The increasing use of alpha/beta Ti-Al alloys in critical aircraft gas turbine engine and airframe applications necessitates the further development of physically-based constitutive models that account for their complex microdeformation mechanisms. Alpha/beta Ti-Al alloys are dual-phase in nature consisting of a mixture of hcp (alpha) and bcc (beta) crystal structures, which through variation in alloying elements and/or processing techniques can be produced in a wide range of microstructural compositions and morphologies. A constitutive model for these materials should address the various sources of material anisotropy and heterogeneity at both the micro and macroscales. The main sources of anisotropy in these materials are the low symmetry of the hcp phase, the texture, the relative strengths of different slip systems, non-planar dislocation core structures, phase distributions, and dislocation substructure evolution. The focus of this work is the development of a 3-D crystal plasticity model for duplex Ti-6Al-4V (Ti-64), an (alpha+beta) alloy. The model is used to study the process of attachment fatigue. Attachment fatigue is a boundary layer phenomenon in which most of the plastic deformation and damage accumulation occurs at depths on the order of tens of microns and encompasses regions of only a few grains into the depth of the material. The use of computational micromechanics-based crystal plasticity models to study attachment fatigue is a relatively new approach. This approach has the potential to offer additional insight to classical homogeneous plasticity models, since the length scales over which relative slip and crack initiation occur during this process is on the order of microstructural dimensions. Emphasis is placed on understanding the effects that texture, slip strength anisotropy, and phase distribution have on the surface and subsurface deformation fields during attachment fatigue. The deformation fields are quantified in terms of cumulative effective plastic strain distributions, plastic strain maps, and plastic strain-based critical plane multiaxial fatigue parameters.

Low-dimensional magnetic systems are of interest due to several new effects and modifications that occur at sizes below the average domain grain boundary within the bulk material. Molecule-like magnetite (Fe3O4) nanoparticles, with sizes ranging from one to two nm were synthesized and characterized in order to investigate new properties arising from quantum size effects. These small systems will provide opportunities to investigate magnetism of zero-dimension systems. A zero-dimensional object is usually called a quantum dot or artificial atom because its electronic states are few and sharply separated in energy, resembling those within an atom. Since the surface to volume ratio is the highest for zero-dimensional systems, most of the changes to magnetic behavior will be observed in ultra-fine magnetic particles. Chemically functional magnetic nanoparticles, comprised of a Fe3O4 magnetite core encased in a thin aliphatic carboxylate, have been prepared by sequential high temperature decomposition of organometallic compounds in a coordinating solvent. In this work, aliphatic carboxylic acid chain length, reaction temperature and duration were varied to produce small core diameters. In order to correlate size effects with changes in particle formation, it is important to have a through understanding of the structural components. This includes studies of the core size, surface effects, decomposition, electronic properties and magnetic behavior. Quantum size effects were observed in the (Fe3O4)X(carboxylate)Y monolayer protected clusters (MPCs) when the average core diameter was ≤ 2.0 nm, evidenced by a blue shifted absorbance band maxima, suggesting the onset of quantum confinement. These (Fe3O4)X(carboxylate)Y MPCs also posses a complex interplay between surface and finite size effects, which govern the magnetic properties of these zero-dimensional systems. These MPCs are all superparamagnetic above their blocking temperatures with total magnetic anisotropy values greater than the bulk value due to an increase in surface and magnetocrystalline anisotropy. A non-linear decrease in saturation magnetization (MS) [Bohr Magneton] per cluster) as a function of the reciprocal of core radius have been attributed to surface effects such as a magnetically inactive layer or an increase in spin disorder as core diameter decreases. The reduced core dimensions of these MPCs make them ideal candidates for further investigation of quantum magnetic systems.

Attention deficit/hyperactivity disorder (ADHD) is a common disease with a worldwide prevalence of 5% on preschool children. It has been reported that ADHD patients have volume variant in partial brain regions. Futhermore, functional magnetic resonance imaging have also been used to detect function variant possibility in particular brain regions. In the last decade, some researchers used diffusion MR imaging to investigate the abnormality of neural fibers in disease involved with central nervous system. In general, the diffusion anisotropy of white matter in both ADHD patients and healthy subjects can be estimated seperately to undergo inter-subject comparison. While previous studies often used the popular ICBM brain template (MNI152), this study applied a self-made template of Taiwan adolescents as the common space of image normailization. In this work, group comparison of diffusion fractional anisotropy was performing by using two methods, TBSS and VBM. Both manners found a decreased FA in white matter of ADHD subjects compared with normal control group. However, regions detected by different methods showed low reproducibility. The areas of significant difference include inferior longitudinal fasciculus¡Binternal capsule¡Bexternal capsule¡Bsuperior longitudinal fasciculus¡Boptic radiation¡Bsuperior frontal¡Bsuperior region of corona radiata¡Bcorticospinal tract¡Bposterior region of corona radiata / superior longitudinal fasciculus¡Bsuperior fronto-occipital fasciculus¡Banterior region of corona radiata¡Bgenu of corpus callosum nerve fibers.

