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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Relationship between fatigue life in the creep-fatigue region and stress-strain response

On the basis of mechanical tests and metallographic studies, strainrange partitioned lives were predicted by introducing stress-strain materials parameters into the Universal Slopes Equation. This was the result of correlating fatigue damage mechanisms and deformation mechanisms operating at elevated temperatures on the basis of observed mechanical and microstructural behavior. Correlation between high temperature fatigue and stress strain properties for nickel base superalloys and stainless steel substantiated the method. Parameters which must be evaluated for PP- and CC- life are the maximum stress achievable under entirely plastic and creep conditions respectively and corresponding inelastic strains, and the two more pairs of stress strain parameters must be ascertained.

Berkovits, A.↗
NASA NTRS1990

Source parameters of earthquakes and intraplate deformation beneath the Shillong Plateau and the northern Indoburman ranges

The fault-plane solutions and focal depths are determined for 17 earthquakes beneath the Shillong Plateau and the northern Indoburman ranges by combining results from the inversion of long-period P and SH waveforms and amplitudes, from polarities of first motions, and from the identification of pP and sP phases on short-period seismograms. Fault-plane solutions of 15 earthquakes show mixtures of thrust and strike-slip faulting, but the P axes for these events are nearly horizontal and consistently oriented N-NE-S-SW. All of these earthquakes occurred at depths greater than 29 km. Beneath the Shillong Plateau, one event occurred at a depth of 52 km. The relatively large depths for earthquakes in an intraplate setting suggest that the Indian lithosphere in this area is especially cold.

Chen, Wang-Ping↗
NASA NTRS1991

A theoretical comparison of the near-optimum design and predicted performance of n/p and p/n indium phosphide homojunction solar cells

Using a detailed simulation model of p(+)nn(+) and n(+)pp(+) indium phosphide (InP) homojunction solar cells, extensive parametric variation computer simulation runs were performed to aid in making near-optimum designs for these two solar cell configurations. The values of all the geometrical and material parameters corresponding to the near-optimal designs of both these configurations are presented. The results of parametric variation runs are presented for each configuration showing how the performance parameters J(sub sc), V(sub oc), and eta vary with each of the cell design parameters for the near-optimally designed cell. Finally, the theoretically obtained results are discussed, and the relative merits and drawbacks of the two configurations are compared.

Goradia, Chandra↗
NASA NTRS1991

Fluid volcanism on Miranda and Ariel - Flow morphology and composition

Several types of volcanic units have been recognized on the icy Uranian satellites Miranda and Ariel. On Miranda, ridges characterized by crest grooves are up to 10 km wide and 500 m high. A continuous flat-topped flow band also 10 km wide and 500 m high forms the outer southern margin of Elsinore Corona, which appears to comprise coalesced flow bands and ridges. On Ariel, in addition to at least one ridge unit similar to those on Miranda, flood plains material has covered the floors of deep chasmata (grabens) and an irregular depression. Flows on both satellites are characterized by linear vent geometries and distinct topographic margins, which indicate extrusion of a relatively viscous material. The topography of the flows can be used to estimate flow viscosity or yield strength using a Bingham plastic model. Extrusion viscosity estimates, incorporating plausible volcanologically based emplacement time scales and a rigid crust correction, range from 10 MP to 1 GP (10 TP in the unlikely absence of a chilled crust). Viscosity estimates are dependent on the assumed emplacement time scale, however, and could be as high as 10 PP, if a solid-state-based time scale is assumed.

