Additive friction stir deposition (AFSD) is a solid-state additive manufacturing (AM) technique that breaks down large constituent particles into more refined and uniformly disturbed microstructure. AFSD was used to print Al-Ce alloys. Current commercial Al-alloys upon elevated temperatures go through dissolution and coarsening of strengthening precipitates causing mechanical degradation of these alloys. Al-Ce alloys do not have this issue as cerium's low solubility restricts dissolution into the aluminum matrix at elevated temperatures, thus giving great thermal stability to the microstructure. Al-Ce alloys lack solid solubility that affects the solid solution strengthening and precipitation strengthening. Al-Ce alloys have limitation at room temperature as they can only reach a maximum of ~65 MPa yield strength. Elements like magnesium have been added to alloy to enable solid solution strengthening, and scandium to enable precipitation strengthening to improve strength before going through the AFSD process. By adding new elements to the Al-Ce alloys, an increase in the yield strength from ~60 MPa to ~200 MPa was achieved before AFSD. The casted alloys form coarse particles that reach 300 µm in size; resulting in stress concentration that causes material fracture before necking, giving >10% ductility. AFSD breaks down these coarse particles to increase strength and ductility increases. The particles were broken down to >20 µm which increased the ductility to 10%. The results of this research shows that Al-Ce alloys are able to reach commercial aluminum alloy mechanical standards of 400 MPa ultimate tensile strength and 10% ductility at room temperature for aerospace applications.

Traumatic brain injuries (TBIs) encompass an array of head trauma caused by diverse mechanisms, including falls, vehicular accidents, and sports-related incidents. These injuries vary from concussions to diffuse axonal injuries. TBIs are characterized by the linear and rotational accelerations of the head during an impact, which are influenced by various factors such as the velocity and location of the impact and the contact surface. Consequently, the accuracy of laboratory tests designed to evaluate protective technologies must closely mirror real-world conditions. This dissertation explores the boundary conditions essential for accurately replicating head impacts in laboratory settings. The research aims to improve the reconstruction of head impacts, concentrating on two main areas: 1) examining various aspects of friction during head impacts and 2) biomechanically characterizing the head impacts sustained by older adults during falls. This study provides insights into the overall influence of friction during head impacts. It investigates the friction coefficients between the helmet's shell and the impact surface, as well as between human heads, headforms, and helmets. Additionally, it assesses how these frictional interactions influence oblique impact kinematics. Defining static and dynamic friction coefficients of the human head and headforms is needed to develop more realistic head impact testing methods, define helmet-head boundary conditions for computer-aided simulations, and provide a framework for cross-comparative analysis between studies that use different headforms and headform alterations. This research also introduces and evaluates the accuracy of a model-based image mapping method to measure head impact speeds from single-view videos in un-calibrated environments. This measurement technique advances our comprehension of head impact kinematics derived from uncalibrated video data. By applying this method, videos of falls involving older adults were analyzed to determine head impact speeds and boundary conditions. The resulting data was used to construct headform impacts, capturing linear and rotational head impact kinematics. These reconstructions can inform the development of biomechanical testing protocols tailored to assess protective gear for older adults, with the goal of reducing fall-related head injuries. / Doctor of Philosophy / Traumatic brain injuries (TBIs) are head injuries that can happen in many ways, such as from falling, car accidents, or playing sports. These injuries can range from mild concussions to more severe cases, brain bleeds, or skull fractures. They happen when the head moves quickly or spins because of a hit, which can be affected by the speed of the impact, where on the head the impact happens, or what the head impacts against. Therefore, the accuracy of laboratory reconstruction head impact tests must closely mirror real-world conditions. This dissertation explores the boundary conditions essential for accurately replicating head impacts in laboratory settings. The research aims to improve the reconstruction of head impacts, concentrating on two main areas: 1) examining various aspects of friction during head impacts and 2) biomechanically characterizing the head impacts sustained by older adults during falls. This study provides insights into the overall influence of friction during head impacts. It investigates the friction coefficients between the helmet's shell and the impact surface, as well as between human heads, headforms, and helmets. Additionally, it assesses how these frictional interactions affect head impacts. Understanding how friction influences head impacts is crucial for improving helmet testing methods and allows for more consistent comparisons across various research studies that use different headform models or modifications. This research also introduces and evaluates a method to calculate head impact speeds by analyzing video footage, even if the video was not taken with special equipment or setup. This approach improves our understanding of head movements during accidents by using video clips of falls, particularly those involving older adults, to determine the head speeds and conditions of the impact. The information gathered from these analyses helps to reconstruct these impacts using a headform to measure injury metrics. These reconstructions are crucial for designing tests that can evaluate safety equipment meant to protect older adults from head injuries during falls.

