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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Micromechanisms of thermomechanical fatigue: A comparison with isothermal fatigue

Thermomechanical Fatigue (TMF) experiments were conducted on Mar-M 200, B-1900, and PWA-1480 (single crystals) over temperature ranges representative of gas turbine airfoil environments. The results were examined from both a phenomenological basis and a micromechanical basis. Depending on constituents present in the superalloy system, certain micromechanisms dominated the crack initiation process and significantly influenced the TMF lives as well as sensitivity of the material to the type TMF cycle imposed. For instance, high temperature cracking around grain boundary carbides in Mar-M 200 resulted in short in-phase TMF lives compared to either out-of-phase or isothermal lives. In single crystal PWA-1480, the type of coating applied was seen to be the controlling factor in determining sensitivity to the type of TMF cycle imposed. Micromechanisms of deformation were observed over the temperature range of interest to the TMF cycles, and provided some insight as to the differences between TMF damage mechanisms and isothermal damage mechanisms. Finally, the applicability of various life prediction models to TMF results was reviewed. Current life prediction models based on isothermal data must be modified before being generally applied to TMF.

Bill, R. C.↗
NASA NTRS1988

Isothermal solidification in a binary alloy melt

A space shuttle experiment employing the General Purpose (Rocket) Furnace (GPF) in its isothermal mode of operation is manifested on MSL-3, circa 1989. The central aim of this experiment is to investigate the effect of reduced gravity levels on the segregation behavior in a slowly, and isothermally, cooled sample of a binary Pb-15 wt percent Sn alloy. This experiment would thus be able to simulate, in a small laboratory sample, about 20 mm dia 60 mm high and weighing about 150 grams, some aspects of the segregation phenomena occurring in large industrial ingots. Ground-based experiments conducted in the single-cavity simulator of the GPF, located at Marshall Space Flight Center (MSFC), in support of the microgravity experiment are described in detail. The results of the MSFC experiments are compared with other related experiments conducted at Case Western Reserve University (CWRU), wherein the isothermal constraints were relaxed.

Laxmanan, V.↗
NASA NTRS1989

Propagating and nonpropagating compression waves in an isothermal atmosphere with uniform horizontal magnetic field

Full analytical solutions to the wave equations for steady vertical compression waves in an isothermal hydrostatic atmosphere with a uniform horizontal magnetic field are presented. It is shown that, in the steady state approach, the behavior of upward waves and downward waves is very different. It is shown that the finding of Thomas (1983), indicating that the cutoff frequency for vertically propagating magnetoacoustic waves in an isothermal atmosphere with a horizontal magnetic field is the same for isothermal atmosphere with no magnetic field, is true only for the downward waves.

Musielak, Z. E.↗
NASA NTRS1990

Thermal barrier coating life and isothermal oxidation of low-pressure plasma-sprayed bond coat alloys

The paper investigates the isothermal oxidation kinetics of Ni-35Cr-6Al-0.95Y, Ni-18Cr-12Al-0.3Y, and Ni-16Cr-6Al-0.3Y low-pressure plasma-sprayed bond coat alloys and examines the effect of these alloys on the thermal barrier coating (TBC) cyclic life. TBC life was examined by cycling substrates coated with the different bond coats and a ZrO2-7 wt pct Y2O3 TBC in an air-rich burner rig flame between 1150 C and room temperature. The oxidation kinetics of the three bond coat alloys was examined by isothermal oxidation of monolithic NJiCrAlY coupons at 1083 C. The Ni-35Cr-6Al-0.95Y alloy exhibits comparatively high isothermal oxidation weight gains and provides the longest TBC life, whereas the Ni-16Cr-6Al-0.3Y alloy had the lowest weight gains and provided the shortest TBC life. The results show that, although bond coat oxidation is known to have a strong detrimental effect on TBC life, it is not the only bond coat factor that determines TBC life.

Brindley, W. J.↗
NASA NTRS1995

Statistical Design in Isothermal Aging of Polyimide Resins

Recent developments in research on polyimides for high temperature applications have led to the synthesis of many new polymers. Among the criteria that determines their thermal oxidative stability, isothermal aging is one of the most important. Isothermal aging studies require that many experimental factors are controlled to provide accurate results. In this article we describe a statistical plan that compares the isothermal stability of several polyimide resins, while minimizing the variations inherent in high-temperature aging studies.

