Theses and Dissertations - Department of Aerospace Engineering and Mechanics

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Generalized Targeting Parameters for the Guidance of Aerospace Vehicles
(University of Alabama Libraries, 1968) McCraney, Richard Marvin; University of Alabama Tuscaloosa
This thesis presents the development of the equations necessary to compute the targeting parameters for the iterative guidance mode for all types of space missions. A brief introduction to the iterative guidance mode is presented as well as the derivation and explanation of the necessary equations to compute the targeting parameters. Experimental verification of the equations is also included.
A study of trailing-edge scalloping on flat plate-membrane airfoil performance
(University of Alabama Libraries, 2009) Hicks, Travis; Hubner, James Paul; University of Alabama Tuscaloosa
The objective of this investigation was to study how membrane scalloping affects the aerodynamic performance of flat, membrane wings at low Reynolds numbers. The onset of membrane vibration was also studied to determine its effects on the aerodynamic performance. This study was performed to improve the efficiency of MAVs (micro air vehicles). The removal of some of the membrane at the trailing edge of the airfoil, which has the highest amplitude of vibration, could significantly decrease the drag without overly affecting the lift. The plates studied were all flat plates with moderate aspect ratio and a repeating cell structure of latex membranes. This allowed the effects of the membrane to be studied with minimal effects from the tip vortices as long as the data was acquired behind the center cell. The onset of membrane vibration was studied using hot-wire anemometry, and the amplitude and frequency of membrane vibration were measured directly using a laser vibrometer. Through a momentum deficit analysis, local drag coefficients were obtained. These results were compared to direct measurements of lift and drag from external balance testing to determine the aerodynamic efficiency. Results showed that the shape of the latex cell has a greater impact on lift than the amount of scalloping and that the amount of scalloping has a greater affect on drag than the cell shape. These effects could allow batten-reinforced membrane wings for MAVs to be more efficient with proper membrane scalloping.
Damage evolution in composite materials under environmental ageing: a stochastic model with experimental validation
(University of Alabama Libraries, 2009) Rahman, Rezwanur; Haque, Anwarul; University of Alabama Tuscaloosa
This work emphasizes on predicting probability density function of damages or "number density of damage" in graphite/epoxy polymer matrix composite materials (PMC) under hygrothermal aging condition. A coupled Forward-Backward Stochastic Differential Equation (FBSDE) is proposed as a mathematical model to predict number density of damages. The FBSDE consists of damage nucleation and annihilation rate in terms of Brownian motion. The uncertainty in damage nucleation and annihilation rate is noticed by proposing these two terms as "Brownian motion with drift". In order to verify the proposed model, a quantitative analysis was carried out on a limited number of graphite/epoxy specimens manufactured by VARTM process. The specimens were kept in a hygrothermal condition with room temperature cycling. A rigorous quantitative analysis of damages was done by optical microscopic inspection at different stages of aging period. The damages were classified based on the criteria of their size. Finally, the experimentally collected data for number density of damages were verified by using the proposed FBSDE. A detail parametric study was carried out using FBSDE and best possible predicted data was validated with the experimental observation. A reasonable estimation was observed from the model output.
Drag measurements across patterned surfaces in a low Reynolds number Couette flow facility
(University of Alabama Libraries, 2009) Johnson, Tyler Jay; Lang, Amy W.; University of Alabama Tuscaloosa
Understanding fluid dynamic drag, and its reduction, has always been a topic of primary concern. Direct drag measurements can be difficult to obtain with low viscosity fluids such as air or water. For this study a new low Reynolds number Couette facility was constructed to investigate surface drag. In this facility, mineral oil was used as the working fluid to increase the shear stress across the surface of the experimental models. A mounted conveyor creates a flow within a Plexiglas tank. The experimental model of a flat or patterned surface was suspended above the moving belt. The experimental plate was attached to linear bearings on a slide system that connects to a force gauge used to measure the drag. Within the gap between the model and moving belt a Couette flow with a linear velocity profile was created. Digital particle image velocimetry was used to confirm the velocity profile. A patterned surface, in this case consisting of 2-D cavities, was embedded into a large portion of a Plexiglas plate. First, the drag across a flat plate of the same size was measured and compared to theoretical values for laminar Couette flow. These results were found to be within 5% of predicted values following CD = 2/Re. The drag for a 2-D embedded square cavity model was then measured and compared to the flat plate results. A drag reduction of up to 20% was found for the lateral rib model at Re ~ 10 or less, with increasing drag reduction as the Re decreases. Lower drag reduction was found up to Re ~ 50, where the difference becomes negligible. Finally, the flow over the 2-D cavities was modeled with a partial slip at the bottom wall. Modeled average partial slip velocities were calculated from the drag measurements and reached values of 10-22% of the belt speed for Re < 10.
