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- Title
- BOUNDING ESTIMATION INTEGRITY RISK FOR LINEAR SYSTEMS WITH STRUCTURED STOCHASTIC MODELING UNCERTAINTY
- Creator
- Langel, Steven Edward
- Date
- 2014, 2014-05
- Description
-
Safety critical estimation applications require quantification of integrity risk, which is the probability of the state estimate error...
Show moreSafety critical estimation applications require quantification of integrity risk, which is the probability of the state estimate error exceeding predefined bounds of acceptability. Integrity risk can only be evaluated when the state estimate error probability density function is precisely known, necessitating stochastic models that exactly describe measurement noise and disturbance inputs. Uncertainty in these models directly results in inaccurate assessments of integrity risk. This dissertation develops the first implementable methods to upper bound integrity risk when the autocorrelation functions of stochastic inputs reside between upper and lower bounding functions. The first part of this work considers real-valued estimation applications that use the Kalman filter or batch weighted least squares estimator. Explicit relations are developed between the estimate error variance and autocorrelation functions using a new generalized covariance matrix derived in this dissertation. From these expressions, two methods are provided to upper bound integrity risk. The first method enables fast computation of a conservative bound, and the second method produces the minimum upper bound via semi-definite optimization. Mixed real/integer estimation applications utilizing integer bootstrapping are the focus of the second part of this work. The integrity risk bound is formally defined as the global solution to a non-convex optimization problem over a polytope. Determination of the polytopic region is difficult, and two bounding approaches are initially developed for a circumscribing hyper-rectangular feasible region. Using an innovative method to define the polytope together with linear programming, a third method is derived to upper bound integrity risk over the true polytopic feasible region.
PH.D in Mechanical and Aerospace Engineering, May 2014
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- Title
- EBSD CHARACTERIZATION OF LOW TEMPERATURE DEFORMATION MECHANISMS IN MODERN ALLOYS
- Creator
- Kozmel, Thomas S Ii
- Date
- 2015, 2015-05
- Description
-
For structural applications, grain re nement has been shown to enhance mechanical properties such as strength, fatigue resistance, and...
Show moreFor structural applications, grain re nement has been shown to enhance mechanical properties such as strength, fatigue resistance, and fracture toughness. Through control of the thermo-mechanical processing parameters, dynamic recrystallization mechanisms were used to produce microstructures consisting of sub-micron grains in 9310 steel, 4140 steel, and Ti-6Al-4V. In both 9310 and 4140 steel, the distribution of carbides throughout the microstructure a ected the ability of the material to dynamically recrystallize and determined the size of the dynamically recrystallized grains. Processing the materials at lower temperatures and higher strain rates resulted in ner dynamically recrystallized grains. Microstructural process models that can be used to estimate the resulting microstructure based on the processing parameters were developed for both 9310 and 4140 steel. Heat treatment studies performed on 9310 steel showed that the sub-micron grain size obtained during deformation could not be retained due to the low equilibrium volume fraction of carbides. Commercially available aluminum alloys were investigated to explain their high strain rate deformation behavior. Alloys such as 2139, 2519, 5083, and 7039 exhibit strain softening after an ultimate strength is reached, followed by a rapid degradation of mechanical properties after a critical strain level has been reached. Microstructural analysis showed that the formation of shear bands typically preceded this rapid degradation in properties. Shear band boundary misorientations increased as a function of equivalent strain in all cases. Precipitation behavior was found to greatly in uence the microstructural response of the alloys. Additionally, precipitation strengthened alloys were found to exhibit similar ow stress behavior, whereas solid solution strengthened alloys exhibited lower ow stresses but higher ductility during dynamic loading. Schmid factor maps demonstrated that shear band formation behavior was in uenced by texturing in these alloys.
Ph.D. in Materials Science and Engineering, May 2015
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- Title
- THERMODYNAMIC PROPERTIES AND PHASE EQUILIBRIA OF SELECTED HEUSLER COMPOUNDS
- Creator
- Yin, Ming
- Date
- 2015, 2015-12
- Description
-
Heusler compounds are ternary intermetallics with many promising properties such as spin polarization and magnetic shape memory effect. A...
Show moreHeusler compounds are ternary intermetallics with many promising properties such as spin polarization and magnetic shape memory effect. A better understanding of their thermodynamic properties facilitates future design and development. Therefore, standard enthalpies of formation and heat capacities from room temperature to 1500 K of selected Heusler compounds X2YZ (X = Co, Fe, Ni, Pd, Rh, Ru; Y = Co, Cu, Fe, Hf, Mn, Ni, Ti, V, Zr; Z = Al, Ga, In, Si, Ge, Sn) and half-Heusler compounds XYSn (X = Au, Co, Fe, Ir, Ni, Pd, Pt, Rh; Y = Hf, Mn, Ti, Zr) were measured using high temperature direct reaction calorimetry. The measured standard enthalpies of formation were compared with those predicted from ab initio calculations and the extended semiempirical Miedema's model. Trends in standard enthalpy of formation with respect to the periodic classification of elements were discussed. The effect of a fourth element (Co, Cu, Fe, Pd; Ti, V; Al, Ga, In, Si, Ge) on the standard enthalpy of formation of Ni2MnSn was also investigated. Lattice parameters of the compounds with an L21 structure were determined using X-ray powder diffraction analysis. Differential scanning calorimetry was used to determine melting points and phase transformation temperatures. Phase relationships were investigated using scanning electron microscopy with an energy dispersive spectrometer. The isothermal section of the Fe-Sn-Ti ternary system at 873 K was established using equilibrated alloys. Three ternary compounds including the Heusler compound Fe2SnTi were observed. A new ternary compound Fe5Sn9Ti6 was reported and the crystal structure of FeSnTi2 was determined for the first time.
Ph.D. in Mechanical, Materials and Aerospace Engineering
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- Title
- EXPERIMENTAL STUDY, TIME-RESOLVED OBSERVATION, AND ANALYSIS OF NANOSECOND LASER ABLATION AND LASER-INDUCED PLASMA
- Creator
- Zhou, Yun
- Date
- 2013, 2013-05
- Description
-
For lasers, there exist many current and potential competitive applications in the areas of manufacturing, materials processing, and so on....