The purpose of the project was to examine fiber optic sensors for the measurement of pressure, skin friction, temperature, and heat flux in high Reynolds number, supersonic flow. Using a standard fiber optic signal conditioning unit (specifically a broadband interferometric system using spectra), the work centered around determining under what conditions these sensors will work effectively and quantifying the total system limitations. An interferometric-based, fiber optic skin friction sensor was developed for the measurement of wall shear stress in complex, supersonic flows. This sensor type was tested successfully in laminar, incompressible flow, and supersonic flow up to Mach 1.92, Mach 2.4 and 3.0 flow, in which the sensor operated with varying success. A micromachined, fiber optic pressure sensor was also tested in these supersonic conditions, also with varying success. The accurate operation of these sensors was found to be tied to the flow conditions and the fiber optic, signal processing system. A correlation was found to exist between the energy of the flow, either through its dynamic pressure or through external disturbances such as shocks or separation, and the noise in the signals, expressed by the variance of the gap estimate, for the pressure and skin friction sensors in these flows. The energy of the flow couples with the mechanical properties of the sensor reducing the fringe contrast of the signal used by the optical signal processing system to determine a gap estimate. As the energy of the flow is increased and the sensor is excited, the fringe contrast is reduced. A practical limit of a normalized fringe contrast of 0.10 was found for producing accurate gap estimates in real flows. A consequence is that there is a limit to the dynamic pressure of the flow for the sensors to operate accurately, which is demonstrated by the varying success of the supersonic wind tunnel tests. This correlation is sensor specific, meaning that sensors can be designed to operate successfully in any flow. Also, the signal processing system, which forms the other end of the total system, could be improved to allow accurate measurements with the current sensors. / Ph. D.

This research produced a new class of skin friction gages that measures wall shear even in shock environments. One test specimen separately measured wall shear and variable-pressure induced moment. Through the investigation of available computational modeling methods, techniques for accurately predicting gage physical responses were developed. The culmination of these model combinations was a design optimization procedure. This procedure was applied to three disparate test conditions: 1) short-duration, high-enthalpy testing, 2) blow-down testing, and 3) flight testing. The resulting optimized gage designs were virtually tested against each set of nominal load conditions. The finalized designs each successfully met their respective test condition constraints while maximizing strain output due to wall shear. These gages limit sources of apparent strain: inertia, temperature gradient, and uniform pressure. A unique use of bellows provided a protective shroud for surface strain gages. Oil fill provided thermal and dynamic damping while eliminating uniform pressure as a source of output voltage. Two Wheatstone bridge configurations were developed to minimize temperature effects first from temperature gradient and then from spatially varying heat flux induced gradient. An inertia limiting technique was developed that parametrically investigated mass and center of gravity impact on strain output. Multiple disciplinary computational simulations of thermal, dynamic, shear, moment, inertia, and instrumentation interaction were developed. Examinations of instrumentation error, settling time, filtering, multiple input dynamic response, and strain gage placement to avoid thermal gradient were conducted. Detailed mechanical drawings for several gages were produced for fabrication and future testing. / Ph. D.

