Дисертації: "Variable and anisotropic composites"

1

Chevalier, Romain. "Modélisation multi-échelles du comportement hygro-mécanique et étude de la stabilité dimensionnelle de structures composites lamellées collées aboutées en Pinus pinaster (Ait.)." Electronic Thesis or Diss., Bordeaux, 2024. http://www.theses.fr/2024BORD0057.

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Cette thèse porte sur le développement de modèles multi-échelles du comportement hygro-mécanique pour l'étude de la stabilité dimensionnelle de lamellés collés aboutés (LCAs) en Pinus pinaster (Ait.). Les structures en bois sont soumises à des variations dimensionnelles dues aux modifications des ambiances climatiques. Dans le cadre des structures LCAs, la stabilité dimensionnelle peut être atteinte via une conception systématique en fonction des propriétés des lamelles constituantes. Dans ce but, un modèle multi-échelles reposant sur une étude bibliographique exhaustive des propriétés du Pinus pinaster (Ait.) a été développé. Ce modèle permet d'obtenir spatialement, grâce à l'utilisation d'une méthode d'homogénéisation numérique et d'un métamodèle basé sur les hypersurfaces Non-Uniform Rational Basis-Spline (NURBS), les propriétés hétérogènes, variables et anisotropes de lamelles de Pinus pinaster (Ait.). De plus, des configurations de LCAs, reposant sur la théorie des stratifiés, ont été proposées et soumises expérimentalement à des variations d'ambiance climatiques. Les déplacements induits sont mesurés grâce à une méthode de corrélation d'images numériques. In fine, une comparaison des déplacements est réalisée qui permet de recommander des configurations de LCAs améliorant la stabilité dimensionnelle des LCAs du groupe Gascogne Bois
This thesis focuses on the development of a multi-scale model of the hygro-mechanical behaviour and the study of the shape stability of glued laminated timber (GLT) made of Pinus pinaster (Ait.). Wooden structures are subject to dimensional variations due to changes in climatic conditions. In the case of GLT structures, shape stability can be achieved through systematic design based on the properties of the constituent lamellae. To this end, a multi-scale model based on an exhaustive bibliographic study of the properties of Pinus pinaster (Ait.) has been developed. Using a numerical homogenisation method and a metamodel based on Non-Uniform Rational Basis-Spline (NURBS) hypersurfaces, this model provides a spatial representation of the heterogeneous, variable, and anisotropic properties of Pinus pinaster (Ait.) lamellae. In addition, configurations of GLTs, based on laminate theory, have been proposed and experimentally subjected to variations in climatic conditions. The induced displacements are measured using a digital image correlation method. Finally, a comparison of the displacements is carried out, enabling the recommendation of GLT configurations that improve the shape stability of the GLTs produced by the Gascogne Bois company

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2

McKernan, Scott John. "Anisotropic tensile probabilistic failure criterion for composites." Thesis, Monterey, California : Naval Postgraduate School, 1990. http://handle.dtic.mil/100.2/ADA237601.

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Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, June 1990.
Thesis Advisor(s): Wu, Edward M. "June 1990." Description based on title screen as viewed on October 16, 2009. Author(s) subject terms: Composite materials, probabilistic failure criterion, combined stress. Includes bibliographical references (p. 151). Also available in print.

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3

Bradford, Ian David Richard. "Finite deformations of highly anisotropic materials." Thesis, University of Nottingham, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334952.

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4

Mohamed, Abdel-Mohsen Onsy. "Performance of an anisotropic clay under variable stresses." Thesis, McGill University, 1986. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=75342.

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In the true triaxial test procedure used for testing laboratory-prepared kaolinite clay samples, undrained (with constant mean stress) strength tests were conducted to study the yield and failure of the clays. The principle concern focussed around the influence of orientation of particle bedding plane on the development of yield and failure characteristics of the clay. As the true triaxial cell permits variations of the three principal stresses, it was possible to study the soil response in any chosen quadrant of the principal space.
As a consequence to what is mentioned above, two types of consolidated undrained true triaxial tests were conducted in this study. In the first type, specimens were trimmed from the block sample with 90, 60, 30 and 0 degree orientation angles of particle's bedding planes; these angles were measured with respect to the direction of the major principal stress axis. For each degree of inclination, specimens were tested with three confining pressures 207, 276 and 345 kPa, and for each value of confining pressure, the loading path was varied from compression to tension.
The degree of dissociation between the stress and strain increment vectors was seen to depend on both initial and stress induced anisotropy.
Most important of all, a constitutive relationship for anisotropic kaolinite clay was derived on the basis of the observed experimental behaviour of soil samples under loading.
Additionally, anisotropy is characterized by a double transformation technique. The first transformation accounts for the directional dependency whilst the second transformation concerns itself with anisotropy of the base vectors. The relative joint invariant principle is used to calculate the degree of dissociation during the loading process. The variation of the dissociation angle during the loading process can be considered as a measure of the evolution of the resultant anisotropy. The model has shown to provide viable predictions of the stress-strain relationships obtained from true triaxial tests on an anisotropic kaolinite clay for: (a) different inclinations of particle's bedding planes, (b) different stress paths in one sector, (c) different stress paths in other sectors, and (d) the failure surfaces for different inclinations of particle's bedding planes in the octahedral plane. (Abstract shortened with permission of author.)

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5

Senan, Anish Sen. "Determination of the Viscoelastic Properties of General Anisotropic Materials." Fogler Library, University of Maine, 2003. http://www.library.umaine.edu/theses/pdf/SenanAS2003.pdf.

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6

Bourn, Steven. "Anisotropic behaviour of magneto-electric coupling in multiferroic composites." Thesis, University of Central Lancashire, 2018. http://clok.uclan.ac.uk/23578/.

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The anisotropy of the direct magnetoelectric effect in textured nickel ferrite/lead zirconate titanate strain mediated bilayer composites has been studied. The magnetic layers of these samples have been crystallographically textured in planes of the form {100}, {110} and {111}. In this study, it is shown that the optimum bias field and the maximum magnetoelectric coupling signal can be controlled by changing the alignment of the applied magnetic field with respect to the magnetocrystalline anisotropy directions. It is also shown that the product of the optimum bias field and the maximum magnetoelectric coupling signal are proportional to the theoretical saturation magnetostriction. The samples have been magnetically characterised using a recommissioned and developed biaxial vibrating sample magnetometer, capable of detecting the component of a sample’s magnetic moment in 2 perpendicular directions and thus determining the net magnetic moment vector of the sample. Coupled with sample rotation this allows insight into the magnetic anisotropy of the sample, which has been compared with a micromagnetic model. A specialist magnetoelectric coupling rig has also been developed to allow application of DC and AC magnetic fields to a sample simultaneously. As part of the magnetic anisotropy study, a modified torque magnetometry method has been developed to enhance the identification of the anisotropy directions in magnetically soft samples, as well as a method by which torque magnetometry can be approximated using the in-field direction component of magnetisation as measured using a standard vibrating sample magnetometer.

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7

Costa, Luan Mayk Torres. "Modélisation micromécanique à variables internes du comportement viscoélastique anisotrope des matériaux hétérogènes : applications aux composites à matrice organique." Electronic Thesis or Diss., Université de Lorraine, 2023. http://www.theses.fr/2023LORR0116.

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L'objectif principal de la thèse consiste à développer une approche micromécanique pour prédire le comportement viscoélastique macroscopique des matériaux hétérogènes à partir des propriétés locales des constituants et de leur microstructure. Les propriétés viscoélastiques effectives sont obtenues par l'utilisation des méthodes d'homogénéisation à champ moyen appropriées. L'approche micromécanique proposée est basée sur une loi constitutive fonctionnelle ayant la forme d'une intégrale de Volterra. Dans un premier temps, nous obtenons une formulation micromécanique à variables internes développée avec le module de relaxation. Dans un autre temps, nous développons une deuxième approche micromécanique cette fois-ci écrite avec le module de fluage, consistant à la formulation duale à la première. À l'aide des techniques des fonctions de Green, nous établissons, dans les deux cas, une équation intégrale décrivant le problème viscoélastique hétérogène. Cette équation principale fait apparaître un terme intégral de volume difficile à évaluer directement, ce qui nous amène à construire une deuxième équation intégrale complémentaire venant s'ajouter à la première. Cette formulation générale définie par ces deux équations intégrales est appliquée au problème classique de l'inclusion viscoélastique d'Eshelby. La solution exacte du problème est obtenue dans le cas d'une morphologie ellipsoïdale arbitraire de l'inclusion ayant un comportement viscoélastique anisotrope et plongée dans une matrice viscoélastique isotrope. Nous utilisons une méthode à variables internes qui considère que l'histoire du matériau est contenue dans son état interne. La résolution de l'approche directement dans l'espace-temps nous conduit à une solution exacte avec un gain appréciable de temps de calcul, en comparant avec les résultats obtenus par les approches héréditaires, traitées dans l'espace de Laplace-Carson. Les deux formulations proposées permettent d'évaluer l'effet du comportement anisotrope et de la morphologie ellipsoïdale des inclusions, tout en permettant également d'étudier l'influence du vieillissement sur le comportement mécanique du composite. Les deux modèles obtenus présentent des résultats cohérents avec ceux disponibles dans la littérature, en permettant un gain de calcul intéressant par rapport aux méthodes existantes
The primary aim of this thesis is to devise a novel micromechanical approach for predicting the macroscopic viscoelastic response of heterogeneous materials. The behavior is achieved through micromechanical modeling that is based on local properties and microstructure. The effective viscoelastic properties are obtained by the use of appropriate mean-field homogenization methods. The mechanical approach is based on a Volterra integral-form functional constitutive law. Firstly, a new internal variable micromechanical formulation is obtained by utilizing the relaxation modulus. Secondly, a second micromechanical approach is developed, which employs the creep modulus and consists to the dual formulation. Using Green's function techniques, we derive integral equations that describe the heterogeneous viscoelastic problem for both cases. The main equation contains a challenging volume integral term, which necessitates the development of a second complementary integral equation. These two integral equations form the general formulation that we apply to the classical Eshelby viscoelastic inclusion problem. We employ an internal variable method that considers the material's history to be contained in its internal state. The approach is solved directly in the time domain, resulting in an exact solution with reduced computation time compared to hereditary approaches processed in the Laplace-Carson space. Our model enables us to evaluate the impact of anisotropic inclusions and to examine the influence of aging behavior on the composite viscoelastic properties. Both approaches proposed in this thesis deliver results that are consistent with those reported in the literature and offer a significant computational advantage over existing methods

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8

Yalcin, Omer Fatih. "A Dynamic Theory For Laminated Composites Consisting Of Anisotropic Layers." Phd thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607116/index.pdf.

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In this thesis, first a higher order dynamic theory for anisotropic thermoelastic plates is developed. Then, based on this plate theory, two dynamic models, discrete and continuum models (DM and CM), are proposed for layered composites consisting of anisotropic thermoelastic layers. Of the two models, CM is more important, which is established in the study of periodic layered composites using smoothing operations. CM has the properties: it contains inherently the interface and Floquet conditions and facilitates the analysis of the composite, in particular, when the number of laminae in the composite is large
it contains all kinds of deformation modes of the layered composite
its validity range for frequencies and wave numbers may be enlarged by increasing, respectively, the orders of the theory and interface conditions. CM is assessed by comparing its prediction with the exact for the spectra of harmonic waves propagating in various directions of a two-phase periodic layered composite, as well as, for transient dynamic response of a composite slab induced by waves propagating perpendicular to layering. A good comparison is observed in the results and it is found that the model predicts very well the periodic structure of spectra with passing and stopping bands for harmonic waves propagating perpendicular to layering. In view of the results, the physical significance of Floquet wave number is also discussed in the study.

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9

Searle, Timothy John. "The manufacture of marine propellers in moulded anisotropic polymer composites." Thesis, University of Plymouth, 1998. http://hdl.handle.net/10026.1/2766.

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This thesis examines the feasibility of manufacturing small marine propellers from continuous fibre reinforced polymer composite materials. An appraisal of some current applications of composite materials in the marine industry is given, together with the moves shown towards the use of composites in the area of propeller design. It has been shown that manufacturing propellers in composite materials is theoretically more cost effective than traditional materials. The manufacturing route investigated is Resin Transfer Moulding, where some detailed investigations have highlighted some of the critical processing parameters necessary for successful production of laminates suitable for propellers and other high performance marine structures. A thorough testing programme of 4 novel designs of composite propeller is reported. Trials at sea on university run vessels has enabled many hours use to be logged, which has shown the fitness for purpose of propellers made from glass reinforced, epoxy composite. Experimental tank testing has helped to shape the remainder of the research by identifying the possibility of using hydroelastic tailoring to improve the efficiency of the propeller when a variety of operating conditions are required from the propulsion system. Further experience is required with respect to the the tooling construction and the life assessment of the propeller. To facilitate appropriate modelling of the propeller, spreadsheet based load prediction models have been used. Finite element analysis (FEA) was used to model the elastic characteristics of one particular design of novel composite propeller. This indicated that traditional geometries may be too stiff to allow significant performance advantages from the anisotropy of the material. However the potential does exist for modified propeller geometries made from composite to give some performance benefit. For specific applications, small marine propellers made from continuous glass fibre reinforced epoxy composite are likely to yield cost savings over traditional propeller materials.