Show moreFor lasers, there exist many current and potential competitive applications in the areas of manufacturing, materials processing, and so on. Nanosecond (ns)-pulsed lasers are often seen in these applications. This dissertation presents the experimental study, time-resolved observation and analysis of nanosecond laser ablation and laser-induced plasma due to ablation, and the analysis is based on physics-based numerical models. Specifically, the following topics have been studied: (1). Plasma induced by ablation using long-duration nanosecond laser pulse Plasma is often produced in laser-material interactions, and may play an important role in many laser-based or laser-assisted technologies, such as laser shock peening (LSP), laser micromachining (through the laser ablation process), laser-induced breakdown spectroscopy (LIBS), and laser propulsion, etc. However, despite the previous work in the literature, the study is still limited for the plasma induced by the ablation process using long nanosecond laser pulses (with durations on the order of ~100 ns). In this dissertation, investigations have been performed on this kind of plasma through fast photography (imaging) and emission spectroscopy techniques, and the experimental results have been analyzed using physics-based computational models. Based on the investigations, the following major interesting findings have been obtained under the investigated conditions: (i) Two high radiation intensity regions are observed in the plasma, and one of them disappears soon after laser pulse ends. The research work shows that this phenomenon is related to the vaporization process from the target surface and the confinement effect of the ambient air. (ii) Laser-induced plasma xvii backward growth phenomenon occurs for target ablation using 100-ns laser pulses, but not for that using 200-ns pulses. The investigation shows that the underlying mechanism is the backward growth of the boundary of the high temperature region without actual backward motion of vapor material. (iii) Under the studied conditions, the core region of the plasma (~100 μm above the laser-ablated target) has relatively low temperatures as compared to some other regions of the plasma, which contradicts the common intuition. (2). Semiconductor ablation utilizing infrared (IR) nanosecond laser pulses Semiconductor micromachining through laser ablation may have the advantages of high spatial resolution, high processing efficiency, good flexibility, and no tool wear. However, most of the prior investigations in the literature on ns laser ablation of semiconductors often employ lasers at the ultraviolet (UV) or visible wavelength. The work using IR ns lasers is limited, but IR ns lasers may often have lower cost and require less external energy consumption to produce the same average laser power output. In this dissertation, the ablation of semiconductors using IR ns lasers has been studied through time-resolved observations (fast photography) and the results have been analyzed utilizing physics-based numerical models. The research work shows that the ablation mechanism under the studied conditions is the surface vaporization process during the early stage followed by the subsequent liquid ejection process that occurs at a later time. The research work and analysis shows that the underlying mechanism for the observed liquid ejection should be due to the spatial gradient of the pressure exerted on the target surface, instead of phase explosion. (3). Laser-induced backside ablation (LIBA) of sapphire with IR ns laser pulses xviii LIBA may potentially provide a good solution for high-quality and highefficiency micromachining of wide-band-gap dielectrics (WBGD). However, the prior work on LIBA of sapphire (which is a very important WBGD material) using low-cost IR ns lasers has been rarely reported. This kind of work has been carried out in this dissertation, and the investigation shows that under the studied conditions LIBA can produce very high material removal efficiency and reasonably good quality of machining without obvious chemical contamination coming from the employed backing layer. The ablation rate and damage threshold are measured under different laser parameters, and the research work may provide useful information for the applications of LIBA in practical areas using IR ns lasers that may often have relatively lower cost than UV or visible ns lasers.
PH.D in Mechanical and Aerospace Engineering, May 2013
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- Title
- INSTANTANEOUS DETECTION OF SPATIAL GRADIENT ERRORS IN DIFFERENTIAL GNSS
- Creator
- Jing, Jing
- Date
- 2014, 2014-12
- Description
-
Differential GNSS (Global Navigation Satellite System) is currently being ex- tended using a high integrity Ground Based Augmentation System ...
Show moreDifferential GNSS (Global Navigation Satellite System) is currently being ex- tended using a high integrity Ground Based Augmentation System (GBAS) to enable civil aircraft precision approach and landing. Differential GPS carrier phase measure- ments between ground-based GBAS reference antennas can provide the means to de- tect and isolate certain signal-in-space (SIS) failures and anomalies that are hazardous to aircraft, most notably ionospheric anomalies and ephemeris failures, which can be characterized as spatial range error gradients. In this research, we develop a monitor capable of instantaneously detecting multi-dimensional spatial gradient faults. The existence of large gradients, while rare, has prompted the development of numerous ground monitors for their detection. One drawback of previously proposed monitors is that their performance for a given ground antenna con guration is de- pendent on how antennas are paired to form measurement differences. In contrast, in this work a new monitor approach is developed to provide consistent detection performance, regardless of how the antennas are paired, by combining measurements from multiple, spatially separated ground antennas through a null space transforma- tion. It is shown that the `null space' monitor signi cantly improves the detection performance over existing fault detection algorithms and enables GBAS to support Category III precision approach and landing. The instantaneous carrier phase monitor cannot detect all gradients due to the presence of integer cycle ambiguities. These ambiguities cannot be resolved because the gradient magnitude is unknown a priori. Furthermore, it has been shown that the performance of such monitors is highly dependent on the reference antenna topol- ogy. The range of detectable gradients for all carrier phase monitors depends on two factors: the number of antennas and their con guration. One can always expand the detection range by using many antennas, but only at greatly increased operational cost. Optimizing antenna con guration has been overlooked as a means to improve performance; simple, heuristic arguments typically prevail in the associated siting decisions. For example, when given four antennas it is generally assumed that they should be sited in a square arrangement to balance performance in all directions. However, such heuristics do not provide the maximum detectable range of gradients, and exploiting the freedom to choose the antenna topology can dramatically expand the detectable range. Due to the presence of carrier phase cycle ambiguities, the re- sulting optimization problem is a constrained, mixed integer nonlinear programming problem. By solving the nonlinear program, the optimal GBAS antenna topology that maximizes the range of detectable gradients can be found for any number of antennas. For ionospheric fronts, the magnitude of the resulting gradient is bounded. However, orbit ephemeris faults can be arbitrarily large. The monitor must be able to detect all large gradients. To detect gradients beyond the capability of the car- rier phase monitor, code phase measurements are integrated into the monitor. Al- though we show this allows for the detection of all gradients, the number and spacing of ground antennas may not be suitable for all ground stations. Therefore, dual frequency carrier phase measurements are also considered to reduce the number of antennas required to achieve the desired detection performance. Finally, a search algorithm is developed to nd the antenna topology that enables the null space mon- itor to detect all hazardous gradients using code and dual frequency carrier phase measurements.
