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At least 505 records · Page 28

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NASA NTRS1991

Recrystallization and superplasticity at 300 C in an aluminum-magnesium alloy

Variations in thermomechanical processing (TMP) which regulate the microstructural characteristics and superplastic response of an Al-10Mg-0.1Zr alloy at 300 C were evaluated. Mechanical property data revealed that the superplastic ductility can be enhanced by simultaneously increasing the total rolling strain, the reduction per pass, and the duration of reheating intervals between passes during isothermal rolling. Texture and microscopy data were consistent with the development of a refined microstructure by recovery-dominated processes, i.e., continuous recrystallization, during the processing. The mechanisms by which a refined substructure can be progressively converted into a fine-grained structure during repeated cycles of deformation and annealing are addressed. A qualitative description of the complex sequence of developments leading to a microstructure better suited to support superplastic response is presented.

Hales, S. J.↗

NASA NTRS1991

Eye-safe coherent laser radar system at 2.1 microns using Tm,Ho:YAG lasers

An eye-safe pulsed coherent laser radar has been developed by using single-frequency Tm,Ho:YAG lasers and heterodyne detection. Returns from a mountainside located 145 km from the laser radar system and the measurement of wind velocity to ranges exceeding 20 km have been demonstrated with transmitted pulse energies of 22 mJ.

Henderson, Sammy W.↗

NASA NTRS1989

Intermediate temperature thermomechanical processing of Al 2090 for superplasticity

The Al-Cu-Li phase diagram indicates that during thermomechanical processing of Al 2090 at 300 C both T(1) and T(2) phases will precipitate. Following work on Al-Mg-X alloys, it is possible that TMP at such temperatures might promote intragranular formation of these phases and promote microstructural refinement during recrystallization. Microstructural analyses reveal that the T(2) phase in Al 2090 may play a similar role to the beta-phase in Al-Mg-X alloys during microstructural evolution. Mechanical property data indicate that Al 2090 can be moderately (215-245 percent) superplastic at 370 C following processing at 300 C.

Hales, S. J.↗

NASA NTRS1991

Injection-seeded operation of a Q-switched Cr,Tm,Ho:YAG laser

Single-frequency Tm,Ho:YAG lasers operating near 2 microns are attractive sources for several applications including eye-safe laser radar (lidar) and pumping of AgGaSe2 parametric oscillators for efficient generation of longer wavelengths. As part of a program to develop a coherent lidar system using Tm,Ho:YAG lasers, a diode laser-pumped tunable CW single-longitudinal-mode (SLM) Cr:Tm:Ho:YAG laser and a flashlamp-pumped single-transverse-mode Q-switched Cr,Tm,Ho:YAG laser were developed. The CW laser was used to injection-seed the flashlamp-pumped laser, resulting in SLM Q-switched output. Operational characteristics of the CW and Q-switched lasers and injection-seeding results are reported.

Henderson, Sammy W.↗

NASA NTRS1991

The X-ray cooling flow in the cluster of galaxies around PKS 2354-35

The identification of the bright, hard X-ray source 4U 0009-33 = 3A 2356-341 = 1H 2355-350 is presently confirmed on the basis of Exosat CMA detector measurements of position and spatial extent. On the basis of these X-ray observations, a strong accretion flow is deduced; the luminosity and core radius imply a central density of at least 0.008/cu cm. The central cooling time is not greater than 5 billion yr, and the mass accretion rate is about 320 solar masses/yr.

Schwartz, Daniel A.↗

NASA NTRS1990

Fabrication of structural components from commercial aluminum alloys using superplastic forming

SPF technology was used to fabricate structural components from the 7475 Al and 8090 Al-Li commercial alloys. Gas-pressurization cycles were established for SPF three-hat stiffener configurations on the basis of uniaxial data and component-geometry considerations. It is established that higher forming rates than the optimum strain rates selected from the uniaxial data for each alloy could be used in the later stages of forming without reducing SPF components' dimensional conformity. Cavitation was precluded through the use of back pressure during forming.

