Showing papers by "Wright-Patterson Air Force Base published in 1990"

Infrared absorption of deep defects in molecular-beam-epitaxial GaAs layers grown at 200 °C: Observation of an EL 2-like defect

Abstract: Infrared optical absorption and Hall-effect techniques were employed to study deep defects in As-rich molecular-beam-epitaxial GaAs layers grown at very low temperature (200 \ifmmode^\circ\else\textdegree\fi{}C). A large ir absorption band was observed between 0.55 eV and the band edge. This band is composed of photoquenchable and photounquenchable components. Photoquenching, thermal recovery from the metastable state, and ir absorption properties of the quenchable defect, of estimated concentration of \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, are identical to those of EL2. On the other hand, the unquenchable defect, of estimated concentration \ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, resembles the isolated ${\mathrm{As}}_{\mathrm{Ga}}$ antisite observed in neutron-irradiated GaAs. Both defects' concentrations, which show different isothermal annealing behavior are reduced by about an order of magnitude upon thermal annealing at 600 \ifmmode^\circ\else\textdegree\fi{}C for 10 min. This reduction is accompanied by an increase of sample resistivity by a few orders of magnitude.

Nanocrystalline titanium-magnesium alloys through mechanical alloying

Abstract: The solid solubility of magnesium in titanium under equilibrium conditions is reported to be extremely small. Mechanical alloying of a mixture of titanium and magnesium powders resulted in the formation of nanocrystalline (10–15 nm in size) grains of Ti–Mg solid solution. This solid solution has a metastable fcc structure with a = 0.426 nm and contains about 3 wt.% (6 at.%) magnesium in it. It is suggested that the fcc structure has formed as a result of the heavy mechanical deformation of the hep structure introduced during milling. High temperature annealing of the metastable solid solution led to its decomposition forming the equilibrium phases, viz., elemental titanium and magnesium.

Vortex Dynamics on a Pitching Delta Wing

Abstract: A study of the dynamic behavior of the leading-edge vortices on a delta wing undergoing oscillatory pitching motions is presented, A sharp-edged, flat-plate, delta wing having a sweep angle of 70 deg was used in this investigation. The wing was sinusoidally pitched about its one-half chord position at reduced frequencies ranging from /r = 2ir/c/i/ = 0.05 to 0.30 at root chord Reynolds numbers between 9xl04 and 3.5 x 10s, for angle-ofattack ranges of a = 29 to 39 deg and a = 0 to 45 deg. During these dynamic motions, visualization of the leadingedge vortices was obtained by injecting TiCl4 through ports located near the model apex. The location of vortex breakdown was recorded using high-speed motion-picture photography. The motion-picture records were analyzed to determine the vortex trajectory and breakdown position as a function of angle of attack. When the wing was sinusoidally pitched, hysteresis was observed in the location of the breakdown position. This hysteresis increased with reduced frequency. The velocity of breakdown propagation along the wing and the phase-lag between model motion and breakdown location were also determined. Detailed information was also obtained on the oscillation of breakdown position in both static and dynamic cases.

High-quality approximation of eigenvalues in structural optimization

Abstract: A new function for approximating natural frequency constraints during structural optimization is presented. The Rayleigh Quotient Approximation (RQA) presented here icreases the quality of the approximate frequency constraint by approximating the modal strain and kinetic energies instead of the frequency eigenvalue itself

Highly Loaded Axial Flow Compressors: History and Current Developments

Abstract: This paper discusses approaches taken over many years to achieve very high loading levels in axial-flow compressors. These efforts have been associated predominantly with aircraft turbine engines. The objective has been to reduce the size and weight of the powerplant, to increase its simplicity and ruggedness, and, whenever possible, to reduce cost. In the introduction, some fundamentals are reviewed that indicate that increased work per stage can only be obtained at a cost of increased Mach number, increased diffusion, or both. The earliest examples cited are some ambitious development programs of the 1950s and 1960s. Some innovative schemes to increase diffusion limits are described that took place in the 1960s and 1970s. Major advancements in dealing with higher Mach number were made in the 1980s. Finally, a few thoughts directed toward potential future developments are presented.

Twin-jet screech suppression

Abstract: A facility test was performed on twin-jet configurations to determine the effectiveness of several concepts in suppressing the supersonic screech tones. Supersonic jet Mach numbers up to 1.75 were tested. The screech suppression concepts were tabs, lateral spacing, axial spacing, and secondary air jets. Acoustic and optical data were obtained. It was found that the twin-jet configuration can result in screech tone amplitudes as much as 20 dB higher than a single jet. Screech tone amplitudes up to 162 dB were measured. Small tabs located at the exit plane were shown to be very effective suppressors if they were large enough or if multiple tabs were installed. Lateral spacing can result in significant tone suppression, however, at certain spacings little suppression was achieved. Axial spacing resulted in essentially no suppression. The secondary air jet was shown to be a very effective suppressor of screech tones from a single jet but was not tested on the twin jet configuration.

