David L. Krause

Structural analyses of Stirling power-convertor heater head for long-term reliability, durability, and performance

Deep space missions require onboard electric power systems with reliable design lifetimes of up to 10-y and beyond. A high-efficiency Stirling radioisotope power system is a prime candidate for future deep space missions and Mars rover applications. To ensure ample durability, the structurally critical Heater Head of the Stirling Power-Convertor has undergone extensive computational analyses of operating temperatures (up to 650 °C), stresses, and creep-resistance of the thin-walled Inconel 718 bill-of-material. Durability predictions are presented in terms of probability of survival. A benchmark structural testing program has commenced to support the analyses. This paper reports the current status of our durability assessments.

Structural Benchmark Testing for Stirling Converter Heater Heads

The National Aeronautics and Space Administration (NASA) has identified high efficiency Stirling technology for potential use on long duration Space Science missions such as Mars rovers, deep space missions, and lunar applications. For the long life times required, a structurally significant design limit for the Stirling convertor heater head is creep deformation induced even under relatively low stress levels at high material temperatures. Conventional investigations of creep behavior adequately rely on experimental results from uniaxial creep specimens, and much creep data is available for the proposed Inconel‐718 (IN‐718) and MarM‐247 nickel‐based superalloy materials of construction. However, very little experimental creep information is available that directly applies to the atypical thin walls, the specific microstructures, and the low stress levels. In addition, the geometry and loading conditions apply multiaxial stress states on the heater head components, far from the conditions of uniaxial test...

Structural Benchmark Creep Testing for the Advanced Stirling Convertor Heater Head

Abstract The National Aeronautics and Space Administration (NASA) has identified the high efficiency Advanced Stirling Radioisotope Generator (ASRG) as a candidate power source for use on long duration Science missions such as lunar applications, Mars rovers, and deep space missions. For the inherent long life times required, a structurally significant design limit for the heater head component of the ASRG Advanced Stirling Convertor (ASC) is creep deformation induced at low stress levels and high temperatures. Demonstrating proof of adequate margins on creep deformation and rupture for the operating conditions and the MarM-247 material of construction is a challenge that the NASA Glenn Research Center is addressing. The combined analytical and experimental program ensures integrity and high reliability of the heater head for its 17-year design life. The life assessment approach starts with an extensive series of uniaxial creep tests on thin MarM-247 specimens that comprise the same chemistry, microstructure, and heat treatment processing as the heater head itself. This effort addresses a scarcity of openly available creep properties for the material as well as for the virtual absence of understanding of the effect on creep properties due to very thin walls, fine grains, low stress levels, and high-temperature fabrication steps. The approach continues with a considerable analytical effort, both deterministically to evaluate the median creep life using nonlinear finite element analysis, and probabilistically to calculate the heater head’s reliability to a higher degree. Finally, the approach includes a substantial structural benchmark creep testing activity to calibrate and validate the analytical work. This last element provides high fidelity testing of prototypical heater head test articles; the testing includes the relevant material issues and the essential multiaxial stress state, and applies prototypical and accelerated temperature profiles for timely results in a highly controlled laboratory environment. This paper focuses on the last element and presents a preliminary methodology for creep rate prediction, the experimental methods, test challenges, and results from benchmark testing of a trial MarM-247 heater head test article. The results compare favorably with the analytical strain predictions. A description of other test findings is provided, and recommendations for future test procedures are suggested. The manuscript concludes with describing the potential impact of the heater head creep life assessment and benchmark testing effort on the ASC program.

Long-term durability analysis of a 100000+HR Stirling power convertor heater head

DOE and NASA have identified Stirling radioisotope power systems (SRPS) as a candidate power system for future deep space exploration missions. As a part of this effort, NASA has initiated a long-term durability project for critical hot section components of the Stirling power convertor to qualify flight hardware. This project will develop a life prediction methodology that utilizes short-term (t<20000 hr) test data to verify long-term (t>100000 hr) design life. The project consists of generating a materials database for the specific heat of alloy, evaluation of critical hermetic sealed joints, life model characterization, and model verification. This paper describes the qualification methodology being developed and provide a status for this effort.

Advanced Stirling Convertor Heater Head Durability and Reliability Quantification

oC) is a key design driver for durability. Inherent uncertainties in the creep behavior of the thin-walled heater head and the variations in the wall thickness, control temperature, and working gas pressure need to be accounted for in the life and reliability prediction. Due to the availability of very limited test data, assuring life and reliability of the HH is a challenging task. The NASA Glenn Research Center (GRC) has adopted an integrated approach combining available uniaxial MarM-247 material behavior testing, HH benchmark testing and advanced analysis in order to demonstrate the integrity, life and reliability of the HH under expected mission conditions. The proposed paper describes analytical aspects of the deterministic and probabilistic approaches and results. The deterministic approach involves development of the creep constitutive model for the MarM-247 (akin to the Oak Ridge National Laboratory master curve model used previously for Inconel 718) and nonlinear finite element analysis to predict the mean life. The probabilistic approach includes evaluation of the effect of design variable uncertainties in material creep behavior, geometry and operating conditions on life and reliability for the expected life. The sensitivity of the uncertainties in the design variables on the heater head reliability is also quantified, and guidelines to improve reliability are discussed.

Characterization of a Viscoplastic Constitutive Model and Its Application for the Finite Element Analyses of a Stirling Space Power Converter Heater 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 converters heater head. Low-cycle fatigue and creep experiments on a nickel-base 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 Freeds 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.

