Each year in the United States over a million people experience a myocardial infarction. The majority of these attacks are caused by coronary artery plaque cap rupture with subsequent thrombus formation. Because rupture is a mechanical event and the tendency of a plaque to rupture is due in part to increases in the mechanical stresses in the fibrous cap, mechanical analyses are important to understanding plaque stability. Histology is the only method capable of identifying plaque features that are associated with vulnerability. Therefore, minimally distorted histologic sections should serve as a basis for constructing the models used in mechanical analyses. Further, because substantial longitudinal variations in geometry and mechanical properties often exist, models should be three-dimensional (3-D). Finally, given the complex geometries of atherosclerotic plaques and the fact that they are composed of different materials, the finite element (FE) method should be used to determine the distribution of stress under physiological loading. Until now, a critical need has existed to determine the distribution of stress in 3-D FE models of human coronary atherosclerotic plaques based on minimally distorted histologic sections. In this research study, a method to measure and correct for distortions caused by acrylic histologic processing was first created. The devised strain-based method yields a limited set of parameters needed for a first order correction. Thus, corrections can be easily implemented using FE methods. Next, a methodology to create 3-D finite FE models of human coronary atherosclerotic plaques based on stable acrylic histologic sections was developed. Models of plaques, ranging in disease severity, were generated using the developed methodology. Lastly, the distributions of stress in these models were obtained and the effects of some plaque features on stresses were determined. Results from this study confirm that morphological description of a plaque is not sufficient to predict plaque rupture. The findings suggest that in many cases the 3-D stress field within a plaque must be known in order to assess plaque stability. Finally, the results show that patient specific models must be developed if the 3-D stress field within a plaque is to be determined.

Modeling of single-walled carbon nanotubes, multi-walled nanotubes and nanotube reinforced polymer composites using both the Finite Element method and the Molecular Dynamic simulation technique is presented. Nanotubes subjected to mechanical loading have been analyzed. Elastic moduli and thermal coefficient of expansion are calculated and their variation with diameter and length is investigated. In particular, the nanotubes are modeled using 3D elastic beam finite elements with six degrees of freedom at each node. The difficulty in modeling multi walled nanotubes is the van der Waal's forces between adjacent layers which are geometrically non linear in nature. These forces are modeled using truss elements. The nanotube-polymer interface in a nano-composite is modeled on a similar basis. While performing the molecular dynamic simulations, the geometric optimization is performed initially to obtain the minimized configuration and then the desired temperature is attained by rescaling the velocities of carbon atoms in the nanotube. Results show that the Young's modulus increases with tube diameter in molecular mechanics whereas decreases in molecular dynamics since the inter-atomic potential due to chemical reactions between the atoms is taken into consideration in molecular dynamics unlike in molecular mechanics.

Low back pain is the most common cause of disability in the United Kingdom with health care costs of more than 1 billion pounds per year. One reason associated with low back pain is the degeneration of intervertebral discs due to loads on the spine. Daily postures such as standing and sitting produce different loads on the discs. Previously, many studies investigated the stress and pressure within the disc in these postures. The results do not agree with each other and the experiments have many limitations. The aim of this project was to assess the feasibility of incorporating magnetic resonance (MR) imaging and finite element (FE) analysis to predict the pressure and stresses developed by different daily postures in an individual. Transient and non-transient subject specific 2D models of nine individuals in standing and sitting were created based on previously acquired MR images. The geometry of these FE models was based on supine MR images. The sitting and standing boundary conditions were calculated by comparing their MR images with the supine posture. The results showed that for six subjects sitting created more intradiscal pressure compared to standing and in one subject standing more than sitting. For two of the subjects the pressure was nearly the same in sitting and standing. Because of the 2D model’s limitations, 3D models of an individual were developed. Both transient and non-transient models of the individual were created. The intradiscal pressure results were three times lower compared to the 2D models. This was due to consideration of out of plane deformation in the 3D models. These results were in the range of in-vivo and in-vitro measurements available in the literature. In conclusion, it was possible to create kinematic transient subject specific FE models based on the MR images in different postures. 2D models provide a method for comparing the postures but 3D models may be more realistic.

