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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Venus gravity - Analysis of Beta Regio

Radio tracking data acquired over Beta Regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived. In addition, a corresponding vertical gravity field was evaluated solely from the topography of the Beta region. A comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line-of-sight gravity maps. It also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone. The effective depth of complete isostatic compensation for the Beta region is estimated to be 330 km, which is somewhat deeper than that found for other areas of Venus.

Esposito, P. B.↗
NASA NTRS1982

Review of factors that influence fretting wear

An assessment is made of presently obtained as well as published results in order to characterize the effects of contact conditions, environmental conditions, and material properties and behavior on fretting wear. The importance of material properties is discussed on the basis of interaction with other factors during fretting. While the isolated effect of contact load, fretting amplitude, temperature and atmosphere have been well explored, especially for the case mild steel, a deficiency is noted in studies of the effects of contact geometry and relative humidity, as well as the interaction of various fretting parameters.

Bill, R. C.↗
NASA NTRS1983

Critique of 'Elastic thickness of the Venus lithosphere estimated from topography and gravity' by A. Cazenave and K. Dominh

A critique of a model of the near-surface structure of Venus by Cazenave and Dominh (1981) based on Pioneer Venus data on gravity and topography is presented. Two objections are raised, the first an assumption of Airy compensation at a depth of 6 km, and the second that a gravity-topography relation is sufficient to study the subsurface characteristics. It is shown that determination of the gravity field along the line of sight, and not along the vertical, is necessary for a valid geophysical model with the available data. It is concluded that the spacecraft data are sufficient to define constraints on models of the subsurface characteristics, but are not sufficient for actually describing the substrate.

Reasenberg, R. D.↗
NASA NTRS1982

Some inelastic effects of thermal cycling on ZrO2-Y2O3 materials

An analysis has been developed which relates the effects of inelastic behavior of Y2O3 stabilized ZrO2 (YSZ) materials. The results show these materials to be sensitive to small changes in temperature and are supported by measurements of inelastic behavior in disk and bar specimens at temperatures as low as 1010 C (1850 F). At higher thermomechanical loads, the test specimens can deform to strains greater than 1 percent and for bar specimens, with sufficient strength, to remain bonded to the metallic substrate.

Hendricks, R. C.↗
NASA NTRS1982

Creep of plasma-sprayed-ZrO2 thermal-barrier coatings

Specimens of plasma-sprayed-zirconia thermal-barrier coatings with three different porosities and different initial particle sizes were deformed in compression at initial loads of 6900, 13,800, and 24,100 kPa (1000, 2000, and 3500 psi) and temperatures of 1100, 1250, and 1400 C. The coatings were stabilized with lime, MgO, and two different concentrations of Y2O3. Creep began as soon as the load was applied and continued at a constantly decreasing rate until the load was removed. Temperature and stabilization had a pronounced effect on creep rate while the stress, particle size, and porosity had a lesser effect. Creep deformation was due to cracking and particle sliding.

Firestone, R. F.↗
NASA NTRS1983

Venus gravity anomalies and their correlations with topography

This report provides a summary of the high-resolution gravity data obtained from the Pioneer Venus Orbiter radio tracking data. Gravity maps, covering a 70 deg latitude band through 360 deg of longitude, are displayed as line-of-sight and vertical gravity. Topography converted to gravity and Bouguer gravity maps are also shown in both systems. Topography to gravity ratios are made over several regions of the planet. There are markedly different ratios for the Aphrodite area as compared to the Beta and Atla areas.

Sjogren, W. L.↗
NASA NTRS1982

Some inelastic effects of thermal cycling on yttria-stabilized zirconia

The effects of inelastic behavior of yttria-stabilized zirconia (YSZ) materials were analyzed. The results show these materials to be sensitive to small changes in temperature and are supported by measurements of inelastic behavior in disc and bar specimens at temperatures as low as 1010 C (1850 F). At higher thermomechanical loadings, the test specimens can deform to strains above 1 percent.