Schenk, Paul M.↗
NASA NTRS1991

G189A modelling of Space Station Freedom's ECLSS

This paper contains a review of the requirements and design of the Environmental Control and Life Support System (ECLSS) for Space Station Freedom; a review of the G189A ECLSS computer program model developed at Boeing for the complete configuration of pressurized elements or volumes; and some significant computed results from this model showing transient performance for subsystems responsible for Temperature and Humidity Control,. Atmosphere Control and Supply, Air Revitalization, and Water Recovery and Management. The computed results presented here are important in assessing the capabilities of the ECLSS equipment in maintaining acceptable levels of temperature, humidity, 02, N2 and (2% in the occupied volumes during crew changeout events. These events involve up to four extra crew members from the Orbiter being located temporarily in the Space Station modules in addition to the normal contingent of four crew members. These additional crew members impose additional demands on the ECLSS equipment, in order to control the levels of the aforementioned quantities. Satisfactory control of all of these quantities was achieved. The maximum computed pp C02 was in the acceptable degraded performance regime. This level is higher than the 3.00 mm Hg upper limit for normal conditions

Environmental Control and Life Support Systems (EC↗
NASA NTRS1992

Implications of new GALLEX results for the Mikheyev-Smirnov-Wolfenstein solution of the solar neutrino problem

We compare the implications for Be-7 and pp neutrinos of the two Mikheyev-Smirnov-Wolfenstein fits to the new GALLEX solar neutrino measurements. Small-mixing-angle solutions tend to suppress the former as electron neutrinos, but not the latter, and large-angle solutions tend to reduce both by about a factor of two. The consequences for BOREXINO and similar solar neutrino-electron scattering experiments are discussed.

Gelb, James M.↗
NASA NTRS1993

Oligomerization reactions of ribonucleotides - The reaction of the 5'-phosphorimidazolide of adenosine with diadenosine pyrophosphate on montmorillonite and other minerals

The overall regiospecificity of the 3',5'-phosphodiester bond formation is shown to be increased from 64 to 83 percent when a 9:1 ratio of ImpA to A5'ppA instead of ImpA alone is reacted in the presence of Na(+)-montmorillonite. The major factor in the regiospecificity increase is the formation of 3',5'-linked A5'ppA3'pA and A5'pp-A3'pA3'pA. The results obtained show that ImpA reacts more readily with A5'ppA than it does with another ImpA molecule and that A5'ppA plays a central role in directing the formation of 3',5'-phosphodiester bonds.

Ferris, James P.↗
NASA NTRS1993

Experimentally reproduced relict enstatite in porphyritic chondrules of enstatite chondrite composition

Experiments are presented that test a model for the origin of porphyritic pyroxene (PP) chondrules in enstatite chondrites that contain phenocrysts of enstatite with blue cathodoluminescence (CL) set in a matrix of radial, dendritic enstatite with red CL. Established one-atmosphere, gas-mixing techniques were used. Relict enstatite phenocrysts with blue CL in a matrix of coarsely radial to dendritic enstatite with red CL were successfully produced. The relict crystals are preserved in runs with a melt time of 36 minutes or less at 1537 C. The relicts remain angular with smooth crystal/melt interfaces, and thus melting has occurred uniformly. Partial melting does occur along fractures produced when the blue CL enstatite was initially grown and cooled through the proto/ortho enstatite transition with the attendent volume change. There is either reaction with the melt and diffusion of Mn and Cr into the blue CL En, or there is an overgrowth of red CL En along the fractures. The bulk of the relicts remain blue. The melt enclosing the relicts crystalized to a coarsely radial to dendritic to micro porphyritic texture comprised of enstatite that has a bright red CL with decreasing melt time. The blue CL En has Mn and Cr contents at or below detection limits of the electron probe as described in earlier studies and in natural blue CL En. In the red CL En in this study, the Mn, Al2O3, and Cr are at previously observed levels and the levels change rapidly.

