This dissertation research focuses on micromechanical modeling and simulations of two-phase heterogeneous materials exhibiting anisotropic and non-uniform microstructures with long-range spatial correlations. Completed work involves development of methodologies for realistic micromechanical analyses of materials using a combination of stereological techniques, two- and three-dimensional digital image processing, and finite element based modeling tools. The methodologies are developed via its applications to two technologically important material systems, namely, discontinuously reinforced aluminum composites containing silicon carbide particles as reinforcement, and boron modified titanium alloys containing in situ formed titanium boride whiskers. Microstructural attributes such as the shape, size, volume fraction, and spatial distribution of the reinforcement phase in these materials were incorporated in the models without any simplifying assumptions. Instrumented indentation was used to determine the constitutive properties of individual microstructural phases. Micromechanical analyses were performed using realistic 2D and 3D models and the results were compared with experimental data. Results indicated that 2D models fail to capture the deformation behavior of these materials and 3D analyses are required for realistic simulations. The effect of clustering of silicon carbide particles and associated porosity on the mechanical response of discontinuously reinforced aluminum composites was investigated using 3D models. Parametric studies were carried out using computer simulated microstructures incorporating realistic microstructural attributes. The intrinsic merit of this research is the development and integration of the required enabling techniques and methodologies for representation, modeling, and simulations of complex geometry of microstructures in two- and three-dimensional space facilitating better understanding of the effects of microstructural geometry on the mechanical behavior of materials.

Composite materials are being widely used in light weight structural applications due to their high specific stiffness and strength properties. However, predicting their mechanical behaviour accurately is a difficult task because of the complicated nature of these heterogeneous materials. This behaviour is not easily modeled with most of existing macro mechanics based models. Designers compensate for the model unknowns in failure predictions by generating overly conservative designs with relatively simple ply stacking sequences, thereby mitigating many of the benefits promised by composites. The research presented in this dissertation was undertaken with the primary goal of providing efficient methodologies for use in the design of composite structures considering inherent material variability and model shortcomings. A micromechanics based methodology is proposed to simulate stiffness, strength, and fatigue behaviour of composites. The computational micromechanics framework is based on the properties of the constituents of composite materials: the fiber, matrix and fiber/matrix interface. This model helps the designer to understand in-depth the failure modes in these materials and design efficient structures utilizing arbitrary layups with a reduced requirement for supporting experimental testing. The only limiting factor in using a micromechanics model is the challenge in obtaining the constituent properties. The overall novelty of this dissertation is to calibrate these constituent properties by integrating the micromechanics approach with a Bayesian statistical model. The early research explored the probabilistic aspects of the constituent properties to calculate the stiffness characteristics of a unidirectional lamina. Then these stochastic stiffness properties were considered as an input to analyze the wing box of a wind turbine blade. Results of this study gave a gateway to map constituent uncertainties to the top-level structure. Next, a stochastic first ply failure load method was developed based on micromechanics and Bayesian inference. Finally, probabilistic SN curves of composite materials were calculated after fatigue model parameter calibration using Bayesian inference. Throughout this research, extensive experimental data sets from literature have been used to calibrate and evaluate the proposed models. The micromechanics based probabilistic framework formulated here is quite general, and applied on the specific application of a wind turbine blade. The procedure may be easily generalized to deal with other structural applications such as storage tanks, pressure vessels, civil structural cladding, unmanned air vehicles, automotive bodies, etc. which can be explored in future work. / Graduate / 0548 / enginer315@gmail.com

Micromechanical approach is employed to investigate the transport and freezing within unsaturated porous media. In unsaturated porous media, water film as well as disjoining pressure are introduced in the transport and freezing problems. In the modeling, it is found that, capillary layer along with pore water dominate the transport at high saturation degree (Sr>10%). However, water film will play a significant role in transport at low saturation degree (Sr<10%), and the diffusion coefficient will be lower than 3 to 4 orders of magnitude than that at higher saturation degree. A micromechanical model of freezing in unsaturated porous media is established. Micromechanical model of freezing is more physical based in nature. That is because different from poromechanical model of freezing media in which ice crystal pressure is introduced, the disjoining pressure of unfrozen water film instead of ice crystal pressure is introduced in the micromechanical model of freezing