The purpose of this work is to reveal the governing mechanisms responsible for the magnetic field-induced i) martensite reorientation in Ni2MnGa single crystals, ii) stress-assisted phase transformation in Ni2MnGa single crystals and iii) phase transformation in NiMnCoIn alloys. The ultimate goal of utilizing these mechanisms is to increase the actuation stress levels in magnetic shape memory alloys (MSMAs). Extensive experimental work on magneto-thermo-mechanical (MTM) characterization of these materials enabled us to i) better understand the ways to increase the actuation stress and strain and decrease the required magnetic field for actuation in MSMAs, ii) determine the effects of main MTM parameters on reversible magnetic field induced phase transformation, such as magnetocrystalline anisotropy energy (MAE), Zeeman energy (ZE), stress hysteresis, thermal hysteresis, critical stress for the stress induced phase transformation and crystal orientation, iii) find out the feasibility of employing polycrystal MSMAs, and iv) formulate a thermodynamical framework to capture the energetics of magnetic field-induced phase transformations in MSMAs. Magnetic shape memory properties of Ni2MnGa single crystals were characterized by monitoring magnetic field-induced strain (MFIS) as a function of compressive stress and stress-induced strain as a function of magnetic field. It is revealed that the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance. The actuation stress of 5 MPa and work output of 157 kJm−3 are obtained by the field-induced variant reorientation in NiMnGa alloys. Reversible and one-way stress-assisted field-induced phase transformations are observed in Ni2MnGa single crystals under low field magnitudes (<0.7T) and resulted in at least an order of magnitude higher actuation stress levels. It is very promising to provide higher work output levels and operating temperatures than variant reorientation mechanisms in NiMnGa alloys. Reversible field-induced phase transformation and shape memory characteristics of NiMnCoIn single crystals are also studied. Reversible field-induced phase transformation is observed only under high magnetic fields (>4T). Necessary magnetic and mechanical conditions, and materials design and selection guidelines are proposed to search for field-induced phase transformation in other ferromagnetic materials that undergo thermoelastic martensitic phase transformation.

Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess several processing challenges due to their anisotropic structural response, the wide variety of deformation textures they exhibit, and limited ductility at room temperature. The aim of this work is to investigate, both experimentally and theoretically, the effect os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number of phases on the flow stress anisotropy and tension compression asymmetry, and the mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered cubic (fcc) phase. Mg and its alloys have high specific strength that can potentially meet the high demand for light weight structural materials and low fuelconsumption in transportation. Zn-Al alloys, on the other hand, can be potential substitutes for several ferrous and non-ferrous materials because of their good mechanical and tribological properties. Both alloys have been successfully processed using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic textures for revealing the relationship between microstructural parameters, crystallographic texture and resulting flow stress anisotropy at room temperature. For AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC) crystal plasticity model. The flow stress anisotropy and tension-compression (T/C) asymmetry of the as received and processed samples at room temperature were measured and predicted using the same VPSC model coupled with a dislocation-based hardening scheme. The governing mechanisms behind these phenomena are revealed as functions of grains size and crystallographic texture. It was found that the variation in flow stress anisotropy and T/C asymmetry among samples can be explained based on the texture that is generated after each processing path. Therefore, it is possible to control the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by controlling the processing route and number of passes, and the selection of processing conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard phase size, morphology, and distribution were found to control the anisotropy in the flow strength and elongation to failure of the ECAE processed samples.

Analytical and numerical models are developed to investigate the effect of orthotropy on the stress distribution in variable thickness solid and annular rotating disks. The plastic treatment is based on Hill&rsquo / s quadratic yield criterion, total deformation theory, and Swift&rsquo / s hardening law. The elastic-plastic stress distributions, residual stresses and radial displacement distributions are obtained after having analysed the cases of rotating solid disk, annular disk with rigid inclusion, annular disk subjected to either internal or external pressure. Thermal loading is also considered for the annular disk with rigid inclusion. Effects of different values of elastic and plastic orthotropy parameters are investigated. It is observed that the elastic orthotropy significantly affects the residual stresses in disks. The most remarkable effect of the plastic orthotropy is observed on the disk with rigid inclusion.

