David L. Krause

Glenn Research Center, Cleveland, Ohio

Ethan J. Brewer

The Ohio State University, Columbus, Ohio

Ralph J. Pawlik

University of Toledo, Toledo, Ohio

Structural Benchmark Creep Testing for Microcast 

MarM-247 Advanced Stirling Convertor E2 Heater 

Head Test Article SN18

NASA/TM—2013-216581

September 2013
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David L. Krause

Glenn Research Center, Cleveland, Ohio

Ethan J. Brewer

The Ohio State University, Columbus, Ohio

Ralph J. Pawlik

University of Toledo, Toledo, Ohio

Structural Benchmark Creep Testing for Microcast 

MarM-247 Advanced Stirling Convertor E2 Heater 

Head Test Article SN18

NASA/TM—2013-216581

September 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors appreciate the inspiring leadership and management at GRC provided by Scott Benson, Dick Shaltens, Lee Mason, 

Wayne Wong, Jeff Schreiber, Scott Wilson, and Lanny Thieme, as well as the full support and technical management of

Drs. Ajay Misra, Steven Arnold, and Michael Nathal. The authors also thank Dr. Sreeramesh Kalluri for his expert experimental 

advice, development of analytical methods, and steadfast reporting of technical progress; Dr. Randy Bowman for his continued 

metallurgical help and expertise; Ashwin Shah and Igor Korovaichuk, Sest, Inc. for their tireless efforts in modeling, analysis, 

and development of reliability and life assessment methodologies; and Frank Bremenour for his technical collaboration and able 

laboratory support for heater head benchmark testing. The Science Mission Directorate at NASA Headquarters provided funding 

to complete the work described herein, and the authors truly are grateful for that enduring commitment.

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Available electronically at http://www.sti.nasa.gov

Trade names and trademarks are used in this report for identifi cation 

only. Their usage does not constitute an offi cial endorsement, 

either expressed or implied, by the National Aeronautics and 

Space Administration.

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-216581 1 

Structural Benchmark Creep Testing for Microcast MarM-247 

Advanced Stirling Convertor E2 Heater Head Test Article SN18 

David L. Krause 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Ethan J. Brewer 

The Ohio State University 

Columbus, Ohio 43210 

Ralph J. Pawlik 

University of Toledo 

Toledo, Ohio 43606 

Abstract 

This report provides test methodology details and qualitative results for the first structural benchmark 

creep test of an Advanced Stirling Convertor (ASC) heater head of ASC-E2 design heritage. The test 

article was recovered from a flight-like Microcast MarM-247 heater head specimen previously used in 

helium permeability testing. The test article was utilized for benchmark creep test rig preparation, wall 

thickness and diametral laser scan hardware metrological developments, and induction heater custom coil 

experiments. In addition, a benchmark creep test was performed, terminated after one week when 

through-thickness cracks propagated at thermocouple weld locations. Following this, it was used to 

develop a unique temperature measurement methodology using contact thermocouples, thereby enabling 

future benchmark testing to be performed without the use of conventional welded thermocouples, proven 

problematic for the alloy. This report includes an overview of heater head structural benchmark creep 

testing, the origin of this particular test article, test configuration developments accomplished using the 

test article, creep predictions for its benchmark creep test, qualitative structural benchmark creep test 

results, and a short summary. 

1.0 Introduction 

The NASA Glenn Research Center (GRC) has provided life and reliability verification and advanced 

development for several technologies used in the Advanced Stirling Radioisotope Generator (ASRG) and 

Advanced Stirling Convertor (ASC) projects. More information about the ASRG and ASC projects and 

the GRC role is available in References 1 to 6. One element of the GRC effort was an analytical creep life 

assessment of the ASC heater head (Fig. 1); it was performed to show that adequate margins exist on 

creep deformation and creep rupture for the long life times and demanding operating conditions of this 

application. There was uncertainty in the validity of the analytical assessment due to complexities of the 

material model and the creep life analyses for this MarM-247 component. To mitigate the risk of 

erroneous results, GRC planned a series of benchmark creep tests using full-scale heater head test articles. 

The test articles were subjected to high temperatures and to stresses ranging from nominal to 

approximately 40 percent higher, to create creep conditions similar to the ASC operating conditions. The 

resulting creep strains measured during testing were compared to predictions made using the analytical 

heater head creep models to verify the methodology. 