Accelerated Life Structural Benchmark Testing for a Stirling Convertor Heater Head

For proposed long‐duration NASA Space Science missions, the Department of Energy, Lockheed Martin, Infinia Corporation, and NASA Glenn Research Center are developing a high‐efficiency, 110‐watt Stirling Radioisotope Generator (SRG110). A structurally significant limit state for the SRG110 heater head component is creep deformation induced at high material temperature and low stress level. Conventional investigations of creep behavior adequately rely on experimental results from uniaxial creep specimens, and a wealth of creep data is available for the Inconel 718 material of construction. However, the specified atypical thin heater head material is fine‐grained with a heat treatment that limits precipitate growth, and little creep property data for this microstructure is available in the literature. In addition, the geometry and loading conditions apply a multiaxial stress state on the component, far from the conditions of uniaxial testing. For these reasons, an extensive experimental investigation is ongo...

Combined experimental and analytical study using cruciform specimen for testing advanced aeropropulsion materials under in-plane biaxial loading

A new in-house test capability has been developed at the NASA Glenn Research Center to conduct highly critical tests in support of major and significant components of the Stirling Radioisotope Generator (SRG). It is to aid the development of analytical life prediction methodology and to experimentally assist in verifying the flight-design components life. Components within the SRG such as the heater head pressure vessel endure a very high temperature environment for a long period of time. Such conditions impose life-limiting failure by means of material creep, a slow gradual increase in strain which leads to an eventual failure of the pressure vessel. To properly evaluate the performance and assist in the design of this component, testing under multiaxial loading setting is essential, since the heater head is subjected to a biaxial state of stress. Thus, the current work undertakes conducting analytical studies under equibiaxial and non-equi-biaxial loadings situations at various temperatures emulating creep environment. These analytical activities will utilize the finite element method to analyze cruciform type specimens both, under linear elastic and creep conditions. And further to calibrate the in-plane biaxial-test system. The specimen finite element model is generated with MSC/Patran [1] and analytical calculations are conducted with MARC and ANSYS finite element codes [2-3]. Complementing these calculations will undertake conducting experimental tests. However, only results pertaining to the analytical studies are reported and their impact on estimating the life of the component is evaluated.

Cruciform specimen design for testing advanced aeropropulsion materials under cyclic in-plane biaxial loading

Investigating material behavior under complex stress states is often done using in-plane biaxial loading approach. Utilizing such techniques requires using cruciform type specimens fabricated from plate material tested by gripping the specimen at four locations and loaded along two orthogonal axes. Servohydraulic systems are generally used in this application which is similar to those used for uniaxial testing. These kind of testing capabilities are currently being conducted at NASA Glenn Research Center via a new in-house testing facility. This is in support of the development of major components for the Stirling Radioisotope Generator (SRG). It is also used to assist in the generation of an analytical life prediction methodology [1] and to experimentally verify the flight-design components life. Further, this work is intended to carry the immediate goal of developing a specimen design that is fully compatible with the in-plane biaxial testing systems installed at NASA Glenn Research Center [2]. Thus, details of the specimen design and its applicability to the ongoing experimental activities are being reported and discussed. Finite element analyses were carried out to optimize the geometry of specimen and to evaluate the stress response under biaxial loading conditions [3, 4]. The material of interest used in this research is nickel based superalloy. The data presented concluded that the specimen can be used to investigate the deformation behavior under general forms of biaxial loading. The provided measurement and observation are limited to 1-in [2.54 cm] diameter circular region at the specimen center.

Experimental Creep Life Assessment for the Advanced Stirling Convertor Heater Head

The United States Department of Energy is planning to develop the Advanced Stirling Radioisotope Generator (ASRG) for the National Aeronautics and Space Administration (NASA) for potential use on future space missions. The ASRG provides substantial efficiency and specific power improvements over radioisotope power systems of heritage designs. The ASRG would use General Purpose Heat Source modules as energy sources and the free-piston Advanced Stirling Convertor (ASC) to convert heat into electrical energy. Lockheed Martin Corporation of Valley Forge, Pennsylvania, is integrating the ASRG systems, and Sunpower, Inc., of Athens, Ohio, is designing and building the ASC. NASA Glenn Research Center of Cleveland, Ohio, manages the Sunpower contract and provides technology development in several areas for the ASC. One area is reliability assessment for the ASC heater head, a critical pressure vessel within which heat is converted into mechanical oscillation of a displacer piston. For high system efficiency, the ASC heater head operates at very high temperature (850 C) and therefore is fabricated from an advanced heat-resistant nickel-based superalloy Microcast MarM-247. Since use of MarM-247 in a thin-walled pressure vessel is atypical, much effort is required to assure that the system will operate reliably for its design life of 17 years. One life-limiting structural response for this application is creep; creep deformation is the accumulation of time-dependent inelastic strain under sustained loading over time. If allowed to progress, the deformation eventually results in creep rupture. Since creep material properties are not available in the open literature, a detailed creep life assessment of the ASC heater head effort is underway. This paper presents an overview of that creep life assessment approach, including the reliability-based creep criteria developed from coupon testing, and the associated heater head deterministic and probabilistic analyses. The approach also includes direct benchmark experimental creep assessment. This element provides high-fidelity creep testing of prototypical heater head test articles to investigate the relevant material issues and multiaxial stress state. Benchmark testing provides required data to evaluate the complex life assessment methodology and to validate that analysis. Results from current benchmark heater head tests and newly developed experimental methods are presented. In the concluding remarks, the test results are shown to compare favorably with the creep strain predictions and are the first experimental evidence for a robust ASC heater head creep life.