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1
Zhang, Yingchun. "Computational study of the transport mechanisms of molecules and ions in solid materials". [College Station, Tex. : Texas A&M University, 2006. http://hdl.handle.net/1969.1/ETD-TAMU-1711.
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Wang, Chao. "A COMPUTATIONAL STUDY OF LINKING SOLID OXIDE FUEL CELL MICROSTRUCTURE PARAMETERS TO CELL PERFORMANCE". Wright State University / OhioLINK, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=wright1377786080.
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Ribeiro-Ayeh, Steven. "Finite element modelling of the mechanics of solid foam materials". Doctoral thesis, Stockholm, 2005. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-154.
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Abbasi, Baharanchi Ahmadreza. "Development of a Two-Fluid Drag Law for Clustered Particles Using Direct Numerical Simulation and Validation through Experiments". FIU Digital Commons, 2015. http://digitalcommons.fiu.edu/etd/2489.
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This dissertation focused on development and utilization of numerical and experimental approaches to improve the CFD modeling of fluidization flow of cohesive micron size particles. The specific objectives of this research were: (1) Developing a cluster prediction mechanism applicable to Two-Fluid Modeling (TFM) of gas-solid systems (2) Developing more accurate drag models for Two-Fluid Modeling (TFM) of gas-solid fluidization flow with the presence of cohesive interparticle forces (3) using the developed model to explore the improvement of accuracy of TFM in simulation of fluidization flow of cohesive powders (4) Understanding the causes and influential factor which led to improvements and quantification of improvements (5) Gathering data from a fast fluidization flow and use these data for benchmark validations. Simulation results with two developed cluster-aware drag models showed that cluster prediction could effectively influence the results in both the first and second cluster-aware models. It was proven that improvement of accuracy of TFM modeling using three versions of the first hybrid model was significant and the best improvements were obtained by using the smallest values of the switch parameter which led to capturing the smallest chances of cluster prediction. In the case of the second hybrid model, dependence of critical model parameter on only Reynolds number led to the fact that improvement of accuracy was significant only in dense section of the fluidized bed. This finding may suggest that a more sophisticated particle resolved DNS model, which can span wide range of solid volume fraction, can be used in the formulation of the cluster-aware drag model. The results of experiment suing high speed imaging indicated the presence of particle clusters in the fluidization flow of FCC inside the riser of FIU-CFB facility. In addition, pressure data was successfully captured along the fluidization column of the facility and used as benchmark validation data for the second hybrid model developed in the present dissertation. It was shown the second hybrid model could predict the pressure data in the dense section of the fluidization column with better accuracy.
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Tofangchi, Mahyari Abbas Ali. "Computational modelling of fracture and damage in poroelastic media". Thesis, McGill University, 1997. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=35426.
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The classical theory of poroelasticity focuses on the coupled response of fluid flow and elastic deformation of porous media saturated with either an incompressible or a compressible pore fluid. The Theory of poroelasticity has been successfully applied to examine time-dependent transient phenomena in a variety of natural and synthetic materials, including geomaterials and biomaterials. The assumption of elastic behaviour of the porous skeleton in these developments is a significant limitation in the application of this theory to brittle geomaterials; which could exhibit non-elastic phenomena either in the form of initiation and extension of discrete fractures, or in the form of initiation and evolution of continuum damage in the porous skeleton. The computational methodology developed in this study examines the effect of development of such defects (fracture or damage) on the fluid transport characteristics and the poroelastic behaviour of saturated geomaterials. The finite element based computational models for fracture and damage phenomena examine two-dimensional plane strain and axisymmetric problems. The classical theory of linear elastic fracture mechanics is extended to examine the timedependent behaviour of local effects at the crack tip in poroelastic media. The numerical procedure accounts for the stress singularity of the effective stress field at the crack tip. The damage model based on the concept of continuum damage mechanics, takes into account the alteration of the stiffness and permeability characteristics of porous material due to development of micromechanical damage in the porous skeleton. The isotropic damage criteria governing the evolution of stiffness and permeability parameters are characterized by the dependency of damage parameters on the distortional strain invariant.As the applications of the theory of poroelasticity diversify, attention needs to be focused on other aspects of importance. The class of transient and steady crack extension in poroelastic media is recognized as an area of interest in geomechanics applications and in energy resources recovery from geological formations. A computational algorithm is developed to examine the transient quasi-static crack extension in poroelastic media where the temporal and spatial variations of boundary conditions governing the displacement, traction and pore pressure fields are taken into account in the incremental analysis. The path of crack extension is established by a mixed-mode crack extension criterion applicable to the porous fabric. The computational modelling of steady state crack extension in poroelastic media at constant velocity is also examined for the plane strain problems. The finite element formulations of the governing equations, which are velocity-dependent, are developed by employing the Galerkin technique. The poroelastic behaviour of material depends on the propagation velocity at the crack tip. The computational schemes developed in this study followed an extensive procedure of verification via known analytical solutions to poroelasticity problems and for limiting cases of initial undrained (t → 0+) and final drained (t → +infinity) elastic responses recovered through analogous problems in classical elasticity.
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Tang, Baobao. "Development of Mathematical and Computational Models to Design Selectively Reinforced Composite Materials". Thesis, University of Louisiana at Lafayette, 2016. http://pqdtopen.proquest.com/#viewpdf?dispub=10163313.
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Different positions of a material used for structures experience different stresses, sometimes at both extremes, when undergoing processing, manufacturing, and serving. Taking the three-point bending as an example, the plate experiences higher stress in the middle span area and lower stress in both sides of the plate. In order to ensure the performance and reduce the cost of the composite, placement of different composite material with different mechanical properties, i.e. selective reinforcement, is proposed.
Very few study has been conducted on selective reinforcement. Therefore, basic understanding on the relationship between the selective reinforcing variables and the overall properties of composite material is still unclear and there is still no clear methodology to design composite materials under different types of loads.
This study started from the analysis of composite laminate under three point bending test. From the mechanical analysis and simulation result of homogeneously reinforced composite materials, it is found that the stress is not evenly distributed on the plate based on through-thickness direction and longitudinal direction. Based on these results, a map for the stress distribution under three point bending was developed. Next, the composite plate was selectively designed using two types of configurations. Mathematical and finite element analysis (FEA) models were built based on these designs. Experimental data from tests of hybrid composite materials was used to verify the mathematical and FEA models. Analysis of the mathematical model indicates that the increase in stiffness of the material at the top and bottom surfaces and middle-span area is the most effective way to improve the flexural modulus in three point bending test. At the end of this study, a complete methodology to perform the selective design was developed.
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Dev, Bodhayan. "Characterization of Ceramic/Glass Composite Seals for Solid Oxide Fuel Cells". The Ohio State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=osu1400847202.
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Tran, Hai Thanh. "Experimental and Computational Study on Fracture Mechanics of Multilayered Structures". Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6595.
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Many devices in electronics are in the form of multilayered structures. These structures can fail catastrophically if they contain defects or cracks. Enhancing their fracture properties is therefore critical to improve the reliability of the systems. The interface-dominated fracture mechanics of multilayered structure was studied using experiments and finite element (FE) modeling by considering two examples: thin films on polymer substrates in flexible electronics and Cu leadframe/epoxy molding compound (EMC) in micro-electronics packaging.
In the first example, aluminum-manganese (Al-Mn) thin films with Mn concentration up to 20.5 at.% were deposited on polyimide (PI) substrates. A variety of phases, including supersaturated fcc (5.2 at.% Mn), duplex fcc and amorphous (11.5 at.% Mn), and completely amorphous phase (20.5 at.% Mn) were obtained by adjusting alloying concentration in the film. In comparison with crystalline and dual phase counterparts, the amorphous thin film exhibits the highest fracture stress and fracture toughness, but limited elongation. Based on a fracture mechanism model, a multilayer scheme was adopted to optimize the ductility and the fracture properties of the amorphous film/PI system. Tensile deformation and subsequent fracture of strained Al-Mn films on PI were investigated experimentally and by FE simulations. It was found that by sandwiching the amorphous film (20.5 at.% Mn) between two ductile copper (Cu) layers, the elongation can be improved by more than ten times, and the interfacial fracture toughness by twenty four times with a limited sacrifice of the film's fracture toughness (less than 18%). This design provides important guidelines to obtain optimized mechanical properties of future flexible electronics devices.
The reliability of amorphous brittle Al-Mn (20.5 at.% Mn) thin films deposited on PI substrates is strongly influenced by the film/substrate interface adhesion. Some strategies to improve the adhesion of the interface were conducted, including roughening the surface of the PI substrate, adding a buffer layer and then tuning its thickness. Tensile testing and FE analysis of amorphous Al-Mn thin films with and without buffer layers coated on intact and plasma etched rough PI were investigated. It was found that by adding a chromium buffer layer of 75 nm on a rough PI substrate, the interface adhesion of the film/substrate can increase by almost twenty times. The obtained results would thus shed light on the interfacial engineering strategies for improving interface adhesion for flexible electronics.
In the second example, a systematic investigation and characterization of the interfacial fracture toughness of the bimaterial Cu leadframe/EMC was carried out. Experiments and FE simulations were used to investigate delamination and interfacial fracture toughness of the biomaterial system. Two dimensional simulations using computational fracture mechanics tools, such as virtual crack closure technique, virtual crack extension and J-integral proved to be computationally cheap and accurate to find the interfacial fracture toughness of the bimaterial structures. The effects of temperature, moisture diffusion and mode-mixity on the interfacial fracture toughness were investigated. Testing temperature and moisture exposure significantly reduce the interfacial fracture toughness, and its relationship with the mode-mixity was achieved by fitting the results with an analytic formula.
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Giardina, Ronald Joseph Jr. "General Nonlinear-Material Elasticity in Classical One-Dimensional Solid Mechanics". ScholarWorks@UNO, 2019. https://scholarworks.uno.edu/td/2666.
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We will create a class of generalized ellipses and explore their ability to define a distance on a space and generate continuous, periodic functions. Connections between these continuous, periodic functions and the generalizations of trigonometric functions known in the literature shall be established along with connections between these generalized ellipses and some spectrahedral projections onto the plane, more specifically the well-known multifocal ellipses. The superellipse, or Lam\'{e} curve, will be a special case of the generalized ellipse. Applications of these generalized ellipses shall be explored with regards to some one-dimensional systems of classical mechanics. We will adopt the Ramberg-Osgood relation for stress and strain ubiquitous in engineering mechanics and define a general internal bending moment for which this expression, and several others, are special cases. We will then apply this general bending moment to some one-dimensional Euler beam-columns along with the continuous, periodic functions we developed with regard to the generalized ellipse. This will allow us to construct new solutions for critical buckling loads of Euler columns and deflections of beam-columns under very general engineering material requirements without some of the usual assumptions associated with the Ramberg-Osgood relation.
