can be controlled to meet a range of design criterions such as performance, weight and cost. The optimisation tool is capable of generating locally varied fibre areal mass and thickness maps that are suitable for manufacture by the robot controlled process. The developed model adopts a multi-scaled finite element approach. Meso-scale simulations are performed to establish size effects in discontinuous fibre composites, to quantify the level of stochastic variability and to determine the representative volume element for a given fibre architecture. A DCFP material database is generated to facilitate macro-scale modelling at the component level. The macro-scale model iteratively redistributes material in order to minimise the total strain energy of the model under prescribed loading conditions. The optimised model is segmented into areas of uniform areal mass, where the zone geometries are tailored to achieve representative material properties according to the meso-scale results, whilst ensuring the design is fit for manufacture. An automotive spare wheel well has been chosen as a demonstrator component, enabling two DCFP architectures to be compared against a continuous glass/carbon fibre NCF design. The first case offers a high performance (high specific stiffness) solution and the second offers a low cost option using high filament count tows. Following optimisation, results suggest that a 3K 25mm fibre length DCFP option can achieve a specific stiffness 52% higher than the glass/carbon baseline design, but for 1.33 times higher material cost. Alternatively, the specific stiffness of a 24K 50mm fibre length DCFP is marginally lower than the first option, but still out-performs the baseline for just 67% of the material cost. The structural optimisation method demonstrates that discontinuous fibre composites can compete against continuous fibre counterparts for semi-structural applications.
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Bond, Michael David. "Multi-scale modelling of discontinuous carbon fibre reinforced composites." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/28879/.
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Discontinuous carbon fibre composites are becoming increasingly popular in the automotive and aerospace sectors, as an alternative to textile-based fibre reinforced composites for both semi-structural and structural components. Materials are highly heterogeneous, with the random architecture leading to uncertainties when modelling and predicting mechanical performance. The microscopic characteristics are known to dominate the strength of the composite, which need to be correctly represented to improve mechanical property predictions at the macroscale. This thesis presents a multi-scale modelling approach that captures the effects of microstructural (filament level) parameters at the macro scale (component level) to predict the mechanical properties of discontinuous composite materials. In the present work, a continuum damage approach has been used to initiate and monitor failure in the models at all scales, via a user defined material (UMAT), allowing strength predictions to be made for the discontinuous material within the ABAQUS solver. Experimental testing of the material constituents (fibre bundle and matrix materials) has been performed to provide input data for the finite element analyses. Micromechanical models have been developed to calculate the properties of fibre bundles, which are used directly at the meso and macroscale. Debonding criterion has also been established and validated which has been used to demonstrate that a small interface, with a thickness of only 1% of the fibre radius, can strongly influence the stress transfer between fibre and matrix materials. Interactions between multiple fibre bundles have been considered at the mesoscale, at a range of bundle orientations and separation distances. As the separation distance between the fibre bundles decreased there was an increase in stiffness 0 f the unit cell (~1.9%) across the bundle orientations considered, however, this also coincided with greater stress concentrations (up to 9.6%) being found in the bundles aligned to the direction of loading. These stress concentrations have been used to produce a comprehensive stiffness reduction scheme at the macro scale to account for the 3D nature of the bundle interactions. A 2D macro scale model is presented for generating discontinuous random fibre architectures consisting of high filament count bundles, with interfacial debonding permitted between the bundle and matrix materials. The fibre bundles are deposited randomly in a 2D plane to provide a representative material. The model has shown that the interface between the bundle and matrix material is critical at short fibre bundle lengths (~5mm) when determining the mechanical properties of the material, with reductions in strength of up to 40% observed at low interfacial shear strengths. The results from the macro scale analysis for discontinuous materials provide predictions within ~10% for tensile stiffness and ~18% for tensile strength when compared with experimental validations.
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Lopez, Delphine. "Comportement d’un thermoplastique renforcé de fibres de verre soumis à des chargements thermo-mécaniques." Thesis, Lorient, 2018. http://www.theses.fr/2018LORIS488/document.
