Theses and Dissertations - Department of Mechanical Engineering

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    Understanding the Process-Structure-Property Relationships of High Strength Aerospace Alloys Processed Via Additive Friction Stir-Deposition
    (University of Alabama Libraries, 2020) Avery, Dustin Zane; Jordon, James B.; Allison, Paul G.; University of Alabama Tuscaloosa
    Additive manufacturing has emerged as the leading forefront alternative technology for fabricating and repairing complex geometry aerospace components. However, a majority of the additive processes are fusion-based, which can create underachieving mechanical responses from materials that are susceptible to hot cracking and phase transformations. A solid-state severe deformation-based additive manufacturing process, Additive Friction Stir-Deposition (AFS-D), offers an innovative solution and a new path to fabricate or repair components to achieve fully-dense depositions with wrought-like mechanical performance. In this work, the process-structure-property relationships will be quantified, through extensive characterization of the microstructural evolution and mechanical response of IN625, a fabricated free-standing deposition of AA7075, and lastly, repaired AA7075 plate additively repaired through the AFS-D process. To quantify the fatigue behavior of the as-deposited IN625, stress-life experiments were conducted, where improved fatigue resistance was observed compared to the feedstock. Post-mortem analysis of the as-deposited IN625 revealed a similar fatigue nucleation and growth mechanism to the feedstock for most of the specimens. Lastly, a microstructure-sensitive fatigue life model was utilized to elucidate structure-property fatigue damage mechanisms. The microstructural characterization of the as-deposited AA7075 employed optical, scanning electron microscope, and electron backscatter diffraction. The as-deposited AA7075 exhibited a refinement of the constituent particles and grains within the microstructure. Additionally, to quantify the fatigue behavior of the as-deposited AA7075, strain-life experiments were conducted, where a reduction in fatigue resistance was observed compared to the heat-treated feedstock. Post-mortem analysis of the as-deposited AA7075 revealed a change in the fatigue nucleation and growth mechanisms compared to the control feedstock. Lastly, a microstructure-sensitive fatigue life model was employed to capture the fatigue life for the first time in AFS-D aluminum alloys. In this work, we quantify the fatigue performance of repaired AA7075. Simulated crack repair was carried out by machining a rounded groove into a plate, which was then additively repaired using the AFS-D process. An extensive microstructural characterization of as-deposited and heat-treated conditions was conducted to elucidate the microstructural evolution of the repaired plate. Additionally, the mechanical performance of the heat-treated repair was then quantified, as well as the fatigue performance, and fatigue crack initiation mechanisms.
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    Using Skin-Mounted Microphones for Reconstructing in Vivo Interskeletal Forces: A Feasibility Study
    (University of Alabama Libraries, 2020) McChristian, Brandon; Shepard, William S; University of Alabama Tuscaloosa
    This study investigates the feasibility of using a non-invasive external measurement technique utilizing microphones on the surface of human skin to enable characterization of forces transferred through bone structures during natural motion. While the measurement of forces within the skeleton during human movement is of great interest to researchers and clinicians alike, the requirement for non-invasive sensors does not allow for direct measurement of these forces. The conventional inverse dynamical method of determining internal bone-on-bone forces in biomechanics can be limiting due to the necessity of performing measurements in a laboratory environment and the reliance on a link-segment model, which tends to propagate error. The novel method investigated in this study involves the measurement of pressure waves that propagate through human soft tissue during dynamic loading of the skeletal frame. This research uses a simplified anatomical test specimen consisting of a hollow aluminum bar cast in ballistic gelatin, representing a femur and the surrounding soft tissues, to experimentally examine the feasibility of this new measurement technique. In these tests, an impact force representing interskeletal forces is applied to the bar with an impact hammer and surface-mounted electret microphones measure the resulting pressure waves transferred to the surface of the ballistic gel. Feasibility of the measurement technique was determined by applying least squares regression fits to measured acoustic autospectral data treated as a function of impact force characteristics in the time-domain. The acoustic autospectral amplitudes and energy in frequency bands were found to be highly correlated with both the peak impact force and impulse. Ultimately, results show that the measurement technique is feasible, thus providing a motivation for the development of more advanced inverse methods utilizing this measurement technique.