Experimental study of boundary layer flow over three dimensional arrays of embedded cavities
(University of Alabama Libraries, 2009) Melnick, Michael Blake; Lang, Amy W.; University of Alabama Tuscaloosa
It is well known that proper surface patterning on a flat plate can decrease drag. Whether it be the dimples on a golf ball or riblets on a flat plate, there are many proven ways to alter a smooth surface to decrease drag. Continuing the previous research related to biomimetic models of shark skin, this research focuses on finding similar, but simpler, geometries that could lead to a decrease in drag. This experiment studies an array of embedded hexagonal cavities on a flat plate in a water tunnel facility. The flow over the modified flat plate generates vortices in the cavities which cause a partial slip condition that can alter boundary layer flow. Fluorescent dye in the cavities was illuminated with a UV lamp to capture images of the flow inside the cavities. Digital particle image velocimetry, which uses a laser to illuminate particles in the flow and captures images with a high speed camera, was used to measure the velocity flow field over the cavities. The flow fields were analyzed for the purpose of studying velocity profiles above the cavities, measuring partial slip velocities, calculating momentum thicknesses and shape factors, as well as determining the difference in boundary layer characteristics of the hexagonal cavities at two different orientations. Finally, the Reynolds stresses over the hexagonal cavities were compared to those over the flat plate under turbulent conditions to attempt to discern the effect of orientation on turbulence augmentation. The results show that the hexagonal cavities cause the boundary layer to transition to turbulence sooner than the flow over a flat plate. Also, in a turbulent flow, the cavities bring higher momentum fluid closer to the wall compared to the flat plate case, which is a proven requirement for the prevention of boundary layer separation. The results also suggested a decrease in the cavity circulation at the downstream portion of the model; confirming this trend should be a focus of future work. If this result is indeed confirmed, only applying this pattern to local regions of incipient separation may prove to be most effective.
Effects of environmental ageing parameters on interfacial bond properties of carbon fiber tow in epoxy resin: an experimental and numerical study
(University of Alabama Libraries, 2009) Vemuganti, Shloka; Haque, Anwarul; University of Alabama Tuscaloosa
The effects of environment on the interfacial bond strength of carbon fiber tow embedded in epoxy resin are being studied through accelerated aging experiments and pull-out tests (PT). The goal is to examine the synergistic effects of hot, cold, wet, dry and stressed environment on interfacial bond strength (IFBS) of carbon/epoxy composites in order to forecast life of composites in severe environments. The PT specimens were prepared by embedding a fixed length of a carbon fiber tow in uncured epoxy. The interfacial debonding between resin and fiber tow is the targeted mode of failure. A spring loaded frame was designed for applying a preset load to multiple PT specimens. All the PT specimens including the frames with the stressed specimens were subjected to accelerated environmental aging conditions for different time intervals. The moisture absorption and desorption data were recorded. The PT tests were then carried out by pulling the carbon fiber tow from the cylindrical resin, mounted in a screw driven MTS testing machine. Average IFBS of approximately 21.5 MPa was observed for the unaged control sample. The hot/wet/stressed (70ºC/3% moisture/2 lb) specimens show a significant degradation (-29.03 %) in bond strength after 176 days of aging time. The degradation due to moisture is seen to be more critical under the influence of temperatures (50°C, 70°C). In order to obtain the maximum interfacial shear strength for various embedded lengths and environmental aging parameters, a numerical analysis has been carried out using a 3D finite element model (FEM). A FEM analysis was done using cohesive elements at the fiber/matrix interface in order to study the environmental aging effects at the interface. The predicted bond strength from the numerical analysis was compared with the experimental data. A reasonable agreement was observed between experimental data and Finite Element Analysis (FEA).