10
Asmadi, Aldi. "Crystal structure prediction : a molecular modellling study of the solid state behaviour of small organic compounds". Thesis, University of Bradford, 2010. http://hdl.handle.net/10454/4441.
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The knowledge of the packing behaviour of small organic compounds in crystal lattices is of great importance for industries dealing with solid state materials. The properties of materials depend on how the molecules arrange themselves in a crystalline environment. Crystal structure prediction provides a theoretical approach through the application of computational strategies to seek possible crystal packing arrangements (or polymorphs) a compound may adopt. Based on the chemical diagrams, this thesis investigates polymorphism of several small organic compounds. Plausible crystal packings of those compounds are generated, and their lattice energies are minimised using molecular mechanics and/or quantum mechanics methods. Most of the work presented here is conducted using two software packages commercially available in this field, Polymorph Predictor of Materials Studio 4.0 and GRACE 1.0. In general, the computational techniques implemented in GRACE are very good at reproducing the geometries of the crystal structures corresponding to the experimental observations of the compounds, in addition to describing their solid state energetics correctly. Complementing the CSP results obtained using GRACE with isostructurality offers a route by which new potential polymorphs of the targeted compounds might be crystallised using the existing experimental data. Based on all calculations in this thesis, four new potential polymorphs for four different compounds, which have not yet been determined experimentally, are predicted to exist and may be obtained under the right crystallisation conditions. One polymorph is expected to crystallise under pressure. The remaining three polymorphs might be obtained by using a seeding technique or the utilisation of suitable tailor made additives.
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Kiwanuka, Robert. "Micro-deformation and texture in engineering materials". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:3c924d01-7501-4d59-bb53-07e6584e50c5.
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This DPhil project is set in the context of single crystal elasticity-plasticity finite element modelling. Its core objective was to develop and implement a methodology for predicting the evolution of texture in single and dual-phase material systems. This core objective has been successfully achieved. Modelling texture evolution entails essentially modelling large deformations (as accurately as possible) and taking account of the deformation mechanisms that cause texture to change. The most important deformation mechanisms are slip and twinning. Slip has been modelled in this project and care has been taken to explore conditions where it is the dominant deformation mechanism for the materials studied. Modelling slip demands that one also models dislocations since slip is assumed to occur by the movement of dislocations. In this project a model for geometrically necessary dislocations has been developed and validated against experimental measurements. A texture homogenisation technique which relies on interpretation of EBSD data in order to allocate orientation frequencies based on representative area fractions has been developed. This has been coupled with a polycrystal plasticity RVE framework allowing for arbitrarily sized RVEs and corresponding allocation of crystallographic orientation. This has enabled input of experimentally measured initial textures into the CPFE model allowing for comparison of predictions against measured post-deformation textures, with good agreement obtained. The effect of texture on polycrystal physical properties has also been studied. It has been confirmed that texture indeed has a significant role in determining the average physical properties of a polycrystal. The thesis contributes to the following areas of micro-mechanics materials research: (i) 3D small deformation crystal plasticity finite element (CPFE) modelling, (ii) geometrically necessary dislocation modelling, (iii) 3D large deformation CPFE modelling, (iv) texture homogenisation methods, (v) single and dual phase texture evolution modelling, (vi) prediction of polycrystal physical properties, (vii) systematic calibration of the power law for slip based on experimental data, and (viii) texture analysis software development (pole figures and Kearns factors).
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Tsou, Nien-Ti. "Compatible domain structures in ferroelectric single crystals". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:2ef69e2d-ec5a-4e1b-814f-b573b1649a58.
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The aim of the current study is to develop an efficient model which can predict low-energy compatible microstructures in ferroelectric bulks and film devices and their dynamic behaviour. The results are expected to assist in the interpretation of microstructure observations and provide a knowledge of the possible domain arrangements that can be used to design future materials with optimum performance. Several recent models of ferroelectric crystals assume low energy domain configurations. They are mainly based on the idea of fine phase mixtures and average compatibility, and can require intensive computation resulting in complex domain configurations which rarely occur in nature. In this research, criteria for the exact compatibility of domain structure in the form of a periodic multi-rank laminate are developed. Exactly compatible structure is expected to be energetically favourable and does not require the concept of a fine mixture to eliminate incompatibilities. The resulting method is a rapid and systematic procedure for finding exactly compatible microstructures. This is then used to explore minimum rank compatible microstructure in various crystal systems and devices. The results reveal routes in polarization and strain spaces along which microstructure can continuously evolve, including poling paths for ferro- electric single crystals. Also, the method is capable to generate all possible exactly compatible laminate configurations for given boundary conditions. It is found that simple configurations are often energetically favourable in conditions where previous approaches would predict more complex domain patterns. Laminate domain patterns in ferroelectrics are classified and corre- lated with observations of domains in single crystals, showing good agreement. The evolution of microstructures under applied mechanical and electrical loads is studied. A variational method, which minimises the overall energy of the crystal is developed. A new concept of transitional “pivot states” is introduced which allows the model to capture the feature that the microstructure in ferroelectric crystal switches between possible domain patterns that are energetically favourable, rather than assuming one particular domain pattern throughout. This model is applied to study the hysteresis responses of barium titanate (BaTiO3) single crystals subjected to a variety of loads. The results have good agreement with experimental data in the literature. The relationship between domain patterns and ferroelectric hysteresis responses is discussed.
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Gunes, Asli. "Synthesis Of Alinite Cement Using Soda Solid Waste". Master's thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612545/index.pdf.
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This study is dedicated to give a production route for a kind of low energy cement called alinite cement using the waste material of soda industry as the main raw material.
Soda solid waste, clay and minor amount of iron ore were mixed with certain quantities and burned at six different burning temperatures of 1050, 1100, 1150, 1200, 1350, and 1450 ºC. The resultant clinkers were investigated by mineralogical and chemical analysis. Mineralogical analyses were performed by X-Ray Diffraction (XRD) technique. XRD analyses revealed the formation of alinite phase in the clinkers. Chemical analyses were performed by X-Ray Fluorescence spectroscopy technique and by wet chemical analysis. Especially, free lime content of the clinkers was searched and an optimum burning temperature was determined. In order to find the compressive strength of the alinite cement, larger amounts of alinite clinker were manufactured in wet rod shape raw mix in a laboratory type of furnace at 1200, 1350 and 1450 º
C. The results have shown that forming alinite phase requires ~6wt % chlorine. Alinite clinker is obtained using soda waste at the temperature range between 1050 and 1200 º
C. However, the free CaO becomes much lower as 0.12 at 1200 º
C. Moreover, a lime saturation factor of 76, which is lower than ordinary Portland clinker is obtained. Satisfactory compressive strength was achieved by gypsum addition.
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Gardner, Kevin Alexander. "Experimental Study of Air Blast and Water Shock Loading on Automotive Body Panels". The Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1468938824.
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Raghavan, Prasanna. "Multi-scale analysis of elastic and debonding composites by an adaptive multi-level computational model". Columbus, Ohio : Ohio State University, 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1073013372.
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Thesis (Ph. D.)--Ohio State University, 2004.Title from first page of PDF file. Document formatted into pages; contains xvi, 162 p.; also includes graphics (some col.). Includes abstract and vita. Advisor: Somnath Ghosh, Dept. of Mechanical Engineering. Includes bibliographical references (p. 155-162).
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Song, Xu. "Modelling residual stresses and deformation in metal at different scales". Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:8c33eaef-306a-456e-bfb3-2b2cf470156d.
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This thesis is devoted to the numerical and experimental investigation of residual stress and deformation in polycrystalline metallic alloys at different scales. The emphasis in the current study is placed on establishing the connection between the simulation of deformation by the Finite Element (FE) method and experimental characterisation by synchrotron X-Ray Diffraction (XRD). Of particular importance is the interpretation of modelling results and their validation by careful comparison with experimental data. The concept of eigenstrain was used extensively throughout the report to study the residual elastic strain distributions and their sources. A pseudo-thermal strain FE procedure was used systematically to simulate the residual stress states in samples and engineering components of different shape and dimensionality. The case of 1-D strain variation was considered using the example of a plastically bent bar. The direct and inverse problems of eigenstrain analysis were solved, and validated experimentally by the use of XRD and EDM slitting methods. A novel 2-D discrete inverse eigenstrain algorithm was proposed and implemented to reconstruct the residual stress distribution in a worn rail head. The link between the residual stress and deformation history was studied via thermo-mechanical modelling of the Linear Friction Welding (LFW) process. To advance the understanding of polycrystalline deformation behaviour across the scales, a crystal plasticity model was employed to simulate the elastic-plastic deformation behaviour of Ti-6Al-4V alloy. A post-processor was developed to extract the average elastic strains for orientation-specific grain groups and to compare them with XRD data. A “peak constructor” post-processor was developed that utilised the knowledge of both the elastic strain and dislocation density. In a further development step, a strain gradient crystal plasticity formulation was adopted to account for the local dislocation evolution. Intra-granular deformation analysis was carried out and micro-beam Laue experimental diffraction technique was used for validation. Thus, local lattice arrangement was studied at the microscopic, intragranular scale. Special attention was paid to the phenomenon of Laue spot “streaking”, indicative of the local lattice misorientation caused by dislocation activity during deformation. The results presented in this thesis contributed to the fundamental understanding of the residual stress and deformation in polycrystalline metallic alloys and lead to more than 20 publications in peer-reviewed journals and conference proceedings, which are listed in the Appendix.
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Kim, Kwanlae. "Domain evolution processes in ferroelectric ceramics". Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:abd786e3-8461-4e75-ae99-2620d08099b1.