A series of direct-measuring skin friction gages were developed for a high-speed, high-temperature environment of the turbulent boundary layer in flows such as that in supersonic combustion ramjet (scramjet) engines, with a progression from free-jet ground tests to a design for an actual hypersonic scramjet-integrated flight vehicle. The designs were non-nulling, with a sensing head that was flush with the model wall and surrounded by a small gap. Thus, the shear force due to the flow along the wall deflects the head, inducing a measurable strain. Strain gages were used to detect the strain. The gages were statically calibrated using a direct force method. The designs were verified by testing in a well-documented Mach 2.4 cold flow. Results of the cold-flow tests were repeatable and within 15% of the value of Cf estimated from simple theory. The first gage design incorporated a cantilever beam with semiconductor strain gages to sense the shear on the floating head. Cooling water was routed both internally and around the external housing in order to control the temperature of the strain gages. This first gage was installed and tested in a rocket-based-combined-cycle (RBCC) engine model operating in the scramjet mode. The free-jet facility provided a Mach 6.4 flow with P0 = 1350 psia (9310 kPa) and T0 = 2800 °R (1555 °K). Local wall temperatures were measured between 850 and 900 °R (472-500 °K). Output from the RBCC scramjet tests was reasonable and repeatable. A second skin friction gage was designed for and tested in a wind tunnel model of the Hyper-X flight vehicle scramjet engine. These unsuccessful tests revealed the need for a radically different skin friction gage design. The third and final skin friction gage was specifically developed to be installed on the Hyper-X flight vehicle. Rather than the cantilever beam and semiconductor strain gages, the third skin friction gage made use of a flexure ring and metal foil strain gages to sense the shear. The water-cooling and oil-fill used on the previous skin friction sensors were eliminated. It was qualified for flight through a rigorous series of environmental tests, including pressure, temperature, vibration, and heat flux tests. Finally, the third skin friction gage was tested in the Hyper-X Engine Model (HXEM), a full-scale-partial-width wind tunnel model of the flight vehicle engine. These tests were conducted at Mach 6.5 enthalpy with P0 = 555 psia (3827 kPa) and h0 = 900 Btu/lbm in a freejet facility. The successful testing in the wind tunnel scramjet model provided the final verification of the gage before installation in the flight vehicle engine. The development, testing, and results of all three skin friction gages are discussed. / Ph. D.

This investigation concerns the design, analysis, and initial testing of a new, two-component wall shear gage for 3D, high-temperature flows. This gage is a direct-measuring, non-nulling design with a round head surrounded by a small gap. Two flexure wheels are used to allow small motions of the floating head. Fiber-optic displacement sensors measure how far the polished faces of counterweights on the wheels move in relation to a fixed housing as the primary measurement system. No viscous damping was required. The gage has both fiber-optic instrumentation and strain gages mounted on the flexures for validation of the newer fiber optics. The sensor is constructed of Haynes 230, a high-temperature nickel alloy. The gage housing is made of 316 stainless steel. All components of the gage in pure fiber-optic form can survive to a temperature of 1073 K. The bonding methods of the backup strain gages limit their maximum temperature to 473 K. The dynamic range of the gage is from 0-500 Pa (0-10g) and higher shears can be measured by changing the floating head size. Extensive use of finite element modeling was critical to the design and analysis of the gage. Static structural, modal, and thermal analyses were performed on the flexures using the ANSYS finite element package. Static finite element analysis predicted the response of the flexures to a given load, and static calibrations using a direct force method confirmed these results. Finite element modal analysis results were within 16.4% for the first mode and within 30% for the second mode when compared with the experimentally determined modes. Vibration characteristics of the gage were determined from experimental free vibration data after the gage was subjected to an impulse. Uncertainties in the finished geometry make this level of error acceptable. A transient thermal analysis examined the effects of a very high heat flux on the exposed head of the gage. The 100,000 W/m2 heat flux used in this analysis is typical of a value in a scramjet engine. The gage can survive for 10 minutes and operate for 3 minutes before a 10% loss in flexure stiffness occurs under these conditions. Repeated cold-flow wind tunnel tests at Mach 2.4 with a stagnation pressure from 3.7-8.2 atm (55-120 psia) and ambient stagnation temperature (Re=6.6x107/m) and Mach 4.0 with a stagnation pressure from 10.2-12.2 atm (150-180 psia) and ambient stagnation temperature (Re=7.4x107/m) were performed in the Virginia Tech Supersonic Wind Tunnel. Some of these tests had the gage intentionally misaligned by 25o to create a virtual 3D flow in this nominally 2D facility. Experimental results gave excellent agreement with semi-empirical prediction methods for both the aligned and 25o experiments. This fiber-optic skin friction gage operated successfully without viscous damping. These tests in the supersonic wind tunnel validated this wall shear gage design concept. / Ph. D.

