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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Lower critical field measurements in YBa2Cu3O(6+x) single crystals

The temperature dependence of the lower critical field in YBa2Cu3O(6+x) single crystals was determined by magnetization measurements with the applied field parallel and perpendicular to the c-axis. Results are compared with data from the literature and fitted to Ginzberg-Landau equations by assuming a linear dependence of the parameter kappa on temperature. A value of 7 plus or minus 2 kOe was estimated for the thermodynamic critical field at T = O by comparison of calculated H (sub c2) values with experimental data from the literature.

Kaiser, D. L.↗
NASA NTRS1991

Critical currents and high-temperature superconductors

Experimental information on critical currents and epitaxial thin films of high temperature superconductors is summarized. Experiments performed to measure critical currents across single grain boundaries are described. A variety of mechanisms responsible for limiting currents in films as well as across grain boundaries are presented and their predictions compared with experimental data.

Chaudhari, P.↗
NASA NTRS1992

Fault tolerance of artificial neural networks with applications in critical systems

This paper investigates the fault tolerance characteristics of time continuous recurrent artificial neural networks (ANN) that can be used to solve optimization problems. The principle of operations and performance of these networks are first illustrated by using well-known model problems like the traveling salesman problem and the assignment problem. The ANNs are then subjected to 13 simultaneous 'stuck at 1' or 'stuck at 0' faults for network sizes of up to 900 'neurons'. The effects of these faults is demonstrated and the cause for the observed fault tolerance is discussed. An application is presented in which a network performs a critical task for a real-time distributed processing system by generating new task allocations during the reconfiguration of the system. The performance degradation of the ANN under the presence of faults is investigated by large-scale simulations, and the potential benefits of delegating a critical task to a fault tolerant network are discussed.

Protzel, Peter W.↗
NASA NTRS1992

Probabilistic Structural Analysis Methods for select space propulsion system components (PSAM). Volume 2: Literature surveys of critical Space Shuttle main engine components

The technical effort and computer code development is summarized. Several formulations for Probabilistic Finite Element Analysis (PFEA) are described with emphasis on the selected formulation. The strategies being implemented in the first-version computer code to perform linear, elastic PFEA is described. The results of a series of select Space Shuttle Main Engine (SSME) component surveys are presented. These results identify the critical components and provide the information necessary for probabilistic structural analysis. Volume 2 is a summary of critical SSME components.

Rajagopal, K. R.↗
NASA NTRS1992

Grain size reduction in granular flows of spheres - The effects of critical impact energy

Methods employed to derive recent kinetic theories for rapid noncomminuting granular flows are extended to homogeneous flows in which a fraction of the repeated collisions produce tiny fractures on the particles' peripheries and gradually reduce their effective diameters. The theory consists of balance equations for mass, momentum, and energy, as well as constitutive relations for the presence tensor and collisional rates of mass and energy lost. The work of Richman and Chou (1989) is improved by incorporating into the constitutive theory the critical impact energy below which no mass loss occurs in a binary collision. The theory is applied to granular shear flows and, for fixed shear rates, predicts the time variations of the solid fraction granular temperature, and induced stresses, as well as their extreme sensitivities to small changes in the critical impact energy.

Richman, M. W.↗
NASA NTRS1992

The susceptibility critical exponent for a nonaqueous ionic binary mixture near a consolute point

We report turbidity measurements of a nonaqueous ionic solution of triethyl n-hexylammonium triethyl n-hexylboride in diphenyl ether. A classical susceptibility critical exponent gamma = 1.01 +/- 0.01 is obtained over the reduced temperature range t between values of 0.1 and 0.0001. The best fits of the sample transmission had a standard deviation of 0.39 percent over this range. Ising and spherical model critical exponents are firmly excluded. The correlation length amplitude xi sub 0 from fitting is 1.0 +/- 0.2 nm which is much larger than values found in neutral fluids and some aqueous binary mixtures.

Zhang, Kai C.↗
NASA NTRS1993

The infeasibility of quantifying the reliability of life-critical real-time software

This paper affirms that the quantification of life-critical software reliability is infeasible using statistical methods, whether these methods are applied to standard software or fault-tolerant software. The classical methods of estimating reliability are shown to lead to exorbitant amounts of testing when applied to life-critical software. Reliability growth models are examined and also shown to be incapable of overcoming the need for excessive amounts of testing. The key assumption of software fault tolerance - separately programmed versions fail independently - is shown to be problematic. This assumption cannot be justified by experimentation in the ultrareliability region, and subjective arguments in its favor are not sufficiently strong to justify it as an axiom. Also, the implications of the recent multiversion software experiments support this affirmation.