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Les composites à matrice polymère sont de plus en plus utilisés dans le secteur automobile. Afin de remplir les conditions exigeantes du cahier des charges vis-à-vis des conditions de mise en service, les pièces en composite doivent maintenir leur forme géométrique sous des conditions thermo-mécaniques parfois extrêmes. Par exemple, un assemblage de hayon composite est soumis à des contraintes mécaniques élevées associées à des variations de température importantes lors des essais de validation du cahier des charges. Les enjeux de la thèse sont axés sur l’aide à la conception dans le domaine quasi-statique de pièces industrielles injectées en thermoplastique renforcé de fibres discontinues. L’amélioration des outils numériques doit permettre un dimensionnement virtuel optimal de ces pièces en anticipant les variations rencontrées en service et les distorsions résiduelles résultantes de chargements thermo-mécaniques. Cette démarche s’appuie sur la connaissance du comportement thermo-mécanique du matériau de l’étude, celui du renfort de hayon, un polypropylène renforcé à 40% en masse de fibres de verre discontinues, et sur la modélisation du comportement de ce type de matériauDiscontinuous fibers reinforced thermoplastic materials have been widely used for several years in the automotive industry. These parts must resist demanding service life conditions and must meet thermo- mechanical specifications. Indeed, structural automotive spare parts have to endure high temperatures, like a few tens of degrees Celsius, for a long duration, at least a few hours. As an example, a structural part of tailgate is subject to high mechanical loading, associated to strong temperature variations, during the validation test, regarding specifications. The purpose of this work is to improve the design of complex industrial parts, like the tailgate in quasi-static domain, by relying on numerical simulation. One of the challenges related to the use of such material, is to have a reliable virtual design of industrial parts by predicting the geometrical variations during service life conditions, and residual strain. Therefore, it is necessary to characterize and to model the thermo-mechanical behavior of the tailgate material, a polypropylene matrix reinforced with discontinuous glass fibers, with a given mass fraction of 40%
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Poumadère, Thomas. "Etude du couplage procédé/propriétés d’un matériau à fibres discontinues de carbone et à matrice époxy mis en oeuvre par un procédé innovant d’injection/transfert." Thesis, Toulouse, ISAE, 2013. http://www.theses.fr/2013ESAE0003.
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Les matériaux composites sont largement utilisés dans l'aéronautique où leurs hautes performances mécaniques combinées à leur légèreté leur permettent de concurrencer les matériaux métalliques. Cependant il est aujourd'hui difficile de fabriquer en série des pièces structurales ayant des formes tridimensionnelles complexes,Si les procédés d'injection de thermoplastiques chargés de fibres courtes (100 à 1 mm) sont bien connus, les études sur l'injection des thermodurcissables à fibres longues sont rares en raison de la difficulté à les faire s'écouler sur plusieurs dizaines de millimètres pour remplir complètement les moules.La société Equip'Aéro Technique a initié des travaux portant sur le développement d'un nouveau procédé de fabrication par injection-transfert (PIMOC) de composites thermodurcissables à fibres longues (> 1 mm) discontinues. Il permet de réaliser en une seule étape des pièces aux formes tridimensionnelles sans usinage.Dans ce travail de thèse le procédé d'injection-transfert a été mis au point et fiabilisé. Ses paramètres principaux ont été identifiés. L'influence des paramètres de fabrication sur les propriétés du matériau a été établie. Les propriétés mécaniques ont ainsi pu être optimisées. Enfin, un modèle de comportement élastique endommageable avec rupture et basé sur une approche multi-critères a été développé dans le but d'initier une méthodologie de dimensionnement de pièces composites à fibres discontinues. Ces critères d'endommagement et de rupture ont été développés en accord avec les observations du comportement mécanique du matériau. L'ensemble des résultats expérimentaux et numériques a été appliqué à la fabrication et au dimensionnement d'un démonstrateur technologiqueComposite materials are widely used in aeronautics. Their high mechanical properties combined to their lightness make it possible for thern to compete With metallic materials. However mass production of complex 3D shape composite structural parts is not usual.Injection process of short fibers (100um à 1 mm) filted thermoplastics is well known. Nevertheless there are few studies about long fibers (>1 mm) filled thermosets. It is very difficult to make the material flow into a closed mold.Equip iAéro Technique carried out research on the deve(opment of a new injection-transfer process (called PIMOC) to manufacture long discontinuous fibers filled thermoset composites. This process makes it possible to produce one shot complex 3D shape parts without machining.ln this work, the injection-transfer process has been developed and is now reliable. Its main parameters have been identified. The influence of manufacuring parameters on material properties have been determined, Thus mechanical properties have been optimizecl. Finally an elastic damage model has been devetoped in order to introduce a methodology or sizing discontinuous fibers composite parts. The model includes failure and is based on a multi-criteria approach. Theses damage and failure criteria have been deveioped according to observations of material mechanical behavior. Experimental and numerical results have been applied for sizing and manufacturing a technical demonstrator
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Nicholls, Tristan Kit. "Adhesive bonding of discontinuous carbon fibre composites : an experimental investigation." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13773/.