Lofgren, Gary E.↗
NASA NTRS1993

Opaque-rich lithology in the Divnoe achondrite: Petrology and origin

An opaque-rich lithology (ORL) makes up to approximately 9 vol. percent of the Divnoe achondrite. It is characterized by enrichment of troilite and pyroxene, compared to the bulk Divnoe, and by the presence of tiny remnants of olivine in low-CA pyroxene and low minor element concentrations in pyroxene. The ORL was formed by local reaction between gaseous sulfur and olivine in the Divnoe. The Divnoe meteorite is an olivine-rich achondrite with subchondritic chemistry and mineralogy. It has a granoblastic coarse-grained olivine groundmass (CGL: coarse-grained lithology) with relatively large (2-10 mm) poikilitic patches (PP) of pyroxene and, rarely, plagioclase. The groundmass also contains an opaque-rich fine-grained lithology (ORL) which comprises approximately 9 vol. percent of the meteorite, displays reaction boundaries with the groundmass, and differs in mineral chemistry from it. Numerous micron- to mm-thick veins of troilite and, rarely, metal cross all the lithologies found in the meteorite. The Divnoe itself appears to represent a sample of an igneous source region.

Petaev, M. I.↗
NASA NTRS1994

Nonlinear stability of oscillatory core-annular flow: A generalized Kuramoto-Sivashinsky equation with time periodic coefficients

In this paper the nonlinear stability of two-phase core-annular flow in a pipe is examined when the acting pressure gradient is modulated by time harmonic oscillations and viscosity stratification and interfacial tension is present. An exact solution of the Navier-Stokes equations is used as the background state to develop an asymptotic theory valid for thin annular layers, which leads to a novel nonlinear evolution describing the spatio-temporal evolution of the interface. The evolution equation is an extension of the equation found for constant pressure gradients and generalizes the Kuramoto-Sivashinsky equation with dispersive effects found by Papageorgiou, Maldarelli & Rumschitzki, Phys. Fluids A 2(3), 1990, pp. 340-352, to a similar system with time periodic coefficients. The distinct regimes of slow and moderate flow are considered and the corresponding evolution is derived. Certain solutions are described analytically in the neighborhood of the first bifurcation point by use of multiple scales asymptotics. Extensive numerical experiments, using dynamical systems ideas, are carried out in order to evaluate the effect of the oscillatory pressure gradient on the solutions in the presence of a constant pressure gradient.

Coward, Adrian V.↗
NASA NTRS1994

Porphyritic versus nonporphyritic chondrules

Chondrules can be divided into two broad textural types: porphyritic and nonporphyritic. Porphyritic chondrules are the most common in most chondrites and range texturally from olivine-rich (PO) to intermediate (POP) to pyroxene-rich (PP). Barred olivine (BO) chondrules can be considered a special case of porphyritic. Compositionally they can be divided into type I and II. Nonporphyritic are less abundant than porphyritic chondrules in most chondrites -- they make up approximately 125% of the chondrules in ordinary chondrites -- and range texturally from glassy (g) to cryptocrystalline (C) to radial pyroxene (RP). Compositionally nonporphyritic differ from porphyritic chondrules and within this group they are very similar to one another. Here we (1) review and contrast the characteristics of the nonporphyritic and porphyritic chondrules; (2) specify some of the problems associated with the origins of the textural and compositional differences between them; and (3) suggest a possible scenario for their origin, which may have important implications for the evolution of chondrules.

Nehru, C. E.↗
NASA NTRS1994

Performance, defect behavior and carrier enhancement in low energy, proton irradiated p+nn+ InP solar cells

The highest AMO efficiency (19.1 percent) InP solar cell consisted of an n+pp+ structure epitaxially grown on a p+ InP substrate. However, the high cost and relative fragility of InP served as motivation for research efforts directed at heteroepitaxial growth of InP on more viable substrates. The highest AMO efficiency (13.7 percent) for this type of cell was achieved using a GaAs substrate. Considering only cost and fracture toughness, Si would be the preferred substrate. The fact that Si is a donor in InP introduces complexities which are necessary in order to avoid the formation of an efficiency limiting counterdiode. One method used to overcome this problem lies in employing an n+p+ tunnel junction in contact with the cell's p region. A simpler method consists of using an n+ substrate and processing the cell in the p+ nn+ configuration. This eliminates the need for a tunnel junction. Unfortunately, the p/n configuration has received relatively little attention the best cell with this geometry having achieved an efficiency of 17 percent. Irradiation of these homoepitaxial cells, with 1 Mev electrons, showed that they were slightly more radiation resistant than diffused junction n/p cells. Additional p/n InP cells have been processed by some activity aimed at diffusion. Currently, there has been some activity aimed at producing heteroepitaxial p+nn+ InP cells using n+ Ge substrates. Since, like Si, Ge is an n-dopant in InP, use of this configuration obviates the need for a tunnel junction. Obviously, before attempting to process heteroepitaxial cells, one must produce a reasonably good homoepitaxial cell. In the present case we focus our attention on homoepitaxially on an n+ Ge substrate.

