Literatura científica selecionada sobre o tema "Gradient-Enhanced"

Owing to continuous miniaturization, many modern high-technology applications such as medical and optical devices, thermal barrier coatings, electronics, micro- and nano-electro mechanical systems (MEMS and NEMS), gems industry and semiconductors increasingly use components with sizes down to a few micrometers and even smaller. Understanding their deformation mechanisms and assessing their mechanical performance help to achieve new insights or design new material systems with superior properties through controlled microstructure at the appropriate scales. However, a fundamental understanding of mechanical response in surface-dominated structures, different than their bulk behaviours, is still elusive. In this thesis, the size effect in a single-crystal Ti alloy (Ti15V3Cr3Al3Sn) is investigated. To achieve this, nanoindentation and micropillar (with a square cross-section) compression tests were carried out in collaboration with Swiss Federal Laboratories for Materials Testing and Research (EMPA), Switzerland. Three-dimensional finite element models of compression and indentation with an implicit time-integration scheme incorporating a strain-gradient crystal-plasticity (SGCP) theory were developed to accurately represent deformation of the studied body-centered cubic metallic material. An appropriate hardening model was implemented to account for strain-hardening of the active slip systems, determined experimentally. The optimized set of parameters characterizing the deformation behaviour of Ti alloy was obtained based on a direct comparison of simulations and the experiments. An enhanced model based on the SGCP theory (EMSGCP), accounting for an initial microstructure of samples in terms of different types of dislocations (statistically stored and geometrically necessary dislocations), was suggested and used in the numerical analysis. This meso-scale continuum theory bridges the gap between the discrete-dislocation dynamics theory, where simulations are performed at strain rates several orders of magnitude higher than those in experiments, and the classical continuum-plasticity theory, which cannot explain the dependence of mechanical response on a specimen s size since there is no length scale in its constitutive description. A case study was performed using a cylindrical pillar to examine, on the one hand, accuracy of the proposed EMSGCP theory and, on the other hand, its universality for different pillar geometries. An extensive numerical study of the size effect in micron-size pillars was also implemented. On the other hand, an anisotropic character of surface topographies around indents along different crystallographic orientations of single crystals obtained in numerical simulations was compared to experimental findings. The size effect in nano-indentation was studied numerically. The differences in the observed hardness values for various indenter types were investigated using the developed EMSGCP theory.

Due to the advent of varied types of masonry systems a comprehensive failure mechanism of masonry essential for the understanding of its behaviour is impossible to be determined from experimental testing. As masonry is predominantly used in wall structures a biaxial stress state dominates its failure mechanism. Biaxial testing will therefore be necessary for each type of masonry, which is expensive and time consuming. A computational method would be advantageous; however masonry is complex to model which requires advanced computational modelling methods. This thesis has formulated a damage mechanics inspired modelling method and has shown that the method effectively determines the failure mechanisms and deformation characteristics of masonry under biaxial states of loading.

Thesis (M.S.)--Mechanical Engineering, Georgia Institute of Technology, 2004.<br>Berthelot, Committee Chair; Lynch, Committee Member; Jacobs, Committee Member. Includes bibliographical references (leaves 111-113).

