A major phase of structural benchmark testing began at the NASA Glenn Research Center (http://www.nasa.gov/glenn) for a critical component of the 110-W Stirling Radioisotope Generator (SRG110). Durability testing of the heater head component was initiated on two test articles under prototypical environmental conditions: Glenn’s special-purpose test rigs subject the heater head specimens to the design operating temperatures and pressure. Under these conditions, the primary life-limiting damage mechanism is creep deformation. The experimental data produced support the development of an analytical life-prediction methodology (ref. 1). The testing will be terminated after approximately 1 year of operation.
The SRG110 is being developed to provide electric power for multimission uses, including possible future long-duration NASA space science missions such as deep-space missions or lunar applications (ref. 2). For these uses, the heater head component must endure high temperature at low stress for a long time. The heater head is designed to minimize the effects of material creep--a slow, gradual increase in the pressure vessel diameter that could result in reduced system performance if not properly designed.
Although creep-limited components have been designed satisfactorily using material properties generated from traditional uniaxial tests, the heater head is subjected to a biaxial state of stress. To supplement the ongoing uniaxial creep tests of flight heat Inconel 718 material (ref. 3), Glenn researchers developed benchmark testing to experimentally evaluate the response to this specific biaxial stress condition. Early test data from previous benchmark testing at Glenn accelerated the creep response in short-duration tests by increasing test pressures to raise the specimens’ stress levels. The current final test phase is not accelerated, and high scatter of the very small magnitude creep strains in the short term can be averaged over the extended 1-year test time.
Twelve-month test of Inconel 718 heater head with axial loading at 650 °C (1200 °F); induction heating coils and diametral extensometer quartz probes are visible.
Long description of figure 1.
The current in-house testing uses two heater-head test articles fabricated to flight prototype specifications. One of these is a "full-up" specimen with structurally significant brazed-on attachments; its test includes an additional externally applied axial load to simulate actual heat source contact loading (see the preceding photograph).
The test facility is located at the Life Prediction Branch’s (http://www.grc.nasa.gov/WWW/LPB/) Structural Benchmark Test Facility. The test stand (see the following photograph) includes two independently operated test rigs with argon pressurization systems capable of 3000 psig. Two 3-kW induction power supplies provide even heating and temperature profiling, and a custom data-acquisition and control system is employed to safely conduct tests and record results.
A final check of thermocouple positions is made at the heater head structural benchmark test stand.
Long description of figure 2.
The benchmark testing was performed in collaboration with Glenn’s Thermal Energy Conversion Branch (http://www.grc.nasa.gov/WWW/TECB/) as part of a Glenn in-house project supporting the development of the SRG110. NASA’s Science Mission Directorate provided funding for this effort. The overall SRG110 project is managed by the Department of Energy. Lockheed Martin and Infinia Corporation are developing the SRG110 for the Department of Energy. Glenn is providing supporting technology development for the SRG110, independent verification and validation testing, and advanced technology efforts.
Find out more about this research:
NASA Glenn Research Center: http://www.nasa.gov/glenn
Glenn’s Thermal Energy Conversion Branch: http://www.grc.nasa.gov/WWW/TECB/
Glenn’s Life Prediction Branch: http://www.grc.nasa.gov/WWW/LPB/
David L. Krause, 216-433-5465, David.L.Krause@nasa.gov; and Dr. Randy R. Bowman, 216-433-3205, Randy.R.Bowman@nasa.gov
Authors: David L. Krause, Dr. Gary R. Halford, and Dr. Randy R. Bowman
Headquarters program office: Science Mission
Programs/Projects: RPS, SRG110
Last updated: October 16, 2006
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