A Stirling radioisotope power system is being developed for potential use on NASA missions, including deep-space missions, Mars rovers, and lunar applications (ref. 1). Advanced Stirling convertors would provide substantial performance and mass benefits for these long-duration missions of up to17 years. In the current design of the Advanced Stirling Radioisotope Generator, the heater head component of the Advanced Stirling Convertor (ASC) is fabricated from INCONEL 718, whereas another version is being developed with a MAR-M 247 (Lockheed Martin) heater head (refs. 2 and 3). The MAR-M 247 material allows increased hot-end temperatures of up to 850 °C (1560 °F), thus increasing efficiency and specific power.
For the long life required, a structurally significant limit for the ASC heater head is creep deformation induced under low stress levels at high material temperatures. Conventional investigations of creep rely on experimental results from uniaxial specimens, and much creep information is available for both materials. However, very little experimental data is available that directly applies to the a typical thin walls, specific microstructures, and low stress levels. In addition, the geometry and loading conditions apply a multiaxial stress state on the part, far from the conditions of uniaxial testing. For these reasons, benchmark testing was developed to accurately assess durability of the heater head. Because testing at prototypical stress requires many months to obtain creep data, initial accelerated creep testing is planned. The results will be used to calibrate deterministic and probabilistic analytical creep models of the heater head.
Conceptual creep rate response is shown for cascade testing of a heater head test article.
Long description of figure 1.
Previous short-term heater head creep tests relied on increased internal pressure to accelerate creep deformation (ref. 4). Safety considerations limited the maximum acceleration because of hazards associated with potential rupture under high pneumatic pressures. In addition, the method produced only one experimental stress-temperature condition for each test. Addressing these issues, the NASA Glenn Research Center developed a “cascade” test procedure. Cascade testing is so called because of the cascade of experimental creep rates produced by a single test article over a wide range of stresses at the temperature of interest. It subjects a large volume of material amenable to creep measurement by using an inductively heated susceptor to create uniform temperature over a major portion of the test article tapered wall (see the preceding figure). In this way, internal pressure creates a large multiaxial stress range at the desired temperature.
The cascade test rig is currently being assembled, with the first test results expected in early 2007. The rig (see the following figure), which is located at the Structural Benchmark Test Facility, includes an argon pressurization system, the induction power supply, diametral extensometers and laser micrometers for strain measurement, and a data-acquisition and control system to safely conduct tests and record results.
Cascade test apparatus for applying heat and measuring creep strains on a heater head test article.
Long description of figure 2.
Under NASA’s Science/Nuclear Power Radioisotope System Development project, the Department of Energy is developing high-efficiency Stirling power systems with Glenn and the Lockheed Martin Corporation (Valley Forge, PA). Sunpower Inc. (Athens, OH) is developing the ASC under a NASA Research Announcement (NRA) award. Glenn manages the NRA project and is providing supporting technology development for the Stirling convertor and the overall generator. The benchmark testing is being performed in collaboration with Glenn’s Thermal Energy Conversion Branch as part of this effort.
Find out more about this research:
Glenn’s Thermal Energy Conversion Branch:
http://www.grc.nasa.gov/WWW/TECB/
Glenn’s Power & In-Space Propulsion Division:
http://www.grc.nasa.gov/WWW/5000/pep/
Glenn’s Mechanics and Life Prediction Branch:
http://www.grc.nasa.gov/WWW/LPB/
NASA Glenn Research Center:
http://www.nasa.gov/glenn/
Last updated: December 14, 2007
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