This paper summarizes a joint study by Gates Aerospace Batteries (GAB) and the Reliability Analysis Center (RAC). This study characterizes the reliability and robustness of the temperature compensated strain gages currently specified for sensing of internal pressure of NiH2 cells. These strain gages are characterized as fully encapsulated, metallic foil grids with known resistance that varies with deformation. The measurable deformation, when typically installed on the hemispherical portion of a NiH2 cell, is proportional to the material stresses as generated by internal cell pressures. The internal pressure sensed in this manner is calibrated to indicate the state-of-charge for the cell. This study analyzes and assesses both robustness and reliability for the basic design of the strain gage, the installation of the strain gage, and the circuitry involved

Klein, Glenn C.

Rash, Donald E., Jr.

English

NASA Technical Reports Server

N93-20515
t
Reliability Study of the NiH2 Strain Gaqe
G/enn C. Klein, Gates Aerospace Batteries; Gainesvi//e, FL
Dona/d E. Rash, Jr., Re/iabifity Ana/ysis Center; Rome, NY
INTRODUCTION
This paper summarizes a joint study by Gates Aerospace Batteries [GAB] and the Reliability
Analysis Center [RAC]. This study characterizes the reliability and robustness of the temperature
compensated strain gages currently specified for sensing of internal pressure of NiH2 cells.
These strain gages are characterized as fully encapsulated, metallic foil grids with known
resistance that varies with deformation. The measurable deformation, when typically installed on
the hemispherical portion of a NiH2 cell, is proportional to the matedal stresses as generated by
internal cell pressures. The internal pressure thus sensed is calibrated to indicate the state-of-
charge for the cell. This study analyzes and assesses both robustness and reliability: for the
basic design of the strain gage, the installation of the strain gage, and the circuitry involved.
DESCRIPTION OF THE STRAIN GAGE
GAB Part Number 3B84010 defines Micro Measurements' Strain Gage Part Number
WK-06-250PD-350. The previous similar part number was 211B2495AB-1. This gage is
characterized as (Reference I.A): dual-element pattern that is fully encapsulated K alloy,
equipped with integral, high-endurance beryllium-copper leadwires. The Carrier Matrix [backing]
is a high-temperature epoxy-phenolic resin system reinforced with glass fibers. WK-Series
gages have the widest temperature range and most extensive environmental capability of any
general-purpose strain gage of the self-temperature-compensated type.
Gage length is 0.250 inches, width is 0.240 inches for the grid pair, and resistance is 350 ohms
+0.4%. Operating Range is nominally -269°C to +400°C for Special or Extended Service, and
-269°C to +290°C for Normal Service. Backing and adhesive life is projected as 5X105 hours
[57 years] at typical Low Earth Orbit [LEO] and Geosynchronous Earth Orbit [GEO] mission
environments. Allowable Strain Limit is 1.5%.
GAB Engineering Specification A15B-815 defines Micro Measurements' M-Bond AE-15 Strain
Gage Installation Kit. The previous similar part number was 283A6484AE-9. This is
characterized as a (Reference 1.B): two-component, 100%-solids epoxy resin system that is
recommended for more critical applications. This system is highly resistant to moisture and most
1992 NASA Aerospace Battery Workshop -553- Nickel-Hydrogen Technologies Session
https://ntrs.nasa.gov/search.jsp?R=19930011326 2020-03-17T08:10:33+00:00Z
chemicals. Typical Elongation Capability is quoted as 10% to 15% at +24°C. A typical set of
strain gages as installed is shown in Figure 1.
PURPOSE OF THE STRAIN GAGE
The two strain gage pairs form a typical four-component Whetstone Bridge that is very sensitive
to minuscule changes in resistance by forming a null-balance system with two active gages
[hard mounted] and two passive gages [soft mounted] for temperature-compensation. Excitation
Voltage is generally 6.4+0.005 volts. This excitation voltage is equivalent to 1.6 KW/M 2, and is
at the lower side of the optimum range.
The output of the strain gage shows as a resistance change as a function of applied strain level.
The strain is directly related to the parent material surface strain, except for the shear -lag of the
bonding adhesive. The parent material [i.e., pressure vessel dome] surface stress is a
proportional, but indirect, measure of the intemal pressure. The internal pressure, created by
hydrogen gas, is proportional to the state-of-charge.
Typical expectations for the time dependent GEO mission environment includes: stability and
repeatability errors less than 1% over 2000 cycles; and, life expectancy of 15 to 16 years.
Different typical expectations for the cycle dependent LEO mission environment includes:
repeatability errors less than 1% over life; bridge output voltage drift less than 0.5% per year;
and, a life expectancy of approaching forty thousand cycles.
FAILURE MODES, FAILURE CAUSES, and FAILURE EFFECTS
Understanding and defining how a specific failure mechanism produces a discrete failure mode
that may effect system operation is important for determining the proper inter-relationship among
the events. A proper understanding of this sequence or chain of events is paramount to
establishing appropriate corrective actions to prevent recurrence.
In addition, the orientation of the analysis, that is whether to concentrate on system response
symptoms or on specific signatures generated by active components, determines both the
