Showing papers by "Wright-Patterson Air Force Base published in 1993"

Saccadic overhead: Information-processing time with and without saccades

Abstract: Information-processing time was compared for serial and spatially distributed visual presentations with performance measures that permit the separation of total time into its during-display and post-display components. For all subjects, there was a significant saccadic overhead, that is, less time was required with the serial format, which allowed data access without eye movements. However, the magnitude of the overhead decreased as task complexity increased. All subjects were able to exercise some control over the distribution of total processing time, trading off short during-display times with longer post-display times and vice versa.

Nonlinear absorption study of a C 60 -toluene solution

Abstract: Nonlinear absorption at 532 nm in a C60–toluene solution by using 8-ns and 30-ps laser pulses is reported. The transmittance for both pulse widths is fluence dependent. A five-level model of C60 is described that yields excellent agreement with both pulse-width data sets for incident fluences as high as approximately 1 J/cm2. Additional phenomena observed at higher fluences indicate that other mechanisms may be active and contribute to optical limiting in this regime. The application of C60 as an optical limiter material is discussed.

Sacroiliac joint fixation guide

Abstract: A novel guide or jig for safe and accurate placement of fixation screws across a sacroiliac joint uses the tip of a sacro-pedicle targeting screw inserted from the rear into a human sacrum as a target for guiding fixation screws. The head of the targeting screw attaches to one end of a straight first arm of a jig member. The other end of the straight arm of the jig member is attached at an angle to a semicircular second arm of the jig member. The semicircular second arm has an open slot to which is attached a bushing assembly which can be moved along and fixed in place anywhere along the slot. A cannulated pin set fits through the bushing assembly. The dimensions of the targeting screw and the jig member, including the angle between the two arms of the jig member and the radius of curvature of the semicircular second arm, are such that the cannulated pin set, when inserted through the bushing assembly, lies along a line drawn as a radius from the tip of the targeting screw. The cannulated pin set is used to guide and insert fixation screws across the sacroiliac joint. The jig insures that at any position of the bushing assembly along the semicircular second arm a fixation screw will be properly guided into position across the joint.

Thermoacoustic loads and fatigue of hypersonic vehicle skin panels

Abstract: A thermo-vibro-acoustic analysis of skin panels for airbreathing hypersonic vehicles is made for a generic trajectory and vehicle design. Aerothermal analysis shows that impingement of the bow shock wave on the vehicle produces fluctuating pressures, and local heat fluxes greatly exceed those due to the attached turbulent boundary. Thermal analysis of carbon-carbon skin panels shows that maximum temperatures will exceed 2700°F (1480°C) at the top of the ascent trajectory. Engine acoustic analysis indicates that sound levels will exceed 170 dB. As a result, loads due to engine acoustics and shock impingement dominate the design of many transatmospheric vehicle skin panels.

Deformation Behavior of an Al2O3─Y3Al5O12 Eutectic Composite in Comparison with Sapphire and YAG

Abstract: The relationship between a composite and its constituents with respect to high-temperature deformation behavior was investigated using the Al2O3-Y3Al5O12 (YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single-crystal sapphire and YAG were studied under identical conditions, using constant-strain-rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG-sapphire interface boundaries may cause the inferior creep behavior at lower strain rates.

Elevated temperature fatigue crack growth in alloy 718—part ii: effects of environmental and material variables

Abstract: Observations concerning the effects of the environment and material variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of deformation characteristics in the crack tip region. The review of the role of material variables has focused on the effects of chemical composition and microstructure parameters including precipitate size and morphology as well as grain size and morphology. These analyses have suggested that the governing mechanism at the crack tip is the degree of homogeneity of plastic deformation and associated slip density. For conditions promoting homogeneous plastic deformation, with a high degree of slip density, the environmental damage contribution is shown to be limited, thus permitting the dominance of cyclic damage effects which are characterized by a transgranular crack growth mode and a lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and related intergranular fracture mode. Considering that the crack growth damage mechanism in alloy 718 ranges from fully cycle dependent to fully environment dependent, conflicting experimental observations under different operating conditions are examined and a sensitizing approach is suggested to increase the alloy resistance to environmental effects.

Air-to-ground training missions: a psychophysiological workload analysis.

Abstract: Psychophysiological measures are used to assess the workload of F4 Phantom aircraft pilots and weapon systems officers (WSOs) during air-to-ground training missions and during the performance of two levels of difficulty of a laboratory tracking task. The bombing range portion of the missions was associated with the highest pilot workload, while the WSO flying the aircraft was the highest workload segment for the WSOs. The pilots' data were found to have a wider range of values for the physiological measures than were found in the WSO data. The different levels of tracking task difficulty produced significant physiological effects but the range of values found for most of the flight segments were much greater. These data demonstrate that extrapolating laboratory data to the flight environment is risky at best. The various physiological measures were differentially sensitive to the different demands of the various flight segments.