 
NASA/TM—2013-216581 2 

Figure 1.—ASC-E2 heater head: 

The SN18 heater head test 

article after completion of 

permeability testing and 

removal of the permeometer 

flange hardware.

The first Microcast MarM-247 heater head test article of the ASC E2 design to undergo structural 

benchmark creep testing was recovered from a heater head specimen previously used in helium 

permeability testing; it was designated with the identification “SN18.” This paper describes its use in 

pretest benchmark test rig preparations, wall thickness and diametral laser scan hardware developments, 

and custom induction heating coil experiments. The paper continues with the description and test results 

from the SN18 short benchmark test run, and finishes with a description of the use of SN18 in developing 

a unique temperature measurement methodology using contact thermocouples. 

The usefulness of results from this testing was dependent on knowledge of the precise conditions 

under which the test article was subjected, namely the stress and temperature incident within the test 

article gage area where creep strain was measured. Accuracy of the stress field was determined by the 

precision of the test article diameter and thickness measurements, and by the use of high-precision 

calibrated transducers in the pneumatic pressurization system. 

Accuracy of the variable temperature field is determined ideally with the application of very many 

weld-attached thermocouples to measure temperatures throughout the extended gage area. However, 

placement of welded thermocouples had negative consequences for this test method. Routing of the 

thermocouple wires and the associated insulators interfered with optical laser measurement of the test 

article strains. The insulating sleeves or thermocouples themselves may alter the temperature field 

resulting from the induction heating method in use. Moreover, thermocouple welds could alter local 

material creep properties. Finally and most importantly, it was discovered that the welds served as crack 

nucleation sites during fatigue loadings under some conditions. This fact resulted in the duration of the 

SN18 benchmark creep test being cut severely when crack growth from certain thermocouple welds 

caused excessive leakage of helium. Crack propagation from thermocouple welds was not experienced in 

earlier testing with the “Mooney 2-10” large-grain MarM-247 heater head test article (Ref. 7). The 

Mooney 2-10 test article was subjected to many pressure cycles during its benchmark creep test and was 

instrumented with a great many welded thermocouples. However, its geometry and material of 

construction were different from that currently proposed in the ASC program as well as actually 

incorporated in the SN18 test article. After the weld cracks limited SN18 to non-pressurized use, it was 

used to develop induction heating coil designs to provide desired heater head temperature gradient 

profiles and to measure those gage area temperatures with high spatial resolution. Later, SN18 was used 

with contact thermocouples to develop a unique temperature measurement methodology that eliminated 

the use of welded thermocouples in the pressurized region. 
NASA/TM—2013-216581 3 

2.0 Structural Benchmark Creep Testing Overview 

Structural benchmark creep testing provided high fidelity testing of prototypical heater head test 

articles and included the relevant material issues and multiaxial stress state in the test article creep 

response. The testing applied the design operating temperature profile for prototypical testing, or 

alternatively an accelerated “cascade” temperature profile (nearly constant high temperature maintained 

throughout the tapered wall gage area of a heater head test article) for more timely accelerated results, in a 

highly controlled laboratory environment. The test articles were pressurized with helium gas, thereby 

inducing a biaxial stress state very nearly the same as that in the complete ASC assembly heater head. 

Because the heater heads have tapered thickness walls, the stress magnitude varied by wall thickness in 

the gage area, even though the test article internal pressure was held constant. A schematic of the test 

setup is shown in Figure 2. 

For accelerated testing using the cascade temperature profile, the thinner-wall, higher-stressed areas 

were temperature-accelerated (relative to the prototypical design temperature profile) to produce larger 

creep strains than nominal. The gage area of interest extended over most of the tapered wall due to 

capture the combined effect of elevated temperature and stress conditions. In the prototypical testing case, 

the critical creep strain measurement location was located at the thick end of the tapered wall, where 

higher creep strain rates existed due to high temperature, even though stresses were lower. 