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    Short-Pulse Welding Technique for Resistance Spot Welding of Aluminum Alloy 6016-T4
    (University of Alabama Libraries, 2020) Schulz, Eric; Balasubramanian, Bharat; Brewer, Luke N.; University of Alabama Tuscaloosa
    The objective of this research is to develop and characterize the use of the short-pulse welding approach for the resistance spot welding (RSW) process on aluminum alloy 6016-T4. Aluminum alloys are increasingly used in automobile body-in-white production concepts to address weight and fuel efficiency requirements. RSW remains a preferred method of joining due to speed, existing equipment, maintenance experience, low cost, and absence of an added joining element. The purpose of the short-pulse welding technique is to improve the consistency of weld quality and to reduce undesired defects including rapid degradation of the welding electrodes, poor surface appearance of the joint, and expulsion of molten material from the welding zone. Additionally, the technique provides significant energy and process time savings to enable a more efficient and flexible application in production. In this work, RSW process simulations and experimental welding tests using two medium frequency direct current (MFDC) welding systems were used to develop the approach. The effects of RSW process parameters on the size of the fusion zone were studied, and it was found that short welding times could be used with the same current and force levels to produce the same welding result. In addition, the use of sharp, rectangular shaped pulses led to more effective nugget development due to more efficient heating of the fusion zone. Further analysis evaluated the electrode wear process for short-pulse RSW of this alloy and the benefits obtained by reduction of pulse width. Finally, combined RSW simulations and experiments examined the solidification and development of fusion zone microstructure using the short-pulse welding approach.
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    Kinetic and Hydrodynamic Simulations of Laser Ablation and Plasma Plume Expansion Induced by Bursts of Short Laser Pulses
    (University of Alabama Libraries, 2020) Ranjbar, Omid A.; Volkov, Alexey N.; University of Alabama Tuscaloosa
    Ablation of materials by nanosecond laser pulses involves expansion of a laser-induced vapor plume into a background gas. The absorption of the incident laser radiation by the plume can substantially decrease the amount of laser energy absorbed directly by the target, and, correspondingly, the amount of the ablated material. This plasma shielding effect limits the overall efficiency of industrial laser systems designed for material removal applications. The goal of the present work is to numerically study the expansion process of plumes induced by irradiation of a metal target by bursts or groups of nanosecond laser pulses and to reveal the implications of the interaction between plumes induced by individual pulses for the efficiency of material removal. The plume expansion induced by irradiation of a copper target in argon background gas is studied based on one- and two-dimensional hybrid computational models that include a hydrodynamic or kinetic model of plasma plumes. The hydrodynamic model is based on finite-difference solution of gas dynamics equations. The kinetic model is implemented in the form of the direct simulation Monte Carlo (DSMC) method. In this work, the generalization of the DSMC method for plasma flows is developed. The effects of laser fluence, spot size, inter-pulse separation, and background gas pressure are thoroughly studied. The numerical simulations of plume expansion induced by a burst of pulses indicate the formation of complicated flow structures with cascades of the primary and secondary shock waves and strong interaction between plumes induced by individual pulses. The simulations reveal the plume accumulation effect when the plumes induced by preceding pulses in a burst change conditions of propagation of plumes generated by subsequent pulses. The degree of plasma shielding increases with increasing number of laser pulses due to the plume accumulation effect. It results in reduction of the effectiveness of material removal by the subsequent pulses. The degrees of the plasma shielding and plume accumulation effects strongly depend on the inter-pulse separation and laser spot size. The trade-off between the plume accumulation and thermal accumulation effects maximizes the ablation depth per pulse at a certain value of the time delay between pulses.
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    Towards Understanding the Effects of Current Input on Fatigue Mechanisms in Resistance Spot Welding of Advanced High Strength Steels
    (University of Alabama Libraries, 2020) Cleek, Conner; Jordon, Brian; Allison, Paul; University of Alabama Tuscaloosa
    In this study, the relationship of welding parameters to fatigue mechanisms is examined in spot welding of advanced high strength steels. Lightweighting efforts in the automotive industry are part of a push for greater fuel economy and improved consumer safety. TBF-1180 is an advanced high strength steel being developed for use in structurally critical components, however its fatigue behavior is not well understood. Electro galvanized TBF-1180 possesses corrosion resistant properties, however the additional zinc layer allows for the possibility of zinc-penetrative liquid metal embrittlement (LME) to occur during resistance spot welding (RSW). Additionally, variations in weld input and correspondingly heat input can affect the performance of welds due to microstructural changes that occur. In this study, the effect of LME and changes in microstructure were assessed in separate experiments for their fatigue impact in TBF-1180. Welds were fabricated in a traditional lap shear geometry in order to investigate the effects of LME, while an hourglass shaped cap geometry was used for welds with microstructural variation. Fatigue testing revealed that for lap-shear coupons containing LME cracks, no deleterious effect was observed. Cap geometry specimens were assessed for performance in a control and a high-current low-time condition, and a significant fatigue knockdown factor was found. Post-mortem fractography on both specimen geometries revealed that fatigue cracks initiated at the inner faying surface, regardless of the presence of LME. Finite element analysis confirmed that the LME cracks in the lap shear weld experience compressive stresses during loading, contributing to the lack of fatigue impact. Experimental conditions used for the cap geometry had lower heat input, which can result in less retained metastable austenite after welding, leading to reduced crack growth resistance. To tests the hypothesis that less retained metastable austenite after welding can cause a reduction in the number of cycles to failure in the spot weld, life prediction were made using fracture mechanics concepts coupled with reported knockdown factors on crack growth rates in relation to the amount of transformed martensite. The life predictions generated with this method strongly matched the observed fatigue behavior for the cap geometry specimens.