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The aim of this doctoral research is to understand domain evolution processes in ferroelectrics using piezoresponse force microscopy (PFM) and Monte Carlo simulation. The results provide improved knowledge of domain evolution processes, and systematic experimental methods for research on domain evolution. There has been extensive previous research on domain evolution in ferroelectrics, but the research was mainly constrained to simple domain patterns. However, ferroelectric domains tend to form complex patterns that generate low-energy domain configurations. In this research, several methods such as statistical analysis of PFM data, ex situ/in situ PFM observation under electrical/mechanical loading and combining PFM with electron backscatter diffraction are employed to study domain evolution processes in complex domain patterns. The results show that domain switching almost always takes place by the evolution of pre-existing domain patterns, rather than direct flipping of polarization. Also the net effect of domain evolution processes follows a primary principle that positive work is done by external loads. But this principle is not always followed for microscopic switching processes. Multiple types of domain switching occur simultaneously, and occasionally an overwriting process involves unfavourable as well as favourable domain switching. Domain switching is significantly constrained by the pre-existing domain patterns. Meanwhile, angle-resolved PFM is developed for the systematic interpretation of PFM signal. Using lateral PFM images taken from multiple sample orientations, angle-resolved PFM maps are generated based on the angle of phase reversal in the PFM signal. The resulting maps reliably show complex domain patterns which may not appear in vertical and lateral PFM images. A model of domain evolution is developed using Monte Carlo simulation. Polarization switching by electric field and mechanical stress in the model is shown to take place via the motion of domain walls between pre-existing domains. Typical domain broadening processes are reproduced through this simulation.
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Salisbury, S. T. Samuel. "The mechanical properties of tendon". Thesis, University of Oxford, 2008. http://ora.ox.ac.uk/objects/uuid:97b73cf6-53bc-4606-b974-a1cdc662e9e8.
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Although the tensile mechanical properties of tendon have been well characterised, the viscoelastic and anisotropic properties remain uncertain. This thesis addresses the anisotropic and viscoelastic material properties of tendon. A method to characterise the three-dimensional shape of tendon is reported and experiments to characterise the fibre-aligned and fibre-transverse viscoelastic properties of tendon are presented. The cross-sectional profiles of bovine digital extensor tendons were determined by a laser-slice method. Linear dimensions were measured within 0.15 mm and cross-sectional areas within 1.7 mm². Tendons were compressed between two glass plates in creep loading at multiple loads. Compression was then modelled in a finite element environment. Tendon was found to be nearly incompressible and reproduction of its isochronal load-displacement curve was achieved with a neo-Hookean material model (E ≃ 0.3 MPa). The fibre-aligned tensile mechanical properties were described using a Quasi-Linear Viscoelastic model. The model was effective at reproducing cyclic loading; however, it was ineffective at predicting stress relaxation outside the scope of data used to fit the model. When all experimental results are considered together, two significant conclusions are made: (1) tendon is much stiffer in fibre-aligned tension than in fibre-transverse compression and (2) the fibre-aligned tensile response is strain dependant, while the transverse response is not.
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Barrera, Cruz Jorge Luis. "A Hierarchical Interface-enriched Finite Element Method for the Simulation of Problems with Complex Morphologies". The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1430838711.
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Ying, Siqi. "On the mesoscale plasticity of nickel-base superalloy single crystals". Thesis, University of Oxford, 2017. http://ora.ox.ac.uk/objects/uuid:9d636959-b59d-4e00-adf4-357b6b6c88af.
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Experimental micromechanics of materials is a branch of science that seeks to build tight connections between composition, structure, processing and performance of materials under specific operating conditions required for particular technology applications. The present project is focused on the development of techniques that use the combination of electron, ion and X-ray microscopies to study the deformation behaviour of a particularly important class of metallic alloys used in the manufacture of aeroengines, namely, the so-called Ni-base superalloys. The complex hierarchical structure of these materials means that their macroscopic response is controlled to a great extent by the phenomena that play out on very fine scales, from angstroms (lattice spacing dimension) to nanometres (precipitates, phase boundaries, dislocations, chemical inhomogeneities) to microns (grains and their boundaries, defects and their clusters, dislocation pileups) to millimetres (component scale). Understanding the fine structure and deformation behaviour requires the development of specially configured experimental setup that allow the observation and quantification of deformation to external loading. In this study, FIB-SEM methods for sample characterization and fabrication were combined with synchrotron-based X-ray diffraction and imaging techniques, and backed up by theoretical analysis and numerical simulation, to elucidate the origins of the strength of these alloys. Micropillar compression tests using in-SEM nanoindentation were used to reveal the size dependence of the apparent strength, and connection was made with the dislocation-mediated crystal slip to provide an explanation of the observed Hall-Petch type dependence with a modified Hall-Petch equation considering both intrinsic and extrinsic characteristic lengths introduced. X-ray scattering was used in the polychromatic micro-Laue mode and using Bragg coherent diffractive imaging to reveal the crystal distortion arising due to plastic deformation. A Discrete dislocation dynamics in the 2.5D formulation was used to obtain a model description of the observed phenomena. The key outcome of the work presented in this thesis lies in the successful development of advanced observational tools and relevant theoretical or computational models for mesoscale plasticity problems for crystal with complex microstructure.
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Zhang, Shu Yan. "High energy white beam X-ray diffraction studies of strains in engineering materials and components". Thesis, University of Oxford, 2008. http://ora.ox.ac.uk/objects/uuid:957786c6-114c-40f1-8ee7-649c8b2522bc.
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The primary aim of this research was to develop and improve the experimental method and data interpretation for strain measurements using diffraction methods to gain a better understanding of micromechanical deformation and anisotropy of lattice strain response. Substantial part of the research was devoted to the development of the laboratory high energy X-ray diffractometer (HEXameter) for bulk residual strain evaluation. White beam energy dispersive X-ray diffraction was chosen as the principal diffraction mode due to its extreme efficiency in utilising X-ray flux and its ability to capture large segments of diffraction patterns. The specimens that have been examined were real engineering components, mechanically deformed specimens and thermally treated specimens, ranging from dynamic in-situ measurements to ex-situ materials engineering. For the real engineering components, a wedge coupon from the trailing edge of a Ti64 wide fan blade and a turbine combustion casing were examined. Among the mechanically deformed specimens that have been measured were shot-peened steel plates, elasto-plastically bent bars of Mg alloy and cold expanded Al disks. Amongst the thermally deformed specimens, laser-formed steel plates, thermal spray coatings, a manual inert gas weld of Al plates, a friction stir weld of Al plates and Ni tubes and a quenched Ni superalloy cylinder used for strain tomography were studied. In-situ loading experiments have also been carried out, such as experiments on pointwise mapping of grain orientation and strain using the 3DXRD microscope at the ESRF and in-situ loading experiments on titanium alloy, rheo-diecast and high pressure diecast Mg alloy, IN718 Ni-base superalloy and Al2024 aluminium alloy. Experimental results from X-Ray diffraction and strain tomography were used to achieve a better understanding of the material properties. Some results were compared with polycrystal Finite Element model predictions. Amongst the most prominent research achievements are the development on the HEXameter laboratory instrument, including: (i) the development of special collimation systems for the detectors and the source tube; (ii) the development of a twin-detector setup (that allows for simultaneous determination of strain in two mutually orthogonal directions); (iii) improved alignment procedures for better performance; and (iv) the adaptation of instrumentation for efficient scanning of both large and small components, that included choosing and adapting translation devices, programming of the translation system and designing sample mounting procedures. In this research several approaches to data treatment were investigated. Quantitative phase analysis, single peak fitting (using custom Matlab routines and GSAS) and full pattern fitting (with individual pattern data refined by GSAS and batch refinement done by invoking GSAS via a Matlab routine) were applied. Different Matlab routines were written for specific experimental setups; and various analysis methods were selected and used for refinement depending on the requirements of the measurement results interpretation. 16 papers were published, ensuring that the results of this thesis are readily available to other researchers in the field.
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Wielewski, Euan. "An investigation into the effects of microstructure and texture on the high strain rate behaviour of Ti-6Al-4V". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:e34d77ae-922d-4f87-a8ab-53da0e5d6e8a.
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The core aim of this research project was to improve understanding of the effects of microstructure and crystallographic texture on the high strain rate plastic deformation behaviour of the industrially important Titanium alloy, Ti-6Al-4V. To facilitate this study, four rolled plates of Ti-6Al-4V, with varying thermo-mechanical processing histories, were provided by TIMET Corp., the world’s largest supplier of Titanium product. To determine the nature of each plate’s microstructure and the crystallographic texture of the dominant α phase, the four Ti-6Al-4V plates were microstructurally characterised using techniques such as optical microscopy and electron backscatter diffraction (EBSD). To determine the effects of the measured microstructures and textures on the strain rate dependent plastic deformation behaviour of the four Ti-6Al-4V plates, uniaxial compression and tension tests were carried out in the three orthogonal material orientations at quasi-static (10^-3 s^-1) and high strain rates (10^3 s^-1) using a standard electro-mechanical test device and split-Hopkinson pressure bars (SHPB), respectively. To provide further understanding of the effects of microstructure and texture on the plastic deformation behaviour of Ti-6Al-4V, this time under complex impact loading conditions, the classic Taylor impact experiment was adapted to include an optical measurement and geometry reconstruction technique. A novel experimental setup was designed that consists of an ultra-high speed camera and mirror arrangement, allowing the Taylor impact specimen to be viewed from multiple angles during the experiment. Using the previously mentioned optical measurement and geometry reconstruction technique, it was then possible to gain valuable, previously unobtainable, data on the deformation history of Taylor impact specimens in-situ, such as the major/minor axes of the anisotropically deforming elliptical specimen cross-sections as a function of time and axial position, true strain as a function of time and axial position, and the true strain rate as a function of axial position. The technique was verified by testing a specimen cut from the in-plane material orientation of a clock-rolled high purity Zirconium plate. The output measurements from a post-deformation image frame were compared with measurements of the recovered specimen made using a coordinate measurement machine (CMM), with analysis showing excellent agreement between the two techniques. The experiment was then carried out on specimens cut from the two orthogonal in-plane material orientations of one of the four Ti-6Al-4V plates. Analysis of the data from these experiments gave significant insight into the plastic deformation behaviour of macroscopically textured Ti-6Al-4V under complex impact loading. Recovered Ti-6Al-4V specimens from the outlined Taylor impact experiments were then sectioned along specific planes and microstructurally characterised using EBSD, with comparisons made between the pre and post-deformation microstructures. From this analysis, and the previously discussed geometry reconstruction technique, insight was gained into the effects of micro-texture on the general anisotropic plastic deformation behaviour of Ti-6Al- 4V plate materials and in particular the role of micro-texture on the formation of deformation twins. Finally, the understanding gained from these experiments, and a detailed review of the literature, was used to inform a novel, physically based material modelling framework, capable of capturing the effects of microstructure and texture on the strain rate and temperature dependent plastic deformation behaviour of Ti-6Al-4V. The model was implemented in the computational software package, MATLAB, and verified by comparison with the mechanical characterisation results from one of the Ti-6Al-4V plates. A number of frameworks are discussed for implementing the new Ti-6Al-4V model within finite element (FE) analysis software packages, such as ABAQUS, LS-DYNA and DEFORM. It is hoped that the new Ti-6Al-4V model can be used to optimise the design of Ti-6Al-4V components and structures for impact loading scenarios.
