Testing capabilities recently developed at the NASA Glenn Research Center are being used to conduct highly vital tests in support of major and significant components of the Stirling Radioisotope Generator (SRG). These tests help in the development of an analytical life-prediction methodology and assist in verifying the life of flight-design components. Key components within the SRG undergo very harsh operating environmental conditions. For example, the heater head pressure vessel must bear high-temperature loads for an extended period of time. Such conditions are very worrisome since they impose life-limiting material creep and a slow gradual increase in strain, which will lead to an eventual breakdown. Efforts are ongoing to design an optimized heater head vessel that meets engine operation requirements. Substantial progress has been made.
Experimental testing under biaxial loading is generally the appropriate approach for properly evaluating the performance, assisting in the design, and understanding the material behavior of heater head components under complex stress states (refs. 1 to 4). Furthermore, the structurally critical cylindrical heater head is made of thin-section wrought Inconel 718 but must operate at temperatures as high as 650 °C. Creep resistance is the primary durability limitation. Experiments under equibiaxial and nonequibiaxial loadings are being performed using cruciform-type specimens at combined thermomechanical loading conditions emulating a creep environment. This is followed by detailed three-dimensional finite-element analyses of the specimen under both elastic and steady-state creep conditions. It is to model the high-temperature creep, to calibrate the accuracy of the specimenís geometry, and to plan for the biaxial testing.
Finite element model of the cruciform specimen and applied loading conditions.
A cruciform plate test specimen design was successfully developed and optimized with overall dimensions of 12 by 12 by 0.625 in. (30.48 by 30.48 by 1.5875 cm) and a gauge area of 2.3 by 2.3 by 0.050 in. (5.842 by 5.842 by 0.127 cm) (ref. 1). The most important feature of the design was the four sets of flexures that were incorporated to partially decouple the applied biaxial loading. Detailed dimensions of the specimen are shown in the preceding figure. This design was tested and checked for compatibility with the in-plane biaxial testing systems installed at Glenn (ref. 1). The desired uniform stress state was achieved in the gauge region (see the following figure). This indicates that this cruciform specimen is suitable for biaxial testing and that these analyses will have a relevant role in evaluating the performance of this key component.
Von Mises stress distribution under equibiaxial loading conditions.
The benchmark testing was performed in collaboration with Glennís Thermal Energy Conversion Branch as part of a Glenn in-house project supporting the development of the SRG110. NASAísScience 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.
Last updated: October 16, 2006
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