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    Smoothed Particle Hydrodynamic Modeling of Solid State Additively Manufactured Aluminum Alloys
    (University of Alabama Libraries, 2021) Stubblefield, George; Allison, Paul G.; University of Alabama Tuscaloosa
    Additive friction stir-deposition (AFS-D) is a nascent additive manufacturing process shown to have better mechanical properties of deposited material than conventional techniques. While significant experimental research has been conducted on AFS-D, relatively little computational research exists for AFS-D. Simulating AFS-D is challenging because traditional finite element approaches fail to accommodate severe deformation. One solution is to use a meshfree framework, such as smoothed particle hydrodynamics (SPH), which better handles large deformation processes. This work aims to create a meshfree framework, improve it, and utilize it to better understand AFS-D and provide predictive power to improve AFS-D processing.Firstly, a meshfree framework was laid out to describe the underlying mechanics and SPH equations. Several AFS-depositions were created while monitoring substrate temperature for use in model calibration. The meshfree framework showed good agreement with the substrate temperature and build profile results. Previously unforeseen phenomena, such as the temperature dip under the stir zone, were revealed in the simulations. Simulations also revealed the relationship between actuator feed rate and processing temperature and plastic strain. To inform future AFS-D research and developments with the meshfree framework, a study was undertaken to compare constitutive models for AFS-D simulations. Two different AA6061 tempers were considered for this study: T6 and O. The constitutive models were calibrated against experimental torsion data at a variety of strain rates and temperatures. Constitutive model selection was found to have a major impact on simulation peak values, temperature, stress, strain, and build profiles. Finally, the meshfree framework was then applied for particle tracking analysis. Two types of depositions were created: one using an anodized feedstock to track oxide distribution in the deposition, which is analogous to material flow from the outside of the feedstock, and one using a copper wire core feedstock to track copper distribution in the deposition, which is analogous to material flow from the inside of the feedstock. Results revealed the tendency of oxides to flow to the retreating side. The copper wire was mainly deposited in a clear line on the advancing side, with some fragments scattered through the deposition. Unique insight into material flow behavior was illustrated with the meshfree framework.
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    Process-Structure-Property-Performance Relationships of Precipitate- and Strain-Hardened Aluminum Alloys as Processed Through Solid-State Additive Manufacturing Process
    (University of Alabama Libraries, 2021) Beck, Sadie Cole; Jordon, J. Brian; Allison, Paul G.; University of Alabama Tuscaloosa
    Additive manufacturing provides alternatives to traditional manufacturing methods. Equipment footprint, energy use, maintenance considerations, component geometries and material selection are all being reconsidered on the rise of additive manufacturing. Aluminum alloys are of particular interest in the additive manufacturing realm because of their strength-to-weight ratio, general availability, and performance in austere environments. However, it’s critical that the strengthening mechanisms that make aluminum alloys so desirable are preserved post additive processing. Additive Friction Stir Deposition (AFSD) is a novel additive manufacturing process that utilizes solid-state plastic deformation to create near-net shaped, layered depositions. Because the process is still being developed, the microstructural and mechanical performance of deposited aluminum alloys have not been fully characterized. In this work, the process-structure-property-performance of a precipitate-hardened (AA6061-T6) and strain-hardened (AA5083-H131) aluminum alloy as processed through AFSD, were quantified. A standard post deposition heat treatment (PDHT) was applied to AA6061 AFSD material, an Al-Mg-Si alloy. The as-deposited material exhibited a refined grain structure, reduced tensile strength from the heat treated feedstock, and increased elongation to failure. The PDHT AFSD material exhibited tensile properties characteristic of a T6 temper through the regrowth of strengthening precipitates. The other material of interest, Al-Mg-Mn alloy (AA5083-H131), a strain-hardened alloy, was processed through AFSD using two methods of machine feeding: recycled chip and solid rod. The thermo-mechanical processing of AFSD resulted in an exchange of strengthening mechanisms – removing the wrought material of strength from strain-hardening and replacing it with grain boundary strengthening. The monotonic tensile results demonstrated a reduced yield strength and comparable elastic modulus and ultimate tensile strength to the AA5083-H131 wrought control. The fatigue results demonstrated comparable fatigue performance, primarily between the recycled chip feedstock and wrought AA5083-H131. A strength model and a multistage fatigue model were employed to capture the tensile and fatigue performance for AFSD AA5083. Dynamic compression testing was performed using a Split-Hopkinson pressure bar to quantify strain rate dependence. Experiments reveal that the flow stress of AA5083-H131 and AA5083 AFSD are dependent on the strain rate under compression loading. Furthermore, resulting mechanical performance was captured by the internal state variable (ISV) plasticity-damage model.