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Hu, Jianan. "A theoretical study of creep deformation mechanisms of Type 316H stainless steel at elevated temperatures". Thesis, University of Oxford, 2015. http://ora.ox.ac.uk/objects/uuid:d956b7ff-9748-408e-a68f-31d4c1d492b5.
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The currently operating Generation II Advanced Gas-Cooled Reactors (AGR) in the nuclear power stations in the UK, mainly built in the 1960s and 1970s, are approaching their designed life. Besides the development of the new generation of reactors, the government is also seeking to extend the life of some AGRs. Creep and failure properties of Type 316H austenitic stainless steels used in some components of AGR at elevated temperature are under investigation in EDF Energy Ltd. However, the current empirical creep models used and examined in EDF Energy have deficiency and demonstrate poor agreement with the experimental data in the operational complex thermal/mechanical conditions. The overall objective of the present research is to improve our general understanding of the creep behaviour of Type 316H stainless steels under various conditions by undertaking theoretical studies and developing a physically based multiscale state variable model taking into account the evolution of different microstructural elements and a range of different internal mechanisms in order to make realistic life prediction. A detailed review shows that different microstructural elements are responsible for the internal deformation mechanisms for engineering alloys such as 316H stainless steels. These include the strengthening effects, associated with forest dislocation junctions, solute atoms and precipitates, and softening effects, associated with recovery of dislocation structure and coarsening of precipitates. All the mechanisms involve interactions between dislocations and different types of obstacles. Thus change in the microstructural state will lead to the change in materials' internal state and influence the mechanical/creep property. Based on these understandings, a multiscale self-consistent model for a polycrystalline material is established, consisting of continuum, crystal plasticity framework and dislocation link length model that allows the detailed dislocation distribution structure and its evolution during deformation to be incorporated. The model captures the interaction between individual slip planes (self- and latent hardening) and between individual grains and the surrounding matrix (plastic mismatch, leading to the residual stress). The state variables associated with all the microstructure elements are identified as the mean spacing between each type of obstacles. The evolution of these state variables are described in a number of physical processes, including the dislocation multiplication and climb-controlled network coarsening and the phase transformation (nucleation, growth and coarsening of different phases). The enhancements to the deformation kinetics at elevated temperature are also presented. Further, several simulations are carried out to validate the established model and further evaluate and interpret various available data measured for 316H stainless steels. Specimens are divided into two groups, respectively ex-service plus laboratory aged (EXLA) with a considerable population of precipitates and solution treated (ST) where precipitates are not present. For the EXLA specimens, the model is used to evaluate the microscopic lattice response, either parallel or perpendicular to the loading direction, subjected to uniaxial tensile and/or compressive loading at ambient temperature, and macroscopic Bauschinger effect, taking into account the effect of pre-loading and pre-crept history. For the ST specimens, the model is used to evaluate the phase transformation in the specimen head volume subjected to pure thermal ageing, and multiple secondary stages observed during uniaxial tensile creep in the specimen gauge volume at various temperatures and stresses. The results and analysis in this thesis improve the fundamental understanding of the relationship between the evolution of microstructure and the creep behaviour of the material. They are also beneficial to the assessment of materials' internal state and further investigation of deformation mechanism for a broader range of temperature and stress.
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Siegkas, Petros. "Static and dynamic performance of Ti foams". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:68938d12-d104-4637-8b08-d1c126ddca84.
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Titanium (Ti) foams of different densities 1622-4100 Kgm-3 made by a powder sintering technique were studied as to their structural and mechanical properties. The foams were tested under static and dynamic loading. The material was tested quasi statically and dynamically under strain rates in the range of 0.001-2500 s-1 and under different loading modes. It was found that strain rate sensitivity is more pronounced in lower density foams. Experiments were complimented by virtual testing. Based on the Voronoi tessellations a computational method was developed to generate stochastic foam geometries. Statistical control was applied to produce geometries with the microstructural characteristics of the tested material. The generated structures were numerically tested under different loading modes and strain rates. Voronoi polyhedrals were used to form the porosity network of the open cell foams. The virtually generated foams replicated the geometrical features of the experimentally tested material. Meshes for finite element simulations were produced. Existing material models were used for the parent material behaviour (sintered Ti) and calibrated to experiments. The virtual foam geometries of different densities were numerically tested quasi statically under uniaxial, biaxial and triaxial loading modes in order to investigate their macroscopic behaviour. Dynamic loading was also applied for compression. Strain rate sensitive and insensitive models were used for the parent material model in order to examine the influence of geometry and material strain rate sensitivity under high rates of deformation. It was found that inertial effects can enhance the strain rate sensitivity for low density foams and numerical predictions for the generated foam geometries were in very good agreement with experimental results. Power laws were established in scaling material properties with density. The study includes: 1. Information on the material behaviour and data for macroscopically modelling this type of foams for a range of densities and under different strain rates. 2. A proposed method for virtually generating foam geometries at a microscopic scale and examine the effect of geometrical characteristics on the macroscopic behaviour of foams.
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Bonfils, Laure. "Characterisation of the high strain rate deformation behaviour of α-β titanium alloys at near-transus temperature". Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:e2507c22-6478-4461-be57-347382a5ee0c.
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The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s-1; then at 4% plastic strain, a strain rate of 2000 s-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.
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Potnis, Prashant. "Single crystal ferroelectrics : macroscopic and microscopic studies". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:96973376-8596-4fc9-9c53-c58379a766a5.
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The aim of this thesis was to improve the understanding of microstructure in single crystal ferroelectrics. This was achieved through macroscopic testing of Lead Magnesium Niobate – Lead Titanate (PMN-PT) and microscopic observations of Barium Titanate (BT) single crystals. Multi-axial polarization rotation tests on PMN-PT showed a gradual increase in the change in dielectric displacement due to ferroelectric switching as the electric field is applied at increasing angles to the initial polarization direction. A relatively high remnant polarization for loading angle near to 90° suggested that PMN-PT is more polarizable in certain directions. Strains measured in two directions, parallel to the electric field and perpendicular to the electric field, showed a noticeable variation on two opposite faces of the specimen suggesting an effect of local domain configurations on macroscopic behaviour. A micromechanical model gave an insight into the switching systems operating in the crystal during the polarization rotation test. Domain structure in BT was mapped using synchrotron X-ray reflection topography. By making use of the angular separation of the diffracted reflections and specimen rocking, different domain types could be unambiguously identified, along with the relative tilts between adjacent domains. Fine needle domains (width ≈ 10μm) were successfully mapped providing a composite topograph directly comparable with optical micrograph. The domain structure was confirmed using other techniques such as piezoresponse force microscopy and atomic force microscopy/scanning electron microscopy and optical observations on the etched crystal. Results show that combined use of multiple techniques is necessary to gain a consistent interpretation of the microstructure. Finally, domain evolution in BT under compressive mechanical loading was observed in-situ using optical and X-ray diffraction techniques providing a series of images that show ferroelastic transition. The domain configurations influence the switching behaviour and constitutive models that can account for such effects need to be developed. Quantitative and qualitative data presented in this thesis can assist model development and validation.
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Wredh, Simon. "Neural Network Based Model Predictive Control of Turbulent Gas-Solid Corner Flow". Thesis, Uppsala universitet, Signaler och system, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-420056.
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Over the past decades, attention has been brought to the importance of indoor air quality and the serious threat of bio-aerosol contamination towards human health. A novel idea to transport hazardous particles away from sensitive areas is to automatically control bio-aerosol concentrations, by utilising airflows from ventilation systems. Regarding this, computational fluid dynamics (CFD) may be employed to investigate the dynamical behaviour of airborne particles, and data-driven methods may be used to estimate and control the complex flow simulations. This thesis presents a methodology for machine-learning based control of particle concentrations in turbulent gas-solid flow. The aim is to reduce concentration levels at a 90 degree corner, through systematic manipulation of underlying two-phase flow dynamics, where an energy constrained inlet airflow rate is used as control variable. A CFD experiment of turbulent gas-solid flow in a two-dimensional corner geometry is simulated using the SST k-omega turbulence model for the gas phase, and drag force based discrete random walk for the solid phase. Validation of the two-phase methodology is performed against a backwards facing step experiment, with a 12.2% error correspondence in maximum negative particle velocity downstream the step. Based on simulation data from the CFD experiment, a linear auto-regressive with exogenous inputs (ARX) model and a non-linear ARX based neural network (NN) is used to identify the temporal relationship between inlet flow rate and corner particle concentration. The results suggest that NN is the preferred approach for output predictions of the two-phase system, with roughly four times higher simulation accuracy compared to ARX. The identified NN model is used in a model predictive control (MPC) framework with linearisation in each time step. It is found that the output concentration can be minimised together with the input energy consumption, by means of tracking specified target trajectories. Control signals from NN-MPC also show good performance in controlling the full CFD model, with improved particle removal capabilities, compared to randomly generated signals. In terms of maximal reduction of particle concentration, the NN-MPC scheme is however outperformed by a manually constructed sine signal. In conclusion, CFD based NN-MPC is a feasible methodology for efficient reduction of particle concentrations in a corner area; particularly, a novel application for removal of indoor bio-aerosols is presented. More generally, the results show that NN-MPC may be a promising approach to turbulent multi-phase flow control.
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Le, Toan T. "A Single-Stage Passive Vibration Isolation System for Scanning Tunneling Microscopy". DigitalCommons@CalPoly, 2021. https://digitalcommons.calpoly.edu/theses/2272.
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Scanning Tunneling Microscopy (STM) uses quantum tunneling effect to study the surfaces of materials on an atomic scale. Since the probe of the microscope is on the order of nanometers away from the surface, the device is prone to noises due to vibrations from the surroundings. To minimize the random noises and floor vibrations, passive vibration isolation is a commonly used technique due to its low cost and simpler design compared to active vibration isolation, especially when the entire vibration isolation system (VIS) stays inside an Ultra High Vacuum (UHV) environment. This research aims to analyze and build a single-stage passive VIS for an STM. The VIS consists of a mass-spring system staying inside an aluminum hollow tube. The mass-spring system is comprised of a circular copper stage suspended by a combination of six extension springs, and the STM stays on top of the copper stage. Magnetic damping with neodymium magnets, which induces eddy currents in the copper conductor, is the primary damping method to reduce the vibrations transferred to the mass-spring system. FEMM and MATLAB® are used to model magnetic flux density and damping coefficients from eddy current effect, which will help determine the necessary damping ratios for the VIS. Viton, which demonstrates a high compatibility with vacuum environments, will also serve as a great damping material between joints and contacts for the housing tube. Viton will be modeled as a Mooney-Rivlin hyperelastic material whose material parameters are previous studied, and Abaqus will be used as a Finite Element Analysis software to study the Viton gaskets’ natural frequencies. The natural frequencies of the aluminum hollow tube will also be investigated through Abaqus.
