Characterization of a Viscoplastic ConstitutiveModel and Its Application for the Finite ElementAnalyses of a Stirling Space Power ConverterHeater Head

NASA has identified the Stirling power converter as a prime candidate for the next generation power system for space applications requiring 60,000 hours of operation. To meet this long-term goal, several critical components of the power converter were analyzed using advanced structural assessment methods. Perhaps the most critical component, because of its geometric complexity and operating environment, was the power converter’s heater head. Low-cycle fatigue and creep experiments on a nickelbase superalloy, Udimet 720 LI (low inclusions) and viscoplastic analyses for the Stirling starfish heater head were conducted. All testing was performed at temperatures of 625 to 820◦C in air. This work was initiated to generate a unique and consistent database in support of a life prediction modeling effort aimed at characterizing Freed’s viscoplastic model and verifying the key damage mechanisms. In general, this paper describes the life assessment of the heater head, which included the characterization of a viscoplastic material model, the thermal and structural analyses of the heater head, and the interpolation of fatigue and creep test results at several elevated temperatures for life prediction purposes.