The heater head structural benchmark creep test facility included two test rigs. Each rig incorporated 

a digital control and data acquisition system, a bottle-fed closed high-purity helium pressurization system, 

an induction heater with custom copper coil or silicon carbide susceptor, a closed-loop cooling system, 

temperature sensing devices, and a pair of vertically translating laser micrometers. References 8 and 9 

provide further discussion of the test apparatus and methods. For prototypical tests, the inductively heated 

susceptor was in a raised position, with the mounting manifold cooling water throttled to produce the 

design operating temperature profile to the extent possible. Strain measurements were concentrated at the 

critical location on the tapered wall. For accelerated testing, a bare induction coil was used at a lower 

level toward the tapered wall thin end, and laser scan measurements for calculation of diametral strains 

along the entire taper were of greater importance. During testing, the laser micrometers provided 

continuous strain measurement at discrete points, whereas intermittent full-length laser scans allowed 

creep characterization across the stress spectrum at selected time increments. 

Figure 2.—Heater head benchmark creep test setup: 

schematic of an ASC heater head with an inductively 

heated susceptor and water-cooled base manifoldflange (bare coil inductor similar). 

 
NASA/TM—2013-216581 4 

Accelerated benchmark creep testing was used to evaluate the effect of the biaxial stress field on 

creep response of the heater head, and it facilitated a comparison with the response predicted from 

accelerated uniaxial test data. In addition, the consequences of the MarM-247 grain microstructure on the 

thin wall creep response, the effects from potentially inhomogeneous heat treatment (if any) on creep 

deformation, and the detection of any circumferentially-directional creep caused by material processing or 

other mechanisms were investigated. 

Both accelerated and prototypical benchmark testing creep strain results were compared to strains 

computed for the experimental conditions (including 3-sigma variations in temperature, pressure, and 

strain measurement) using procedures similar to those used for the heater head creep life assessment. If 

the cumulative creep strains fell within a three-sigma range of the calculated median value, indicating 

approximately 0.999 reliability, then the tests verified the heater head creep life assessment methodology. 

If the cumulative creep strains did not, re-investigation and fine-tuning of the analysis model would be 

called for to reflect more accurately the experimental setup and uncertainties. The heater head life 

assessment methodology then would be modified to incorporate any required systemic changes. 

Reference 10 presents a more complete description of the life assessment approach as well as an overview 

of heater head life criteria. 

3.0 Origin of the SN18 Test Article 

The SN18 test article was the first Microcast MarM-247 E2 design heater head to undergo 

permeability testing, which measures the rate that helium gas under high pressure passes through the thin 

walls (Fig. 3). After its use in the permeometer was complete, it was provided on August 7, 2008 as an 

early heater head benchmark test article to develop further the benchmark test methods and to obtain 

Microcast MarM-247 multiaxial creep test data. As received, the surface condition was questionable and 

had non-uniform oxidation; it was documented with photomicrographs, but no further analysis of the 

oxidation was performed. 

Figure 3.—Helium permeability test configuration: schematic of an 

earlier ASC heater head design with brazed-on permeometer 

mounting flange. 
NASA/TM—2013-216581 5 

To separate the heater head from its permeometer flange, vacuum furnace de-brazing procedures were 

developed (Fig. 4). Two initial attempts at de-brazing in a furnace were made using a suspension wire and 

70 lb of weight. The braze material melted away at 1100 °C, but the stainless steel inner permeometer 

mounting flange cylinder and the MarM-247 outer wall did not separate, possibly due to differential 

growth from the mismatched thermal expansion materials (Fig. 5). Additional attempts at separation were 

made at temperatures up to 1180 °C and with slots machined into the braze ring, along with other 

machined configurations, but, in the end, debrazing was not successful. Therefore, the bulk flange was 

separated from the heater head using precise electrical discharge machining. The remaining internal 

stainless steel ring was then completely removed, leaving a surface (Fig. 6) that could be sealed with the 

o-ring in the benchmark test rig manifold-flange. 

Figure 4.—SN18 in vacuum de-brazing furnace: Heating up to 

1100 C along with 70 lb of tensile force failed to separate SN18 

from the permeometer flange brazed to the rejector end. 

Figure 5.—SN18 heater 

head: Vacuum furnace 

de-brazing attempts 

failed to separate the 

heater head from its 

permeometer mounting 

flange. 

Figure 6.—SN18 test article: 

Condition of rejector-end flange 

after removal of permeometer 

flange using electric discharge 

machining. 
NASA/TM—2013-216581 6 

4.0 Benchmark Creep Test Preparation 

Prior to starting the benchmark test of the SN18 heater head, it was check-fitted to the Benchmark 

Test Rig #3 hardware to assure that the newly exposed rejector end would fit into the rig manifold-flange 

and could maintain a pressure seal. After that was demonstrated, the test article was de-mounted, and 

precise room temperature wall thickness measurements were made using a fixture fabricated for that 

specific purpose; the fixture is shown in Figure 7. This fixture had a precision of 0.00005 in. (0.001 mm). 