La prévision de la nucléation et de la propagation des fissures est essentielle pour décrire la réponse des structures dans des conditions de chargement complexes. On observe l'apparition de microfissures diffuses avant la formation d'une macrofissure. Dans le cas de matériaux quasi-fragiles, on observe un comportement adoucissant lié à une perte progressive de rigidité. D'un point de vue thermodynamique, ce comportement peut être décrit de manière continue par une variable d'état d'endommagement. Cependant, les modèles d'endommagement locaux conduisent inévitablement à un problème aux valeurs limites mal posé. Dans un contexte d'éléments finis, les résultats numériques dépendent donc du maillage. Les modèles d'endommagement non locaux permettent d'obtenir des résultats indépendants du maillage en introduisant des interactions de voisinage par le biais d'une longueur interne. Les approches non locales classiques considèrent des interactions isotropes et constantes, ce qui ne permet pas de reproduire correctement l'ensemble du processus de dégradation. Des approches prenant en compte des interactions évolutives existent et peuvent mieux décrire le comportement de la fissuration.Cette thèse vise à fournir des aspects théoriques et numériques pour le développement de modèles d'endommagement à gradient implicite avec interactions évolutives. Tout d'abord, les modèles non-locaux sont étudiés et comparés en analysant les effets de bord et la diffusion de l'endommagement dans un essai d'écaillage unidimensionnel en dynamique explicite.L'approche non-locale Eikonale est étudiée, où les interactions évolutives sont considérées par le biais d'une métrique riemannienne dépendante de l'endommagement. La version avec gradient de ce modèle (ENLG) est ensuite dérivée d'un cadre micromorphe basé sur la géométrie différentielle, conduisant à une expression de dissipation respectant au second principe de la thermodynamique. Une formulation variationnelle simplifiée est développée pour évaluer les capacités du modèle dans des simulations numériques quasi-statiques bidimensionnelles avec endommagement isotrope. Enfin, la régularisation ENLG est couplée à un modèle d'endommagement anisotrope prenant en compte un tenseur d'endommagement de second ordre. L'anisotropie induite est naturellement prise en compte dans le comportement et dans les interactions évolutives. Des simulations en deux et trois dimensions sont étudiées et comparées aux résultats expérimentaux existants dans la littérature, tout en soulignant les aspects numériques associés. Une analyse détaillée décrit les avantages de la prise en compte de l'endommagement anisotrope et des interactions anisotropes dépendantes de l'endommagement<br>Predicting the cracking nucleation and propagation is essential to describe structural response under complex loading conditions. Diffuse micro-cracks are observed to appear before coalescing into a macro-crack. In the case of quasi-brittle materials, strain-softening behavior is observed and is related to a progressive loss of stiffness. From a thermodynamics viewpoint, this can be described in a continuum way by a damage state variable.However, local continuum damage mechanics models inevitably lead to an ill-posed rate equilibrium problem. In a finite element context, numerical results are, therefore, mesh-dependent. Non-local damage models can recover mesh-independent results by introducing neighborhood interactions through an internal length. Classic non-local approaches consider isotropic and constant interactions, which cannot reproduce the entire degradation process appropriately. Evolving interaction approaches exist and may better describe the cracking behavior. This thesis aims to provide theoretical and numerical aspects for developing evolving interactions gradient-enhanced damage models. Firstly, non-local models are studied and compared by analyzing boundary effects and damage diffusion in a one-dimensional explicit dynamics spalling test.The Eikonal non-local approach is given attention, where evolving interactions are considered through a damage-dependent Riemannian metric. The gradient-enhanced version of this model (ENLG) is then derived from a differential geometry-based micromorphic framework, leading to a dissipation expression fulfilling thermodynamics second principle. A simplified variational formulation is developed to evaluate the model's capabilities in two-dimensional isotropic damage quasi-static numerical simulations. Finally, the ENLG regularization is coupled to an anisotropic damage model considering a second-order damage tensor. Damage-induced anisotropy is naturally considered in the behavior and the evolving interactions. Simulations in two and three-dimensional contexts are studied and compared to existing experimental results from the literature while highlighting the numerical aspects involved. A detailed analysis describes the advantages of considering anisotropic damage and damage-dependent anisotropic interactions

Gold nanoclusters possess both theoretical and practical importance in the development of ultrasensitive biosensors based on surface-enhanced Raman spectroscopy (SERS). Manipulation of gold nanoclusters in a predictable and reproducible manner for the application of refined biochemical analysis still remains challenging. In this study, high-purity gold nanoclusters are isolated via a simple method based on density gradient centrifugation. Three distinct bands including monomers, small aggregates (2-4 nanospheres), and large aggregates (>5 nanospheres) can be separated via density gradient centrifugation. The isolated gold nanoclusters greatly enhance the Raman intensity of the trapped dye molecules such that single nanocluster detection is feasible. To develop a gold nanoparticle-based biosensor for influenza virus, effort was also made to modify recognition moieties such as aptamers to gold nanoparticles via distinct approaches. The increase of hydraulic diameter and the shift of optical absorbance spectrum indicate the success of surface modification to gold nanoparticles.<br>Master of Science