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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Analytical and experimental investigation of fatigue in a sheet specimen with an interference-fit bolt

A fatigue analysis, based on finite-element calculations and fatigue tests, was conducted for an aluminum-alloy sheet specimen with a steel interference-fit bolt. The stress analysis of the region near the bolt hole showed that the beneficial effect of an interference-fit bolt can be interpreted as the combined result of two effects: (1) load transfer through the bolt and (2) the compressive interference stresses in the sheet. Results of the fatigue tests show that progressively higher interference levels produced longer fatigue lives. The tests also show that a high level of interference prevents fretting at the bolt-sheet interface and that interferences larger than this level produced little additional improvement in fatigue life.

Crews, J. H., Jr.↗
NASA NTRS1976

Observations on the relationship of structure to the mechanical properties of thin TD-NiCr sheet

A study of the relationship between structure and mechanical properties of thin TD-NiCr sheet indicated that the elevated temperature tensile, stress-rupture, and creep strength properties are dependent on grain aspect ratio and sheet thickness. In general, the strength properties increase with increasing grain aspect ratio and sheet thickness. Tensile testing revealed an absence of ductility at elevated temperatures (not less than 1144 K). Significant creep damage as determined by subsequent tensile testing at room temperature occurs after very small amounts (less than 0.1%) of prior creep deformation over the temperature range 1144-1477 K. A threshold stress for creep appears to exist. Creep exposure below the threshold stress at T not less than 1366 K results in almost full retention of room temperature tensile properties.

Whittenberger, J. D.↗
NASA NTRS1977

Interplanetary current sheets at 1 AU

The structure and nature of 'discontinuities' in the interplanetary magnetic field at 1 AU in the period from March 18 to April 9, 1971, is determined by using high-resolution magnetic-field measurements from Explorer 43. The discontinuities that were selected for this analysis occurred under a variety of interplanetary conditions at an average rate of 0.5/hour. Both tangential and rotational discontinuities were identified. Tangential discontinuities were observed every day, even among Alfvenic fluctuations. In particular, on one day during which Alfvenic fluctuations were intense and persistent in a high-speed stream, tangential discontinuities (TD's) were seen throughout the day at an average rate of 0.5/hour; rotational discontinuities (RD's) were also observed during this day at a higher than usual rate, the ratio of TD's to RD's being approximately one. The structure of most of the current sheets was simple and ordered; i.e., the magnetic field usually changed smoothly and monotonically from one side of the current sheet to the other. The thickness distributions of the TD's and RD's with very smooth current sheets were similar. The average thickness of the RD's was 1200 km (13 proton Larmor radii), and the average thickness of the TD's was 1300 km (12 proton Larmor radii).

Burlaga, L. F.↗
NASA NTRS1977

Real-time testing of titanium sheet and extrusion coupon specimens subjected to Mach 2.7 supersonic cruise aircraft wing stresses and temperatures

The accuracy of three accelerated flight-by-flight test methods for material selection, and fatigue substantiation of supersonic cruise aircraft structure was studied. The real time stresses and temperatures applied to the specimens were representative of the service conditions in the lower surface of a Mach 2.7 supersonic cruise aircraft wing root structure. Each real time flight lasted about 65 minutes, including about one hour at (500 F) in the cruise condition. Center notched coupon specimens from six titanium materials were tested: mill-annealed, duplex-annealed, and triplex-annealed Ti-8Al-1Mo-1V sheets; mill-annealed Ti-8Al-1Mo-1V extrusion; mill-annealed Ti-6Al-4V sheet; and solution-treated and aged Ti-6Al-4V extrusion. For duplex-annealed Ti-8Al-1Mo-1V sheet, specimens with single spotweld were also tested. The test results were studied in conjunction with other related data from the literature for: material selection, structural fabrication, fatigue resistance of supersonic cruise aircraft structure, and fatigue test acceleration procedures for supersonic cruise aircraft.

Lunde, T.↗
NASA NTRS1977

Systematic study of plasma flow during plasma sheet thinnings

On the basis of a study of Imp 6 measurements of plasma flow, it is concluded that there is no clear indication of a predominance of tailward plasma flow beyond about X = -15 R sub E in the midnight sector of the plasma sheet during the expansive phase of a substorm. In fact, it is shown statistically that sunward plasma flow is more frequently observed in the midnight sector within about 30 R sub E from the earth than in any other direction during plasma sheet thinning at the substorm expansion. This result supports the conclusion that there is no definite evidence for the formation of a reconnection neutral line in the near-earth plasma sheet during most substorms.

