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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Basal hydraulic conditions of Ice Stream B

Fifteen boreholes have been drilled to the base of Ice Stream B in the vicinity of UpB Camp. The boreholes are spread over an area of about 500 x 1000 m. Several till cores were retrieved from the bottom of the 1000-m-deep holes. Laboratory tests using a simple shear box revealed a yield strength of basal till of 2 kPa. This agrees well with in-situ measurements using a shear vane. Since the average basal shear stress of Ice Stream B with a surface slope of 0.1 degree is about 20 kPa, the ice stream cannot be supported by till that weak. Additional support for this conclusion comes from the basal water pressure that has been measured in all boreholes as soon as the hot water drill reached bottom. In several boreholes, the water pressure has been continuously monitored; in two of them, over several years. The water pressure varies but stays within 1 bar of flotation where ice overburden pressure and water pressure are equal. The ratio of water and overburden pressure lies between 0.986 and 1.002. This is an extremely high value as compared to other fast-moving ice masses; e.g., Variegated Glacier in surge has a ratio of 0.8, and Columbia Glacier - a fast-moving tidewater glacier - has a ratio of 0.9. It implies that water flow under the glacier occurs in a thin film and not in conduits that would drain away water too rapidly. It also implies that basal sliding must be very effective. Water flow under the glacier was measured in a salt-injection experiment where a salt pulse was released at the bottom of a borehole while 60 m down-glacier, the electrical resistance was measured between two other boreholes. A flow velocity of 7 mm/s was obtained.

Engelhardt, Hermann↗
NASA NTRS1943

Tests of Aluminum-alloy Stiffened-sheet Specimens Cut from an Airplane Wing

The specimens used in the present tests were cut from an actual airplane wing of the stressed-skin type. The specimens thus obtained were not representative of the usual type of laboratory specimens because the stiffeners were not exactly parallel nor evenly spaced and, in one case, the skin consisted of pieces of sheet of different thicknesses. The test data obtained indicate that the buckling strain of stiffened curved sheet can be computed with reasonable accuracy by the equation given by Wenzek. The ultimate loads of the specimens when tested as flat sheet were within +/-11 percent of the product of the compressive yield strength and the cross-sectional area of the stiffeners. A rivet spacing equal to 98 times the sheet thickness was a source of weakness, and rivet spacings up to 36 times the sheet thickness appeared satisfactory.

Holt, Marshall↗
NASA NTRS1943

Description of stress-strain curves by three parameters

A simple formula is suggested for describing the stress-strain curve in terms of three parameters; namely, Young's modulus and two secant yield strengths. Dimensionless charts are derived from this formula for determining the stress-strain curve, the tangent modulus, and the reduced modulus of a material for which these three parameters are given. Comparison with the tensile and compressive data on aluminum-alloy, stainless-steel, and carbon-steel sheet in NACA Technical Note No. 840 indicates that the formula is adequate for most of these materials. The formula does not describe the behavior of alclad sheet, which shows a marked change in slope at low stress. It seems probable that more than three parameters will be necessary to represent such stress-strain curves adequately.

Ramberg, Walter↗
NASA NTRS1993

Fractal dimensions of rampart impact craters on Mars

Ejecta blanket morphologies of Martian rampart craters may yield important clues to the atmospheric densities during impact, and the nature of target materials (e.g., hard rock, fine-grained sediments, presence of volatiles). In general, the morphologies of such craters suggest emplacement by a fluidized, ground hugging flow instead of ballistic emplacement by dry ejecta. We have quantitatively characterized the shape of the margins of the ejecta blankets of 15 rampart craters using fractal geometry. Our preliminary results suggest that the craters are fractals and are self-similar over scales of approximately 0.1 km to 30 km. Fractal dimensions (a measure of the extent to which a line fills a plane) range from 1.06 to 1.31. No correlations of fractal dimension with target type, elevation, or crater size were observed, though the data base is small. The range in fractal dimension and lack of correlation may be due to a complex interplay of target properties (grain size, volatile content), atmospheric pressure, and crater size. The mere fact that the ejecta margins are fractals, however, indicates that viscosity and yield strength of the ejecta were at least as low as those of basalts, because silicic lava flows are not generally fractals.