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The excellent specific stiffness and strength of carbon fibre reinforced polymer composites means that the automotive sector has been investigating methods of implementing these materials into structurally demanding applications. The work detailed within this thesis supports ongoing research at the University of Nottingham into the automated manufacture of discontinuous carbon fibre reinforced polymer composite materials. Advances in the automation of composites manufacturing has meant that methods to effectively join these materials is required. This work provides a fundamental understanding of the differences that result from the adhesive bonding of a discontinuous fibre composite (DFC) compared to conventional fibre reinforced composite materials. The main objective of the project was to characterise the behaviour of adhesively bonded DFC adherends. Using a single lap shear joint geometry, an optimised fibre architecture and joint geometry was identified with a 2-part low temperature curing epoxy adhesive being characterised for industrial application. To further improve the performance of the DFC substrates, a fibre alignment technique was implemented that achieved properties comparable to those of more traditional non-crimp fabric composites. From the experimental investigations conducted, the use of discontinuous carbon fibre reinforced composites in bonded assemblies shows promise with the potential for use in structural applications.
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Burn, David T. "Long discontinuous carbon fibre/polypropylene composites for high volume automotive applications." Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/33665/.
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The volume of fibre reinforced composites is increasing within the automotive industry, as stringent emissions legislation and consumer demands for improved fuel economy are encouraging manufacturers to reduce vehicular mass. Moreover, the falling cost of carbon fibre has meant that these composites are now being considered for semi-structural and structural components for medium-volume (+50,000ppa) applications in Euro Market Segments E and F (Jaguar XF, BMW 7 Series, Mercedes S-Class). The use of thermoplastic matrices with carbon fibre enables cycle times of less than one minute, creating opportunities for high volume manufacture of high specific stiffness components. However, the interfacial adhesion between these materials has been shown to be poor. This thesis seeks to identify whether polypropylene combined with long, discontinuous carbon fibres at high volume fractions, are suitable for high volume, semi-structural applications within the automotive industry. In particular, fibres recovered using two different recycling methods have been considered, as a potential route for reducing future material costs. Interfacial characterisation has been performed using the microbond method to investigate the quality of adhesion between the fibre and matrix, where the effects of sizing removal and introduction of a coupling agent have been considered. Fibre surface topology and chemistry have been examined to interpret data collected from interfacial testing, in addition to fibre strength measurements to assess the validity of the microbond method for high interface strength systems. A tow coating rig has been developed to produce partially pre-impregnated carbon fibre/polypropylene tows. The continuous coated tow has been chopped and processed into random fibre composites using non-isothermal compression moulding, and mechanical properties of the moulded panels have been characterised. The interface strength between sized and desized (pseudo-recycled) carbon fibre and unmodified polypropylene has been found to be poor. A 295% increase in interfacial shear strength (IFSS) is observed with the addition of 2wt.% maleic anhydride to the polypropylene, between the matrix and epoxy-sized carbon fibres. An increase of up to 353% in IFSS is observed for the desized fibres. These improvements can be attributed to chemical bonding as a result of esterification of hydroxyl groups on the carbon fibre surface, with anhydride functionalities of the coupling agent. Additionally, interactions occur between the nitrogen containing groups on the desized fibre surface and the anhydride carbonyl groups in the matrix. Surface roughness is not found to significantly contribute to interface strength. Good interfacial bonding has therefore been observed between polypropylene and sized carbon fibre due to the addition of a coupling agent at 2wt.%, which allows the low cost polymer to be combined with commercially available fibre. Long, discontinuous carbon fibre/polypropylene composites have been characterised in this study at volume fractions that have not previously been reported. Mechanical property characterisation has shown linear increases in stiffness with increasing fibre volume fraction. The specific stiffness of carbon fibre/polypropylene (0.45Vf) is comparable to the carbon fibre/epoxy benchmark. A plateau is observed for both strength and impact strength above volume fractions of 0.25, due to increased void content. The specific strength of the long fibre carbon fibre/polypropylene system can be improved further to a certain extent, by optimising the processing conditions to minimise trapped air.