Lui, A. T. Y.↗
NASA NTRS1978

Plasma flows and magnetic field vectors in the plasma sheet during substorms

A detailed study of the plasma flow and the magnetic field vector in the plasma sheet during magnetospheric substorms is made to determine whether plasma flows are field-aligned or crossfield. It is shown that there is generally a large magnetic field-aligned component in the rapid plasma flow observed in the plasma sheet during substorms. In particular, the larger the observed flow speed, the closer the observed flow direction is aligned with the magnetic field line. There is no clear association between the plasma flow direction and the sign of the Bz component of the magnetic field during plasma sheet thinnings at substorms. The rapid plasma flows observed in the magnetotail are predominantly magnetic field-aligned.

Lui, A. T. Y.↗
NASA NTRS1977

Multiple-satellite studies of magnetospheric substorms: Plasma sheet recovery and the poleward leap of auroral-zone activity

Particle observations from pairs of satellites (Ogo 5, Vela 4A and 5B, Imp 3) during the recovery of plasma sheet thickness late in substorms were examined. Six of the nine events occurred within about 5 min in locations near the estimated position of the neutral sheet, but over wide ranges of east-west and radial separations. The time of occurrence and spatial extent of the recovery were related to the onset (defined by ground Pi 2 pulsations) and approximate location (estimated from ground mid-latitude magnetic signatures) of substorm expansions. It was found that the plasma sheet recovery occurred 10 - 30 min after the last in a series of Pi bursts, which were interpreted to indicate that the recovery was not due directly to a late, high latitude substorm expansion. The recovery was also observed to occur after the substorm current wedge had moved into the evening sector and to extend far to the east of the center of the last preceding substorm expansion.

Pytte, T.↗
NASA NTRS1977

Passage of a weak vortex sheet through an oblique shock

The two-dimensional problem of the passage of a free vortex sheet of weak strength through an oblique shock wave of finite strength is investigated. Conditions are established which define the changes of the strength and shape of the vortex sheet after its passage through the shock wave in terms of the shock strength and the angle between the shock wave and the vortex sheet.

Ting, L.↗
NASA NTRS1978

A time dependent model of the Jovian current sheet

Analysis of vector helium magnetometer measurements during the Pioneer 10 outbound pass through the Jovian magnetosphere reveals that the average location of the near-equatorial current sheet is a surface whose shape is determined by the velocity at which the magnetic perturbations produced by a rotating tilted dipole propagate to large radial distances. A model is presented which is a surface encircling the planet and rotating rigidly with it. Based on a linear fit to the times of the current sheet crossings, a formula is derived for the surface representing the average position of the near-equatorial current sheet in the Jovian magnetosphere.

Kivelson, M. G.↗
NASA NTRS1978

Structure of current sheets in magnetic holes at 1 AU

Current density profiles in several types of interplanetary magnetic holes have been calculated using high-resolution Imp 6 magnetic field data (12.5 vector measurements/s), assuming that the currents flow in planar sheets and that the magnetic field varies only in the direction normal to the sheet. The planarity was verified in four holes which were observed by two suitably spaced spacecraft. Four types of simple magnetic holes are discussed, in which B varies nearly monotonically on each side of the hole. In two of the holes, B varies in intensity but not in direction as a result of currents normal to B. In the other two holes, B changes in both magnitude and direction as a result of currents both normal and parallel to B. The observed structures are found to be qualitatively consistent with the models of Burlaga and Lemaire (1978). Examples of complex irregular magnetic holes are also presented, and they are shown to contain multiple current sheets in which currents flow parallel to one another at various angles with respect to B. There is no model of such magnetic holes at present.

Fitzenreiter, R. J.↗
NASA NTRS1978

A two dimensional analysis of sheet resistance and contact resistance effects in solar cells

A computer model is developed to study the two-dimensional nature of the sheet resistance and contact resistance effects in solar cells. A major result is the demonstration that the distributed nature of the semiconductor sheet resistance causes the terminal dark I-V characteristics to exhibit an exp(qV/2kT) type dependence even when the one-dimensional characteristics of the cell exhibit an exp(qV/kT) type voltage dependence. The analytical model which is developed provides an easy method for estimating the sheet resistance of a solar cell from the terminal I-V data.