Ph.D. in Mechanical and Aerospace Engineering, December 2014
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- Title
- DIRECT DIFFEOMORPHIC REPARAMETERIZATION FOR CORRESPONDENCE OPTIMIZATION IN STATISTICAL SHAPE MODELING
- Creator
- Li, Kang
- Date
- 2015, 2015-05
- Description
-
This dissertation proposes an efficient optimization approach for obtaining shape correspondence across a group of objects for statistical...
Show moreThis dissertation proposes an efficient optimization approach for obtaining shape correspondence across a group of objects for statistical shape modeling. With each shape represented in a B-spline based parametric form, the correspondence across the shape population is cast as an issue of seeking a reparametrization for each shape so that a quality measure of the resulting shape correspondence across the group is optimized. The quality measure is the description length of covariance matrix of the shape population, with landmarks sampled on each shape. The movement of landmarks on each B-spline shape is controlled by the reparameterization of the B-spline shape. The reparameterization itself is also represented with B-splines and B-spline coefficients are used as optimization parameters. We have developed formulations for ensuring the bijectivity of the reparameterization. A gradient-based optimization approach is developed, including techniques such as constraint aggregation and adjoint senstivity for efficient, direct di↵eomorphic reparameterization of landmarks to improve the group-wise shape correspondence. Numerical experiments on both synthetic and real 2D and 3D data sets demonstrate the efficiency and e↵ectiveness of the proposed approach.This dissertation proposes an efficient optimization approach for obtaining shape correspondence across a group of objects for statistical shape modeling. With each shape represented in a B-spline based parametric form, the correspondence across the shape population is cast as an issue of seeking a reparametrization for each shape so that a quality measure of the resulting shape correspondence across the group is optimized. The quality measure is the description length of covariance matrix of the shape population, with landmarks sampled on each shape. The movement of landmarks on each B-spline shape is controlled by the reparameterization of the B-spline shape. The reparameterization itself is also represented with B-splines and B-spline coefficients are used as optimization parameters. We have developed formulations for ensuring the bijectivity of the reparameterization. A gradient-based optimization approach is developed, including techniques such as constraint aggregation and adjoint senstivity for efficient, direct di↵eomorphic reparameterization of landmarks to improve the group-wise shape correspondence. Numerical experiments on both synthetic and real 2D and 3D data sets demonstrate the efficiency and e↵ectiveness of the proposed approach.
Ph.D. in Mechanical and Aerospace Engineering, May 2015
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- Title
- ASYMPTOTIC SIMILARITY IN TURBULENT BOUNDARY LAYERS
- Creator
- Duncan, Richard D.
- Date
- 2011-05-10, 2011-05
- Description
-
The turbulent boundary layer is one of the most fundamental and important applications of fluid mechanics. Despite great practical interest...
Show moreThe turbulent boundary layer is one of the most fundamental and important applications of fluid mechanics. Despite great practical interest and its direct impact on frictional drag among its many important consequences, no theory absent of significant inference or assumption exists. Numerical simulations and empirical guidance are used to produce models and adequate predictions, but even minor improvements in modeling parameters or physical understanding could translate into significant improvements in the efficiency of aerodynamic and hydrodynamic vehicles. Classically, turbulent boundary layers and fully-developed turbulent channels and pipes are considered members of the same “family,” with similar “inner” versus “outer” descriptions. However, recent advances in experiments, simulations, and data processing have questioned this, and, as a result, their fundamental physics. To address a full range of pressure gradient boundary layers, a new approach to the governing equations and physical description of wall-bounded flows is formulated, using a two variable similarity approach and many of the tools of the classical method with slight but significant variations. A new set of similarity requirements for the characteristic scales of the problem is found, and when these requirements are applied to the classical “inner” and “outer” scales, a “similarity map” is developed providing a clear prediction of what flow conditions should result in self-similar forms. An empirical model with a small number of parameters and a form reminiscent of Coles’ “wall plus wake” is developed for the streamwise Reynolds stress, and shown to fit experimental and numerical data from a number of turbulent boundary layers as well as other wall-bounded flows. It appears from this model and its scaling using the free-stream velocity that the true asymptotic form of u′2 may not become self-evident until Re ≈ 275, 000 or δ+ ≈ 105, if not higher. A perturbation expansion made possible by the novel inclusion of the scaled streamwise coordinate is used to make an excellent prediction of the shear Reynolds stress in zero pressure gradient boundary layers and channel flows, requiring only a streamwise mean velocity profile and the new similarity map. Extension to other flows is promising, though more information about the normal Reynolds stresses is needed. This expansion is further used to infer a three layer structure in the turbulent boundary layer, and modified two layer structure in fully-developed flows, by using the classical inner and logarithmic profiles to determine which portions of the boundary layer are dominated by viscosity, inertia, or turbulence. A new inner function for U+ is developed, based on the three layer description, providing a much more simplified representative form of the streamwise mean velocity nearest the wall.
Ph.D. in Mechanical and Aerospace Engineering, May 2011
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- Title
- MECHANICAL PROPERTIES AND SINTERING MECHANISMS OF POWDER METALLURGY TI6AL4V
- Creator
- Xu, Xiaoyan
- Date
- 2013, 2013-05
- Description
-
Titanium has been identified as one of the key materials with a high strength to weight ratio that can reduce the weight of components and...
