Made available in DSpace on 2014-06-11T19:28:35Z (GMT). No. of bitstreams: 0 Previous issue date: 2004-04Bitstream added on 2014-06-13T20:58:36Z : No. of bitstreams: 1 elisei_cca_me_guara.pdf: 1405191 bytes, checksum: 4108c22cd51274171b8447b8d08e9498 (MD5) / Universidade Estadual Paulista (UNESP) / Foram caracterizados os comportamentos eletroquímicos e avaliadas as resistências à corrosão das ligas aeronáuticas 2024-T351 e 7050-T7451 em soluções aquosas de cloreto contendo cromato, molibdato e tungstato. Foram realizados ensaios de corrosão não-eletroquímicos de imersão prolongada acompanhados de análise metalográfica de superfície por microscopia óptica e identificação dos produtos de corrosão por difratometria de raios-X. A análise quantitativa de superfícies das ligas após a imersão, indica que os pites formados têm áreas médias similares. Os pites são mais largos do que profundos e de geometria, predominantemente, cônica ou quase-cônica e irregular. Em todos os produtos de corrosão de cada liga foi encontrado hidróxido de alumínio, em suas diferentes formas cristalinas. Medidas de perda de dureza, como uma conseqüência da deterioração superficial, também foram determinadas. Além disso, ensaios eletroquímicos como medidas de potencial em circuito aberto, curvas de polarização e voltametria cíclica complementaram este estudo. Em meio aerado os resultados obtidos mediante medidas eletroquímicas são consistentes com aqueles obtidos nos ensaios de imersão, em particular o efeito do CrO42- e do MoO42-. O WO42- mostrou-se agressivo em períodos prolongados de imersão. Apesar dos ensaios revelarem uma redução parcial de MoO42- em ambas as ligas, o efeito desse oxi-ânion parece ser diferente sobre cada liga. Em meio desaerado as ligas apresentam passivação em todos os eletrólitos. A adição dos oxi-ânions não modificou significativamente o potencial de pite para a liga 7050, enquanto que para a liga 2024 ele foi deslocado levemente para valores mais positivos. / It has been characterized the electrochemical behavior and evaluated the 2024-T351 and 7050-T7451 aircraft alloys corrosion resistance in chloride aqueous solutions containing chromate, molybdate and tungstate. It has been carried out non-electrochemical long immersion corrosion testings accompanied by surface metalography analysis achieved by light microscopy and corrosion products identification by X-ray difratometry. Surfaces quantitative analysis upon the alloys after immersion, indicates that formed pits have similar average area. Pits are widther than deeper and own predominantly a conical or quasi-conical and irregular geometry. In all corrosion products of each alloy it has been found aluminum hydroxide in its different crystalline ways. Hardness loss measurements have also been determined. In addition, electrochemical testings such as open circuit potential measures, polarization curves and cyclical voltammetry have completed this study. In aerated means the obtained results before electrochemical mesurements are similar to those obtained in the immersion tests, in particular CrO42- and MoO42- effects. WO42- has been found to be aggressive in very long immersion period. Though tests display a MoO42- partial reduction in both alloys, this oxi-anion effect seems to be different upon each alloy. In de-aerated means alloys present passivation in all eletrolytes. Oxi-anion addition has not changed significantly pit potential for 7050 alloy, while for 2024 alloy it has been dislocated, slightly, for more positive values.

<p dir="ltr">Inverse problems in infinite dimensions are ubiquitously encountered across the scien- tific disciplines. These problems are defined by the need to reconstruct continuous fields from incomplete, noisy measurements, which oftentimes leads to ill-posed problems. Almost universally, the solutions to these problems are constructed in a Bayesian framework. How- ever, in the infinite-dimensional setting, the theory is largely restricted to the Gaussian case, and the treatment of prior physical knowledge is lacking. We develop a new framework for Bayesian reconstruction of infinite-dimensional fields which encodes our physical knowledge directly into the prior, while remaining in the continuous setting. We then prove various characteristics of the method, including situations in which the problems we study have unique solutions under our framework. Finally, we develop numerical sampling schemes to characterize the various objects involved.</p>