The thickness measurements were made for 18 locations at each of nine axial positions (162 total distinct 

measurements) and were generally within the most stringent tolerance specified (Fig. 8). The primary 

purpose of the measurements was to enable accurate computation of benchmark test stresses for analysis 

of the benchmark test results and for validation of the probabilistic analytical assessment methodology. 

The as-built measurements also were used to determine orientation of the test article in Rig #3, so that 

creep strain behavior at the wall thickness extrema, i.e., local stress minimum and maximum locations, 

would be captured by the benchmark test instrumentation. Similarly, the measurements were used to 

determine where the diameter and profile laser scans would be taken. 

Following the wall thickness measurements, the SN18 test article was instrumented with Type K 

welded thermocouples and additional gage area top and bottom extent markers, which were used to 

delineate clearly the gage area in laser scans. Next, calibrations of the two in-situ laser micrometers at 

benchmark test Rig #3 were verified using National Institute of Standards and Technology targets. Then, 

verification of calibrations was completed for the axial displacement sensors, linear variable differential 

transformers (LVDT’s), using a very high precision, calibrated, electronic dial indicator. Finally, the test 

article was installed at the rig manifold-flange with an internal, high-purity, alumina volume plug, 

thermocouples were connected to the data acquisition system, and the high-purity helium pneumatic 

system was connected to the manifold. 

Figure 7.—Wall thickness measurements: A fixture was 

designed and fabricated to accurately measure heater 

head test article wall thicknesses at various axial 

positions. 

Figure 8.—SN18 wall thickness measurements: 162 

measurements were made at nine total axial positions 

and generally were within the stringent fabrication 

tolerances. 

 

Axial Location

Wall Thickness 

SN18 Wall Thickness

Specified

Measured
NASA/TM—2013-216581 7 

Pretest preparations for the diametral scans were made using the X and Y in-situ lasers (Fig. 9). A 

study was completed to evaluate the effects on diameter measurement fidelity due to the internal laser 

scanning frequency and changes in internal averaging samples value, data acquisition rate, axial direction 

of scanning, and axial rate of scanning. Optimized settings resulted in scan noise levels approximately an 

order of magnitude better than the configuration used for previous benchmark creep testing. That 

facilitated interpretation of the elastic and creep response test data. Following the study, the diametral 

laser scans of the test article at room temperature and ambient pressure were completed. To measure the 

test article elastic strain response to pressure, SN18 was charged with the helium system up to the 

maximum test pressure, and in-situ scans again were completed at room temperature (Fig. 10). Note that 

this pressure is much higher than the heater head design peak (mean plus cyclic) pressure. Using the 

diametral laser scans from ambient and test pressures, the circumferential engineering strains in the gage 

area were calculated by dividing their difference by the original diameters. A plot of these measurements 

along with theoretical strains calculated by first principles is shown in Figure 11. 

Figure 9.—In-situ laser scanning: SN18 was instrumented with 

thermo-couples, mounted to the Rig #3 manifold-flange, and 

scanned using in-situ laser micrometers. 

Figure 10.—SN18 diametral laser scans: In-situ laser 

micrometer scans of the test article relate outside 

diameter to axial position in the gage area (Y-axis 

scans shown at 0 and maximum test pressure). 

Figure 11.—SN18 experimental elastic strains: Diametral 

strains due to the test pressure are calculated from the 

diametral scans by dividing the difference of the 0 and 

maximum test pressure scans by the diameters 

obtained from the unpressurized scan.
NASA/TM—2013-216581 8 

To prepare for the SN18 benchmark creep test, temperature gradient experimentation using an 

inductively heated susceptor was performed with the #1B “Thermal Standard” at Rig #3. Various heating 

trials were conducted with the power source transformer traps, susceptor, coil, insulation, and geometry, 

seeking to optimize the accelerated “cascade test” temperature profile to be used for the test. It was found 

that the 3 kW power source could not drive the original silicon carbide susceptor to attain 850 °C over the 

desired gage area of the heater head (Fig. 12(a)). A new shorter susceptor with a double-wrap induction 

coil was fabricated and tested (Fig. 12(b)) but also failed to attain the 850 °C cascade profile. In fact, no 

coil configuration using a silicon carbide susceptor provided a heating gradient to overcome the watercooled, cold-end heat sink and the natural effects of convective air cooling. 

