Cruciform Specimen Used To Assess Long-Term Creep for Characterizing Advanced Aerospace Materials Under In-Plane Biaxial Loading

The Department of Energy and NASA are developing a high-efficiency Stirling radioisotope power system for potential use on NASA missions, including deep-space missions, Mars rovers, and lunar applications (refs. 1and 2). A key qualification criterion for flight hardware is long-term durability for the critical hot-section components of the power convertor. One such critical component is the power convertor heater head. The heater head is a high-temperature pressure vessel that transfers heat to and from the working gas of the convertor, which is typically helium. The efficiency of a successful heater head design depends on balancing specific requirements, such as having thin walls for minimizing heat conduction losses between the hot and cold ends and having thick walls to lower the stresses and thus improve creep resistance for durability. In the current design of the Advanced Stirling Radioisotope Generator (ASRG), the heater head of the Advanced Stirling Convertor (ASC) is fabricated from the nickel-base superalloy INCONEL 718 (Special Metals Welding Products Company). Another version of the ASC is being developed with a MAR-M 247 (Lockheed Martin) heater head, which allows increased hot-end temperatures and, thus, increased efficiency and specific power. The vessel walls are subjected to temperatures as high as 650 °C (1200 °F) with INCONEL 718 and as high as 850 °C (1560 °F) with MAR-M 247, for lifetimes up to 17 years (refs. 3 and 4).

Material behavior under complex stress states is often investigated using an in-plane biaxial loading approach. Utilizing this technique requires cruciform-type specimens fabricated from plate material. The specimen is gripped at four locations and loaded along two orthogonal axes in servohydraulic systems. These testing capabilities currently exist at the NASA Glenn Research Center, where biaxial cruciform specimen testing is supporting the formulation and verification of an analytical life-prediction methodology for the flight-design heater head.

This work is intended to continue the development of a specimen design that is fully compatible with the in-plane biaxial testing systems at Glenn (ref. 5). Details of the specimen design and its applicability to the ongoing experimental activities are reported and discussed in reference 5.

Analytical activities at Glenn led to a thorough finite element analysis to optimize the geometry of a cruciform specimen made of a nickel-base superalloy, INCONEL 718, and evaluating the stress response under biaxial loading conditions (ref. 6). Results reported indicated that the specimen can be used to investigate deformation behavior under general forms of biaxial loading, provided measurement and observation are limited to a 1-in.- (2.54-cm-) diameter circular region at the specimen’s center. However, the conditions could be more complex in experiments investigating strength and fracture behavior.

Von Mises stress distribution under equibiaxial loading conditions.

The preceding figure shows a nearly uniform stress state in the test region under linear elastic, equibiaxial loading, demonstrating the applicability of the specimen design for equibiaxial loading. A similar analysis was performed for nonequibiaxial loading under linear elastic conditions. Additional analyses were carried out under steady-state creep to further validate the biaxial test loading conditions with a heater head test specimen called Bitec1. The following figure shows the x-component of creep strain within the test section. More details are noted in the figure on the right, where the x-component of creep strain is reported for different time intervals as a function of the axial distance. The analyses were carried out for 750 hr, and the results indicate that the creep strain, as expected, increases with time. The analytical activities are progressing as planned to support ongoing benchmark tests on ASC heater heads and upcoming biaxial experiments on cruciform specimens.

Left: x-component of creep strain after 750 hr of nonequibiaxial loading. Right: x-component of creep strain after different time intervals of nonequibiaxial loading.

References

1. Schreiber, J.: Developmental Considerations on the Free-Piston Stirling Power Convertor for Use in Space. AIAA-2006-4015, 2006.
2. Chan, J.; Wood, J.G.; and Schreiber, J.G.: Development of Advanced Stirling Radioisotope Generator for Space Exploration. Proceedings of Space Technology and Applications International Forum (STAIF-2007), M.S. El-Genk, ed., American Institute of Physics, Melville, NY, Feb. 11-15, 2007.
3. Bowman, Randy R.: Long-Term Creep Assessment of a Thin-Walled Inconel 718 Stirling Power-Convertor Heater Head. Proc. Intersoc. Energy Convers. Eng. Conf., vol. 1, 2001, pp. 435-440.
4. Johnson, A.E.: Creep Under Complex Stress Systems at Elevated Temperatures. Proc. Inst. Mech. Eng. Proc., vol. 164, 1951, pp. 432-447.
5. Abdul-Aziz, Ali; and Krause, David: Combined Experimental and Analytical Study Using Cruciform Specimen for Testing Advanced Aeropropulsion Materials Under In-Plane Biaxial Loading. Proc. SPIE Int. Soc. Opt. Eng., vol. 6176, 2006.
6. Ellis, John R.; and Abdul-Aziz, Ali: Specimen Designs for Testing Advanced Aeropropulsion Materials Under In-Plane Biaxial Loading. NASA/TM--2003-212090, 2003. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2003/TM-2003-212090.html

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