Mechanical Characterization of SCS-6/Ti-6-4 Metal Matrix Composite

Abstract: Off-axis tension tests were performed on SCS-6/Ti-6-4 metal matrix com posite. A one-parameter plasticity model was used to characterize the elastic-plastic prop erties. In addition, a micromechanical model was developed assuming elastic fiber and elastic-plastic matrix properties. This model was employed to relate the apparent yielding with the fiber/matrix separation in the MMC. From the micromechanical model, the fiber/matrix interfacial bond strength was estimated, and with the aid of a damage model, the nonlinear off-axis stress-strain curves were accurately predicted.

Thermochemical Processing of Titanium Alloys

Abstract: As the performance requirements of structures and devices increase, new and improved materials and processes are required. One such technique is thermochemical processing, which involves the use of hydrogen as a temporary alloying element. Thermochemical processing significantly enhances both the fabricability and mechanical behavior of titanium alloys.

Health Monitoring Aircraft

Abstract: A description is given of the health monitoring aircraft concept, which will incorporate innovative sensors, AI, and advanced analytical techniques to provivide real-time and continual aircraft health assessment. According to this concept, all flight-critical structures will be evaluated for integrity as part of the automated preflight checklist. The pilot will be given a visual display of the health of all systems prior to takeoff. Any in-flight change in the health of the aircraft wil be displayed along with recommended action. In order to achieve these capabilities, state-of-the-art structural integrity computer programs, together with AI/neutral network decision-making software, will be incorporated as part of the aircraft computing capability. The life history of the aircraft will be continually and automaticaly updated so that acccurate structural integrity assessments can be made.

High resolution scanning electron microscopy of PAN-based and pich-based carbon fibre

Abstract: On compare les fibres de carbone obtenues a partir de polyacrylonitrile et de brai du point de vue des proprietes physicomecaniques et de la microstructure

Modeling, Control, and Design of Flexible Structures: A Survey

Abstract: Large structures used in space applications, are made flexible to reduce the high cost of lifting mass to orbit. These highly flexible structures will inevitably be vulnerable to dynamic excitations caused by the slewing/pointing maneuvers, thermal transients, operations of on-board coolers and generators, etc. Thus the aspects of modeling and control become extremely important for the safe and effective operation of large flexible structures (LFS’s). The theoretical, computational, and practical methodologies developed during the last decade along with the latest trends in this area are described. The research work addressing the aspects of model reduction, passive and active control, hierarchical/decentralized control, integrated structural and control design, optimal design, and sensor/actuator location selection problem for large flexible structures is discussed.

Robustness Improvement of Actively Controlled Structures through Structural Modifications

Abstract: The parameter variations introduced by the analysis model, uncertain material properties, or optimization may adversely influence the stability and performance characteristics of a closed-loop controlled structure. The improvement of robustness of actively controlled structures through structural modifications is considered in this work. The stability and performance robustness indices are defined as measures of robustness of actively controlled structures. The integrated structural/control design problem is considered as a multiobjective optimization problem, in which three objectives—structural weight, stability robustness index, and performance robustness index—are considered for minimization. The utility function, lexicographic, and goal programming methods are applied to solve the multiobjective nonlinear programming problem. Two examples, a two-bar truss and two-bay truss, are considered to demonstrate the procedure. HERE has been a dramatic increase in the past decade in the use of active control systems to improve structural performance.1'2 The major challenge in the field of active control of structures is in the design of control systems for very large space structures. These structures are by nature distributed parameter systems with multiple inputs (controls) and a continuum of outputs (displacements). The finite-ele- ment method is commonly used for the description of these structures. This is a source of parameter errors and truncated (or reduced order) models in the system. In addition, the structural properties of large space structures cannot be tested before they are put into orbit and, hence, sizeable uncertain- ties exist in modal parameters. A great deal of research is currently in progress on develop- ing methods for the simultaneous (integrated) design of the structure and the control system. The weight of the structure was minimized, with constraints on the distribution of the eigenvalues and/or damping ratio of the closed-loop system by Khot et al.3 Miller and Shim4 considered the simultaneous minimization, in structural and control variables, of the sum of structural weight and the infinite horizon linear regulator quadratic control cost. The structure/control system optimiza- tion problem was formulated by Khot et al.,5 with constraints on the closed-loop eigenvalue distribution and the minimum Frobenious norm of the control gains. It can be seen that in all the above works the consideration of robustness of the control system has been ignored. The parameter variations introduced by the analysis model, uncertain material properties, or optimization may adversely influence the stability and performance characteristics of the control system. The robustness is an extremely important feature of a feedback control design. A robust control design is one that satisfactorily meets the system specifications, even in the presence of parameter uncertainties and other modeling errors. Since the system specifications could be in terms of