Multiscale simulation of polymer nano-composites (PNC) using molecular dynamics (MD) and generalized interpolation material point method (GIMP)
(University of Alabama Libraries, 2010) Nair, Abilash Rajendran; Roy, Samit; University of Alabama Tuscaloosa
Recent mechanical characterization experiments with pultruded E-Glass / polypropylene (PP) and compression molded E-Glass/Nylon-6 composite samples with 3-4 weight% nanoclay and baseline polymer (polymer without nanoclay) confirmed significant improvements in compressive strength (~122%) and shear strength (~60%) in the nanoclay modified nanocomposites, in comparison with baseline properties. Uniaxial tensile tests showed a small increase in tensile strength (~3.4%) with 3 wt% nanoclay loading. While the synergistic reinforcing influence of nanoparticle reinforcement is obvious, a simple rule-of-mixtures approach fails to quantify the dramatic increase in mechanical properties. Consequently, there is an immediate need to investigate and understand the mechanisms at the nanoscale that are responsible for such unprecedented strength enhancements. In this work, an innovative and effective method to model nano-structured components in a thermoplastic polymer matrix is proposed. Effort will be directed towards finding fundamental answers to the reasons for significant changes in mechanical properties of nanoparticle-reinforced thermoplastic composites. This research ensues a multiscale modeling approach in which (a) a concurrent simulations scheme is developed to visualize atomistic behavior of polymer molecules as a function of continuum scale loading conditions and (b) a novel nanoscale damage mechanics model is proposed to capture the constitutive behavior of polymer nano composites (PNC). The proposed research will contribute towards the understanding of advanced nanostructured composite materials, which should subsequently benefit the composites manufacturing industry.
Life prediction of composite materials subjected to long term mechanical/environmental loading condition
(University of Alabama Libraries, 2010) Singh, Sushil; Roy, Samit; University of Alabama Tuscaloosa
A multi-scale mechanism-based life prediction model is developed for high-temperature polymer matrix composites (HTPMC) under thermo-oxidative aging conditions. Life prediction model is based on stiffness and strength degradation in unidirectional HTPMC under accelerated thermo-oxidative aging condition. A multi-scale model based on continuum damage mechanics to predict stiffness degradation and progressive failure due to degradation of inter-laminar shear strength is developed for unidirectional composite. Using continuum damage mechanics one can relate the behavior of composites at micro-level (representative volume element) to the macro-level (structural element). Thermo-oxidative aging is simulated with diffusion-reaction model in which temperature, oxygen concentration and weight loss effects are considered. For fiber/matrix debond growth simulation, a model based on Darcy's laws for oxygen permeation in the fiber-matrix interface is employed, that, when coupled with polymer shrinkage, provides a mechanism for permeation-controlled debond growth in HTPMC. Viscoelastic regularization in the constitutive equations of the cohesive layer used in this model not only mitigates numerical instability, but also enables the analysis to follow load-deflection behavior beyond the point of peak failure load. Benchmark of model prediction with experiment was carried out to establish proof-of-concept.