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10

Belijar, Guillaume. "Anisotropic composite elaboration and modeling : toward materials adapted to systems." Thesis, Toulouse 3, 2016. http://www.theses.fr/2016TOU30353/document.

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L'objectif de ces travaux de thèse a été de démontrer la possibilité, en se basant sur une approche prédictive, de contrôler avec précision la fonctionnalisation d'un matériau composite, d'isotrope à anisotrope, sous l'application d'un champ électrique. Ces derniers matériaux présentent en effet un fort potentiel pour des applications futures telles que des condensateurs intégrés ou bien encore des composites conducteurs thermiques. Une première approche théorique des différentes forces et mécanismes entrant en jeux lors de l'élaboration de composites anisotropes par chaînage a permis d'identifier les paramètres impactant le procédé d'élaboration. A la suite de cette étude théorique, un modèle de formation de chaînes de particules sous champ électrique a été développé afin de prédire la dynamique de chaînage. Le modèle choisi (méthode moment dipolaire effectif) a permis la simulation de plus de 4500 particules. Les paramètres ayant au préalable été identifiés ont ensuite été mesurés. Pour la permittivité des particules, une méthode de mesure diélectrophorétique a été développée, ce qui est une première dans le cas de particules céramiques. L'élaboration des composites anisotropes a été couplé avec un suivi novateur, en temps réel, d'un marqueur (permittivité) de la formation de chaînes, permettant d'obtenir la dynamique de structuration des particules. Afin de valider l'aspect prédictif du modèle numérique, une comparaison a été effectuée entre la dynamique mesurée et simulée. Les résultats obtenus ont démontré une très bonne fiabilité des prédictions du modèle, même si des progrès sont encore réalisables aux faibles taux de chargement. Dans un dernier temps, une preuve de concept a été démontrée, de la réalisation de composites anisotropes dont les particules sont alignées perpendiculairement au champ électrique
This study was aimed to demonstrate the possibility, based on a predictive approach, to tailor the structure of a composite from isotropic to anisotropic when applying an electric field. This composites have great potential for future applications such as embed capacitors or thermally conductive composites. A theoretical approach of the forces and mechanisms acting in the elaboration of anisotropic composites by chaining allowed identifying the key parameters. Based on this approach a model of particle chaining under electric field was established to predict the structuration dynamics. This model (effective dipole moment) allowed simulating more than 4500 particles. The parameters previously identified were then measured, and for the particle permittivity, a dielectrophoretic measurement method was developed, which was a first for ceramic particles. The elaboration of anisotropic composites was coupled to a novel on-line monitoring of a chaining marker (permittivity), allowing to obtain the structuration dynamics. To validate the predictive aspect of the model, experimental and numerical dynamics were compared showing the robustness and accuracy of the model, even if improvement is still possible at low filler content. In the last part, a proof of concept was demonstrated of the elaboration of anisotropic composites with fillers oriented normally to the direction of the electric field

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11

Panesar, Ajit S. "Multistable morphing composites using variable angle tows (VAT)." Thesis, University of Bristol, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.574264.

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The continual need to better aircraft performance is increasingly prompting designers towards the realisation of morphing structures. One such enabling morphing technology are "multistable composites", and interest in them stems from the fact that they are able to sustain significant changes in shape without the need for a continuous power supply. This research benefits from the tow-steering technique to develop laminates with curved fibre-paths or Variable Angle Tows (VAT) in a ply (i.e. exploiting the anisotropy of composites), to introduce residual thermal stresses capable of imparting bistability. The principle idea is that VAT laminates can impart bistability with the new feature of ensuring fibre continuity within the wider structure facilitating structural integration (i.e. blending of lay-ups across components). Additional structural strength is shown to be imparted due to the load path continuity achieved via the tow-steering technique. The thermally induced bistable behaviour of VAT laminates is investigated through Finite Element (FE) modelling and experimental studies. The effect on the stable shapes for variations in the resin layers, fibre volume fraction (Vf), resin thermal expansion and laminate thickness are reported and found to be influential (from high to low) in the same quoted order. An approach, aiding the development of well- defined finite element models that are capable of predicting the bistable behaviour of manufactured laminates, is presented. It is shown that despite the inherent variabilities in laminates, a shell model capturing sufficient detail (i.e. resin layer[s), ply thicknesses and the Vf of a ply) is successful in predicting the cured shape(s). An optimisation based design framework aimed at realising the morphing potential of VAT laminates is presented. A morphing alternative to the plain flap is achieved, and the two potential solutions: one aiming to maximise the deployed flap-angle, and the other to attain maximum flap-angle increment, are discussed. Moreover, a good agreement exists between the FE predictions and the experimental observations for the manufactured laminates, demonstrating the feasibility of the morphing flap concept.

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12

Choi, Hyung Jip. "On iso- and nonisothermal crack problems of a layered anisotropic elastic medium." Diss., Virginia Polytechnic Institute and State University, 1991. http://hdl.handle.net/10919/53606.

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The iso- and non-isothermal crack problems of layered fiber-reinforced composite materials are investigated within the framework of linear anisotropic thermoelasticity and under the state of generalized plane deformation. The crack is assumed to be parallel to the layer bounding surfaces. By employing the Fourier integral transform technique and the flexibility/stiffness matrix formulation, the current mixed boundary value problems are reduced to solving a set of simultaneous singular integral equations with Cauchy-type kernels. The crack·tip stress intensity factors are then defined in terms of the solution of the integral equations. Numerical results are presented addressing the salient and unique features for a class of crack problems involving highly anisotropic fibrous composite materials. Specifically, the cases of a crack embedded i) within a homogeneous and anisotropic slab, ii) between two bonded dissimilar anisotropic half-spaces and iii) within the matrix-rich interlaminar region of a generally laminated anisotropic slab are considered. The effects of relative crack size, crack location and fiber volume fraction on the stress intensity factors are examined as a function of über angle. For the case of layered composites, the matrix-rich interlaminar region is modeled as a separate interlayer. As the interlayer thickness approaches zero, the interlaminar crack model illustrates no smooth transition to the ideal interface crack model of zero interlayer thickness which exhibits oscillatory stress singularities. The mixed-mode crack tip response is shown to involve the simultaneous presence of three fracture modes. It is demonstrated that the corresponding values of stress intensity factors are strongly influenced by the laminate stacking sequence and layer orientation. In addition, the partially insulated crack surface condition is observed to alleviate the severity of thermally-induced stress fields near the crack tip.
Ph. D.

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13

Barakati, Amir. "Dynamic interactions of electromagnetic and mechanical fields in electrically conductive anisotropic composites." Diss., University of Iowa, 2012. https://ir.uiowa.edu/etd/3562.

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Recent advances in manufacturing of multifunctional materials have provided opportunities to develop structures that possess superior mechanical properties with other concurrent capabilities such as sensing, self-healing, electromagnetic and heat functionality. The idea is to fabricate components that can integrate multiple capabilities in order to develop lighter and more efficient structures. In this regard, due to their combined structural and electrical functionalities, electrically conductive carbon fiber reinforced polymer (CFRP) matrix composites have been used in a wide variety of applications in most of which they are exposed to unwanted impact-like mechanical loads. Experimental data have suggested that the application of an electromagnetic field at the moment of the impact can significantly reduce the damage in CFRP composites. However, the observations still need to be investigated carefully for practical applications. Furthermore, as the nature of the interactions between the electro-magneto-thermo-mechanical fields is very complicated, no analytical solutions can be found in the literature for the problem. In the present thesis, the effects of coupling between the electromagnetic and mechanical fields in electrically conductive anisotropic composite plates are studied. In particular, carbon fiber polymer matrix (CFRP) composites subjected to an impact-like mechanical load, pulsed electric current, and immersed in the magnetic field of constant magnitude are considered. The analysis is based on simultaneous solving of the system of nonlinear partial differential equations, including equations of motion and Maxwell's equations. Physics-based hypotheses for electro-magneto-mechanical coupling in transversely isotropic composite plates and dimension reduction solution procedures for the nonlinear system of the governing equations have been used to reduce the three-dimensional system to a two-dimensional (2D) form. A numerical solution procedure for the resulting 2D nonlinear mixed system of hyperbolic and parabolic partial differential equations has been developed, which consists of a sequential application of time and spatial integrations and quasilinearization. Extensive computational analysis of the response of the CFRP composite plates subjected to concurrent applications of different electromagnetic and mechanical loads has been conducted. The results of this work verify the results of the previous experimental studies on the subject and yield some suggestions for the characteristics of the electromagnetic load to create an optimum impact response of the composite.

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14

Schclar, Noemi Alejandra. "Application of the Boundary Element Method to the structural analysis of three dimensional anisotropic material." Thesis, University of Portsmouth, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334408.

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15

Li, Renfu. "A Study of Interface Crack Branching in Dissimilar Anisotropic Bimaterial Composites Including Thermal." Diss., Georgia Institute of Technology, 2004. http://hdl.handle.net/1853/4893.

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The interface crack branching phenomena, including thermal effects, has been investigated by using complex variable method and Stroh's dislocation theory, extended to thermo-elasticity in matrix notation. As one of the most catastrophic failure modes in structures like laminated and sandwich composites in aerospace and marine construction, thin film in electronic packaging, rotators in high speed engine of aircraft and reactor in nuclear power station, the study of interface crack branching has become a topic not only having theoretical importance, but also having practical significance. A unified approach is presented to address the thermoelastic interface crack problems in dissimilar anisotropic bimaterial composites, and a compact closed form solution is formulated by analytical continuation principle of complex analysis. Employing the contour integral method, an explicit solution to the interaction between the dislocations and the interface crack is obtained. By modeling the branched portion as a continuous distribution of the dislocations, the thermoelastic interface crack branching problem is then converted to a set of semi-coupled singular integral equations and solved by Gauss-Jacobi integration schemes. The influence of material property mismatches between the two constituents and the thermal loading effects on the interface crack branching are demonstrated by extensive numerical simulation. Some useful criteria for predicting the interface crack branching growth and guidance for optimal composites design are suggested. Further, a contact model to eliminate the overlapping between the two surfaces of an interface crack is also proposed and some new parameters which could influence the interpenetrating phenomena are also discovered. The technique to extend the current method to three dimensional problems is also outlined. Furthermore, the C++ source code has been implemented to manipulate the complicated complex operations for numerically solving the singular integral equations in complex matrix form.

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16

Vel, Senthil S. "Analytical Solutions for the Deformation of Anisotropic Elastic and Piezothermoelastic Laminated Plates." Diss., Virginia Tech, 1998. http://hdl.handle.net/10919/30003.

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The Eshelby-Stroh formalism is used to analyze the generalized plane strain quasistatic deformations of an anisotropic, linear elastic laminated plate.The formulation admits any set of boundary conditions on the edges and long faces of the laminate. Each lamina may be generally anisotropic with as many as 21 independent elastic constants. The three dimensional governing differential equations are satisfied at every point of the body.The boundary conditions and interface continuity conditions are satisfied in the sense of a Fourier series. Results are presented for three sample problems to illustrate the versatility of the method. The solution methodology is generalized to study the deformation of finite rectangular plates subjected to arbitrary boundary conditions. The effect of truncation of the series on the accuracy of the solution is carefully examined. Results are presented for thick plates with two opposite edges simply supported and the other two subjected to eight different boundary conditions. The results are compared with three different plate theories.The solution exhibits boundary layers at the edges except when they are simply supported. Results are presented in tabular form for different sets of edge boundary conditions to facilitate comparisons with predictions from various plate theories and finite element formulations. The Eshelby-Stroh formalism is also extended to study the generalized plane deformations of piezothermoelastic laminated plates. The method is capable of analyzing laminated plates with embedded piezothermoelastic patches. Results are presented for a thermoelastic problem and laminated elastic plates with piezothermoelastic lamina attached to its top surface. When a PZT actuator patch is attached to an elastic cantilever substrate, it is observed that the transverse shear stress and transverse normal stress are very large at the corners of the PZT-substrate interface. This dissertation is organized in the form of three self-contained chapters each of which will be submitted for possible publication in a journal.
Ph. D.

