by Siu Wing Hon. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references. / by Siu Wing Hon. / Acknowledgement --- p.i / Abstract --- p.ii / Publication List --- p.iv / Chapter 1 --- Introduction --- p.1 / References --- p.6 / Chapter 2 --- Real-Space Renormalization Group (RG) Theory in Electrical Conduction --- p.9 / Chapter 2.1 --- Scale Invariance --- p.10 / Chapter 2.2 --- Critical Exponents --- p.14 / Chapter 2.3 --- Alternative View-Point of RG Theory --- p.15 / References --- p.18 / Chapter 3 --- "Weakly Nonlinear Composites: Critical Behavior, Flicker Noise and Crossover Behavior" --- p.19 / Chapter 3.1 --- Introduction --- p.19 / Chapter 3.2 --- Formalism --- p.20 / Chapter 3.3 --- Critical Exponents by RG Method --- p.22 / Chapter 3.4 --- Connection to Flicker Noise Problem and Crossover Behavior --- p.25 / Chapter 3.5 --- Discussions and Conclusions --- p.27 / References --- p.28 / Chapter 4 --- Critical Behavior of Strongly Nonlinear Composites --- p.30 / Chapter 4.1 --- Introduction --- p.30 / Chapter 4.2 --- Formalism --- p.31 / Chapter 4.3 --- Applications of RG Theory to Strongly Nonlinear Composites --- p.32 / Chapter 4.4 --- Connections with Links-Nodes-Blobs picture --- p.36 / Chapter 4.5 --- Discussions and Conclusions --- p.39 / References --- p.41 / Chapter 5 --- "Enhanced Nonlinear Response of Superconductor-Normal-conductor Composite Wires, Strips and Rods" --- p.43 / Chapter 5.1 --- Introduction --- p.43 / Chapter 5.2 --- Formalism --- p.45 / Chapter 5.3 --- Linear and Nonlinear Responses of Composite Wires --- p.46 / Chapter 5.4 --- Linear and Nonlinear Response of Composite Strips --- p.49 / Chapter 5.5 --- Linear and Nonlinear Responses of Composite Rods --- p.56 / Chapter 5.6 --- Scaling Behaviors --- p.59 / Chapter 5.7 --- Discussions and Conclusions --- p.63 / References --- p.64 / Chapter 6 --- Renormalized Effective Medium Theory for Weakly Nonlinear Composites --- p.66 / Chapter 6.1 --- Introduction --- p.66 / Chapter 6.2 --- Weakly Nonlinear Conductance Network --- p.69 / Chapter 6.3 --- Simulation --- p.70 / Chapter 6.4 --- Effective Medium Approximation --- p.76 / Chapter 6.5 --- Renormalized Effective Medium Approximation --- p.79 / Chapter 6.6 --- Discussion and Conclusions --- p.81 / References --- p.83 / Chapter 7 --- Conclusions --- p.86 / Chapter A --- Derivation of Voltage-Summation Formulas --- p.88 / Chapter B --- Effective Linear and Nonlinear Response of 2 x 2 cell --- p.92 / Chapter C --- Duality Symmetry in 2D Network --- p.97 / Chapter D --- Derivation of Effective-Medium Approximation --- p.99

Os materiais compostos de fibras longas vêm se destacando entre o conjunto de novos materiais em desenvolvimento, com larga aplicação na indústria e também na construção civil. Elabora. se aqui um texto introdutório no campo das placas laminadas constituídas de lâminas em compostos de fibras longas. A partir das propriedades dos materiais constituintes, fibra e matriz, são apresentadas as teorias para a determinação das propriedades equivalentes das lâminas. A teoria da anisotropia elástica é formulada e a teoria dos laminados em flexão é desenvolvida. Conclusões com respeito ao acoplamento de esforços em tais estruturas são mostradas. Os modos básicos de ruptura da fibra, matriz e da interface entre ambas são apresentados, bem como os modos básicos de delaminação. Alguns critérios de ruptura são mostrados, assim como um critério de escoamento. Ao fim alguns exemplos ilustram a análise pelo método dos elementos finitos de tais estruturas feita por programa disponível em literatura, com a implementação de subrotina para análise da temperatura. / Long fiber composite materials have been standing out among the group of new materials in development with wide industrial application and also in civil construccion. Here is elaborated an introductory text in laminated plates field constituted of long fiber composite laminae. Starting from constituent materials properties, fiber and matrix, the theories for determination of laminae equivalent properties are presented. Elastic anisotropy theory is formulated and the bending laminated theory is developed. Conclusions about the coupling efforts in such structures are showed. Fiber, matrix and the interface between both failure basic modes are presented, as well as delamination basic modes. Some failure criteria are showed, as well as a yielding criteria. At the end some examples illustrate the finite element method analysis of such structures made by available literature programme with temperature analysis subroutine implementation.

