Polymer nanocomposites (PNCs), mixtures of nanometer-sized particles and polymeric matrices, have attracted continuing interest over the past few decades, primarily because they offer the promise of significant property improvements relative to the pure polymer. It is now commonly accepted in the community that the spatial organization of nanoparticles (NPs) in the polymer host plays a critical role in determining the macroscopic properties of the resulting PNCs. However, till date there is still dearth of cost-effective methods for controlling the dispersion of NPs in polymeric hosts. In this dissertation, we are dedicated to developing practically simple and thus commercially relevant strategies to controllably disperse NPs into synthetic polymer matrices (both amorphous and semicrystalline). We first investigate the influence of casting solvent on the NP spatial organization and the thermomechanical properties in a strongly attractive PNC consisting of bare silica NPs and poly(2-vinylpyridine) (P2VP) hosts cast from two different solvents - methylethylketone (MEK) or pyridine. In MEK, we show that P2VP strongly adsorbs onto the silica surface, creating a stable bound polymer layer and thus helping sterically stabilize the NPs against agglomeration. On the contrary, in pyridine, P2VP does not adsorb on the silica NPs, and the phase behavior in such case is a subtle balance among electrostatic repulsion, polymer-induced depletion attraction, and the kinetic slowdown of diffusion-limited NP aggregation. Using Brillouin light scattering, we further show that in pyridine-cast films, there is a single acoustic phonon, implying a homogeneous mixture of silica and P2VP on the mesoscopic scales. However, in MEK-cast samples, two longitudinal and two transverse acoustic phonons are probed at high particle content, reminiscent of two metastable microscopic phases. These solvent-induced differences in the elastic mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state locally approach equilibrium upon annealing. Next, to disperse silica NPs into an energetically unfavorable polystyrene (PS) matrix in a controllable fashion, we have proposed a simple and robust strategy of adsorbing a monolayer of PS-b-P2VP block copolymer onto the silica surface, where the short P2VP block is densely coated around the silica particles and thus helps to reduce the inter-core attraction while the long PS block provides a miscible interface with the matrix chains. As a result, we have found that the silica particles can be uniformly dispersed in the PS matrices at a low grafting density of 0.01 chains/nm2. Even more interestingly, we have shown that the BCP coated NPs are remarkably better dispersed than the ones tethered with bimodal PS-P2VP brushes at comparable PS grafting characteristics. This finding can be reconciled by the fact that in the case of BCP adsorption, each NP is more uniformly coated by a P2VP monolayer driven by the strongly favorable silica-P2VP interactions. Since each P2VP block is connected to a PS chain we conjecture that these adsorbed systems are closer to the limit of spatially uniform sparse brush coverage than the chemically grafted case. Finally, we have examined the interplay between NP organization and polymer crystallization in a melt-miscible model semicrystalline nanocomposite comprised of poly(methyl methacrylate) or poly(methyl acrylate) grafted silica NPs in poly(ethyleneoxide) matrices. Here we have achieved active NP organization at a length scale of 10-100 nm by isothermal polymer crystallization. We have shown that the melt-miscible spherical NPs are engulfed by the polymer crystals and remain spatially well-dispersed for crystallization faster than a critical growth rate (G > Gc ~ 0.1 um/s). However, anisotropic sheet-like NP ordering results for slower G - the NPs are preferentially segregated into the interlamellar zone of the multiscale, hierarchical polymer crystal structure spanning lamellae (10-50 nm), fibrils (um) and spherulites (mm). This NP ordering is found to favorably impact the elastic modulus while leaving fracture toughness unaffected. We thus conclude that polymer crystal growth kinetics coupled to the unusual morphology of semicrystalline polymers represent a novel handle for in-situ fabricating hierarchical, anisotropic NP structures in a synthetic semicrystalline polymer, which could inspire significant applications.

