It has been observed that carbon nanotube (CNT)-polymer nanocomposite material has observable piezoresistive effect, that is to say that changes in applied strain may induce measurable changes in resistance. The first focus of the work is on modeling the piezoresistive response of the CNT-polymer nanocomposites by using computational micromechanics techniques based on finite element analysis. The in-plane, axial, the three dimensional piezoresistive responses of the CNT-polymer nanocomposites are studied by using 2D, axisymmetric, and 3D electromechanically coupled and multiscale finite element models. The microscale mechanisms that may have a substantial influence on the overall piezoresistivity of the nanocomposites, i.e. the electrical tunneling effect and the inherent piezoresistivity of the CNT, are included in microscale RVEs in order to understand their influence on macroscale piezoresistive response in terms of both the normalized change in effective resistivity and the corresponding effective gauge factor under applied strain. The computational results are used to better understand the driving mechanisms for the observed piezoresistive response of the material. The second focus of the work is on modeling the piezoresistive response of fuzzy fiber reinforced polymer composites by applying a 3D multiscale micromechanics model based on finite element analysis. Through explicitly accounting for the local piezoresistive response of the anisotropic interphase region, the piezoresistive responses of the overall fuzzy fiber reinforced polymer composites are obtained. The modeling results not only provide a possible explanation for the small gauge factors as observed in experiments, but also give guidance for the manufacture of fuzzy fiber reinforced polymer composites in order to achieve large, consistent, and predictable gauge factors. The third focus of the work is on modeling the coupled effect between continuum damage and piezoresistivity in the CNT-polymer nanocomposites by using computational micromechanics techniques based on a concurrent multiscale finite element analysis. The results show that there is a good correlation between continuum damage and piezoresistive response of the nanocomposites, which gives theoretical and modeling support for the use of CNT-polymer nanocomposites in structural health monitoring (SHM) applications for damage detections. / Ph. D.

In this study, a micromechanics-based method is developed to predict the off-axis strength of unidirectional linear elastic laminae. These composites fail by matrix cracking along a plane parallel to the fiber direction. The stresses in the matrix are calculated using a local stress analysis based on a concentric cylinder model. This model consists of a unique fiber embedded in matrix; both constituents are represented by cylinders. A finite element model is also constructed and the results of the two models compared. The stresses and strains from the concentric cylinder model are averaged over the volume of the matrix and used in a local failure function. This failure function has the form of a reduced and normalized strain energy density function where only transverse and shear terms are considered. The off-axis strength prediction method is validated using data from the literature. This failure function will be used in the near future for composites with a matrix having nonlinear properties. Experimental tensile tests on steel-cord/rubber laminae and laminates as well as on the nonlinear rubber matrix were performed. Stress-strain behavior and off-axis strength data were obtained. An approach for off-axis strength prediction for these laminae is defined based on a finite element stress analysis. The finite element analysis approach is motivated by the one used for linear composites. / Master of Science

Les activités industrielles modernes (stockage de déchets nucléaires, puits géothermiques, centrales nucléaires, …) peuvent solliciter les matériaux cimentaires dans des conditions extrêmes, par exemple à des températures supérieures à 200 °C. Ce niveau de température va induire des phénomènes de déshydratation au sein de la pâte, impactant notamment les C-S-H, hydrate majoritaire à l'origine de la cohésion mécanique. L'effet de cette déshydratation sur les propriétés mécaniques et de transport a ainsi fait l'objet de ce travail de thèse. Afin d'appréhender ces effets, il convient de prendre en compte le caractère hétérogène, poreux et multi-échelle de ces matériaux. Pour cela, la micromécanique et les outils d'homogénéisation basés sur la solution d'Eshelby ont été utilisés. Par ailleurs, pour accompagner cette modélisation multi-échelle, des essais mécaniques basés sur la théorie des milieux poreux ont été menés. La mesure des modules de compressibilité, de la perméabilité et de la porosité sous confinement ont permis d'étudier les mécanismes de dégradation de ces matériaux lors de sollicitations thermiques jusqu'à 400°C / The modern industrial activities (storage of nuclear waste, geothermal wells, nuclear power plants, ...) can submit cementitious materials to some extreme conditions, for example at temperatures above 200 ° C. This level of temperature will induce phenomena of dehydration in the cement paste, particularly impacting the CSH hydrates which led to the mechanical cohesion. The effects of these temperatures on the mechanical and transport propertes have been the subject of this thesis.To understand these effects, we need to take into account the heterogeneous, porous, multi-scale aspects of these materials. To do this, micromechanics and homogenization tools based on the Eshelby problem's solution were used. Moreover, to support this multi-scale modeling, mechanical testing based on the theory of porous media were conducted. The measurements of modulus compressibility, permeability and porosity under confining pressure were used to investigate the mechanisms of degradation of these materials during thermal loads up to 400 ° C