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    Synthesis and Analysis of Tensegrity Mechanisms
    (University of Alabama Libraries, 2021) Gotberg, Clayton Russell; Vikas, Vishesh; University of Alabama Tuscaloosa
    Tensegrity systems combine antagonistic tensile and compressive members. The computational cost of synthesis and form-finding greatly increases with the complexity of a tensegrity structure. One solution, the use of modules which can be combined to create a more complex structure (tensegrity primitives), has been investigated using highly symmetric primitives of a few canonical types. This thesis determines the multistable region of an example planar tensegrity-adjacent mechanism, measures the feasibility of various parameters for shape control and demonstrates a method for stacking any number of such mechanisms, then develops a search method which exhaustively determines all possible tensegrities with a specified cable graph and with varying additional constraints. To demonstrate the viability of these methods, full catalogues of tensegrity primitives for a host of simple cable graphs are generated.
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    Microstructural and Mechanical Properties of a Solid-State Additive Manufactured Magnesium Alloy
    (University of Alabama Libraries, 2021) Robinson, Thomas Wilkes; Allison, Paul G.; Jordon, James B.; University of Alabama Tuscaloosa
    In recent years, additive manufacturing (AM) has gained prominence in rapid prototyping and production of structural components with complex geometries. Magnesium alloys, whose strength-to-weight ratio is superior compared to steel and aluminum alloys, have shown potential in lightweighting applications. However, commercial beam-based AM technologies have limited success with magnesium alloys due to vaporization and hot cracking. Therefore, as an alternative approach, we propose the use of a near net-shape solid-state additive manufacturing process, Additive Friction Stir Deposition (AFSD), to fabricate magnesium alloys in bulk. In this study, a parametric investigation was performed to quantify the effect of process parameters on AFSD build quality including volumetric defects and surface quality in magnesium alloy AZ31. In order to understand the effect of the AFSD process on structural integrity in the magnesium alloy AZ31, in-depth microstructure and mechanical property characterization was conducted on a bulk AFSD build fabricated with optimized process parameters. Results of the microstructure analysis of the as-deposited AFSD build revealed bulk microstructure similar to wrought magnesium alloy AZ31 plate. Additionally, similar hardness measurements were found in AFSD build compared to control wrought specimens. While tensile test results of the as-deposited AFSD build exhibited a 20 percent drop in yield strength, nearly identical ultimate strength was observed compared to the wrought control. The experimental results of this study illustrate the potential of using the AFSD process to additively manufacture Mg alloys for load bearing structural components with achieving wrought-like microstructure and mechanical properties.
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    Mesoscale Structural and Mechanical Simulations of Cross-Linked Carbon Nanotube Materials
    (University of Alabama Libraries, 2021) Banna, MD Abu Horaira; Volkov, Alexey N.; University of Alabama Tuscaloosa
    Relatively poor mechanical properties of carbon nanotube (CNT) bulk materials can be improved by formation of bonds or covalent cross-links (CLs) between nanotubes. In this work, an “effective bond model” of covalent CLs between carbon nanotubes is developed for mesoscopic simulations of cross-linked CNT materials. A general approach for fitting the CL model parameters based on results of atomistic simulations is developed. The best-fit parameters of the CL model are found. The developed effective bond model of CLs is included into a dynamic mesoscopic model of CNT materials, where each nanotube is represented in the form of a chain of stretchable cylindrical segments. The mesoscopic force field in this model accounts for stretching and bending of CNTs, van der Waals interaction between nanotubes, and inter-tube CLs. The model is applied to generate and equilibrate in silico pristine and cross-linked CNT fiber and film samples with structural characteristics close to observed in experiments. The structural parameters of CNT fibers and films, including the average bundle size, Herman orientation factor, and tortuosity, are calculated. The quasi-static simulations of large-scale cross-linked CNT films are performed to reveal the load transfer mechanism, as well as effects of CNT length, CL density, material density, and network morphology on mechanical properties under conditions of quasi-static deformation. It is found that stretching of CNT segments is the dominant mode of load transfer in cross-linked CNT film during their stretching, while bending and buckling is the dominant mode of load-transfer during compression. Both tensile modulus and strength of CNT films increase strongly with increasing CNT length. The effect of the nanotube length on mechanical properties, however, is altered by the density of CLs. The mutual effect of the nanotube length and CL density on modulus and strength is described by power scaling laws, where the modulus and strength are functions of the average number of CLs per nanotube, i.e., the product of the CNT length and CL linear density. The exponents in the scaling laws for the modulus and strength are strongly different from each other. The material density of the film samples weakly affects the specific mechanical properties. The dispersion of nanotubes in the films without formation of thick bundles results in the few-fold increase of the modulus and strength. In qualitative agreement with experimental observations, the in-plane compression of a large thin CNT film results in collective bending of nanotubes and folding of the whole film with minor irreversible structural changes. Depending on the CNT length, the reliefs of the compressed films vary from quasi-one-dimensional wavy surface to complex two-dimensional landscape.