Show moreTitanium has been identified as one of the key materials with a high strength to weight ratio that can reduce the weight of components and thereby reduce energy consumption. Single press and sinter as a powder metallurgy technique has the potential to provide cost effective components. Armstrong prealloyed Ti6Al4V, HDH prealloyed Ti6Al4V, HDH blended Ti6Al4V powder and their mixtures were pressed and sintered at different conditions. The chemistry, mechanical and microstructural properties have been investigated to establish optimum processing parameters. Sintered parts were sent to Oshkosh Truck to test and compared with aluminum and steel parts. The Titanium and Ti6Al4V parts were successfully applied and tested. All the specimens passed the load test without failures. The sintering mechanisms of Armstrong prealloyed Ti6Al4V powder were investigated. At relative sintered densities of 75% to 90% (around 900°C), surface diffusion cooperate with grain boundary diffusion, which leads to densification of the powder compact. Around 900°C, grain boundary diffusion controls the sintering process. At 1000°C, boundary diffusion made little contribution to the densification of the Ti6Al4V powder compact. Above 900°C and below 91% sintered density, boundary diffusion controls sintering. Lattice diffusion dominates the densification process at higher temperatures (1100°C~1300°C). The sintering of master alloy blended Ti6Al4V powder has been investigated in order to elucidate the mechanism of sintering. Both blended powder compacts and diffusion couples were investigated using backscattered imaging and energy xvi dispersive analysis to determine the phases present and diffusion path on sintering at 1000ºC and 1100ºC. It is shown that transient liquid phase sintering does not occur and the reason for the rapid sintering of this material is due to enhanced diffusion kinetics resulting from a combination of the concentration gradient and stress induced by a phase transformation in the ternary system.
PH.D in Materials Science and Engineering, May 2013
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- Title
- EUTECTIC γ(NI)/γ′(NI3AL)-δ(NI3NB) POLYCRYSTALLINE NICKEL-BASE SUPERALLOYS: CHEMISTRY, PROCESSING, MICROSTRUCTURE AND PROPERTIES
- Creator
- Xie, Mengtao
- Date
- 2012-12-03, 2012-12
- Description
-
Directionally solidified γ(Ni)/γ′(Ni3Al)-δ(Ni3Nb) eutectic alloys possess attrac- tive high temperature mechanical properties and were...
Show moreDirectionally solidified γ(Ni)/γ′(Ni3Al)-δ(Ni3Nb) eutectic alloys possess attrac- tive high temperature mechanical properties and were considered as candidate tur- bine blade materials. Currently, the properties of polycrystalline γ/γ′-δ alloys are of interest as they inherit many advantageous attributes from the directionally solidi- fied γ/γ′-δ alloys, including high volume fraction of reinforcing phases, exceptional thermal stability and resistance to segregation-induced defect formation. If these at- tributes are properly harnessed, these γ/γ′-δ eutectic alloys might provide a unique solution to the problems experienced by traditional γ/γ′ polycrystalline Ni-base su- peralloys. This thesis is therefore dedicated towards the development of a funda- mental understanding of this novel class of eutectic alloys from several important perspectives. To enrich our understanding of this alloy system, this thesis will first be focused on quantifying the specific effect of individual alloying element on this γ/γ′-δ eutectic system. A set of quaternary Ni-Cr-Al-Nb alloy compositions with increasing levels of Chromium(Cr) was designed to investigate the detailed influence of this element on the primary phase formation, solidus and liquidus temperatures and γ-δ eutectic morphology. The alloying effect of Tantalum(Ta), which shares many similarities to Niobium(Nb), was studied by designing a matrix of multi-component γ/γ′-δ alloy compositions with nominally the same overall (Ta+Nb) content but varying Ta/Nb ratios. Here, different solidification segregation and solid state partitioning behaviors of Ta and Nb in this γ/γ′-δ eutectic system will be discussed, as well as the influ- ence of Ta/Nb ratio on solidification characteristics and equilibrium/non-equilibrium phase volume fractions. Thermodynamic calculations using the Computherm Pandat database (PanNi7) were compared to experimental results in these investigations. The second part of this thesis will aim to provide a more general understand- xvii ing of the effect of various alloying elements, including Cr, Co, Al, Ti, Mo, W, Ta and Nb, on this γ/γ′-δ system. A large number of experimental γ/γ′-δ alloys covering a broad range of compositions was selected for the analysis in this study. Important alloy attributes, such as primary phase formation, overall δ volume fraction, phase transformation temperatures and ternary eutectic initiation, were quantitatively char- acterized as a function of individual alloying element concentrations or combined con- tent of more elements. Linear regression analysis was performed to reveal the relative effectiveness of these elements on this eutectic system. Meanwhile, an extensive com- parison between the experimental observations and Pandat predictions was provided to critically evaluate the strength and weakness of existing thermodynamic database model in predicting trends in this eutectic alloy system with substantially higher Nb content compared to traditional γ/γ′ superalloys. The last part of this thesis emphasizes the development of cast and wrought manufacturing processes for cast γ/γ′-δ eutectic alloys as a cost effective alternative to the powder metallurgy route. Hot rolling of workpieces encapsulated within a steel can was performed on a simple model cast γ/γ′-δ alloy (897) to stimulate the ingot to billet. The influence of different deformation levels on breaking down the dendritic structure and promoting fine and homogenized microstructure was investi- gated. The mechanical soundness associated with different microstructures generated by different hot rolling processes was compared via compression and creep testing. Microstructural parameters that contribute to better mechanical properties will be discussed.
PH.D in Materials Science and Engineering, December 2012
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- Title
- LASER MICROMACHINING, SINTERING, AND LASER-INDUCED PLASMA DEBURRING
- Creator
- Gao, Yibo
- Date
- 2013, 2013-12
- Description
-
Lasers can provide non-mechanical-contact, localized and concentrated energy input to materials with controlled durations and high spatial...
Show moreLasers can provide non-mechanical-contact, localized and concentrated energy input to materials with controlled durations and high spatial resolutions down to a few microns or less. Therefore, lasers have more and more applications in manufacturing and materials processing, such as laser micromachining (which is to create micro-scale features through laser-induced material removal) and laser sintering. Despite the previous research work in the literature, many laser-based manufacturing and materials processing areas still require lots of further research work. Specifically, the following topics will be investigated in the research work in this thesis: nanosecond-pulsed laser ablation of silicon carbide at an infrared wavelength, nanosecond laser-induced plasma deburring, two-step nanosecond laser surface texturing, and the fabrication of carbon nanotube (CNT)-ceramic composites through the laser sintering process.
