Thesis (Ph. D.)--Drexel University, 2006. / Includes abstract and vita. Includes bibliographical references.

Abstract Micro-electromechanical systems (MEMS) are subject to inevitable and inherent uncertainties in their dimensional and material parameters. Those lead to variability in their performance and reliability. Manufacturing processes leave substantial variability in the shape and geometry of the device due to its small dimensions and high feature complexity, while the material properties of a component are inherently subject to scattering. The effects of these variations have to be considered and a modeling methodology is needed in order to ensure required MEMS performance under uncertainties. Furthermore, in the design of high-Q micro-resonators, dissipation mechanisms may have detrimental effects on the quality factor (Q). One of the major dissipation phenomena to consider is thermoelastic damping, so that performances are directly related to the thermoelastic quality factor, which has to be predicted accurately. The purpose of this research is to develop a numerical method to analyze the effects of geometric and material property random variations on the thermoelastic quality factor of micro-resonators. The extension of the Perturbation Stochastic Finite Element Method (PSFEM) to the analysis of strongly coupled multiphysic phenomena allows the quantification of the influence of uncertainties, making available a new efficient numerical tool to MEMS designers. Résumé Dans le domaine des microsystèmes électromécaniques (MEMS), les micro-résonateurs jouent un rôle important pour le développement de micro-capteurs de plus en plus précis (ex : micro-accéléromètres). Dans cette optique daugmentation de la précision, les pertes dénergie qui limitent les performances des micro-résonateurs doivent être identifiées et quantifiées. Le facteur limitant des micro-résonateurs actuels est leur facteur de qualité thermo-élastique, qui doit donc être prédit de manière précise. De plus, suite à la tendance actuelle de miniaturisation et complexification accrues des MEMS, les sources de dispersions sont très nombreuses, à la fois sur les constantes physiques des matériaux utilisés et sur les paramètres géométriques. La mise au point doutils numériques permettant de prendre en compte les incertitudes de manière efficace est donc primordiale afin daméliorer les prestations densemble du microsystème et dassurer un certain niveau de robustesse et de fiabilité. Le but de cette recherche est de développer une méthode numérique pour analyser les effets des variations aléatoires des propriétés matérielles et géométriques sur le facteur de qualité thermo-élastique de micro-résonateurs. Pour ce faire, lapproche dite perturbative de la méthode des éléments finis stochastiques (PSFEM) est étendue à lanalyse de phénomènes multiphysiques fortement couplés, fournissant ainsi aux acteurs de lindustrie des MEMS un nouvel outil de conception efficace.

¡§Phononic crystal,¡¨ a binary-composite medium composed of a square array of parallel circular steel cylinders in a air matrix is studied. Phononic crystal exists full band-gaps phenomenon which is caused by strongly constructive interference of Bragg reflection in their acoustic transmission spectrum. The Bragg reflection theorem is also a basis for searching the full band-gaps in this thesis. This thesis applies the boundary element simulation software BEASY to analyze the sound field characteristics of solid/fluid composite medium, phononic crystal. The forbidden bands of the band gap are shown by the relative amplitude in the incidence before and after. First, the study by Varadan and Faran aims at scattering sound field of the single rigid sphere and the circular cylinder in water which constructed a simulation of the boundary element model. It is compared to under the different kr change result of its scattering sound field and it has demonstrated that our simulation work was feasible. Second, the study constructs the boundary element model for a two-dimensional phononic crystal which was studied by Sánchez-Pérez etc. with experimentation, constituted of rectangular and triangular array of parallel circular stainless steel and aluminum cylinders in air. The study is compared with the forbidden bands of the band gap in the reference which performs the simulations with the mono-frequency by sweep. The full band gaps are determined from the combination of the results in both the [100] and [110] direction. Finally, the study aimed at the scattering pattern of sound field in phononic crystal to make discussion. In order to understand the sound source acts on the phononic crystal, the status of the sound pressure is distributed over the spatial. So it could get up to reduce the influence of the noise by way of the improvement the structure in phononic crystal. The study has successfully shown the boundary element simulation for the solid/fluid phononic crystal. The study of experiment in the reference is compared with the BEM simulation in this thesis. The results have demonstrated that the boundary element method is a good tool for the design of phononic crystal in application to new type sound absorption (isolation) material in the future.

In this study, a modified two-dimensional axisymmetric finite element model is used to analyze the deflections of the piezoelectric actuator of valveless micropump after being driven by applied voltage. And the volume change of the pump chamber caused by the deformation of the piezoelectric actuator is calculated. We expect these analyses will help the design of piezoelectric valveless micropump. This model is able to analyze the piezoelectric materials which can transform the mechanical energy to electric energy and vice versa by properly assuming the three displacement fields and including the electrical potential as the fourth degree of freedom. Comparisons of some examples are made between the present work and those available in the literature to validate the exactitude and the feasibility of the present work. Furthermore, the inspections of the variations of the deflections will be carried out by changing the geometrical dimensions of the piezoelectric actuators under the same driven voltage.

Modeling of the time-dependent behavior of cement paste has always been a difficulty. In the past, viscoelastic behavior of cementitious materials has been primarily attributed to the viscoelastic properties of C-S-H components. Recent experimental results show that C-S-H may not exhibit as much creep and relaxation as previously thought. This requires new consideration of different mechanisms leading to the viscoelastic behavior of cement paste. Thus the objective of this thesis is to build a computational model using finite element method to predict the viscoelastic behavior of cement paste, and using this model, virtual tests can be carried out to improve understanding of the mechanisms of viscoelastic behavior. The primary finding from this thesis is that the apparent viscoelastic behavior due to dissolution of load bearing phases is substantial. The dissolution process occurring during the hydration reaction can change the stress distribution inside cementitious materials, resulting in an apparent viscoelastic behavior of the whole cementitious materials. This finding requires new consideration of mechanisms of time-dependent behavior of cementitious materials regarding the dissolution process of cement paste.

