Showing papers by "Wright-Patterson Air Force Base published in 2006"

Adaptive Terminal Guidance for Hypervelocity Impact in Specified Direction

Abstract: The problem of guiding a hypersonic gliding vehicle in the terminal phase to a target location is considered. In addition to the constraints on its final position coordinates, the vehicle must also impact the target from a specified direction with very high precision. The proposed 3-dimensional guidance laws take simple proportional forms. The analysis establishes that with appropriately selected guidance parameters the 3-dimensional guided trajectory will satisfy these impact requirements. We provide the conditions for the initial on-line selection of the guidance law parameters for the given impact direction requirement. The vehicle dynamics are explicitly taken into account in the realization of guidance commands. To ensure high accuracy in the impact angle conditions in an operational environment, we develop closed-loop nonlinear adaptation laws for the guidance parameters. We present the complete guidance logic and associated analysis. Simulation results are provided to demonstrate the effectiveness and accuracy of the proposed terminal guidance approach. I. Introduction Recent interests in developing on-demand global-reach payload delivery capability have brought to the forefront a number of underlying technological challenges. Such operations will involve responsive launch, autonomous entry flight, and precision terminal maneuvers. In certain scenarios the mission requirements call for the payload to impact the target location from a specific direction with supersonic speed. One example is to impact the target in a direction perpendicular to the tangent plane of the terrain at the target. The terminal guidance system will be responsible for directing the vehicle to the target and achieving the desired impact direction. The impact precision requirements under the scenarios considered are very high and stringent. For instance, the required Circular Error Probable (CEP) of the impact distance is just 3-meter. 1 The errors of the impact angles are desired to be within 0.5 deg. The very high speeds throughout the terminal phase only make it considerably more difficult to achieve these levels of precision. Yet cost considerations dictate that the terminal guidance algorithm should be relatively simple and computationally tractable for real-time operations. While a number of guidance methods can guide the vehicle to the target, not many address the unique need for impact from a specific direction. One method that can is the so-called “dive-line” guidance approach in Ref. 2. In this method one or more lines intersecting the Earth are established. The final dive-line intersects the target, and its direction can be set to the desired direction. The vehicle’s velocity vector is

Comparison of high-voltage ac and pulsed operation of a surface dielectric barrier discharge

Abstract: A surface dielectric barrier discharge (DBD) in atmospheric pressure air was excited either by low frequency (0.3–2 kHz) high-voltage ac or by short, high-voltage pulses at repetition rates from 50 to 600 pulses s−1. The short-pulse excited discharge was more diffuse and did not have the pronounced bright multiple cathode spots observed in the ac excited discharge. The discharge voltage, current and average power deposited into the discharge were calculated for both types of excitation. As a measure of plasma-chemical efficiency, the ozone number density was measured by UV absorption as a function of average deposited power. The density of ozone produced by ac excitation did not increase so rapidly as that produced by short-pulse excitation as a function of average power, with a maximum measured density of ~3 × 1015 cm−3 at 25 W. The maximum ozone production achieved by short-pulse excitation was ~8.5 × 1015 cm−3 at 20 W, which was four times greater than that achieved by ac excitation at the same power level.

Peptide-Mediated Formation of Single-Wall Carbon Nanotube Composites

Abstract: The formation of silica- and titania-coated single-wall carbon nanotubes (SWNTs) using a mutlifunctional peptide to both suspend SWNTs and direct the precipitation of silica and titania at room temperature is demonstrated.

Combustion enhancement via stabilized piecewise nonequilibrium gliding arc plasma discharge

Abstract: A new piecewise nonequilibrium gliding arc plasma discharge integrated with a counterflow flame burner was developed and validated to study the effect of a plasma discharge on the combustion enhancement of methane-air diffusion flames. The results showed that the new system provided a well-defined flame geometry for the understanding of the basic mechanism of the plasma-flame interaction. It was shown that with a plasma discharge of the airstream, up to a 220% increase in the extinction strain rate was possible at low-power inputs. The impacts of thermal and nonthermal mechanisms on the combustion enhancement was examined by direct comparison of measured temperature profiles via Rayleigh scattering thermometry and OH number density profiles via planar laser-induced fluorescence (calibrated with absorption) with detailed numerical simulations at elevated air temperatures and radical addition. It was shown that the predicted extinction limits and temperature and OH distributions of the diffusion flames, with only an increase in air temperature, agreed well with the experimental results. These results suggested that the effect of a stabilized piecewise nonequilibrium gliding arc plasma discharge of air at low air temperatures on a diffusion flame was dominated by thermal effects.