success of the analysis and the effectiveness of remedial actions. Failure Mode: what aspect,
condition, or position is of concern; in what manner does the failure manifest itself. Failure
Cause [or Failure Mechanism]: what particular component or part prompts the failure mode to
occur and what likelihood of occurrence exists. Failure Effects: what are the effects of the failure,
if any, at the interface, on the system, or on the overall mission performance?
1992 NASA Aerospace Battery Workshop -554- Nickel-Hydrogen Technologies Session
ANALYSIS of FAILURE MODES
Other than a substantial accumulation of fatigue characteristics within Tech Note TN-508-1
(Reference 1.C) to be discussed later, the manufacture has not performed any reliability
analyses or assessments. MIL-HDBK-217E does not directly address strain gages for the
purpose of reliability prediction. Accordingly, a reliability study was performed by RAC
(Reference 2) for a specific contractual obligation. The study included a detailed Failure Modes
Effects Analysis [FMEA]; a Worst Case Analysis [WCA]; and, a Circuit Stress Analysis [CSA].
Failure Modes Effects Analysis
Generally, the FMEA contains the largest single source of information on discrete failure events.
The FMEA involves the listing of potential failure modes, their causes, and the effects upon the
components, subsystems, and subsequent systems. The FMEA is a "bottoms-up" analysis of the
product design characteristics relative to the planned fabrication, test, and inspection process.
This analysis ensures that the resultant product meets the intended need, expectation, and
performance goals. When potential failure modes are identified, corrective action can be
initiated to eliminate them or continually reduce their risk [or potential occurrence].
This present FMEA encompasses the design, fabrication, and use of the strain gage installation
as applied to the GAB NiH2 cell. This analysis covers the use of the strain gage within specific
conditions of environment and use of the host cell as it transits throughout test, integration, and
the launch and mission environments. The incorporated Failure Modes Effects Analysis of the
strain gage contains substantially more detail than a typical FMEA. The following headings are
contained within this analysis [Table 1]:
FMEA No.
Item Name
Part Number
Quantity of Parts
Part Function
Failure Mode
Failure Causes
Failure Cause
Failure Detection and Verification
Corrective Action -Short Term
Corrective Action -Long Term
Failure Effect on the Mission
Failure Effect on the System
Failure Effect at the Interface
A more typical FMEA for a NiH2 cell in similar isolated analysis details only two generic failure
modes for the strain gage. Those failure modes are cited as: [1] Loss of signal; and,
[2] Inaccurate signal. The present FMEA treats the strain gage and installation in isolation upon
the cell. As such, the analysis provides details conceming every component, sub-component,
and material used during installation of the strain gage on the cell assembly.
1992 NASA Aerospace Battery Workshop -555- Nickel-Hydrogen Technologies Session
Circuit Stress Analysis
The strain gage and installation do not contain the discrete electrical or electronic piece parts
that may be subjected to the typical stress derating and application review. Thus, the CSA
portion of the RAC study [Reference 2] analyzed installation and application stresses. These
stresses were used to predict the base failure rate for the strain gage installation.
The analysis predicted the base failure rate for the strain gage by two different methods. A
thermistor model, utilizing handbook principles, predicted a failure rate of 0.13 failures per
million hours. RAC databases indicate a failure rate of 0.128 per million hours for a pair of
NiChrome resistors. The handbook failure model for the forty-four hand soldered connections in
the strain gage subsystem predicted 0.1144 failures per million hours. The total failure rate for
the strain gage installation was predicted at 0.2444 failures per million hours.
Worst Case Analysis
The WCA is used to predict the change in performance parameters if all constituent parts were
to operate at their extreme stress value, or at the extreme of design tolerance. The addition of
the resultant worst case values will provide an end-of-mission performance extreme.
Subsequently, insight is gained then as to which extreme values may be modified to reduce
inherent risk.
Thus the WCA portion of the RAC study (Reference 2) analyzed environmental profiles and
strain gage attributes to predict resistance changes over the mission life. The analysis showed
that the 29.3mW that must be dissipated is only 17% of its maximum allowable for high accuracy
and only 2% when mounted on the cell as a heat sink. The analysis further predicted a variation
in output readings of 1.04% at 5°F at the end of a potential 16.5 year GEO mission.
ANALYSIS of FATIGUE
Expectations for the time dependent GEO mission environment typically includes 2000 cycles
over a life expectancy of 15 to 16 years. Expectations for the cycle dependent LEO mission
environment includes 30 to 40 thousand cycles approaching a life expectancy of 5 years. The
cycle dependency of the LEO mission environment is one area not previously analyzed for the
strain gage installation. Thus, the increased cycle requirement for LEO versus GEO mission
environments raises the specter of fatigue. Henceforth, our discussion is centered about two
specific areas: [1] Fatigue Analysis of the strain gage proper; and [2] Fatigue Analysis of the
1992 NASA Aerospace Battery Workshop -556- Nickel-Hydrogen Technologies Session