Une grande partie de la structure des avions modernes est fabriquée à partir des dernières générations de matériaux composites. Le principal défi des constructeurs d’avions réside dans la taille importante des pièces en composites. D’où la nécessité de développer de nouvelles technologies de fabrication telle que la dépose automatique de préimprégné. La qualité des pièces fabriquées dépend de nombreux facteurs dont notamment la maîtrise de la rhéologie de la résine du préimprégné. Cette dernière évolue fortement avec les conditions environnementales et les conditions de fabrication. Elle détermine pour partie, les instabilités de la matière particulièrement le long des trajectoires curvilignes. Ces instabilités, qui se présentent sous forme de cloques, engendrent après cuisson des pièces avec des défauts de repliement des couches dans les stratifiés. L’objectif principal de cette thèse de doctorat est l’étude du comportement mécanique d’un ruban de préimprégné prenant en compte la rhéologie de la matière ainsi que sa pégosité dans le but de mieux comprendre les conditions d’apparition des cloques en trajectoire curviligne. Une partie de mes travaux est consacrée à la caractérisation des propriétés physico-chimiques et mécaniques de la matière. Une seconde partie, complémentaire à la première, est dédiée au développement d’un modèle de comportement mécanique d’une plaque sur fondation élastique représentant le ruban de préimprégné et soumise à une rotation imposée. La méthode de Ritz a été utilisée pour l'obtention de solutions explicites des charges critiques de flambement. Il a été montré que les plus faibles rayons de courbure sans formation de cloques en trajectoire curviligne dans le plan sont obtenus avec un faible écrasement du rouleau et un niveau de collant élevé. / The evolution of composite materials in aerospace has been spectacular over the last thirty years. The main advantage provided by composites is weight gain compared to other materials. However, new aluminum lithium alloys may slow this trend because of their specific properties and exceptional manufacturing costs lower than those of composites. Much parts of the structure of the A380 are made from the latest generation of composite materials with organic matrix. The Airbus A380 is the first plane of the manufacturer composed by a center wing box made by composite reinforced with carbon fibers and organic matrix. And like the devices that preceded it, the center wing box, rudder, horizontal stabilizer and the elevators were designed with composites reinforced with carbon fibers and organic matrix. But the main challenge with the new aircraft is the size of parts ; the size of the horizontal stabilizer of the A380 for example, is similar to the wing of the A310. Hence the need to develop the intensive use of automatic prepreg lay up technology. The use of semi-finished prepregs for the manufacture of parts allows standardizing production. Indeed, these products exhibit with the desired rate of reinforcement on the final parts, all the delicate operations of pumping excess resin during the implementation of parts, as the case in the RTM process (Resin Transfer Molding) are eliminated. The quality of parts produced using automatic lay up depends on many factors including the accuracy of trajectory lay up, the control of rheology of the resin contained in the prepreg and behavior at the interface of the material and tooling. The rheology of the resin is changing considerably with environmental conditions (temperature, humidity) and processing conditions (speed of removal, compaction strength). It determines in part and in combination with the tackiness of the prepreg, the instabilities of the material particularly along curved trajectories. In curvilinear lay up, the rotation of the tow placement head imposes to this portion of slit tape, a curvature generating a circular bending phenomenon.The outer portion of the tape is then stretched while the inside is compressed. When the radius of curvature of the tow placement head is low, the result is a phenomenon of local buckling of the slit tape. The corresponding layer waviness (See figure 1) is visible after the passage of the roller so instantly or delayed, depending on the conditions of lay up (speed lay up,compaction strength, temperature, humidity) and the tackiness of the material. Thus, the formation of the layer waviness is a local phenomenon that appears in the material between the tow placement head and tools. It is this area of the material defined by the form of plate, and the extent of which depends on the compaction strength and flexibility of the roller which will be the model proposed in this communication. It is supposed that this plate is based on elastic foundation in order to reflect the tack of the material. The formation mechanism of ply waviness is very few studied and remains unclear since these defects appear at different steps in the manufacturing process. Some studies recommend, without justification, to not exceed some radius of curvature to avoid buckling [1]. Interactions between tooling and material in relation with lay up conditions were studied [2]. In this paper we study the mechanical behavior of a slit tape taking into account the rheology of the material so that his tack in order to better understand the conditions of appearance of layer waviness during circular trajectories. At first, we will present some results from the identification of physico-chemical and mechanical properties of prepreg carbon / epoxy and then we will explain the evolutions observed. A second part, complements the first,will be dedicated to the development of a model of mechanical behavior of a plate based onelastic foundation, representing the slit tape which is subjected to a rotation imposed. The mechanical model of the slit tape will be based on the theory of plate under the hypotheses of Kirchhoff-Love and Von-Karman. Determining the critical buckling load is carried by the Ritz method. The simulation results using different boundary conditions will be analyzed and compared to real cases.