Hendricks, R. C.↗
NASA NTRS1984

Preliminary study of thermomechanical fatigue of polycrystalline MAR-M 200

Thermomechanical fatigue (TMF) experiments were conducted on polycrystalline MAR-M 200 over a cyclic temperature range of 500 to 1000 C. Inelastic strain ranges of 0.03 to 0.2 percent were imposed on the specimens. The TMF lives were found to be significantly shorter than isothermal low-cycle-fatigue (LCF) life at the maximum cycle temperature, and in-phase cycling was more damaging than out-of-phase cycling. Extensive crack tip oxidation appeared to play a role in promoting the severity of in-phase cycling. Carbide particle - matrix interface cracking was also observed after in-phase TMF cycling. The applicability of various life prediction models to the TMF results obtained was assessed. It was concluded that current life prediction models based on isothermal data as input must be modified to be applicable to the TMF results.

Bill, R. C.↗
NASA NTRS1984

Method of fabricating an abradable gas path seal

The thermal shock resistance of a ceramic layer is improved. The invention is particularly directed to an improved abradable lining that is deposited on shroud forming a gas path in turbomachinery. Improved thermal shock resistance of a shroud is effected through the deliberate introduction of benign cracks. These are microcracks which will not propagate appreciably upon exposure to the thermal shock environment in which a turbine seal must function. Laser surface fusion treatment is used to introduce these microcracks. The ceramic surface is laser scanned to form a continuous dense layer. As this layer cools and solidifies, shrinkage results in the formation of a very fine crack network. The presence of this deliberately introduced fine crack network precludes the formation of a catastrophic crack during thermal shock exposure.

Bill, R. C.↗
NASA NTRS1984

Low cycle fatigue behavior of conventionally cast MAR-M 200 AT 1000 deg C

The low cycle fatigue behavior of the nickel-based superalloy MAR-M 200 in conventionally cast form was studied at 1000 C. Continuous cycling tests, without hold times, were conducted with inelastic strain ranges of from 0.04 to 0.33 percent. Tests were also conducted which included a hold time at peak strain in either tension or compression. For the conditions studied, it was determined that imposition of hold times did not significantly affect the fatigue life. Also, for continuous cycling tests, increasing or decreasing the cycle frequency did not affect life. Metallographic analysis revealed that the most significant damage mechanism involved environmentally assisted intergranular crack initiation and propagation, regardless of the cycle type. Changes in the gamma morphology (rafting and rod formation) were observed, but did not significantly affect the failure.

Milligan, W. W.↗
NASA NTRS1984

Venus - Ishtar gravity anomaly

The gravity anomaly associated with Ishtar Terra on Venus is characterized, comparing line-of-sight acceleration profiles derived by differentiating Pioneer Venus Orbiter Doppler residual profiles with an Airy-compensated topographic model. The results are presented in graphs and maps, confirming the preliminary findings of Phillips et al. (1979). The isostatic compensation depth is found to be 150 + or - 30 km.

Sjogren, W. L.↗
NASA NTRS1982

Life prediction and constitutive models for anisotropic materials

The intent of this program is to develop a basic understanding of cyclic creep-fatigue deformation mechanisms and damage accumulation, a capability for reliable life prediction, and the ability to model the constitutive behavior of anisotropic single crystal (SC) and directionally solidified or recrystallized (DSR) comprise the program, and the work breakdown for each option reflects a distinct concern for two classes of anisotropic materials, SC and DSR materials, at temperatures encountered in the primary gas path (airfoil temperatures), and at temperatures typical of the blade root attachment and shank area. Work directed toward the higher temperature area of concern in the primary gas path includes effects of coatings on the behavior and properties of the materials of interest. The blade root attachment work areas will address the effects of stress concentrations associated with attachment features.

Bill, R. C.↗
NASA NTRS1984

Low cycle fatigue of MAR-M 200 single crystals at 760 and 870 deg C

Fully reversed low cycle fatigue tests were conducted on single crystals of the nickel-base superalloys Mar-M 200 at 760 C and 870 C. At 760 C, planar slip (octahedral) lead to orientation-dependent strain hardening and cyclic lives. Multiple slip crystals strain hardened the most, resulting in relatively high stress ranges and low lives. Single slip crystals strain hardened the least, resulting in relatively low stress ranges and higher lives. A preferential crack initiation site which was related to slip plane geometry was observed in single slip orientated crystals. At 870 C, the trends were quite different, and the slip character was much more homogeneous. As the tensile axis orientation deviated from 001 , the stress ranges increased and the cyclic lives decreased. Two possible mechanisms were proposed to explain the behavior: one is based on Takeuchi and Kuramoto's cube cross-slip model, and the other is based on orientation-dependent creep rates.