In order for microfluidics devices to be marketable, they must be inexpensive and easy to use. Two projects were pursued in this study for this purpose. The first was the design of a chip alignment system for visual feedback, in which a two-layer microfluidic chip was placed under a camera and an image processing and linear algebra program aligned a computer model to it. The system then translated the new locations of air valves and could detect valve activation in a chip filled with food coloring. The second was the design of a cheap, portable system to detect phosphorus in water. This system could not be completed due to time constraints, but the methods were detailed, and design ideas were laid out for future work. / by Ian Gerstel. / Thesis (M.S.C.S.)--Florida Atlantic University, 2010. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2010. Mode of access: World Wide Web.

Fraturas são descontinuidades físicas, presentes em diversos materiais utilizados na engenharia, e são responsáveis pela redução da resistência e da rigidez global dos materiais. Tratando-se de fraturas de pequena dimensão, é possível definir a existência de duas escalas: a escala microscópica, onde as fraturas são visíveis, e a escala macroscópica, onde o material fraturado é homogêneo. Maghous et al. (2010) utilizaram a micromecânica para expor o tensor de rigidez homogeneizado para materiais elásticos fraturados, fazendo a ressalva de que fraturas transmitem esforços por suas faces. Utilizando os conceitos formulados por Maghous, Lorenci (2013) ampliou sua aplicação, estendendo à distribuição aleatória das fraturas. Utilizando o mesmo procedimento realizado por Lorenci, determinou-se os tensores de rigidez homogeneizados para materiais elásticos fraturados, os quais foram empregados para formular as condições de propagação de fraturas para materiais elásticos. Conceitualmente, a condição de propagação de fraturas em meios elásticos é formulada com base em conceitos clássicos da termodinâmica, baseados na dissipação de energia. Tratando-se de meios viscoelásticos, a dissipação de energia adquire um novo termo denominado de dissipação viscosa. Nguyen (2010) estabeleceu uma condição de propagação de fissuras em meios viscoelásticos, entretanto, as fissuras admitidas por Nguyen não são responsáveis pela transferência de esforços. Para estender a análise de Nguyen ao caso de fraturas, foi necessário determinar os tensores de relaxação do material viscoelástico fraturado, estes tensores foram obtidos combinando-se os tensores elásticos homogeneizados com os conceitos da transformada de Carson-Laplace, admitindo que as fraturas não se propagam ao longo do tempo. Com base no tensor de relaxação isótropo homogeneizado, determinou-se um modelo reológico equivalente que represente o material viscoelástico fraturado assumindo diferentes modelos reológicos para a matriz e para fraturas. Por fim, analisou-se as condições de propagação de fraturas em meios viscoelásticos de duas formas: de forma aproximada (apurando os estudos realizados por Nguyen) e de forma homogeneizada (admitindo que a propagação de fraturas se dá na escala macroscópica). / responsible for reducing the overall strength and stiffness of the material. In the case of small fractures, is possible set two scales: a microscopic scale, where fractures are visible, and the macroscopic scale, where the fractured material is homogeneous. Maghous et al. (2010) used the micromechanics to expose the homogenized stiffness tensor for fractured elastic materials, making the observation that fractures transmit efforts by their faces. Using the concepts formulated by Maghous, Lorenci (2013) expanded its application, extending to a random distribution of fractures. Using the same procedure performed by Lorenci, the homogenized stiffness tensor was determined for fractured elastic materials, which were employed to formulate the fracture propagation conditions for elastic materials. Conceptually, the fracture propagation conditions for elastic means is made based on classical concepts of thermodynamics, based on the energy dissipation. In the case of viscoelastic means, the energy dissipation acquires a new term called viscous dissipation. Nguyen (2010) established a condition of crack propagation in viscoelastic means, however, the Nguyen’s cracks are not responsible for the transfer of efforts. To extend Nguyen analysis to the case of fractures, was necessary to determine the relaxation tensor for viscoelastic fractured materials, these tensors are obtained by combining the homogenized elastic tensor to the concepts of the Carson- Laplace transform, assuming that the fractures are not propagate over time. Based on the isotropic homogenized relaxation tensors, was determined an equivalent rheological model representing the fractured viscoelastic material assuming different rheological models for matrix and fractures. Finally, was analyzed the fracture propagation conditions in viscoelastic means in two ways: in an approximate way (improving the studies conducted by Nguyen) and homogenized form (assuming that the propagation of fractures occurs at the macroscopic scale).