Ching, Delwyn↗
NASA NTRS1993

Collisions of small Spacewatch asteroids with the Earth

The discovery, with the Spacewatch Telescope, of 8 Earth-approaching objects smaller than 100 m in diameter has been reported. I have calculated the probability and velocity of collision with Earth for each of these objects. Using a code we have successfully employed to model the Tunguska and Revelstoke atmospheric explosions, I have simulated the entry of these objects into Earth's atmosphere, assuming iron, stony, carbonaceous, and cometary compositions. The smallest of these objects, with tens to hundreds of kilotons of kinetic energy, pose a substantial threat at the surface only if they are iron objects. An object is taken to 'pose a substantial threat' if it either craters the ground, or explodes in the atmosphere with sufficient energy at low enough altitude to create an overpressure at the surface capable of felling trees or destroying buildings. Larger objects, with energies greater than about a megaton, devastate the surface regardless of whether they are of iron, stony, or carbonaceous composition. Iron objects crater the ground, whereas stony and carbonaceous objects explode in the atmosphere low enough to fell trees and damage buildings over thousands of square kilometers. Spacewatch objects, if presumed to be of carbonaceous composition, are as dangerous as their stony counterparts, as the former objects' lower yield strengths (and hence, higher explosion altitudes) are roughly compensated by their larger masses (as derived from their lower albedos for a given observed magnitude). Although comets are intrinsically less dangerous than asteroids, the 90 m diameter Spacewatch objects would devastate hundreds of square kilometers at the surface, even if cometary.

Chyba, C. F.↗
NASA NTRS1993

Experimental segregation of iron-nickel metal, iron-sulfide, and olivine in a thermal gradient: Preliminary results

Speculation about the possible mechanisms for core formation in small asteroids raises more questions than answers. Petrologic evidence from iron meteorites, pallasites, and astronomical observations of M asteroids suggests that many small bodies were capable of core formation. Recent work by Taylor reviews the geochemical evidence and examines the possible physical/mechanical constraints on segregation processes. Taylor's evaluation suggests that extensive silicate partial melting (preferably 50 vol. percent or greater) is required before metal can segregate from the surrounding silicate and form a metal core. The arguments for large degrees of silicate partial melting are two-fold: (1) elemental trends in iron meteorites require that the metal was at is liquidus; and (2) experimental observations of metal/sulfide inclusions in partially molten silicate meteorites show that the metal/sulfide tends to form spherules in the liquid silicate due to surface tension effects. Taylor points out that for these metal spherules to sink through a silicate mush, high degrees of silicate partial melting are required to lower the silicate yield strength. Although some qualitative experimental data exists, little is actually known about the behavior of metals and liquid sulfides dispersed in silicate systems. In addition, we have been impressed with the ability of cumulative olivine to expel trapped liquid when placed in a thermal gradient. Consequently, we undertook to accomplish the following: (1) experimentally evaluate the potential for metal/sulfide/silicate segregation in a thermal gradient; and (2) obtain quantitative data of the wetting parameters of metal-sulfide melts among silicate grains.

Jurewicz, Stephen R.↗
NASA NTRS1993

A comparison of calculated and measured rheological properties of crystallising lavas in the field and in the laboratory

Models of most magmatic processes, including realistic models of planetary lava flows require accurate data on the rheological properties of magma. Previous studies suggest that field and laboratory rheological properties of Hawaiian lavas can be calculated from their physico-chemical properties using a non-Newtonian rheology model. The present study uses new measurements of the rheological properties of crystallizing lavas to show that this is also true for lavas from Mount Etna. Rheological measurements on quenched Etna basalts were made in a specially designed furnace using a Haake Rotovisco viscometer attached to a spindle which has been designed to eliminate slippage at the melt-spindle interface. Using this spindle, we have made measurements at lower temperatures than other workers in this field. From these measurements, Mount Etna lavas are Newtonian at temperatures above 1120 C and they are thixotropic pseudoplastic fluids with a yield strength at lower temperatures. The close agreement between calculated and measured rheology over the temperature range 1084 - 1125 C support the use of the non-Newtonian rheology model in future modeling of planetary lava flows.