Butler, Ricky W.↗
NASA NTRS1993

Formal verification of algorithms for critical systems

We describe our experience with formal, machine-checked verification of algorithms for critical applications, concentrating on a Byzantine fault-tolerant algorithm for synchronizing the clocks in the replicated computers of a digital flight control system. First, we explain the problems encountered in unsynchronized systems and the necessity, and criticality, of fault-tolerant synchronization. We give an overview of one such algorithm, and of the arguments for its correctness. Next, we describe a verification of the algorithm that we performed using our EHDM system for formal specification and verification. We indicate the errors we found in the published analysis of the algorithm, and other benefits that we derived from the verification. Based on our experience, we derive some key requirements for a formal specification and verification system adequate to the task of verifying algorithms of the type considered. Finally, we summarize our conclusions regarding the benefits of formal verification in this domain, and the capabilities required of verification systems in order to realize those benefits.

Rushby, John M.↗
NASA NTRS1993

Specific heats and thermodynamic critical fields in Zn-doped YBa2Cu3O(7-x) according to an induced-pairing model

Electronic specific heats and thermodynamic critical fields are calculated in a mean-field version of an induced-pairing model for superconductivity, and compared with results of Loram et al. (1990) on YBa2(Cu(1-y)Zn(y))3O(7-x). This model involves induction of pairing of holes in a wideband by strongly bound electronlike pairs. It is assumed that the planar hole concentration for no Zn addition is close to, but slightly higher than, that for the maximum Tc, and that it increases by 0.015 per planar Cu ion for each increase of y by 0.01. Parameters of the model are taken to be the same as in a previous publication in which energy gaps were discussed, except that an effective hybridization parameter is adjusted for each Zn concentration to give agreement with the observed Tc. Results are presented for y = 0.0, 0.01, and 0.03. The agreement with experiment is good for thermodynamic critical fields, and is fair for specific heats. For specimens with larger y, with relatively low T(c)s, it is argued that the model should be supplemented to include effects of a BCS-type interaction amongst the wideband carriers.

Eagles, D. M.↗
NASA NTRS1949

Critical combinations of shear and direct axial stress for curved rectangular panels

A solution is presented for the problem of the buckling of curved rectangular panels subjected to combined shear and direct axial stress. Charts giving theoretical critical combinations of shear and direct axial stress are presented for panels having five different length-width ratios. Because the actual critical compressive stress of rectangular panels having substantial curvature is known to be much lower than the theoretical value, a semiempirical method of analysis of curved panels subjected to combined shear and direct axial stress is presented for use in design. (author

PLATES - STRESSES, SHEARING CURVED↗
NASA NTRS1947

Study by the Prandtl-Glauert method of compressibility effects and critical Mach number for ellipsoids of various aspect ratios and thickness ratios

By using the Prandtl-Glauert method that is valid for three-dimensional flow problems, the value of the maximum incremental velocity for compressible flow about thin ellipsoids at zero angle of attack is calculated as a function of the Mach number for various aspect ratios and thickness ratios. The critical Mach numbers of the various ellipsoids are also determined. The results indicate an increase in critical Mach number with decrease in aspect ratio which is large enough to explain experimental results on low-aspect-ratio wings at zero lift.

Hess, Robert V↗
NASA NTRS1951

Critical velocities of ultracentrifuges

The Euler equations for the rotation of a solid body are applied to the problem of the motion of ultracentrifuges. Particular attention is paid to the problem of the passage of an ultracentrifuge rotor through the critical velocity. The factors that affect the passage of rotors through the critical point are stressed.

Sokolov, V I↗
NASA NTRS1993

Measurement and analysis of critical CTOA for an aluminum alloy sheet

The stable tearing behavior of thin sheets of 2024-T3 aluminum alloy was investigated for middle crack tension, M(T), and compact tension, C(T), specimens. The surface crack-tip opening angle (CTOA), applied loads, crack extension, and local displacements were measured. A critical CTOA fracture criterion was incorporated into a two-dimensional, elastic plastic finite element code and used to simulate the experimental fracture behavior. The CTOA measurements and observations of the fracture surfaces have shown that large values for surface CTOA were observed for small crack extensions (less than the sheet thickness); substantial tunneling of the crack was associated with small crack extensions; crack tunneling in the M(T) specimen was less than that observed in the C(T) configuration; for larger crack extensions, the measured CTOA values were determined to be approximately 6 degrees for both the M(T) and C(T) configuration; and for larger crack extensions, crack tunneling remained constant. The two-dimensional finite element predictions of fracture behavior assumed a constant critical CTOA value of 6 degrees and accounted for local crack tip constraint with a plane strain core of elements ahead of the crack tip. The plane strain core extended 5 mm above the crack plane. The simulations were within +/- 4 percent of the maximum applied load for the C(T) tests within 2 percent for the M(T) tests.