Orientador: Marcelo dos Santos Pereira / Banca: Herman Jacobus Cornelis Voorwlad / Banca: Antonio Jorge Abdalla / Resumo: Foram caracterizados os comportamentos eletroquímicos e avaliadas as resistências à corrosão das ligas aeronáuticas 2024-T351 e 7050-T7451 em soluções aquosas de cloreto contendo cromato, molibdato e tungstato. Foram realizados ensaios de corrosão não-eletroquímicos de imersão prolongada acompanhados de análise metalográfica de superfície por microscopia óptica e identificação dos produtos de corrosão por difratometria de raios-X. A análise quantitativa de superfícies das ligas após a imersão, indica que os pites formados têm áreas médias similares. Os pites são mais largos do que profundos e de geometria, predominantemente, cônica ou quase-cônica e irregular. Em todos os produtos de corrosão de cada liga foi encontrado hidróxido de alumínio, em suas diferentes formas cristalinas. Medidas de perda de dureza, como uma conseqüência da deterioração superficial, também foram determinadas. Além disso, ensaios eletroquímicos como medidas de potencial em circuito aberto, curvas de polarização e voltametria cíclica complementaram este estudo. Em meio aerado os resultados obtidos mediante medidas eletroquímicas são consistentes com aqueles obtidos nos ensaios de imersão, em particular o efeito do CrO42- e do MoO42-. O WO42- mostrou-se agressivo em períodos prolongados de imersão. Apesar dos ensaios revelarem uma redução parcial de MoO42- em ambas as ligas, o efeito desse oxi-ânion parece ser diferente sobre cada liga. Em meio desaerado as ligas apresentam passivação em todos os eletrólitos. A adição dos oxi-ânions não modificou significativamente o potencial de pite para a liga 7050, enquanto que para a liga 2024 ele foi deslocado levemente para valores mais positivos. / Abstract: It has been characterized the electrochemical behavior and evaluated the 2024-T351 and 7050-T7451 aircraft alloys corrosion resistance in chloride aqueous solutions containing chromate, molybdate and tungstate. It has been carried out non-electrochemical long immersion corrosion testings accompanied by surface metalography analysis achieved by light microscopy and corrosion products identification by X-ray difratometry. Surfaces quantitative analysis upon the alloys after immersion, indicates that formed pits have similar average area. Pits are widther than deeper and own predominantly a conical or quasi-conical and irregular geometry. In all corrosion products of each alloy it has been found aluminum hydroxide in its different crystalline ways. Hardness loss measurements have also been determined. In addition, electrochemical testings such as open circuit potential measures, polarization curves and cyclical voltammetry have completed this study. In aerated means the obtained results before electrochemical mesurements are similar to those obtained in the immersion tests, in particular CrO42- and MoO42- effects. WO42- has been found to be aggressive in very long immersion period. Though tests display a MoO42- partial reduction in both alloys, this oxi-anion effect seems to be different upon each alloy. In de-aerated means alloys present passivation in all eletrolytes. Oxi-anion addition has not changed significantly pit potential for 7050 alloy, while for 2024 alloy it has been dislocated, slightly, for more positive values. / Mestre

<p>  </p> <p>Architectured metamaterials are artificial systems with unique structural characteristics. They show distinct deformation behaviors and improved mechanical properties compared to regular materials. For example, mechanical metamaterials demonstrate negative Poisson's ratios, whereas regular materials have positive values. In theory, the auxetic behaviors arise from periodic cellular architectures regardless of the materials utilized. While this premise is mostly true for macroscopic metamaterials, it may not work well at a very small lengthscale since chemistry may play a critical role in nanostructures. However, this fundamental idea has not been addressed due to the lack of powerful manufacturing strategies at the nanoscale. The majority of architectured metamaterials are manufactured from top down with their unit size of microns or larger. On the other hand, there are also molecular auxetics which are natural crystals and thus are not designable. Therefore, there is a significant gap in lengthscale from 10 nm to 1 µm. DNA self-assembly is a bottom-up approach that can construct complex nanostructures based on sequence complementarity. Examples include DNA origami structures and DNA tile assemblies. This dissertation bridges the gap in the lengthscale by introducing nanoscale auxetic units from DNA and investigates relevant structural properties and mechanical behaviors. This study addresses the premise of metamaterials and elucidates the structure-property relation. The findings from this work formulate design principles for DNA based auxetic metastructures. </p> <p>In this work, we built several two-dimensional (2D) auxetic nanostructures from wireframe DNA origami. They serve as the model systems to demonstrate the feasibility of constructing nanoscale auxetics via DNA self-assembly. DNA origami structures are commonly constructed by a long ‘scaffold’ strand with many ‘staple’ oligonucleotides. Since the DNA metastructures are too small to directly apply external forces, we implemented chemical deformation by inserting ‘jack’ edges. Like a car jack, the length of the jack edges can be modulated via two-step DNA reactions: toehold-mediated strand displacement and annealing with a new set of jack staples. The DNA nanostructures reconfigure accordingly. To complement the experiment, we performed molecular dynamics (MD) simulations based on coarse-grained models using an open-source oxDNA platform. In the numerical computation, external loads were directly applied to deform the metastructures, providing details of structural deformation. We discovered that the auxetic behaviors of DNA metamaterials can be estimated by architectural designs, however the material properties are also crucial in the structures and deformations. Our mechanistic study provided general design guidelines for 2D auxetic DNA metamaterials. We also designed and constructed a Hoberman flight ring from DNA, a simplified planar version of Hoberman sphere. This structure consists of six equilateral triangles that are topologically organized into two layers, resembling a trefoil knot. The DNA flight ring deploys upon external forces, expanding (open state) or contracting (closed state) by sliding the two layers of triangles. This is the first synthetic deployable nanostructure and offers a versatile platform for topological research.</p> <p>This thesis also investigates 3D effects in DNA assemblies and related mechanics. We used a DNA origami tile designed with an intrinsic twist as a model system and explored its cyclization process using MD simulations. The numerical computation revealed the detailed process where the structure untwists and curves for cyclization simultaneously under external forces. The force and energy required to overcome the initial curvature and cause the 3D deformation were also calculated. The results agree well with the previous experiment and theory, further verifying the simulation method. Direct mechanical forces and DNA responses were realized experimentally with 3D DNA crystals built from triangular DNA tiles. Nanoindentation was performed on macroscopic ligated crystals using atomic force microscopy (AFM). MD simulations were performed in parallel, which revealed the full spectrum of several distinct deformation modes from linear elasticity to structural failure. The combined experiment, computation, and theoretical calculation showed that the complex behaviors can only be understood fully by considering the structure and its components. </p> <p>The scientific findings from this thesis should contribute to the construction of auxetic metastructures, the design methods for DNA based metamaterials as well as the prediction of their structural properties and mechanical behaviors. This thesis will pave the way for building architectured materials from DNA with tailored properties and functionalities, opening the door for new opportunities and unique applications.</p>