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17

Batarseh, Melanie Turkett. "Formation of anisotropic hollow fiber membranes via thermally induced phase separation /." Digital version accessible at:, 1999. http://wwwlib.umi.com/cr/utexas/main.

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18

VIGNOLI, LUCAS LISBOA. "A STUDY OF STRESS CONCENTRATION EFFECTS IN ANISOTROPIC MATERIALS APPLIED TO UNIDIRECTIONAL LAMINATE COMPOSITES." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2016. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=27721@1.

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PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO
Entalhes e mudanças bruscas de geometria são indispensáveis na prática, mas geram uma perturbação no campo de tensões e são responsáveis pela falha da maioria dos componentes estruturais. O presente trabalho tem por objetivo estudar o efeito de concentração de tensão em materiais compósitos. O formalismo de Stroh é utilizado para obter a solução analítica da distribuição de tensão na borda de furos elípticos em placas infinitas anisotrópicas sob tensões nominais aplicados genéricas no plano. A teoria clássica dos laminados é aplicada para obter propriedades equivalentes de laminados simétricos de tal forma que o mesmo possa ser considerado uma placa ortotrópica homogênea de rigidez equivalente. Os critérios de Tsai-Wu, Puck e LaRC05 são estudados pelos seus destacados desempenhos no WWFE (World-Wide Failure Exercise) e aplicados a diversas condições de carregamentos para furos circulares e elípticos para diferentes laminados. O estado multiaxial da distribuição de tensões na borda do furo causado pelo efeito da espessura é estudado analiticamente considerando a hipótese limite de deformação plana. A análise de placas finitas é realizada utilizando o software comercial de elementos finitos ANSYS considerando-se tensão plana para comparar soluções aproximadas para as mesmas encontradas na literatura. Por último, um estudo com base na micromecânica utilizando o modelo de Halpin-Tsai para estimar as propriedades de uma lâmina em função da fração volumétrica das fibras é apresentado para avaliar a importância da mesma na concentração de tensão.
Notches and abrupt geometry variations are unavoidable in practice, but they result in stress field irregularities and are the reason for failure in majority of structural components. The aim of the present work is to study stress concentration on composite materials. To accomplish, the Stroh formalism is introduced to obtain the analytical solution of the stress distribution around the border an elliptical hole in an infinity plate subjected to general in-plane applied nominal stresses. The classical laminate theory is used to obtain equivalent properties of symmetric laminates since it could be modeled as a homogeneous plate with equivalent stiffness. Tsai-Wu, Puck and LaRC05 criteria are discussed in detail and applied for different load conditions for laminate plates with circular and elliptical holes. The multiaxial stress distribution along the hole border caused by the thickness effect is studied using the plane strain hypothesis. Finite plates are analyzed using the commercial finite element package ANSYS considering plane stress hypothesis to compare the approximation solutions available in literature. At last, a micromechanics based approach using the Halpin-Tsai model to estimate the lamina mechanical properties according to the fiber volumetric fraction is presented to evaluate its influence on stress concentration.

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19

Tatsumi, Mio. "Studies on Novel Anisotropic Polymer Composites Synthesized from Mesomorphic Colloidal Suspensions of Cellulose Nanocrystals." Kyoto University, 2015. http://hdl.handle.net/2433/202725.

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Kyoto University (京都大学)
0048
新制・課程博士
博士(農学)
甲第19320号
農博第2141号
新制||農||1036(附属図書館)
学位論文||H28||N4948(農学部図書室)
32322
京都大学大学院農学研究科森林科学専攻
(主査)教授西尾嘉之, 教授木村恒久, 教授髙野俊幸
学位規則第4条第1項該当

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20

Setoodeh, Shahriar. "Optimal Design of Variable-Stiffness Fiber-Reinforced Composites Using Cellular Automata." Diss., Virginia Tech, 2005. http://hdl.handle.net/10919/29204.

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The growing number of applications of composite materials in aerospace and naval structures along with advancements in manufacturing technologies demand continuous innovations in the design of composite structures. In the traditional design of composite laminates, fiber orientation angles are constant for each layer and are usually limited to 0, 90, and Â±45 degrees. To fully benefit from the directional properties of composite laminates, such limitations have to be removed. The concept of variable-stiffness laminates allows the stiffness properties to vary spatially over the laminate. Through tailoring of fiber orientations and laminate thickness spatially in an optimal fashion, mechanical properties of a part can be improved. In this thesis, the optimal design of variable-stiffness fiber-reinforced composite laminates is studied using an emerging numerical engineering optimization scheme based on the cellular automata paradigm. A cellular automaton (CA) based design scheme uses local update rules for both field variables (displacements) and design variables (lay-up configuration and laminate density measure) in an iterative fashion to convergence to an optimal design. In the present work, the displacements are updated based on the principle of local equilibrium and the design variables are updated according to the optimality criteria for minimum compliance design. A closed form displacement update rule for constant thickness isotropic continua is derived, while for the general anisotropic continua with variable thickness a numeric update rule is used. Combined lay-up and topology design of variable-stiffness flat laminates is performed under the action of in-plane loads and bending loads. An optimality criteria based formulation is used to obtain local design rules for minimum compliance design subject to a volume constraint. It is shown that the design rule splits into a two step application. In the first step an optimal lay-up configuration is computed and in the second step the density measure is obtained. The spatial lay-up design problem is formulated using both fiber angles and lamination parameters as design variables. A weighted average formulation is used to handle multiple load case designs. Numerical studies investigate the performance of the proposed design methodology. The optimal lay-up configuration is independent of the lattice density with more details emerging as the density is increased. Moreover, combined topology and lay-up designs are free of checkerboard patterns. The lay-up design problem is also solved using lamination parameters instead of the fiber orientation angles. The use of lamination parameters has two key features: first, the convexity of the minimization problem guarantees a global minimum; second, for both in-plane and bending problems it limits the number of design variables to four regardless of the actual number of layers, thereby simplifying the optimization task. Moreover, it improves the convergence rate of the iterative design scheme as compared to using fiber angles as design variables. Design parametrization using lamination parameters provides a theoretically better design, however, manufacturability of the designs is not certain. The cases of general, balanced symmetric, and balanced symmetric with equal thickness layers are studied separately. The feasible domain for laminates with equal thickness layers is presented for an increasing number of layers. A restricted problem is proposed that maintains the convexity of the design space for laminates with equal thickness layers. A recursive formulation for computing fiber angles for this case is also presented. On the computational side of the effort, a parallel version of the present CA formulation is implemented on message passing multiprocessor clusters. A standard parallel implementation does not converge for an increased number of processors. Detailed analysis revealed that the convergence problem is due to a Jacobi type iteration scheme, and a pure Gauss-Seidel type iteration through a pipeline implementation completely resolved the convergence problem. Timing results giving the speedup for the pipeline implementation were obtained for up to 260 processors. This work was supported by Grant NAG-1-01105 from NASA Langley Research Center. Special thanks to our project monitor Dr. Damodar R. Ambur for his technical guidance.
Ph. D.

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21

FALLAHI, NASIM. "Analysis and Optimization of Variable Angle Tow Composites Through Unified Formulation." Doctoral thesis, Politecnico di Torino, 2021. http://hdl.handle.net/11583/2875739.

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22

Yang, Fan. "An anisotropic model of damage mechanics for inelastic behaviors of fiber reinforced composite laminates /." [Hong Kong] : University of Hong Kong, 1992. http://sunzi.lib.hku.hk/hkuto/record.jsp?B13349429.

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23

Rodopoulos, C. A. "Fatigue studies under constant and variable amplitude loading in MMCs." Thesis, University of Sheffield, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245575.

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24

Rodeghiero, Giacomo. "Complex Anisotropic Panels and Fast Electromagnetic Imaging – CAP-FELIM." Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112223.

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Le Contrôle Non Destructif (CND) de matériaux composites multicouches pour des problèmes de qualité, viabilité, sécurité et disponibilité des systèmes qui impliquent des pièces fabriquées dans les industries aéronautiques et de l’automobile est devenu une tâche essentielle aujourd’hui. L'objectif visé par cette thèse est l’imagerie électromagnétique de structures complexes multicouches anisotropes, de plus en plus utilisées dans des applications, et encore source de sérieux défis à l'étape de leur modélisation et encore plus à l'étape souvent en enfance de leur imagerie. En utilisant une vaste gamme de fréquences, qui va des courants de Foucault jusqu’aux micro-ondes, il y a un fort besoin de rendre disponibles des procédures de modélisation et d'imagerie qui sont robustes, rapides, précises et utiles à la décision des utilisateurs finaux sur des défauts potentiels, tant donc en basse fréquence (BF) (matériaux conducteurs, type fibre de carbone) qu’en haute fréquence (HF) (matériaux diélectriques, type fibre de verre). De plus, il est important d'obtenir des résultats en des temps brefs. Cependant, cela nécessite la connaissance d’une réponse précise à des sources externes aux multicouches, en considérant les couches des composites comme non endommagées ou endommagées : on parle donc de solution du problème direct, avec le cas particulier de sources élémentaires conduisant aux dyades de Green (DGF). La modélisation et la simulation numérique du problème direct sont gérés principalement via une solution au premier ordre de la formulation intégrale de contraste de source impliquant le tenseur de dépolarisation des défauts, quand ceux-ci sont assez petits vis-à-vis de l’épaisseur de peau locale (cas BF) ou de la longueur d'onde locale (cas HF). La précision des DGF doit nécessairement être assurée alors, même si les sources se situent loin de l'origine, ce qui donne un spectre de dyades qui oscille très rapidement. La technique d'interpolation-intégration dite de Padua-Domínguez est ainsi introduite dans le but d'évaluer de façon efficace des intégrales fortement oscillantes.Néanmoins, les matériaux composites peuvent souffrir de divers défauts, lors du processus de fabrication ou pendant leurs utilisations. Vides d’air, cavités remplies de liquide, fissures, etc., peuvent affecter le fonctionnement correct des structures composites. Il est donc indispensable de pouvoir détecter la présence des défauts. Ici, l’insistance est sur la méthode bien connue d’imagerie dite MUltiple SIgnal Classification (MUSIC), qui est basée sur la décomposition en valeurs singulières (SVD) des DGF ; celle-ci est développée afin de localiser les positions de multiples petits défauts volumiques en interaction faible enfouis dans des milieux anisotropes uniaxiaux. Le principal inconvénient de la méthode MUSIC est cependant sa sensibilité par rapport au bruit. Par conséquent, des méthodes MUSIC avec une résolution améliorée et la Recursively Applied and Projected (RAP) MUSIC sont introduites afin de surmonter un tel inconvénient de l'algorithme standard et de fournir des résultats de qualité avec une meilleure résolution. De nombreuses simulations numériques illustrent ces investigations
Non-Destructive Testing/Evaluation (NdT/E) of multi-layered composite materials for problems of quality, viability, safety and availability of systems involving manufactured parts (in aeronautics and in automotive industry, as a good example) has become an interesting and challenging task nowadays. The focus of the PhD thesis is on the electromagnetic (EM) imaging of complex anisotropic multi-slab composite panels as increasingly encountered in applications, yet source of strong challenges at modeling stage and even more at often-in-infancy imaging stage. From eddy-currents to microwaves, there is a strong need to make available modeling and imaging procedures that are robust, fast, accurate and useful to potential end-users’ decision about potential defects both at low-frequency (LF) (conductive materials, carbon-fiber like) and high-frequency (HF) (dielectric materials, glass-fiber like). Moreover, it is important to get the results in close-to-real-time. However, this requires an accurate response to external sources of the multilayers, considering the layers which these composite structures are made of as undamaged or damaged. The modeling at forward stage is managed via a first-order solution involving the dyadic Green’s functions (DGF) of the layers along with the depolarization tensor of the assumed defects when they are small enough vis-à-vis the skin depth (LF case) or the wavelength (HF case). The accuracy of the DGF has to be ensured even if the sources lie far away from the origin, which yields a fast-oscillating spectrum of the dyads. The Padua-Domínguez interpolation-integration technique is introduced herein in order to evaluate in an effective fashion fast-oscillating integrals.Damages or disorders, which these composite structures may suffer from, are of many kinds. One could mention voids, fluid-filled cavities or uniaxial defects with obvious impacts on the electromagnetic and geometric parameters of the multilayers. That is, the task to make available to end-users imaging algorithms tailored to detect the presence of defects. The well-known standard MUltiple SIgnal Classification (MUSIC) algorithm, which is based on the Singular Value Decomposition (SVD) of such DGF, is here applied to localize the positions of small multiple defects with weak interaction embedded in anisotropic uniaxial media. The main drawback of MUSIC is its sensitivity with respect to the noise. Therefore, MUSIC with enhanced resolution and Recursively Applied and Projected (RAP) MUSIC are introduced to overcome such a drawback of the standard algorithm and to provide quality results with better resolution

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25

楊帆 and Fan Yang. "An anisotropic model of damage mechanics for inelastic behaviors of fiber reinforced composite laminates." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 1992. http://hub.hku.hk/bib/B31233314.