Characterization and modeling of the effect of environmental degradation on flexural strength of carbon/epoxy composites
(University of Alabama Libraries, 2010) Chawla, Sneha Anil; Roy, Samit; University of Alabama Tuscaloosa
A mechanism-based modeling methodology has developed for prediction of long-term durability of composites for emerging facilities in different climatic zones. The objective of the research was to develop a predictive tool using the Arrhenius principles adapted to the TTS (Time Temperature Superposition) to measure degradation of carbon-fiber/epoxy composite under hygrothermal exposure and applied tensile stress. The hygrothermal conditions capture the synergistic effects of field exposure and extreme temperatures, viz., hot/dry, hot/wet, cold/dry, and cold/wet. Short term tests were performed to determine the flexural strength of environmentally aged composite specimens in accordance with ASTM D2344-84 and ASTM D7264 respectively. Carbon/epoxy specimens of [02/902]2s configuration were manufactured for flexure tests using Vacuum Assisted Resin Transfer Molding (VARTM). A unique strain fixture was designed to apply constant strain on the specimens during ageing and applied a simple methodology to eliminate excessive creep in the specimens. A two-dimensional cohesive layer constitutive model with a cubic traction-separation law has being developed in order to predict the life of the composite under hygrothermal conditions. The model simulated the test conditions and predicted the progressive failure mechanism of the specimen as observed in the tests, under various loading conditions. The model also incorporated synergistic interactions between temperature, moisture and stress effects and predicted degradation in strength and stiffness as a function of different ageing conditions and ageing times. Model predictions have been benchmarked using test data.
Investigation of Taylor impact test of isotropic and anisotropic material through geometrical characteristics of specimens
(University of Alabama Libraries, 2010) Cao, Zhiyi; Barkey, Mark E.; University of Alabama Tuscaloosa
In this thesis, high strain rate properties of isotropic material (a copper alloy) and anisotropic material (2195-T8 aluminum-lithium alloy) are investigated using Taylor impact tests. Coordinate measuring machines (CMMs) are used to measure the shape of specimens after the deformation. The geometrical data enables us to determine the plastic distributions and the dynamic yield stresses of specimens. A raise in yield strength is found in the copper alloy during the impact. It means that material properties of the copper alloy are sensitive to high strain rate. Yet such phenomenon is not found in the 2195-T8 aluminum-lithium alloy. Based on the uniaxial compression strain state in the barreling regions of the specimens, the dynamic yield stresses in the rolling, transverse and short transverse directions are obtained for the 2195-T8 aluminum-lithium alloy. This enables us to determine the anisotropic coefficients in Hill's criterion and carry out the finite element analysis. The dependencies of fracture are also investigated. It is found that the fracture is sensitive to maximum shear stress, equivalent plastic strain and stress triaxiality.
Impact of a biologically-inspired tail assembly on drag reduction for lighter-than-air near-space platforms
(University of Alabama Libraries, 2010) Cottingham, Jason Lambert; Baker, John; University of Alabama Tuscaloosa
The near space region is a vastly underused region of our atmosphere due to the extreme conditions present but makes up approximately 75% of the usable atmosphere. Although there are no weather patterns like rain, storms and clouds, the winds that are present at high altitudes are very challenging to overcome. Many have researched and designed high altitude platforms for exploration, but few have succeeded in experimental aircraft tests at altitude. Due to the low density, temperature and pressure in this region, fixed wing flight is almost impossible but lighter than air technology can be utilized to help overcome some of those challenges. Long endurance flight in the near space region is beginning to be heavily researched due to the many advantages of the capability from civilian uses to military uses. With the ever present winds in the near space region, lighter than air technology used in conjunction with its station keeping ability becomes an almost unconquerable task. This research focuses on using the adaptations of nature to help solve the power requirement challenge surrounding high altitude/long endurance flight. With the use of a biologically inspired tail, the drag on an airship can be decreased, and thus the power required for station keeping abilities. Using computational modeling, a complete environmental model was created to represent the aspects of the atmosphere that directly affect airship flight. A model of a basic shaped airship and an airship fitted with a tail inspired by avian flight were created to be used in the environmental model. The approach used in this research in one approach that could be used to significantly reduce the drag on a high altitude airship and help enable the creation of viable near space vehicles.