Wetting, which addresses either spontaneous or forced spreading of liquids on a solid surface, is a ubiquitous phenomenon in nature and can be observed by us on a daily basis, e.g., rain drops falling on a windshield and lubricants protecting our corneas. The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. Generally, wetting is governed by the dynamic and static behaviors of the three-phase contact line. Therefore, a deep insight into the dynamics and statics of three-phase contact line at nanoscale is necessary for the technological advancement in nanotechnology and nanoscience. This dissertation aims to understand the dynamic wetting under a molecular kinetic framework and resolve the reconfiguration of liquid molecules at the molecular region of contact line. Water spreading on polytetrafluoroethylene surfaces is selected as a classical example to study the dynamic behaviors of three-phase contact line. To accommodate the moving contact line paradox, the excess free energy is considered to be dissipated in the form of molecular dissipation. As-formed contact line friction/dissipation coefficient is calculated for water interacting with PTFE surfaces with varying structures and is found to be on the same order of magnitude with dynamic viscosity. From an ab initio perspective, contact line friction is decomposed into contributions from solid-liquid retarding and viscous damping. A mathematical model is established to generalize the overall friction between a droplet and a solid surface, which is able to clarify the static-to-kinetic transition of solid-liquid friction without introducing contact angle hysteresis. Moreover, drag reduction on lotus-leaf-like surface is accounted for as well. For the first time, the concept of contact line friction is used in the rational design of a superhydrophobic condenser surface for continuous dropwise condensation. We focus on the transport and reconfiguration of liquid molecules confined by a solid wall to shed light on the morphology of the molecular region of a three-phase contact line. A governing equation, which originates from the free energy analysis of a nonuniform monocomponent system, is derived to describe the patterned oscillations of liquid density. By comparing to the Reynolds transport theorem, we find that the oscillatory profiles of interfacial liquids are indeed governed in a combined manner by self-diffusion, surface-induced convection and shifted glass transition. Particularly for interfacial water, the solid confining effects give rise to a bifurcating configuration of hydrogen bonds. Such unique configuration consists of repetitive layer-by-layer water sheets with intra-layer hydrogen bonds and inter-layer defects. Molecular dynamics simulations on the interfacial configuration of water on solid surfaces reveal a quadratic dependence of adhesion on solid-liquid affinity, which bridges the gap between macroscopic interfacial properties and microscopic parameters. / Doctor of Philosophy / The study of wetting phenomena can be traced back to the observation of water rising in a capillary tube by Hauksbee in 1706 and still remains as a hot topic, since it lays the foundation for a wide spectrum of applications, such as fluid mechanics, surface chemistry, micro/nanofluidic devices, and phase change heat transfer enhancement. The conventional hydrodynamic analysis with no slip boundary condition predicts a diverging shear stress at the contact line as well as an unbounded shear force exerted on the solid surface. To accommodate this paradox, different mechanism and models have been proposed to clarify the slip between a moving contact line and a solid surface. Although almost all models yield reasonable agreement with experimental observations or numerical simulations, it is still difficult to pick up a specific model using only macroscopic properties and experiment-accessible quantities, because the energy dissipation mechanism during dynamic wetting is not identified and the contact line deforms over different length scales. In this dissertation, we ascribe the energy dissipation in dynamic wetting to contact line friction/dissipation under the framework of molecular kinetic theory, as it is assumed that the contact line is constantly oscillating around its equilibrium position. By decomposing contact line friction into two contributions: solid-liquid retarding and viscous damping, we are able to derive a universal model for the contact line friction. This model predicts a decaying solid-liquid friction on superhydrophobic surfaces, corresponding to the lotus effect. In the meantime, this model is able to clarify the recently-discovered static-to-kinetic transition of frictional force between a sessile drop and a solid surface. Later, we