PH.D in Mechanical and Aerospace Engineering, December 2013
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- Title
- DETECTING GNSS SPOOFING ATTACKS USING INS COUPLING
- Creator
- Tanil, Cagatay
- Date
- 2016, 2016-12
- Description
-
Vulnerability of Global Navigation Satellite Systems (GNSS) users to signal spoofing is a critical threat to positioning integrity, especially...
Show moreVulnerability of Global Navigation Satellite Systems (GNSS) users to signal spoofing is a critical threat to positioning integrity, especially in aviation applications, where the consequences are potentially catastrophic. In response, this research describes and evaluates a new approach to directly detect spoofing using integrated Inertial Navigation Systems (INS) and fault detection concepts based on integrity monitoring. The monitors developed here can be implemented into positioning systems using INS/GNSS integration via 1) tightly-coupled, 2) loosely-coupled, and 3) uncoupled schemes. New evaluation methods enable the statistical computation of integrity risk resulting from a worst-case spoofing attack – without needing to simulate an unmanageably large number of individual aircraft approaches. Integrity risk is an absolute measure of safety and a well-established metric in aircraft navigation. A novel closed-form solution to the worst-case time sequence of GNSS signals is derived to maximize the integrity risk for each monitor and used in the covariance analyses. This methodology tests the performance of the monitors against the most sophisticated spoofers, capable of tracking the aircraft position – for example, by means of remote tracking or onboard sensing. Another contribution is a comprehensive closed-loop model that encapsulates the vehicle and compensator (estimator and controller) dynamics. A sensitivity analysis uses this model to quantify the leveraging impact of the vehicle’s dynamic responses (e.g., to wind gusts, or to autopilot’s acceleration commands) on the monitor’s detection capability. The performance of the monitors is evaluated for two safety-critical terminal area navigation applications: 1) autonomous shipboard landing and 2) Boeing 747 (B747) landing assisted with Ground Based Augmentation Systems (GBAS). It is demonstrated that for both systems, the monitors are capable of meeting the most stringent precision approach and landing integrity requirements of the International Civil Aviation Organization (ICAO). The statistical evaluation methods developed here can be used as a baseline procedure in the Federal Aviation Administration’s (FAA) certification of spoof-free navigation systems. The final contribution is an investigation of INS sensor quality on detection performance. This determines the minimum sensor requirements to perform standalone GNSS positioning in general en route applications with guaranteed spoofing detection integrity.
Ph.D. in Mechanical and Aerospace Engineering, December 2016
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- Title
- HOT DUCTILITY BEHAVIOR IN THE CONTINUOUS CASTING OF NIOBIUM-BEARING MICROALLOYED STEELS
- Creator
- Jansto, Steven George
- Date
- 2013, 2013-12
- Description
-
Over 200 million tons of Niobium-bearing steels were continuously cast and hot rolled globally in 2012. These Nb-bearing plate, bar and sheet...
Show moreOver 200 million tons of Niobium-bearing steels were continuously cast and hot rolled globally in 2012. These Nb-bearing plate, bar and sheet products are manufactured throughout the world. The melting and casting practices to assure production of crack-free slabs, billets and blooms of high surface quality is presented. Much has been published about the traditional ductility trough associated with higher carbon equivalent steels with and without microalloy additions of Nb, V and/or Ti. The focus has primarily been on the chemistry and optimal composition based upon the literature review. However, the steelmaking and process metallurgy parameters are rarely correlated to the hot ductility behavior. Most studies involve laboratory produced heats, but this research is based upon industrial produced samples.The hot ductility troughs associated with simple carbon-manganese steels can also result in surface and internal quality issues if certain steelmaking and casting parameters are not followed. Although high carbon equivalent steels exhibit inherently lower hot ductility behavior, as measured by percent reduction in area at elevated temperature, these steels still exhibit sufficient ductility to satisfactorily meet the unbending stress and strain gradients existing in the straightening section of most casters. The % Reduction in Area significantly overstates the minimum ductility required for crack-free casting of Nb-bearing steels. The relationship between the steelmaking and caster operation and the resultant slab quality is related through the hot ductility behavior. This global Nb-bearing continuous casting steel research study introduces the strain energy as an improved xxi measure of the hot ductiity behavior. The incidence of cracking is primarily related to the steelmaking and caster process parameters and steel chemistry is secondary. These parameters include the elemental residual chemistry level, superheat variation, transfer ladle temperature stratification, oscillation frequency and stroke, mould flux incompatibility, casting speed fluctuation, and excessive secondary cooling.
PH.D in Materials Science and Engineering, December 2013
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- Title
- A NOVEL METHOD FOR THE IMPLEMENTATION OF STRUCTURAL CONTACT IN FINITE ELEMENT METHODS OFFERING SIMPLIFIED TREATMENT OF ENERGY DISSIPATION
- Creator
- Grudzinski, James John
- Date
- 2012-04-24, 2012-05
- Description
-
A novel method for implementing contact/impact in an implicit nite element formulation is presented. The method uses the ideas of buoyancy to...
Show moreA novel method for implementing contact/impact in an implicit nite element formulation is presented. The method uses the ideas of buoyancy to enforce the normal contact constraint and a velocity dependent force to model energy dissipation. Upon contact (penetration) a normal force equal to the depth of penetration times a target weight density (di erent and much larger than the actual material weight density) creates a normal pressure on the contacting body. In addition to the buoyancy force, the penetrating surface area is subjected to a drag-like force that acts in a direction opposite the velocity vector of the penetrating node of the contacting body . This rate dependence is broken up into components tangential and normal to the target surface. The normal component of the drag performs two functions. First it provides for an energy absorbing mechanism similar to a coe cient of restitution for modeling non-conservative systems. Secondly, it can provide damping (analogous to mathematical damping) which can aid in solution convergence. The tangential component of the damping force serves the function of modeling friction in a simpli ed manner. The method applies contact forces in the manner of external forces and as such lends itself well to simpli ed contact detection schemes which rely on functional representation of bodies. The method is described and demonstrated through several examples including a comparison to experimental data.