Construction of a multi-functional cryogenic propellant tank with cross-linked silica aerogel
(University of Alabama Libraries, 2010) Reinheimer, Preston Glenn; Roy, Samit; University of Alabama Tuscaloosa
Aerogels are low-density nanostructured porous materials, whose practical applications have been limited by their poor mechanical properties. Crosslinking the nanoparticle building blocks of silica aerogels with polymeric tethers increases both the modulus and the strength significantly. The polymer coating preserves the mesoporous structure of the silica framework while retaining its low thermal conductivity. The uniqueness of crosslinked silica aerogel has load carrying capabilities in which are determined in tensile, compression and flexural bending tests. Crosslinked silica aerogel testing displays specific compressive strength of 389000 Nm/Kg. Ballistic testing of crosslinked silica aerogel also corroborates its mechanical properties displaying a ballistic limit up to 80 m/s. Its thermal conductivity at 0.041 W/mK supports the use of crosslinked silica aerogel in cryogenic fuel cell applications. Manufacturing practices have been evaluated to obtain an optimal process which reduces time, money and difficulty.
Processing and characterization of unidirectional thermoplastic nanocomposites
(University of Alabama Libraries, 2010) Narasimhan, Kameshwaran; Roy, Samit; University of Alabama Tuscaloosa
The manufacture of continuous fibre-reinforced thermoplastic nanocomposites is discussed for the case of E-Glass reinforced polypropylene (PP) matrix and for E-Glass reinforced Polyamide-6 (Nylon-6), with and without dispersed nanoclay (montmorillonite) platelets. The E-Glass/PP nanocomposite was manufactured using pultrusion, whereas the E-Glass/Nylon-6 nanocomposite was manufactured using compression molding. Mechanical characterization of nanocomposites were performed and compared with traditional microcomposites. Compressive as well as shear strength of nanocomposites was improved by improving the yield strength of the surrounding matrix through the dispersion of nanoclay. Significant improvements were achieved in compressive strength and shear strength with relatively low nanoclay loadings. Initially, polypropylene with and without nanoclay were melt intercalated using a single-screw extruder and the pultruded nanocomposite was fabricated using extruded pre-impregnated (pre-preg) tapes. Compression tests were performed as mandated by ASTM guidelines. SEM and TEM characterization revealed presence of nanoclay in an intercalated and partially exfoliated morphology. Mechanical tests confirmed significant improvements in compressive strength (~122% at 10% nanoclay loading) and shear strength (~60% at 3% nanoclay loading) in modified pultruded E-Glass/PP nanocomposites in comparison with baseline properties. Uniaxial tensile tests showed a small increase in tensile strength (~3.4%) with 3% nanoclay loading. Subsequently, E-Glass/Nylon-6 nanocomposite panels were manufactured by compression molding. Compression tests were performed according to IITRI guidelines, whereas short beam shear and uni-axial tensile tests were performed according to ASTM standards. Mechanical tests confirmed strength enhancement with nanoclay addition, with a significant improvement in compressive strength (50% at 4% nanoclay loading) and shear strength (~36% at 4% nanoclay loading) when compared with the baseline E-Glass/Nylon-6. Uni-axial tensile tests resulted in a small increase in tensile strength (~3.2%) with 4% nanoclay loading. Also, hygrothermal ageing (50 °C and 100% RH) of baseline and nanoclay modified (4%) E-Glass/Nylon-6 were studied. It was observed that the moisture diffusion process followed Fickian diffusion. E-Glass/Nylon-6 modified with 4% nanoclay loading showed improved barrier performance with a significant reduction (~30%) in moisture uptake compared to baseline E-Glass/Nylon-6 composites. Significant improvement in mechanical properties was also observed in hygrothermally aged nanocomposite specimens when compared with the aged baseline composite.