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26

Saeidi, Farid. "Hygrothermal Fracture Analysis Of Fibrous Composites With Variable Fiber Spacing Using Jk-integral." Master's thesis, METU, 2013. http://etd.lib.metu.edu.tr/upload/12615370/index.pdf.

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In this study, a Jk-integral based computational method will be developed to conduct fracture analysis of fibrous composite laminates that possess variable fiber spacing. This study will be carried out for the fibrous composites exposed to not only thermal but also hygroscopic boundary condition, which results hygrothermal load. Formulation of the Jk-integral will be carried out by using the constitutive relations of plane orthotropic hygrothermoelasticity. One of the most important challenges of this study is to change Jk-integral formulation into domain independent form, because dealing with infinitely small domains in solving the integral would be frustrating. Developed form of Jk-integral will be merged to ANSYS, a finite element analysis software. Numerical results will be generated so as to assess the influence of variable fiber spacing on the modes I and II stress intensity factors, energy release rate, and the T-stress. For validation and comparison, some of the results are also obtained using Displacement Correlation Technique (DCT).

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27

Lebedev, Gor. "Composites multiferroїques pour dispositifs magnéto-électriques intégrés". Phd thesis, Université de Grenoble, 2012. http://tel.archives-ouvertes.fr/tel-00782536.

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Ce travail de thèse porte sur l'étude de composites magnétoélectriques laminaires dans le but de réaliser des dispositifs innovants intégrés sur silicium tel que l'inductance RF variable. Grâce au couplage mécanique entre des couches adjacentes magnétostrictive ultra douce et piézoélectrique, il est possible d'obtenir un couplage magnétoélectrique indirect qui est supérieur de plusieurs ordres de grandeur à celui des matériaux multiferroïques naturels. Dans un premier temps, nous avons utilisé l'approche phénoménologique basée sur les énergies pour décrire le panorama des effets attendus dans des composites magnétoélectriques laminaires (multicouches). Ensuite, des composites magnétoélectriques macroscopiques à base de substrats piézoélectriques de type MFC et de couches minces de FeCoB ont été réalisés. L'étude du couplage magnétoélectrique en fonction de la composition de FeCoB a permis de déterminer les propriétés clés des matériaux, notamment le rapport λs/Ms, qui sont essentielles pour obtenir un effet magnétoélectrique élevé. Un coefficient magnétoélectrique record de 250 V∙cm‐1Oe‐1 a été obtenu. Par ailleurs, un microscope à effet Kerr a été spécialement développé pour pouvoir observer de manière quasi-instantanée la modification de la structure en domaines sous l'effet de la tension électrique dans ces composites. Pour la première fois, l'observation directe de la rotation de l'axe facile d'aimantation sous commande électrique a été réalisée. La deuxième partie de ce manuscrit est consacrée à la conception, simulation, fabrication et caractérisation d'un dispositif MEMS hybride d'inductance variable intégrée. Ce dispositif exploite l'effet magnétoélectrique indirect entre un élément moteur en PZT (sol gel) et un élément inductif à base de FeCoB. Etant donné le caractère multiphysique hors norme de ce dispositif, un ensemble de tests électriques, mécaniques, optiques et magnétiques a été déployé tout au long de la fabrication. Les résultats concluent à une preuve de concept partiellement fonctionnelle en raison principalement d'une mauvaise gestion des contraintes internes liées à la fabrication. Les pistes d'amélioration aux niveaux du design, des matériaux et des procédés sont identifiées.

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28

Fayazbakhsh, Kazem. "The impact of gaps and overlaps on variable stiffness composites manufactured by automated fiber placement." Thesis, McGill University, 2014. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=121454.

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Variable stiffness design of composites, where the fiber orientation changes within the plane of laminates, can improve the structural performance of the laminates, and provide a trade-off between design objectives. Laminates with variable stiffness design can be manufactured with Automated Fiber Placement (AFP) technology. During the manufacturing, certain defects, in the form of gaps and/or overlaps, often emerge and hence affect the structural performance of the composite laminates. The amount of change in structural properties depends on the laminates design and manufacturing parameters. Therefore, calculating the actual improvement in the structural properties of variable stiffness composite laminates, when the effect of defects is considered, has been a challenge. In this thesis, first, to fully exploit the benefits of variable stiffness design, simultaneous maximization of in-plane stiffness and buckling load for composite laminates with a constant curvature fiber path is investigated using a surrogate-based genetic algorithm. It is shown that varying the fiber orientation perpendicular to the loading direction can increase buckling load up to 111%. However, considering the minimum turning radius constraint imposed by AFP, the maximum achievable improvement in buckling load reduces to 57%, due to the limited fiber steering. Then, a methodology is developed to locate gaps and overlaps and calculate their area percentages. It is found that by increasing the number of tows inside a course, when the tow width is kept constant, the gap and overlap area percentages are significantly reduced. Smaller reductions in gap and overlap area percentages are obtained when the tow width increases and the number of tows is kept constant. Finally, a strategy, namely the Defect Layer Method, is proposed to build the Finite Element model of variable stiffness laminates with gaps and overlaps. This method is proved to be computationally efficient and more precise compared to the available method in the literature. Finite Element Analysis of two selected variable stiffness laminates is performed and it is found that overlaps increase their structural performance, while gaps deteriorate the mechanical properties. Considering the presence of overlaps, the maximum achievable improvement in buckling load can be increased to 105% compared to 56%, where the overlaps are ignored. Considering the presence of gaps within the laminate, the improvement in buckling load is limited to 40%. Modified Pareto fronts considering the effect of gaps or overlaps are obtained to provide important design guidelines of direct interest to industry.
La conception de composites à rigidité variable (où l'orientation des fibres change avec le plan du stratifié) peut améliorer les performances mécaniques du stratifié, et permettre un meilleur compromis dans les designs. Les stratifiés à rigidité variable peuvent être obtenus à l'aide du procédé de dépose automatique de fibres (AFP). Souvent, lors de la fabrication, certains défauts (espaces ou recouvrements) apparaissent et affectent les performances mécaniques du stratifié. La perte de propriétés mécanique dépend des paramètres de conception et des paramètres du procédé. Ainsi, l'objectif de ce travail de thèse est de prédire l'amélioration effective des propriétés mécaniques d'un composite stratifié à rigidité variable, en tenant compte de ces défauts. D'abord, afin d'exploiter maximum les bénéfices du design à rigidité variable, une optimisation du parcours de dépose permet de maximiser la rigidité dans le plan et la charge maximal admissible en flambage. Un rayon de courbure constant du parcours de dépose est étudié à l'aide d'un algorithme génétique. Les fibres orientées perpendiculairement à la direction de chargement peuvent augmenter la charge de flambage jusqu'à 111%. Toutefois, en considérant le rayon de courbure minimum admissible par la tête de dépose AFP, l'amélioration maximum réalisable en terme de charge de flambage se réduit à 57%. Ensuite, une méthodologie est développée pour localiser les espaces et les recouvrements et calculer leurs pourcentages surfaciques. Il est constaté qu'une augmentation du nombre de mèches par course, en conservant une largeur de mèche constante, réduit significativement le pourcentage d'espaces et recouvrements. Une réduction inférieure est obtenue si la largeur de la mèche est augmentée en conservant le même nombre total de mèches. Enfin, une stratégie, la méthode de la couche défectueuse (Defect Layer Method), est proposée pour construire un modèle d'éléments finis pour les composites à rigidité variable avec espaces et recouvrements. Cette méthode est efficace numériquement et plus précise que les méthodes proposées dans la littérature. Une analyse par éléments finis de deux matériaux composites à rigidité variable spécifique est proposée. Elle montre que les recouvrements améliorent les performances mécaniques tandis que les espaces les réduisent. En considérant le cas d'une dépose avec recouvrements, l'amélioration maximale réalisable en terme de charge de flambage est de 105% (à comparer à 56% lorsque les recouvrements sont ignorés). En considérant le cas d'une dépose avec espace, l'amélioration en charge de flambement est limitée à 40%. Des frontières d'efficacité de Pareto prennent en compte les effets des espaces et des recouvrements sont obtenues. Elles constituent d'importants principes de conception qui ont un intérêt direct pour l'industrie.

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29

Sen, Ozge. "Transient Dynamic Response Of Viscoelastic Cylinders Enclosed In Filament Wound Cylindrical Composites." Phd thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606412/index.pdf.

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In this study, transient dynamic response of viscoelastic cylinders enclosed in filament wound cylindrical composites is investigated. Thermal effects, in addition to mechanical effects, are taken into consideration. A generalized thermoelasticity theory which incorporates the temperature rate among the constitutive variables and is referred to as temperature-rate dependent thermoelasticity theory is employed. This theory predicts finite heat propagation speeds. The body considered in this thesis consists of n+1-layers, the inner layer being viscoelastic, while the outer fiber reinforced composite medium consist of n-different generally orthotropic, homogeneous and elastic layers. In each ply, the fiber orientation angle may be different. The body is a hollow circular cylinder with a finite thickness in the radial direction, whereas it extends to infinity in the axial direction. The multilayered medium is subjected to uniform time-dependent dynamic inputs at the inner and/or outer surfaces. The body is assumed to be initially at rest. The layers are assumed to be perfectly bonded to each other. The case in which the inner surface of the viscoelastic cylinder is a moving boundary is further investigated in this study. This is similar to the solid propellant rocket motor cases. The solid propellant is modelled as a viscoelastic material which in turn is modelled as standard linear solid
whereas, the rocket motor case is a fiber-reinforced filament wound cylindrical composite. Method of characteristics is employed to obtain the solutions. Method of characteristics is suitable because the governing equations are hyperbolic. The method is amenable to numerical integration and different boundary, interface and initial conditions can be handled easily.

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30

BEN, ZINEB TARAK. "Analyse des zones a forts gradients de contraintes dans les plaques composites elancees a profil variable." Paris, ENSAM, 1996. http://www.theses.fr/1996ENAM0004.

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Il s'agit d'etudier l'influence d'une variation rapide de l'epaisseur d'une plaque composite verre-epoxy soumise a un chargement de flexion pure. Le profil de la zone a epaisseur variable est defini par deux parametres geometriques que nous faisons evoluer pour etudier leur influence. Un modele numerique par elements finis a deux echelles a ete developpe. La premiere est macroscopique et concerne la majeure partie a epaisseur constante. Elle permet de calculer le comportement global de l'eprouvette. La deuxieme est mesoscopique et represente les domaines a epaisseur variable. Elle permet le calcul de l'etat local de concentration de contraintes. Une loi de transition est introduite au niveau des frontieres entre les deux zones a echelle de modelisation differente. Pour valider ce modele et etudier les types de rupture, une campagne experimentale a ete menee. Elle a consiste a caracteriser le composite et a solliciter en flexion pure des eprouvettes composites presentant la forme adoptee lors de la modelisation. Elles sont fabriquees par le procede rtm. La sollicitation de flexion pure est assuree grace a un banc d'essais entierement concu au sein du laboratoire et adapte aux structures flexibles. Une bonne correlation entre les resultats numeriques et experimentaux a ete observee

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31

Huber, Armin [Verfasser], and Markus [Akademischer Betreuer] Sause. "Numerical Modeling of Guided Waves in Anisotropic Composites with Application to Air-coupled Ultrasonic Inspection / Armin Huber ; Betreuer: Markus Sause." Augsburg : Universität Augsburg, 2021. http://d-nb.info/1230755314/34.

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32

Parra, Martinez Juan Pablo. "On multilayered system dynamics and waves in anisotropic poroelastic media." Doctoral thesis, KTH, VinnExcellence Center for ECO2 Vehicle design, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-195801.

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The mechanical and acoustic anisotropy of media is a governing factor in the behaviour of multilayered systems including such media. The understanding of the mechanisms conditioning the dynamic behaviour of multilayered systems is of paramount importance. In particular, the intrinsic anisotropy of poroelastic media presents a potential for the optimal design of systems for multifunctional performances. Indeed, these multilayered systems are bound by stiffness, thermal and acoustic performance constraints in simultaneously. A plane wave method is presented to study the influence of material orientation in the dynamic behaviour of multilayered systems composed of anisotropic poroelastic media. The method is applied to a system composed of an anisotropic open-celled melamine foam core in between two metal sheets. This particular multilayered configuration allows to shed light on phenomena intrinsic to layers composed of anisotropic poroelastic materials, such as the frequency shift of the fundamental resonance of the panel, or the compression-shear coupling effects taking place in the poroelastic core layers. The latter phenomena is of particular importance, as it is evidenced on the unconventional polarisation of waves in anisotropic poroelastic media. Finally, the method is adapted to the optimisation of multilayered systems for acoustic performance. the design variables are consequently the core material orientations with respect to the global coordinate system. The solutions to the optimisation problem are analysed in terms of dynamic behaviour, thus allowing to correlate acoustic performance of the overall structure, and the response of each individual layer.