Figure 12.—Heating trials: To provide 

the flat cascade temperature profile, 

several induction configurations were 

tried. 
NASA/TM—2013-216581 9 

With this experience, several bare induction coils were fabricated in an attempt to expand the 

temperature axial range while still providing the required uniform heating, the clearance for the 

micrometers’ laser planes, and a margin on overheating the cold end o-ring seal. A three-turn coil 

(Fig. 12(c)) did not provide adequate induction. For the final, successful configuration, a bare, five-turn, 

copper tube induction coil was developed (Fig. 12(d)). This provided the best distribution of temperature 

over the length of the heater head tapered wall gage area. Based on the readings from 29 welded 

thermocouples on the Thermal Standard, the gage area temperatures were within 50 °C of the desired 

cascade profile for 80 percent of the gage area (Fig. 13). While the highly instrumented Thermal Standard 

was installed, three additional accelerated temperature gradients (at 825, 875, and 900 C) were obtained 

for potential use in the SN18 heater head cascade test. 

Pretest steady-state creep rate (SSCR) calculations were completed for the SN18 heater head test 

article to best predict the appropriate benchmark test internal pressure that would result in reaching the 

onset of tertiary creep for the most critical gage area location within the planned test duration of six 

months (Table 1). Both the ORNL master curve creep model and the simplified Arrhenius creep model 

were employed, using the test article’s maximum principal stresses based on the precise test article wall 

thickness and the diameter measurements described previously. Results were determined for the 

experimental cascade temperatures measured from the Thermal Standard tests for the four temperature 

profiles with nominal values of 825, 850, 875, and 900 C. The combination of the 850 C profile at a 

medium high internal pressure was selected for the SN18 cascade benchmark test. Note that the SSCR 

calculations performed using the simplified Arrhenius creep model predicted smaller creep rates than those 

by the master curve creep model in these four cases. A comparison of the calculated SSCR’s for the entire 

gage area using the master curve and Arrhenius creep models for the 850 °C profile is shown in Figure 14. 

Figure 13.—Cascade temperature profile: Comparison of the 

measured temperature profile using a bare 5-turn coil with the 

#1B Thermal Standard, and the accelerated cascade 

temperature profile. 

TABLE 1.—CALCULATED TEST PARAMETERS FOR CASCADE 

BENCHMARK CREEP TEST 

Nominal cascade 

test temperature, 

°C 

Heater head’s 

internal pressure 

Master Curve 

Model—Max. 

SSCRa

 (/day) 

Arrhenius 

Model—Max. 

SSCR (/day) 

825 high 80 30 

850 med. high 80 26 

875 med. low 80 15 

900 low 80 6 a

80 /day steady state creep rate (SSCR) results in accumulated creep strain of 1.45% in six months 
NASA/TM—2013-216581 10 

Figure 14.—Steady-state creep rate prediction for SN18: The master curve and 

simplified Arrhenius model creep rate predictions for the measured 850 C 

cascade temperature profile of the Thermal Standard and the chosen medium 

high test internal pressure. 

5.0 Benchmark Creep Testing and Results 

Accelerated benchmark testing of the SN18 heater head test article began on April 3, 2009, using the 

nominal 850 C cascade temperature profile and a medium high internal pressure (Fig. 15). As described 

previously, the MarM-247 master curve creep model was used with the calculated maximum principal 

stresses to predict that the onset of tertiary creep in the heater head gage area would began within the test 

period of six months. Before starting the SN18 test, the temperature profile was checked using a reduced 

number of thermocouples on SN18 (compared to the Thermal Standard) for identical heating settings 

obtained earlier using the Thermal Standard. The SN18 temperature profile closely matched that of the 

Thermal Standard but with slightly higher temperatures at the hot end of the gage area and slightly lower 

temperatures at the bottom, thinner-walled end. Using the master curve model to re-predict the creep 

response with the actual SN18 temperature profile, the result was that the maximum predicted steady state 

creep rate in the gage area was 48 microstrain per day, only 60 percent of the desired rate to complete the 

test in six months (Fig. 16). The changed temperature profile may have been a result of the difference in 

external geometry between the SN18 test article of ASC-E2 design and the #1B Thermal Standard test 

article of an earlier design, which had a different hot end configuration. A judgment was made to pursue 

the test with the obtained profile and to alter either temperature or pressure in the future if the observed 

creep rates were deemed too low. It was calculated that increasing the peak temperature approximately 