Luminescent coating image analysis on a three dimensional grid
(University of Alabama Libraries, 2010) Esirgemez, Ergin; Hubner, James Paul; University of Alabama Tuscaloosa
The luminescent photoelastic coating (LPC) technique is a method to measure the full-field strain on three-dimensional (3D) structural components. A luminescent dye within a photoelastic binder is excited with circular polarized light, and the corresponding emission intensity for coating is detected via a CCD camera. Images are then processed to find the relative change in emission with respect to camera analyzer position, and subsequently analyzed to determine maximum in-plane shear strain. Image alignment plays a crucial role to obtain accurate measurements, especially when implementing an oblique excitation approach to separate the principal strains while accounting for non-strain related polarization changes due to surface inclination. Image warping methods in the image two-dimensional (2D) coordinate system provides reasonable results for 2D or simple 3D specimens; however, for complex 3D structures with moderate movement or deflection in the field-of-view, the accuracy and efficiency of these methods are not optimal. An alternative approach is to perform the analysis on a 3D grid representation of the structures. This study will research the merit of such an approach and develop the analysis procedures to separate the principal strains on 3D structures. The theoretical results will be compared to experimental data from a 2D and a 3D specimens while assessing the accuracy of the approach.
Experimental and computational investigation of the effect of environmental degradation on interlaminar shear strength of carbon/epoxy composites
(University of Alabama Libraries, 2010) Akepati, Avinash Reddy; Roy, Samit; University of Alabama Tuscaloosa
The effect of environmental and loading conditions on the degradation of Interlaminar Shear Strength (ILSS) of the carbon-epoxy composite specimens was studied in the current research. The hygrothermal conditions capture the synergistic effects of field exposure and extreme temperatures. Short beam shear tests (SBST) were performed to determine the Interlaminar Shear Strength (ILSS) of environmentally aged composite specimens in accordance with ASTM D2344-84. Initially, a standard two-dimensional cohesive layer constitutive model with a cubic traction-separation law was employed in order to simulate the experiment using an in-house Finite Element Analysis code (NOVA-3D). Numerical instabilities encountered using the standard elastic cohesive layer model were overcome by incorporating viscoelastic regularization in the constitutive equations of the cohesive layer. This modification also enabled the analysis to continue beyond the point of peak failure load. The model was able to accurately simulate the load-deflection behavior of most of the SBST specimens aged under various hygrothermal and synergistically applied stress conditions. Further, the effect of displacement rate on ILSS of specimens was studied using NOVA-3D. A PC based life prediction software, Composite Performance Predictive Tool (CPPT), was developed using a mechanism-based degradation model in order to predict the life of a composite structure under given environmental and loading conditions. The software was benchmarked using the test data.
Microhardness, strength and strain field characterization of self-reacting friction stir weld and friction plug welds of dissimilar aluminum alloys
(University of Alabama Libraries, 2011) Horton, Karla Renee; Barkey, Mark E.; University of Alabama Tuscaloosa
Friction stir welding (FSW) is a solid state welding process with potential advantages for aerospace and automotive industries dealing with light alloys. Self-reacting friction stir welding (SR-FSW) is one variation of the FSW process being developed at the National Aeronautics and Space Administration (NASA) for use in the fabrication of propellant tanks. Friction plug welding is used to seal the exit hole that remains in a circumferential SR-FSW. This work reports on material properties and strain patterns developed in a SR-FSW with a friction plug weld. Specifically, this study examines the behavior of a SR-FSW formed between an AA2014-T6 plate on the advancing side and an AA2219-T87 plate on the retreating side and a SR-FSW (AA2014-T6 to AA2219-T87) with a 2219-T87 plug weld. This study presents the results of a characterization of the micro-hardness, joint strength, and strain field characterization of SR-FSW and FPW joints tested at room temperature and cryogenic temperatures. The initial weld microstructure analysis showed a nugget region with fine grains and a displaced weld seam from the advancing side past the thermo-mechanical affected zone (TMAZ) into the