Recent Advances in Gamma Titanium Aluminide Alloys

Abstract: Gamma titanium aluminide alloys of current interest are two-phase alloys consisting of γ-TiAl phase as the matrix and a α2-Ti3Al phase as the second phase. The properties of these alloys depend on alloy composition, processing, microstructure, and their combination. Two major microstructural constituents are gamma grains and lamellar grains, the latter of which contain alternate layers of gamma (γ) and alpha-2 (α2) thin plates. The relative amounts and distribution of these two constituents are the main factors controlling mechanical properties. This paper reviews our current understanding of the composition/microstructure/property relationships. An extended discussion will be made on the fundamental aspects of the formation of lamellar structure during cooling and the evolution of microstructure occurring during thermomechanical treatments.

A study of the reaction zone in an SiC fiber-reinforced titanium alloy composite

Abstract: Results on several aspects of a SiC fiber-reinforced IMI-829 (α-titanium alloy) metal matrix composite (MMC) are presented. Scanning Auger microscopy (SAM) of SiC fibers reveals a high concentration of oxygen which varies across the diameter of the fibers. It also shows that composition of SiC changes across the diameter and, for the most part, is carbon-rich nonstoichiometric SiC. Transmission electron microscopy (TEM) of thin MMC foils shows the presence of TiC and TiSi2 in the reaction zone. Postfabrication thermal exposures of MMC s at 975 °C lead to void formation in the reaction zone. Concentration profiles of various elements across the reaction zone reveal a buildup of Zr, Nb, and Si and a decrease of Ti, Al, and Sn in the matrix around the reaction zone. Void formation in the reaction zone has been explained by the relatively high flow of Si atoms to the matrix leading to an accumulation of vacancies in the reaction zone which condense to form voids. In addition, an enhancement of hardness in the matrix around the reaction zone has been attributed to a strengthening of the matrix by solid solution and precipitation hardening, together with a contribution from residual stresses.

Robust control design with real-parameter uncertainty and unmodeled dynamics

Abstract: This paper presents a design methodology for the synthesis of a robust controller that accounts for both unmodeled dynamics and structured real-parameter uncertainty for multiple-input/multiple-output systems. The unmodeled dynamics are assumed to be characterized as a single block dynamic uncertainty at a point in the closed-loop system. In a design aimed at constraining both the H^ norm of a certain disturbance transfer matrix and a quadratic Gaussian performance index under their respective bounds, a surrogate system may be formed by modeling the structured real-parameter uncertainty as additional noise inputs and additional weights at the existing noise inputs and measurement outputs of the system. Application of a Riccati equation approach to this surrogate system then yields a robust controller that, when used in the actual system, will result in a closed-loop system that has the same H^ bound and quadratic Gaussian performance index bound as the surrogate system, even in the presence of given real-parameter variations.

Rim seal experiments and analysis for turbine applications

Abstract: An experimental investigation was conducted to determine the sealing effectiveness and the aerodynamic characteristics of four rim seal models for a number of flow conditions. The experiments were conducted to obtain an extended data base for advanced turbine rim seal design. The class of rim seals investigated are those found on the downstream side of the rotor where the boundary layer on the disk is pumped directly into the seal gap. The experiments were conducted at disk tangential Reynolds numbers up to 5.1×10 6 with a simulated gas path flow across the top of the seal

Effects of social-system factors on absenteeism, turnover, and job performance

Abstract: Changing business environments have prompted organizational an alysts to issue a call for new forms of organization capable of meeting the business challenges of the 90s (Lawler, 1986). Lawler (1986) con tends that technological and workforce changes affecting American business organizations auger a shift towards participative systems of management. These developments have also kindled renewed interest in socio technical systems theory (Trist & Bamforth, 1951). Socio-technical in terventionists key upon the complementary nature of social and techni cal systems. Because these frameworks assume that behavioral change will accompany social-system change, the current study focuses on the relationships between social-system factors and workplace attitudes, withdrawal motives, and performance behavior. Three aspects of the psycho-social task environment were investi gated in the current study, participation in decision making (PDM), group cohesiveness, and communication climate. Researchers have fre quently speculated on the consequences of these social-system factors for organizational behavior. Proponents of participative management contend that it creates a climate of trust, commitment, and motivation (Sashkin, 1984). Its

Subacute and Subchronic Toxicity of Ethylene Glycol Administered in Drinking Water to Sprague-Dawley Rats