Observations on the faceted initiation site in the dwell-fatigue tested Ti-6242 alloy : Crystallographic orientation and size effects

Abstract: A novel use of the electron backscattered diffraction (EBSD) characterization technique for study of fracture has been demonstrated. This new approach has been employed for characterization and analysis that contribute to the understanding of crack initiation in Ti-6242 under dwell-fatigue loading conditions. A faceted crack initiation site is typically observed on the dwell-fatigue fracture surface of Ti-6242. The level of microtexture has a major influence on the dwell-fatigue failures in near-α titanium alloys, such as Ti-6242. In this study, serial sectioning and EBSD techniques were used to obtain the orientation images of almost the entire specimen cross section at different depths below the fracture surface. The orientation images are color coded on three different bases: the angle between the loading axis and basal plane normal, the Schmid factor for prism slip, and the Schmid factor for basal slip. The aim was to determine the important aspects of the crystallographic orientation and the size of the microtextured region that is associated with the faceted initiation site. The results of this study are used to explain the possible locations of crack initiation in a test specimen under dwell-fatigue loading condition. These results are also used to better understand the role of size of microtextured regions in determining which crack will outgrow the other cracks (for the case of multiple cracking typically observed in the alloy of current study under the dwell-fatigue loading conditions) to become the dominant crack that leads to eventual specimen failure. This understanding has important practical implications because the dominant crack effectively determines the specimen life.

Progress in ZnO materials and devices

Abstract: ZnO is a wide-band-gap semiconductor material that is now being developed for many applications, including ultraviolet (UV) light-emitting diodes, UV photodetectors, transparent thin-film transistors, and gas sensors. It can be grown as boules, as thin films, or as nanostructures of many types and shapes. However, as with any useful semiconductor material, its electrical and optical properties are controlled by impurities and defects. Here, we consider various important donor-type impurities, such as H, Al, Ga, and In, and acceptor-type impurities, such as N, P, As, and Sb. We also examine the effects of a few common point defects, including Zn interstitials, Zn vacancies, O vacancies, and complexes of each. The main experimental techniques of interest here include temperature-dependent Hall-effect and low-temperature photoluminescence measurements, because they alone can provide donor and acceptor concentrations and donor energies. The important topic of p-type ZnO is also considered in some detail.

Characterization of metal and metal alloy films as contact materials in MEMS switches

Abstract: This study presents a basic step toward the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between two important parameters, resistivity and hardness, since they provide the guidelines and assessment of contact resistance, wear, deformation and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals, platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au), were investigated. The interrelationship between resistivity and hardness was established for three levels of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh) and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Polymeric Nanolayers as Actuators for Ultrasensitive Thermal Bimorphs

Abstract: Polymeric nanolayers are introduced here as active, thermal-stress mediating structures facilitating extremely sensitive thermal detection based upon the thermomechanical response of a bimaterial polymer-silicon microcantilever. To maximize the bimaterial bending effect, the microcantilever bimorph is composed of stiff polysilicon, with a strongly adhered polymer deposited via plasma-enhanced chemical vapor deposition. The polymer layers with thickness ranging from 20 to 200 nm possess a rapid and pronounced response to temperature fluctuations due to intrinsic sensitive thermal behavior. We show that by taking advantage of the thermal stresses generated by the huge mismatch of material properties in the polymer-silicon bimorph, unprecedented thermal sensitivities can be achieved. In fact, the temperature resolution of our bimaterial microcantilevers approaches 0.2 mK with thermal sensitivity reaching 2 nm/mK; both parameters are more than an order of magnitude better than the current metal-ceramic design. This new hybrid platform overcomes the inherently limited sensitivity of current sensor designs and provides the basis to develop the ultimate uncooled IR microsensor with unsurpassable sensitivity.

Microswitches with sputtered Au, AuPd,Au-on-AuPt, and AuPtCu alloy electric contacts

Abstract: This paper is the first to report on a new analytic model for predicting microcontact resistance and the design, fabrication, and testing of microelectromechanical systems (MEMS) metal contact switches with sputtered bimetallic (i.e., gold (Au)-on-Au-platinum (Pt), (Au-on-Au-(6.3at%)Pt)), binary alloy (i.e., Au-palladium (Pd), (Au-(3.7at%)Pd)), and ternary alloy (i.e., Au-Pt-copper (Cu), (Au-(5.0at%)Pt-(0.5at%)Cu)) electric contacts. The microswitches with bimetallic and binary alloy contacts resulted in contact resistance values between 1-2Omega. Preliminary reliability testing indicates a 3times increase in switching lifetime when compared to microswitches with sputtered Au electric contacts. The ternary alloy exhibited approximately a 6times increase in switch lifetime with contact resistance values ranging from approximately 0.2-1.8Omega

The Transverse Humeral Ligament: A Separate Anatomical Structure or a Continuation of the Osseous Attachment of the Rotator Cuff?