<p dir="ltr">Repeated loading and unloading cycles lead to the formation of strain in the material which causes initiation of the crack formation this phenomenon is called fatigue. Fatigue properties are critical for structures subject to cyclic load; hence fatigue analysis is used to predict the life of the material. Fatigue analysis plays an important role in optimizing the design of the 3D printed material and predicting the fatigue life of the 3D printed component.</p><p><br></p><p dir="ltr">The main objective of this thesis is to predict the fatigue behavior of different microstructures of Ti-64 titanium alloy by using the PRISMS-Fatigue open-source framework. To achieve this goal Ti-64 microstructure models were created using programming scripts, then the structures were exported to a finite element visualization software package, with all the required properties embedded in the pipeline. The PRISMS-Fatigue framework is used to conduct a fatigue analysis on 3D printed materials, using the Fatigue Indicator Parameters (FIP), which measure the driving force of fatigue crack formation in the microstructurally small crack growth.</p><p><br></p><p dir="ltr">Three different microstructures, i.e., cubic equiaxed, random equiaxed, and rolled equiaxed microstructures, are analyzed. The FIP results show that the cubic equiaxed grains have the best fatigue resistance due to their isotropic structural characteristics. Additionally, the grain size effect using 1 and 10 micrometers is investigated. The results show that the 1 micrometer grain size cubic equiaxed microstructure has a better fatigue resistance because as grains are small and they have a higher mechanical strength.</p>

<p>Controllable continuous feeding of biomass feedstock in a biorefinery is critical to upscaling current ethanol conversion techniques to a commercial scale. Mechanical pretreatment of biomass feedstock performed using a compression feed screw (CFS) improves the ethanol yield but is subject to flowability issues, especially the plugging of biomass. The mechanical behavior, and hence, the flowability of biomass feedstock, is strongly affected by several factors, including preparation method, moisture content, physical composition, and particle size distribution. In addition, the current design of CFS is guided by limited experimentation and even fewer theoretical correlations. This thesis aims at developing computational methods to model the flow of densified feedstock in a CFS and experimental techniques to characterize the mechanical properties required for the model. We adopted a modified Drucker-Prager Cap constitutive (mDPC) law for milled corn stover (a widely used feedstock for bioethanol production) to model the material’s rate-independent bulk behavior in a CFS. The mDPC elastoplastic law captures the frictional shear and permanent volumetric changes in corn stover using a continuous porosity-dependent yield surface. The parameters of the mDPC model are calibrated using a unified set of single-ended die compaction and multiple shear failure tests. In addition, we quantified the changes in the mDPC parameters with moisture content up to the water-holding capacity of corn stover particles. A Coupled Eulerian-Lagrangian Finite Element Method model developed for the CFS geometry predicts the deformation of the material using the calibrated mDPC parameters. We model the interaction between the material and the CFS surface using a Coulomb wall friction coefficient calibrated using the Janssen-Walker method for a punch and die system. A laboratory-scale compression feed screw is designed and fabricated to characterize the flow of dense granular materials in collaboration with undergraduate students in the School of Mechanical Engineering. FEM model predictions of feeding torque and mass flow rate are validated against the laboratory-scale feeder for microcrystalline cellulose Avicel PH-200 and milled corn stover. The model predictions agree with the experiments for Avicel PH-200 but have a higher error in the case of corn stover. Some physical effects, such as shear hardening and particle erosion observed in milled corn stover, are not captured using the current implementation of the mDPC model, which explains the different model accuracies for both materials. The continuum model is used to uncover material density distribution, torque, and pressure inside the CFS, otherwise challenging through experiments. The FEM model showed a significantly higher sensitivity of the feeder performance to two material properties, namely the hydrostatic yield stress and the wall friction coefficient. The characterized variation of material properties with moisture content and the effect of each material property on the feeder performance provide strategies to engineer the feedstock for better flowability. Further, the continuum model offers a method to study design changes before manufacturing the equipment. Finally, we propose the possibility of a reduced-order analytical model based on the critical material properties and the material deformation mechanism demonstrated by the FEM model.</p>