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Xiao, Zhaofei. "Advancements in discontinuous carbon fibre composite technologies for high-volume manufacturing processes." Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/51942/.
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Fengler, Benedikt [Verfasser]. "Manufacturing-constrained multi-objective optimization of local patch reinforcements for discontinuous fiber reinforced composite parts / Benedikt Fengler." Karlsruhe : KIT-Bibliothek, 2019. http://d-nb.info/1176022628/34.
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Fengler, Benedikt [Verfasser]. "Manufacturing-constrained multi-objective optimization of local patch reinforcements for discontinuous fiber reinforced composite parts / Benedikt Fengler." Karlsruhe : KIT Scientific Publishing, 2021. http://d-nb.info/1229623698/34.
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Gadoury, Pascal. "Finite Element Modeling and Multivariate Optimization Over Fibre Orientation and Volume Fraction of Fibre Composite Parts Aimed at Minimizing Targeted Displacements." Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/26122.
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A software program was written that implements a finite element analysis (FEA) solution as the basis of an optimization function used for guiding the inverse design problem of aligning fibres, minimizing displacements in a fibre-reinforced polymer composite part in response to a given loading condition, for various part geometries.
The FEA solution makes use of the superlinear RGNTet4 element, which includes 3 displacement and 3 rotational degrees of freedom at 4 nodes. Convergence testing verified the accuracy of the solver versus symbolic results for simple cases.
Multivariate optimization over fibre orientations and volume fractions was carried out for a simple test case using the NLOpt nonlinear optimization library. Both derivative-free and gradient-based algorithms were tested. Low-Storage Broyden-Fletcher-Goldfarb-Shannon was the most effective algorithm.
Four more complex cases were examined, and by varying fibre orientations, reductions of 48%, 66%, 58% and 32% were achieved in displacements at the loaded nodes.
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Lin, Shih Hsiung, and 林世雄. "Research on the characteristics of continuous and discontinuous fiber reinforced composites." Thesis, 1996. http://ndltd.ncl.edu.tw/handle/29634415335998292376.
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(8780756), Imad A. Hanhan. "Investigating damage in discontinuous fiber composites through coupled in-situ X-ray tomography experiments and simulations." Thesis, 2020.
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Composite materials have become widely used in engineering applications, in order to reduce the overall weight of structures while retaining their required strength. Due to their light weight, relatively high stiffness properties, and formability into complex shapes, discontinuous fiber composites are advantageous for producing small and medium size components. However, qualifying their mechanical properties can be expensive, and therefore there is a need to improve predictive capabilities to help reduce the overall cost of large scale testing. To address this challenge, a composite material consisting of discontinuous glass fibers in a polypropylene matrix is studied at the microstructural level through coupled experiments and simulations, in order to uncover the mechanisms that cause microvoids to initiate and progress, as well as certain fiber breakage events to occur, during macroscopic tension. Specifically, this work coupled in-situ X-ray micro computed tomography (μ-CT) experiments with a finite element simulation of the exact microstructure to enable a 3D study that tracked damage initiation and propagation, and computed the local stresses and strains in the microstructure. In order to have a comprehensive 3D understanding of the evolution of the microstructure, high fidelity characterization procedures were developed and applied to the μ-CT images in order to understand the exact morphology of the microstructure. To aid in this process, ModLayer - an interactive image processing tool - was created as a MATLAB executable, and the 3D microstructural feature detection techniques were compared to traditional destructive optical microscopy techniques. For damage initiation, this work showed how high hydrostatic stresses in the matrix can be used as a metric to explain and predict the exact locations of microvoid nucleation within the composite’s microstructure. From a damage propagation standpoint, matrix cracking - a mechanism that has been notably difficult to predict because of its apparent stochastic nature - was studied during damage propagation. The analysis revealed the role of shear stress in fiber mediated flat matrix cracking, and the role of hydrostatic stress in fiber-avoidance conoidal matrix cracking. Overall, a sub-fiber simulation and an in-situ experimental analysis provided the microstructural physical phenomena that govern certain damage initiation and progression mechanisms, further enabling the strength and failure predictions of short fiber thermoplastic composites.
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(11201085), Ronald F. Agyei. "INVESTIGATING DAMAGE IN SHORT FIBER REINFORCED COMPOSITES." Thesis, 2021.