(1) A new model for the mass transfer in Direct Contact Membrane Distillation (DCMD) process is developed. The model is based on Direct Simulation Monte Carlo (DSMC) method. It avoids the over simplification of the resistance mechanisms and hence, give more accurate prediction. The model is validated by means of experiments. The influences of the main parameters in DCMD are also studied, including temperature difference between the feed side and the permeate side, the membrane's thickness and the pore size. Moreover, it is proposed to use aerogel as the membrane material. It is shown that the aerogel's hydrophobic property, low thermal conductivity and high porosity offer a much improved performance over the commonly used membrane material PTFE. The fresh water productivity can reach 10.0 kg/m2 per day. / (2) A new energy harvesting method for converting thermal energy to kinetic energy is proposed. This method is based on the rarefied gas phenomenon called Knudsen effect. By Knudsen effect, a gas flow can be generated from temperature difference. In order to generate Knudsen effect, a special material, aerogel, is used. It is a porous material full of holes of dozens of nanometers. Using Direct Simulation Monte Carlo (DSMC) simulation, it is shown that Knudsen effect still works under atmosphere pressure with aerogel material. Accordingly, a device is designed. Based on the numerical simulation, the device can generate about 70 W kinetic energy when driven by a solar panel with intensity of 1 kW/m2. / (3) A solar desalination system is designed. This system is based on a combination of Knudsen compressor and simple solar still. The Knudsen effect is generated from the aerogel driven by solar radiation. As a result, the system operates at lower pressure resulting in enhanced water evaporation process. Based on the simulation, the evaporation rate is significantly increased. It is found that in a typical summer day in tropic region like Hong Kong, such a system can generate about 5 kg fresh water per 1 m2 solar still per day. This number is about 30% higher than the simple direct solar still. Moreover, the proposed technology can be readily combined with other technologies such as condensation heat recovery to further improve the fresh water productivity. The optimal working condition is also studied. / Energy and water are two of the most important issues in the world today. The social and economic health of the world depends on sustainable supply of both energy and water. Especially, these two critical resources are always inextricably linked. To solve the emerging crisis of energy and water, renewable energy technologies is the key. On the other hand, recent advances in Micro-Electro-Mechanical Systems (MEMS) technology have opened new ways for us to use micro/nano scale physical and chemical effects. It is no doubted that the combination of the renewable energy technologies and micro/nano technologies will have great potential and there are plenty of room to explore. / The research presented in this thesis focuses on extending the micro scale effect to the macroscopic applications. Based on this idea, a new energy harvesting method and two new water desalination technologies are proposed, with computer simulations and experiment validations. These include: / Zhang, Peng. / Adviser: Ruxu Du. / Source: Dissertation Abstracts International, Volume: 72-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves 123-135). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. Ann Arbor, MI : ProQuest Information and Learning Company, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Composite materials reinforced with particles or fibers are widely used in industrial applications due to their good mechanical, thermal, and electrical properties. Consequently, for the scientific community as well as the industry, an important challenge is to understand the relationship between the microstruture and the macroscopic response in order to design composite materials with optimised properties. In this thesis, we study a class of inclusion-reinforced multi-phase composites. Our main objective is to develop a micromechanical model and the corresponding numerical algorithms which enable the simulation of the rate-independent mechanical response. The proposed model is based on an incremental Hill-type formulation and uses the two-step Mori-Tanaka/Voigt mean-field homogenisation schemes. The crucial issues of the choice of reference comparison materials and Eshelby's tensor computation are examined In parallel, an experimental study consisting in four-point bending tests performed on plain concrete and steel fiber-reinforced concrete (SFRC) specimens, is carried out with the aim of achieving an appropriate modelling of SFRC, and collecting data for the validation of our model predictions. The accuracy and the efficiency of the proposed approach are evaluated through numerical simulations. Several discriminating tests of concrete, metal, and polymer matrix composites are carried out. A two-scale approach is developed in order to simulate, within reasonable CPU time and memory usage, the response of realistic structures under complex loadings. In many cases our estimates are validated against finite element computations and experimental results.