QC 20161110

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33

Leite, Rubim Rafael. "Graphene oxide sheets confined within anisotropic fluid matrices." Thesis, Bordeaux, 2018. http://www.theses.fr/2018BORD0209/document.

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Dès sa découverte, le graphène oxydé (GO), le plus accessible des précurseurs du graphène, a été largement utilisé pour des applications en science et technologie. La motivation de ce travail est d'étudier, d'un point de vue fondamental, le couplage entre des bicouches amphiphiles auto-associées (lesquelles peuvent être vues comme une matrice anisotrope formée d'objets bidimensionnels) et un objet lui-même bidimensionnel, en l'occurrence le feuillet de graphène oxydé, quand ils sont dispersés dans un solvant commun. La compétition entre les élasticités intrinsèques des bicouches et des feuillets de GO, ainsi que les interactions directes bicouche-bicouche, bicouche-GO et GO-GO, permet d'envisager un riche polymorphisme en fonction de la composition du système. Après avoir développé une procédure destinée à contrôler, dans une gamme étendue de teneur en GO, le système binaire GO-eau, le domaine confiné des dispersions aqueuses de GO a été exploré et, par la suite, le diagramme de phases ternaire a été construit. Les systèmes obtenus ont été caractérisés par des techniques comme la microscopie optique et la diffusion du rayonnement (diffusion dynamique de la lumière et diffusion des rayons-X aux petits angles). Les propriétés élastiques et thermodynamiques ont été décrites par l'application de modèles initialement conçus pour les phases lamellaires à deux constituants et adaptés dans le cadre de cette étude
Since the discovery of graphene oxide (GO), the most accessible of the precursors of graphene, this material has been widely studied for applications in science and technology. The motivation of this work is to study with a fundamental perspective the coupling between amphiphilic bilayers, which can be seen as an anisotropic matrix formed of two-dimensional objects, and another two-dimensional object, namely the graphene oxide sheet when they are dispersed in a common solvent. The competition between the intrinsic elasticities of the bilayers and GO sheets, as well as between direct bilayer-bilayer, bilayer-GO and GO-GO interactions allows us to envisage a rich polymorphism, depending on the composition of the system. Following the development of a dedicated procedure for controlling in an extended range of GO content the binary GO-water system, the confined domain of aqueous GO dispersions was first investigated, and the ternary phase diagram then constructed. The obtained systems have been characterised, using techniques such as optical microscopy, light and x-ray scattering. Elastic and thermodynamic properties have been described by applying, and adapting to the scope of this study, models for two-component lamellar stacks
Desde sua descoberta, o grafeno oxidado (GO), o mais acessível dos precursores do grafeno,tem sido amplamente utilizado para aplicações na ciéncia e tecnologia. A motivação destetrabalho é de estudar, de um ponto de vista fundamental, o acoplamento entre bicamadas anfifílicas auto-organizadas (que podem ser vistas como uma matriz anisotrópica formada por objetos bidimensionais) e um objeto ele mesmo bidimensional, neste caso a folha de óxido de grafeno, quando estão dispersados em um solvente comum.A competição entre as elasticidades intrínsecas das bicamas e das folhas de GO, assimcomo as interaçãoes diretas bicamada-bicamada, bicamada-GO e GO-GO, permitem esperar um rico polimorfismo em função da composição do sistema. Seguindo o desenvolvimento de um procedimento destinado ao controle, em um intervalo extendido da quantidade de GO, o sistema binário GO-água, o domínio confinado de dispersões aquosas de GO foi explorado e, em seguida, o diagrama de fases ternário contruído.Os sistemas obtidos foram caracterizados por t_ecnicas como microscopia ótica, espalhamento dinâmico de luz e espalhamento de raios-x à baixos ângulos. As propriedadeselásticas e termodinâmicas foram descritas pela aplicação de modelos inicialmente concebidos para fases lamelares à dois constituintes e adaptados ao escopo deste estudo

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34

Bacco, Giacomo. "Advanced Design and Optimization of Anisotropic Synchronous Machines." Doctoral thesis, Università degli studi di Padova, 2019. http://hdl.handle.net/11577/3423172.

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This work covers many research aspects of anisotropic synchronous motors, which are synchronous reluctance (SyR), permanent magnet assisted synchronous reluctance (PMaSyR) and interior permanent magnet (IPM) machines. In fact, all these kinds of machines exhibit quite a strong reluctance torque component, hence the name anisotropic. From the early 2000s, the design of electric machines started to deeply rely on finite element analysis (FEA) coupled to automatic optimization algorithms. This workflow enabled the machine designer to make fewer initial sizing hypotheses and to explore a wider design space. The drawbacks of this approach are that the time required is long and that the computational resources needed are quite large. However, the computing performances have always been improving over the years, especially when multi-processor architectures became widespread. Therefore nowadays it is common to employ tens or even hundreds of cores on cluster PCs to perform FEA during optimization runs. The thesis is structured as follows. The first part gives the background knowledge needed to develop the topics covered in the following. This comprehends an introduction to the machines studied, some general knowledge about magnetic materials, some basic concepts about the differential evolution (DE) algorithm, and the drawing of fluid rotor flux-barriers. The second part deals with the analytical modeling of SyR and PMaSyR machines. The complete model is nonlinear and may become convoluted to develop especially in an industrial environment. Therefore, using simplifying assumptions, a handful of simple equations can be derived. This simple model is also extended and applied to asymmetric rotor structures, which try to compensate torque harmonics. The third part focuses on applied multi-objective optimizations coupled to FEA for many different case studies. In particular, a SyR motor (SyRM) for pumping applications is optimized, prototyped and tested. Then, a feasibility study on a very low speed PMaSyR motor is carried out through multi-objective optimization. After that, high speed SyRMs are studied and optimized to understand the power limits of this kind of machine. Finally, the DE multi-objective optimization algorithm is also applied to improve the sensorless-control capabilities of anisotropic machines by design.
Questo lavoro analizza molti aspetti di ricerca dei motori sincroni anisotropi, che includono le macchine sincrone a riluttanza pura (SyR), a riluttanza assistita da magneti (PMaSyR) e le macchine a magneti permanenti interni (IPM). Infatti, tutte queste macchine esibiscono una forte componente di riluttanza, da cui il nome anisotrope. Dai primi anni 2000, la progettazione di macchine elettriche ha cominciato a basarsi in modo consistente sull’analisi agli elementi finiti (FEA) accoppiata ad algoritmi di ottimizzazione automatici. Questo flusso di lavoro permette al progettista di fare un minor numero di ipotesi preliminari e di esplorare uno spazio di progetto più ampio. Gli svantaggi di questo approccio sono che il tempo richiesto è lungo e che le risorse computazionali richieste possono essere elevate. Tuttavia, le prestazioni dei computer migliorano di anno in anno, e in particolar modo con la diffusione delle architetture a multi-processore. Pertanto oggigiorno è comune impiegare decine o persino centinaia di core su cluster di PC per effettuare analisi agli elementi finiti durante un’ottimizzazione. La tesi è strutturata nel seguente modo. La prima parte copre le conoscenze di base necessarie a sviluppare gli argomenti trattati nel seguito. C’è quindi un’introduzione alle macchine studiate, delle conoscenze generali sui materiali magnetici e ferromagnetici, alcuni concetti di base sull’algoritmo di ottimizzazione differential evolution (DE) utilizzato, e il disegno delle barriere fluide dei rotori di macchine a riluttanza. Nella seconda parte si sono sviluppati modelli analitici di macchine SyR e PMaSyR. Il modello completo è non lineare e può diventare abbastanza complesso da sviluppare, specialmente in un contesto industriale. Pertanto, usando alcune ipotesi semplificative, si possono derivare alcune semplici equazioni di progetto. Questo modello semplice è anche esteso e applicato a strutture di rotore asimmetriche, che tentano di compensare alcune armoniche di coppia. La terza parte si concentra sull’applicazioni di ottimizzazioni multiobiettivo accoppiate a FEA per alcuni casi di studio. In particolare, si è ottimizzato, prototipato e testato un motore SyRper pompe centrifughe. Poi, è stato condotto uno studio di fattibilità per un motore PMaSyR attraverso ottimizzazioni multi-obiettivo. Dopodiché si sono studiati motori SyRper alte velocità e si sono dedotti i limiti di potenza di questa macchina. Infine l’ottimizzazione DE multi-obiettivo è stata anche applicata per migliorare le capacità di controllo sensorless delle macchine anisotrope già in fase di progetto.

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35

Bagault, Caroline. "Mechanical contact for layered anisotropic materials using a semi-analytical method." Phd thesis, INSA de Lyon, 2013. http://tel.archives-ouvertes.fr/tel-00940377.

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Fretting and wear are recurrent problems in the field of aeronautics. Contacts the blade / disk at the compressor or high-pressure turbine aircraft engines, for example, are subjected to high stresses at high temperatures. The challenge for manufacturers is to maximize the lifetime of these components and be able to predict crack initiation. To improve handling parts, coatings are used to protect them. Materials and their mechanical properties have a direct impact on the contact and the lifetime. The choice of materials, number of layers, the thickness of the order are therefore essential. By their composition (fibers, single crystals), elaboration (extrusion) or their mode of deposition, the hypothesis to consider homogeneous isotropic materials is too simplistic. Anisotropy is an important parameter to take into account in the design. Composite materials are increasingly used in the aeronautic. In this context, this thesis aims to study the behavior of anisotropic homogeneous materials, focusing on the influence of the main parameters mechanical characteristics of a material to better understand their effects. We focus on the Young's modulus (or modulus of elasticity), the module Coulomb (or shear modulus) and the Poisson's ratio, and values in different directions. As expected, the Young's modulus in the direction normal to the contact plays an important role in determining the pressure profile. However, the influence of Young modulus in the plane tangent to the contact is not to neglect it also alters the shape of the contact area. The orientation of the material with respect to the contact is therefore a parameter to take into consideration, it can directly reduce or enhance the effect of Young's modulus in a direction privileged. The module Coulomb and Poisson's ratio were also analyzed. As a result they significantly affect the contact. These results are confirmed in the case of a coated solid, unlike the effects of coating and substrate can compensate. The impact properties of the coating will be even more important than it is thick. The scale of the contact relative to the materials used is also important on the pressure profiles. A comparison between the model anisotropic homogeneous and isotropic heterogeneous model have been performed. At mesoscopic scale, the composite is composed of a matrix with fibers that induce pressure peaks while at the macroscopic level, the composite material is seen as a homogeneous material, the pressure profiles are smoothed.

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36

Ducoulombier, Nicolas. "Anisotropic concrete : 3D priting of concrete reinforced by long fibers, process, characterisation et modelisation." Thesis, Paris Est, 2020. http://www.theses.fr/2020PESC2070.