The compositional dependence of antiphase-boundary energies and the mechanism of anomalous flow in Ni3 Al alloys

Abstract: The theory of the anomalous flow behaviour of Ll2 compounds has developed over the last 30 years. This theory has a foundation in the early estimates of the crystallographic anisotropy of antiphase-boundary (APB) energy in these compounds. In spite of this critical aspect of the theory, it is only in the last 5 years that electron microscopy has been employed to quantify the APB energies and to determine the detailed nature of dislocation structures at each stage of deformation. The present study examined binary Ni3Al single crystals having compositions which span the single-phase region, and selected ternary compositions expected to have a dominant influence on either {001} or {111} plane APB energies. Crystals were deformed in compression at an orientation near the [001] direction over the temperature range from — 196 to 700°C. Weak-beam dark-field microscopy was employed to quantify the APB energies and the APB energy anisotropy over the composition range. Experimental uncertainties associated...

Elevated temperature fatigue crack growth in alloy 718—part i: effects of mechanical variables

Abstract: — In this paper observations concerning the effects of mechanical variables on the crack growth process in alloy 718 are reviewed and analyzed on the basis of the related deformation characteristics in the crack tip region. The variables included temperature, frequency, wave shape, hold time, load ratio and load interaction. These analyses have suggested that the role of each parameter in the acceleration of crack tip damage is governed mainly by their relative influence on the nature of the corresponding plastic deformation and associated slip line density. On the basis of this view (which assumes crack growth damage covers the range from cyclic- to fully time-dependent processes), the interactive effects of loading parameters are discussed when considering the corresponding fracture mode. Conflicting experimental observations under different operating conditions are examined.

Phase transformations and microstructure evolution in sol-gel derived yttrium-aluminum garnet films

Abstract: Diphasic yttrium-aluminum garnet (Y3Al5O12, YAG) sols were made by hydrolysis of aluminum and yttrium isopropoxides. The sols were gelled across TEM grids to make films. The films were heat-treated up to 1550 °C for as long as 300 h. Heat-treatments of bulk gel were also done. Microstructure and phase evolution were observed by TEM. Some observations were done in situ in a TEM hot-stage. YAG fraction and grain size, matrix grain size, nuclei/area, and film thickness were measured. Bulk samples were characterized by x-ray, DTA, and TGA. Yttrium-aluminum monoclinic (YAM) and transition alumina appeared at 800 °C. YAG nucleated between 800 °C and 950 °C. Nucleation was weakly correlated with the transient presence of YAlO3 garnet, and was eventually site-saturated at 0.3/μm3. The change in grain growth rate of the YAM and transition alumina matrix correlated with the change in the growth rate of YAG. Between 850 °C and 1000 °C YAG growth had t1/2 dependence and 280 kJ/mole activation energy. Below 850 °C nucleation was continuous, and growth had t0.85 dependence. Above 1000 °C YAG growth had t1/4 dependence, and the matrix grains coarsened with t1/4 dependence. Thicker films reacted faster because the nuclei/area and the growth rate after nucleation scaled with thickness. YAG growth was accompanied by formation of 20–100 nm subgrains. In the late stages of matrix grain coarsening there was also some reaction to YAG by a different process. Nucleation and growth kinetics are compared with other systems. Possible mechanisms are discussed.

Pharmacokinetic Modeling of Trichloroethylene and Trichloroacetic Acid in Humans

Abstract: The development and application of appropriate physiologically based pharmacokinetic (PBPK) models of chemical contaminants will provide a rational basis for risk assessment extrapolation. Trichloroethylene (TCE) is a widespread contaminant found in soil, groundwater, and the atmosphere. Exposures to TCE and its metabolites have been found to be carcinogenic in rodents. In this study, a PBPK model for TCE and its major metabolite, trichloroacetic acid (TCA), is developed for humans. The model parameters, estimated from the relevant published literature on human exposures to TCE and its metabolites, are described. Key parameters describing the metabolism of TCE and the kinetics of TCA were estimated by optimization. The optimization was accomplished by simultaneously matching model predictions to observations of TCE concentrations in blood and exhaled breath, TCA plasma concentrations, and urinary TCA excretion from five published studies. The optimized human PBPK model provides an excellent description of TCE and TCA kinetics. The predictions were especially good for TCA plasma concentrations following repeated TCE inhalation, an exposure scenario similar to that occurring in the workplace. The human PBPK model can be used to estimate dose metrics resulting from TCE exposures and is therefore useful when considering the estimation of human health risks associated with such exposures.