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In contrast to traditional steel and aluminum, short fiber reinforced polymer composites (SFRCs) provide promising alternatives in material selection for automotive and aerospace applications due to their potential to decrease weight while maintaining excellent mechanical properties. However, uncertainties about the influence of complex microstructures and defects on mechanical response have prevented widespread adoption of material models for
SFRCs. In order to build confidence in models’ predictions requires deepened insight into the heterogenous damage mechanisms. Therefore, this research takes a micro-mechanics standpoint of assessing the damage behavior of SFRCs, particularly micro-void nucleation at the fiber tips, by passing information of microstructural attributes within neighborhoods of incipient damage and non-damage sites, into a framework that establishes correlations between the microstructural information and damage. To achieve this, in-situ x-ray tomography of the gauge sections of two cylindrical injection molded dog-bone specimens, composed of E-glass fibers in a polypropylene matrix, was conducted while the specimens were monotonically loaded until failure. This was followed by (i) the development of microstructural characterization frameworks for segmenting fiber and porosity features in 3D images, (ii) the development of a digital volume correlation informed damage detection framework that confines search spaces of potential damage sites, and (iii) the use of a Gaussian process classification framework to explore the dependency of micro-void nucleation on neighboring microstructural defects by ranking each of their contributions. Specifically, the analysis considered microstructural metrics related to the closest fiber, the closest pore, and the local stiffness, and the results demonstrated that less stiff resin rich areas were more relevant for micro-void nucleation than clustered fiber tips, T-intersections of fibers, or varying porosity volumes. This analysis provides a ranking of microstructural metrics that induce microvoid nucleation, which can be helpful for modelers to validate their predictions on proclivity of damage initiation in the presence of wide distributions of microstructural features and
manufacturing defects.
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Juang, Jia-Hao, and 江家豪. "Investigation on Mechanical Properties and Fracture Behavior by Nanoscale Graphene Discontinuous Carbon Fiber Reinforced Copolymer (Epoxy/Benzoxazine) Composite." Thesis, 2015. http://ndltd.ncl.edu.tw/handle/k9985k.
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碩士國立清華大學
動力機械工程學系
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This study is focuses on the characteristics of the benzoxazine/epoxy copolymer matrix, combined thegraphene and micro-scale short carbon fibers to be reinforcement. In order to investigate the reinforced mechanism of the nano-scale and micro-scale additive, the interfacial properties, mechanical behavior, and fatigue failure would be realized on the mulit-scale reinforced composite. The researches includes: (1) Different contents of benzoxazine resin in Epoxy resin, (2) graphene and Micro-scale short carbon fibers concentration, (3) the interaction of multi-scale reinforcement material concentration This research aims to discuss the effect of the mixture of nano-scale graphene and micro-scale short carbon fiber, notably intensifies the the mixture of multi-scale interface, improving and increasing both mechanical properties and dynamic fatigue life. The investigation includes:(1)Different Benzoxazine resin concentration in Epoxy,(2)Nano-scale graphene and Micro-scale short carbon fibers concentration.(3)Multi-scale reinforcement material concentration. In the matrix experiment, the results indicate that the value of mechanical strength increases with the content of benzoxazine increased. From the results, the 20wt% benzoxazine/epoxy significantly improves the mechanical strength up to 21.37% improvement in tensile strength; 33.45% improvement in flexural strength; 13.86% improvement in flexural modulus; 38.03% improvement in resistance of water absorption. However, because of the property benzoxazine is more brittle than epoxy, the impact strength of benzoxazine/epoxy copolymer reduces about 55%. The research shows that addition of graphene in the optimum content of 20wt% benzoxazine/epoxy composite 14.18% improvement in tensile strength, 22.83% improvement in the resistance of water absorption by adding the 0.5wt% graphene; 2.46% improvement in flexural strength, and 8.05% improvement in flexural modulus by adding the 0.25wt% graphene. It is showed that adding 8wt% micro-scale short carbon fiber has the best enhancement to the mechanical properties: increasing 22.93% in flexural strength and 9.84% in the resistance of water absorption. Finally, the optimum content of GNPs-0.5wt%/SF-8wt%/ Benzoxazine/ Epoxy composites preform the best enhancement to the properties about 22.62% improvement in tensile strength, 12.2% improvement in flexural strength, and 39.07% improvement in impact strength, 3.2-3.8 times improvement in torsion fatigue life. Because of the reason of short carbon fiber will produce a lot of micro-cracks in the fiber-end, the stress concentration would influence the ductility and lead to cracks propagation. In the result, the flexural tests reveal about 33.84% downtrend in flexural modulus.