Abstract: BackgroundNo study to date has isolated the anatomical nature of the transverse humeral ligament and its relationship to the biceps tendon and the anterosuperior portion of the rotator cuff.HypothesisThere is no separate identifiable transverse humeral ligament, but rather the fibers covering the intertubercular groove are composed of a sling formed by fibers from the subscapularis and supraspinatus tendons.Study DesignDescriptive laboratory study.MethodsA total of 14 shoulder examinations were performed on 7 matched pairs of fresh-frozen cadaveric shoulders. Magnetic resonance imaging scans were performed, followed by gross and microscopic anatomical dissection.ResultsIn the location of the transverse humeral ligament, magnetic resonance imaging and gross dissection revealed the continuation of superficial fibers of the subscapularis tendon from the tendon body across the intertubercular groove to attach to the greater tuberosity, whereas deeper fibers of the subscapularis tendon inserted on the lesser t...

Dynamic-coarsening behavior of an α/β titanium alloy

Abstract: The dynamic-coarsening behavior of Ti-6Al-4V with an equiaxed α microstructure was established via isothermal hot-compression testing of cylindrical samples cut from an ultra-fine-grain-size (UFG) billet. Compression experiments were conducted at 900 and 955 °C, strain rates between 10−4 and 1 s−1, and imposed true strains between 0 and 1.4. Following deformation, quantitative metallography revealed marked coarsening of the primary α particles at low strain rates (10−4 and 10−3 s−1). The dynamic-coarsening rate followed rn vs time kinetics, in which n was between 2 and 3, or behavior between those of bulk-diffusion and interface-reaction controlled. An examination of the temperature and strain-rate dependence of theoretical coarsening rates, however, strongly suggested that bulk diffusion (with n=3) was more important. The dynamic-coarsening behavior was also interpreted in the context of the observed plastic-flow behavior. At low strain rates, high values of the strain-rate sensitivity (m>0.5) and the overall shape of log stress-log strain rate plots indicated that the majority of the imposed strain was accommodated by grain-boundary sliding (gbs) and only a small amount via dislocation glide/climb processes. In addition, an analysis of the flow hardening that accompanied dynamic coarsening indicated that the flow stress varied approximately linearly with the α particle size, thus providing support for models based on gbs accommodation by dislocation activity in grain-mantle regions.

Micromechanical Modeling of Viscoelastic Properties of Carbon Nanotube-Reinforced Polymer Composites

Abstract: A micromechanics model is developed for predicting the linearly viscoelastic properties of carbon nanotube-reinforced polymer composites. By employing the Correspondence Principle in viscoelasticity, the Mori-Tanaka method is extended to the Carson domain. The inversion of the creep compliances from the Carson (transformed) domain to the time (physical) domain is accomplished numerically by using a recently developed multi-precision algorithm. The new micromechanics model is validated by comparing with existing experimental data. By applying the presently developed model, a parametric study for the creep behavior of carbon nanotube-reinforced polymer composites is conducted, with testing temperature, nanotube aspect ratio, nanotube volume fraction and nanotube orientation as the controlling parameters. For composites having unidirectionally aligned nanotubes, numerical results indicate that the increase of the nanotube aspect ratio significantly enhances their axial creep resistance but has insignificant ...

Vision-based UAV flight control and obstacle avoidance

Abstract: In this work, we explore various ideas and approaches to deal with the inherent uncertainty and image noise in motion analysis, and develop a low-complexity, accurate and reliable scheme to estimate the motion fields from UAV navigation videos. The motion field information allows us to accurately estimate ego-motion parameters of the UAV and refine (or correct) the motion measurements from other sensors. Based on the motion field information, we also compute the range map for objects in the scene. Once we have accurate knowledge about the vehicle motion and its navigation environment (range map), control and guidance laws can be designed to navigate the UAV between way points and avoid obstacles.

Carbon ad-dimer defects in carbon nanotubes.

Abstract: The adsorption of carbon dimers on carbon nanotubes leads to a rich spectrum of structures and electronic structure modifications. Barriers for the formation of carbon dimer induced defects are calculated and found to be considerably lower than those for the Stone-Wales defect. The electronic states introduced by the ad-dimers depend on defect structure and tube type and size. Multiple carbon ad-dimers provide a route to structural engineering of patterned tubes that may be of interest for nanoelectronics.