Ph.D. in Mechanical and Aerospace Engineering, May 2012
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- Title
- KILOMETER-SPACED GNSS ARRAY FOR IONOSPHERIC IRREGULARITY MONITORING
- Creator
- Su, Yang
- Date
- 2017, 2017-05
- Description
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This dissertation presents automated, systematic data collection, processing, and analysis methods for studying the spatial-temporal...
Show moreThis dissertation presents automated, systematic data collection, processing, and analysis methods for studying the spatial-temporal properties of Global Navigation Satellite Systems (GNSS) scintillations produced by ionospheric irregularities at high latitudes using a closely spaced multi-receiver array deployed in the northern auroral zone. The main contributions include 1) automated scintillation monitoring, 2) estimation of drift and anisotropy of the irregularities, 3) error analysis of the drift estimates, and 4) multi-instrument study of the ionosphere. A radiowave propagating through the ionosphere, consisting of ionized plasma, may su↵er from rapid signal amplitude and/or phase fluctuations known as scintillation. Caused by non-uniform structures in the ionosphere, intense scintillation can lead to GNSS navigation and high-frequency (HF) communication failures. With specialized GNSS receivers, scintillation can be studied to better understand the structure and dynamics of the ionospheric irregularities, which can be parameterized by altitude, drift motion, anisotropy of the shape, horizontal spatial extent and their time evolution. To study the structuring and motion of ionospheric irregularities at the sub-kilometer scale sizes that produce L-band scintillations, a closely-spaced GNSS array has been established in the auroral zone at Poker Flat Research Range, Alaska to investigate high latitude scintillation and irregularities. Routinely collecting lowrate scintillation statistics, the array database also provides 100 Hz power and phase data for each channel at L1/L2C frequency. In this work, a survey of seasonal and hourly dependence of L1 scintillation events over the course of a year is discussed. To efficiently and systematically study scintillation events, an automated low-rate scintillation detection routine is established and performed for each day by screening the phase scintillation index. The spaced-receiver technique is applied to cross-correlated phase and power measurements from GNSS receivers. Results of horizontal drift velocities and anisotropy ellipses derived from the parameters are shown for several detected events. Results show the possibility of routinely quantifying ionospheric irregularities by drifts and anisotropy. Error analysis on estimated properties is performed to further evaluate the estimation quality. Uncertainties are quantified by ensemble simulation of noise on the phase signals carried through to the observations of the spaced-receiver linear system. These covariances are then propagated through to uncertainties on drifts. A case study of a single scintillating satellite observed by the array is used to demonstrate the uncertainty estimation process. The distributed array is used in coordination with other measuring techniques such as incoherent scatter radar and optical all-sky imagers. These scintillations are correlated with auroral activity, based on all-sky camera images. Measurements and uncertainty estimates made over a 30-minute period are made and compared to a collocated incoherent scatter radar, and show good agreement in horizontal drift speed and direction during periods of scintillation for cases when the characteristic velocity is less than the drift velocity. The methods demonstrated are extensible to other zones and other GNSS arrays of varying size, number, ground distribution, and transmitter frequency.
Ph.D. in Mechanical and Aerospace Engineeering, May 2017
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- Title
- SYNTHESIS OF NANOPLATE STRUCTURES IN NI-BASED ALLOYS VIA DISCONTINUOUS PRECIPITATION
- Creator
- Zhou, Yang
- Date
- 2017, 2017-07
- Description
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Nanostructured materials are of increasing interest due to their potential for use in numerous applications including catalysis and filtration...
Show moreNanostructured materials are of increasing interest due to their potential for use in numerous applications including catalysis and filtration. Methods of synthesizing nanomaterials often include chemical techniques producing individual particles, which gives rise to issues of agglomeration and containment, and the processes are difficult to scale-up. This work is directed at addressing issues of the occurrence and kinetics of self-assembly and structural alignment of nanostructures, which involves the discontinuous precipitation transformation followed by selective dissolution. This synthesis method has been demonstrated using a Ni-49at.%Co-12at.%Al alloy. A complete discontinuous transformation in the Ni 49-12 alloy can be achieved in times as short as 30min. Thus, the large-scale production of such structures is feasible using conventional heat treatment facilities. The synthesis technique is generally applicable to any alloy system in which DP goes to completion and one phase can be selectively removed. The nanoplates are self-assembled, self-supported and well aligned if the precipitate is coherent with the matrix. In order to control the process it is necessary to understand the phase equilibria involving γ, γ′ and β phases in the Al-Co-Ni system. This has been investigated both experimentally and computationally. The isothermal sections at 1100°C and 800°C as well as a partial liquidus projection were determined which result in modifications to previously published work. Comparing the experimental results with the calculated results using Thermo-Calc (TCNI8), there is reasonable agreement. The predicted separation of the γ phase into a two-phase ferromagnetic and paramagnetic region has never been observed experimentally in this or other published work. Future work should explore the ferromagnetic and paramagnetic phase separation as predicted using Thermo-Calc to verify its occurrence in this system. The nanostructured material produced by complete DP transformation may have interesting mechanical properties and these have been studied to a limited extent in this work. The hardness tests show that with decreasing annealing temperature to 600°C, the hardness of certain samples increases significantly due to the precipitation of second phase. The tensile and creep properties of alloy samples with DP phase present were also investigated. The results imply that the occurrence of DP will have a favorable effect on the tensile strength of the sample while lowering the ductility at the same time. A method to generate serrated grain boundaries based on the DP transformation is proposed. Such structures are expected to increase the creep resistance. This was found to be the case in a limited temperature and loading range. Higher temperature or stress levels lead to DP occurrence and cause negative effects on the creep resistance compared to conventionally processed material. The Curie temperature in the Al-Co-Ni alloy system was also studied and determined using a combined magnetic TG and DSC method. A ternary contour map of the Curie temperature has been constructed. From the contour map, the Curie temperature was seen to decrease from high Co, low Al content samples to low Co, high Al samples bypassing a platform at mid Co content. A nonlinear surface fitting was made through a Exponential2D model, the function is helpful for the prediction of Curie temperature of γ phase in Al-Co-Ni alloy system. This method also provides a novel idea of detecting phase transformation and precipitation through thermo-magnetically analyzing the magnetic behavior of the alloys.
