NASA Contractor Report 202316
The Development of a PdCr Integral Weldable
Strain Measurement System Based on NASA
Lewis PdCr/Pt Strain Sensor for User-Friendly
Elevated Temperature Strain Measurements
S.P. Wnuk, Jr. and V.P. Wnuk
Hitec Products, Inc.
Lewis Research Center
Under Contract C-86401-D
National Aeronautics and
Trade names or manufacturers' names are used in this report for identification
only. This usage does not constitute an official endorsement, either expressed
or implied, by the National Aeronautics and Space Administration.
THE DEVELOPMENT OF A PdCr INTEGRAL WELDABLE STRAIN
MEASUREMENT SYSTEM BASED ON NASA LEWIS PdCr/Pt
STRAIN SENSOR FOR USER-FRIENDLY ELEVATED
TEMPERATURE STRAIN MEASUREMENTS
S.P. Wnuk, Jr. and V.P. Wnuk
Hitec Products, Inc.
This report describes the development of a user firiendly weldable strain gage employing the NASA Lewis PdCr/
Pt wire strain sensor. The NASA sensors are preattached to Hastelloy X or Titanium alloy shims using flame spray
techniques developed under previous NASA programs. The weldable sensors are then prestabilized for 50 hr at
1440 °F in air. A weldable terminal and high temperature cable is then connected to the sensor and the assembly is
precalibrated over the full test temperature range. Calibrated resistors are inserted into a bridge completion module
at the cool end of the cable to condition the sensor in half or full bridge configuration. The sensor is attached to the
structure using a common capacitive discharge spot welder. No additional high temperature stabilization or calibra-
tion is required. The resultant device is a precalibrated strain transducer which can be plugged into any common
variety strain instrumentation.
The development of PdCr wire strain gages make possible static strain measurements to 1400 °F. The develop-
ment of these gages revolutionized attachment techniques rendering all prior art obsolete. A totally new and compre-
hensively different set of procedures and new materials were developed. In order to use these gages successfully,
every strain gage installer must undergo a thorough reorientation, abandon familiar materials and procedures, and
adopt completely new practices, the details of which are discussed in references 1 and 2.
In addition to strict observance of installation procedures, the PdCr strain gages must be prestabilized at elevated
temperatures (1440 "F for 50 hr). Care must be exercised not to overtemp the gages because a momentary excursion
above the critical temperature (ref. 7) will destroy the gage compensation. The special flame sprayed alumina/zirco-
nia ceramic matrix provides superior resistance stability. However, excessive under temperature will result in insuf-
ficient stabilization of the PdCr and resistance changes less than expected. This causes an unbalanced bridge, thus an
improper value for the balast resistor Rg, (either too low or too high) which distorts the apparent strain curve. An Rg
value which produces a near perfect balanced bridge produces the least amount of strain. Therefore, it is imperative
to hold stabilization temperatures at each gage location within the specified tolerance. This is difficult to do on large
Therefore, the installation of PdCr/Pt gages onto thin shims which can be prestabiUzed in a closely controlled
oven is an attractive alternative for several reasons:
1 . Gages are installed by trained technicians in a closely controlled laboratory environment.
2. Gages are stabiUzed in a very closely controlled programmable furnace.
3. With a terminal and high temperature cable attached, the gages on a shim can be precalibrated for Rg over the
desired temperature range.
4. With the selected Rg value inserted into a miniature signal conditioning module at the cool end of the high
temperature cable; the gage can be precalibrated for apparent strain versus temperature. Using the calibration
method developed by Hofstotter (ref. 3) the gage can be calibrated on a sample coupon of test article material.
The weldable gage consists of a free filament PdCr/Pt strain gage flame spray (shown in fig. 1 ) bonded to a 5 mil
thick shim using bonding procedures described in reference 2. A thin metalized precoat of nickel chrome aluminum
alloy is applied to the gage bonding area of the shim to enhance ceramic bonding. Rokide HTZ rod, a special flame
spray rod developed specifically for use with PdCr is used for gage attachment. A margin of shim material extends
around the perimeter of the gage and is the weld area used to attach the shim to the test structure. The gage is
attached to the structure using capacitive discharge spot welding equipment ordinarily used for this purpose.
The gage is available in two sizes, a small, 60 Q unit and a standard 120 Q size. The 60 Q. units are only 0.5-in.
long by 0.4-in. wide. See table I for nomenclature and dimensions. Two shim materials are standard, Hastelloy X
for compensation on 6 ppm/°F materials and Ti6A14V for compensation on Titanium matrix composites.
Although available without cables, the gages are usually supplied with an integral weldable terminal, high tem-
perature cable, and a bridge completion module on the cold end of the cable.