Sutter, James K.↗
NASA NTRS1969

Isothermal drop calorimeter provides measurements for alpha active, pyrophoric materials

Isothermal drop calorimeter measures the heat content of intensely alpha active and pyrophoric materials in inert atmospheres. It consists of a furnace, calorimeter, and aluminum isothermal jacket contained within an inert-atmosphere glove box, which permits the use of unencapsulated materials without exposing personnel to alpha contamination.

Savage, H.↗
NASA NTRS1975

Isothermal blast wave model of supernova remnants

The validity of the 'adiabatic' assumption in supernova-remnant calculations is examined, and the alternative extreme of an isothermal blast wave is explored. It is concluded that, because of thermal conductivity, the large temperature gradients predicted by the adiabatic model probably are not maintained in nature. Self-similar solutions to the hydrodynamic equations for an isothermal blast wave have been found and studied. These solutions are then used to determine the relationship between X-ray observations and inferred parameters of supernova remnants. A comparison of the present results with those for the adiabatic model indicates differences which are less than present observational uncertainties. It is concluded that most parameters of supernova remnants inferred from X-ray measurements are relatively insensitive to the specifics of the blast-wave model.

Solinger, A.↗
NASA NTRS1979

Phase change in liquid face seals. II - Isothermal and adiabatic bounds with real fluids

Analytical studies of phase change effects in parallel and tapered liquid face seals are presented. An isothermal and adiabatic model of low Reynolds number flow are considered by numerical integration of the descriptive equations for a real fluid, and its thermodynamic properties are calculated for each step, using a computer program for the steam tables or fluid thermodynamic properties. It was shown that for low leakage rate the isothermal model is more accurate and for high leakage rates the adiabatic model is more accurate; that both models yield the same conclusions regarding stability; and that the transient of collapse is described by the adiabatic model which predicts a catastrophic collapse and then either failure or explosive return to a larger film thickness value. Finally, it is shown that converging seals may become unstable and the mass leakage rate is reduced significantly below the all liquid value when boiling occurs.

Hughes, W. F.↗
NASA NTRS1980

Fragmentation in rotating isothermal protostellar clouds

The paper reports results of an extensive set of three-dimensional hydrodynamic calculations performed to investigate the susceptibility of rotating clouds to gravitational fragmentation; only isothermal collapse sequences were considered. It is found that rotating isothermal gas clouds are unstable to fragmentation under a wide range of conditions. The degree of instability and the mode (ring vs. blob) of fragmentation is sensitive to alpha, but insensitive to beta. The initial amplitude of a perturbation does not appear to be crucial; fragmentation should occur eventually even for low-amplitude initial NAPs.

Bodenheimer, P.↗
NASA NTRS1980

Estimation of uncertainty in isotherms deduced from Hugoniots resulting from shockwave generated defects

The problem of how defect concentrations produced in a shock experiment affect the isothermal equation of state is considered. From the few experimental results available that bear directly on this question, it appears the effect could be substantial, i.e., the pressure in material free of defects may be 10-25% lower at a given volume than the isothermal pressure deduced from Hugoniot data on the assumption that defects are negligible in the shocked material. This correction caused by the presence of defects is called the defect pressure.

Ruoff, A. L.↗
NASA NTRS1982

Collapse of accreting, rotating, isothermal, interstellar clouds

The evolutions of the envelopes of collapsing, accreting, isothermal clouds have been numerically calculated for both spherically symmetric and rotating (axisymmetric) clouds. The results provide a cohesive picture of isothermal collapse, and their relationship to previous numerical calculations and similarity solutions is discussed. Even with a large initial rotation rate, the majority of the cloud envelope is accreted, in one case leaving behind a large-scale circulation current. The calculations are performed for both initially uniform density and centrally condensed clouds. Density and velocity profiles for a wide variety of observed systems are compared with those obtained in this study, providing a preliminary assessment of the stage of evolution and initial structure for the observed systems.