Pinkerton, Harry↗
NASA NTRS1992

Indentability of conventional and negative Poisson's ratio foams

The indentation resistance of foams, both of conventional structure and of reentrant structure giving rise to negative Poisson's ratio, is studied using holographic interferometry. In holographic indentation tests, reentrant foams had higher yield strength and lower stiffness than conventional foams of the same original relative density. Calculated energy absorption for dynamic impact is considerably higher for reentrant foam than conventional foam.

Lakes, R. S.↗
NASA NTRS1993

Hypervelocity impact on shielded plates

A ballistic limit equation for hypervelocity impact on thin plates is derived analytically. This equation applies to cases of impulsive impact on a plate that is protected by a multi-shock shield, and it is valid in the range of velocity above 6 km/s. Experimental tests were conducted at the NASA Johnson Space Center on square aluminum plates. Comparing the center deflections of these plates with the theoretical deflections of a rigid-plastic plate subjected to a blast load, one determines the dynamic yield strength of the plate material. The analysis is based on a theory for the expansion of the fragmented projectile and on a simple failure criterion. Curves are presented for the critical projectile radius versus the projectile velocity, and for the critical plate thickness versus the velocity. These curves are in good agreement with curves that have been generated empirically.

Smith, James P.↗
NASA NTRS1994

The effect of fabrication on corrosion in aluminum 2195: Environmental and microstructural considerations

Aluminum alloys containing lithium are particularly attractive to the aerospace structural designer. Lithium's density is only 0.53 g/cc, thus an addition of one weight percent lithium not only increases yield strength, but decreases the density by almost three percent while increasing the modulus by over six percent. The fact that lithium improves these physical properties simultaneously has led to intense study and development of the alloy system. Heretofore, problems in large scale alloy production have retarded commercial development. During the last fifteen years, advances in production technology have rekindled interest in Al-Li alloys, and aluminum suppliers have developed many candidate aerospace materials. However, if these alloys are to be employed successfully, a more complete understanding of their nonequilibrium metallurgy is required. Peel and Starke have each pointed out that an understanding of the weldability of these alloys is a critical step in their implementation. This study addresses the critical lack of information on the environmental compatibility of welded Al 2195 components. Corrosion data for these systems is incomplete, particularly for welded materials exposed to sea water or sea water condensate.

Walsh, Daniel W.↗
NASA NTRS1992

Indentability of conventional and negative Poisson's ratio foams

The indentation resistance of foams, both of conventional structure and of re-entrant structure giving rise to negative Poisson's ratio, is studied using holographic interferometry. In holographic indentation tests, re-entrant foams had higher yield strengths sigma(sub y) and lower stiffness E than conventional foams of the same original relative density. Calculated energy absorption for dynamic impact is considerably higher for re-entrant foam than conventional foam.

Lakes, R. S.↗
NASA NTRS1995

Advanced Small Rocket Chambers. Option 3: 110 1Bf Ir-Re Rocket, Volume 1

This report describes the AJ10-221, a high performance Iridium-coated Rhenium (Ir-Re) 110 lbf (490N) welded rocket chamber with 286:1 area ratio nozzle. This engine was designed, built, and hot fired for over 6 hours on this program. It demonstrated an I(s) of 321.8 sec, which is 10 sec higher than conventional 110 lbf silicide coated Cb chambers now in use. The approach used in this portion of the program was to demonstrate the performance improvement that can be made by the elimination of fuel film cooling and the use of a high temperature (4000F) (2200C) iridium-coated rhenium (Ir-Re) rocket chamber. Detailed thermal, performance, mechanical, and dynamic design analyses of the full engine were conducted by Aerojet. Two Ir-Re chambers were built to the Aerojet design by Ultramet, using the chemical vapor deposition (CVD) process. Incorporation of a secondary mixing device or Boundary Layer Trip (BLT) within the combustion chamber (Aerojet Patents 4882904 and 4936091) results in improvement in flow uniformity, and a significant life and performance increase. The 110 lbf engine design was verified in bolt-up hardware tests at sea level and altitude. The effects of injector design on performance were studied. Two duplicate injectors were fabricated matching the preferred design and were demonstrated to be interchangeable in operation. One of these units was fabricated matching the preferred design and was demonstrated to be interchangeable in operation. One of these units was welded into a flight type thruster which was tested for an accumulated duration of 22,590 sec in 93 firings, one of which was a continuous burn of two hours. A design deficiency in the C-103 nozzle near the Re-Cb transition joint was discovered, studied and corrected design has been prepared. Workhardening studies have been conducted to investigate methods for increasing the low yield strength of the Re in the annealed conditions. An advanced 490N high performance engine has been demonstrated which, when proven to be capable of withstanding launch vibration, is ready for flight qualification.