The gage may be attached to the terminal located axially or transversely to the cable. See figure 4. The terminal
consists of a weldable base shim with three high purity alumina insulators and the hot end of the cable strap welded
to the base shim (fig. 2). The three cable wires are threaded through the insulators and bonded to the inside of the
insulator using a high alumina ceramic cement. The bonding prevents movement of the conductors when the cable is
twisted, pulled or vibrated. The ceramic cement is also applied to the exterior of the insulators to bond them to each
other and to the weld straps. The cement is heat cured prior to attachment of the terminal to the gage.
The terminal is attached to the gage with two wire flexures welded to the edge of the gage shim and the edge of
the terminal. The flextures consist of 10 mil diameter Nichrome* wires bent in a "U" shape. The flextures are used
to hold the gage and terminal together without imposing stresses on the PdCr/Hoskins weld joint. The 3 mil PdCr
leads are spot welded to the Hoskins alloy conductors. The high temperature cable consists of three number 25
AWG Hoskins alloy 875 conductors individually insulated with Nextel^ fiber-braided insulation and held together
with a tightly wound Nextel braid over the three individual cables. The cable is heat cleaned prior to assembly. A
small PC board module (fig. 3) which contains the bridge completion resistors is attached to the cool end of the high
temperature cable. The gage with terminal, and completion module can be temperature calibrated at the user's facil-
ity or they may be supplied precalibrated.
Determination of R,
The first step in the calibration process is the determination of Rg and other bridge completion resistor values.
The gage is mounted in a calibration fixture attached to a sample of test material (if calibration on a specific material
is desired), or it can be calibrated on the shim only. Referring to the Sketch below, Rjj, R23 and R]3 are measured at
points (T) . (2) . and (3) at room temperature and at maximum temperature using a meter with at least 2 decimal
rb © "^4 Ri3
R is measured at end of leads at points M J ,(2Jand(^33.
*T.M. Driver Harris
tjM 3M Co.
From the resistance measurements, R3 is calculated using the equations given in reference 4. A sample calcula-
tion is included in the appendix. The calculated value of Rg is verified using a temporary bridge completion network
of precision decade resistors such as the General Radio model 1432 or Vishay model V-40. Rg is placed in series
with the compensating gage. The other two resistors completing the full bridge also utilize precision decade resis-
tors. The resistor adjacent to R is set at 120 Si (or 60 Q for 60 Q gages), and the resistor Rj is adjusted to balance
the bridge. Bridge balance is read out on a Vishay P-3500 strain indicator. The bridge balance resistor within the
indicator is disconnected for this procedure.
With the gage connected to the precision resistor network the gage on the test bar (Hastelloy X for 6 ppm/°F
gages) is placed into a cool furnace. It is necessary to keep the gage shim flat during calibration. In addition to tack
welding the four comers, the gage may be clamped to the test bar using the methods developed by Hofstotter
(ref. 3). A type K thermocouple is also spot welded to the test bar and connected to the X axis of an X-Y plotter. The
temperature is also read out on a digital meter. A thin layer of alumina felt insulation is placed over the gage to pre-
vent spurious signals due to thermal convection within the furnace. The output of the P-3500 strain indicator is con-
nected into the Y axis of the X-Y plotter.
Strain readings are taken every 100 °F and an analog plot is made of the apparent strain calibration on the X-Y
plotter. Slow furnace heat up and cool down rates are used to ehminate thermal stresses within the test bar. Thermal
EMFs are checked periodically by turning off bridge power momentarily. If the apparent strain calibration is satis-
factory, a permanent bridge completion module is made up using adjustable bridge completion resistors available
from strain gage manufacturers. The values of these adjustable resistors are made identical to the values on the pre-
cision decade resistors. If the apparent strain curve is not satisfactory, Rg is re-adjusted and another calibration run
is made. This process is repeated, if necessary, until a satisfactory calibration curve is achieved. The adjustable resis-
tors are inserted into the bridge completion module in place of the precision decade resistors and calibration curve
recorded. A second cycle is run to check repeatabihty and to verify that the maximum and zero return strain readings
are repeatable. The gage is then removed from the test bar and packaged for shipment along with the analog calibra-
Figure 2 is a photograph of the weldable gage, terminal, and high temperature cable. Figure 3 is a photograph of
the bridge completion module. Figure 4 shows two sketches of the weldable strain gage assembly with half bridge
and full bridge signal conditioning modules. A typical analog plot of apparent strain versus temperature recorded on
an X-Y plotter is shown in figure 5. Note that the zero shift at room temperature is only 22 he, after a thermal cycle
to 1380 °F.