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Ce travail s’intéresse au renforcement des matériaux cimentaire mis en œuvre par fabrication additive à grande échelle. Ce nouveau procédé permet une complexité géométrique importante, généralement fortement consommatrice de moyens matériels et humains. De plus, il rend théoriquement possible l’industrialisation de la fabrication d’éléments constructifs singuliers, par exemple optimisés par répondre à un chargement mécanique donné. Cependant, il n’existe à l’heure actuelle aucune méthode de renforcement standardisée permettant d’obtenir la résistance en traction et la ductilité nécessaire pour leur utilisation dans les structures des bâtiments. Ce qui limite fortement leur utilisation dans la pratique.Si de nombreuses méthodes sont envisagées dans la littérature pour le renforcement des matériaux cimentaires mis en œuvre par impression 3D, celles-ci sont généralement calquées sur les méthodes traditionnelles du renforcement : bétons fibrés, armatures passives et câble de précontraintes. Ce travail de thèse propose un procédé de renforcement alternatif, breveté au cours de ce travail de thèse, qui tire parti de la spécificité du procédé d’extrusion. De nombreux renforts continus sont en effet insérés dans la filière d’extrusion, appelé ici tête d’impression et entrainé par le débit du matériau cimentaire, ce dernier fournissant la force nécessaire aux déroulements des renforts continus. Le matériau extrudé est alors un composite unidirectionnel à matrice cimentaire renforcé par de nombreuses fibres continues alignées selon la direction du parcours d’impression.Ce travail définit alors le cahier des charges du procédé en termes de propriétés rhéologiques de la matrice cimentaire au moment du dépôt et le type de renfort à privilégier permettant l’obtention d’une bonne adhérence des renforts à la matrice cimentaire, nécessaire au développement d’un renforcement significatif en traction. Le comportement mécanique de l’interface est par ailleurs étudié précisément grâce aux développements d’essais micromécaniques dédiées et l'observation de l’endommagement par microtomographie aux rayons X. Les perspective de ce travail sont la caractérisation et la modélisation multi-échelles du comportement du composite à matrice cimentaire et la proposition de systèmes constructifs innovants
This work focuses on the reinforcement strategies for large scale additive manufacturing of cementitious materials. This new process allows an important geometrical complexity for constructive elements, generally consuming a lot of material and human resources. In addition, it makes it theoretically possible to industrialize the manufacture of singular constructive elements, for example optimized to meet a given mechanical load. However, there is currently no standardized reinforcement method for obtaining the tensile strength and ductility required for their use in building structures. This severely limits their use in practice.While many reinforcement methods are considered in the literature for the 3D-printed cementitious materials, they are a direct transcription of the traditional reinforcement methods such as fibre-reinforced concrete, passive reinforcement and post-tension method. This thesis work proposes an alternative reinforcement process, patented during this thesis work, which takes advantage of the specificity of the extrusion process. Many continuous reinforcements can be inserted before the extrusion die and driven by the flow of the cementitious material, the latter providing the force necessary for the unwinding of each individual continuous reinforcements. The extruded material is then a unidirectional cementitious matrix composite reinforced by many continuous fibers aligned in the direction of the printing path.This work then defines the specifications of the process in terms of rheological properties of the cementitious matrix at the time of deposition and the type of reinforcement to be preferred, allowing good cohesion between the reinforcements and the cementitious matrix necessary for the development of a significant tensile reinforcement. The mechanical behaviour of the interface is also precisely studied thanks to the development of dedicated micromechanical tests and the observation of the damage by X-ray microtomography. The perspectives of this work are the characterization and multi-scale modeling of the behavior of the cementitious matrix composite and the proposal of innovative constructive systems

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37

Post, Nathan L. "Reliability based design methodology incorporating residual strength prediction of structural fiber reinforced polymer composites under stochastic variable amplitude fatigue loading." Diss., Virginia Tech, 2008. http://hdl.handle.net/10919/26492.

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The research presented in this dissertation furthers the state of the art for reliability-based design of composite structures subjected to high cycle variable amplitude (spectrum) fatigue loads. The focus is on fatigue analyses for axially loaded fiber reinforced polymer (FRP) composites that contain a significant proportion of fibers in the loading direction and thus have fiber-direction dominated failure. The four papers presented in this dissertation describe the logical progression used to develop an improved reliability-based methodology for fatigue-critical design. Throughout the analysis extensive experimental fatigue data on several material systems was used to verify the assumptions and suggest the path forward. A comparison of 12 fatigue model approaches from the literature showed that a simple linear residual strength approach (Broutman and Sahu) provides an improvement in fatigue life prediction compared to the Palmgren-Miner rule, while more complex residual strength models did not consistently improve on Broutman and Sahu. Evaluation of the effect of load history randomness on fatigue life was made using experimental results for spectra in terms of the first order autocorrelation of the stress events. For approximately reversed Rayleigh distributed fatigue loading, load sequence was not critical in the material behavior. Based on observations of empirical data and evaluation of the micro-mechanics deterioration and failure phenomena of FRP composites under fatigue loading, a new residual strength model for the tension and compression under any load history was proposed. Then this model was implemented in a stochastic framework and a method was proposed to enable calculation of the load and resistance factor design (LRFD) parameters for realistic reliabilities with relatively few computations. The proposed approach has significant advantages over traditional lifetime-damage-sum-based reliability analysis and provides a significant step toward enabling more accurate reliability-based design with composite materials.
Ph. D.

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38

Blanco, Villaverde Norbert. "Variable mixed-mode delamination in composite laminates under fatigue conditions: testing & analysis." Doctoral thesis, Universitat de Girona, 2005. http://hdl.handle.net/10803/7760.

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La majoria de les fallades en elements estructurals són degudes a càrrega per fatiga. En conseqüència, la fatiga mecànica és un factor clau per al disseny d'elements mecànics. En el cas de materials compòsits laminats, el procés de fallada per fatiga inclou diferents mecanismes de dany que resulten en la degradació del material. Un dels mecanismes de dany més importants és la delaminació entre capes del laminat. En el cas de components aeronàutics, les plaques de composit estan exposades a impactes i les delaminacions apareixen facilment en un laminat després d'un impacte. Molts components fets de compòsit tenen formes corbes, superposició de capes i capes amb diferents orientacions que fan que la delaminació es propagui en un mode mixt que depen de la grandària de la delaminació. És a dir, les delaminacions generalment es propaguen en mode mixt variable. És per això que és important desenvolupar nous mètodes per caracteritzar el creixement subcrític en mode mixt per fatiga de les delaminacions. El principal objectiu d'aquest treball és la caracterització del creixement en mode mixt variable de les delaminacions en compòsits laminats per efecte de càrregues a fatiga. Amb aquest fi, es proposa un nou model per al creixement per fatiga de la delaminació en mode mixt. Contràriament als models ja existents, el model que es proposa es formula d'acord a la variació no-monotònica dels paràmetres de propagació amb el mode mixt observada en diferents resultats experimentals. A més, es du a terme un anàlisi de l'assaig mixed-mode end load split (MMELS), la característica més important del qual és la variació del mode mixt a mesura que la delaminació creix. Per a aquest anàlisi, es tenen em compte dos mètodes teòrics presents en la literatura. No obstant, les expressions resultants per l'assaig MMELS no són equivalents i les diferències entre els dos mètodes poden ser importants, fins a 50 vegades. Per aquest motiu, en aquest treball es porta a terme un anàlisi alternatiu més acurat del MMELS per tal d'establir una comparació. Aquest anàlisi alternatiu es basa en el mètode dels elements finits i virtual crack closure technique (VCCT). D'aquest anàlisi en resulten importants aspectes a considerar per a la bona caracterització de materials utilitzant l'assaig MMELS. Durant l'estudi s'ha dissenyat i construït un utillatge per l'assaig MMELS. Per a la caracterització experimental de la propagació per fatiga de delaminacions en mode mixt variable s'utilitzen diferents provetes de laminats carboni/epoxy essencialment unidireccionals. També es du a terme un anàlisi fractogràfic d'algunes de les superfícies de fractura per delaminació. Els resultats experimentals són comparats amb les prediccions del model proposat per la propagació per fatiga d'esquerdes interlaminars.
Most of the failures in structural elements in use are a consequence of mechanical fatigue. Therefore, fatigue is a decisive factor in designing durable mechanical elements. In laminated composite materials, the fatigue process involves different damage mechanisms that result in the degradation of the material. One of the most important damage mechanisms is the delamination between plies of the laminate. In aeronautical applications, composite plates are sensitive to impact and delamination occurs readily in composite laminates on impact. Many composite components have curved shapes, tapered thickness and plies with different orientations, which make the delamination grow with a mode mix that depends on the extent of the crack. Thus, delaminations generally grow under varying mode mix. It is therefore important to develop methods that can characterise subcritical, mixed-mode growth in fatigue delamination. The main objective of the present investigation is the characterisation of the variable mixed-mode delamination in composite laminates under fatigue conditions. To this end, a mixed-mode fatigue delamination model is proposed. Oppositely to the mixed-mode fatigue delamination models present in the literature, the proposed model takes into account the non-monotonic variation of the propagation parameters with the mode mix observed in different experimental data. Moreover, the mixed-mode end load split (MMELS) test, which main characteristic is that the propagation mode of the interlaminar crack varies with the crack extent, is analysed. Two theoretical approaches present in the literature are considered. However, the resulting expressions for the MMELS test are not equivalent and the differences between approaches can be up to 50 times. A more accurate alternative analysis of the MMELS test is carried out in the present study for comparison. The alternative analysis is based on the finite element method and the virtual crack closure technique. Significant findings are found for precise materials characterisation using the MMELS test. A MMELS test rig is also designed and built. Different specimens of essentially unidirectional carbon/epoxy laminates are tested for the experimental characterisation of fatigue delamination under varying mode mix. A fractographic analysis is also conducted in some of the delaminated fracture surfaces. The experimental results are compared to the predictions of a proposed model for the fatigue propagation of interlaminar cracks.

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39

Zoukel, Abdelhalim. "Etude des phénomènes d’interaction faisceau d’électrons-gaz-matière dans un MEB à pression variable : Applications aux matériaux composites (polymères, céramiques et métaux)." Thesis, Lille 1, 2013. http://www.theses.fr/2013LIL10161/document.

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La microscopie électronique à balayage est une technique essentielle pour la caractérisation des matériaux. La nouvelle génération de MEB connue sous le nom de MEB à pression variable (aussi appelée MEB environnemental) permet de travailler dans des conditions moins drastiques de pression et de tension. Cependant, l’imagerie et la micro-analyse chimique rencontrent un défi majeur en ce qui concerne la diffusion du faisceau d'électrons primaires par les atomes/molécules du milieu gazeux. Ce phénomène de diffusion (skirt) conduit à l'apparition de plusieurs artéfacts au-delà de ceux qui sont familiers dans un MEB conventionnel. Le principal artéfact reconnu est la dégradation de la résolution spatiale qui est délimitée par le volume d'interaction en mode high-vacuum. Les objectifs de la recherche rapportés ici sont les suivants: (i) étudier l'ampleur et l'étendue de la fraction du faisceau d'électrons diffusée. (ii) le développement d'une méthodologie originale et nouvelle, afin de faire face à l'effet du skirt sur la résolution spatiale. L'efficacité de cette étude est démontrée par sa capacité à quantifier les effets de certains paramètres expérimentaux sur la dégradation de la résolution spatiale. En outre, la nouvelle méthodologie proposée est un atout précieux pour garder la résolution spatiale ultime obtenue en mode high-vacuum. Cela dépend fortement du nouveau volume d'interaction (appelé le volume d'interaction en mode low-vacuum) créé à la fois par la fraction du faisceau d'électrons diffusée et la fraction non-diffusée
Scanning electron microscope (SEM) is an essential technique to characterize materials. The new generation of SEMs known as a variable pressure SEM (also named environmental SEM) allows to work under less drastic conditions of pressure and voltage. However, the imaging and chemical microanalysis face a major challenge with regard to the scattering of the primary electron beam by the atoms/molecules of the gas medium. This phenomenon of beam skirting leads to the appearance of several artifacts beyond those familiar in conventional SEM. The main recognized artifact is the degradation of the spatial resolution which is delineated by the high-vacuum interaction volume. The objectives of the research reported herein were: (i) to study the magnitude and the extent of the electron beam skirt. (ii) and the development of an original and new methodology in order to deal with the effect of the electron beam skirt on the spatial resolution. The effectiveness of this study is demonstrated by its ability to quantify the effects of some experimental parameters on the degradation of the spatial resolution. Further, the new methodology proposed is a valuable asset to keep the ultimate spatial resolution obtained at high vacuum mode. This depend strongly on the new interaction volume (called the low-vacuum interaction volume) created by both scattered and unscattered fraction of the electron beam

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40

Zoukel, Abdelhalim. "Etude des phénomènes d’interaction faisceau d’électrons-gaz-matière dans un MEB à pression variable : Applications aux matériaux composites (polymères, céramiques et métaux)." Electronic Thesis or Diss., Lille 1, 2013. http://www.theses.fr/2013LIL10161.