Textile composites have a wide variety of applications in the aerospace, sports, automobile, marine and medical industries. Due to the availability of a variety of textile architectures and numerous parameters associated with each, optimal design through extensive experimental testing is not practical. Predictive tools are needed to perform virtual experiments of various options. The focus of this research is to develop a better understanding of linear elastic response, plasticity and material damage induced nonlinear behavior and mechanics of load flow in textile composites. Textile composites exhibit multiple scales of complexity. The various textile behaviors are analyzed using a two-scale finite element modeling. A framework to allow use of a wide variety of damage initiation and growth models is proposed. Plasticity induced non-linear behavior of 2x2 braided composites is investigated using a modeling approach based on Hill’s yield function for orthotropic materials. The mechanics of load flow in textile composites is demonstrated using special non-standard postprocessing techniques that not only highlight the important details, but also transform the extensive amount of output data into comprehensible modes of behavior. The investigations show that the damage models differ from each other in terms of amount of degradation as well as the properties to be degraded under a particular failure mode. When compared with experimental data, predictions of some models match well for glass/epoxy composite whereas other’s match well for carbon/epoxy composites. However, all the models predicted very similar response when damage factors were made similar, which shows that the magnitude of damage factors are very important. Full 3D as well as equivalent tape laminate predictions lie within the range of the experimental data for a wide variety of braided composites with different material systems, which validated the plasticity analysis. Conclusions about the effect of fiber type on the degree of plasticity induced non-linearity in a ±25° braid depend on the measure of non-linearity. Investigations about the mechanics of load flow in textile composites bring new insights about the textile behavior. For example, the reasons for existence of transverse shear stress under uni-axial loading and occurrence of stress concentrations at certain locations were explained.

The hot-workability of Titanium (Ti) alloys is of current interest to the aerospace industry due to its widespread application in the design of strong and light-weight aircraft structural components and engine parts. Motivated by the need for accurate simulation of large scale plastic deformation in metals that exhibit macroscopic plastic anisotropy, such as Ti, a constitutive model is developed for anisotropic materials undergoing plastic deformation coupled with ductile damage in the form of internal cavitation. The model is developed from a rigorous micromechanical basis, following well-known previous works in the field. The model incorporates the porosity and void aspect ratio as internal damage variables, and seeks to provide a more accurate prediction of damage growth compared to previous existing models. A closed form expression for the macroscopic yield locus is derived using a Hill-Mandel homogenization and limit analysis of a porous representative volume element. Analytical expressions are also developed for the evolution of the internal variables, porosity and void shape. The developed yield criterion is validated by comparison to numerically determined yield loci for specific anisotropic materials, using a numerical limit analysis technique developed herein. The evolution laws for the internal variables are validated by comparison with direct finite element simulations of porous unit cells. Comparison with previously published results in the literature indicates that the new model yields better agreement with the numerically determined yield loci for a wide range of loading paths. Use of the new model in continuum finite element simulations of ductile fracture may be expected to lead to improved predictions for damage evolution and fracture modes in plastically anisotropic materials.

Asphalt pavements experience damage due to traffic loading under various environmental conditions. Damage can be caused by viscopl microcracks, fracture due to fatigue cracking, or fracture due to thermal cracking. Asphalt pavements have the capability to remedi s damage depending on binder surface and rheological properties, filler surface properties, and length of rest periods. Asphalt mastic (asphalt and fine aggregates) properties play an important role in controlling damage and healing. This dissertation development of a comprehensive methodology to characterize damage and healing in asphalt mastics and mixtures. The methodology reli ctive imaging techniques (X-ray CT), principles of continuum damage mechanics, and principles of micromechanics. The X-ray CT yield meter that quantifies the percentage of cracks and air voids in a specimen. The continuum damage model parameters are derived from p between applied stress and pseudo strain. The micromechanics model relates the damaged mastic modulus to a reference undamaged mo ationship is a function of internal structure properties (void size, film thickness, and percentage of voids), binder modulus, aggr and bond energy between binder and aggregates. The internal structure parameters are all obtained using X-ray CT and correlated. The developed methodology was used to characterize damage in asphalt mastic and mixture specimens tested using the Dynamic Mechanic A) and dynamic creep test. The damage parameter measured using X-ray CT correlated very well with the predictions of the continuum ics models. All damage parameters were able to reflect the accumulation of damage under cyclic loading and were also able to captur of moisture conditioning on damage. Although this dissertation focused on fatigue cracking at room temperatures, the methodology d used to assess damage due to different mechanisms such as permanent deformation and low temperature cracking.