La transformation de phase influence considérablement les propriétés thermomécaniques des matériaux métastables. Elle est prise en considération dans des modèles numériques qui simule le comportement de ces derniers dans des codes de calcul et qui nécessitent des résultats expérimentaux. Ainsi, ces travaux concernent la caractérisation du comportement axial et multiaxial de deux aciers iTRIP à savoir l’acier 301L et l’acier 304L ainsi qu’un alliage à mémoire de forme à base de CuAlBe. Cette caractérisation est couplée avec un suivi de la cinétique des transformation de phase entreprise par la mesure de la résistance électrique. Le premier chapitre est une étude bibliographique des deux classes de matériaux métastables cités précédemment ainsi que du phénomène de transformation de phase et les techniques permettant sa caractérisation. Le deuxième chapitre, s’intéresse aux essais uniaxiaux thermomécaniques réalisés sur un acier iTRIP afin de valider la méthode de dosage de phase. Aussi, les essais thermomécaniques multiaxiaux réalisés sur des éprouvettes tubulaires en acier iTRIP 304L y seront présentés. Le troisième chapitre est consacré aux essais uniaxiaux réalisés sur des éprouvettes de CuAlBe ainsi qu’au dosage de phase des cas biphasé et triphasé. Aussi, la validation de l’hypothèse de linéarité entre la fraction volumique de martensite et la déformation de transformation équivalente dans le cas de chargements multiaxiaux proportionnels et non proportionnels est faite. / Phase transformation considerably influences the thermomechanical properties of metastable materials. This is reflected in the numerical model that simulates the behavior of these materials for the calculation codes and require experiments.Thus, the present work concerns the characterization of the axial and multi-axial behavior of two iTRIP steels, 301L steel and 304L steel in addition to a shape memory alloy based on CuAlBe. This characterization is coupled with monitoring of phase transformation kinetics through the measurement of the electrical resistance.The first chapter is a bibliographic study of the two classes of metastable materials mentioned above as well as the phase transformation phenomenon and its characterization techniques. The second chapter deals with uniaxial thermomechanical tests on a steel iTRIP to validate the phase assay. The multiaxial thermomechanical testing performed on specimens tubular steel 304L iTrip will be presented.The third chapter is devoted to uniaxial tests performed on CuAlBe spicemens and realization phase doping in a three-phase case. Also, the validation of the assumption of linearity between the martensite volume fraction and the equivalent transformation strain in the case of proportional and non-proportional loading is done.

All materials are heterogeneous at various scales of observation. The influence of material heterogeneity on nonuniform response and microstructure evolution can have profound impact on continuum thermomechanical response at macroscopic "engineering" scales. In many cases, it is necessary to treat this behavior as a multiscale process. This research developed a hierarchical multiscale approach for modeling microstructure evolution. A theoretical framework for the hierarchical homogenization of inelastic response of heterogeneous materials was developed with a special focus on scale invariance principles needed to assure physical consistency across scales. Within this multiscale framework, the second gradient is used as a nonlocal kinematic link between the response of a material point at the coarse scale and the response of a neighborhood of material points at the fine scale. Kinematic consistency between two scales results in specific requirements for constraints on the fluctuation field. A multiscale internal state variable (ISV) constitutive theory is developed that is couched in the coarse scale intermediate configuration and from which an important new concept in scale transitions emerges, namely scale invariance of dissipation. At the fine scale, the material is treated using finite element models of statistical volume elements of microstructure. The coarse scale is treated using a mixed-field finite element approach. The coarse scale constitutive equations are implemented in a finite deformation hyperelastic inelastic integration scheme developed for second gradient constitutive models. An example problem based on an idealized porous microstructure is presented to illustrate the approach and highlight its predictive utility. This example and a few variations are explored to address the boundary-value-problem dependent nature of length scale parameters employed in nonlocal continuum theories. Finally, strategies for developing meaningful kinematic ISVs, free energy functions, and the associated evolution kinetics are presented. These strategies are centered on the goal of accurately representing the energy stored and dissipated during irreversible processes.