nugget region. The displaced material shared the same hardness as the parent material. Dynamic recrystallization was observed in the SR-FSW zone and the displaced weld seam region. The welds revealed a fine grain structure in the SR-FSW zone with a sharp demarcation seen on the advancing side and fairly diffuse flow observed on the retreating side. The parent material hardness is 145 HV_700g with a drop in hardness starting at the HAZ to 130 HV_700g . The hardness further drops in the TMAZ to118 HV_700g with an increase representing a dispersed interface of AA2014-T6 material to 135 HV_700g . The hardness then drops significantly within the nugget region to 85 HV_700g followed by an increase through the retreating side TMAZ into the HAZ to 135 HV_700g . There was a sharp increase in the hardness value within the nugget region with the samples that were PWHT showing an increase of 58%. The welded joints were tested for ultimate strength. The testing variations included two specimen widths, two plug sizes (M3 and M5), room temperature and cryogenic testing, and post weld heat treated (PWHT) samples. Initial welds had an average ultimate strength of 370 MPa. There was a slight drop from initial weld strength to plug weld strength of approximately 13.8 MPa was observed with M3 plug strength approximately equal to M5 plug strength. The PWHT strengths at room temperature were slightly higher than non-PWHT of 13.8-20.7 MPa and PWHT strengths were equal to non-PWHT at cryogenic temperature. Non-PWHT had a cryogenic strength enhancement approximately 59.2 MPa and PWHT had a cryogenic strength enhancement of approximately 57.2 MPa in the M3 and M5 plugs. Within the subsets of data collected no major statistical significance in strength behavior was observed between the samples tested at room temperature or between the subsets tested at room temperature or between the subsets tested at cryogenic temperature. In almost all cases, failure occurred on the retreating side of the weld which corresponds to the softer material (AA2219-T87). Exceptions were characterized with flaws (weld defects) in the sample. In these cases, failure occurred on the advancing side, the side where flaws were detected. Ductile fracture was noted in most all samples. Digital image correlation using the ARAMIS system was used to define strain patterns in the weld joint. Strain accumulation was observed in the weld along the retreating side and around the plug. ARAMIS data in comparison to extensometer data shows a very reasonable comparison. The ARAMIS strain gage data showed the retreating side of the major diameter has a greater yield than the advancing side. This behavior is identical to the external electrical resistance strain gages.
Analysis of beveled semi-elliptical surface cracks in friction stir plug welded plates made of Al 2195 alloy
(University of Alabama Libraries, 2011) Vadakke Veetil, Rahul; Barkey, Mark E.; University of Alabama Tuscaloosa
Friction Stir Welding (FSW) is a solid state joining process primarily used for Al alloys. Friction Stir Plug Welding (FPW) is a process in which a tapered shaped plug is friction stir welded into the hole that was left in the welded part when the initial FSW tool was removed. A rectangular plate made of Al 2195 alloy with a friction welded plug and containing a semi-elliptical surface crack was analyzed using the help of the software `FEA Crack'. Three different crack depths of deep cracks as well as shallow cracks were considered in identical plates of a quarter inch thickness. The depths were 0.08, 0.13 and 0.18 inches for deep cracks and 0.008, 0.013 and 0.018 inches for shallow cracks. A uniaxial tensile load of 1 psi was applied on one end surface with the opposite surface being fixed. For each depth, four different crack arc lengths were considered which were of 15, 30, 60 and 90 degrees. For each of these cases, the crack tube containing the crack was rotated around the plug having an inner bevel, in steps of 10 degree starting from the base position to the 90 degree (vertical) position with an additional case of 45 degree rotation in between. The stress intensity factor K was plotted against the crack front angle. The average and maximum K factor values were also plotted for each of the main crack lengths against the crack rotation angle. The same procedure was employed for shallow cracks. The results were validated using Newman-Raju equations for semi elliptical surface cracks. Non dimensional K factor plots were also made for different cases of both deep and shallow surface cracks. Researchers studying the surface cracks can now get an estimate of the value of stress intensity factor for the crack length, depth of crack and also the angular position of the crack around the plug by interpolating my results.