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Asadikiya, Mohammad. "Thermodynamic Investigation of Yttria-Stabilized Zirconia (YSZ) System". FIU Digital Commons, 2017. https://digitalcommons.fiu.edu/etd/3550.
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The yttria-stabilized zirconia (YSZ) system has been extensively studied because of its critical applications, like solid oxide fuel cells (SOFCs), oxygen sensors, and jet engines. However, there are still important questions that need to be answered and significant thermodynamic information that needs to be provided for this system. There is no predictive tool for the ionic conductivity of the cubic-YSZ (c-YSZ), as an electrolyte in SOFCs. In addition, no quantitative diagram is available regarding the oxygen ion mobility in c-YSZ, which is highly effective on its ionic conductivity. Moreover, there is no applicable phase stability diagram for the nano-YSZ, which is applied in oxygen sensors. Phase diagrams are critical tools to design new applications of materials. Furthermore, even after extensive studies on the thermodynamic database of the YSZ system, the zirconia-rich side of the system shows considerable uncertainties regarding the phase equilibria, which can make the application designs unreliable.
During this dissertation, the CALPHAD (CALculation of PHase Diagrams) approach was applied to provide a predictive diagram for the ionic conductivity of the c-YSZ system. The oxygen ion mobility, activation energy, and pre-exponential factor were also predicted.
In addition, the CALPHAD approach was utilized to predict the Gibbs energy of bulk YSZ at different temperatures. The surface energy of each polymorph was then added to the predicted Gibbs energy of bulk YSZ to obtain the total Gibbs energy of nano-YSZ. Therefore, a 3-D phase stability diagram for the nano-YSZ system was provided, by which the stability range of each polymorph versus temperature and particle size are presented.
Re-assessment of the thermodynamic database of the YSZ system was done by applying the CALPHAD approach. All of the available thermochemical and phase equilibria data were evaluated carefully and the most reliable ones were selected for the Gibbs energy optimization process. The results calculated by the optimized thermodynamic database showed good agreement with the selected experimental data, particularly on the zirconia-rich side of the system.
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Sui, Tan. "Thermal-mechanical behaviour of the hierarchical structure of human dental tissue". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:2c8e9604-ec4b-4cfa-b6df-fff3e6579492.
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Human dental tissues are fascinating nano-structured hierarchical materials that combine organic and mineral phases in an intricate and ingenious way to obtain remarkable combinations of mechanical strength, thermal endurance, wear resistance and chemical stability. Attempts to imitate and emulate this performance have been made since time immemorial, in order to provide replacement (e.g. in dental prosthodontics) or to develop artificial materials with similar characteristics (e.g. light armour). The key objectives of the present project are to understand the structure-property relationships that underlie the integrity of natural materials, human dental tissues in particular, and the multi-scale architecture of mineralized tissues and its evolution under thermal treatment and mechanical loading. The final objective is to derive ideas for designing and manufacturing novel artificial materials serving biomimetic purposes. The objectives are achieved using the combination of a range of characterization techniques, with particular attention paid to the synchrotron X-ray scattering (Small- and Wide-Angle X-ray Scattering, SAXS and WAXS) and imaging techniques (Micro Computed Tomography), as well as microscopy techniques such as Environmental Scanning Electron Microscopy (ESEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Mechanical properties were characterized by nanoindentation and photoelasticity; and thermal analysis was carried out via thermogravimetric analysis (TGA). Experimental observations were critically examined and matched by advanced numerical simulation of the tissue under thermal-mechanical loading. SAXS and WAXS provided the initial basis for elucidating the structure-property relationships in human dentine and enamel through in situ experimentation. Four principal types of experiment were used to examine the thermal and mechanical behaviour of the hierarchical structure of human dental tissue and contributed to the Chapters of this thesis: (i) In situ elastic strain evolution under loading within the hydroxyapatite (HAp) in both dentine and enamel. An improved multi-scale Eshelby inclusion model was proposed taking into account the two-level hierarchical structure, and was validated against the experimental strain evaluation data. The achieved agreement indicates that the multi-scale model accurately reflects the structural arrangement of human dental tissue and its response to applied forces. (ii) The morphology of the dentine-enamel junction (DEJ) was examined by a range of techniques, including X-ray imaging and diffraction. The transition of mechanical properties across the DEJ was evaluated by the high resolution mapping and in situ compression measurement, followed by a brief description of the thermal behaviour of DEJ. The results show that DEJ is a narrow band of material with graded structure and mechanical properties, rather than a discrete interface. (iii) Further investigation regarding the thermo-mechanical structure-property relationships in human dental tissues was carried out by nanoindentation mapping of the nano-mechanical properties in ex situ thermally treated dental tissues. (iv) In order to understand the details of the thermal behaviour, in situ heat treatment was carried out on both human dental tissues and synthetic HAp crystallites. For the first time the in situ ultrastructural alteration of natural and synthetic HAp crystallites was captured in these experiments. The results presented in this thesis contribute to the fundamental understanding of the structure-property integrity mechanisms of natural materials, human dental tissues in particular. These results were reported in several first author publications in peer-reviewed journals, conference proceedings, and a book chapter.
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Hardie, Christopher David. "Micro-mechanics of irradiated Fe-Cr alloys for fusion reactors". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:a3ac36ba-ca6f-4129-8f37-f1278ef8a559.
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In the absence of a fusion neutron source, research on the structural integrity of materials in the fusion environment relies on current fission data and simulation methods. Through investigation of the Fe-Cr system, this detailed study explores the challenges and limitations in the use of currently available radiation sources for fusion materials research. An investigation of ion-irradiated Fe12%Cr using nanoindentation with a cube corner, Berkovich and spherical tip, and micro-cantilever testing with two different geometries, highlighted that the measurement of irradiation hardening was largely dependent on the type of test used. Selected methods were used for the comparison of Fe6%Cr irradiated by ions and neutrons to a dose of 1.7dpa at a temperature of 288°C. Micro-cantilever tests of the Fe6%Cr alloy with beam depths of 400 to 7000nm, identified that size effects may significantly obscure irradiation hardening and that these effects are dependent on radiation conditions. Irradiation hardening in the neutron-irradiated alloy was approximately double that of the ion-irradiated alloy and exhibited increased work hardening. Similar differences in hardening were observed in an Fe5%Cr alloy after ion-irradiation to a dose of 0.6dpa at 400°C and doses rates of 6 x 10-4dpa/s and 3 x 10-5dpa/s. Identified by APT, it was shown that increased irradiation hardening was likely to be caused by the enhanced segregation of Cr observed in the alloy irradiated with the lower dose rate. These observations have significant implications for future fusion materials research in terms of the simulation of fusion relevant radiation conditions and micro-mechanical testing.
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Parrinello, Antonino. "A rate-pressure-dependent thermodynamically-consistent phase field model for the description of failure patterns in dynamic brittle fracture". Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:c6590f4f-f4e2-40e3-ada1-49ba35c2a594.
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The investigation of failure in brittle materials, subjected to dynamic transient loading conditions, represents one of the ongoing challenges in the mechanics community. Progresses on this front are required to support the design of engineering components which are employed in applications involving extreme operational regimes. To this purpose, this thesis is devoted to the development of a framework which provides the capabilities to model how crack patterns form and evolve in brittle materials and how they affect the quantitative description of failure. The proposed model is developed within the context of diffusive interfaces which are at the basis of a new class of theories named phase field models. In this work, a set of additional features is proposed to expand their domain of applicability to the modelling of (i) rate and (ii) pressure dependent effects. The path towards the achievement of the first goal has been traced on the desire to account for micro-inertia effects associated with high rates of loading. Pressure dependency has been addressed by postulating a mode-of-failure transition law whose scaling depends upon the local material triaxiality. The governing equations have been derived within a thermodynamically-consistent framework supplemented by the employment of a micro-forces balance approach. The numerical implementation has been carried out within an updated lagrangian finite element scheme with explicit time integration. A series of benchmarks will be provided to appraise the model capabilities in predicting rate-pressure-dependent crack initiation and propagation. Results will be compared against experimental evidences which closely resemble the boundary value problems examined in this work. Concurrently, the design and optimization of a complimentary, improved, experimental characterization platform, based on the split Hopkinson pressure bar, will be presented as a mean for further validation and calibration.
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Xi, Jiahe. "Cardiac mechanical model personalisation and its clinical applications". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:0db4cf52-4f64-4ee0-8933-3fb49d64aee6.
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An increasingly important research area within the field of cardiac modelling is the development and study of methods of model-based parameter estimation from clinical measurements of cardiac function. This provides a powerful approach for the quantification of cardiac function, with the potential to ultimately lead to the improved stratification and treatment of individuals with pathological myocardial mechanics. In particular, the diastolic function (i.e., blood filling) of left ventricle (LV) is affected by its capacity for relaxation, or the decay in residual active tension (AT) whose inhibition limits the relaxation of the LV chamber, which in turn affects its compliance (or its reciprocal, stiffness). The clinical determination of these two factors, corresponding to the diastolic residual AT and passive constitutive parameters (stiffness) in the cardiac mechanical model, is thus essential for assessing LV diastolic function. However these parameters are difficult to be assessed in vivo, and the traditional criterion to diagnose diastolic dysfunction is subject to many limitations and controversies. In this context, the objective of this study is to develop model-based applicable methodologies to estimate in vivo, from 4D imaging measurements and LV cavity pressure recordings, these clinically relevant parameters (passive stiffness and active diastolic residual tension) in computational cardiac mechanical models, which enable the quantification of key clinical indices characterising cardiac diastolic dysfunction. Firstly, a sequential data assimilation framework has been developed, covering various types of existing Kalman filters, outlined in chapter 3. Based on these developments, chapter 4 demonstrates that the novel reduced-order unscented Kalman filter can accurately retrieve the homogeneous and regionally varying constitutive parameters from the synthetic noisy motion measurements. This work has been published in Xi et al. 2011a. Secondly, this thesis has investigated the development of methods that can be applied to clinical practise, which has, in turn, introduced additional difficulties and opportunities. This thesis has presented the first study, to our best knowledge, in literature estimating human constitutive parameters using clinical data, and demonstrated, for the first time, that while an end-diastolic MR measurement does not constrain the mechanical parameters uniquely, it does provide a potentially robust indicator of myocardial stiffness. This work has been published in Xi et al. 2011b. However, an unresolved issue in patients with diastolic dysfunction is that the estimation of myocardial stiffness cannot be decoupled from diastolic residual AT because of the impaired ventricular relaxation during diastole. To further address this problem, chapter 6 presents the first study to estimate diastolic parameters of the left ventricle (LV) from cine and tagged MRI measurements and LV cavity pressure recordings, separating the passive myocardial constitutive properties and diastolic residual AT. We apply this framework to three clinical cases, and the results show that the estimated constitutive parameters and residual active tension appear to be a promising candidate to delineate healthy and pathological cases. This work has been published in Xi et al. 2012a. Nevertheless, the need to invasively acquire LV pressure measurement limits the wide application of this approach. Chapter 7 addresses this issue by analysing the feasibility of using two kinds of non-invasively available pressure measurements for the purpose of inverse parameter estimation. The work has been submitted for publication in Xi et al. 2012b.