strain gage installation. "Strain gage installation" may be more correctly referred to as the strain
gage mount and periphery installations including the circuitry.
Analysis of Strain Gage Proper
GAB has adopted and applied the manufacturer's recommendations as stated in Tech Note
TN 505 (Reference I.D) for both the strain gage and the installation. In addition, the
manufacture has performed substantial testing of strain gages to determine their fatigue
characteristics. The following information is paraphrased from their Tech-Note [Reference 1. C].
The metal foils used in strain gages are prone, as are all metals, to fatigue damage when
cyclically strained at sufficiently high amplitude. In general, larger grid areas result in higher
fatigue life, while higher resistance result in lower fatigue life. Micro Measurements monitors
three parameters on strain gages during fatigue testing: "super-sensitivity," gage factor change,
and zero-shift. Super-sensitivity results from cracks that are just forming, and that are open only
during the tension portion of the loading cycle. Super-sensitivity can only be detected and
monitored by using an oscilloscope. Fatigue cracks can also cause increases in the tension
gage factor; however, they are easily detected since the compression value will be much lower.
For purely dynamic strain measurements, zero-shift is relatively incidental, and strain gages can
be considered functionally adequate until fatigue damage has progressed almost to the stage of
super-sensitivity. Generally, Nominal Fatigue Life is based upon a zero-shift of 100_;.
Figure 2 illustrates those fatigue stress test results. Numerically, Micro Measurements cites a
Fatigue Life of 106 cycles for a Strain Level of _..2.400_, and 107 cycles for a Strain Level of
:_?.200t_s. This fatigue life data is based on fully reversed strain levels. As a generalized
approximation, this data can be used for unidirectional strains, or various mean-strains by taking
the indicated peak amplitude and derating by 10 percent. As an example, +1500_s would be
approximately equivalent in gage fatigue damage to strain levels of +2700 to 01_;, or
0 to -2700[Lt_;, or +2500/-200_;. However, a mean-strain that increases in a tensile direction
during cycling will lead to a much earlier failure.
A typical GAB design destined for a LEO mission environment yielded the following
characteristics. Intemal cell pressure varies according to MCP[1-DOD], where MCP is the
maximum cell pressure and DOD is the depth-of-discharge. For a typical LEO mission:
MCP = 950 PSIG @ BOL [beginning of life];
MCP = 1000 PSIG @ EOL [end of life]; and,
DOD = 30% for a nominal 89 A-H.
1992 NASA Aerospace Battery Workshop -557- Nickel-Hydrogen Technologies Session
The Worst Case pressure varies between 700 to 1000 PSIG [850+150 PSIG]. Therefore, for a
minimum thickness of 0.019 inches the strain varies from 1595 to 22771_ [or, 1936+341_E].
Thus, 5X104 cycles at 1595 to 22771_E:appear well below the manufacturer's point of concern.
Analysis of Strain Gage Mount and Periphery Installations
Appendix A of Tech-Note TN-508-1 provides numerous installation
Maximum
Notes and
1.
2.
3.
4.
5.
recommendations for
Strain Gage Fatigue Life. This appendix refers to an additional series of both Tech
Tech Tips for hands-on installation techniques and tips. These tips and hints include:
Avoiding excess adhesive films;
Soft solders with low melt points;
Using auxiliary bondable terminals;
Leadwire attachment techniques; and,
Use of overcoatings.
GAB has adapted all the installation techniques into their current MCD's. The installation
process is controlled and basic instruction techniques were provided by Micro Measurements.
The soldering process is certified to NHBB 5300.513A-1].
CONCLUSIONS and RECOMMENDATIONS
1. The expanded Failure Modes & Effects Analysis, the Circuit Stress Analysis, and the Worst
Case Analysis each show the design, fabrication and installation of the strain gage to be
conservative in view of the manufacture's available equipment list and installation
recommendations.
2. The Root Cause of numerous Failure Modes identified within the FMEA could be traced to
potential fatigue damage. The Fatigue Analysis of the strain gage shows the gage usage and
environment to be well below even the manufacturer's points of minimum concern. Significant
test data exists for the prediction of fatigue life of the strain gage. However, this gage
installation and periphery, while following all possible recommendations, have not been tested
for fatigue life.
3. The end result of this analysis is the recommendation for two life test regimes for the strain
gage and installation. A LEO test of 2000 cycles at Room Temperature has already been
scheduled for completion by year end. A GEO test is being devised with expected completion
by Mid-1993. Success and failure criteria are being determined, and test results will be reported
in a later paper.
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REFERENCES
1. Strain Gage Technology Manual; Measurements Group -Vishay, Raleigh, NC; 1991:
A. Catalog 500, Parts A & B.
B. Catalog A-110-5.
C. Tech Note TN-508-1, "Fatigue Characteristics of Micro-Measurements Strain Gages."
D. Tech Note TN-505, "Strain Gage Selection Cdteda, Procedures, Recommendations."
2. Reliability Study of the NiH2 Strain Gage; Reliability Analysis Center, Rome, NY; 1992.
1992 NASA Aerospace Battery Workshop -567- Nickel-Hydrogen Technologies Session



Reliability study of the NiH2 strain gage

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