Technique for increasing the data rate in a spread spectrum data link

Abstract: A method for increasing the bit rate of a data link is to select two additional 31-bit chip code patterns that are orthogonal to the present two chip codes, and to each other. This method will not require any more bandwidth that the present 10 MHz used. This method suggests that each of the four chip code patterns are assigned a two bit value i.e.: 00, 01, 10, 11. At present, the two correlated chip codes represent data in a pulse position method. No information is transferred by determining which of the two chip codes actually correlated. This new method suggests each of the four chip code patterns will still perform the pulse position modulation and also provide two additional bits of data. These additional two bits of data will up the data rate of the link by 100 percent. Alternatively, the data rate may be increased by coding the datasuch that a reduction in duty cycle is realized as well as an increase in the data rate. Variations of the coding scheme avoid repeating a chip code in successive windows to reduce the effects of multipath propagation.

Aircraft power unit with elective mechanical coupling

Abstract: An integrated aircraft power unit providing the electrical and other forms of energy needed during auxiliary power unit operating conditions-while the aircraft is on the ground, and also providing airborne or emergency power including high altitude flight emergency conditions power wherein the power unit uses an oxidant supply carried by the aircraft. In this high altitude operating condition the power unit provides lower electrical output, but lower losses and improved efficiency over previous power units. The disclosed power unit includes separate electromechanical transducers or motor generator devices coupled to a segregated pair of rotational shaft members, one for the inlet or compressor fan and one for the outlet or exhaust turbine fan with electrical coupling and then friction clutch coupling of these shafts accomplished for low altitude high energy output operation of the power unit. The described power unit is also capable of providing these aircraft energy needs with a minimum of physical changes from conventional integrated aircraft power units.

Evaluating the risk of liver cancer in humans exposed to trichloroethylene using physiological models.

Abstract: Trichloroethylene (TCE) is a widespread environmental pollutant. TCE is classified as a rodent carcinogen by the U.S. Environmental Protection Agency (EPA). Using the rodent cancer bioassay findings and estimates of metabolized dose, the EPA has estimated lifetime exposure cancer risks for humans that ingest TCE in drinking water or inhale TCE. In this study, a physiologically based pharmacokinetic (PB-PK) model for mice was used to simulate selected gavage and inhalation bioassays with TCE. Plausible dose-metrics thought to be linked with the mechanism of action for TCE carcinogenesis were selected. These dose-metrics, adjusted to reflect an average amount per day for a lifetime, were metabolism of TCE (AMET, mg/kg/day) and systemic concentration of TCA (AUCTCA, mg/L/day). These dose-metrics were then used in a linearized multistage model to estimate AMET and AUCTCA values that correspond to liver cancer risks of 1 in 1 million in mice. A human PB-PK model for TCE was then used to predict TCE concentrations in drinking water and air that would provide AMET and AUCTCA values equal to the predicted mice AMET and AUCTCA values that correspond to liver cancer risks of 1 in 1 million. For the dose-metrics, AMET and AUCTCA, the TCE concentrations in air were 10.0 and 0.1 ppb TCE (continuous exposure), respectively, and in water, 7 and 4 micrograms TCE/L, respectively.

Multiobjective Optimization of Large-Scale Structures

Abstract: This paper presents a multiobjective optimization algorithm based on generalized compound scaling techniques. The algorithm handles any number of objective functions, similar to handling behavior constraints. This technique generates a partial Pareto set while solving the optimization problem. A reliability-ba sed decision criterion is used for selecting the best compromise design. The example cases considered in this work include various disciplines in airframe structures, such as stress, displacement, and frequency with hundreds of design variables and constraints. This paper also discusses the concept of Pareto-optima l solutions in the context of a multiobjective structural optimization problem and the commonly used methods of generating Pareto-optimal solutions.

New AsGa related center in GaAs.

Abstract: A new center related to ${\mathrm{As}}_{\mathrm{Ga}}$ has been found at relatively high concentrations (${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) in semi-insulating (2\ifmmode\times\else\texttimes\fi{}${10}^{7}$ \ensuremath{\Omega} cm) molecular beam epitaxial GaAs grown at 400 \ifmmode^\circ\else\textdegree\fi{}C. Although the ir photoquenching and thermal recovery characteristics are nearly identical to those of EL2, the thermal activation energy is only 0.65\ifmmode\pm\else\textpm\fi{}0.01 eV, much lower than the EL2 value of 0.75\ifmmode\pm\else\textpm\fi{}0.01 eV. Other properties which are different include the electron-capture barrier energy, hyperfine constant, and magnetic circular dichroism spectrum.