Polymer nanocomposites (PNCs) have gained significant interest because they have outstanding performance that allows cost reduction, weight reduction, and product improvement. This research study focuses on the manufacture and characterization of PNCs in order to explore their potential in automotive applications. More specifically, polypropylene (PP) nanocomposites reinforced with xGnP and nanokaolin were fabricated by manufacturing methods that optimize their performances. Exfoliated graphite nanoplatelets (xGnP) are promising nanofillers that are cost effective and multifunctional with superior mechanical, thermo-mechanical and electrical properties. Nanokaolin is a newly introduced natural mineral mind in Georgia that has not been studied as of now. PNCs reinforced with these two nanofillers were characterized in terms of mechanical, thermo-mechanical, and various other properties, and then compared to talc- reinforced PP composites, which are the current state of the art for rear bumpers used by Honda Motor. Characterization results indicated that xGnP had better performance than talc and nanokaolin. Furthermore, the addition of xGnP introduces electrical conductivity in the PNCs, leading to more potential uses for PNCs in automotive applications such as the ability to be electrostatic painted. In order to fabricate PNCs with a desired conductivity value, there is need for a design tool that can predict electrical conductivity. Existing electrical conductivity models were examined in terms of model characteristics and parameters, and model predictions were compared to the experimental data. The percolation threshold is the most important parameter in these models, but it is difficult to determine experimentally, that is why a correlation between thermo-mechanical properties and electrical conductivity is also investigated in this study.

Magnesium alloys have a great potential in automotive industries, compared to steel and aluminium (Al), Magnesium (Mg) is much lighter and this weight reduction improves fuel efficiency and lowers green gas emission. Due to its hexagonal crystal structure, magnesium has poor ductility at room temperature. Magnesium's ductility improves significantly above about 200°C due to thermal activation of additional slip systems. This has lead to efforts to form auto-body panels with commercial AZ31 magnesium sheet at elevated temperatures. In this work, various AZ31 magnesium alloy materials were used to investigate the influence of deformation conditions on texture and microstructure. Moreover, it is to define the correlation between formability and different deformation mechanisms. / It was observed that only basal slip and twinning contributed to room temperature deformation. As deformation temperature increased, an increase in ductility in Mg contributed to dynamic recrystallization occurring readily at elevated temperatures (≥300°C). Even coarse grain material experienced significant tensile elongation due grain refinement. Depending on temperature and strain rate, different deformation mechanisms were activated and lead to different failure modes (moderate necking, cavity, strong necking). More specifically, deformation at elevated temperature in the low-strain-rate regime with stress exponent n about 2-3 and activation energy close to grain-boundary diffusion of Mg (Q = 92 kJ/mol) is characteristic of GBS. Deformation at elevated temperature in the high strain rate regime showed that the stress exponent increased to a value close to 5 and that the activation energy was consistent with the one for Mg self-diffusion (135 kJ/mol) and for diffusion of Al in Mg (143 kJ/mol). This was indicative of a dislocation creep deformation mechanism. Plus the six-fold symmetric patterns of the {1 100} and {1120} pole figures and the splitting of basal plane distribution are another indication of slip mechanism or of dislocation creep mechanism. / The optimum deformation behavior for AZ31 sheet was found to be for the material with fine grain microstructure. The highest elongation of 265% was obtained with the material having initial grain size of 8 mum. In addition, strain-rate sensitivity, which is a good indication of material's ductility, also was the highest in material with 8 mum grain size. As a common trend, the strain-rate sensitivity increased with decreasing strain rate, increasing temperature and decreasing grain size. / In terms of drawability of AZ31 sheet, the deformation controlled by GBS resulted in a fair drawability/formability property with r-value about 1 whereas a deformation mechanism controlled by dislocation creep showed a good drawability with r-value above 1.5. Due to activation of additional slip systems (non-basal <a> and <c+a>), the thinning of the sheet was prevented, in particular at deformation conditions of 450°C with 0.1s-1 where r-value was highest. This deformation condition might suggest good forming process parameters, especially for deep drawing, for the commercial AZ31 sheet under investigation. A preliminary study of Forming Limit Diagram for AZ31 sheet was performed by the Limit Dome Height test method at 300°C. The FLD0 of AZ31 was found to be 67%; the part depth of biaxial forming was 1.86 in; and the maximum LDH varied from 2.4 to 2.6 in.