Weinberg, I.↗
NASA NTRS1993

Classification and soil moisture determination of agricultural fields

During the Mac-Europe campaign of 1991 several SAR (Synthetic Aperature Radar) experiments were carried out in the Flevoland test area in the Netherlands. The test site consists of a forested and an agricultural area with more than 15 different crop types. The experiments took place in June and July (mid to late growing season). The area was monitored by the spaceborne C-band VV polarized ERS-1, the Dutch airborne PHARS with similar frequency and polarization and the three-frequency PP-, L-, and C-band) polarimetric AIRSAR system of NASA/JPL. The last system passed over on June 15, 3, 12, and 28. The last two dates coincided with the overpasses of the PHARS and the ERS-1. Comparison of the results showed that backscattering coefficients from the three systems agree quite well. In this paper we present the results of a study of crop type classification (section 2) and soil moisture determination in the agricultural area (section 3). For these studies we used field averaged Stokes matrices extracted from the AIRSAR data (processor version 3.55 or 3.56).

Vandenbroek, A. C.↗
NASA NTRS1995

Effects of prescattering attenuation of spectropolarimetric line profiles

We present the wavelength dependent linear polarization and position angle variations arising from single Thomson scattering of continuum stellar radiation in an axisymmetric, hot, planar disk, which is in bulk motion. The wavelength dependence arises through the combination of Doppler shifted line attenuation, prior to scattering, and the Doppler redistribution of the scattered radiation due to the bulk and thermal motions of the disk electrons. We find that when the scattering electrons are cold, there are large polarization and position angle variations across the resulting spectrapolarimetric absorption line profile-agreeing with previous investigations. However, when the thermal Doppler velocities of the scattering electrons are comparable to or greater than the electron bulk velocity (as is of ten the case), then the scattered line profile is broadened and the amplitude of the polarimetric variations are considerably reduced. The properties of the electron thermal smearing function are such that the equivalent width of any scattered spectrapolarimetric feature is independent on the electron temperature-a consequence of the conservation of areas under convolution. An important result of this is that for an axisymmetric circumstellar envelope the position angle variations will average to zero across any line formed through the above process. We expect that the effect of multiple line blending will yield a depolarization equivalent width is given by thesum of the individual equivalent widths and the position angle variations will average to zero. The implications of this are that the large depolarizations observed in the UV spectra of pi Aquarii, zeta Tauri and PP Carinae by the Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) can only be explained with prescattering attenuation of stellar flux if there are a sufficient number of multiple overlapping lines in that part of the spectrum.

Wood, Kenneth↗
NASA NTRS1995

Astro-D observations of flares: Detecting the impulsive phase

The flare star Proxima Centauri was observed by the satellite for approximately 50 ks and a number of flares were successfully detected. The scientific results were presented at several meetings and were discussed in articles, primarily in 'Solar-like M-Class X-ray Flares on Proxima Centauri Observed by the ASCA Satellite' by Haisch, Antunes and Schmitt, Science, Vol. 268, pp. 1327-1329, attached to the report.

