Self-consistent charge densities at isolated planar defects in metals

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Scattering by wave-bearing surfaces under fluid loading

Wave-bearing surfaces and compressible fluids are often adjacent, the subsequent interactions are of substantial interest in structural acoustics, acoustic microscopy, seismology and many other fields. Here we take a broad view and discuss a variety of problems, both time harmonic and transient, which are amenable to exact solution. These in turn highlight physical effects and can additionally form the basis of asymptotic solutions. In structural acoustics the interaction of plate waves with defects is Cl major source of underwater noise. A model problem of two semi-infinite elastic plates (made of different material) joined in a variety of ways is considered for obliquely incident flexural plate waves. Asymptotic results for 'light' and 'heavy' fluid loading are extracted. In addition reciprocity and power flow relations, besides being of independent interest, provide a useful check on the results. There are many closely related problems involving a fluid loaded elastic solid. The situation here is somewhat similar, but often more complicated, due to the number of waves that an elastic solid supports, mode conversion at interfaces, and interfacial waves. We first address the scattering effects of low frequency waves by very small interfacial defects, that is, small relative to a typical wavelength. In this limit, and in related water wave or acoustic work, matched asymptotic expansions are used. An important aspect, that has not been noticed before, is the natural separation that occurs in the inner problem into fluid and solid pieces. A matching argument may now be used to give a useful physical interpretation of these defects and far field directivity patterns show the distinctive beaming that occurs along the Rayleigh angles in the light fluid loading limit. In many areas of interest embedded defects are imaged by pulses and we therefore require a transient analysis. In this case our problem involves a combination of compressional and shear source loadings beneath a fluid-solid interface. The exact solution is found and a full asymptotic analysis of this solution is performed with an emphasis upon wavefront expansions and leaky waves, and in particular, for 'light' and 'moderate' fluid loading. In some situations, when the sources are near the interface, a pseudo-compressional wavefront is generated and the limit as the loading approaches the interface is investigated. These non-geometric wave arrivals may be important in seismology and elastic wave studies related to the non-destructive evaluation of structures. This study is generalised to investigate the dynamic stress loading of subsurface cracks in either homogeneous or non-homogeneous media. An iterative method of solution based on physical considerations is developed and quantities of interest such as the scattered displacement fields and the stress intensity factors are determined. The problems considered here are ideally suited to analysis by transform methods and the Wiener-Hopf and Cagniard-de Hoop techniques

Experimentelle und theoretische Untersuchungen zu Inkompatibilitäts- und Versetzungsaufstauungsspannungen an Korngrenzen unter Berücksichtigung der elastischen und plastischen Anisotropie