Under consideration is the finite-size scaling of two thermomechanical responses of random heterogeneous materials. Stochastic mechanics is applied here to the modeling of heterogeneous materials in order to construct the constitutive relations. Such relations (e.g. Hooke's Law in elasticity or Fourier's Law in heat transfer) are well-established under spatial homogeneity assumption of continuum mechanics, where the Representative Volume Element (RVE) is the fundamental concept. The key question is what is the size L of RVE? According to the separation of scales assumption, L must be bounded according to d&lt;L&lt;&lt;LMacro where d is the microscale (or average size of heterogeneity), and LMacro is the macroscale of a continuum mechanics problem. Statistically, for spatially ergodic heterogeneous materials, when the mesoscale is equal to or bigger than the scale of the RVE, the elements of the material can be considered homogenized. In order to attain the said homogenization, two conditions must be satisfied: (a) the microstructure's statistics must be spatially homogeneous and ergodic; and (b) the material's effective constitutive response must be the same under uniform boundary conditions of essential (Dirichlet) and natural (Neumann) types. / In the first part of this work, the finite-size scaling trend to RVE of the Darcy law for Stokesian flow is studied for the case of random porous media, without invoking any periodic structure assumptions, but only assuming the microstructure's statistics to be spatially homogeneous and ergodic. By analogy to the existing methodology in thermomechanics of solid random media, the Hill-Mandel condition for the Darcy flow velocity and pressure gradient fields was first formulated. Under uniform essential and natural boundary conditions, two variational principles are developed based on minimum potential energy and complementary energy. Then, the partitioning method was applied, leading to scale dependent hierarchies on effective (RVE level) permeability. The proof shows that the ensemble average of permeability has an upper bound under essential boundary conditions and a lower bound under uniform natural boundary conditions. / To quantitatively assess the scaling convergence towards the RVE, these hierarchical trends were numerically obtained for various porosities of random disk systems, where the disk centers were generated by a planar Poisson process with inhibition. Overall, the results showed that the higher the density of random disks---or, equivalently, the narrower the micro-channels in the system---the smaller the size of RVE pertaining to the Darcy law. / In the second part of this work, the finite-size scaling of effective thermoelastic properties of random microstructures were considered from Statistical to Representative Volume Element (RVE). Similarly, under the assumption that the microstructure's statistics are spatially homogeneous and ergodic, the SVE is set-up on a mesoscale, i.e. any scale finite relative to the microstructural length scale. The Hill condition generalized to thermoelasticity dictates uniform essential and natural boundary conditions, which, with the help of two variational principles, led to scale dependent hierarchies of mesoscale bounds on effective (RVE level) properties: thermal expansion strain coefficient and stress coefficient, effective stiffness, and specific heats. Due to the presence of a non-quadratic term in the energy formulas, the mesoscale bounds for the thermal expansion are more complicated than those for the stiffness tensor and the heat capacity. To quantitatively assess the scaling trend towards the RVE, the hierarchies are computed for a planar matrix-inclusion composite, with inclusions (of circular disk shape) located at points of a planar, hard-core Poisson point field. Overall, while the RVE is attained exactly on scales infinitely large relative to microscale, depending on the microstructural parameters, the random fluctuations in the SVE response become very weak on scales an order of magnitude larger than the microscale, thus already approximating the RVE. / Based on the above studies, further work on homogenization of heterogeneous materials is outlined at the end of the thesis. / Keywords: Representative Volume Element (RVE), heterogeneous media, permeability, thermal expansion, mesoscale, microstructure.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Sitaraman, Suresh; Committee Member: Peak, Russell; Committee Member: Ume, Charles