Boss, A. P.↗
NASA NTRS1983

Multiple transonic solutions with a new class of shock transitions in steady isothermal solar and stellar winds

It is shown that a new class of shock transitions arises in the transonic solutions of the steady isothermal solar wind equations when momentum deposition and/or nonradial flow tube divergence give rise to multiple critical points in the flow. These shock transitions between critical solutions occur for a certain range of the parameters which characterize the momentum deposition function. The isothermal wind equations allow multiple transonic solutions in the presence of such shock transitions, yielding a continuous solution passing through an inner critical point and solutions involving a shock transition between critical solutions. It is determined that these multiple transonic solutions have the same flow speed at the base but different supersonic flow speeds at infinity. It is found that the nonradial flow tube divergence and momentum addition are equivalent, which gives rise to multiple critical points and hence to multiple transonic solutions with shock transitions. In addition, the physical relevance of these properties are examined for astrophysical systems such as the inner solar wind, flows in extragalactic jets, and accretion discs.

Habbal, S. R.↗
NASA NTRS1985

The physical implications of an isothermal model for the hot intracluster medium

X-ray fluxes from HEAO-1 A2 and Einstein Imaging Proportional Counter (IPC) observations of clusters of galaxies were used to constrain the parameter beta in the isothermal surface brightness profile. Beta is found primarily to have values between .50 and .75 for 15 clusters. Eight of these objects have values of beta previously measured using imaging observations. For these clusters good agreement is found with the values reported here implying that this profile is a good description of the surface brightness out to 8 to 10 core radii. The total gas mass and radial distribution (assuming spherical symmetry) within the cluster resulting from the isothermal model imply an extended halo of hot gas which has 30 to 60% of the virial mass for some clusters.

Hendriksen, M. J.↗
NASA NTRS1988

Viscous pulsational instability of the transonic region of isothermal geometrically thin accretion discs. I - Analytical results

Viscous instability of the transonic region of the conventional geometrically thin alpha-type accretion disks is examined analytically. For simplicity, isothermal disks and isothermal perturbations are assumed. It is found that when the value of alpha is larger than a critical value the disk is unstable against two types of perturbations. One is local propagating perturbations of inertial acoustic waves. Results suggest the possibility that unstable perturbations develop to overstable global oscillations which are restricted only in the innermost region of the disk. The other is standing growing perturbations localized just at the transonic point. The cause of these instabilities is that the azimuthal component of the Lagrangian velocity variation associated with the perturbations becomes in phase with the variation of the viscous stress force. Because of this phase matching work is done on perturbations, and they are amplified.

Kato, Shoji↗
NASA NTRS1989

Isothermal life prediction of composite lamina using a damage mechanics approach

A method for predicting isothermal plastic fatigue life of a composite lamina is presented in which both fibers and matrix are isotropic materials. In general, the fatigue resistances of the matrix, fibers, and interfacial material must be known in order to predict composite fatigue life. Composite fatigue life is predicted using only the matrix fatigue resistance due to inelasticity micromechanisms. The effect of the fiber orientation on loading direction is accounted for while predicting composite life. The application is currently limited to isothermal cases where the internal thermal stresses that might arise from thermal strain mismatch between fibers and matrix are negligible. The theory is formulated to predict the fatigue life of a composite lamina under either load or strain control. It is applied currently to predict the life of tungsten-copper composite lamina at 260 C under tension-tension load control. The calculated life of the lamina is in good agreement with available composite low cycle fatigue data.

Abuelfoutouh, Nader M.↗
NASA NTRS1987

High temperature isothermal and cyclic oxidation behavior of a single crystal Ni base superalloy

High-temperature oxidation behavior of a Ni-base superalloy, PWA1480, developed for use as single-crystal blades and vanes in advanced gas turbines, has been assessed. Isothermal and cyclic oxidation tests up to 200 hours were conducted at 1323, 1410, and 1473 K. The alloy was evaluated as to sample weight change, type of scale formed, type and amount of spall, and microstructural changes. The oxidation attack was more severe in cyclic as compared with isothermal oxidation. Also, enormous growth of the gamma-prime precipitate was observed at 1423 K during cyclic oxidation.

Hebsur, M. G.↗
NASA NTRS1990

Isothermal Calorimeter

Pressure-feedback signal indicates rate of heating. Improved isothermal calorimeter measures rate of heating in object under test. Called "isothermal" because chamber holding object and its environment maintained at or near constant temperature to minimize spurious tranfers of heat introducing errors into measurements. When item under test generates heat, rate of boiling and pressure in inner chamber increase. Servo-valve opens wider to maintain preset differential pressure. Valve-control voltage used as measure of rate of heating.

Rowlette, John J.↗