Milligan, W. W.↗
NASA NTRS1985

Venus Gravity: Global Field Results

The Pioneer Venus Orbiter has provided a vast amount of gravity data, since March 1979. High resolution results have revealed the high correlation between topography and gravity. These data were acquired at relatively low spacecraft altitudes (150 km) where atmospheric effects and high frequency variations were significant and modelling with global spherical harmonics was difficult. During 1982 data were acquired over a complete Venus rotation where the lowest altitudes were 1000 km or more, thus removing atmospherics and high frequency gravity effects. These data that have been reduced to produce a tenth degree and order spherical harmonic model of the global gravity field of Venus. The reduction technique used a least squares where 78 independent arcs of data were combined in a simultaneous inversion. The tenth degree and order gravity field solution has 117 parameters describing the spherical harmonic coefficients. This provides approximately 1800 km feature resolution. These coefficients are presented as a geoid map.

Mottinger, N. A.↗
NASA NTRS1985

Multiaxial and thermomechanical fatigue considerations in damage tolerant design

In considering damage tolerant design concepts for gas turbine hot section components, several challenging concerns arise: Complex multiaxial loading situations are encountered; Thermomechanical fatigue loading involving very wide temperature ranges is imposed on components; Some hot section materials are extremely anisotropic; and coatings and environmental interactions play an important role in crack propagation. The effects of multiaxiality and thermomechanical fatigue are considered from the standpoint of their impact on damage tolerant design concepts. Recently obtained research results as well as results from the open literature are examined and their implications for damage tolerant design are discussed. Three important needs required to advance analytical capabilities in support of damage tolerant design become readily apparent: (1) a theoretical basis to account for the effect of nonproportional loading (mechanical and mechanical/thermal); (2) the development of practical crack growth parameters that are applicable to thermomechanical fatigue situations; and (3) the development of crack growth models that address multiple crack failures.

Leese, G. E.↗
NASA NTRS1984

A dynamic model of Venus's gravity field

Unlike Earth, long wavelength gravity anomalies and topography correlate well on Venus. Venus's admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either Airy or Pratt isostasy, but is consistent with dynamic support from mantle convection. A model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve. On Earth, the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle. These viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs. The different effective viscosity profiles for Earth and Venus may reflect their convective styles, with tectonism and mantle heat transport dominated by hot plumes on Venus and by subducted slabs on Earth. Convection at degree 2 appears much stronger on Earth than on Venus. A degree 2 convective structure may be unstable on Venus, but may have been stabilized on Earth by the insulating effects of the Pangean supercontinental assemblage.

Kiefer, W. S.↗
NASA NTRS1984

Human factors in aircraft incidents - Results of a 7-year study (Andre Allard Memorial Lecture)

It is pointed out that nearly all fatal aircraft accidents are preventable, and that most such accidents are due to human error. The present discussion is concerned with the results of a seven-year study of the data collected by the NASA Aviation Safety Reporting System (ASRS). The Aviation Safety Reporting System was designed to stimulate as large a flow as possible of information regarding errors and operational problems in the conduct of air operations. It was implemented in April, 1976. In the following 7.5 years, 35,000 reports have been received from pilots, controllers, and the armed forces. Human errors are found in more than 80 percent of these reports. Attention is given to the types of events reported, possible causal factors in incidents, the relationship of incidents and accidents, and sources of error in the data. ASRS reports include sufficient detail to permit authorities to institute changes in the national aviation system designed to minimize the likelihood of human error, and to insulate the system against the effects of errors.

Billings, C. E.↗
NASA NTRS1985

Venus gravity - A harmonic analysis and geophysical implications

An improved theoretical model of Venusian global gravity has been obtained by fitting a tenth degree spherical harmonic series to 78 orbital arcs of Doppler tracking data from the Pioneer Venus Orbiter. Maps of the free-air anomaly and its formal error are presented. Isostatic anomaly and 'Geoid' maps are also presented, and their geophysical implications are discussed in details. Comparison with equivalent resolution topographic models reveals a strong correlation between long wavelength gravity and the topography of Venus. Analysis of the second degree harmonics showed two aspects of the orientation of the inertial axes of Venus: (1) a significant (about three degrees) departure of the axis of greatest inertia from the rotational axis; and (2) a near alignment of the axis of least inertia with the location of the subterrestrial point at the time of the next inferior conjunction with earth (December 16, 2101). A series of contour maps of the Venusian free-air anomalies is provided.

Mottinger, N. A.↗