Molecular statics simulations of core structures and motion of dislocations in NiAl

Abstract: Possible dislocation core structures in NiAl were studied using molecular statics calculations with an empirical many-body interatomic potential developed specifically for NiAl within the embedded-atom method. The cores were examined under stress, and the stress required to initiate motion was used to estimate the relative mobility of the dislocations. Dislocations with Burgers vectors of a〈001〉, a〈110〉 and a〈111〉 and of edge, screw and mixed characters were studied. The glide planes examined included the {110}, {112} and {100} planes. The results were compared with reported experimental observations and the simulation results were found to be in good agreement and/or consistent with most experimental observations to date. The results suggest that atomic simulations may offer a significant advantage over the continuum models in inderstanding the slip response of NiAl.

Static tests of jet fuel thermal and oxidative stability

Abstract: Jet fuels and a jet fuel surrogate have been thermally stressed to simulate the time/temperature history of aircraft fuel handling systems. Surrogate fuels were used to develop quantitative measurement techniques to assess fuel stability in static tests and compare the results with flowing tests. A variety of experimental techniques including Fourier transform infrared (IR), gas chromatography with atom-sensitive atomic-emission detection and high-pressure liquid chromatography have been used to study stressed and unstressed fuels. Quantitative and qualitative measurements of the deposits and the fuels are presented. In general, the static tests described here indicate that there is good agreement between static and flowing tests concerning the quality of a fuel. However, to adequately assess fuel stability, the availability of oxygen must be limited. Arbitrarily increasing the oxygen availability is likely to yield results which are not applicable to oxygen-starved stressing processes. Furthermore, contrary to expectations, the rate at which a fuel oxidizes is shown to be inversely related to the rate of formation of insoluble products.

The Effect of Pressure Gradients on Transition Zone Length in Hypersonic Boundary Layers.

Abstract: : Boundary layer transition was measured in zero, favorable, and adverse pressure gradients at Mach 8 using heat transfer. Models consisted of 7 degrees half angle forecones 0.4826 m long, followed by flared or ogive aft bodies 0.5334 m long. The flares and ogives produced constant pressure gradients. For the cases examined, favorable pressure gradients delay transition and adverse pressure gradients promote transition, but transition zone lengths are shorter in favorable pressure gradient. Results of the effect of adverse pressure gradient on transition zone lengths were inconclusive.

An analytical model for current transport in AlGaAs/GaAs abrupt HBTs with a setback layer

Abstract: An intrinsic setback layer (or spacer) is frequently used in abrupt heterojunctions to improve the emitter injection efficiency and to reduce the impurity out-diffusion from the heavily doped base to the emitter. We have developed an analytical model to predict the d.c. performance of the AlGaAs/GaAs abrupt heterojunction bipolar transistor (HBT) with a setback layer. The effects of different setback layer thicknesses on the collector and base currents are studied in detail. Our results suggest that the presence of the setback layer can improve injection efficiency, but it can also increase the base current. Furthermore, for the device considered and parameters used, it was shown that a setback layer with a 100 A thickness can yield the highest current gain and that a setback layer thicker than 100 A can actually degrade the HBT performance. The model predictions compare favorably with the results obtained from solving numerically the Poisson and continuity equations including the nonuniform spatial band distribution as well as carrier degeneracy.

Kilohertz AgGaSe 2 optical parametric oscillator pumped at 2 μm

Abstract: A high-repetition-rate high-average-power AgGaSe2 doubly resonant optical parametric oscillator has been demonstrated by using a 2-μm pump laser operating at 2.5 and 5 kHz. Energy conversion and slope efficiencies of 23% and 32%, respectively, were achieved. Sustained average power outputs as high as 740 mW were measured. Evidence of optical parametric oscillator output saturation and damage site formation at the crystal output face are effects attributed to thermal lensing in our AgGaSe2 crystal.

Performance Characteristics of Brush Seals for Limited-Life Engines

Abstract: Brush seals are potential replacements for air-to-air labyrinth seals in gas turbine engines. An investigation has been conducted to determine the performance characteristics of brush seals for application in limited-life gas turbine engines. An elevated temperature, rotating test rig was designed and built to test labyrinth and brush seals in simulated subsonic and supersonic engine conditions. Results from initial tests for subsonic applications demonstrated that brush seals exhibit appreciably lower leakage compared to labyrinth seals, and thus offer significant engine performance improvements. Performance results have been obtained showing the effect of various brush seal parameters, including: initial interference, backplate gap, and multiple brush seals in series.