18 C would provide creep acceleration to restore the creep rate to 80 microstrain per day if needed. 

In addition, before starting the SN18 test, several issues arose with new rig control software. This 

required that the heating system be controlled by one computer while most of the thermocouples be 

recorded on a separate computer. The arrangement degraded test article protection slightly, because, if 

limits set for permissive circuits including monitored thermocouples were exceeded, the controls would 

shut down the helium pressurization system but not the furnace. However, this would leave the test article 

in a safe state, with no applied mechanical stress or creep-inducing conditions. Nonetheless, surface oxide 

growth would continue and therefore add to the accumulated oxidation hours, which were being logged 

separately. 

 
NASA/TM—2013-216581 11 

The SN18 test was interrupted after 73 hr duration to integrate fully the temperature recording and 

induction heater automated shutdowns with revised test control system software. Upon restart, the test ran 

without incident for a total of 187 hr. Using the scanning lasers in fixed position extensometer mode, realtime creep strain measurements were made at a critical position in the gage area; the measurements were 

corrected for the growth of an oxidation layer as described in the Appendix. The predicted creep rate at 

this location was 40 microstrain/day. The X-axis laser was discovered after test completion to have been 

influenced by thermocouple leads that displaced upon heating, interfering with the beam impinging on the 

test article outside wall. This led to incorrect results for the X-axis. 

Figure 15.—SN18 structural benchmark creep test: This 

test started on April 3, 2009 using the five-turn bare 

coil and 850 C cascade temperature profile with an 

applied medium high internal helium pressure. 

Figure 16.—Creep prediction for SN18 test temperatures: This revised creep 

rate prediction was made using SN18 measured temperatures. 

 
NASA/TM—2013-216581 12 

Figure 17.—Laser extensometer data for SN18 creep test: The 

Y-axis laser provided a signal indicating an average creep 

rate much higher than predicted; the X-axis reading may 

have been corrupted by interference from the thermocouple 

instrumentation. 

Figure 18.—SN18 diametral laser scans: Using scans from the in-situ laser micrometers, the pre- and post-test 

gage area diameter versus axial position plots are compared for SN18’s X-axis (upper) and Y-axis (lower). 

The test was terminated on April 13, 2009 to investigate increased consumption of helium gas; the 

accumulated total creep test time was 187.4 hr, and the total oxidation time was 190.4 hr. The Y-axis laser 

recorded a maximum creep strain rate that was significantly higher than the predicted rate (Fig. 17). The 

heating coil was removed and in-situ laser scans were completed to record the accumulated creep strains 

throughout the gage area (Fig. 18). Measurements were also recorded with the Instron uniaxial frame 

laser micrometer system to supplement the in-situ laser data. Similar to the calculation of elastic strains, 

the gage area creep strains where then calculated, but an allowance for oxide layer growth was subtracted 

from the post-test diameter data (Fig. 19). The analysis was performed using a Matlab script developed 

in-house, which permitted varied levels of smoothing but preserved the raw data for use in any future 

probabilistic analyses. Results of the analysis indicated that the maximum average X- and Y-axes creep 

strains and maximum average creep rates developed in the gage area were also higher than the prediction. 

In addition, the gage area maximums were somewhat greater than the stationary laser extensometers had 

measured, indicating that the extensometers were not positioned precisely at the location of maximum creep. 

As previously described, the pre-test creep calculations predicted a maximum gage area SSCR of 

48 microstrain/day, using the master curve creep law method. Although lower than the measured rate, this 

was quite an accurate prediction of the creep response, considering both the historically large scatter in 

experimental creep rates, and the fact that the measured creep rate may have included primary creep if it 

existed. Unfortunately, the test did not run long enough to distinguish a transition to a secondary, steadystate creep strain rate. 

0 50 100 150 200

Laser Diameter Measurement

Test Duration (hours)

SN18 Benchmark Test: 001-002

X Laser

Y Laser

predicted SSCR (40 /day)

(thermocouple 

insulator 

interference)
NASA/TM—2013-216581 13 

Figure 19.—SN18 Gage Area Creep Strains: Gage area creep strains 

were determined using Matlab, and they were significantly higher than 

predicted. 