Die mechanischen Eigenschaften von metallischen Materialien hängen stark vom Versetzungsverhalten ab, wie z.B. der Dichte, der Verteilung, der Nukleation und der Beweglichkeit von Versetzungen, sowie den Wechselwirkungen zwischen Versetzungen und Korngrenzen (KG). Das Hauptziel dieser Arbeit ist die Untersuchung der Auswirkungen von elastischen und plastischen Anisotropien auf die Versetzungs-KG Wechselwirkung unter Berücksichtigung der komplexen Eigenschaften von KG, sowie der Einflüsse der Misorientierung und der freien Oberflächen. Um dieses Ziel zu erreichen, wurde ein auf dem L-E-S-Formalismus basierender analytischer Ansatz verwendet, der die elastischen Felder einzelner gerader Versetzungen und unterschiedlicher Versetzungskonfigurationen an KG in anisotropen homogenen Medien, Halbräumen, Bi- und Tri-Materialien unter möglicher Berücksichtigung freier Oberflächeneffekte liefert. Die Tri-Material-Konfiguration erlaubt die Berücksichtigung einer KG mit der Dicke ungleich Null im Nanometerbereich und eines spezifischen Steifigkeitstensors für die KG. Die Konfiguration mit zwei freien Oberflächen wurde zur Untersuchung von Größeneffekten verwendet. Die Auswirkungen der anisotropen Elastizität, der kristallographischen Orientierung, der KG-Steifigkeit und der freien Oberflächen wurden für den Fall einer einzelnen Versetzung sowie für einen Versetzungsaufstau in einem Ni-Bikristall mit Bildkraft- bzw. Aufstaulängenanalysen untersucht. Parallel dazu wurden in-situ Mikrodruckversuche an Ni und α-Messung Bikristallen, welche durch FIB-Bearbeitung hergestellt wurden, kombiniert mit SEM, AFM und EBSD Untersuchungen durchgeführt. Die Druckversuche wurde mit einer geringen Dehnung durchgeführt, bis Gleitlinien beobachtet wurden oder die Fließspannung erreicht wurde. Dann wurden die räumlichen Variationen der Gleitstufenhöhe an den lokalisierten Gleitbändern, welche an der KG endeten, mittels AFM gemessen und die Verteilung der Burgers-Vektoren in dem Versetzungsaufstau bestimmt. Diese Versetzungsverteilung wurde dann mit den experimentell gemessenen Parametern simuliert, indem die Auswirkungen von Misorientierung, KG-Steifigkeit, freien Oberflächen, Inkompatibilitätsspannungen und kritischer Kraft berücksichtigt wurden. Insbesondere wurden die Inkompatibilitätsspannungen mit Hilfe von CPFEM-Simulationen analysiert und die Dicke der KG wurde mit atomistischen Simulationen mittels LAMMPS bestimmt.Les propriétés mécaniques des matériaux métalliques dépendent fortement du comportement des dislocations, telles que la densité, la distribution, la nucléation et la mobilité des dislocations ainsi que les interactions entre les dislocations et les joints de grain (JDGs). L'objectif principal de cette thèse est d'étudier les effets des anisotropies élastiques et plastiques sur l'interaction de dislocations-JDG en considérant les propriétés complexes des JDGs, les effets de désorientation et les effets de surfaces libres. Pour atteindre cet objectif, une approche analytique basée sur le formalisme L-E-S a été étudiée, qui fournit les champs élastiques des dislocations droites simples et des différents empilements de dislocations aux JDGs dans des milieux homogènes anisotropes, des demi-espaces, des bi- et tri-matériaux tout en considérant éventuellement les effets de surface libre. La configuration tri-matériaux permet d'envisager une épaisseur non nulle de l'ordre du nanomètre et un tenseur de rigidité spécifique pour la région du JDG. La configuration à deux surfaces libres a été utilisée pour étudier les effets de taille. Les effets de l'élasticité anisotrope, de l'orientation cristallographique, de la rigidité du JDG et des surfaces libres ont été étudiés dans le cas d'une seule dislocation et des empilements