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Avachat, Siddharth. "Experimental and numerical analyses of dynamic deformation and failure in marine structures subjected to underwater impulsive loads". Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/44904.
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The need to protect marine structures from the high-intensity impulsive loads created by underwater explosions has stimulated renewed interest in the mechanical response of sandwich structures. The objective of this combined numerical and experimental study is to analyze the dynamic response of composite sandwich structures and develop material-structure-property relations and design criteria for improving the blast-resistance of marine structures. Configurations analyzed include polymer foam core structures with planar geometries. A novel experimental facility to generate high-intensity underwater impulsive loads and carry out in-situ measurements of dynamic deformations in marine structures is developed. Experiments are supported by fully dynamic finite-element simulations which account for the effects of fluid-structure interaction, and the constitutive and damage response of E-glass/polyester composites and PVC foams.
Results indicate that the core-density has a significant influence on dynamic deformations and failure modes. Polymeric foams experience considerable rate-effects and exhibit extensive shear cracking and collapse under high-magnitude multi-axial underwater impulsive loads. In structures with identical masses, low-density foam cores consistently outperform high-density foam cores, undergoing lesser deflections and transmitting smaller impulses. Calculations reveal a significant difference between the response of air-backed and water-backed structures. Water-backed structures undergo much greater damage and consequently need to absorb a much larger amount of energy than air-backed structures. The impulses transmitted through water-backed structures have significant implications for structural design. The thickness of the facesheets is varied under the conditions of constant material properties and core dimensions. The results reveal an optimal thickness of the facesheets which maximizes energy absorption in the core and minimizes the overall deflection of the structure.
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Hill, Theresa Y. "Understanding Drop-on-Demand Inkjet Process Characteristics in the Application of Printing Micro Solid Oxide Fuel Cells". Wright State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=wright156167105938597.
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Mulvihill, Daniel Martin. "Studies of frictional interface behaviour : experiments and modelling". Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:54e18b10-1167-40f1-9dc9-0ca529a56f34.
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Predictive models of structures containing frictional joints presently suffer from poor descriptions of interface behaviour at the joints. This thesis aims to address this shortfall by furthering the physical understanding of parameters affecting interface behaviour such as friction and contact stiffness. Aspects of friction and contact stiffness relevant to the characterisation of fretting joints are investigated by a combined modelling and experimental approach. Friction and wear behaviour in gross-slip fretting are investigated by in-line and rotational fretting tests. New 3D topography parameters are found to be useful in the analysis of surfaces during fretting. Wear-scar shape is found to be dependent on material. A phenomenon whereby friction increases during the gross-slip phase of individual cycles is found to be due to wear-scar interaction primarily through the interference of local features distributed over the contact area. These features are similar in size to the applied fretting stroke. A simple model to explain the behaviour is put forward which shows that wear-scar shape determines the form of the friction variation. A finite-element (FE) model of the interaction of an elastic-plastic asperity junction is used to predict sliding friction coefficients. The modelling differs from previous work by: permitting greater asperity overlaps, enforcing an interface shear strength, and allowing material failure. The results are also used to predict friction coefficients for a stochastic rough surface. The magnitudes of the predicted friction coefficients are generally representative of experimental measurements. Results suggest that friction arises from both plasticity and tangential interface adhesion. Contact stiffness is studied for both fretting and non-fretting. A technique to isolate the true interface stiffness from results derived from load-deflection data is developed by comparing experimental and FE results. In the fretting wear case, comparison of tangential contact stiffness results in the literature with FE results reveals an interface whose compliance dominates the response to the extent that stiffness is proportional to contact area. In fretting tests such as this, wear debris is thought to be a factor contributing to high interface compliance. Non-fretting experiments performed here show that, at higher pressures, interface domination is reduced as the contact approaches the smooth case. Experiments are performed where contact stiffness is measured simultaneously by both ultrasound and digital image correlation. The effect of normal and tangential loading upon the contact stiffness (normal and tangential) is investigated. Experimental evidence showing that ultrasound measures an ‘unloading’ stiffness while DIC measures a ‘loading’ stiffness is obtained for the case of tangential loading where the ‘DIC stiffness’ decreases with increasing tangential load whereas the ‘ultrasound stiffness’ remains approximately constant. On average, ultrasound gives magnitudes 3.5 and 2.5 times stiffer than the DIC results for the normal and tangential stiffness cases, respectively. The difference in magnitudes can largely be physically explained, and is relatively small considering the significant differences between the techniques. Therefore, both methods can claim to give valid measurements of contact stiffness – though each has its own limitations which are outlined herein.
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Hofmann, Felix. "Probing the deformation of ductile polycrystals by synchrotron X-ray micro-diffraction". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:4a5db5b9-1673-40bf-a25f-2e09a572a108.
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Microscopic beams of penetrating synchrotron radiation provide a unique tool for the analysis of material structure and deformation. This thesis describes my contributions to the development of new synchrotron X-ray micro-beam diffraction experimental techniques and data interpretation, and the use of experimental results for the validation of material deformation models. To study deeply buried material volumes in thick samples, the micro-beam Laue technique was extended to higher photon energies. Through-thickness resolution was achieved either by a wire scanning approach similar to Differential Aperture X-ray Microscopy (DAXM), or by applying tomographic reconstruction principles to grain-specific Laue pattern intensity. Both techniques gave promising first results. For reliable micro-beam Laue diffraction measurements of elastic strains in individual grains of a polycrystal, understanding of the error sources is vital. A novel simulation-based error analysis framework allowed the assessment of individual contributions to the total measurement error. This provides a rational basis for the further improvement of experimental setups. For direct comparison of experimental measurements and dislocation dynamics simulations, diffraction post-processing of dislocation models in two and three dimensions was developed. Simulated diffraction patterns of two-dimensional dislocation cell/wall type structures captured correctly some of the features observed experimentally in reciprocal space maps of a large-grained, lightly deformed aluminium alloy sample. Crystal lattice rotations computed from three-dimensional dislocation dynamics simulations of a Frank-Read source showed anisotropic orientation spread similar to that observed in micro-beam Laue experiments. For the experimental study of crystal lattice distortion, a novel technique was proposed that combines micro-beam Laue diffraction with scanning white-beam topography. Diffraction topography allows the study of lattice rotation at scales smaller than the scanning beam size. The new technique makes it possible to apply classical topography methods to deformed samples.
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Tavares, Renato Normandia. "Simulação numérica da convecção mista em cavidade preenchida com meio poroso heterogêneo e homogêneo". Universidade Tecnológica Federal do Paraná, 2016. http://repositorio.utfpr.edu.br/jspui/handle/1/1657.
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No presente trabalho é apresentada a modelagem e solução numérica da convecção mista em cavidade aquecida por baixo com o topo deslizante, preenchida com meio poroso heterogêneo e homogêneo. Na abordagem heterogênea, o domínio do sólido é representado por blocos condutores de calor igualmente espaçados; a fase fluido circunda os blocos, limitada pelas paredes da cavidade. A abordagem homogênea ou poro-contínua é caracterizada através da porosidade e da permeabilidade da cavidade. As equações de conservação da massa, quantidade de movimento e energia são obtidas, adimensionalizadas e generalizadas de modo a representarem tanto o modelo contínuo quanto o poro-contínuo. A solução numérica é obtida através do método dos volumes finitos. As equações são discretizadas via esquema QUICK e é utilizado o algoritmo SIMPLE para o acoplamento pressão - velocidade. Visando o regime laminar, os parâmetros do escoamento são mantidos no intervalo de 102≤Re≤103 e 103≤Ra≤106 tanto para a abordagem heterogênea, quanto para a homogênea. Nas configurações testadas para o modelo contínuo, 9, 16, 36 e 64 blocos são considerados para cada combinação de Re e Ra e a porosidade microscópica é mantida constante φ=0,64 . No modelo poro-contínuo o número de Darcy (Da) é definido em função do número de blocos da cavidade heterogênea e da porosidade φ. Resultados numéricos do estudo comparativo entre a abordagem microscópica e a macroscópica são apresentados. Como resultado, correlações para o Nusselt médio para os modelos contínuo e poro-contínuo são obtidas em função do Ra modificado para cada Re.In this work is presented mixed convection heat transfer inside a lid-driven cavity heated from below and filled with heterogeneous and homogeneous porous medium. In the heterogeneous approach, the solid domain is represented by heat conductive equally spaced blocks; the fluid phase surrounds the blocks being limited by the cavity walls. The homogeneous or pore-continuum approach is characterized by the cavity porosity and permeability. Generalized mass, momentum and energy conservation equations are obtained in dimensionless form to represent both the continuum and the pore-continuum models. The numerical solution is obtained via the finite volume method. QUICK interpolation scheme is set for numerical treatment of the advection terms and SIMPLE algorithm is applied for pressure-velocity coupling. Aiming the laminar regime, the flow parameters are kept in the range of 102≤Re≤103 and 103≤Ra≤106 for both the heterogeneous and homogeneous approaches. In the tested configurations for the continuous model, 9, 16, 36, and 64 blocks are considered for each combination of Re and Ra being the microscopic porosity set as constant φ=0,64 . For the pore-continuum model the Darcy number (Da) is set according to the number of blocks in the heterogeneous cavity and the φ. Numerical results of the comparative study between the microscopic and macroscopic approaches are presented. As a result, average Nusselt number equations for the continuum and the pore continuum models as a function of Ra and Re are obtained.
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McKellar, Dougan Kelk. "A dislocation model of plasticity with particular application to fatigue crack closure". Thesis, University of Oxford, 2001. http://ora.ox.ac.uk/objects/uuid:45183b90-017f-4ac1-9550-94772a0ca88b.