Повний текст джерела

Анотація:

La microscopie électronique à balayage est une technique essentielle pour la caractérisation des matériaux. La nouvelle génération de MEB connue sous le nom de MEB à pression variable (aussi appelée MEB environnemental) permet de travailler dans des conditions moins drastiques de pression et de tension. Cependant, l’imagerie et la micro-analyse chimique rencontrent un défi majeur en ce qui concerne la diffusion du faisceau d'électrons primaires par les atomes/molécules du milieu gazeux. Ce phénomène de diffusion (skirt) conduit à l'apparition de plusieurs artéfacts au-delà de ceux qui sont familiers dans un MEB conventionnel. Le principal artéfact reconnu est la dégradation de la résolution spatiale qui est délimitée par le volume d'interaction en mode high-vacuum. Les objectifs de la recherche rapportés ici sont les suivants: (i) étudier l'ampleur et l'étendue de la fraction du faisceau d'électrons diffusée. (ii) le développement d'une méthodologie originale et nouvelle, afin de faire face à l'effet du skirt sur la résolution spatiale. L'efficacité de cette étude est démontrée par sa capacité à quantifier les effets de certains paramètres expérimentaux sur la dégradation de la résolution spatiale. En outre, la nouvelle méthodologie proposée est un atout précieux pour garder la résolution spatiale ultime obtenue en mode high-vacuum. Cela dépend fortement du nouveau volume d'interaction (appelé le volume d'interaction en mode low-vacuum) créé à la fois par la fraction du faisceau d'électrons diffusée et la fraction non-diffusée
Scanning electron microscope (SEM) is an essential technique to characterize materials. The new generation of SEMs known as a variable pressure SEM (also named environmental SEM) allows to work under less drastic conditions of pressure and voltage. However, the imaging and chemical microanalysis face a major challenge with regard to the scattering of the primary electron beam by the atoms/molecules of the gas medium. This phenomenon of beam skirting leads to the appearance of several artifacts beyond those familiar in conventional SEM. The main recognized artifact is the degradation of the spatial resolution which is delineated by the high-vacuum interaction volume. The objectives of the research reported herein were: (i) to study the magnitude and the extent of the electron beam skirt. (ii) and the development of an original and new methodology in order to deal with the effect of the electron beam skirt on the spatial resolution. The effectiveness of this study is demonstrated by its ability to quantify the effects of some experimental parameters on the degradation of the spatial resolution. Further, the new methodology proposed is a valuable asset to keep the ultimate spatial resolution obtained at high vacuum mode. This depend strongly on the new interaction volume (called the low-vacuum interaction volume) created by both scattered and unscattered fraction of the electron beam

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41

Marchetti, Fabien. "Modelling and characterisation of anisotropic multilayered plates on a wide frequency range." Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEI130.

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Le présent travail de thèse traite de la modélisation vibratoire et de la caractérisation dynamique de matériaux multicouches anisotropes. Dans le premier chapitre, une classification de quelques modèles analytiques de multicouche et techniques de caractérisation expérimentales de structures planes est établie sous la forme d'une synthèse bibliographique. Elle regroupe les récents résultats publiés autour du sujet. Le second chapitre propose une extension d'un modèle équivalent de multicouche au cas des structures anisotropes. Ce modèle décrit les premières courbes de dispersion de la structure et tient compte des phénomènes de cisaillement pouvant agir sur le mode de flexion en hautes fréquences. Le principal avantage du modèle réside dans sa simplicité et sa rapidité grâce à son nombre de variables cinématiques indépendant du nombre de couches. Les caractéristiques du multicouche sont définies par les 5 rigidités de flexion d'une plaque équivalente mince qui est comparée à ce dernier. L'extension de modèle est validée à l'aide d'une étude expérimentale réalisée sur des plaques composites en fibres de carbone. Un intérêt tout particulier est porté sur la modélisation de l'amortissement structurel. Une définition énergétique du facteur de perte, décrite par une formulation spatiale et temporelle de l'atténuation, est comparée à une définition équivalente. Une nouvelle définition de la formulation spatiale est proposée dans le cas de structures très amorties et est comparée aux résultats de la littérature. Dans le troisième chapitre, la technique de caractérisation RIC est étendue au cas des structures anisotropes. Cette méthode inverse s'appuie sur l'analyse du champ de déplacement de la structure et possède, grâce à son aspect local, des atouts non négligeables pour des applications industrielles. La méthodologie est tout d'abord présentée pour l'identification de sources (objectif initial de la méthode) et est ensuite adaptée à la caractérisation. Plusieurs applications numériques et expérimentales sont présentées afin de valider cette extension. Enfin, le quatrième chapitre porte sur la caractérisation expérimentale d'un sandwich en nids d'abeille sur une large bande fréquentielle (1 à 300 kHz). Le comportement dynamique complexe de cette structure épaisse est décrit au travers des résultats prometteurs fournis par les simulations de notre modèle et les estimations de RIC. Ces résultats sont également comparés à ceux d'un modèle de référence et d'autres méthodes de caractérisation
This PhD thesis concerns the modelling and the dynamic characterisation of anisotropic multilayered structures. In the first chapter, a classification of some analytical models and experimental methods of characterisation is presented in the form of a bibliographical synthesis where the main published results are gathered. The second chapter introduces an extension of an equivalent model of multilayered structures to the case of anisotropic materials. This model describes the first dispersion curves of the structure and considers the shear phenomena that can affect the bending mode in high frequencies. The main advantage of the model lies in its simplicity and rapidity thanks to a number of kinematic variables independent of the number of layers. The characteristics of the multilayer are defined by the 5 flexural rigidities of a thin equivalent plate which is compared to the multilayer. The results of the model are validated by means of an experimental study on carbon fiber composite plates. A particular interest is dedicated to the modelling of structural damping. An energetic loss factor, based on a temporal and spatial definition of the attenuation, is compared to an equivalent one. A new definition of the spatial attenuation is suggested for high damped structures and is compared to literature. In the third chapter, the methodology of the characterisation technique CFAT is adapted for anisotropic plates. This inverse method is based on the displacement field analysis of the structure and has, by means of its local aspect, relevant advantages for industrial applications. This adaptation is, as a first step, introduced for the identification of sources (initial objective of the method) and applied, in a second step, for the characterisation. Several numerical and experimental applications are presented to validate the results of the method. Finally, the fourth chapter deals with the experimental characterisation of a honey comb sandwich on a high frequency range (1 to 300 kHz). The complex dynamic behaviour of this thick structure is described through the promissing results given by the simulations of our model and the estimations of RIC. These results are also compared to the ones of a reference model and other characterisation methods

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42

Le, Thi Huyen Cham. "Robust variable kinematics plate ﬁnite elements for composite structures". Thesis, Paris 10, 2019. http://faraway.parisnanterre.fr/login?url=http://bdr.parisnanterre.fr/theses/intranet/2019/2019PA100053/2019PA100053.pdf.

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Le but de ce travail est de développer deux nouveaux éléments ﬁnis quadrilatéraux à quatre et à huit nœuds implantés dans le code commercial de calcul par Eléments Finis (EF) Abaqus pour plaques composites. Les modèles plaques à cinématique variable sont formulés dans le cadre de la formulation uniﬁée de Carrera (CUF), qui regroupent des descriptions de type: couche équivalente (ESL) et couches discrètes (LW), avec les variables déﬁnies par des polynômes jusqu’au 4ème ordre suivant épaisseur z. Les deux formulations variationnelles sont utilisées pour dériver les matrices des éléments ﬁnis: le Principe des Déplacements Virtuels (PVD) et le Théorème Variationnel Mixte de Reissner (RMVT). Grâce à la technique de condensation statique, une formulation hybride basée sur le RMVT est introduite. Aﬁn d’éliminer la pathologie de verrouillage en cisaillement transverse, deux approximations compatibles pour le champ de déformations de cisaillement transverse indépendantes en z, notée QC4 et CL8, sont étendues aux éléments plaques à cinématique variable basés sur CUF. De plus, les QC4S et CL8S interpolations sont également introduites pour les contraintes de cisaillement transverse dans les éléments mixtes et les éléments hybrides. Les résultats numériques comparés à ceux disponibles dans la littérature montrent que les FE proposés sont eﬃcaces pour modéliser des éléments ﬁnis robustes
The aim of this work is the development of two classes of new four-node and eightnode quadrilateral ﬁnite elements implemented into the commercial ﬁnite element (FE) code Abaqus for composite plates. Variable kinematics plate models are formulated in the framework of Carrera’s Uniﬁed Formulation (CUF), which encompasses Equivalent Single Layer (ESL) as well as Layer-Wise (LW) models, with the variables that are deﬁned by polynomials up to 4th order along the thickness direction z. The two classes refer to two variational formulations that are employed to derive the ﬁnite elements matrices, namely the Principle of Virtual Displacement (PVD) and Reissner’s Mixed Variational Theorem (RMVT). Thanks to the static condensation technique, a Hybrid formulation based on the RMVT is derived. For the purpose of eliminating the shear locking pathology, two ﬁeld compatible approximations for only the z−constant transverse shear strain terms, referred to as QC4 and CL8 interpolations, are extended to all variable kinematics CUF plate elements. Moreover, the QC4S and CL8S interpolations, are also introduced for the transverse shear stress ﬁeld within RMVT-based and Hybrid mixed-based elements. Numerical results in comparison with those available in literature show that the proposed FEs are eﬃcient for modeling a robust ﬁnite elements

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43

Jeancolas, Antoine. "Étude expérimentale et modélisation micromécanique du comportement de composites hybrides : optimisation de la conductivité thermique." Thesis, Université de Lorraine, 2018. http://www.theses.fr/2018LORR0285/document.

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L’augmentation de la puissance électrique des composants électroniques pose le problème de la dissipation de la chaleur générée. Les boîtiers électriques doivent permettre la dissipation de cette chaleur en conservant une isolation électrique. La solution retenue pour évacuer la chaleur par transfert thermique consiste en matériaux composites dont les renforts par leur structuration vont améliorer la conductivité thermique. Des composites à matrice polymère ont été choisis pour leur aptitude de mise en forme. La conductivité thermique et l’isolation électrique sont assurées par des charges céramiques. Les méthodes d’homogénéisation donnent des pistes d’amélioration du comportement de composites en fonction des propriétés de leurs constituants, de leur géométrie et de leur distribution. Elles fournissent ainsi une formulation optimisée de matériaux répondant à certaines caractéristiques issues de cahiers des charges émanant du partenaire industriel (Institut de Soudure). La conductivité thermique attendue des composites impose une forte fraction volumique de charges pour compenser le caractère isolant de la matrice polymère. Des méthodes d’homogénéisation ont été développées pour prédire la conductivité thermique effective pour de forts taux de charges (supérieur à 20%) et des contrastes élevés de conductivité thermique. La présence d’une interphase provenant d’incompatibilités fortes entre les composants doit également être modélisée
The increase of electronic components in the integrated circuits and the required electrical power set the question of the dissipation of the heat generated. The electrical box must favor the heat dissipation while maintaining electrical insulation. The solution chosen to transfer the heat is to develop composite materials whose reinforcements by their structure will improve the thermal conductivity. Polymer-based composite materials were chosen for their building ability. Thermal conductivity and electrical insulation are insured by ceramic reinforcements. The homogenization methods allow to improve the composites’ design according to the properties of their constituents, their geometry and their distribution. They thus provide an optimized formulation of materials satisfying the characteristics emanating from the industrial partner (‘Institut de Soudure’). The expected thermal conductivity of the composites imposes a high volume fraction of reinforcements to counterbalance the insulating polymer matrix. Homogenization methods have been developed to provide predictions of effective thermal conductivity for high (greater than 20%) reinforcement rates and high thermal conductivity contrasts. The presence of an interphase resulting from strong physico-chemical incompatibilities between the components must also be modeled

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Nguyen, Van Quang. "Méthodes d'éclatement basées sur les distances de Bregman pour les inclusions monotones composites et l'optimisation." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066183/document.

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Le but de cette thèse est d'élaborer des méthodes d'éclatement basées sur les distances de Bregman pour la résolution d'inclusions monotones composites dans les espaces de Banach réels réflexifs. Ces résultats nous permettent d'étendre de nombreuses techniques, jusqu'alors limitées aux espaces hilbertiens. De plus, même dans le cadre restreint d'espaces euclidiens, ils donnent lieu à de nouvelles méthodes de décomposition qui peuvent s'avérer plus avantageuses numériquement que les méthodes classiques basées sur la distance euclidienne. Des applications numériques en traitement de l'image sont proposées
The goal of this thesis is to design splitting methods based on Bregman distances for solving composite monotone inclusions in reflexive real Banach spaces. These results allow us to extend many techniques that were so far limited to Hilbert spaces. Furthermore, even when restricted to Euclidean spaces, they provide new splitting methods that may be more avantageous numerically than the classical methods based on the Euclidean distance. Numerical applications in image processing are proposed

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45

Jeancolas, Antoine. "Étude expérimentale et modélisation micromécanique du comportement de composites hybrides : optimisation de la conductivité thermique." Electronic Thesis or Diss., Université de Lorraine, 2018. http://www.theses.fr/2018LORR0285.