Ph.D. in Materials Science and Engineering, July 2017
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- Title
- THE VAPORIZATION PHENOMENA OF FUEL DROPLETS EXPOSED TO ASYMMETRIC RADIANT HEATING USING PLANAR LASER-INDUCED FLUORESCENCE
- Creator
- Ammigan, Kavin
- Date
- 2012-04-17, 2012-05
- Description
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Droplet vaporization under asymmetric conditions is prevalent in many combustion related devices where fuel droplets may either experience...
Show moreDroplet vaporization under asymmetric conditions is prevalent in many combustion related devices where fuel droplets may either experience asymmetric thermal radiant heating or travel in velocity and temperature gradients. Asymmetric radiant heating is particularly common in spray flames, counter-flow diffusion flames, regions close to the walls of conventional combustion chambers and more importantly in liquid-fueled microcombustors. In this study, experiments are carried out to observe how droplets vaporize when exposed to asymmetric radiant heating. The experimental set-up consists of applying radiant heating, through a radiant panel heater, to one face of a monodisperse droplet stream while using the planar-laser induced fluorescence (PLIF) diagnostic tool to reveal the spatial vapor distribution around vaporizing droplets. Since most fuels are made up of multiple components, bicomponent droplets are also investigated. Pure acetone droplets as well as mixtures of acetone/alkanes (octane and hexane) and acetone/alcohols (ethanol and 2-propanol) droplets are investigated. Results in the form of PLIF images, reveal asymmetric vapor distributions around the droplets with the apparent induction of Stefan flow from the irradiated droplet surface. Such phenomena have not previously been reported in the literature and have relevance to the overall fuel vaporization process as well as subsequent ignition and pollutant formation processes. To further investigate the experimental results, a convective and radiative heat transfer model is employed to simulate the droplets under corresponding experimental conditions. Results from the model show convective cooling and a strong thermal radiation absorption near the droplets’ surface. The induced asymmetric Stefan flow observed experimentally is therefore a consequence of the high thermal radiation absorption at the droplets’ surface. This study gives both experimental and theoretical results of the vaporization phenomena of asymmetrically irradiated fuel droplets with varying compositions, diameters and irradiation temperatures.
Ph.D. in Mechanical and Aerospace Engineering, May 2012
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- Title
- ANALYSIS AND CONTROL OF COMPRESSION-IGNITION AND SPARK-IGNITED ENGINES OPERATING WITH DUAL-FUEL COMBUSTION STRATEGY
- Creator
- Kassa, Mateos
- Date
- 2017, 2017-07
- Description
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In recent years, the implementation of a dual-fuel combustion strategy has been explored as a means to improve the thermal efficiencies of...
Show moreIn recent years, the implementation of a dual-fuel combustion strategy has been explored as a means to improve the thermal efficiencies of internal combustion engines while simultaneously reducing their emissions. The dual-fuel combustion strategy was introduced in compression ignition engines to control the combustion phasing by varying the proportion of two simultaneously injected fuels, and altering the combustion timing. The dual-fuel injection strategy also allowed to extend the load limitation of advanced combustion engines, since the two injected fuel ignite in succession reducing the high peak pressures that generally act as a limiting factor. In spark-ignited (SI) engine, the implementation of a dual-fuel combustion strategy serves as an alternative approach to avoid knock (the inadvertent auto-ignition of the fuel mixture). Although conventional engines rely on delaying spark timing to avoid knocking cycles (which significantly reduces the thermal efficiency), the dual-fuel SI engine rely on the simultaneous injection of a low knock resistance and high knock resistance fuel to dynamically adjust the fuel resistance to knock as required. The dual-fuel SI engine thereby successfully suppresses knock without compromising the engine efficiency. Despite the benefits of the dual-fuel combustion strategy, several challenges arise in its implementation, especially when it is implemented along with other advanced combustion strategy leveraging variable valve timing, exhaust gas recirculation, turbocharging, and so forth. This study explores some of these challenges and addresses them from a control standpoint. Cylinder-to-cylinder variations is identified as one of the main challenges. An in-cylinder oxygen estimation strategy and modification to the conventional fueling strategy are proposed as approaches to reduce the combustion variations. In SI engines, the valve dynamics in transient operations are shown to negatively impact the dual-fuel control strategy. The effect of the valve timing on knock propensity and the resulting effect on the fueling strategy is investigated. Finally, the dual-fuel SI engine relies on measurements of the combustion intensity to adjust the fuel split between the low RON and high RON fuel. The implementation of a conventional knock control is shown to be counterintuitive for dual-fuel SI engines due to the highly reactive nature of the controller and the deterministic approach that assumes cycle-to-cycle correlation of the combustion intensity. Statistical investigation of the combustion intensity metric is conducted to identify key properties that can be leveraged for more effective control strategy.
Ph.D. in Mechanical, Material and Aerospace Engineering
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- Title
- CYCLIC THERMAL TREATMENT
- Creator
- Gu, Sijie
- Date
- 2015, 2015-12
- Description
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Cyclic thermal treatment has the potential to improve energy efficiency of thermal processing. It has been shown that in some cases, the...