Three-dimensional woven composites were developed to improve the through-thickness properties which conventional two-dimensional laminate composites currently lack. However, these textile composites generally show lower in-plane mechanical properties due to fibre crimping, and also encounter modelling difficulties due to the complex geometries. In this thesis, the static and fatigue mechanical behaviour of several types of 3D woven composites were experimentally characterised, the influence of the weave architecture on the mechanical performance was revealed, and meso/macro scale numerical models with improved failure criteria were developed to simulate the tensile behaviour of the 3D woven composites. The mechanical characterisation was conducted on six woven structures under tension, compression, and flexural loading, and were also carried out on two weaves under open-hole quasi-static tensile and fatigue loading. Digital image correlation and thermoelastic stress analysis were used to characterise the strain and damage development during static and fatigue loading. The testing results showed that the angle-interlock weave W-3 had higher in-plane quasi-static properties, lower notch sensitivity, higher fatigue damage resistance, but lower delamination resistance. The meso-scale model was developed on the unit cell of the woven structure and the macro-scale model (mosaic model) was created on the testing samples. Both un-notched and notched tensile behaviour were modelled for the angle-interlock weave W-3 and a one-by-one orthogonal weave W-1, and the difference between the predicted and experimental results was within 16% for the unit cell models and within 21% for the mosaic models. A modified failure criterion was developed to better simulate the damage behaviour of the notched macro-scale model and improved the predicted notched strength by 10-20%. Whilst further experimental investigation and improvement in the modelling techniques are still required, the data presented in this thesis provided an essential update for the current 3D woven composites research, and the presented models offered the potential to predict the damage behaviour of large 3D woven structures.

<div>Polymer bonded explosives (PBXs) consist of energetic material (EM) crystals embedded inside a polymeric binder. These are highly heterogeneous structures designed to explode under controlled conditions. However, accidental ignition of PBXs leading to deflagration, or even detonation, may take place due to non-shock stimulus such as low velocity impacts and vibration. Thus, assessing the safety of PBXs under non-shock stimulus is very important.</div><div><br></div><div>The ignition in PBXs depends on several microstructural features which include mechanical properties of EM particles and polymeric binder, as well as the adhesive properties of interface between EM particles and binder. It is also sensitive to initial defects in EM particles including cracks or voids. EM particle size distribution, distance between particles and their relative location are also shown to be affecting the ignition behavior of PBXs. This study focuses on PBX composition consisting of HMX as EM and Sylgard or HTPB as polymeric binder. Among several mechanisms of hot-spot formation, this study focuses on frictional heating at cracks or debonded surfaces.</div><div><br></div><div>Finite element simulations are performed on a domain containing a single EM particle embedded inside polymer binder under compressive and tensile loading at 10 m/s. The effect of the binder properties and the particle surface properties, on damage evolution and corresponding temperature rise due to frictional heat generation, is investigated. Two binders, Sylgard and HTPB, while two surface qualities for HMX particle, low and high, are compared. The adhesion strength of the particle-polymer interface is varied and damage evolution is qualitatively compared with experimental results to estimate interfacial energy release rate for HMX-Sylgard and HMX-HTPB interfaces. Simulations of two HMX particles inside Sylgard binder, subjected to vibration loading, are performed to analyze the effect of particle-particle distance and relative location of particles on the damage evolution and frictional heating in the particles.</div><div><br></div><div>The results of impact simulations show that the low surface quality HMX particle inside HTPB is likely to propagate cracks as compared to high surface quality particle. The HMX particle inside Sylgard shows crack propagation irrespective of particle surface quality. The impact simulations with the lower stiffness binder do not show a significant increase in temperature after impact. A polymer with higher stiffness induces more particle damage under impact contributing to a larger temperature rise. Furthermore, high quality surface and higher adhesion strength induces larger stresses and increase the temperature rise. The vibration simulations show that a small particle is less likely to damage when it is shielded by a large particle irrespective of its distance, within 40-200$\mu$m, from the large particle. However, the small particle is likely to damage when it is in parallel to the large particle with respect to loading. The temperature rise in the small particle is higher than the larger particle only in case of parallel configuration. The adhesion between the particles and the polymer has a direct effect on the formation of hot-spots due to friction and through local increase of compressive stresses that may cause a surge in heat generation.</div><div><br></div><div>The energetic materials often show anisotropy in elastic and crystalline properties. Fracture in HMX along the preferred cleavage plane is considered. Anisotropy in the elastic constants is also incorporated in the fracture model. The dependence of pressure on temperature is considered using Mie-Gruneisen equation of state which is shown to be important for damage evolution in HMX at impact velocity of 100 m/s.</div>