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The ability to predict fatigue crack growth rates is essential in safety critical systems. The discovery of fatigue crack closure in 1970 caused a flourish of research in attempts to simulate this behaviour, which crucially affects crack growth rates. Historically, crack tip plasticity models have been based on one-dimensional rays of plasticity emanating from the crack tip, either co-linear with the crack (for the case of plane stress), or at a chosen angle in the plane of analysis (for plane strain). In this thesis, one such model for plane stress, developed to predict fatigue crack closure, has been refined. It is applied to a study of the relationship between the apparent stress intensity range (easily calculated using linear elastic fracture mechanics), and the true stress intensity range, which includes the effects of plasticity induced fatigue crack closure. Results are presented for all load cases for a finite crack in an infinite plane, and a method is demonstrated which allows the calculation of the true stress intensity range for a growing crack, based only on the apparent stress intensity range for a static crack. Although the yield criterion is satisfied along the plastic ray, these one-dimensional plasticity models violate the yield criterion in the area immediately surrounding the plasticity ray. An area plasticity model is therefore required in order to model the plasticity more accurately. This thesis develops such a model by distributing dislocations over an area. Use of the model reveals that current methods for incremental plasticity algorithms using distributed dislocations produce an over-constrained system, due to misleading assumptions concerning the normality condition. A method is presented which allows the system an extra degree of freedom; this requires the introduction of a parameter, derived using the Prandtl-Reuss flow rule, which relates the magnitude of slip on complementary shear planes. The method is applied to two problems, confirming its validity.
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Moravej, Mohammadtaghi. "Investigating Scale Effects on Analytical Methods of Predicting Peak Wind Loads on Buildings". FIU Digital Commons, 2018. https://digitalcommons.fiu.edu/etd/3799.
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Large-scale testing of low-rise buildings or components of tall buildings is essential as it provides more representative information about the realistic wind effects than the typical small scale studies, but as the model size increases, relatively less large-scale turbulence in the upcoming flow can be generated. This results in a turbulence power spectrum lacking low-frequency turbulence content. This deficiency is known to have significant effects on the estimated peak wind loads.
To overcome these limitations, the method of Partial Turbulence Simulation (PTS) has been developed recently in the FIU Wall of Wind lab to analytically compensate for the effects of the missing low-frequency content of the spectrum. This method requires post-test analysis procedures and is based on the quasi-steady assumptions. The current study was an effort to enhance that technique by investigating the effect of scaling and the range of applicability of the method by considering the limitations risen from the underlying theory, and to simplify the 2DPTS (includes both in-plane components of the turbulence) by proposing a weighted average method. Investigating the effect of Reynolds number on peak aerodynamic pressures was another objective of the study.
The results from five tested building models show as the model size was increased, PTS results showed a better agreement with the available field data from TTU building. Although for the smaller models (i.e., 1:100,1:50) almost a full range of turbulence spectrum was present, the highest peaks observed at full-scale were not reproduced, which apparently was because of the Reynolds number effect. The most accurate results were obtained when the PTS was used in the case with highest Reynolds number, which was the1:6 scale model with a less than 5% blockage and a xLum/bm ratio of 0.78. Besides that, the results showed that the weighted average PTS method can be used in lieu of the 2DPTS approach. So to achieve the most accurate results, a large-scale test followed by a PTS peak estimation method deemed to be the desirable approach which also allows the xLum/bm values much smaller than the ASCE recommended numbers.
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Carter, Justin B. "Vibration and Aeroelastic Prediction of Multi-Material Structures based on 3D-Printed Viscoelastic Polymers". Miami University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=miami1627048967306654.
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Tong, Xiaolong. "A Constitutive Model for Crushable Polymer Foams Used in Sandwich Panels: Theory and FEA Application". University of Akron / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1596806015399848.
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Arthington, Matthew Reginald. "Photogrammetric techniques for characterisation of anisotropic mechanical properties of Ti-6Al-4V". Thesis, University of Oxford, 2010. http://ora.ox.ac.uk/objects/uuid:51e4f4d9-75e2-4784-9fbf-103d07496e23.
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The principal aims of this research have been the development of photogrammetric techniques for the measurement of anisotropic deformation in uniaxially loaded cylindrical specimens. This has been achieved through the use of calibrated cameras and the application of edge detection and multiple view geometry. The techniques have been demonstrated at quasi-static strain rates, 10^-3 s^-1, using a screw-driven loading device and high strain rates, 10^3 s^-1, using Split Hopkinson Bars. The materials that have been measured using the technique are nearlyisotropic steel, anisotropic cross-rolled Ti-6Al-4V and anisotropic clock-rolled commercially pure Zr. These techniques allow the surface shapes of specimens that deform elliptically to be completely tracked and measured in situ during loading. This has allowed the measurement of properties that could not have been recorded before, including true direct stress and the ratio of transverse strains in principal material directions, at quasi-static and elevated strain rates, in tension and compression. The techniques have been validated by measuring elliptical prisms of various aspect ratios and independently measuring interrupted specimens using a coordinate measurement machine. A secondary aim of this research has been to improve the characterisation of the anisotropic mechanical properties of cross-rolled Ti-6Al-4V using the techniques developed. In particular, the uniaxial yield stresses, hardening properties and the associated anisotropic deformation behaviour along the principal material directions, have all been recorded in detail not seen before. Significant findings include: higher yield stresses in-plane than in the through-thickness direction in both tension and compression, and the near transverse-isotropy of the through-thickness direction for loading conditions other than quasi-static tension, where significant anisotropy was observed.
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Wahlström, Dennis. "Probabilistic Multidisciplinary Design Optimization on a high-pressure sandwich wall in a rocket engine application". Thesis, Umeå universitet, Institutionen för fysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-138480.
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A need to find better achievement has always been required in the space industrythrough time. Advanced technologies are provided to accomplish goals for humanityfor space explorer and space missions, to apprehend answers and widen knowledges. These are the goals of improvement, and in this thesis, is to strive and demandto understand and improve the mass of a space nozzle, utilized in an upperstage of space mission, with an expander cycle engine. The study is carried out by creating design of experiment using Latin HypercubeSampling (LHS) with a consideration to number of design and simulation expense.A surrogate model based optimization with Multidisciplinary Design Optimization(MDO) method for two different approaches, Analytical Target Cascading (ATC) and Multidisciplinary Feasible (MDF) are used for comparison and emend the conclusion. In the optimization, three different limitations are being investigated, designspace limit, industrial limit and industrial limit with tolerance. Optimized results have shown an incompatibility between two optimization approaches, ATC and MDF which are expected to be similar, but for the two limitations, design space limit and industrial limit appear to be less agreeable. The ATC formalist in this case dictates by the main objective, where the children/subproblems only focus to find a solution that satisfies the main objective and its constraint. For the MDF, the main objective function is described as a single function and solved subject to all the constraints. Furthermore, the problem is not divided into subproblems as in the ATC. Surrogate model based optimization, its solution influences by the accuracy ofthe model, and this is being investigated with another DoE. A DoE of the full factorial analysis is created and selected to study in a region near the optimal solution.In such region, the result has evidently shown to be quite accurate for almost allthe surrogate models, except for max temperature, damage and strain at the hottestregion, with the largest common impact on inner wall thickness of the space nozzle. Results of the new structure of the space nozzle have shown an improvement of mass by ≈ 50%, ≈ 15% and ≈ -4%, for the three different limitations, design spacelimit, industrial limit and industrial limit with tolerance, relative to a reference value,and ≈ 10%, ≈ 35% and ≈ 25% cheaper to manufacture accordingly to the defined producibility model.
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Legrand, Romain. "Acoustique - étude et utilisation de nouvelles sources et transducteurs aux longueurs d'onde nanométriques". Phd thesis, Université Pierre et Marie Curie - Paris VI, 2013. http://tel.archives-ouvertes.fr/tel-00945558.
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Ce manuscrit porte sur l'étude de transducteur acoustique couvrant la plage de fréquence allant de 100 GHz à 1 THz. Un laser impulsionnel pico-seconde est à l'origine de ces hautes fréquences. Les structures utilisées pour la génération sont un empilement bicouches GaAs/AlAs. La sensibilité de génération et de détection d'onde acoustique en fonction de la longueur d'onde est étudiée expérimentalement. Afin d'améliorer l'efficacité de transduction, une micro-cavité optique est utilisée pour augmenter l'interaction entre le rayonnement laser et le super-réseau. Ces transducteurs opto-acoustiques sont utilisés pour mesurer les temps de vie des phonons subterahertz et terahertz. Une anomalie dans le comportement en fréquence des temps de vie a été mise en évidence dans l'arséniure de gallium. L'utilisation de courbes de dispersion issues de calculs ab initio associée à l'approche de Landau-Rumer pour les temps de vie ultra-sonores permet d'expliquer ce comportement inattendu.
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"Efficient Extended Finite Element Algorithms for Strongly and Weakly Discontinuous Entities with Complex Internal Geometries". Doctoral diss., 2015. http://hdl.handle.net/2286/R.I.36386.
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abstract: The objective of this research is to develop robust, accurate, and adaptive algorithms in the framework of the extended finite element method (XFEM) for fracture analysis of highly heterogeneous materials with complex internal geometries. A key contribution of this work is the creation of novel methods designed to automate the incorporation of high-resolution data, e.g. from X-ray tomography, that can be used to better interpret the enormous volume of data generated in modern in-situ experimental testing. Thus new algorithms were developed for automating analysis of complex microstructures characterized by segmented tomographic images.
A centrality-based geometry segmentation algorithm was developed to accurately identify discrete inclusions and particles in composite materials where limitations in imaging resolution leads to spurious connections between particles in close contact.To allow for this algorithm to successfully segment geometry independently of particle size and shape, a relative centrality metric was defined to allow for a threshold centrality criterion for removal of voxels that spuriously connect distinct geometries.
To automate incorporation of microstructural information from high-resolution images, two methods were developed that initialize signed distance fields on adaptively-refined finite element meshes. The first method utilizes a level set evolution equation that is directly solved on the finite element mesh through Galerkins method. The evolution equation is formulated to produce a signed distance field that matches geometry defined by a set of voxels segmented from tomographic images. The method achieves optimal convergence for the order of elements used. In a second approach, the fast marching method is employed to initialize a distance field on a uniform grid which is then projected by least squares onto a finite element mesh. This latter approach is shown to be superior in speed and accuracy.