Abstract: Subacute (10-day) and subchronic (90-day) toxicity studies of ethylene glycol (EG) were conducted in male and female Sprague-Dawley rats to provide the U.S. Environmental Protection Agency's (EPA) Office of Drinking Water with toxicity data for final preparation of a Health Advisory for the chemical. Ethylene glycol was administered in drinking water at concentrations of 0.5, 1.0, 2.0, and 4.0% for both sexes in the 10-day study. Based on a projected consumption rate of 100 ml/kg/day, the respective doses on a mg/kg/day basis would be 554, 1108, 2216, and 4432. These dose levels were also used in the 90-day study for females, but dose levels for the males in the 90-day study were 0.25, 0.5, 1.0, and 2.0% (227, 554, 1108, and 2216 mg/kg/day). At time of sacrifice necropsies were performed and tissues were prepared for histological evaluation. Blood samples were taken for hematology and clinical chemistry determinations. Body weights were measured weekly. Water and food consumption were determined three times weekly. No mortality occurred in the 10-day study. In the 90-day study 8/10 females and 2/10 males in the high dose group died prior to sacrifice. Body weights were suppressed in a dose response fashion for males and females. Hemoglobin, hematocrit, erythrocytes, and leukocytes were all significantly decreased in female rats receiving 4% EG for 10 days. The most significant histopathological findings, seen predominantly in males, were kidney lesions which included calcium oxalate crystals in tubules and pelvic epithelium; tubular dilation and degeneration; intratubular proteinaceous material; and inflammation in tubules and pelvic epithelium. At the same dose of ethylene glycol, males had more kidney lesions and much higher incidence and severity of lesions than the females.

Evaluation of brush seals for limited-life engines

Abstract: Brush seals are a relatively new concept for replacing labyrinth seals in gas turbine engines. An evaluation was performed to assess the potential of brush seals for limited-life gas turbine engines. A rotating rig was designed and built to test labyrinth and brush seals over a range of simulated engine conditions. An initial set of brush seals was rig-tested to determine leakage and wear performance and identify potential optimum configurations. The measured results showed that brush seals offer significant improvements over labyrinth seals with a factor of three or more reduction in leakage flow. Brush seals exhibit an initial wear-in period but retain significantly reduced leakage over labyrinth seals for times exceeding most limited-life engine applications. Consequently, brush seals offer the potential to precisely meter cooling/leakage air, thereby decreasing parasitic leakage and improving fuel consumption and thrust. Thus, brush seals are a definite candidate for replacing labyrinth seals in gas turbine engines.

Strapdown Astro‐Inertial Navigation Utilizing the Optical Wide‐angle Lens Startracker

Abstract: This paper discusses the application of strapdown stellar inertial systems as autonomous navigators for manned aircraft, ships, missiles, and remote piloted vehicles. It analyzes the implications of gyroscopic performance, artificial stellar image stabilization, star density, sky visibility, and sky background irradiance considerations for system performance. It concludes that a high-precision, reliable, low-cost stellar inertial system can be achieved by eliminating gimbals and combining a strapdown INS with an optical wide-angle lens startracker (OWLS).

Numerical simulation of an F-16A at angle of attack

Abstract: The transonic flowfield around an F-16A fighter configuration at a moderate incidence angle is simulated by solving the Navier-Stokes equations on a single-block grid. The numerical solution matches experimental freestream conditions with a mach number of 0.85, 16 degrees angle of attack, and a characteristic Reynolds number of 12.75 million. MacCormack's explicit algorithm is used in conjuction with a local time step and consecutive mesh refinement procedure to accelerate numerical convergence. The Baldwin-Lomax algebraic model provides turbulent closure. Computed surface pressure distributions and the aircraft lift coefficient compare favorably with wind tunnel data. The drag coefficient in the simulation overpredicts the experimental value by 8 percent.

Development of a soluble lubricity additive for perfluoropolyalkylether fluids

Abstract: A new lubricity additive for perfluoropolyalkylether (PFPAE) fluids was synthesized. A linear, unbranched fluid formulated with this additive was evaluated under boundary lubrication and elastohydrodynamic (EHD) lubrications. Performance of the base fluid imporved significantly with the incorporation of this additive.

Principles and Performance of a PC-Based Program for Simulation of Double-Axis X-Ray Rocking Curves of Thin Epitaxial Films

Abstract: PC-based software for the rapid simulation of double-axis X-ray rocking curves from epitaxial thin films by solution of the Takagi-Taupin equations is described. The principles of the mathematical model are discussed. Graded layers and interface roughness are treated by piece-wise approximation to linear or quadratic functions and fractional relaxation of each epilayer may be included. The reliability of the data-bases incororated are examined and the requirement for internal consistency demonstrated. Bench-mark tests are reported for various PC-compatible microcomputers. The shift in epilayer peak position, which occurs experimentally on reducing layer thickness at constant composition, is predicted in the simulations and compared with other simulated data in the literature. Detailed studies of generated data have been undertaken and compared with independent simulations and experimental data. Agreement is excellent.