used the concept of contact line friction in the droplet growth process in dropwise condensation so as to promote the rational design of superhydrophobic condenser surfaces for sustainable dropwise condensation. As the morphology of a contact line is dependent on the length scale of interest, we focus on the molecular region of contact line. We study the transport and structural change of liquid molecules that are several molecular layers away from the solid surface. It is found that liquid molecules in this region experience patterned density oscillations, which cannot be simply attributed to the random deviations from continuum limit. By combining free energy analysis and Reynolds transport theorem, it is demonstrated that the omnipresent density oscillations arise from the collective effects of self-diffusion, surface-induced convection and shifted glass transition. For liquid water, we propose a bifurcating hydrogen bonding network in contrast to its tetrahedron configuration in bulk water.

Additive friction stir deposition (AFSD) is an emerging solid-state metal additive manufacturing technology that uses deformation bonding to create near-net shape 3D components. As a developing technology, a deeper understanding of the processing science is necessary to establish the process-structure relationships and enable improved control of the as-printed microstructure and material properties. AFSD provides a unique opportunity to explore the friction stir fundamentals via direct observation of the material during processing. This work explores the relationship between the processing parameters (e.g., tool rotation rate Ω, tool velocity V, and material feed rate F) and the thermomechanical history of the material by process monitoring of i) the temperature evolution, ii) the force evolution, and iii) the interfacial contact state between the tool and deposited material. Empirical trends are established for the peak temperature with respect to the processing conditions for Cu and Al-Mg-Si, but a key difference is noted in the form of the power law relationship: Ω/V for Cu and Ω2/V for Al-Mg-Si. Similarly, the normal force Fz for both materials correlates to V and inversely with Ω. For Cu both parameters show comparable influence on the normal force, whereas Ω is more impactful than V for Al-Mg-Si. On the other hand, the torque Mz trends for Al-Mg-Si are consistent with the normal force trends, however for Cu there is no direct correlation between the processing parameters and the torque. These distinct relationships and thermomechanical histories are directly linked to the contact states observed during deformation monitoring of the two material systems. In Cu, the interfacial contact between the material and tool head is characterized by a full slipping condition (δ=1). In this case, interfacial friction is the dominant heat generation mechanism and compression is the primary deformation mechanism. In Al-Mg-Si, the interfacial contact is characterized by a partial slipping/sticking condition (0<δ<1), so both interfacial friction and plastic energy dissipation are important mechanisms for heat generation and material deformation. Finally, an investigation into the contact evolution at different processing parameters shows that the fraction of sticking is critically dependent on the processing parameters which has many implications on the thermomechanical processing history. / Doctor of Philosophy / Additive manufacturing or three-dimensional (3D) printing technologies have been lauded for their ability to fabricate complex geometries and multi-material parts with reduced material waste. Of particular interest is the use of metal additive manufacturing for repair and fabrication of industrial and structural components. This work focuses on characterizing the thermomechanical processing history for a developing technology Additive Friction Stir Deposition (AFSD). AFSD is solid-state additive manufacturing technology that uses frictional heat and mechanical mixing to fabricate 3D metal components. From a fundamental materials science perspective, it is imperative to understand the processing history of a material to be able to predict the performance and properties of a manufactured part. Through the use of infrared imaging, thermocouples, force sensors, and video monitoring this work is able to establish quantitative relationships between the equipment processing parameters and the processing history for Cu and Al. This work shows that there is a fundamental difference in how these two materials are processed during AFSD. In the future, these quantitative relationships can be used to validate modeling efforts and improve manufacturing quality of parts produced via friction stir techniques.