Dawicke, D. S.↗
NASA NTRS1993

Critically evaluated/distributed database of IRAS LRS spectra

Accomplishments under this grant effort include: successful scientific utilization of the IRAS Low Resolution Spectrometer (LRS) database of over 150,000 scans of 7-23 micron spectra for over 50,000 celestial sources; publication in refereed journal of an additional 486 critically evaluated spectra of sources brighter than 20 Jy, completing the LRS ATLAS (Olnon and Raimond 1986 A&A) uniformly to that level, and production of an additional 1,830 critically evaluated spectra of sources brighter than 10 Jy; creation and maintenance of on-line, remotely accessible LRS spectra of over 7500 sources; cooperation with Astrophysics Data System personnel for transitioning this LRS database to the ADS access system after funding for this project expires; and publication of research highlights, which include a systematic variation of the shapes of LRS silicate features among stars of differing IRAS broad-band colors, maser characteristics and light curve asymmetries, all correlated with the chemical and physical development and processing of solid phase materials, and preliminary evidence for silicate profile variations in individual stars as a function of visual light curve phase.

Stencel, R. E.↗
NASA NTRS1993

A critical evaluation of theories for predicting microcracking in composite laminates

We present experimental results on 21 different layups of Hercules AS4 carbon fiber/3501-6 epoxy laminates. All laminates had 90 deg plies; some had them in the middle, while some had them on a free surface. During tensile loading, the first form of damage in all laminates was microcracking of the 90 deg plies. For each laminate, we recorded both the crack density and the complete distribution of crack spacings as a function of the applied load. By rearranging various microcracking theories, we developed a master-curve approach that permitted plotting the results from all laminates on a single plot. By comparing master-curve plots for different theories, it was possible to critically evaluate the quality of those theories. We found that a critical-energy-release-rate criterion calculated using a 2D variational stress analysis gave the best results. All microcracking theories based on a strength-failure criteria gave poor results. All microcracking theories using 1D stress analyses, regardless of the failure criterion, also gave poor results.

Nairn, John A.↗
NASA NTRS1994

Orientation effects on the measurement and analysis of critical CTOA in an aluminum alloy sheet

Fracture tests were conducted on 76.2mm wide, 2.3mm thick middle crack tension (M(T)) specimens machined from 2024-T3 aluminum sheet. The specimens were tested on the T-L orientation and comparisons were made to similar tests conducted in the L-T orientation. Measurement of critical crack tip opening angle (CTOA), applied stress, and crack front shape were made as a function of crack extension. A two-dimensional, elastic-plastic finite element analysis was used to simulate the fracture behavior for both orientations. The results indicate that the T-L orientation had a 10 percent lower stress at fracture than similar tests conducted in the L-T orientation. Correspondingly, the critical CTOA in the T-L tests reached a constant value of 4.7 degrees after 2-3mm of crack extension and the L-T tests reached a value of 6 degrees. The fracture surfaces of the T-L specimens were observed to remain flat, while those of the L-T specimens transitioned to a 45 degree slant fracture after about 2-3mm of crack extension. The tunneling behavior of the two orientations also differed; the T-L specimens reached a deeply tunneled stabilized crack front shape while, the L-T specimens were observed to have only a small amount of tunneling once the crack began to grow on the 45 degree slant. The two-dimensional, elastic-plastic finite element analysis was able to simulate the fracture behavior for both the T-L and L-T orientations.

Sutton, M. A.↗
NASA NTRS1992

MISSION: Mission and Safety Critical Support Environment. Executive overview

For mission and safety critical systems it is necessary to: improve definition, evolution and sustenance techniques; lower development and maintenance costs; support safe, timely and affordable system modifications; and support fault tolerance and survivability. The goal of the MISSION project is to lay the foundation for a new generation of integrated systems software providing a unified infrastructure for mission and safety critical applications and systems. This will involve the definition of a common, modular target architecture and a supporting infrastructure.

Mckay, Charles↗
NASA NTRS1995

Scheduling time-critical graphics on multiple processors

This paper describes an algorithm for the scheduling of time-critical rendering and computation tasks on single- and multiple-processor architectures, with minimal pipelining. It was developed to manage scientific visualization scenes consisting of hundreds of objects, each of which can be computed and displayed at thousands of possible resolution levels. The algorithm generates the time-critical schedule using progressive-refinement techniques; it always returns a feasible schedule and, when allowed to run to completion, produces a near-optimal schedule which takes advantage of almost the entire multiple-processor system.

Meyer, Tom W.↗