Turbulent separation control effects of mako shark skin samples on a naca 4412 hydrofoil
(University of Alabama Libraries, 2011) Smith, Jonathon Andrew; Lang, Amy W.; University of Alabama Tuscaloosa
Shark skin is investigated as a means of passive flow separation control due to its preferential flow direction and the potential for its scales to obstruct low-momentum backflow resulting from an adverse pressure gradient. In this study, the effect of the scales on flow reversal is observed in a tripped turbulent boundary layer by comparing the flow over a NACA 4412 hydrofoil with a smooth, painted surface to that over the same hydrofoil with samples of mako shark skin affixed to its upper surface. These samples were taken from the shark's flank region because the scales at that location have been shown to have the greatest angle of bristling, and thus the best potential for separation control. All sets of flow data in this study were obtained using Time-Resolved Digital Particle Image Velocimetry and recorded at multiple angles of attack (between 8 and 16 degrees) and two Reynolds numbers. The flow was primarily analyzed by means of the backflow coefficient (a value based on the percentage of time that flow in a region over the hydrofoil is reversed), average backflow magnitude, and the time history of instantaneous flow velocity values at specific points in the boundary layer over the hydrofoil models. Results showed that at angles of attack of 12° and below, the shark skin generated a slightly larger, higher magnitude region of reversed flow than was seen over the painted surface. At an angle of attack of 16°, the backflow region of the shark skin surface was significantly reduced in size and magnitude compared to that of the painted surface. These results support the hypothesis that in order for the scales to be an effective means of flow control, sufficient shear must be present in the backflow to cause them to bristle fully.
A finite element analysis of a boa constrictor skull and the design of a jaw bone transducer
(University of Alabama Libraries, 2011) Tadi, Nava Sundeep Avinash Reddy; Barkey, Mark E.; University of Alabama Tuscaloosa
The purpose of this research is to analyze the finite element model of a Boa constrictor skull and design a transducer at the jaw bone to measure the forces applied during feeding. The concepts in mechanics are applied to simulate and study the behavior of jaws while feeding.. In this study, the images of the skull of a boa constrictor were scanned to obtain the precise size and shape of each of the bones inside. The specimen was scanned using the X-ray micro tomography procedure. This process left several regions in the finite element model unconnected. These regions are corrected using the software HYPERMESH. The loading and boundary conditions along with the various properties like the Elastic modulus, Poisson's ratio are given to the FE model in ABAQUS before analyzing it. Electrical resistance strain gauges are simulated to have attached to the jawbone at two different points. The strain measured is then used to find out the forces that caused it, by using the various concepts and principles in solid and continuum mechanics. Electrical resistance strain gauges, numerical methods, moments of inertia are some of the commonly used terms in this analysis.
Development of a water-cooled LDV probe for rocket/gas-turbine engine environments
(University of Alabama Libraries, 2011) Davis, Jason Allen; Olcmen, Semih M.; University of Alabama Tuscaloosa
Measurements of the flow velocity within rockets and gas-turbines prove to be a difficult problem due to the high temperature, high pressure, and high noise levels encountered. Velocity measurements in such environments are required to understand the physics of the flow field to improve the design and efficiency of these engine types. The design of a water-cooled jacket intended for use in conjunction with an existing novel, miniature fiber-optic LDV probe to make velocity measurements in these extreme engine environments is the topic of this research. The jacket design uses off-the-shelf materials and machining techniques in order to reduce cost and simplify manufacturing. The jacket was designed and drawn using the 3D parametric modeling software called SolidWorks. The SolidWorks CFD add-in, FloWorks, was used to numerically simulate the insertion of the water-cooled jacket into the afterburner or augmentor section of an air-breathing gas-turbine engine operating at 2000 K static temperature. The augmentor diameter was chosen to be 18 inches based on the existing General Electric J-85 jet engine augmentor. Different configurations in which the combinations of jacket insertion distance, augmentor flow velocity Mach number, and alternate jacket tip designs were examined and analyzed in order to determine the operating conditions that will permit velocity measurements representative of the undisturbed freestream velocity. Computational results indicate that while a cooling water flow rate of 3 GPM allows for the successful operation of the jacket within the augmenter at Mach 0.7 and 2000 K, Mach numbers of 0.5 and below exhibit flow field disturbances that will allow for LDV measurements representative of the freestream.

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