Hales, S. J.↗

NASA NTRS1990

The roles of Zr and Mn in processing and superplasticity of Al-Mg alloys

Processing studies have been conducted on two alloys, of nominal compositions Al-10Mg-0.1Zr or Al-10Mg-0.5Mn, in order to clarify the role of the dispersoid forming Zr or Mn additions. Mechanical property data reveal that the Mn-containing alloy has a lower maximum elongation but exhibits superplastic response over a broader range of temperature. Microstructural investigations and texture analyses were utilized to assess the effect of the presence of Al8Mg5 precipitates in combination with either Al3Zr or Al6Mn dispersoid particles during isothermal rolling at 300 C and subsequent tensile deformation at temperatures from 200-425 C.

Mcnelley, Terry R.↗

NASA NTRS1991

The application of statistically designed experiments to resistance spot welding

State-of-the-art Resistance Spot Welding (RSW) equipment has the potential to permit realtime monitoring of operations through advances in computerized process control. In order to realize adaptive feedback capabilities, it is necessary to establish correlations among process variables, welder outputs, and weldment properties. The initial step toward achieving this goal must involve assessment of the effect of specific process inputs and the interactions among these variables on spot weld characteristics. This investigation evaluated these effects through the application of a statistically designed experiment to the RSW process. A half-factorial, Taguchi L sub 16 design was used to understand and refine a RSW schedule developed for welding dissimilar aluminum-lithium alloys of different thickness. The baseline schedule had been established previously by traditional trial and error methods based on engineering judgment and one-factor-at-a-time studies. A hierarchy of inputs with respect to each other was established, and the significance of these inputs with respect to experimental noise was determined. Useful insight was gained into the effect of interactions among process variables, particularly with respect to weldment defects. The effects of equipment related changes associated with disassembly and recalibration were also identified. In spite of an apparent decrease in equipment performance, a significant improvement in the maximum strength for defect-free welds compared to the baseline schedule was achieved.

Hafley, Robert A.↗

NASA NTRS1990

Relative effects on stratospheric ozone of halogenated methanes and ethanes of social and industrial interest

Four atmospheric modeling groups have calculated relative effects of several halocarbons (chlorofluorocarbons (CFC's)-11, 12, 113, 114, and 115; hydrochlorofluorocarbons (HCFC's) 22, 123, 124, 141b, and 142b; hydrofluorocarbons (HFC's) 125, 134a, 143a, and 152a, carbon tetrachloride; and methyl chloroform) on stratospheric ozone. Effects on stratospheric ozone were calculated for each compound and normalized relative to the effect of CFC-11. These models include the representations for homogeneous physical and chemical processes in the middle atmosphere but do no account for either heterogeneous chemistry or polar dynamics which are important in the spring time loss of ozone over Antarctica. Relative calculated effects using a range of models compare reasonably well. Within the limits of the uncertainties of these model results, compounds now under consideration as functional replacements for fully halogenated compounds have modeled stratospheric ozone reductions of 10 percent or less of that of CFC-11. Sensitivity analyses examined the sensitivity of relative calculated effects to levels of other trace gases, assumed transport in the models, and latitudinal and seasonal local dependencies. Relative effects on polar ozone are discussed in the context of evolving information on the special processes affecting ozone, especially during polar winter-springtime. Lastly, the time dependency of relative effects were calculated.

Fisher, Donald A.↗

NASA NTRS1990

Relative effects on global warming of halogenated methanes and ethanes of social and industrial interest

The relative potential global warming effects for several halocarbons (chlorofluorocarbons (CFC's)-11, 12, 113, 114, and 115; hydrochlorofluorocarbons (HCFC's) 22, 123, 124, 141b, and 142b; and hydrofluorocarbons (HFC's) 125, 134a, 143a, and 152a; carbon tetrachloride; and methyl chloroform) were calculated by two atmospheric modeling groups. These calculations were based on atmospheric chemistry and radiative convective models to determine the chemical profiles and the radiative processes. The resulting relative greenhouse warming when normalized to the effect of CFC-11 agree reasonably well as long as we account for differences between modeled lifetimes. Differences among results are discussed. Sensitivity of relative warming values is determined with respect to trace gas levels assumed. Transient relative global warming effects are analyzed.