de dislocations dans un bi-cristal de Ni avec l'analyse des forces images et de la longueur d'empilements, respectivement. En parallèle, des essais in-situ de compression sur des bi-cristaux de Ni et de α-laiton de taille micronique réalisés par usinage au FIB et des observations couplant MEB, AFM et EBSD ont été effectués. L'essai de compression a été réalisé avec une faible déformation jusqu'à ce que les lignes de glissement soient observées ou que la limite d'élasticité soit atteinte. Ensuite, les variations spatiales de la hauteur des marches dûs aux bandes de glissement localisées se terminant au JDG ont été mesurées par AFM pour déterminer la distribution du vecteur de Burgers dans l'empilement de dislocations. Cette distribution a ensuite été simulée par la configuration de l'empilement de dislocations dans des bi-cristaux avec les paramètres mesurés expérimentalement en considérant l'effet de la désorientation, de la rigidité du JDG, des surfaces libres, des contraintes d'incompatibilités et de la force critique. En particulier, les contraintes d'incompatibilités ont été analysées à l'aide de simulations CP-MEF et l'épaisseur du JDG a été simulée à l'aide de simulations atomistique avec LAMMPS.The mechanical properties of metallic materials strongly depend on the dislocation behavior, such as the density, the distribution, the nucleation and the mobility of dislocations as well as the interactions between dislocations and grain boundaries (GB). The main objective of this thesis is to study the effects of elastic and plastic anisotropies on the dislocation-GB interaction considering complex properties of GBs, misorientation effects and free surfaces effects. To reach this objective, an analytical approach based on the L-E-S formalism was investigated, which provides the elastic fields of single straight dislocations and different dislocation pile-ups at GBs in anisotropic homogeneous media, half-spaces, bi- and tri-materials while possibly considering free surface effects. The tri-material configuration allows considering a non-zero thickness in the nanometer range and a specific stiffness tensor for the GB region. The configuration with two free surfaces was used to study size effects. The effects of anisotropic elasticity, crystallographic orientation, GB stiffness and free surfaces were studied in the case of a single dislocation and dislocation pile-ups in a Ni bi-crystals with image forces and pile-ups length analyses, respectively. In parallel, in-situ compression tests on micron-sized Ni and α-Brass bi-crystals produced from FIB machining and observations coupling SEM, AFM and EBSD were performed. The compression test was performed with a low strain until slip lines were observed or yield stress was reached. Then, step height spatial variations due to localized slip bands terminating at GB were measured by AFM to determine the Burgers vector distribution in the dislocation pile-up. This distribution was then simulated by dislocation pile-up configuration in bi-crystals with the experimentally measured parameters by considering the effect of misorientation, GB stiffness, free surfaces, incompatibility stresses and critical force. In particular, the incompatibility stresses were analyzed using CPFEM simulations and the thickness of GB was simulated using atomistic simulations with LAMMPS.French Ministry of Higher Education and Scientific Research, French-German University (UFA-DFH), Experimental Methodology in Materials Science (MWW) in Saarland University, French State (ANR) through the program “Investment in the future” (LabEx “DAMAS” referenced as ANR-11-LABX-0008-01), Deutsche Forschungsgemeinschaft DFG for the financing of the AFM microscope, Grant No. INST 256/455-1 FUG