L’utilisation de structures fines est croissante dans bon nombre d’industries. En ce sens, leur représentation mécanique et optimisation est un enjeu majeur de la recherche actuelle. De façon classique, l’optimisation s’effectue avec un critère de contrainte, obtenue à partir d’une modélisation éléments-finis en déplacements. L’idée de ces travaux est de construire un modèle éléments-finis mixte déplacements-contraintes et de développer des méthodes de réduction adaptées, de façon à améliorer l’efficacité des méthodes d’optimisation existantes. On construit d’une part deux modèles élément-finis mixtes déplacements-contraintes généralisées, pour des analyses dynamiques de structures “plaque” fines et épaisses. Ces derniers présentent l’avantage de donner des résultats identiques aux modèles classiques en déplacements, avec un meilleur temps de reconstruction des champs de contraintes. Cependant, ils s’avèrent être délicats pour plusieurs raisons : la taille des matrices associées, la difficulté de faire une analyse modale rapide, et un temps d’assemblage accru. C’est la raison pour laquelle nous développons par la suite des méthodes de sous-structuration et de double synthèse modale spécifiquement dédiées aux modèles mixtes. L’idée est d’utiliser des bases modales tirées du modèle équivalent en déplacements pour composer une nouvelle base mixte réduite. Dix méthodes sont implémentées, basées sur des modes encastrés, libres et de branche, parmi lesquelles certaines s’avèrent très efficaces pour réduire le nombre de degrés de liberté du système mixte, sans passer par ses modes propres. Enfin, nous intégrons les modèles mixtes sous-structurés sous forme de super- éléments mixtes dans un algorithme génétique, dans le but de mener une optimisation multi-objectif de structures “plaque” académiques sous chargement dynamique, avec critères de contrainte et paramètres d’épaisseur. Les modèles précédemment définis sont ainsi paramétrés en épaisseur, et ne nécessitent plus d’être ré-assemblés pour chaque configuration. Nous disposons désormais d’un modèle mixte “plaque”, qui conserve les avantages d’un accès direct aux contraintes, tout en étant affranchi de sa taille importante par le biais des méthodes de réduction, et de son assemblage grâce au paramétrage. Il en résulte des modèles mécaniques originaux et efficaces, permettant de réduire les coûts de calcul des algorithmes d’optimisation classiques. Ce type de méthode, couplé à de puissants algorithmes génétiques, permet d’avoir une bonne vue d’ensemble des solutions optimales, et laissent augurer des perspectives intéressantes pour une utilisation industrielle. / The use of thin structures is increasing in many industries. Their mechanical representation and optimization is therefore a major challenge in modern research. Usually, the optimization is done with a stress criterion which is determined through displacements finite-element model. The idea of this work is to build a mixed displacements-stresses finite-element model and to develop adapted reduction procedures, in order to improve the efficiency of existing optimization methods. On the one hand, we build two mixed displacements-generalized stresses finite element models, for thin and thick dynamic plate structures analysis. They afford the advantage of giving identical results as classical displacements models with a better computational time to re-build the stress fields. Nevertheless, they turn out to be tricky for some reasons : the bigger matrices size, the difficulty of modal analysis and an assembling time higher. That is the reason why we develop afterwards some sub-structuring methods and double modal synthesis specifically dedicated to mixed models. The idea is to use modal basis taken from the equivalent displacement model so as to build a new mixed reduced basis. Ten methods are implemented, based on fixed modes, free modes, and branch modes. Some of them turn out to be very efficient to drastically reduce the amount of degrees of freedom of the mixed model, without using its eigenmodes. Finally, we embed the sub-structured mixed model in the form of Mixed Super- Element in a genetic algorithm, with the aim of conducting a multi-objective optimization of academic plate structures under dynamic loads, with stresses criterion and thicknesses parameters. The models previously defined are configured with thicknesses as parameters, and therefore don’t need to be re-assembled for each configuration. We now dispose of a powerful thickness-parametrized mixed reduced plate finite element model : it keeps the advantages of an easy access to the stresses and is free of its important size thanks to the reduction method and of its assembling thanks to the parametrization. The result is an original and efficient mechanical model that reduces the computational cost of classical optimization algorithms. That type of model, coupled with powerful genetic algorithms, permits a global optimization with a good overview of the solutions and promises interesting perspectives for industrial uses.