Haisch, Bernhard↗
NASA NTRS1995

Reflectometer distance measurement between parallel conductive plates

This report presents an analytic and experimental investigation of the measurement problem in which a reflectometer is used to determine the distance to a target that is a highly conductive surface parallel to the reflectometer antenna ground plane. These parallel surfaces constitute a waveguide (WG) which can contribute parasitic perturbations that seriously degrade the accuracy of the measurements. Two distinct parallel-plate-waveguide (PPWG) phenomena are described, and their effects on both frequency and time-domain reflectometers are considered. The time-domain processing approach was found to be superior to a representative frequency-domain phase-measurement approach because of less susceptibility to perturbations produced by edge reflections and immunity to phase capture. Experimental results are presented which show that a simple radiating system modification can suppress parallel-plate (PP) propagation. The addition of a thin layer of lossy mu-metal 'magnetic absorber' to the antenna ground plane allowed a measurement accuracy of 0.025 cm (0.01 in.) when a vector network analyzer (VNA) is used as a time-domain reflectometer.

Hearn, Chase P.↗
NASA NTRS1998

Laboratory Evaluation and Application of Microwave Absorption Properties Under Simulated Conditions for Planetary Atmospheres

Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments, entry probe radio signal absorption measurements, and earth-based radio astronomical observations can be used to infer abundances of microwave absorbing constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. The use of theoretically-derived microwave absorption properties for such atmospheric constituents, or using laboratory measurements of such properties taken under environmental conditions which are significantly different than those of the planetary atmosphere being studied, often leads to significant misinterpretation of available opacity data. For example, laboratory measurements completed recently by Kolodner and Steffes (ICARUS 132, pp. 151-169, March 1998, attached as Appendix A) under this grant (NAGS-4190), have shown that the opacity from gaseous H2SO4 under simulated Venus conditions is best described by a different formalism than was previously used. The recognition of the need to make such laboratory measurements of simulated planetary atmospheres over a range of temperatures and pressures which correspond to the altitudes probed by both spacecraft entry probe and orbiter radio occultation experiments and by radio astronomical observations, and over a range of frequencies which correspond to those used in such experiments, has led to the development of a facility at Georgia Tech which is capable of making such measurements. It has been the goal of this investigation to conduct such measurements and to apply the results to a wide range of planetary observations, both spacecraft and earth-based, in order to determine the identity and abundance profiles of constituents in those planetary atmospheres.

Steffes, Paul G.↗
NASA NTRS1997

Intensive Exercise Training During Bed Rest Attenuates Deconditioning

Intensive exercise training during bed rest attenuates deconditioning. Med. Sci. Sports Exerc., Vol. 29, No. 2, pp. 207-215, 1997. A 30-d 6 deg head-down bed rest project was conducted to evaluate variable high-intensity, short-duration, isotonic cycle ergometer exercise (ITE) training and high-intensity intermittent resistive isokinetic exercise (IKE) training regimens designed to maintain peak VO2 and muscle mass, strength, and endurance at ambulatory control levels throughout prolonged bed rest. Other elements of the deconditioning (adaptive) syndrome, such as proprioception, psychological performance, hypovolemia, water balance, body composition, and orthostatic tolerance, were also measured. Major findings are summarized in this paper. Compared with response during bed rest of the no exercise (NOE) control group: the ITE training regimen (a) maintained work capacity (peak VO2), (b) maintained plasma and red cell volumes, (c) induced positive body water balance, (d) decreased quality of sleep and mental concentration, and (e) had no effect on the decrease in orthostatic tolerance; the IKE training regimen (f) attenuated the decrease in peak VO2 by 50%, (g) attenuated loss of red cell volume by 40% but had no effect on loss of plasma volume, (b) induced positive body water balance, (i) had no adverse effect on quality of sleep or concentration, and 0) had no effect on the decrease in orthostatic tolerance. These findings suggest that various elements of the deconditioning syndrome can be manipulated by duration and intensity of ITE or IKE training regimens and that several different training protocols will be required to maintain or restore physiological and psychological performance of individuals confined to prolonged bed rest.

Greenleaf, John E.↗