Development of High Barrier Nylon Based Multilayer Films

Les films multicouches sont composés d’une couche de coeur possédant de bonnes propriétés barrières et mécaniques, prise en sandwich entre deux couches de polyoléfines. Cette composition est couramment utilisée dans l’industrie des emballages alimentaires afin d’améliorer les propriétés mécaniques et barrières à l’oxygène et à l’humidité des films. Au cours de cette étude, des films multicouches à base de nylon aromatique (MXD6), aliphatique (PA6) et leurs nanocomposites, à hautes propriétés barrières ont été développés. Les performances thermiques, barrières (oxygène et vapeur d’eau) et mécaniques des films multicouches ont été comparées entre elles, en faisant varier la couche de coeur (PA6, MXD6 ou leurs nanocomposites). Dans la première partie de ce travail, des films de nylon aliphatique (PA6), de nylon aromatique (MXD6) ainsi que leurs nanocomposites, préparé par polymérisation in-situ avec 4wt% d’argile, ont été extrudés par calandrage à l’aide d’une extrudeuse de laboratoire et refroidis rapidement à l’aide de couteaux d’air. Les propriétés rhéologiques, cristallines, thermiques, barrières et mécaniques des résines pures et des films monocouches extrudés ont été étudiées et comparées. Dans la seconde partie de ce travail, les films monocouches produits ont été étirés uniaxialement à 110 ºC avec un rapport d’étirage variant de 1.5 à 5. L’alignement de l’argile généré par l’étirement des films de nanocomposites ont été mesurés à l’aide de trois techniques différentes : déconvolution des pics en FTIR, soustraction spectrale interactives en FTIR, et diffraction aux Rayons X. Il a été déterminé que les particules d’argile sont principalement orientées dans la direction machine (MD) et que leur orientation est améliorée sous l’effet de l’étirement uniaxial. L’effet des changements d’orientation des cristaux pour toutes les phases III cristallines et amorphes a été examiné à l’aide de la diffraction aux rayons X et de l’analyse trichroique des spectres FTIR. Basé sur les modèles WAXD et les résultats en FTIR, il a été possible de proposer un model schématique afin de décrire le mécanisme de cristallisation du nylon en présence d’argile. Dans la troisième partie de cette étude, l’effet de l’étirement uniaxial sur la structure cristalline, les propriétés thermiques, mécaniques et barrières à l’oxygène des nylons aromatiques et aliphatiques ainsi que de leur nanocomposites ont été étudiés et comparées. Finalement, les films multicouches contenant en couche de coeur, le PA6, ou le MXD6, ou leur nanocomposites ont été produits en utilisant une unité de coextrusion calandrage de laboratoire. Les couches sandwitch de peau sont en LLDPE, et agissent comme des couches barrières à l’humidité. Des films multicouches à 5 couches (une couche supplémentaire a été ajouté de chaque côté entre la couche de coeur et de peau afin d’en améliorer la compatibilité) ont également été produits. Durant le procédé, les paramètres de production ont été optimisés afin d’éliminer les instabilités interfaciales et d’améliorer l’uniformité des films multicouches. Les films produits ont été caractérisés et comparés. ----------- Insufficient barrier properties of commercial thermoplastics to the permeation of atmospheric gases such as oxygen and water vapor are a major problem in the packaging industry. In particular, a high oxygen permeation rate reduces the shelf life of packaged food products, which results in higher costs for food processors and retail customers. Multilayer films having a core layer with good barrier and mechanical properties coextruded between two polyolefin layers have been used in food packaging industry to improve the mechanical performance and the barrier properties. Nylon is an engineering thermoplastic used in flexible packaging due to its high stiffness, toughness, tensile strength, flex crack and puncture resistance as well as low oxygen transmission rate. The objective of this research was fundamental understanding on the differences between properties of monolayer and multilayer aromatic and aliphatic nylon films and their nanocomposites. In the selection of the aromatic and aliphatic nylons, particular attention paid to the oxygen barrier properties of the films as the targeted application is for food packaging and this property plays a critical role in determining shelf life of packed product. Resin characteristics particularly the rheological and thermal properties, morphology, molecular orientation, ability to crystallize (i.e. fast or slow crystallization rate), type of crystalline structure are the key factors for the production of the precursor films with appropriate crystallinity and orientation, which in turn control the final film properties. The extent of nanoclay intercalation and exfoliation, crystal structure, crystallinity, thermal, rheological, barrier and mechanical properties of polyamide 6 (PA6), poly (m-xylene adipamide) (MXD6) and their in-situ polymerized nanocomposites with 4 wt% clay were studied and compared. Dynamic rheological measurements confirmed a strong interfacial interaction between the silicate platelets and the MXD6 chains. A longer relaxation V time for the MXD6, which was related to its higher intermolecular interactions compared to the PA6, resulted in a slower rate of crystallization and lower crystallinity in the former. It was found that due to the stronger polymer chain interaction of the MXD6, there was a lower free volume and gas diffusion path for the MXD6 nanocomposite film compared to the PA6 nanocomposite. In the second part of this project, the precursor monolayer films were uniaxially stretched at 110 ºC with draw ratios varying from 1.5 to 5. The clay alignment was measured with three different techniques: FTIR peak deconvolution, FTIR interactive spectral subtraction and X-ray diffraction. It was found that the clay platelets are mainly oriented in the machine direction (MD) and their orientation improved upon uniaxial stretching. The changes in orientation of crystal axes of all the crystalline phases and amorphous region of the aromatic and the aliphatic nylons and their nanocomposites were examined using X-ray diffraction and Trichroic Infrared analyses. Based on the WAXD patterns and FTIR results, schematic models were proposed to describe the crystallization mechanism of the nylon in the presence of the clay platelets. The crystalline and amorphous orientations as well as the clay alignment significantly affect the performance of the stretched films. In the third step of this study, the structural development of the aliphatic and aromatic nylons and their nanocomposite films during uniaxial stretching was investigated. The effect of uniaxial drawing on the morphology, crystallinity, thermal, mechanical and oxygen barrier properties of the polyamide 6 and the MXD6 as well as their insitu polymerized nanocomposites were studied. A significant enhancement in the Young’s modulus and tensile strength of the uniaxially stretched aliphatic and aromatic nylons was observed. The oxygen permeability and oxygen diffusion through the nylon nanocomposite films VI were predicted with theoretical models and with incorporating structural parameters such as the crystalline phase orientation, clay aspect ratio and clay orientation. In the last phase, coextruded multilayer films with the PA6 and MXD6 nylons as well as their in-situ polymerized nanocomposites, as an oxygen barrier layer (core), and a linear low-density polyethylene (LLDPE) as the moisture barrier layers (skin) with the adjacent tie were produced and characterized. The effect of core layer material on the thermal, optical, barrier and mechanical properties of the coextruded multilayer films has been investigated

Stress generation during the processing of epoxy-carbon composites.