La réaction alcali-silice est une réaction endogène du béton qui peut contribuer à diminuer la durée de vie d'ouvrages coûteux. La modélisation est nécessaire pour pouvoir évaluer la durée de vie restante des ouvrages atteints. L'alcali-réaction provoque un gonflement du béton dû à une fissuration microscopique sous pression de produits de réaction qui sont des gels gonflant par absorption d'eau. Si le béton est chargé, la fissuration microscopique se développe en fonction du chargement local du béton, ce qui induit une anisotropie de comportement et de déformation du béton. L'objectif de notre travail est de simuler, à partir d'hypothèses simples sur les mécanismes réactionnels en jeu, pour une classe de granulats à réactivité rapide, le déroulement de la fissuration du béton au niveau microscopique, de façon à estimer les déformations et les propriétés mécaniques du béton attaqué. Nous utilisons pour cela une description micromécanique du béton qui nous permet de calculer les propriétés mécaniques et les déformations en fonction de l'état de fissuration, et un critère énergétique de fissuration de façon à faire évoluer l'état de fissuration. Le fonctionnement du modèle est testé sur de nombreux cas qui font apparaître que l'utilisation d'un critère de rupture énergétique en micromécanique est bien adaptée à l'alcali-réaction. L'identification des paramètres du modèle sur des essais en laboratoire donne de bons résultats pour des chargements en dessous de 10 MPa, mais conduit à des estimations très élevées des énergies mécaniques. Le modèle a en effet une tendance à surestimer l'anisotropie du gonflement qui est compensée par l'augmentation de l'énergie surfacique de fissuration / The alkali-silica reaction is an endogenous chemical reaction affecting concrete. Therefore, it is important to model the effects of the reaction so as to estimate the life span of the attacked structures. The reaction leads to a microscopic cracking, due to the pressure of the reaction products which swell by absorption of water, inducing swelling of the concrete. If the concrete is mechanically loaded, the orientation of the microscopic cracking is affected by the local stress state, which induces anisotropy of the mechanical properties and deformations of the concrete. Our work aims at simulating, starting from simple assumptions about the reaction mechanisms, and for a class of fast-reacting aggregates, the development of cracking at the microscopic scale, so as to estimate the deformations and mechanical properties of the attacked concrete. In this purpose, we use a micromechanical description of the concrete, thanks to which we can compute the mechanical properties and deformations from the state of cracking of the concrete. In addition to that, we use an energy fracture criterion to determine the evolution of cracking as the attack progresses. We tested our model on numerous cases. These tests show that this description is well adapted to studying alkali-silica reaction. The identification of the parameters using laboratory experiments yielded good results as far as compression stresses are below 10 MPa. However, the fracture energies identified are greater than accepted values for concretes. We think that our model overestimates the anisotropy of the reaction, which is balanced by higher fracture energies in the identification

<div>The service life of aircraft structure, primarily composed of aluminum alloys, is markedly lower when galvanic corrosion is present due to early crack initiation at localized pitting, with the likelihood of cracking being higher at pits spanning several microns. To understand the joint effect that the mechanical and chemical behavior of AA7050-T7451 have on the evolution of corrosion prior and until its transition to cracking, the microstructure, constituent particles, mechanical strains, and the corrosion morphology were experimentally characterized using high-resolution methods and the mechanical stresses are computationally modeled at the micrometer level using a FFT-based crystal plasticity framework. </div><div><br></div><div>The material was corroded under both mechanically loaded and unloaded conditions under different corrosion intervals to properly capture the evolution of corrosion before, during, and after particle fallout. For the events prior to cracking, statistical cross-correlations between the mechanical state of the material and the corrosion morphology were performed to understand the mechanisms driving corrosion at its various stages. For the cracking event and its subsequent growth, the joint analysis of strains and stresses obtained from 3D crystal plasticity models were used to calculate Fatigue Indicator Parameters (FIPs) that can quantitatively give an insight of the major mechanisms driving crack initiation and growth in pre-corroded materials. The development of micromechanical models that account for both the environmental degradation and the microstructure in the material allowed to accurately predict the location of crack initiation arising from pits, which has been a longstanding problem in the field of corrosion. It is concluded that both corrosion growth and its transition to cracking are multivariable events, where corrosion growth is jointly driven by the local chemistry and the micromechanics, and crack initiation is driven by the coupled interaction between the corrosion geometry and the micromechanics.</div><div><br></div>