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    SLS Production Friction Stir Plugs by Additive Friction Stir Deposition Aluminum 2219
    (University of Alabama Libraries, 2021) Anderson, Kathryn; Daniewicz, Steve; Amaro, Robert; University of Alabama Tuscaloosa
    The self-reacting friction stir welding (SR-FSW) method is extensively used in NASA’s current generation rocket, the Space Launch System (SLS). The initialization and termination of welds created by the SR-FSW process produce holes resulting from the removal of the weld tool assembly. These holes must subsequently be filled by the use of a separate process. These holes pose a mission-critical engineering challenge in the production of the SLS rocket. The current method for sealing the holes is the Friction Pull Plug Welding (FPPW) process, where a conical piece of material is spun and plunged into the remaining hole. The solid-state additive friction stir-deposition (AFS-D) process can create pull plugs with tailored microstructures that can increase the reliability of the current FPPW method.This work furthers the understanding of using AFS-D AA2219 material as a replacement for the material currently being used in the FPPW method. The impacts of this research are as follows: 1. The ability for NASA to predict the deformation response of AFS-D AA2219 material produced by any process parameter set intended for use in the SLS 2. An understanding of the effects of the AFS-D process on AFS-D AA2219, including deformation response, precipitation hardening effects, and cyclic material properties 3. An increased reliability in the plug/plate assembly because of more consistent properties between the base material and FPPW, enabling the SLS to fly. This is achieved through the creation and calibration of a micromechanical model that captures the effects of microstructure on the deformation response of AFS-D pull plugs as a function of the manufacturing process parameters. Ultimately, this work will provide the SLS engineering team with the necessary information to support a change in the FPPW process which will reduce the time of construction by mitigating the need for FPPW repairs.
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    Analysis and Elicitation of Electroencephalogram Data Pertaining to High Alert and Stressful Situations: Source Localization Through the Inverse Problem
    (University of Alabama Libraries, 2021) Heim, Isaac C; Fonseca, Daniel J.; University of Alabama Tuscaloosa
    This dissertation work deals with the design and development of a fuzzy controller to analyze electroencephalogram (EEG) data. The fuzzy controller made use of the multiple functions associated with the different regions of the brain to correlate multiple Brodmann areas to multiple outputs. This controller was designed to adapt to any data imported into it. The current framework implemented supports a math study and a police officer study. The rules for the interactions of the Brodmann areas have been set up for these applications, detailing how well the police subjects’ brains exhibited behavior indicative to activation relating to vision, memory, shape/distance, hearing/sound, and theory of mind. The math subjects’ outputs were attuned to their related study which involved transcranial direct current stimulation (tDCS), which is a form of neurostimulation. Anode affinity, cathode affinity, calculation, memory, and decision making were the outputs focused on for the math study. This task is best suited to a fuzzy controller since interactions between Brodmann areas can be analyzed and the contributions of each area accounted for.The goal of the controller was to determine long-term behavior of the subjects with repeated sampling. With each addition of data, the controller was able to develop new bounds related to the current condition of the data in the study. Processing this data was accomplished by the creation of an automated filtering script for EEGLAB in MATLAB. The script was designed to rapidly load and filter the files associated with any given dataset. These files were also automatically prepared for analysis with a program called Low Resolution Brain Electromagnetic Tomography i.e. (LORETA). LORETA was used to solve the inverse problem, which involves identifying where the signals from the surface electrodes originated within the brain through a process called source localization. Once the sources of the EEG signals were located, they were associated with the Brodmann areas. The fuzzy controller then processed this information to automatically generate heat maps which displayed information such as normalized data, z-score, and rankings. Each set of scores displays how the subject's brain was acting, which lined up with the expected results.
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    Low-Temperature Plasma Enhanced Chemical Looping System
    (University of Alabama Libraries, 2021) Varadarajan Ranganathan, Rajagopalan; Uddi, Mruthunjaya; Wang, Ruigang; University of Alabama Tuscaloosa
    Carbon Capture Utilization and Storage (CCUS) is considered a key technology in reducing carbon footprint across the globe. In this thesis, Chemical Looping System is used to implement CCUS in converting greenhouse gas emissions such as carbon dioxide and methane to value-added chemicals. The industrial processes to produce chemicals generally work at higher temperatures. To reduce the operating temperature, the critical area of the study approached in this thesis are nanomaterials and plasma assistance. By using advanced material preparation methods, nanomaterials are produced in this thesis to enhance the catalytic activity. Plasma plays a significant role in enhancing the reaction by breaking down the input molecules into ions, electrons, radicals, vibrationally, rotationally, and electronically excited molecules. Plasma creates a synergistic effect by interacting between the catalyst and input gases. The first part of the thesis is concentrated on the experimentation of a plasma chemical looping system using different oxygen carrier material for the redox process involving dry reforming and water splitting. To begin with, an experimental setup is developed and a Quadrupole mass spectrometer is applied to understand the time-based evolution of various product species during these processes. Significant production of chemicals of interest such as syngas and hydrogen at lower temperatures is demonstrated. Finally, the characterization of materials studied using different techniques. In the second part of the thesis, to understand the critical plasma parameter for chemical looping experiment, which is temperature, an ab-initio setup is developed. Optical techniques such as Rayleigh scattering and optical emission spectroscopy are used to conduct a parametric investigation in a two-dimensional plane. The results obtained are used to understand the distribution of temperature in the plasma during the reduction and the oxidation step.