Fisher, Donald A.↗

NASA NTRS1992

Anthropomorphic teleoperation: Controlling remote manipulators with the DataGlove

A two phase effort was conducted to assess the capabilities and limitations of the DataGlove, a lightweight glove input device that can output signals in real-time based on hand shape, orientation, and movement. The first phase was a period for system integration, checkout, and familiarization in a virtual environment. The second phase was a formal experiment using the DataGlove as input device to control the protoflight manipulator arm (PFMA) - a large telerobotic arm with an 8-ft reach. The first phase was used to explore and understand how the DataGlove functions in a virtual environment, build a virtual PFMA, and consider and select a reasonable teleoperation control methodology. Twelve volunteers (six males and six females) participated in a 2 x 3 (x 2) full-factorial formal experiment using the DataGlove to control the PFMA in a simple retraction, slewing, and insertion task. Two within-subjects variables, time delay (0, 1, and 2 seconds) and PFMA wrist flexibility (rigid/flexible), were manipulated. Gender served as a blocking variable. A main effect of time delay was found for slewing and total task times. Correlations among questionnaire responses, and between questionnaire responses and session mean scores and gender were computed. The experimental data were also compared with data collected in another study that used a six degree-of-freedom handcontroller to control the PFMA in the same task. It was concluded that the DataGlove is a legitimate teleoperations input device that provides a natural, intuitive user interface. From an operational point of view, it compares favorably with other 'standard' telerobotic input devices and should be considered in future trades in teleoperation systems' designs.

Hale, J. P., II↗

NASA NTRS1992

Assessment of a head-mounted miniature monitor

Two experiments were conducted to assess the capabilities and limitations of the Private Eye, a miniature, head-mounted monitor. The first experiment compared the Private Eye with a cathode ray tube (CRT) and hard copy in both a constrained and unconstrained work envelope. The task was a simulated maintenance and assembly task that required frequent reference to the displayed information. A main effect of presentation media indicated faster placement times using the CRT as compared with hard copy. There were no significant differences between the Private Eye and either the CRT or hard copy for identification, placement, or total task times. The goal of the second experiment was to determine the effects of various local visual parameters on the ability of the user to accurately perceive the information of the Private Eye. The task was an interactive video game. No significant performance differences were found under either bright or dark ambient illumination environments nor with either visually simple or complex task backgrounds. Glare reflected off of the bezel surrounding the monitor did degrade performance. It was concluded that this head-mounted, miniature monitor could serve a useful role for in situ operations, especially in microgravity environments.

Hale, J. P., II↗

NASA NTRS1992

Climate forcing by anthropogenic aerosols

Although long considered to be of marginal importance to global climate change, tropospheric aerosol contributes substantially to radiative forcing, and anthropogenic sulfate aerosol, in particular, has imposed a major perturbation to this forcing. Both the direct scattering of short-wavelength solar radiation and the modification of the shortwave reflective properties of clouds by sulfate aerosol particles increase planetary albedo, thereby exerting a cooling influence on the planet. Current climate forcing due to anthropogenic sulfate is estimated to be -1 to -2 watts per square meter, globally averaged. This perturbation is comparable in magnitude to current anthropogenic greenhouse gas forcing but opposite in sign. Thus, the aerosol forcing has likely offset global greenhouse warming to a substantial degree. However, differences in geographical and seasonal distributions of these forcings preclude any simple compensation. Aerosol effects must be taken into account in evaluating anthropogenic influences on past, current, and projected future climate and in formulating policy regarding controls on emission of greenhouse gases and sulfur dioxide. Resolution of such policy issues requires integrated research on the magnitude and geographical distribution of aerosol climate forcing and on the controlling chemical and physical processes.