Improved knowledge of human nerve function and recruitment would enable innovation in the Biomedical Engineering field. Better understanding holds the potential for greater integration between devices and the nervous system as well as the ability to develop therapeutic devices to treat conditions affecting the nervous system. This work presents a three-dimensional volume conductor model of the human arm for coupling with code describing nerve membrane characteristics. The model utilizes an inhomogeneous medium composed of bone, muscle, skin, nerve, artery, and vein. Dielectric properties of each tissue were collected from the literature and applied to corresponding material subdomains. Both a fully anatomical version and a simplified version are presented. The computational model for this study was developed in COMSOL and formatted to be coupled with SPICE netlist code. Limitations to this model due to computational power as well as future work are discussed. The final model incorporated both anatomically correct geometries and simplified geometries to enhance computational power. A stationary study was performed implementing a boundary current source through the surface of a conventionally placed electrode. Results from the volume conductor study are presented and validated through previous studies.

Detection of unexpectedly high concentrations of agricultural pollutants in ground water have inspired investigations of the role of preferential movement of chemicals through agricultural soils. This research focuses on preferential flow and solute transport processes and the effects of agricultural management practices on these processes. Experimental methods for monitoring preferential flow and solute transport in the field as well as a stochastic, physically-based model for predicting water flow and transport of non-reactive chemicals in heterogeneous soils with naturally occurring macropores were developed and evaluated. Field experiments, aimed at monitoring the occurrence of preferential flow and solute transport, were conducted in a conventionally-tilled and a no-till soybean field in the Coastal Plain of Virginia. A rainfall simulator was used to apply a one-hour storm at rates of 5.0, 6.5 and 7.5 cm/hr to six 1.83 by 1.83 m plots. Chloride was added to the water to serve as a non-reactive tracer. Electrical conductivity equipment provided a useful method for monitoring solute transport. The moisture and solute conditions, observed during a 28-hour period after the start of the rainfall event, clearly indicated the occurrence of preferential flow and solute movement in the field plots. The variability of the solute concentrations in the field plots was generally higher in the no-till plots than in the conventionally-tilled plots. The plots that received rain at 6.5 and 7.5 cm/hr showed more variability than the plots that received rain at 5 cm/hr. The observed solute concentrations indicated that if the solute transport would have taken place by advection only, 61% of the solute transport in the conventionally-tilled plots and 50% of the solute transport in the no-till plots could be attributed to preferential flow. A physically-based, finite element model for simulating flow and solute transport in variably-saturated soils with macropores (MICMAC) was developed. Flow and solute transport are described by the Richards' equation and the convection-dispersion equation. Flow in the macropores is described by the Hagen-Poiseuille equation. An axisymmetric coordinate system is used to simulate the flow and solute transport from the macropore into the surrounding soil matrix, assuming a vertically oriented, surface-vented, cylindrical macropore. Flow and solute transport between the macropore and the soil matrix are driven by the pressure head at the macropore-matrix boundary. To assess the natural heterogeneity of the soil properties a stochastic component was added to the model. Flow and solute transport at the field scale were simulated by regarding the field as a collection of statistically independent, non-interacting vertical soil columns, using Monte Carlo simulation. The sensitivity analysis of the model indicated that, for a soil with macropores, the model is most sensitive to the saturated water content of the soil matrix, the initial moisture content, and the rainfall rate. The model is not very sensitive to the macropore dimensions. Examination of the stochastic approach indicated that the representation of a heterogeneous field as a collection of non-interacting stream columns may substantially underestimate water and solute leaching. A change of 5% in the soil properties of the neighboring soil columns may underpredict the solute leaching, 24 hours after a rainstorm, by 157% for a soil column with a macropore, and by 58% for a soil column without a macropore. These differences decreased to 47% and 8%, respectively, 168 hours after the rainfall. Field application of the model suggested that the model underestimates the leaching of water and solutes from the root zone. However, the computed results were substantially better than the results obtained when no preferential flow component was included in the model. The model performed best under conditions that favored preferential flow, i.e., a high rainfall rate and high initial moisture conditions. The simulated and observed solute concentrations in the root zone agreed reasonably well, although the maxima of the observed data were generally higher than those of the simulated data. / Ph. D.