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    Coupled Flexural-Torsional Vibration Analysis of a Double-Cantilever Structure for Nanomaching Application
    (University of Alabama Libraries, 2021) Zargarani, Anahita; Mahmoodi, Nima; University of Alabama Tuscaloosa
    This dissertation aims to investigate the coupled flexural-torsional vibrations of a piezoelectrically-actuated double-cantilever structure for nanomachining applications. The structure of interest consists of two identical Euler-Bernoulli cantilever beams connected by a rigid tip connection at their free ends. The double-cantilever structure in this study vibrates in two distinct modes: flexural mode or coupled flexural-torsional mode. The flexural mode refers to the in-phase flexural vibrations of the two cantilever beams resulting in transverse motion of the tip connection, while the coupled flexural-torsional mode refers to the coupled flexural-torsional vibrations of the cantilever beams resulting in the rotational motion of the tip connection. The latter is the main interest of this research. The governing equations of motion and boundary conditions are developed using Hamilton’s principle. Two uncoupled equations are found for each beam: one corresponding to the flexural vibrations and the other one corresponding to the torsional vibrations of the cantilever beam. The characteristic equations for both the flexural and the coupled flexural-torsional vibration modes are derived and solved to find the corresponding natural frequencies. The orthogonality condition among the mode shapes is derived and utilized to determine the modal coefficients corresponding to each mode of vibration. The time response to the forced vibrations of the structure is found using the Galerkin approximation method. The effects of the dimensional parameters, including the length of the cantilever beams and the length of the tip connection, and the piezoelectric input voltage on the natural frequencies and the amplitude of vibrations of the structure are analyzed. An experimental setup consisting of a piezoelectric double-cantilever structure is designed and utilized to verify the analytical results. First, the coupled flexural-torsional fundamental frequencies of the structure with various configurations are obtained experimentally, which are in good agreement with the analytically-determined values. Moreover, the experimental results verify the analytical results stating that the natural frequencies of the structure decrease as either the length of the cantilever beams or the length of the tip connection is increased. Next, the amplitudes of the coupled flexural-torsional vibrations of different configurations of the structure excited at their natural frequencies with a range of input voltages are obtained. The results of the effect of the dimensional parameters and the piezoelectric input voltage on the angle of rotation of the tip connection are presented.
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    Design and Modeling of a Cable-Driven System for Delivering Balance Perturbations
    (University of Alabama Libraries, 2021) Sung, Gugyeong; Martelli, Dario; University of Alabama Tuscaloosa
    This thesis explores the design and modeling of a cable-driven system able to provide balance perturbations for rehabilitation purposes. These balance perturbations can be used for both rehabilitation and prevention of falls. Many current rehabilitation strategies focus on exoskeletal designs with rigid links. These systems are costly, can inhibit subjects' movements, and can induce subjects to make less effort by not challenging balance enough in an effective way. Cable-driven rehabilitation is an alternative solution that can be more cost-effective as well as addressing some of the issues inherent in previous systems.The cable-driven system developed in this thesis provides balance perturbations with a stepping force in one direction via a DC motor that is connected through a cable and a load cell to a harness worn by the subject. Before testing on human subjects, the motor was tested by attaching the system to a heavy object to measure the force of the stall torque during various duty cycles. Through this testing, the motor was found to provide perturbations of up to a maximum of approximately 100 N within 130 milliseconds. Once the force of the stall torque was determined, the system was tested on eight healthy adults with the harness secured to the participants waist and the cable pulling for 0.5 seconds in the posterior direction parallel to the floor. When tested on human subjects, the average measured force was up to approximately 17% lower than the desired force, but the control panel can be recalibrated according to the force measured for improved accuracy in the future. Overall, the system was shown to be a successful method for providing waist-pull force perturbations to human subjects.