The stresses generated during the processing of carbon fibre/epoxy resin composite materials are predicted using the finite element method and classical lamination theory. Elastic material behaviour is assumed. Emphasis is placed on the residual microstresses which have been less-extensively studied than other aspects of the stress generation process.Temperature and stress distributions are modelled through the thickness of laminates assuming cooling from the cure temperature of 190°C. At the microlevel the effect of varying fibre volume fraction, interfibre distance, packing geometry and fibre diameter are studied. Random and regular fibre arrays are considered.It is found that the residual stresses are generated almost entirely due to the differing properties of the fibre and the matrix and the anisotropy of the fibres, rather than any temperature gradients within the materials. At the macrolevel maximum stresses (10-100 MPa) are calculated in the transverse layers of multidirectional laminates. At the microlevel maximum stresses (10-100 MPa) are predicted at the fibre/matrix interface. The exact values depend on the assumed laminate stacking sequence and distribution of fibres, respectively. The maximum values of the microstresses are found to be approximately inversely proportional to the minimum interfibre distance and proportional to the fibre diameter. This implies that, at the shorter minimum interfibre distances typical of more realistic random arrays, the maximum stress values are greater. When the macrostress and microstress fields are superimposed it is predicted that cracks will form at some of the fibre/matrix interfaces and propagate outwards into the matrix.Observations of laminate samples under the electron microscope show no such cracking to occur, rather in a few localised regions, cracking around the fibre/matrix interface is apparent. It is suggested that in these regions the interface is weak and fails due to the weaker radial stress. Otherwise it is suggested that cracking is not observed due to a visco-elastic/visco-plastic behaviour of the matrix, the presence of an interlayer at the fibre/matrix interface with properties different to that in the matrix away from the interface and a crack suppressing mechanism resulting from the interaction of adjacent plies. The latter effect is most significant for thin plies.It is proposed that regular packing of the fibres, which precludes low interfibre distances, will prevent microcracking. Hexagonal packing is preferred since this achieves the highest volume fraction and thus the highest strengths. Sizings applied to the fibres which improve the fibre/matrix adhesion, and react/diffuse into the matrix to produce a flexible interlayer, will improve the strength and impact resistance of these composites. In multidirectional laminates thin transverse layers, less than 0.5 mm are advised

Crystal Plasticity at Micro- and Nano-scale Dimensions

The present collection of articles focuses on the mechanical strength properties at micro- and nanoscale dimensions of body-centered cubic, face-centered cubic and hexagonal close-packed crystal structures. The advent of micro-pillar test specimens is shown to provide a new dimensional scale for the investigation of crystal deformation properties. The ultra-small dimensional scale at which these properties are measured is shown to approach the atomic-scale level at which model dislocation mechanics descriptions of crystal slip and deformation twinning behaviors are proposed to be operative, including the achievement of atomic force microscopic measurements of dislocation pile-up interactions with crystal grain boundaries or with hard surface coatings. A special advantage of engineering designs made at such small crystal and polycrystalline dimensions is the achievement of an approximate order-of-magnitude increase in mechanical strength levels. Reasonable extrapolation of macro-scale continuum mechanics descriptions of crystal strength properties at micro- to nano-indentation hardness measurements are demonstrated, in addition to reports on persistent slip band observations and fatigue cracking behaviors. High-entropy alloy, superalloy and energetic crystal properties are reported along with descriptions of deformation rate sensitivities, grain boundary structures, nano-cutting, void nucleation/growth micromechanics and micro-composite electrical properties

Nano-Materials By Design and Their Applications in Infrastructure

For utilizing nano materials in infrastructure applications, they should exist in huge quantities, in low cost, and they should be applied efficiently to achieve the ultimate goal design, based on fundamental mechanical and physical laws. To achieve this goal multiscale characterization and modeling is needed to deliver our thorough application effectively. The main objective in this research is to implement multiscale characterization of the nano-micro-meso-macro properties of advanced polymeric and cementitious nanocomposites, to correlate mechanical properties to morphology. This include a proposed joint experimental-theoretical approach