Jassowski, Donald M.↗
NASA NTRS1995

Advanced small rocket chambers. Option 3: 110 1bf Ir-Re rocket, volume 2

This is the second part of a two-part report that describes the AJ10-221, a high performance iridium-coated rhenium (Ir-Re) 110 lbf (490N) welded rocket chamber with 286:1 area ratio nozzle. This engine was designed, built, and hot fired for over 6 hours on this program. It demonstrated an I(s) of 321.8 sec, which is 10 sec higher than conventional 110 lbf silicide coated Cb chambers now in use. The approach used in this portion of the program was to demonstrate the performance improvement that can be made by the elimination of fuel film cooling and the use of a high temperature (4000 F) (2200 C) iridium-coated rhenium (Ir-Re) rocket chamber. Detailed thermal, performance, mechanical, and dynamic design analyses of the full engine were conducted by Aerojet. Two Ir-Re chambers were built to the Aerojet design by Ultramet, using the chemical vapor deposition (CVD) process. Incorporation of a secondary mixing device or Boundary Layer Trip (BLT) within the combustion chamber (Aerojet Patents 4882904 and 4936091) results in improvement in flow uniformity, and a significant life and performance increase. The 110 lbf engine design was verified in bolt-up hardware tests at sea level and altitude. The effects of injector design on performance were studied. Two duplicate injectors were fabricated matching the preferred design and were demonstrated to be interchangeable in operation. One of these units were welded into a flight type thruster which was tested for an accumulated duration of 22,590 sec in 93 firings, one of which was a continuous burn of two hours. A design deficiency in the C-103 nozzle near the Re-Cb transition joint was discovered, studied and corrected design has been prepared. Workhardening studies have been conducted to investigate methods for increasing the low yield strength of the Re in the annealed conditions. An advanced 490N high performance engine has been demonstrated which, when proven to be capable of withstanding launch vibration, is ready for flight qualification.

Jassowski, Donald M.↗
NASA NTRS1995

Lithospheric bending at subduction zones based on depth soundings and satellite gravity

A global study of trench flexure was performed by simultaneously modeling 117 bathymetric profiles (original depth soundings) and satellite-derived gravity profiles. A thin, elastic plate flexure model was fit to each bathymetry/gravity profile by minimization of the L(sub 1) norm. The six model parameters were regional depth, regional gravity, trench axis location, flexural wavelength, flexural amplitude, and lithospheric density. A regional tilt parameter was not required after correcting for age-related trend using a new high-resolution age map. Estimates of the density parameter confirm that most outer rises are uncompensated. We find that flexural wavelength is not an accurate estimate of plate thickness because of the high curvatures observed at a majority of trenches. As in previous studies, we find that the gravity data favor a longer-wavelength flexure than the bathymetry data. A joint topography-gravity modeling scheme and fit criteria are used to limit acceptable parameter values to models for which topography and gravity yield consistent results. Even after the elastic thicknesses are converted to mechanical thicknesses using the yield strength envelope model, residual scatter obscures the systematic increase of mechanical thickness with age; perhaps this reflects the combination of uncertainties inherent in estimating flexural wavelength, such as extreme inelastic bending and accumulated thermoelastic stress. The bending moment needed to support the trench and outer rise topography increases by a factor of 10 as lithospheric age increases from 20 to 150 Ma; this reflects the increase in saturation bending moment that the lithosphere can maintain. Using a stiff, dry-olivine rheology, we find that the lithosphere of the GDH1 thermal model (Stein and Stein, 1992) is too hot and thin to maintain the observed bending moments. Moreover, the regional depth seaward of the oldest trenches (approximately 150 Ma) exceeds the GDH1 model depths by about 400 m.