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    Exploration of high efficiency pathways in dual fuel low temperature combustion engines
    (University of Alabama Libraries, 2020-12) Jha, Prabhat Ranjan; Srinivasan, Kalyan; University of Alabama Tuscaloosa
    It's crucial to use advanced combustion strategies to increase efficiency and decrease engine-out pollutants because of the compelling need to reduce the global carbon footprint. This dissertation proposes dual fuel low-temperature combustion as a viable strategy to decrease engine-out emissions and increase the thermal efficiency of future heavy-duty internal combustion (IC) engines. In dual fuel combustion, a low reactivity fuel (e.g. methane, propane) is ignited by a high reactivity fuel (diesel) in a compression-ignited engine. Generally, the energy fraction of low reactivity fuel is maintained at much higher levels than the energy fraction of the high reactivity fuel. For a properly calibrated engine, combustion occurs at lean and low-temperature conditions (LTC). This decreases the chances of the formation of soot and oxides of nitrogen within the engine. However, at low load conditions, this type of combustion results in high hydrocarbon and carbon monoxide emissions. The first part of this research experimentally examines the effect of methane (a natural gas surrogate) substitution on early injection dual fuel combustion at representative low loads of 3.3 and 5.0 bar BMEPs in a single-cylinder compression ignition engine (SCRE). Gaseous methane fumigated into the intake manifold at various methane energy fractions was ignited using a high-pressure diesel pilot injection at 310 CAD. Cyclic combustion variations at both loads were also analyzed to obtain further insights into the combustion process and identify opportunities to further improve fuel conversion efficiencies at low load operation. In the second part, the cyclic variations in dual fuel combustion of three different low reactivity fuels (methane, propane, and gasoline) ignited using a high-pressure diesel pilot injection was examined and the challenges and opportunities in utilizing methane, propane, and gasoline in diesel ignited dual fuel combustion, as well as strategies for mitigating cyclic variations, were explored. Finally, in the third part a CFD model was created for diesel methane dual fuel LTC. The validated model was used to investigate the effect of methane on diesel autoignition and various spray targeting strategies were explored to mitigate high hydrocarbon and carbon monoxide emissions at low load conditions.
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    Fundamental characterization of the additive friction stir-deposition process via two commercial aluminum alloys
    (University of Alabama Libraries, 2021) Phillips, Brandon James; Allison, Paul G.; Jordon, James B.; University of Alabama Tuscaloosa
    Additive manufacturing is a rapidly growing industry with numerous technologies to suit a variety of applications. However, each application has its own inherent flaws and niches. Aluminum alloys are difficult for the more popular fusion-based additive manufacturing techniques due to the intense thermal gradients generating distortion and selective vaporization of alloying elements. To subjugate some of the issues, the solid-state Additive Friction Stir-Deposition (AFS-D) method was proposed to produce high quality, defect free aluminum deposits. This work investigates the process-structure-property relationships of two popular commercial aluminum alloys employed extensively by consumer, transportation, and defense industries. The first work on process-deformation characteristics of AA6061 were evaluated by producing microhardness profiles taken from the cross-section of builds to produce relationships between mechanical characteristics and machine parameters. Resulting average hardness values were plotted against the processing window and used to determine comparative samples for microstructural analysis. Electron backscatter diffraction and transmission electron microscopy was conducted to characterize the microstructural evolution of depositions. This study provides a succinct, multiscale characterization of as-deposited AFS-D AA6061 to expound the effect of the high-shear solid-state AM process. The subsequent investigation on AA6061 is the first investigation of the process-structure-property relationships of AFS-D in an overlapping, parallel raster deposit. In particular, the deposit produced in this work explores the influence of severe plastic deformation on the as-deposited microstructure and tensile response of material that overlaps in parallel layers at the outer edge of the tool. This study sought to determine the viability of producing large scale structural components larger than the track-width of the AFS-D tool. The final study quantifies the microstructural evolution and consequential tensile response of AA5083. A brief examination of the effect of AFS-D processing parameters was undertaken to determine preferential processing conditions for a larger, free-standing AA5083 structure. Optical and scanning electron microscopy evaluate the microstructural evolution of particles and grain morphology. Tensile properties were evaluated in the longitudinal and vertical build directions, and subsequent fractography discovered the influence of lubrication on the variable mechanical response.
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    Investigation of turbulent jets, sprays, and supercritical mixing by time resolved rainbow schlieren deflectometry
    (University of Alabama Libraries, 2020-12) Wanstall, Christopher Taber; Bittle, Joshua A.; Agrawal, Ajay K.; University of Alabama Tuscaloosa
    As technological advancements have enabled higher efficiency compression ignition (CI) engines (diesel engines) by operating at higher pressures and temperatures, the mechanisms of fuel-air mixing are postulated to change. Thus, there is a greater need for experimental data to develop and verify new phenomena. However, the fast time scales and harsh environments associated with CI engines present many experimental difficulties and requires high quality non-intrusive diagnostics to obtain useful data. The diagnostics itself must be validated prior to its implementation in an engine environment. The purpose of this research is to advance rainbow schlieren deflectometry (RSD) to study turbulent fuel sprays including the effects of supercritical mixing in CI engine environments. This objective is met by developing and applying quantitative RSD, for the first time, to three separate aspects of fuel sprays: 1) turbulent mixing, 2) phase boundaries, and 3) non-ideal gas mixing. First, quantitative RSD is demonstrated for turbulent mixing in a canonical helium jet and validated with Rayleigh scattering data in the literature. Second, RSD is applied to high pressure multi-phase fuel sprays, and new methodologies are developed to distinguish the liquid region from the vapor region and to quantify the in-between region on a probabilistic basis. The results of this study demonstrate that RSD can be used as a single diagnostic to measure both liquid and vapor boundaries. Third, the optical-to-thermodynamic relations associated with refractometry are investigated under non-ideal gas mixing conditions to develop a generalized relationship valid for both ideal or non-ideal gas mixing. The last part of this research develops the theoretical tools to understand and experimentally realize supercritical mixing at diesel conditions. Guided by the theoretical analysis, RSD is applied to compare supercritical versus transcritical mixing at CI conditions using a constant pressure test rig. It is concluded that fully supercritical mixing offers several benefits in diesel engines including faster mixing times (50% higher velocities) and the absence of droplets which cause high levels of particulate emissions. However, achieving fully supercritical mixing is difficult in practice because of the requirements of fuels with low critical temperature and/or substantial fuel preheating.
