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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 433 records · Page 24
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

How Gaps in Time-Series Data Affect Asteroseismic Interpretation

Most pulsating white dwarf stars pulsate with many periods, each of which is a probe of their interior, which has made asteroseismolgy of these stars an active field. However, disentangling the multiple periodicities requires long, uninterrupted strings of data. We briefly describe the history of multi-site observing campaigns that culminated in the development of the Whole Earth Telescope in the late 1980s that still functions today. Through examples from the May 1990 campaign on GD 358, we show how critical it is to eliminate periodic gaps in data to greatly reduce aliasing in Fourier Transforms normally used to analyze the frequency content of pulsating white dwarfs. We close with a brief description of space satellite-based data, along with the advantages and disadvantages of these data compared to ground-based data.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1970

Generation of long time creep data on refractory alloys at elevated temperatures

Creep tests were conducted on two tantalum alloys (ASTAR 811C and T-111 alloy), on a molybdenum alloy (TZM), and on CVD tungsten. The T-111 alloy 1% creep life data have been subjected to Manson's station function analysis, and the progress on this analysis is described. In another test program, the behavior of T-111 alloy with continuously varying temperatures and stresses has been studied. The results indicated that the previously described analysis predicts the observed creep behavior with reasonable accuracy. In addition to the T-111 test program, conventional 1% creep life data have been obtained for ASTAR 811C alloy. Previously observed effects of heat treatment on the creep strength of this material have been discussed and a model involving carbide strengthening primarily at the grain boundaries, rather than in a classical dispersion hardening mechanism, has been proposed to explain the observed results.

Sheffler, K. D.↗
NASA NTRS1971

Generation of long time creep data on refractory alloys at elevated temperatures

Ultrahigh vacuum creep tests were performed on tungsten, molybdenum, and tantalum alloys to develop design creep data and to evaluate the influence of liquid lithium exposure on the creep resistance of a tantalum alloy. Test conditions were generally selected to provide 1% creep in 1000 to 10,000 hours, with the test temperatures ranging between 1600 and 2900 F (1144 K and 1866 K). One percent creep life data from a tantalum-base T-111 alloy (Ta-8%W-2%Hf) were analyzed using a station function method to provide an improved parametric representation of the T-111 data. In addition, the minimum creep rate data from an ASTAR 811C alloy (Ta-8%W-1%Re-0.7%Hf0.025%C) were analyzed to determine the stress and temperature dependence of creep rate. Results of this analysis indicated that the activation energy for creep decreased from about 150 Kcal/mole (5130 J/mole) above 2400 F (1589 K) to about 110 Kcal/mole (3760 J/mole) below 2000 F (1361 K). This temperature range corresponds to the range where the creep mechanism changes from grain boundary sliding to intragranular creep.

Sheffler, K. D.↗