Les matériaux céramiques présentent généralement des propriétés mécaniques très intéressantes pour la réalisation de blindages. Ce sont des matériaux très durs et pourtant légers. Les plaques de blindages en céramique sont classiquement mises en forme par pressage à haute température de poudres, ce qui limite la taille et la forme des réalisations tout en impliquant un coût élevé. Une alternative pour produire ces pièces est le moulage d’un composite constitué de particules de céramiques dans une matrice époxy. Ce procédé permet de réduire le coût des pièces tout en autorisant des géométries plus complexes et des dimensions plus importantes.Le comportement mécanique de ce type de matériau dépend de multiples paramètres de conception : propriétés mécaniques des constituants (matrice polymère et particules céramiques), proportion volumique des deux phases, taille et distribution spatiale des particules ou encore l’adhésion entre les constituants. L’objectif de la thèse est d’évaluer l’influence de ces paramètres sur les propriétés d’usage du matériau. Pour ce faire, une analyse multi-échelle du matériau sous sollicitations quasi-statique et dynamique est réalisée.Plus précisément, les propriétés statiques et dynamiques du composite à renforts particulaires ont été déterminées pour différentes combinaisons de ces paramètres de conception. En particulier, le mécanisme de décohésion particule/matrice a été spécifiquement étudié. Les approches de Modèles de Zone Cohésive (CZM) et de Mécanique de la Rupture Finie (FFM) ont été utilisées pour modéliser ce phénomène et un fort effet de taille des particules a été observé. / Ceramics have extensively been used for ballistic protection in the last decades. The combination of their mechanical properties makes them very interesting for armouring. Indeed, they exhibit a high hardness, large compression strength, high stiffness and low density. Ceramic armouring plates are commonly manufactured through a sintering process, where ceramic powders are pressed at high temperatures. This manufacturing process tends to limit the size and shape of components and imparts high costs. On the other hand, moulding using a polymer matrix composite provides an alternative process for developing lower cost parts whilst accommodating increased complexity of geometry and size.However, the mechanical behaviour of such a material is not completely known and depends on multiple design parameters: the mechanical properties of the phases, their volume fraction, the size and spatial distributions of the particles, and the adhesion between the components. The objective of the thesis is to evaluate the influence of the main morphological parameters on the overall mechanical properties, emphasising the influence of the particle/matrix adhesion. To do so, both numerical and experimental multiscale analyses of the material under quasi-static and dynamic loadings were carried out.More precisely, static and dynamic properties of the particle-reinforced composite have been determined for different combinations of the design variables. In particular, attention has been dedicated to the particle/matrix decohesion mechanism. Cohesive zone models (CZM) and Finite Fracture Mechanics (FFM) approaches were used to model this phenomenon and a strong effect of the particle size on debonding was observed.

Geotechnical engineers are commonly faced with the need to perform ground improvement techniques to achieve the necessary bearing capacity for a project. Some of the most common techniques involve the excavation and replenishment of problematic geomaterial with one of better quality. Common projects, such as road embankments and retaining walls, also require the selection of backfill material. The guidelines for selecting backfill material are typically limited to complying with certain gradation bands, relative densities and allowable fines content. Round-grained silica sand, and beach sand from Boca Raton, FL, were used to generate a total of 16 binary granular mixtures containing different amounts of finer material, for which a series of direct shear tests were conducted. Based on the experimental results, it may be possible to provide an alternative criteria for selecting backfill material based on granulometric parameters and the amount of finer material. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2016. / FAU Electronic Theses and Dissertations Collection