Image charge detector (ICD) technology has existed for decades. However, not until recently has an ICD been proposed for use in space exploration, specifically for studying the characteristics of the dust on Mars. Characterizing the dust on Mars is crucial for designing equipment to aid manned missions. It also improves our understanding of Mars' climate and weather systems. An ICD utilizing printed circuit board (PCB) electrodes, coupled with a custom differential amplifier, is best suited for this type of measurement because of its light weight, simplicity, and noise performance. The noise floor of our particular amplifier is measured to be 1030 e- and simulated to be as low as 140 e-. Both of these measurements are taken without averaging. To further verify and understand this device, I developed a novel simulation method using ANSYS Maxwell 3D to simulate the interaction between the charged particle and the electrodes of the ICD. The results from this simulation are then easily passed to Cadence where we can clearly see the response of the custom amplifier to the charged particle. This knowledge is used to study various types of electrode geometry for improved noise performance, as well as understand how particle trajectory affect the resulting signal. Once the validity of the Maxwell simulation is established, I use it, along with experimental data and a mathematical model based on conformal mapping, to optimize the ICD for noise performance. I find that the maximum noise performance does not lie in simply increasing the number of sensing stages, as was previously thought. The optimum number of stages is a function of the parasitic capacitance of the amplifier, with the greater parasitic capacitance leading to the greater number of stages for the optimum.

No description available.

Articular cartilage degeneration, called osteoarthritis, in the hip joint is a serious condition that affects millions of individuals yearly, with limited clinical solutions available to prevent or slow progression of damage. Additionally, the effects of high-risk factors (e.g. obesity, soft and hard tissue injuries, abnormal joint alignment, amputations) on the progression of osteoarthritis are not fully understood. Therefore, the objective of this thesis is to generate a finite element model for predicting osteochondral tissue stress and strain in the human hip joint during gait, with a future goal of using this model in clinically relevant studies aimed at prevention, treatment, and rehabilitation of OC injuries. A subject specific finite element model (FEM) was developed from computerized tomography images, using rigid bones and linear elastic isotropic material properties for cartilage as a first step in model development. Peak contact pressures of 8.0 to 10.6 MPa and contact areas of 576 to 1010 mm2 were predicted by this FEM during the stance phase of gait. This model was validated with in vitro measurements and found to be in good agreement with experimentally measured contact pressures, and fair agreement with measured contact areas.

This thesis is concerned with model-based non-rigid registration of single-modality magnetic resonance images of compressed and uncompressed breast tissue in breast cancer diagnostic/interventional imaging. First, a volumetric registration algorithm is developed which solves the registration as a state estimation problem. Using a static deformation model. To reduce computations, the similarity measure is calculated at some specific points called control points. These control points can be from a low resolution image grid or any irregular image grid. Our numerical analysis has shown that control points placed in the area without much information; i.e with small or no changes in image intensity, yield negligible deformation. Therefore, the selection of the control points can significantly impact the accuracy and computation complexity of the registration algorithms. An extension of the speeded up robust features (SURF) to 3D is proposed for enhanced selection of the control points in deformable image registration. The impact of this new control point selection method on the performance of the registration algorithm is analyzed by comparing it to the case where regular grid control points are used. The results show that the number of control points could be reduced by a factor of ten with new selection methodology without sacrificing performance. Second image registration method is proposed in which, based on a segmented pre-operative image, a deformation model of the breast tissue is developed and discretized in the spatial domain using the method of finite elements. The compression of the preoperative image is modeled by applying smooth forces on the surface of the breast where compression plates are placed. Image registration is accomplished by formulating and solving an optimization problem. The cost function is a similarity measure between the deformed preoperative image and intra-operative image computed at some control point and the decision variables are the tissue interaction forces. / Thesis / Master of Applied Science (MASc)