Figure 20.—Helium leaks at thermocouple welds: The 

growth of five cracks at these locations led to 

termination of the SN18 benchmark creep test. 

Figure 21.—SN18 internal oxidation bands: Two bands of 

increased oxidation were observed at areas with 

temperatures of 200 to 400 C and 800 to 850 C. 

The increased consumption of helium gas in the test was investigated and found to be leakage of 

helium through the test article thin walls at thermocouple locations. Optical microscopy was used to 

detect five through-thickness cracks in the test article (Fig. 20). All cracks initiated at thermocouple or 

gage-area-marker weld locations on a plane just below the gage area; the cracks had propagated up to one 

millimeter in length. At this location, the temperature was 600 C, and the tapered wall thickness was 

small, resulting in a high stress level. Because of their location and spacing, it is believed that the 

presence of these defects did not affect the test article strain measurements. 

Due to the observed rapid progression of crack growth during the five cycles of full pressurization 

that SN18 endured, it is felt that the five cracked areas must be not only sealed, but also structurally 

reinforced, if any future creep testing with this test article is to be performed. Inspection of the interior 

surfaces of SN18 showed two bands with slightly increased oxidation. The bands corresponded 

approximately to temperature ranges of 200 to 400 C and 800 to 850 C (Fig. 21). The internal 

environment of the test article during testing had very low oxygen content, as the test procedures included 
NASA/TM—2013-216581 14 

vacuum purging several times and the use of high-purity helium gas. The oxidation was thought to have 

negligible effect on creep response of the test article. 

Additional benchmark creep testing of the SN18 test article was not performed due to the need to 

make wall repairs, the likelihood that further cracking would occur at other welds, and constraints of the 

project schedule. The test article was used productively as a thermal standard for a subsequent heater head 

benchmark test (Ref. 11), and for an extensive study to develop a contact thermocouple procedure that 

eliminated use of welded thermocouples for heater head temperature measurement (Ref. 12). 

6.0 Summary 

The Glenn Research Center conducted an analytical creep life assessment of the heater head 

component of the Advanced Stirling Convertor for the Lockheed Martin Advanced Stirling Radioisotope 

Generator. This life assessment defined several creep response criteria to substantiate component lifetime 

based on computational results that relied on material creep properties derived from uniaxial creep testing 

of the heater head material. The heater head component was required to meet the creep criteria with high 

reliability as determined by probabilistic analysis. Due to the complexity of the creep life assessment, a 

heater head structural benchmark creep testing program was established to validate this methodology. 

The SN18 test article was an early benchmark creep test article salvaged from a previously completed 

helium permeability test. It served as a useful tool to develop structural benchmark creep testing methods 

and to provide the first multiaxial creep test results for the Microcast MarM-247 material, the heater head 

material of construction. The use of SN18 minimized potential negative effects on test results from initial 

future creep test articles of much higher value. It also instigated development of a new temperature 

measurement technology that replaced otherwise overly intrusive and damaging thermocouple 

instrumentation. Creep results from the SN18 test article under the imposed accelerated cascade test 

conditions provided the first verification of the analytical life assessment procedures within expected 

experimental noise. The SN18 benchmark creep testing experience proved to be important and valuable. 

References 

1. Chan, Jack, Wood, J. Gary, and Schreiber, Jeffrey G.: Development of Advanced Stirling 

Radioisotope Generator for Space Exploration. NASA/TM—2007-214806, 2007. 

2. Shaltens, Richard K., and Wong, Wayne A.: Advanced Stirling Technology Development at NASA 

Glenn Research Center. NASA/TM—2007-214930, 2007. 

3. Wong, Wayne A., Wood, J. Gary, and Wilson, Kyle: Advanced Stirling Convertor (ASC) - From 

Technology Development to Future Flight Product. NASA/TM—2008-215282, 2008. 

4. Schreiber, Jeffrey G. and Thieme, Lanny G.: GRC Supporting Technology for NASA’s Advanced 

Stirling Radioisotope Generator (ASRG). Proceedings of the Space Technology and Applications 

International Forum (STAIF-2008), edited by M.S. El-Genk, American Institute of Physics 

Conference Proceedings 969, Melville, New York, 2008, pp. 582-592. 