DISCUSSION OF RESULTS
Possibly the most common problem faced by today's experimental engineer is obtaining enough time to perform
the testing. The development of the precalibrated weldable strain gage assembly goes a long way to easing the bur-
den of the test engineer. While one might be interested in how calibrations are done, project time schedules often
preclude the test engineer from conducting extensive apparent strain calibrations on the test article. Often users are
not interested in conducting temperature calibrations; they want to install the instrumentation and run the test. The
development of the precalibrated weldable strain gage assembly allows the user to do just that - install the strain
gage and run the test.
1 . A major benefit of the preinstalled strain gage is that the stabilization treatment takes place with the PdCr/Pt
wires in contact with a 96 percent alumina, 4 percent zirconia ceramic matrix. This zirconia oxide additive to alu-
mina significantly reduces oxidation of the wire as compared to the same wires oxidized in air (refs. 5 and 6).
2. Another benefit of precalibrated weldable gages is improved accuracy. It is extremely difficult in practice to
generate accurate apparent strain calibration curves on the structure because it is virtually impossible to maintain
isothermal conditions within the structure during heat up and cool down. Temperature gradients cause thermal
stresses which result in a hysteresis between heat up and cool down cycles. It is only when the structure is reduced
in size to that of a gage on a shim (which is also well insulated during this test) that the hysteresis between heat up
and cool down disappears.
3. The greatest benefit of the precalibrated weldable strain gage is that it is user friendly. The test engineer is not
expected to have the expertise of the strain gage vendor. He/she should not have to be a strain gage guru in order to
achieve good results. A calibrated gage with understandable specifications and easy to follow installation instruc-
tions with no unexpected, unforeseen operational surprises, is essential for good results.
A user friendly weldable strain measurement system based on the NASA Lewis PdCr/Pt wire strain gage has been
developed for high temperature strain measurements up to 1400 °F.
The strain measurement system developed under this and other NASA Lewis contracts has been made commer-
cially available in accordance with the Space Act Agreements between NASA Lewis and Hitec Products, Inc.
The writers wish to acknowledge the key role of Lynda A. Murray in her untiring effort making the NASA PdCr/Pt
strain gages which are at the heart of the program's success.
Calculate value for ballast resistor Rd from reference below:
with R,,, R13 and R23 readings taken at temperature T. kg^ =
n _i_ R 1?
— — — — at "0" or room temperature
AT = Tempeature at T - T^,; T^ = room temperature.
I^c„ ■ AT
j^ ^ R23 + R13 ~Rl2
(Readings taken at temperature T)
R-t-t + Rn — Ri
, _ rt23jviM3~iM2.
For gages bonded to Alumina specimen:
(Readings taken at room temperature)
T R12 R23 ^13
To=73°F 131.4 22.6 138.9
Tt=1125°F 146.3 36.4 167.2
_ R12+R13-R23 _ 131.4 + 138.9-22.6
R _ Rl2+Ri3-R23 ^ 146.3 + 167.2 -36.4 ^
^^2 2 ■
AT = 1125 -73 = 1052
_ RgT-Rgp _ 138.55-123.85
Ac„ ■ AT
^ _ R23+Ri3-Ri2 ^ 36.4 + 167.2-146.3 ^,„^^
"■^ 2 2 ■
^ _ R23 + R13-R12 _ 22.6+138.9-131.4
= "^i ±^ = 0.000858
Using a decade resistor, set Rg = 100.2Q
R is measured at end of leads at points (V) ,(V) and (V).
The 2nd decade is set at 120.0n and the bridge is balanced by adjusting the adjacent decade, which resulted in
The gage is placed in the furnace and experimental determination of the apparent strain between room tempera-
ture and 1 100 °F is made as follows. A Vishay P3500 Strain Indicator, with GF set to 1.30, is used to record strain,
and a chromel alumel thermocouple is used to measure temperature.
Jih-Fen Lei, D.R. Englund and C. Croom: "The Temperature Compensation Techniques for a PdCr Resistance
Strain Gage," Society for Experimental Mechanics, Fall Conference 1991.
1. Lei, J.F., Wnuk, S.P., Jr., "A Hame-Sprayed Resistance Strain Gage for High-Temperature Applications,"
Journal of Thermal Spray Technology, Vol. 3, No. 3, Sept. 1994, pp. 305-309.
2. Wnuk, S.P. Jr., "Final report on Procedure for Installation of PdCr Gages by Flame Spraying," NASA
CR-195389, Oct. 1994.
3. Hofstotter, P, "The Use of Encapsulated High-Temperature Strain Gages at Temperatures up to 315 °C,"
Experimental Techniques, August 1985.