Charlson, R. J.↗

NASA NTRS1992

Study of explosive and nonexplosive cyclogenesis during FGGE

Explosive cyclogenesis during the winter of the First Global GARP Experiment (January-February 1979) is analyzed using the revised European Centre for Medium Range Weather Forecasts (ECMWF) analyses. Explosive cyclogenesis is defined as a decrease in the sea level pressure at the rate of 1 mb/h for at least 12 h. Diagnostics for 23 explosively developing cases and 16 nonexplosive cases are evaluated. Parameters compared include the dry static stability, low-level relative vorticity, vorticity advection, upper-level divergence, and kinematic vertical velocities. These parameters are compared statistically at the initial, 12-, and 24-h time periods. Parameters for which the explosive and nonexplosive cyclone ensembles were statistically separable are the kinematic vertical velocity and the upper-level divergence and vorticity advection. The strong upper-level processes for the explosive cases at the initial time indicate the importance of upper-tropospheric features in producing the stronger vertical motions and more rapid cyclogenesis.

Wash, Carlyle H.↗

NASA NTRS1990

Continuous recrystallization during thermomechanical processing of a superplastic Al-10Mg-0.1Zr alloy

Microstructural evolution via static continuous recrystallization during thermomechanical processing of an Al-Mg-Zr alloy is addressed. Mechanical property data demonstrated that as-rolled material was capable of superplastic response without further treatment. Further, superplastic ductility at 300 C was enhanced by a factor of five by increasing the reheating time between rolling passes during processing also at 300 C. This enhanced ductility was associated with a Cu-texture and a microstructure consisting of predominantly high-angle boundaries. Processing to minimize recovery resulted in a strong Brass-texture component, a predominantly low-angle boundary microstructure and poorer ductility.

Hales, S. J.↗

NASA NTRS1992

Atomic oxygen effects on thin film space coatings studied by spectroscopic ellipsometry, atomic force microscopy, and laser light scattering

The University of Nebraska is currently evaluating Low Earth Orbit (LEO) simulation techniques as well as a variety of thin film protective coatings to withstand atomic oxygen (AO) degradation. Both oxygen plasma ashers and an electron cyclotron resonance (ECR) source are being used for LEO simulation. Thin film coatings are characterized by optical techniques including Variable Angle Spectroscopic Ellipsometry, Optical spectrophotometry, and laser light scatterometry. Atomic Force Microscopy (AFM) is also used to characterize surface morphology. Results on diamondlike carbon (DLC) films show that DLC degrades with simulated AO exposure at a rate comparable to Kapton polyimide. Since DLC is not as susceptible to environmental factors such as moisture absorption, it could potentially provide more accurate measurements of AO fluence on short space flights.

Synowicki, R. A.↗

NASA NTRS1992

Electrodynamics payloads for small rockets

Totally integrated design facilitates electrical cleanliness and light weight, which are necessary in subsonic parachute-borne payloads for electrodynamics investigations. 'Blunt' probes measure ion conductivity, as do Gerdien condensers. Recent finite-element computer analyses combining flow and electrodynamics have resolved problems in determining ion densities and mobilities from Gerdien data. Three-axis electric fields are measured with deployable boom-mounted electrodes from dc through VLF. Splitting the cylindrical payload with an insulator and measuring the current between halves has provided a vertical Maxwell current detector mechanically rigid enough to measure, at ELF, energy related to coupling. A nose tip 'Smith' probe turbulence measurement is usually performed on ascent. Other instrumentation, such as photo-ionization sources and X-ray detectors, can also be included. These electrodynamic measurement payloads are about one meter in length and have a mass of about 9 kg. They can be launched with an Orion-class or smaller vehicle.

Croskey, C. L.↗

NASA NTRS1991

Virtual reality applications to automated rendezvous and capture

Virtual Reality (VR) is a rapidly developing Human/Computer Interface (HCI) technology. The evolution of high-speed graphics processors and development of specialized anthropomorphic user interface devices, that more fully involve the human senses, have enabled VR technology. Recently, the maturity of this technology has reached a level where it can be used as a tool in a variety of applications. This paper provides an overview of: VR technology, VR activities at Marshall Space Flight Center (MSFC), applications of VR to Automated Rendezvous and Capture (AR&C), and identifies areas of VR technology that requires further development.

Hale, Joseph↗