This paper introduces an alternative technique for the development of microfluidic systems for biotechnology based on mechanical machining and ultrasonic welding. Advantages of this approach over existing prototyping approaches involving the rapid development of tooling include: (a) short cycle time, (b) design flexibility, and (c) low cost manufacturing. In addition, the process provides a migration path to high volume production. A limitation of this system is that it cannot practically produce microchannels smaller than about 250 μm (0.010 in). However, for many biological cell-based biosensors, this feature scale seems well suited based on cell viability results. Several issues are discussed relevant to this approach, including bond strength, seal leakage, and sterilization. / Graduation date: 2002

This study introduces a multiscale model for analyzing nonlinear thermo-viscoelastic responses of particulate composites. A simplified micromechanical model consisting of four sub-cells, i.e., one particle and three matrix sub-cells is formulated to obtain the effective thermal and mechanical properties and time-dependent response of the composites. The particle and matrix constituents are made of isotropic homogeneous viscoelastic bodies undergoing small deformation gradients. Perfect bonds are assumed along the sub-cell⁰́₉s interfaces. The coupling between the thermal and mechanical response is attributed to the dissipation of energy due to the viscoelastic deformation and temperature dependent material parameters in the viscoelastic constitutive model. The micromechanical relations are formulated in terms of incremental average field quantities, i.e., stress, strain, heat flux and temperature gradient, in the sub-cells. The effective mechanical properties and coefficient of thermal expansion are derived by satisfying displacement- and traction continuities at the interfaces during the thermo-viscoelastic deformations. The effective thermal conductivity is formulated by imposing heat flux- and temperature continuities at the subcells⁰́₉ interfaces. The expression of the effective specific heat at a constant stress is also established. A time integration algorithm for simultaneously solving the equations that govern heat conduction and thermoviscoelastic deformations of isotropic materials is developed. The algorithm is then incorporated within each sub-cell of the micromechanical model together with the macroscopic energy equation to determine the effective coupled thermoviscoelastic response of the particulate composite. The numerical formulation is implemented within the ABAQUS, general purpose displacement based FE software, allowing for analyzing coupled heat conduction and deformations of composite structures. Experimental data on the effective thermal properties and time dependent responses of particulate composites available in the literature are used to verify the micromechanical model formulation. The multiscale model capability is also examined by comparing the field variables, i.e., temperature, displacement, stresses and strains, obtained from heterogeneous and homogeneous composite structures, during the transient heat conduction and deformations. Examples of coupled thermoviscoelastic analyses of particulate composites and functionally graded structures are also presented. The present micromechanical modeling approach is found to be computationally efficient and shows good agreement with experiments in predicting the effective thermo-mechanical response of particulate composites and functionally graded materials. Our analyses forecast a better design for creep resistant and less dissipative structures using particulate composites and functionally graded materials.

Se ha llevado a cabo:A) Un estudio de las propiedades básicas del suelo no saturado (tipo limo) a escala microestructural, incluyendo esqueleto sólido, fase liquida y fase gaseosa. B) Elaboración de un modelo conceptual discontinuo de comportamiento. La geometría interna esta construida por partículas, agua retenida en forma de meniscos alrededor de los contactos y aire ocupando los restantes poros. La interfase liquido gas esta gobernada por la succión y la tensión superficial. Los meniscos tienen un efecto rigidizador del esqueleto.Se observa la posible redistribución de la humedad y del aire, quedando acoplados en definitiva los aspectos mecánicos y de flujo.C) En base a lo anterior, implementación de un modelo numérico discontinuo de ensayo de comportamiento basado en ordenador. Se ha adaptado y modificado el D.E.M. de P.A. Cundall (diferencias finitas explicitas en el tiempo), indicado en el caso que se trata, con no-linealidades geométricas. Se han desarrollado importantes algoritmos de control geométrico. D) Verificación parcial del modelo y aplicación a casos típicos en suelo no saturado, especialmente el colapso.En conjunto es una herramienta útil para su estudio y permite apreciar algunos interesantes mecanismos de actuación de la succión a escala microscópica.