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    Heat conduction using green’s functions: partial pipe heating, a novel numerical method, and inverse heat conduction
    (University of Alabama Libraries, 2020-12) Samadi, Forooza; Woodbury, Keith K. W.; University of Alabama Tuscaloosa
    Finding the unknowns in directly inaccessible areas is one of the challenging problems in engineering applications. The fluid flow or thermal conditions of the flow inside the pipe are examples of such unknowns. Developing analytical models that provide explicit mathematical formulas is a mathematically efficient and desirable way of dealing with these challenges. This dissertation focuses on developing an analytical solution that can find the temperature distribution in radial and axial directions in the pipe wall. Partial heating along with Green’s functions is used to develop this solution. Working with Greens functions sparked the idea of using them as building blocks for a novel numerical method for solving the heat conduction equation. In the second chapter, the transient temperature response inside the pipe to the partial heating on it is found using GFs and the solution is verified using a few intrinsic verification principles. This analytical solution, then, is used in developing a non-invasive method for measuring the flow rate in pipes by measuring the temperature at a single point. Optimal experiment design methods are used to find the optimal location and time duration for performing the measurement. Chapter 3 is about developing a novel numerical solution to the linear heat conduction equation. This method uses superposition of exact solutions (SES), obtained using GFs, to evaluate the temperature and heat flux at any point of the 1-D domain. The SES method is not sensitive to the size of the grids and is much more accurate than the conventional Crank Nicolson (CN) method. This method is extended to the cases in which the thermal properties vary with temperature, later, in Chapter 4. The results confirm the grid independence of the SES and show high accuracy in its prediction when the time step is not large. One of the applications of the analytical solution obtained herein is using them in solving inverse problems. Inverse heat conduction problems (IHCPs) are used to estimate unknown heat flux functions through measuring temperatures far from active surfaces. The second part of this dissertation (Chapters 5 and 6) focuses on generalizing and optimizing two IHCP solution methods.
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    Towards elucidating the process-structure-performance relationships of lightweight structural alloys
    (University of Alabama Libraries, 2020-12) Rutherford, Benjamin Andrew; Jordon, James B.; Allison, Paul G.; University of Alabama Tuscaloosa
    Additive manufacturing processes have become a leading technology for research innovation. Additive manufacturing offers the capacity to fabricate complex, near net shape components and the possibility to repair existing components. The vast majority of additive processes are fusion-based, relying on melting and solidification, which can lead to poor mechanical performance due to intense thermal gradients leading to solidification cracking and columnar dendritic grain growth. Additive Friction Stir-Deposition (AFS-D) is a novel technique that implements solid-state severe deformation to create depositions additively. As such, the AFS-D process offers potential to fabricate fully dense components with wrought-like mechanical performance and microstructure. Likewise, AFS-D avoids the intense thermal gradients of fusion welding that leads to solidification cracking in lightweight materials in aerospace applications, such as aluminum alloys. In this research, the process-structure-property relationship is quantified by means of microstructure characterization and mechanical evaluation of AFS-D AA6061. To understand the process-structure-property relationships of AFS-D as-deposited AA6061, test specimens in two orthogonal directions, longitudinal and build, were subjected to quasi-static monotonic tension and strain-controlled fatigue testing. Microstructural evaluation revealed the refinement of constituent particles in AFS-D AA6061, in addition to dynamic recrystallization and grain refinement. Mechanical results indicated homogeneous strength between the two directions investigated at a similar strength to wrought AA6061-O, and fatigue performance similar to the wrought in the longitudinal direction. Microstructural examination of a standard heat treatment for the T6 temper of AA6061 on AFS-D AA6061 was conducted. This led to mechanical performance superior in strength to the control wrought AA6061-T651 in monotonic tension tests, and similar fatigue performance to the as-deposited AFS-D AA6061 and wrought AA6061. Fractography revealed an evolution in the deformation behavior for post deposition heat treated AFS-D AA6061 compared to the as-deposited specimens. Lastly, the mean strain effects of heat treated AFS-D AA6061 and a lightweight rolled aerospace aluminum alloy, AA2099-T83, are quantified and captured in this work. The tensile stable cycle mean stress proved detrimental to the fatigue performance of aluminum alloys. A modified strain-based Morrow model is proposed in this work that successfully captures the effect of tensile mean strain loading conditions on these aluminum alloys.