An investigation was conducted to design instruments to directly measure skin friction along the chamber walls of supersonic combustor models. Measurements were made in a combustor at the General Applied Science Laboratory (GASL) and in the Direct Connect Arcjet Facility (DCAF) supersonic combustor at the NASA AMES Research Center. Flow conditions in the high enthalpy combustor models ranged from total pressures of 275-800 psia (1900-5550 kPa) and total temperatures from 5800-8400 R (3222-4667 K). This gives enthalpies in the range of 1700-3300 BTU/Ib_m (3950-7660 KJ/kg) and simulated flight Mach number from 9 to 13. A direct force measurement device was used to measure the small tangential shear force resulting from the flow passing over a non-intrusive floating element. The floating head is mounted to a stiff cantilever beam arrangement with deflection due to the shear force on the order of 0.0005 in (0.0125 mm). This small deflection allows the balance to be a non-nulling type. Several measurements were conducted in cold supersonic flows to verify the concept and establish accuracy and repeatability. This balance design includes actively controlled cooling of the floating sensor head temperature through an internal cooling system to eliminate nonuniform temperature effects between the head and the surrounding chamber wall. This enabled the device to be suitable for shear force measurement in very hot flows. The key to this device is the use of a quartz tube cantilever with strain gages bonded at orthogonal positions directly on the surface at the base. A symmetric fluid flow was developed inside the quartz tube to provide cooling to the backside of the floating head. Bench tests showed that this did not influence the force measurement. Numerical heat transfer calculations were conducted for design feasibility and analysis, and to determine the effectiveness of the active cooling of the floating head. Analysis of the measurement uncertainty in cold supersonic flow tests show that uncertainty under 8% is achievable, but variations in the balance cooling during a particular test raised uncertainty up to 20% in these very hot flows during the early tests. Improvements to the strain gages and balance cooling reduced uncertainty for the later tests to under 15%. / Ph. D.

"Hot-tests" form a vital facet towards the end of the production line of modern automotive plants, where the condition of the engine is checked by running it for a short period of time, to ensure its performance under standard operating conditions. The duration of hot-tests for small engines varies from 20-75 seconds. In the conventional procedure, about 10-30 grams of lubricant (for pre-coating) is used with about 650ml of standard oil for engine testing. However, about 1-3 oz. of oil is lost per engine, as it cannot be sucked out of the crankcase after the hot tests. The loss of 1-3 oz. of oil leads to a significant loss in revenue, over the large number of engines manufactured. It also causes a potential safety and environmental hazard due to leakage of lubricant during shipping or upon first use in a particular application. The goal of this project is to conduct "no-oil" hot tests using less than 10 grams of specially formulated lubricants for pretreatment. Implementation of this procedure for conducting the hot tests in the manufacturing facility would save revenue and eliminate potential hazards mentioned above in addition to cutting down on manpower and/or machinery used for handling the engine oil. An experimental study of pre-treatment of interacting interfaces of engine components, with specially formulated lubricants, for no-oil hot tests is presented. This study includes sixteen tests performed on the production line of Tecumseh's small engine manufacturing plant. The formulated lubricants were made up of tribopolymer formers, i.e., monomers, which were used in previous tribopolymerization studies. Tribopolymerization is defined as the planned or intentional formation of protective polymeric films directly and continuously on rubbing surfaces to reduce damage and wear by the use of minor concentrations of selected compounds capable of forming polymeric films in situ. This study entailed the investigation of the anti-wear properties of the formulated lubricants on a high temperature pin-on-disk machine and subsequent selection of lubricants exhibiting superior performance for use in the engine tests. The no-oil hot-tests performed at Virginia Tech and on the assembly line exhibited the superior anti-scuffing/anti-wear properties of the specially formulated lubricants, to warrant their use on the production line in the near future. / Master of Science