Levitt, Daniel A.↗
NASA NTRS1994

The isostatic state of Mead crater

We have analyzed high-resolution Magellan Doppler tracking data over Mead crater, using both line-of-sight and spherical harmonic methods, and have found a negative gravity anomaly of about 4-5 mgal (at spacecraft altitude, 182 km). This is consistent with no isostatic compensation of the present topography; the uncertainty in the analysis allows perhaps as much as 30% compensation at shallow dpeths (approximately 25 km). This is similar to observations of large craters on Earth, which are not generally compensated, but contrasts with at least some lunar basins which are inferred to have large Moho uplifts and corresponding positive Bouguer anomalies. An uncompensated load of this size requires a lithosphere with an effective elastic lithosphere thickness greater than 30 km. In order for the crust-mantle boundary not to have participated in the deformation associated with the collapse of the transient cavity during the creation of the crater, the yield strength near the top of the mantle must have been significantly higher on Earth and Venus than on the Moon at the time of basin formation. This might be due to increased strength against frictional sliding at the higher confining pressures within the larger planets. Alternatively, the thinner crusts of Earth and Venus compared to that of the Moon may result in higher creep strength of the upper mantle at shallower depths.

Banerdt, W. B.↗
NASA NTRS1994

Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano-Puna Volcanic Complex

The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14-km-long coulee with a volume of at least 26 cu km. It is the largest of a group of several closely similar dacitic lavas erupted during a recent (less than 100,000 year old) magmatic episode in the Altiplano-Puna Volcanic Complex (APVC; 21-24 deg S) of the Centra; Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced approximately 1 cu km of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted approximately 22.5 cu km of magma in the form of a composite coulee covering approximately 53 sq km with a 400-m-high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6-km-long, 3-km-wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal-rich high-K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare snidine, and oxides. Phenocryst contents reach 40-60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20-25%) and phase 3 (approximately 40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preeruptive zonation of the magma body in composition, temperature (approximately 840 C), fO2 (19(exp -11), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurements suggest that the Chao lavas had internal plastic viscosities of 10(exp 10) to 10(exp 12) Pa s, apparent viscosities of 10(exp 9) Pa s, surface viscosities of 10(exp 15) to 10(exp 24) Pa s, and a yield strength of 8 x 10(exp 5) Pa. These estimates indicate that Chao would have exhibited largely similar rheological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20 deg to 3 deg) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 cu m/s for short periods. Four other lavas in the vicinity with volumes of approximately 5 cu km closely resemble Chao and are probably comagnetic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.

De Silva, S. L.↗
NASA NTRS1995

Development of practical high temperature superconducting wire for electric power application

The technology of high temperature superconductivity has gone from beyond mere scientific curiousity into the manufacturing environment. Single lengths of multifilamentary wire are now produced that are over 200 meters long and that carry over 13 amperes at 77 K. Short-sample critical current densities approach 5 x 104 A/sq cm at 77 K. Conductor requirements such as high critical current density in a magnetic field, strain-tolerant sheathing materials, and other engineering properties are addressed. A new process for fabricating round BSCCO-2212 wire has produced wires with critical current densities as high as 165,000 A/sq cm at 4.2 K and 53,000 A/sq cm at 40 K. This process eliminates the costly, multiple pressing and rolling steps that are commonly used to develop texture in the wires. New multifilamentary wires with strengthened sheathing materials have shown improved yield strengths up to a factor of five better than those made with pure silver. Many electric power devices require the wire to be formed into coils for production of strong magnetic fields. Requirements for coils and magnets for electric power applications are described.

Hawsey, Robert A.↗
NASA NTRS1996

Neutron diffraction measurements and modeling of residual strains in metal matrix composites

Neutron diffraction measurements at room temperature are used to characterize the residual strains in tungsten fiber-reinforced copper matrix, tungsten fiber-reinforced Kanthal matrix, and diamond particulate-reinforced copper matrix composites. Results of finite element modeling are compared with the neutron diffraction data. In tungsten/Kanthal composites, the fibers are in compression, the matrix is in tension, and the thermal residual strains are a strong function of the volume fraction of fibers. In copper matrix composites, the matrix is in tension and the stresses are independent of the volume fraction of tungsten fibers or diamond particles and the assumed stress free temperature because of the low yield strength of the matrix phase.

Saigal, A.↗