Made available in DSpace on 2015-05-08T15:00:01Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 1709663 bytes, checksum: 35ce26ec1cf65f8e0507ea2d1c7adfd0 (MD5) Previous issue date: 2009-10-13 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Cu-Al-Ni and Cu-Al-Be shape memory alloys were cast under ambient atmosphere, and characterized thought optical microscopy, X-ray diffraction, and differential scanning calorimetry. Superelasticity, and shape recovery were evaluated using mechanical tensile test. Thermomechanical properties of shape memory alloys used were obtained under different temperature range, in which was possible to conclude that Cu-Al-Ni alloy can be used as mechanical actuators for temperatures above 90ºC, as well as this alloy in particular should not be considered for application in temperatures below 30ºC. Differently of CuAlBe alloy that can be used in low temperatures, as well as in temperature superiors to 0ºC, as, for example, mechanical actuators. / Ligas Cu-Al-Ni e Cu-Al-Be forma elaboradas sob atmosfera ambiente e caracterizadas por microscopia, difração de R-X e Calorimetria Diferencial de varredura. Foram realizados nestas ligas ensaios de tração, recuperação de forma e superelasticidade. As propriedades termomecânicas destas ligas realizadas em diferentes temperaturas permitiu concluir que as ligas Cu-Al-Ni podem ser utilizadas como atuadores mecânicos para temperaturas acima de 90°C e que estas ligas não devem ser empregadas em temperaturas abaixo de 30°C. Podemos concluir também que as ligas Cu-Al-be podem ser utilizadas como atuadores mecânicos em temperaturas superiores a 0°C.

Depuis quelques années, les polyoléfines et en particulier le Polyéthylène présentent un intérêt économique qui se traduit par un marché en croissance constante. Cependant, il est souvent nécessaire d’améliorer leurs propriétés d’usage notamment pour des problématiques liées à l’industrie automobile. Dans ce contexte, une amélioration des propriétés thermomécaniques et barrière aux vapeurs et liquides du Polyéthylène sur une gamme de température étendue constitue un nouveau challenge scientifique et environnemental. Dans cette étude, nous avons tout d’abord étudié l’impact de charges inorganiques et d’un recuit à 125°C sous air sur les propriétés thermomécaniques d’un HDPE. Le deuxième axe de recherche s’est concentré sur l’amélioration des propriétés barrière aux essences de ce polymère en favorisant les charges lamellaires pour leur haut facteur de forme induisant un effet de tortuosité élevé. L’impact de ce type de charges sur les phénomènes de sorption et d’extraction a été étudié dans le but de définir une formulation à base HDPE optimale pour limiter la perte physique d’oligomères et d’antioxydants. Le but de ces travaux étant de mieux comprendre les différents mécanismes mis en jeu, nous avons cherché à établir les relations Structure / Morphologie / Propriétés pour les deux axes d’étude développés / In the recent years, the use of polyolefin and especially Polyethylene are of economic interest resulting in a growing attention concerning the improvement of its properties of use, especially for automotive application. In this context, the reinforcement of its thermomechanical properties and the improvement of its barrier properties to different fuels over an extended temperature range constitute a major scientific and environmental challenge. In this study, we first decided to analyze the impact of silica particles and of an annealing at 125°C under air atmosphere on thermomechanical properties of a HDPE. The second area of research was focused on the improvement of its barrier properties to fuels by the addition of lamellar charges known for their high aspect ratio inducing a tortuosity effect. The impact of these particles on sorption and extraction phenomena was also studied to determine an optimal HDPE-based formulation in order to limit the physical loss of oligomers and antioxidants. The guideline of this project was the determination of Structure / Morphology / Properties relationships to have a better understanding of the involved mechanisms

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