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Sheffler, K. D.

Publications and source records attributed to Sheffler, K. D..

33 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Influence of creep damage on the low cycle thermal-mechanical fatigue behavior of two tantalum base alloys

Low cycle fatigue tests have been performed on the tantalum base alloys T-111 and ASTAR 811C with synchronized, independently programmed temperature and strain cycling. The thermal-mechanical cycles applied fell into three basic categories: these were isothermal cycling, in-phase thermal cycling, and out-of-phase thermal cycling. In-phase cycling was defined as tensile deformation associated with high temperature and compressive deformation with low temperature, while out-of-phase thermal cycling was defined as the reverse case. The in-phase thermal cycling had a pronounced detrimental influence on the fatigue life of both alloys, with the life reduction being greater in the solid solution strengthened T-111 alloy than in the carbide strengthened ASTAR 811C alloy. The out-of-phase tests also showed pronounced effects on the fatigue life of both alloys, although not as dramatic.

Sheffler, K. D.↗
NASA NTRS1972

Interactions between creep, fatigue and strain aging in two refractory alloys

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-111) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first stage creep strain and the second stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency and A ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strain-aging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials). By combining two analytical expressions which are normally used to describe creep and strain aging behavior, an expression was developed which correctly described the influence of temperature, frequency, and A ratio on the TZC creep rate acceleration.

Sheffler, K. D.↗
NASA NTRS1972

Interactions between creep, fatigue, and strain-aging in two refractory metal alloys.

The application of low-amplitude, high-frequency fatigue vibrations during creep testing of two strain-aging refractory alloys (molybdenum-base TZC and tantalum-base T-111) significantly reduced the creep strength of these materials. This strength reduction caused dramatic increases in both the first-stage creep strain and the second-stage creep rate. The magnitude of the creep rate acceleration varied directly with both frequency and A ratio (ratio of alternating to mean stress), and also varied with temperature, being greatest in the range where the strain-aging phenomenon was most prominent. It was concluded that the creep rate acceleration resulted from a negative strain rate sensitivity which is associated with the strain-aging phenomenon in these materials. (A negative rate sensitivity causes flow stress to decrease with increasing strain rate, instead of increasing as in normal materials.)

Sheffler, K. D.↗
NASA NTRS1971

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

Ultrahigh vacuum creep tests were conducted on tantalum, tungsten, and molybdenum alloys to develop creep data and to evaluate the influence of liquid lithium exposure on the creep behavior of the tantalum alloy T-111. Test conditions were generally selected to provide 1% creep in 1000 to 10,000 hours, with test temperatures ranging from 1600 to 2912 F (1144 to 1873 K). Tests on T-111 specimens exposed to vacuum and to liquid lithium for various times and temperatures showed large reductions in creep strength for exposures in the 1800 to 1900 F (1255 to 1310 K) range. Tests on the ASTAR 811c showed a significant influence of grain size on the creep strength of this material, with larger grain size specimens being significantly stronger above the equicohesive temperature of 2000 F (1366 K).

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.↗
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.↗