<p>Mechanisms of sex chromosome dosage compensation vary widely between different vertebrate species. All eutherians, including humans, other primates, and rodents, undergo random X chromosome inactivation in early female embryos, a process by which the majority of the genes on one X chromosome in the female are silenced (inactive X, Xi) to create a transcription level matching that of the single X chromosome in males. Random inactivation of the placental embryo is initiated from a region on the X chromosome called the X-inactivation center (XIC in humans and Xic in mice), thus implicating this region as the key chromosomal element in distinguishing random from imprinted X inactivation during mammalian evolution. This invites a comparative genomic approach to explore the organization and evolution of this region throughout mammalian lineages</p><p>Patterns of X inactivation are genetically determined, as indicated by non-random patterns of inactivation in mice heterozygous for the X-linked X controlling element (Xce) locus, the molecular and genomic basis of which is unknown. Using QTL mapping in Xce heterozygous mice, we previously identified a 1.85Mb candidate region for Xce. This candidate region contains the X inactivation center (Xic), including the critical X inactivation genes Xist and Tsix. To explore the genomic organization of this region in C57BL/6J (B6), we identified extensive large (>5Kb) inverted and non-inverted segmental duplications lying greater than 350Kb proximal to Xist. Investigating these segmental duplications further, we then compared copy number and sequence variant differences among strains carrying different Xce alleles to identify candidate variants in a subportion of the interval that correlate with specific Xce alleles. </p><p>The Xce candidate region was then compared to the corresponding region of the X chromosome from several other species. Notably, the segmental duplications within the mouse Xce region are maintained positionally through the other species over at least 105 MYA, although they do not share the same DNA in the copy variant. </p><p>These and future experiments should provide detailed characterization of the Xce candidate region and an opportunity to address the role that these sequence signatures may play in the earliest stages of X inactivation when the two X chromosomes are distinguished from one another.</p> / Dissertation

Advancements in computational power and micro-imaging has allowed the creation of finite element (FE) models on a microstructural level that can represent complex skeletal structures. These µFE models can analyze the structural integrity of individual trabeculae and may be used to model the impact of complex pathologies on skeletal stability. This thesis aims to: 1) optimize the histological identification of microdamage in healthy and mixed metastatic whole rat vertebrae, 2) quantify trabecular level stress and strain using µFE models and deformable registration generated from µCT data and 3) evaluate stress and strain in µFE models comparing undamaged regions with areas of mechanically induced microdamage. This novel technique allows the histological identification of microdamage in whole vertebrae with accurate alignment to 3D μCT data sets. In the μFE models, significantly higher stresses and strains were found in areas of damaged bone in both healthy and metastatically involved vertebrae.

Advancements in computational power and micro-imaging has allowed the creation of finite element (FE) models on a microstructural level that can represent complex skeletal structures. These µFE models can analyze the structural integrity of individual trabeculae and may be used to model the impact of complex pathologies on skeletal stability. This thesis aims to: 1) optimize the histological identification of microdamage in healthy and mixed metastatic whole rat vertebrae, 2) quantify trabecular level stress and strain using µFE models and deformable registration generated from µCT data and 3) evaluate stress and strain in µFE models comparing undamaged regions with areas of mechanically induced microdamage. This novel technique allows the histological identification of microdamage in whole vertebrae with accurate alignment to 3D μCT data sets. In the μFE models, significantly higher stresses and strains were found in areas of damaged bone in both healthy and metastatically involved vertebrae.

A new three-dimensional finite solid element (3D FE) model for dowel-type wood connections was developed using the concept of a beam on a nonlinear wood foundation, which addresses the intricate wood crushing behaviour under the connector in a dowel type connection. In order to implement the concept of wood foundation with solid elements, a 3D FE wood foundation model was defined within a prescribed foundation zone surrounding the dowel. Based on anisotropic plasticity material theory, the material model for the foundation zone was developed using effective foundation material constants that were defined from dowel-embedment test data. New 3D FE single nail connection models were developed that incorporated the wood foundation model. The 3D wood foundation model was justified and validated using dowel-embedment test data with a range of dowel diameters, from a 2.5-mm nail to a25.4-mm bolt. The connection models provided successful results in simulating the characteristics of load-slip behaviour that were experimentally observed. Based on the success of the single nail connection models, several applications of the3D FE connection models were investigated including statistical wood material models, bolted connection models and a multiple nail connection model. Throughout the application studies, discussion of the benefits and limitations of the new model approach using the 3D FE wood foundation are presented. Also, future areas of study are proposed in order to improve the 3D FE dowel-type wood connections models.

Many soft food materials, including vegetable shortening, exhibit complex rheological behaviour with properties that resemble those of a solid and a liquid simultaneously. The fundamental parameters used to describe the rheological response of vegetable shortening were obtained from uniaxial compression tests, including monotonic and cyclic compression, as well as creep and stress relaxation tests. The fundamental parameters obtained from the various compression tests were then used in two mechanical models (viscoelastic and elasto-visco-plastic) to predict the compression and conical indentation response of vegetable shortening. The accuracy of the two models was studied with the help of the commercially available finite element analysis software package Abaqus. It was determined that the viscoelastic model was not suitable for the prediction of the rheological response of shortening. On the other hand, the proposed elasto-visco-plastic model predicted with reasonable accuracy the uniaxial compression and indentation experimental response of vegetable shortening. / February 2009