Lastly, extended finite element method simulations are performed for the analysis of particle fracture in metal matrix composites with realistic particle geometries initialized from X-ray tomographic data. In the simulations, particles fracture probabilistically through a Weibull strength distribution. The model is verified through comparisons with the experimentally-measured stress-strain response of the material as well as analysis of the fracture. Further, simulations are then performed to analyze the effect of mesh sensitivity, the effect of fracture of particles on their neighbors, and the role of a particles shape on its fracture probability.Dissertation/Thesis
Doctoral Dissertation Aerospace Engineering 2015
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McMahon, James Joseph. "Energy release rate in a cracked elastic-plastic solid". Thesis, 1993. http://hdl.handle.net/1911/16651.
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To predict the growth of a crack in a body, Griffith introduced, for linear elastic materials, the concept of the energy release rate. According to this concept, crack growth occurs when the work done on a body by the applied surface tractions minus the increase in the strain energy of the body is equal to the energy required to break the material bonds in front of the crack. This approach has been widely accepted as a criterion for crack growth. For elastic-plastic materials, the concept of the energy release rate is not as well established.
In this thesis, a general criterion for crack growth in elastic-plastic materials is proposed. A general constitutive theory is used, and a new work postulate, which imposes a finite limit on the amount of work that can be recovered from an elastic-plastic body, is proposed. For a certain class of elastic-plastic materials, this finite limit is directly related to a potential function.
Next, an elastic-plastic body containing an edge crack is considered, and an energy balance at the crack tip is derived. This energy balance states that quantities defined at the crack tip are equal to the sum of the rate of work of the applied surface tractions, the rate of recoverable work defined inside the body, and a term that is a measure of the rate of work dissipated by plastic deformations. Finally, an interpretation of each term in the energy balance is given, and the proposed criterion for crack growth is discussed. This new criterion takes into account the dissipative nature of plastic deformations.
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(8817533), Hadi Shagerdi Esmaeeli. "MULTISCALE THERMAL AND MECHANICAL ANALYSIS OF DAMAGE DEVELOPMENT IN CEMENTITIOUS COMPOSITES". Thesis, 2020.
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The exceptional long-term performance of concrete is a primary reason that this material represents a significant portion of the construction industry. However, a portion of this construction material is prone to premature deterioration for multi-physical durability issues such as internal frost damage, restrained shrinkage damage, and aggregate susceptibility to fracture. Since each durability issue is associated with a unique damage mechanism, this study aims at investigating the underlying physical mechanisms individually by characterizing the mechanical and thermal properties development and indicating how each unique damage mechanism may compromise the properties development over the design life of the material.
The first contribution of this work is on the characterization of thermal behavior of porous media (e.g., cement-based material) with a complex solid-fluid coupling subject to thermal cycling. By combining Young-Kelvin-Laplace equation with a computational heat transfer approach, we can calculate the contributions of (i) pore pressure development associated with solidification and melting of pore fluid, (ii) pore size distribution, and (iii) equilibrium phase diagram of multiple phase change materials, to the thermal response of porous mortar and concrete during freezing/thawing cycles. Our first finding indicates that the impact of pore size (and curvature) on freezing is relatively insignificant, while the effect of pore size is much more significant during melting. The fluid inside pores smaller than 5 nm (i.e., gel pores) has a relatively small contribution in the macroscopic freeze-thaw behavior of mortar specimens within the temperature range used in this study (i.e., +24 °C to -35 °C). Our second finding shows that porous cementitious composites containing lightweight aggregates (LWAs) impregnated with an organic phase change material (PCM) as thermal energy storage (TES) agents have the significant capability of improving the freeze-thaw performance. We also find that the phase transitions associated with the freezing/melting of PCM occur gradually over a narrow temperature range (rather than an instantaneous event). The pore size effect of LWA on freezing and melting behavior of PCM is found to be relatively small. Through validation of simulation results with lab-scale experimental data, we then employ the model to investigate the effectiveness of PCMs with various transition temperatures on reducing the impact of freeze-thaw cycling within concrete pavements located in different regions of United States.
The second contribution of this work is on quantification of mechanical properties development of cementitious composites across multiple length scales, and two damage mechanisms associated with aggregate fracture and restrained shrinkage cracking that lead to compromising the long-term durability of the material. The former issue is addressed by combining finite element method-based numerical tools, computational homogenization techniques, and analytical methods, where we observe a competing fracture mechanism for early- age cracking at two length scales of mortar (meso-level) and concrete (macro-level). When the tensile strength of the cement paste is lower than the tensile strength of the aggregate phase, the crack propagates across the paste. When the tensile strength of the cement paste exceeds that of the aggregate, the cracks begin to deflect and propagate through the aggregates. As such, a critical degree of hydration (associated with a particular time) exists below which the cement paste phase is weaker than the aggregate phase at the onset of hydration. This has implications on the inference of kinetic based parameters from mechanical testing (e.g., activation energy). Next, we focus on digital fabrication of a cement paste structure with controlled architecture to allow for mitigating the intrinsic damage induced by inherent shrinkage behavior followed by extrinsic damage exerted by external loading. Our findings show that the interfaces between the printed filaments tend to behave as the first layer of protection by enabling the structure to accommodate the damage by deflecting the microcrack propagation into the stable configuration of interfaces fabricated between the filaments of first and second layers. This fracture behavior promotes the damage localization within the first layer (i.e., sacrificial layer), without sacrificing the overall strength of specimen by inhibiting the microcrack advancement into the neighboring layers, promoting a novel damage localization mechanism. This study is undertaken to characterize the shrinkage-induced internal damage in 7-day 3D-printed and cast specimens qualitatively using X-ray microtomography (μCT) technique in conjunction with multiple mechanical testing, and finite element numerical modeling. As the final step, the second layer of protection is introduced by offering an enhanced damage resistance property through employing bioinspired Bouligand architectures, promoting a damage delocalization mechanism throughout the specimen. This novel integration of damage localization-delocalization mechanisms allows the material to enhance its flaw tolerant properties and long-term durability characteristics, where the reduction in the modulus of rupture (MOR) of hardened cement paste (hcp) elements with restrained shrinkage racking has been significantly improved by ~ 25% when compared to their conventionally cast hcp counterparts.
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"Novel Methodology for Atomistically Informed Multiscale Modeling of Advanced Composites". Doctoral diss., 2018. http://hdl.handle.net/2286/R.I.49129.
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abstract: With the maturity of advanced composites as feasible structural materials for various applications there is a critical need to solve the challenge of designing these material systems for optimal performance. However, determining superior design methods requires a deep understanding of the material-structure properties at various length scales. Due to the length-scale dependent behavior of advanced composites, multiscale modeling techniques may be used to describe the dominant mechanisms of damage and failure in these material systems. With polymer matrix fiber composites and nanocomposites it becomes essential to include even the atomic length scale, where the resin-hardener-nanofiller molecules interact, in the multiscale modeling framework. Additionally, sources of variability are also critical to be included in these models due to the important role of uncertainty in advance composite behavior. Such a methodology should be able to describe length scale dependent mechanisms in a computationally efficient manner for the analysis of practical composite structures.
In the research presented in this dissertation, a comprehensive nano to macro multiscale framework is developed for the mechanical and multifunctional analysis of advanced composite materials and structures. An atomistically informed statistical multiscale model is developed for linear problems, to estimate and scale elastic properties of carbon fiber reinforced polymer composites (CFRPs) and carbon nanotube (CNT) enhanced CFRPs using information from molecular dynamics simulation of the resin-hardener-nanofiller nanoscale system. For modeling inelastic processes, an atomistically informed coupled damage-plasticity model is developed using the framework of continuum damage mechanics, where fundamental nanoscale covalent bond disassociation information is scaled up as a continuum scale damage identifying parameter. This damage model is coupled with a nanocomposite microstructure generation algorithm to study the sub-microscale damage mechanisms in CNT/CFRP microstructures. It is further integrated in a generalized method of cells (GMC) micromechanics model to create a low-fidelity computationally efficient nonlinear multiscale method with imperfect interfaces between the fiber and matrix, where the interface behavior is adopted from nanoscale MD simulations. This algorithm is used to understand damage mechanisms in adhesively bonded composite joints as a case study for the comprehensive nano to macroscale structural analysis of practical composites structures. At each length scale sources of variability are identified, characterized, and included in the multiscale modeling framework.Dissertation/Thesis
Doctoral Dissertation Aerospace Engineering 2018
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Zheng, LiLi. "Micromechanical Studies of Intergranular Strain and Lattice Misorientation Fields and Comparisons to Advanced Diffraction Measurements". 2011. http://trace.tennessee.edu/utk_graddiss/1245.
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Inhomogeneous deformation fields arising from the grain-grain interactions in polycrystalline materials have been evaluated using a crystal plasticity finite element method and extensively compared to neutron diffraction measurements under fatigue crack growth conditions. The roles of intergranular deformation anisotropy, grain boundary damage, and non-common deformation mechanisms (such as twinning for hexagonal close packed crystals) are systematically evaluated. The lattice misorientation field can be used to determine the intragranular deformation behavior in polycrystals or to describe the deformation inhomogeneity due to dislocation plasticity in single crystals. The study of indentation-induced lattice misorientation fields in single crystals sheds lights on the understanding of the scale-dependent plasticity mechanisms.
A two-scale micromechanical analysis is performed to study the lattice strain distributions near a fatigue crack tip. The experimental finding of vanishing residual intergranular strain in polycrystals as the increase of the fully reversed loading cycles suggests the intergranular damage be the dominant failure mechanism. Our model predictions are compared to in situ neutron diffraction measurements of Ni-based superalloys under fatigue crack growth conditions. Predicted and measured lattice strains in the vicinity of fatigue crack tips illustrate the important roles played by the intergranular damage and the surrounding plasticity in fatigue growth.
Motivated by the synchrotron x-ray measurements of lattice rotation fields in single crystals under indentation, the effect of the orientation of slip systems on the 2D wedge indentation of a model single crystal is investigated. Furthermore, the crystallographic orientations of the indented solids are gradually rotated, resulting changes of lattice misorientation patterns under the indenter. These 2D simulations, as well as a 3D Berkovich indentation simulation, suggest a kinematic relationship between the lattice misorientation and crystalline slip fields.
Advanced structural materials such as light-weighted materials, nanocrystalline metals/alloys, and hierarchically structured alloys often encounter unconventional deformation mechanisms. The convolution of crystalline slip and deformation twin are considered in the hexagonal close packed polycrystals. Specifically, we have determined the lattice strain distributions near fatigue crack tips in Zircaloy-4, and the role of tensile-twins on intergranular strain evolution in a wrought Mg alloy, which compare favorable to available neutron diffraction measurements.