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Анотація:

L’augmentation de la puissance électrique des composants électroniques pose le problème de la dissipation de la chaleur générée. Les boîtiers électriques doivent permettre la dissipation de cette chaleur en conservant une isolation électrique. La solution retenue pour évacuer la chaleur par transfert thermique consiste en matériaux composites dont les renforts par leur structuration vont améliorer la conductivité thermique. Des composites à matrice polymère ont été choisis pour leur aptitude de mise en forme. La conductivité thermique et l’isolation électrique sont assurées par des charges céramiques. Les méthodes d’homogénéisation donnent des pistes d’amélioration du comportement de composites en fonction des propriétés de leurs constituants, de leur géométrie et de leur distribution. Elles fournissent ainsi une formulation optimisée de matériaux répondant à certaines caractéristiques issues de cahiers des charges émanant du partenaire industriel (Institut de Soudure). La conductivité thermique attendue des composites impose une forte fraction volumique de charges pour compenser le caractère isolant de la matrice polymère. Des méthodes d’homogénéisation ont été développées pour prédire la conductivité thermique effective pour de forts taux de charges (supérieur à 20%) et des contrastes élevés de conductivité thermique. La présence d’une interphase provenant d’incompatibilités fortes entre les composants doit également être modélisée
The increase of electronic components in the integrated circuits and the required electrical power set the question of the dissipation of the heat generated. The electrical box must favor the heat dissipation while maintaining electrical insulation. The solution chosen to transfer the heat is to develop composite materials whose reinforcements by their structure will improve the thermal conductivity. Polymer-based composite materials were chosen for their building ability. Thermal conductivity and electrical insulation are insured by ceramic reinforcements. The homogenization methods allow to improve the composites’ design according to the properties of their constituents, their geometry and their distribution. They thus provide an optimized formulation of materials satisfying the characteristics emanating from the industrial partner (‘Institut de Soudure’). The expected thermal conductivity of the composites imposes a high volume fraction of reinforcements to counterbalance the insulating polymer matrix. Homogenization methods have been developed to provide predictions of effective thermal conductivity for high (greater than 20%) reinforcement rates and high thermal conductivity contrasts. The presence of an interphase resulting from strong physico-chemical incompatibilities between the components must also be modeled

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Lasseigne, Alexis. "Optimization of variable-thickness composite structures. Application to a CROR blade." Thesis, Lyon, 2016. http://www.theses.fr/2016LYSEM006/document.

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Cette thèse aborde la problématique de la conception optimale de structures composites stratifiées d’épaisseur variable. Les variables d’empilement définissent un problème d’optimisation combinatoire et des espaces de décisions de grande taille et potentiellement multimodaux. Les algorithmes d’optimisation stochastiques permettent de traiter ce type de problème et de tirer profit des performances et de l’anisotropie des plis composites pour l’allègement des structures composites stratifiées. Le but de cette étude est double : (i) développer un algorithme d’optimisation dédié aux composites stratifiés d’épaisseur variable et (ii) estimer le potentiel des composites stratifiés pour la maîtrise des performances aérodynamiques d’une pale de CROR composite.Dans la première partie de cette thèse, un algorithme évolutionnaire est spécialisé pour l’optimisation de tables de drapage et la gestion d’un ensemble de règles de conception représentatif des pratiques de l’industrie. Pour se faire, un encodage spécifique des solutions est proposé et des opérateurs de variations spécialisés sont développés.Dans la deuxième partie, l’algorithme est enrichi d’une technique de guidage basée sur l’exploitation d’un espace auxiliaire afin d'accroître son efficacité et d’intégrer davantage de connaissances des composites dans la résolution du problème.Finalement, la méthode est appliquée pour la conception d’une pale de CROR composite à l’échelle de la maquette de soufflerie. Au préalable, des processus itératifs de mise à froid et mise à chaud de la pale sont mis en place afin d’estimer la forme de la pale au repos et l’état de contraintes dans la pale en fonctionnement
This thesis deals with the optimal design of variable-thickness laminated composite structures. The stacking variables define a combinatorial optimization problem and large decision spaces which are potentially multimodal. Stochastic optimization algorithms allow solving this type of problem and allow taking advantage from the performance and the anisotropic nature of unidirectional composite plies to lighten laminated composite structures.The purpose of this study is twofold: (i) developing an optimization algorithm dedicated to variable-thickness laminated composites and (ii) assessing the potential of laminated composites in influencing the aerodynamic performances of a composite CROR blade.Firstly, an evolutionary algorithm is specialized in order to optimize layup tables and handle a set of design guidelines which is representative of industrial practices. In this purpose, a specific encoding of the solutions is suggested and specialized variation operators are developed.Secondly, the algorithm is enriched with a guiding technique based on the exploitation of an auxiliary space in order to improve its efficiency and to include further composites-related knowledge for the resolution of the problem.Finally, the method is applied for the design of a reduced-scale composite CROR blade intended for wind-tunnel testing. Beforehand, iterative processes are implemented to estimate the shape of the non-operating blade and the stress state within the operating blade

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Androuin, Guillaume. "Caractérisation de la propagation de délaminage en fatigue : essais à résonance et sollicitation à amplitude variable." Thesis, Toulouse, ISAE, 2018. http://www.theses.fr/2018ESAE0004/document.

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Dans le domaine aéronautique, la détection des défauts sur structures composites sefait lors des phases de maintenance. Celles-ci sont prévues de manière à détecter lesdéfauts avant que ceux-ci ne deviennent critiques pour la tenue des structures en service.L’optimisation des intervalles de maintenance représente donc un fort intérêt que ce soitau niveau financier ou opérationnel. Les dommages des structures composites sont aujourd’hui traités par la non-propagationdes défauts : lorsque ceux-ci apparaissent et qu’ils sont détectés, les pièces sont immédiatementréparées ou remplacées. Afin d’améliorer ces intervalles de maintenance, lecomportement des défauts soumis aux spectres de charges aéronautique nécessite d’êtrecaractérisé. Cela permettrait une transition vers une philosophie de tolérance aux dommages,en définissant des phases de propagation non-critique pour la tenue mécanique desstructures. Dans ce contexte, la propagation de délaminage est étudiée sous chargement de fatigue.En mode I de propagation, les effets de la fréquence de sollicitation et du rapport de chargesur la propagation de délaminage en fatigue sont déterminés. Ceux-ci amorcent l’étudede la propagation de délaminage sous spectre de charges à niveau variable. Les effets del’historique de chargement sont mis en évidence. En mode II de propagation, les effets de la fréquence de sollicitation sur la propagationde délaminage sont abordés pour quatre matériaux à l’aide d’un montage d’essaisvibratoires dédié. Une analyse thermique des essais est également conduite. De plus, leseffets du rapport de charge sont déterminés pour ce mode de propagation
The detection of damage in composite aeronautical structures is carried out duringmaintenance phases. These operations are planned so that damage is detected beforereaching any critical size for the in-service structures. The optimization of maintenanceintervals is therefore of great interest both financially and operationally. Damage in composites structures is currently dealt through the no-growth approach :when damage appears and is detected, the affected parts are immediately repaired orreplaced. In order to improve the maintenance schedule, the behaviour of defects subjectedto aeronautical load spectra needs to be characterized. This would enable a transitiontowards a damage tolerance philosophy with a slow-growth approach by defining noncriticalpropagation phases for the mechanical strength of structures. In this context, delamination propagation under fatigue loading is studied. The effectsof loading frequency and load ratio on fatigue delamination propagation are determinedin mode I. Then a study of delamination propagation under complex load spectra isconducted for blocks at different loading amplitudes. Effects of the loading history arehighlighted. For the propagation in mode II, loading frequency effects on delamination propagationare investigated for four different composite materials using a dedicated vibration testingdevice. A thermal analysis is also conducted during high frequency tests. In addition, loadratio effects are determined for this propagation mode

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Yang, Guanda [Verfasser], Dirk [Akademischer Betreuer] Schubert, Fritjof [Akademischer Betreuer] Nilsson, Dirk [Gutachter] Schubert, Fritjof [Gutachter] Nilsson, Georg [Gutachter] Fischer, and Kyle [Gutachter] Webber. "Big Data Analysis and Simulation Platform for Anisotropic Electrically Conductive Composites -Validation Utilizing PMMA and Carbon Filler / Guanda Yang ; Gutachter: Dirk Schubert, Fritjof Nilsson, Georg Fischer, Kyle Webber ; Dirk Schubert, Fritjof Nilsson." Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2021. http://d-nb.info/1228214735/34.

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Akoussan, Komlan. "Modélisation et conception de structures composites viscoélastiques à haut pouvoir amortissant." Thesis, Université de Lorraine, 2015. http://www.theses.fr/2015LORR0188/document.

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L’objectif de ce travail est de développer des outils numériques utilisables dans la détermination de manière exacte des propriétés modales des structures sandwichs viscoélastiques composites au vue de la conception des structures sandwichs viscoélastiques légères mais à haut pouvoir amortissant. Pour cela nous avons tout d’abord développé un outil générique implémenté en Matlab pour la détermination des propriétés modales en vibration libre des plaques sandwichs viscoélastiques dont les faces sont en stratifié de plusieurs couches orientées dans diverses directions. L’intérêt de cet outil, basé sur une formulation éléments finis, réside dans sa capacité à prendre en compte l’anisotropie des couches composites, la non linéarité matérielle de la couche viscoélastique traduit par diverses lois viscoélastiques dépendant de la fréquence ainsi que diverses conditions aux limites. La résolution du problème aux valeurs propres non linéaires complexes se fait par le couplage entre la technique d’homotopie, la méthode asymptotique numérique et la différentiation automatique. Ensuite pour permettre une étude continue des effets d’un paramètre de modélisation sur les propriétés modales des sandwichs viscoélastiques, nous avons proposé une méthode générique de résolution de problème résiduel non linéaire aux valeurs propres complexes possédant en plus de la dépendance en fréquence introduite par la couche viscoélastique du coeur, la dépendance du paramètre de modélisation qui décrit un intervalle d’étude bien spécifique. Cette résolution est basée sur la méthode asymptotique numérique, la différentiation automatique, la technique d’homotopie et la continuation et prend en compte diverses lois viscoélastiques. Nous proposons après cela, deux formulations distinctes pour étudier les effets, sur les propriétés amortissantes, de deux paramètres de modélisation qui sont importants dans la conception de sandwichs viscoélastiques à haut pouvoir amortissement. Le premier est l’orientation des fibres des composites dans la référence du sandwich et le second est l’épaisseur des couches qui lorsqu’elles sont bien définies permettent d’obtenir non seulement des structures sandwichs à haut pouvoir amortissant mais très légères. Les équations fortement non linéaires aux valeurs propres complexes obtenues dans ces formulations sont résolues par la nouvelle méthode de résolution d’équation résiduelle développée. Des comparaisons avec des résultats discrets sont faites ainsi que les temps de calcul pour montrer non seulement l’utilité de ces deux formulations mais également celle de la méthode de résolution d’équations résiduelles non linéaires aux valeurs propres complexes à double dépendance
Modeling and design of composite viscoelastic structures with high damping powerThe aim of this thesis is to develop numerical tools to determine accurately damping properties of composite sandwich structures for the design of lightweight viscoelastic sandwichs structures with high damping power. In a first step, we developed a generic tool implemented in Matlab for determining damping properties in free vibration of viscoelastic sandwich plates with laminate faces composed of multilayers. The advantage of this tool, which is based on a finite element formulation, is its ability to take into account the anisotropy of composite layers, the material non-linearity of the viscoelastic core induiced by the frequency-dependent viscoelastic laws and various boundary conditions . The nonlinear complex eigenvalues problem is solved by coupling homotopy technic, asymptotic numerical method and automatic differentiation. Then for the continuous study of a modeling parameter on damping properties of viscoelastic sandwichs, we proposed a generic method to solve the nonlinear residual complex eigenvalues problem which has in addition to the frequency dependence introduced by the viscoelastic core, a modeling parameter dependence that describes a very specific study interval. This resolution is based on asymptotic numerical method, automatic differentiation, homotopy technique and continuation technic and takes into account various viscoelastic laws. We propose after that, two separate formulations to study effects on the damping properties according to two modeling parameters which are important in the design of high viscoelastic sandwichs with high damping power. The first is laminate fibers orientation in the sandwich reference and the second is layers thickness which when they are well defined allow to obtain not only sandwich structures with high damping power but also very light. The highly nonlinear complex eigenvalues problems obtained in these formulations are solved by the new method of resolution of eigenvalue residual problem with two nonlinearity developed before. Comparisons with discrete results and computation time are made to show the usefulness of these two formulations and of the new method of solving nonlinear complex eigenvalues residual problem of two dependances

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Ridha, Hashem M. "The effect of using variable curing light types and intensities on the parameters of a mathematical model that predicts the depth of the cure of light-activated dental composites." Connect to resource online, 2009. http://hdl.handle.net/1805/2082.

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Thesis (M.S.D.)--Indiana University School of Dentistry, 2009.
Title from PDF t.p. (viewed Feb. 5, 2010) Advisor(s): John A. Levon, Chair of the Research Committee, Carl J. Andres, Tien-Min Gabriel Chu, David Brown, Suteera Hovijitra. Curriculum vitae. Includes abstract. Includes bibliographical references (leaves 66-69).

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Дисертації з теми "Variable and anisotropic composites"

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