<p>Continuous casting is a vital process in the production of semi-finished steel, converting molten metal into solid form. Primary cooling, a critical stage of this process, uses water to cool the solidifying shell as it descends through the mold. The quality of the final cast product is significantly influenced by the fluid flow characteristics during this phase. Given the inherent complexities and costs associated with experimental studies in this area, numerical modeling has emerged as a crucial tool for understanding, predicting, and optimizing fluid flow dynamics within the mold. This research focuses on the implications of argon injection within the mold during the primary cooling stage of the continuous casting process.</p><p>In this thesis, a comprehensive computational investigation of the transportation, entrapment, and fluid dynamic effects of argon injection is presented. Through an exploration of bubble sizes, SEN submergence depths, and slide gate openings, the study reveals how these parameters can significantly influence the casting process.</p><p>The research finds that argon bubble size plays a critical role in determining bubble trajectories and residence times, with smaller bubbles showing a longer residence time and increased boundary interaction due to the dominance of drag forces. The submergence depth of the submerged entry nozzle (SEN) also influences these factors, with deeper submergence resulting in longer bubble trajectories and greater residence times. The study highlights how bubble diameter impacts their entrapment probability, with bubbles ranging from 0.3mm to 0.6mm being most prone to entrapment.</p><p>The effects of argon injection on fluid flow within the SEN demonstrate an enhancement of turbulence, thus promoting a uniform outflow. However, excessively high argon flow rates risk a critical reduction in meniscus velocity, which could lead to overcooling. The research further elucidates the influence of argon on X-velocity near the mold's narrow faces, indicating a potential method for controlling dendritic growth and enhancing the final product quality.</p><p>This work underlines the complex and multifaceted impacts of argon injection on the continuous casting process. It suggests that through careful manipulation of argon bubble size, SEN submergence depth, and slide gate opening, it is possible to optimize the transportation and entrapment of argon bubbles, manage fluid flow dynamics, and ultimately, improve the quality of the cast product.</p>

<p>  </p> <p>In this study, we aimed to explore the potential of integrating wind turbines into tall buildings to harness wind energy in urban areas. Advanced computer simulations will be used to analyze the complex wind patterns and turbulence around tall buildings. We will also study the optimization of wind turbine placement to maximize energy production. We focus on two types of wind turbines, the savonius and a modified savonius, using the Myring formula. We evaluated their performance in turbulent urban areas using computational fluid dynamics simulations. The simulations will also help us understand the wind flow behavior around tall buildings, informing wind turbine placement optimization.</p> <p>Our findings contribute to the understanding of urban wind energy production. This may lead to further advancements in wind turbine design and application in urban environments, promoting sustainable and clean energy production in densely populated areas.</p> <p>We also evaluate the economic feasibility of wind power as an energy source and its potential for commercial applications. Our study's insights are significant for wind energy research, urban planning, and sustainable energy production in cities.</p> <p>To achieve our objectives, we will use state-of-the-art computational tools such as the ANSYS Fluent Student software and the Steady Reynolds Averaged Navier-Stokes (SRANS) K-ε model and K-ω SST models for simulating wind flow around tall buildings.</p> <p>In summary, the goal of this research is to develop a methodology for integrating wind turbines into tall urban buildings to harness wind energy potential. This will contribute to the understanding of urban wind energy production and its economic feasibility for commercial applications.</p>