Abstract not available

The aim of this study was to develop a new single-part healing system for self-healing composites. The self-healing approach to composite repair has been developed in the last two decades and means that a damaged area can be repaired by material already housed within the structure. The background and development of self-healing has been reviewed. The two main self-healing mechanisms are discussed. To date only two part self healing systems have been examined. These require diffusion of the separate constituents to a single location in order to effect cure and restore strength. Single part adhesives do not have this disadvantage and are therefore very attractive. Several candidate single-part adhesive or resin systems were considered and discussed according to the critical requirements of a self-healing system. A series of experiments was undertaken to evaluate the possibility of candidate adhesive systems being effective for self-healing by focusing on the determination of storage stability and bonding efficiency. The results of storage stability testing showed that the stability of cyanoacrylate and polyurethane adhesives was poor. However silane and polystyrene cements showed good storage stability. Very low bonding efficiency was achieved with polystyrene cement but a 22% strength recovery was obtained with the silane 3-[tris(trimethylsiloxy)silyl]-propylamine. Suggestions for further research into single-part healing systems are also given.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 291-304).

No description available.

Natural fibers are gaining popularity as reinforcement materials for thermoset resins over the last two decades. Natural fibers are inexpensive, abundant, renewable and environmentally friendly. Kenaf fibers are one of the natural fibers that can potentially be used for reinforcing unsaturated polyester (UPE). As a polymer matrix, UPE enjoys a 40% market share of all the thermoset composites. This widespread application is due to many favorable characteristics including low cost, ease of cure at room temperature, ease of molding, a good balance of mechanical, electrical and chemical properties. One of the barriers for the full utilization of the kenaf fiber reinforced UPE composites, however, is the poor interfacial adhesion between the natural fibers and the UPE resins. The good interfacial adhesion between kenaf fibers and UPE matrix is essential for generating the desired properties of kenaf-UPE composites for most of the end applications. Use of a coupling agent is one of the most effective ways of improving the interfacial adhesion. In this study, six novel effective coupling agents were developed and investigated for kenaf-UPE composites: DIH-HEA, MFA, NMA, AESO-DIH, AESO-MDI, and AESO-PMDI. All the coupling agents were able to improve the interfacial adhesion between kanaf and UPE resins. The coupling agents were found to significantly enhance the flexural properties and water resistance of the kenaf-UPE composites. Fourier transform infrared spectroscopy (FTIR) confirmed all the coupling agents were covalently bonded onto kenaf fibers. Scanning electron microscopy (SEM) images of the composites revealed the improved interfacial adhesion between kanaf fibers and UPE resins. / Graduation date: 2013

In this dissertation, experimental investigations and theoretical studies on the stochastic matrix cracking evolution under static and fatigue loading in composite laminates with defects are presented. The presented work demonstrates a methodology that accounts for the statistically distributed defects in damage mechanics models for the assessment of the integrity of composites and for the structural design of composites. The experimental study deals with the mechanisms of the formation of a single crack on a micro-scale and the stochastic process for the multiplication of cracks on a macro-scale. The defects introduced by the manufacturing processes are found to have significant effect on the matrix cracking evolution. Influenced by the distributed defects, the initiation and multiplication of cracks evolve in a stochastic way. The experimental study on the in-plane shear stress finds the detrimental effect of the shear stress on the fatigue performance of composite laminates. Combined with the transverse tensile stress, the in-plane shear stress induces multiple inclined microcracks in the matrix, which enhance the initiation and propagation of the major matrix cracks. Based on the experimental investigations, a statistical model for the stochastic matrix cracking evolution on the macro-scale is developed. Simulations based on the statistical model yield accurate predictions for both static and fatigue loading compared to the experimental data. The Weibull distribution of the static strength is estimated by the statistical model by comparing against the experimental crack density data. The estimated Weibull distribution of the static strength provides an efficient approach to characterize the manufacturing quality of composite laminates. Compared to deterministic approaches, the Weibull distribution of the static strength provides comprehensive information of the strength property of composite laminates.

Graduation date: 1993