Show moreCyclic thermal treatment has the potential to improve energy efficiency of thermal processing. It has been shown that in some cases, the productivity was enhanced by the cyclic thermal treatment operation. In order to investigate the cyclic thermal treatment effect, Copper-Nickel interdiffusion couples were investigated. When the Cu-Ni interdiffusion couple showed positive results, the cyclic thermal treatment was applied to pack carburization and gas carburization of steel. The Cu-Ni interdiffusion couples were annealed with different time-temperature profiles for 5 days. There are three types of time-temperature profile; isothermal, symmetric, and asymmetric cyclic thermal treatment. After thermal treatment, concentration-distance profiles were. Based on the concentration-distance profile, the interdiffusion coefficients of different time-temperature profiles were calculated. The interdiffusion coefficient of the diffusion couple with a ramp rate of 1°C/min had a higher diffusion coefficient than that of the diffusion couple annealed isothermally at the equivalent temperature, 863°C, which means that cyclic thermal treatment has the effect of accelerating diffusion. When the ramp rate was 5ºC/min interdiffusion coefficients were higher than that of the diffusion couple annealed isothermally at the maximum temperature. However, when the ramp rate was increased to 10°C/min, the diffusion coefficient decreased to almost the same as the interdiffusion coefficient of the diffusion couple at the equivalent temperature. After achieving a promising result for the Cu-Ni diffusion couples, we expanded the cyclic thermal treatment to carburizing. The temperature range for cyclic pack carburization was 850° to 950°C. Increasing the cyclic ramp rate resulted in an increase in the case depth. Due to the setup of the pack carburization, the maximum cooling rate achievable is 5°C/min. In order to reach a higher ramp rate, an induction heating gas carburization system was setup. The temperature range for the cyclic induction heat gas carburization was 850°C to 950°C. For the cyclic induction heat gas carburization with increase in ramp rate, the case depth increased. The sample induction gas carburized at a ramp rate of 20°C/min had a deeper case depth than the sample induction gas carburized isothermally at 904.4°C, the equivalent temperature. The first test showed the sample induction gas carburized with a ramp rate of 20°C/min had a deeper case depth than the sample induction gas carburized isothermally at 950° C. With this we draw the conclusion that the cyclic induction gas carburization can achieve a deeper case depth than the isothermal at equivalent temperature induction gas carburization.
Ph.D. in Materials Science and Engineering, December 2015
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- Title
- A NANO-STRUCTURED CERAMIC/POLYMER COMPOSITE FILM FOR ELECTRONIC INTERCONNECTIONS
- Creator
- Harwath, Frank
- Date
- 2016, 2016-05
- Description
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Separable electrical interconnections are a ubiquitous part of modern life and for technical reasons are currently based on the use of gold....
Show moreSeparable electrical interconnections are a ubiquitous part of modern life and for technical reasons are currently based on the use of gold. Since gold is a commodity and subject to significant price fluctuations there is a need for separable interconnects not based on gold. Polymer/ceramic films were produced from various polymer precursors with loadings of multi-wall nanotubes (MWNT) and inert fillers. A variety of applications means were employed with the best success being achieved by means of a modified doctor blade. Pyrolysis was conducted in an inert atmosphere at 1 bar at a range of temperatures in a tube furnace. Pyrolysis was also conducted using a fiber laser. The modulus of the film is estimated to be 71.8 MPa with an ultimate tensile strength of 179 MPa based on hardness tests and anisotropic crack dimensions which developed as a result of uniaxial stress induced during application of the precursor. Uniaxial stress improved film adhesion regardless of filler type or level. Modification of film characteristics after pyrolysis was attempted using spark plasma sintering (SPS). Electrical testing displayed a percolation threshold above loadings of 1% (wt) of MWNTs where there is a significant drop in electrical resistivity. Further reductions in contact resistance were demonstrated up to 2% loading of MWNTs. The level of contact resistance achieved (<10) for a separable contact, in conjunction with a gold plated contact representative of most electronic connectors, indicates that an acceptable level of contact resistance may be achieved using these materials. Characterization of the film using attenuated total reflectance (ATR), xray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy point to a morphology which is dominated by crystallites joined by regions of aliphatic carbon chains. Work function measurements were consistent with highly ordered pyrolytic graphite. (HOPG)
Ph.D. in Materials Science and Engineering, May 2016
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- Title
- DEVELOPMENT OF ADVANCED PACKAGING, FABRICATION AND RELIABILITY METHODS FOR SUBMINIATURE IMPLANTABLE NEURAL PROSTHESES
- Creator
- Kim, Taehyung
- Date
- 2011-08, 2011-07
- Description
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The main purpose of the present thesis is to study the critical reliability issues of polymeric encapsulated electronic devices for visual...
Show moreThe main purpose of the present thesis is to study the critical reliability issues of polymeric encapsulated electronic devices for visual prosthesis. Visual prosthesis has been compatibly studied over twenty years in many professional research labs. The electric design of circuit for the visual prosthesis has been well studied so far. However, the device packaging design and reliability have been not studied extensively. In particular, failure analysis and reliability using non-hermetic packaging for long lifetime visual prosthesis have been poorly studied. The first step of the research for this thesis was to create a mechanical design of a visual prosthesis prototype. The prototype device was created by fabricate a multi-layer structure consisting of electrodes, substrate, and integrated circuit chip with silicone encapsulated packaging. The interconnection between the chip and electrodes used Au wire and Al bonding pad. The prototype devices were tested in a liquid water. This water environment is not a common reliability test for commercial electronic packaging. Relative humidity testing is a common and widely used testing methods, but the visual prosthesis device cannot be applied to gas phase relative humidity testing due to the environment condition. Water absorption in polymeric material in the liquid water is higher than in the vapor water. After water penetrated interconnection interface, the Au-Al intermetallic compound becomes oxidized and generates bonding die open failure. From these unusual testing results, we were conformed the failure mechanism and predicted the lifetime using Au-Al imtermetallic growth pattern and oxidation. Additional discussions include transition metal ions in CSF to expect the other failure mechanisms. Wafer and packaging level xiii failure mechanisms by Cu and Fe ions are also discussed in this thesis. The end of this thesis discuss possible fabrication processing to protect overall external environment effects for polymeric packaging visual prosthesis. Overall this thesis study, which uses polymeric packaging electronic devices for bio-implant research, concluded that devices may not make good reliability devices in the CSF environment. The corrosion, oxidation and metallization on or in metal surfaces and interconnection interfaces should continually be studied to produce longer lasting electronic devices over 30 years. The reliability of the visual prosthesis has not been well studied in any other professional research labs, so this thesis may give or address some initial guidelines to help save time for the project decision in the future to develop advanced hermetic packaging for bioimplantable electronic devices.
Ph.D. in Materials Science and Engineering, July 2011
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