Effectiveness of z-pins in preventing delamination of co-cured composite joints on the example of a double cantilever test

Abstract: The effectiveness of z-pins in co-cured joints is illustrated on the model of a composite double cantilever beam (DCB) subject to a standard fracture toughness test. A comprehensive solution is presented in the paper accounting for a broad spectrum of issues that affect the problem. They include the accurate evaluation of the rotational constraint provided by the intact section of DCB, possible transverse shear deformation in the delaminated section, and effects of uniform and nonuniform temperature on the response. A simple criterion for the effectiveness of z-pins in co-cured joint is introduced and its application is illustrated on numerous examples. As follows from the analysis, z-pinning is an effective method of enhancing delamination resistance of composite joints. Even a very small volume fraction of z-pins (less than 1.5%) may arrest delamination in co-cured composite joints.

Modeling the tensile properties in β-processed α/β Ti alloys

Abstract: The development of a set of computational tools that permit microstructurally based predictions for the tensile properties of commercially important titanium alloys, such as Ti-6Al-4V, is a valuable step toward the accelerated maturation of materials. This paper will discuss the development of neural network models based on a Bayesian framework to predict the yield and ultimate tensile strengths of Ti-6Al-4V at room temperature. The development of such rules-based model requires the population of extensive databases, which in the present case are microstructurally based. The steps involved in database development include producing controlled variations of the microstructure using novel approaches to heat treatments, the use of standardized stereology protocols to characterize and quantify microstructural features rapidly, and mechanical testing of the heat-treated specimens. These databases have been used to train and test neural network models for prediction of tensile properties. In addition, these models have been used to identify the influence of individual microstructural features on the tensile properties, consequently guiding the efforts toward development of more robust mechanistically based models. Based on the neural network model, it is possible to investigate the influence of individual microstructural features on the tensile properties, and in certain cases these dependencies can point toward unrecognized phenomena. For example, the apparently unexpected trend of increase in tensile strength with increasing prior β-grain size has led to the determination of the pronounced role of the basketweave microstructure in strengthening these alloys, especially in case of larger prior β grains.

YBCO/Ag boundary resistivity in YBCO tapes with metallic substrates

Abstract: The quality of the contact between a YBCO layer and the protective silver coating is an important parameter affecting the current transfer between YBCO and the normal metal. We studied experimentally the quality of this contact in 6 mm wide YBCO-coated conductor with a critical current of ~60 A at 77 K. The measured current transfer length for the original sample of a YBCO-coated conductor covered by ~3 µm thick silver and for the same sample additionally laminated by a 25 µm thin copper layer was 0.19 and 0.47 mm, respectively. The contact resistivity determined from these experiments was between 3.7 × 10−11 and 7.0 × 10−12 Ω m2. A direct measurement of the resistance between two overlapped tapes soldered by indium yielded values between 2.5 × 10−12 and 5 × 10−12 Ω m2. The boundary resistance and current transfer length are important parameters for the design of the optimal tape architecture for coils and windings, especially for stability issues.

Calculation of two-photon absorption spectra of donor-π-acceptor compounds in solution using quadratic response time-dependent density functional theory

Abstract: Linear and quadratic response time-dependent density functional theories have been applied to calculate the photophysical properties of donor-π-acceptor molecules which are known to have large nonlinear absorption. The linear absorption and two-photon absorption spectra predicted using hybrid functionals, including the Coulomb-attenuated model, with continuum solvation models are reported and compared to experiment and to previous theoretical predictions. While the quadratic response with these functionals overestimated the TPA cross sections relative to experiment when a Gaussian linewidth function was used, a fairly good agreement was obtained when a Lorentzian linewidth function was applied. In addition, the comparison of the TPA cross sections calculated by the sum over states with those calculated by the two-state approximation indicates the importance of the higher energy states in TPA, particularly in nondegenerate experiments.

A 3-D Monte-Carlo (Potts) model for recrystallization and grain growth in polycrystalline materials

Abstract: The evolution of microstructure during annealing of deformed polycrystalline metallic materials was modeled using a 3-D Monte-Carlo (Potts) approach in which the initial microstructure, texture (ODF), spatial distribution of the stored energy of deformation, and nucleation mechanism were carefully quantified. The modeling technique was tested using several special cases of 3-D stored-energy distributions and different textures of the as-deformed state.