5. Wong, Wayne A., Wilson, Kyle, Smith, Eddie, and Collins, Josh: Pathfinding the Flight Advanced 

Stirling Convertor Design with the ASC-E3. Proceedings of the 10th International Energy 

Conversion Engineering Conference, AIAA-2012-4251, American Institute of Aeronautics and 

Astronautics, Reston, VA, 2012, pp. 1-10. 

6. Lewandowski, Edward J.: Testing of the Advanced Stirling Radioisotope Generator Engineering 

Unit at NASA Glenn Research Center. Proceedings of the 10th International Energy Conversion 

Engineering Conference, AIAA-2012-4253, American Institute of Aeronautics and Astronautics, 

Reston, VA, 2012, pp. 1-11. 

7. Krause, David L.: Results for Structural Benchmark Creep Testing of a Large Grain MarM-247 

Advanced Stirling Convertor Heater Head Test Article. NASA/TM—2009-215620, 2009. 

 
NASA/TM—2013-216581 15 

8. Krause, David L., Kalluri, Sreeramesh, Bowman, Randy R., and Shah, Ashwin R.: Structural 

Benchmark Creep Testing for the Advanced Stirling Convertor Heater Head. Proceedings of the 6th 

International Energy Conversion Engineering Conference, AIAA 2008-5774, American Institute of 

Aeronautics and Astronautics, Reston, VA, 2008, pp. 732-741. 

9. Krause, David L., Kalluri, Sreeramesh, and Bowman, Randy R.: Structural Benchmark Testing for 

Stirling Convertor Heater Heads. Proceedings of the Space Technology and Applications 

International Forum (STAIF–2007), edited by M.S. El-Genk, American Institute of Physics 

Conference Proceedings 880, Melville, NY, 2007, pp. 297-304. 

10. Shah, Ashwin R., Korovaichuk, Igor, Krause, David L., and Kalluri, Sreeramesh: Advanced Stirling 

Convertor Heater Head Durability and Reliability Quantification. Proceedings of the 6th 

International Energy Conversion Engineering Conference, AIAA 2008-5772, American Institute of 

Aeronautics and Astronautics, Reston, VA, 2008, pp. 167-174. 

11. Krause, David L.; Kalluri, Sreeramesh; Shah, Ashwin R.; and Korovaichuk, Igor: Experimental 

Creep Life Assessment for the Advanced Stirling Convertor Heater Head. Proceedings of the 8th 

International Energy Conversion Engineering Conference, AIAA Conference Proceedings 2010-

7093, Reston, VA, 2010. 

12. Brewer, Ethan J., Pawlik, Ralph J., and Krause, David L.: Contact Thermocouple Methodology and 

Evaluation for Temperature Measurement in the Laboratory. NASA/TM—2013-216580, 2013. 

 
 
NASA/TM—2013-216581 17 

Appendix—Correction of Creep Strain Measurements for Oxidation Layer Growth 

Structural benchmark creep testing of heater heads was performed at high temperature and in air, 

subjecting the test articles to an oxidizing environment. For the ASC heater heads fabricated from 

MarM-247, the exterior test article surfaces oxidized slightly; the growth of this oxidation layer was 

accounted for when calculating heater head creep strains based on optical laser measurements of the test 

article diameters. 

To quantify the MarM-247 oxidation layer growth over time, a small experimental study was 

completed. Based on the ready availability of materials, one large-grain MarM-247 and two Microcast 

MarM-247 rectangular oxidation specimens were exposed to an 850 °C air environment in a metallurgy 

furnace. One large-grain MarM-247 specimen was held at ambient conditions and served as a control 

specimen. A plot of the results from measurements made over the exposure period of 5036 hr is shown in 

Figure 22. 

When diametral laser measurements of heater head test articles were made, the total exposure time 

was noted, and the average oxidation layer thickness measured in this study for that exposure time was 

subtracted from each radius; the resulting difference was the test article diameter corrected for oxidation 

layer growth. When the original, unexposed test article diameter was subtracted from this number, and 

then divided by the original, unexposed diameter, a measure of the creep strain was obtained that was 

corrected for oxidation layer growth. 

Figure 22.—MarM-247 oxidation layer growth: Results from measurements of oxidation layer growth of 

MarM-247 during exposure to air at 850 C.