4. Lei, J.F., Englund, D.R., Croom, C, "The Temperature Compensation Techniques for a PdCr Resistance Strain
Gage," Society for Experimental Mechanics, Fall Conference 1991.
5. Lei, Jih-Fen, "Protective Coats for High Temperature Strain Gages", NASA Technical Briefs, p. 94, Sept. 1993.
6. Leca, L., "Physical and Structural Properties Of Palladium-Chromium Layers Obtained by Cathodic Sputtering:
Resitive Element for a High Temperature Strain Gage," Note Technique 1995-14, Office National D'Etudes
Et De Recherches Aerospatiales, 29 Avenue de la Division Leclirc, 92320 Chatillon (France).
7. Lei, Jih-Fen, Greer, L.C., III, and Oberle, L.G., "Evaluation of PdCr Wires for Strain Gage Application," NASA
TM-106857, Feb. 1995.
— PALLAOnjM CHROME WELDABLE STRAIN GAGES
HB WAPd-06- 1 30-In-SPdW
HB W APd-06- 1 30-Ti-SPdW
HB WAPd- 1 2-300-In-SPdW
HB W APd- 1 3-300-Ti-SPdW
Leads are 0.07 mm diameter PdCr, 25 mm long.
0.03 mm diam
compensator — 4*-
0.03 mm diam
Figure 1 .—Free filament PdCr strain gage (6x larger than actual size).
Figure 2. — Photograph of PdCr weldable gage with terminal and cable.
Figure 3. — Photograph of bridge completion module.
k 0.50 in. »j
PdCr strain gage
Weldable terminal .005 in
thick Hastelloy x or 300
seriies stainless steel
3 conductor #25
Hoskins 875 Nextel
Figure 4. — High temperature PdCr/R weldable strain gage with temiinal, cable, and bridge completion module.
3 conductor #25
Hoskins 875 Nextel
PdCr strain gage
gage #5-50700 w/2' cable
G.F. + 1.2 Ri =120.0
Rb = 68.2 R2 = 162.6
82 °F 200 °F
400 °F 600 °F 800 °F 1000 °F 1200 °F
Figure 5.— Analog plot of apparent strain versus temperature.
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1. AGENCY USE ONLY (/.eai/e Man/c)
2. REPORT DATE
REPORT TYPE AND DATES COVERED
Final Contractor Report
4. TfTLE AND SUBTrtLE
The Development of a PdCr Integral Weldable Strain Measurement System
Based on NASA Lewis PdCr/Pt Strain Sensor for User-Friendly Elevated
Temperature Strain Measurements
S.P. Wnuk, Jr. and V.P. Wnuk
5. FUNDING NUMBERS
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
Hitec Products, Inc.
100 Park Street
Ayer, Massachusetts 01432
8. PERFORMING ORGANIZATION
9. SPONSORING/MONrrORING AGENCY NAME(S) AND ADDRESS(ES)
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44 1 35 - 3 1 9 1
AGENCY REPORT NUMBER
11. SUPPLEMENTARY NOTES
Project Manager, Jih-Fen Lei, Instrumentation and Controls Division, NASA Lewis Research Center, organization
code 5510, (216) 433-3922.
12a. DISTRIBUTION/AVAILABILrTY STATEMENT
Unclassified - Unlimited
Subject Category 19
This publication is available from the NASA Center for AeroS pace Information, (301)621-0390.
12b. DISTRIBUTION CODE
13. ABSTRACT fMax/mum 200 words;
This report describes the development of a user friendly weldable strain gage employing the NASA Lewis PdCr/Pt wire
strain sensor. The NASA sensors are pre-attached to Hastelloy X or Titanium alloy shims using flame spray techniques
developed under previous NASA programs. The weldable sensors are then pre-stabilized for 50 hours at 780 °C in air.
A weldable terminal and high temperature cable is then connected to the sensor and the assembly is pre-calibrated over
the full test temperature range. Calibrated resistors are inserted into a bridge completion module at the cool end of the
cable to condition the sensor in half or full bridge configuration. The sensor is attached to the structure using a common
capacitive discharge spot welder. No additional high temperature stabilization or calibration is required. The resultant
device is a pre-calibrated strain transducer which can be plugged into any common variety strain instrumentation.
14. SUBJECT TERMS
Weldable gage; Strain gage; PdCr alloy; High temperature strain measurement
15. NUMBER OF PAGES
16. PRICE CODE
17. SECURfTY CLASSIFICATION
18. SECURfFY CLASSIFICATION
OF THIS PAGE
19. SECURITY CLASSIFICATION
20. LIMrrATION OF ABSTRACT
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