Thesis (M. Phil.)--University of Hong Kong, 1990.

No description available.

Thesis (Ph. D.)--University of Hong Kong, 1989.

A test fixture for combined tension-compression cyclic testing of unidirectional composites was designed and characterized using 606l-O aluminum specimens. The elastoplastic response of graphite/aluminum l5° off-axis and 90° specimens under tension-compression cyclic loading was subsequently investigated at three temperatures, -l50°F, room temperature and 250°F. The test results showed that the tensile response was predominantly elastoplastic, whereas the compressive response could not be characterized exclusively on the basis of the classical plasticity theory. Secondary dissipative mechanisms caused by inherent voids in the material's microstmcture had an apparent influence on the elastoplastic behavior in compression. At different test temperatures, the initial yield stress in tension and compression were translated in the tension direction with increasing temperature. This is believed to be caused by residual stresses induced in each phase of the composite. The micromechanics model proposed by Aboudi was subsequently employed to correlate the experimental and analytical results at room temperature. A semi-inverse methodology was incorporated to determine the in-situ properties of the constituents. Comparison between the analytical and experimental results showed good agreement for monotonic tensile response. For tension-compression cyclic loading, fairly good correlation was obtained for l5° specimens, but poor for 90° specimens. The major cause of the discrepancy is suggested to be caused by the secondary dissipative mechanisms. / Master of Science

A constitutive model based on rate-independent elastoplasticity concepts is developed to simulate the behavior of geologic materials under arbitrary three-dimensional stress paths, stress reversals and cyclic loading. The model accounts for the various factors such as friction, stress path, stress history, induced anisotropy and initial anisotropy that influence the behavior of geologic materials. A hierarchical approach is adapted whereby models of progressively increasing sophistication are developed from a basic isotropic-hardening associative model. The influence of the above factors is captured by modifying the basic model for anisotropic (kinematic) hardening and deviation from normality (nonassociativeness). Both anisotropic hardening and deviation from normality are incorporated by introducing into the formulation a second order tensor whose evolution is governed by the level of induced anisotropy in the material. In the stress-space this formulation may be interpreted as a translating potential surface Q that moves in a fixed field of isotropic yield surfaces. The location of the translating surface in the stress-space, at any stage of the deformation, is given by the 'induced anisotropy' tensor. A measure to represent the level of induced anisotropy in the material is defined. The validity of this representation is investigated based on a series of special stress path tests in the cubical triaxial device on samples of Leighton Buzzard sand. The significant parameters of the models are defined and determined for three sands based on results of conventional laboratory test results. The model is verified with respect to laboratory multiaxial test data under various paths of loading, unloading, reloading and cyclic loading.

This PhD thesis consists of five complementary chapters. Chapters 2 through 4 constitute the basis of research papers to be published subsequently. These three chapters summarize the state of a single crystal undergoing elastoplastic deformation. The studies presented in this thesis primarily deal with experimental and computational concepts that enable the calculation, measurement and extraction of the spatially resolved net Burgers density vector and the geometrically necessary dislocation densities (GNDs), which reveal the small scale continuum characteristics of a single crystal in the elastoplastic state. The calculation methodology of a new validation parameter, β, which is the orientation of the net Burgers density vector, is given in chapter 2. This new parameter, β, enables us to validate the elastic-plastic constitutive relations. Since the existing methods used for validation cannot give direct information about the state of the material, the β-variable is introduced for elastic- plastic constitutive models. β-fields, which are essentially contour maps of β-variables on two dimensional spatial coordinates, are used to monitor the activity regions of effective slip systems. Chapters 2 through 4 present a comprehensive analysis of the spatially resolved net Burgers density vector, along with the length scale characterization of dislocation structures and validation of constitutive relations. The studies presented in this work are the outcome of experimental and computational research. The experimental work consists of the indentation of a nickel single crystal deformed through a quasi-statically applied line load parallel to the [110] crystallographic orientation. The line load was applied onto (001) surface of the single crystal by a tungsten carbide wedge indenter with a 90◦ included angle. A two-dimensional deformation field on an indented single crystal, in which the only non-zero lattice rotation occurs in the plane of deformation and only three effective in-plane slip systems are activated, was investigated. The mid-section of the deformed single crystal was exposed by EDM and polished electrochemically. The in-plane lattice rotations were measured by high-resolution electron backscattered diffraction (HR-EBSD). The Nye's dislocation density components, lattice curvatures, GNDs and net Burgers density vectors were calculated. Therefore, the β- variable and the β-fields are calculated both experimentally, analytically and numerically in Chapter 2. A qualitative comparison of the three methods showed that the β-field obtained from experimental measurements agrees with those obtained from analytical and numerical methods. The directions of the net Burgers density vector, which are used to determine the boundaries of the slip activity regions, are also given in Chapter 2. Chapter 3 mainly deals with the hardening parameters associated with strain hardening rules utilized in finite element simulations, and investigates the sensitivity of the β-variable to parameters such as latent hardening ratio, initial hardening modulus and saturation strength. The study revealed that a change in the saturation strength has a significant effect on both magnitude of the β-variable and the boundary of the slip activity regions. Chapter 4 presents a length scale analysis associated with dislocation structures such as cell size and cell wall width. The methods presented in this chapter employ the SEM- based continuum method and Fourier Analysis. As-measured GNDs are extracted along the local crystallographic traces, and a quasi-periodic arrangement of dislocation structures is obtained. The extracted GND functions are truncated, interpolated, and filtered. Finally, Fourier Transform is applied to obtain a relationship between cell size and cell wall width of the dislocation structures. The results are compared with those obtained by TEM micrographs. Whereas TEM micrographs characterize the dislocation structures in small scale, the method that is presented in this chapter provides multi scale characterization, which is an order of magnitude larger. Concluding remarks and recommendations for future studies are given in Chapter 5.