Fang, C. R.↗
NASA NTRS1979

Radial deformation of the solar current sheet as a cause of geomagnetic storms

It is suggested that the solar current sheet, extending from a coronal streamer, develops a large-scale radial deformation, at times with a very steep gradient at the earth's distance. The associated magnetic field lines (namely, the interplanetary magnetic field (IMF) lines) are expected to have also a large gradient in the vicinity of the current sheet. It is also suggested that some of the major geomagnetic storms occur when the earth is located in the region where IMF field lines have a large dip angle with respect to the ecliptic plane for an extended period (6-48 h), as a result of a steep radial deformation of the current sheet.

Akasofu, S.-I.↗
NASA NTRS1980

Origin of the warped heliospheric current sheet

The warped heliospheric current sheet for early 1976 is calculated from the observed photospheric magnetic field by a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity for early 1976 obtained at several locations in the heliosphere by Helios 1, Helios 2, Pioneer 11, and at the earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large-scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field and that 'ballerina skirt' effects may add small scale ripples.

Wilcox, J. M.↗
NASA NTRS1980

A semi-analytic approach to the self induced motion of vortex sheets

The rolling up of the trailing vortex sheet produced by a wing of finite span was calculated as a series expansion in time. For a vorticity distribution corresponding to a wing with cusped tips, the shape of the sheet was found by summing the series using Pade approximants. The sheet remains analytic for some time but ultimately develops an exponential spiral at the tips. The centroid of vorticity was conserved to high accuracy.

Schwartz, L. W.↗
NASA NTRS1981

Multiple crossings of a very thin plasma sheet in the Earth's magnetotail

High resolution magnetic field, plasma and energetic particle data from the IMP-8 spacecraft were studied for multiple crossings of the Earth's magnetotail plasma sheet when it becomes thin during magnetospheric substorms. Traversals recur on a time scale of several minutes and they are associated with high velocity plasma flows that are usually directed tailward but are occasionally directed earthward for brief intervals. Observations are explained by rapid oscillations of a plasma sheet that is only a few thousand km thick, a dimension comparable to the gyroradius of energetic protons. Differences in the angular distributions of the two energies indicate that the higher energy protons are preferentially located on field lines deeper in the tail lobe. A neutral line acceleration model is supported tailward streaming energetic electrons which are occasionally present at the lobe plasma sheet interface.

Fairfield, D. H.↗
NASA NTRS1980

The origin of the warped heliospheric current sheet

The warped heliospheric current sheet in early 1976 was calculated from the observed photospheric magnetic field using a potential field method. Comparisons with measurements of the interplanetary magnetic field polarity in early 1976 obtained at several locations in the heliosphere at Helios 1, Helios 2, Pioneer 11 and Earth show a rather detailed agreement between the computed current sheet and the observations. It appears that the large scale structure of the warped heliospheric current sheet is determined by the structure of the photospheric magnetic field, and that "ballerina skirt" effects may add small scale ripples.

Wilcox, J. M.↗
NASA NTRS1980

Magnetohydrodynamic shock propagation in the vicinity of a magnetic neutral sheet

This paper reports a numerical investigation of the propagation of magnetohydrodynamic (MHD) shocks in the vicinity of magnetic neutral sheets. The attenuation of a shock after passing through a neutral sheet has been evaluated (assuming infinite electrical conductivity). In a parameter study, values of shock speed, polytropic index, plasma beta, and neutral-sheet thickness which are representative of solar coronal conditions have been examined. If solar cosmic rays are accelerated in association with a flare-induced shock (as seems most likely), then our results suggest that the spatial structure of solar particle sources will be influenced by helmet streamers. Such streamers are most readily detectable by H alpha filaments in the underlying chromosphere.

Steinolfson, R. S.↗
NASA NTRS1981

Energetic ion composition of the plasma sheet

Data obtained from the energetic ion mass spectrometer experiment on Isee 1 in the distant plasma sheet are presented. These data show that (1) the plasma sheet has a significant and variable ionospheric component (H(+) and O(+)) representing from 10% to more than 50% of the total number density and (2) there is more than one process responsible for the energization of solar wind plasma (H(+) and He(++)) to plasma sheet energies.

Peterson, W. K.↗