Deep drawing operations are crucial for metal forming operations and manufacturing. Obtaining a defect free final product with the desired mechanical properties is very important for fulfilling the customer expectations and market competitions. Wrinkling is one of the fatal and most frequent defects that must be prevented. This study focuses on understanding the phenomenon of wrinkling and probable precautions that can be applied. In this study, dynamic &ndash / explicit commercial finite element code is used to simulate deep drawing process. The numerical experiments are compared with NUMISHEET benchmarks in order to verify the reliability of the finite element code and analysis parameters. In order to understand plastic wrinkling, the effect of blank holder force is investigated. Axisymmetrical numerical models of a cup are investigated with different blank holder forces. Wrinkling instability is illustrated in energy diagrams of the process. Effect of anisotropy on wrinkling is also discussed by comparing isotropic and anisotropic numerical experiments with the material as steel. Different drawbead models, both equivalent and physical, are implied to the problem and results are discussed. Besides numerical analysis, experimental verification is also conducted as conventional deep drawing operation by a hydraulic press. This yields to the ability to understand the effect of blank thickness on wrinkling formation through numerical and experimental analyses. The wave formations of different sized blanks with four different thicknesses are illustrated.

Anisotropy in engineering properties of soils occurs due to the depositional process forming the soil fabric and/or different directional stresses in soil history. This study investigates the anisotropy in undrained shear strength and drained compressibility of preconsolidated, stiff and fissured Ankara Clay. The compressibility behavior is determined using standard oedometer testing while the shear strength anisotropy is investigated through large diameter unconsolidated-undrained triaxial testing on undisturbed samples taken in vertical and horizontal directions from several deep excavation sites along the Konya Road in &Ccedil / ukurambar-Balgat Area, Ankara. According to the results achieved, Ankara Clay is slightly anisotropic in compressibility, with an anisotropy ratio between 0.72 and 1.17 in terms of coefficient of volume compressibility for several pressure ranges between 50 kPa and 1600 kPa. On the other hand, while a slight anisotropy in undrained shear strength at a ratio ranging between 0.87 and 1.19 in terms of deviator stress can be observed in Ankara Clay, considering the great variation in the test results of samples in same direction which mostly overlaps with the range of results obtained in the other direction, it has been concluded that the Ankara Clay located in this area can be regarded as isotropic in terms of shear strength for practical purposes.

Attention deficit hyperactivity disorder (ADHD) is a neurobehavior developmental disorder that affects around 7.5% of Taiwan children. With the use of magnetic resonance imaging , many results have been reported that ADHD patients have volume atrophy in gray matter and dysfunction in couples of cortical regions. In recent years, diffusion MR imaging with diffusion-sensitizing gradients has been used to investigate the abnormality of neural fibers in disease involved with central nervous system. In this study, the anisotropy of white matter in both ADHD patients and age-matched healthy subjects was estimated using diffusion tensor imaging to undergo inter-subject comparison. In this work, a significant decrease (FWE-corrected p-value <0.05) of FA values has been found in white matter of adolescents diagnosed as ADHD patients, compared with normal controls group. The areas that confirmed by two different algorithms of inter-subject comparison are mainly diffused on white matter region, including middle cerebellar peduncle, left inferior longitudinal fasciculus, internal capsule, left optic radiation, external capsule, splenium of the corpus callosum, superior longitudinal fasciculus, superior frontal and parietal-occipital nerve fibers.

This work is a study of complex many-body systems with non-trivial interactions. Many such systems can be described with models that are much simpler than the real thing but which can still give good insight into the behavior of realistic systems. We take a look at two such systems. The first part looks at a model that elucidates the variety of magnetic phases observed in rare-earth heterostructures at low temperatures: the six-state clock model. We use an ANNNI-like model Hamiltonian that has a three-dimensional parameter space and yields two-dimensional multiphase regions in this space. A low-temperature expansion of the free energy reveals an example of Villain's `order from disorder' [81, 60] when an infinitesimal temperature breaks the ground-state degeneracy. The next part of our work describes biological systems. Using ECIS (Electric Cell-Substrate Impedance Sensing), we are able to extract complex impedance time series from a confluent layer of live cells. We use simple statistics to characterize the behavior of cells in these experiments. We compare experiment with models of fractional Brownian motion and random walks with persistence. We next detect differences in the behavior of single cell types in a toxic environment. Finally, we develop a very simple model of micromotion that helps explain the types of interactions responsible for the long-term and short-term correlations seen in the power spectra and autocorrelation curves extracted from the times series produced from the experiments.