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

No description available.

Thesis (Ph. D.)--Florida State University, 2004. / Advisor: Dr. N. Chandra, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed June 15, 2004). Includes bibliographical references.

L’analyse du comportement des pieux est un problème complexe du fait de la diversité des phénomènes qui gouvernent le comportement du sol et en particulier celui se trouvant au voisinage du pieu. Ce dernier dépend particulièrement du procédé d’installation du pieu qui peut parfois engendrer des déformations de très grande amplitude dans le sol entre autre phénomènes. L’objectif de ce travail est de mettre en place un outil de modélisation pour évaluer le comportement des pieux sous chargements axiaux en développant des modèles représentant les phénomènes physiques pertinents et de les intégrer numériquement dans un logiciel aux éléments finis utilisant des algorithmes robustes et efficaces. Pour pouvoir modéliser la phase d’installation statique ou dynamique d’un pieu quelques problématiques doivent être considérées. Premièrement, étant donné que pendant l’installation d’un pieu des déformations finies (non-infinitésimal) ont lieu au niveau de l’interface, une formulation eulérienne avec un taux logarithmique des déformations a été adoptée pour prendre en compte le fait que l’hypothèse des déformations infinitésimales n’est plus valable. En plus, le modèle constitutive doit prendre en compte le comportement physique du sol lorsqu’il est soumis à des déplacements d’une magnitude élevée. Le dernier est constitué, entre autres, par le phénomène de l’écrasement des grains ce qui influence beaucoup le comportement volumique du sol et finalement sa résistance au cisaillement ou en d’autres termes le frottement mobilisé. Ce phénomène a été modélisé en introduisant une variable d’écrouissage supplémentaire au modèle de comportement élastoplastique de l’ECP. Les critères d’admissibilité thermodynamique ont été vérifiés pour le modèle constitutive original ainsi que pour le modèle révisé. Des simulations numériques ont été faites pour les deux types d’installation, monotone et pseudo-dynamique (cyclique) et les résultats ont été analysés en détail. Finalement, la dégradation de la résistance au cisaillement au niveau du fût des pieux est un phénomène typique qui se produit pendant le chargement cyclique des fondations composées par des pieux. Le modèle constitutive pour pouvoir bien reproduire ce phénomène n’est pas simple à définir car le chemin de chargement suivi par l’interface sol-pieu est très dépendant du comportement volumique du sol qui à son tours dépend de l’histoire de chargement et des conditions aux limites du problème. Une étude détaillée de toutes les composantes du comportement du pieu pendant ce type de chargement a été effectuée afin de mettre en évidence l’influence de l’histoire de chargement sur la résistance au cisaillement et l’apparition des phénomènes tels que la fatigue du frottement. / The analysis of pile behavior is a complex problem due to the diversity of the phenomena governing the soil behavior and particularly that of the neighboring soil. The objective of this work was to develop a valid modeling tool to evaluate piles’ behavior under axial loads by developing a pertinent mechanical model supported in a robust finite element program which would successfully reproduce the soil behaviour under extreme monotonic and cyclic shear strain. This is done to allow for the numerical modelling of the installation procedure of pile foundations and continued loading of high amplitude cyclic paths. In order to model the installation phase of a monotonic, jacked or dynamic pile foundation some issues must be addressed. Finite deformations take place whilst the pile is put in place requiring an adjustment in the mechanical formulation of the model at the interface level to take into account that the small deformations (rotations and strain) hypothesis is no longer valid. Moreover, the constitutive model must take into account the physical behaviour of the soil when subjected to high order of magnitude displacements. This includes the phenomenon of grain breakage, also referred to as particle crushing, which greatly influences the volumetric behaviour of soil as thus reflecting of shear stress mobilization. The elastoplastic ECP model has therefore been enhanced by introducing an internal variable taking into account the breakage mechanism. The thermodynamic admissibility criteria are verified for the original and revised constitutive models. Both monotonic and pseudo-dynamic installation procedures were numerically simulated and the results thoroughly analysed. Finally, the cyclic shear resistance degradation at the pile shaft is a commonly occurring phenomenon during continued cyclic loading of pile foundation (friction fatigue). The constitutive modelling of this phenomenon, however, is not a straightforward matter. The stress path followed by the thin layer at the soil-pile interface level is known to be directly related to the volumetric behaviour due to the boundary conditions of the problem. A comprehensive analysis of all the components of the behaviour of soil during this stage was object of study in this work.