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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. 
Ayer, Massachusetts 



February 1997 



Prepared for 

Lewis Research Center 

Under Contract C-86401-D 




National Aeronautics and 
Space Administration 



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. 
Ayer, Massachusetts 



SUMMARY 

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. 

INTRODUCTION 

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 
structures. 

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. 



GAGE DESCRIPTION 

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, 



B 



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 
place resolution. 



C 
Rg C 



■9 



© 



rb © "^4 Ri3 



C 



R23I 



re 



© 




i2o.on 



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. 



Calibration 

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- 
tion curve. 



RESULTS 

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. 



BENEHTS 

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. 



CONCLUSIONS 

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. 



Technology Transfer 

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. 



ACKNOWLEDGMENTS 

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. 



APPENDIX 



SAMPLE CALCULATION 



Calculate value for ballast resistor Rd from reference below: 



dtc-Ag 



Rp 



Oo 



where 



A _:^IxZ^ 



I^goAT 



A Rl2+Rl3~R23 



with R,,, R13 and R23 readings taken at temperature T. kg^ = 



n _i_ R 1? 

— — — — at "0" or room temperature 

2 



AT = Tempeature at T - T^,; T^ = room temperature. 



dc = 



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 



Therefore, 



R„ 



_ R12+R13-R23 _ 131.4 + 138.9-22.6 



123.85 



R _ Rl2+Ri3-R23 ^ 146.3 + 167.2 -36.4 ^ 
^^2 2 ■ 



AT = 1125 -73 = 1052 



_ RgT-Rgp _ 138.55-123.85 



6b -AT 



123.85x1052 



0.000112 



dc 



Rct-Rcq 
Ac„ ■ AT 



^ _ R23+Ri3-Ri2 ^ 36.4 + 167.2-146.3 ^,„^^ 
"■^ 2 2 ■ 



^ _ R23 + R13-R12 _ 22.6+138.9-131.4 



= 15.05 



Then 



28.65-15.05 
= "^i ±^ = 0.000858 

15.05x1052 



Rf 



lic„Uc-(ig 



_ 15.05(0.000858-0.000112) 
0.000112 



R3=100.2Q 



Using a decade resistor, set Rg = 100.2Q 



Re 




i2o.oa 



111.67ft 



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 
111.67Q. 

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. 



70°F 
150°F 
200°F 
300°F 
400°F 
460°F 
500°F 
600°F 
700°F 
800°F 
900°F 
1000°F 
1100°F 



US 

0000 
-630 
-975 
-1525 
-1850 
-1912 
-1880 
-1675 
-1275 
-720 
0000 
-1-485 
-1-260 



Max(-) 



APPENDIX REFERENCE 

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. 

REFERENCES 



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. 



TABLE I. 


— PALLAOnjM CHROME WELDABLE STRAIN GAGES 






Designation 


Resistance, W 


Compensation 


Carrier 


L, 

mm 


w, 

mm 


Sensor 


Thermometer 


HB WAPd-06- 1 30-In-SPdW 


60 


4 


Inconel 


Hastelloy X 
Shim 


12.5 


10 


HB W APd-06- 1 30-Ti-SPdW 


68 


4 


TMC* 


Ti6A14V 
Shim 


12.5 


10 


HB WAPd- 1 2-300-In-SPdW 


120 


8 


Inconel 


Hastelloy X 
Shim 


20 


11 


HB W APd- 1 3-300-Ti-SPdW 


135 


8 


TMC* 


Ti6A14V 

Shim 


20 


11 



20 mm 

■« ► 



11 mm 



I 



120Xlweldable 
(actual size) 



12.5 mm 

■* ► 



10 mm 



t 



60fl weldable 
(actual size) 



Leads are 0.07 mm diameter PdCr, 25 mm long. 



Tape carrier 



0.03 mm diam 
ZGSPt 
compensator — 4*- 




0.03 mm diam 
PdCr strain 
sensor 



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. 



Gage axis 



k 0.50 in. »j 



u 



1^ 



<^ 







Weldable 60n 
PdCr strain gage 



Weldable terminal .005 in 
thick Hastelloy x or 300 
seriies stainless steel 



3 conductor #25 
Hoskins 875 Nextel 
braid insulation 



Half bridge 
completion 
module 



Figure 4. — High temperature PdCr/R weldable strain gage with temiinal, cable, and bridge completion module. 




Weldable 
6011 PdCr 
strain gage 



axis 



Weldable 
terminal 
.005 in. 
thick 

hastelloy x 
or 300 
series 
stainless 
steel 



3 conductor #25 
Hoskins 875 Nextel 
braid insulation 



Full bridge 
completion 
module 



10 



c 
(0 



Temperature 





Strain 




readings, 




°F 


US 


PdCr strain gage 
gage #5-50700 w/2' cable 


82 
200 
400 


0000 
-1556 
-2711 


G.F. + 1.2 Ri =120.0 


600 


-2660 


Rb = 68.2 R2 = 162.6 


800 


-1950 




1000 


-1007 




1200 


-630 




1380 


+304 




648 


-2746 




134 


-529 




82 


+22 




I I 

82 °F 200 °F 



400 °F 600 °F 800 °F 1000 °F 1200 °F 

Figure 5.— Analog plot of apparent strain versus temperature. 



1380 °F 



11 



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1. AGENCY USE ONLY (/.eai/e Man/c) 



2. REPORT DATE 

February 1997 



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 



6. AUTHOR(S) 



S.P. Wnuk, Jr. and V.P. Wnuk 



5. FUNDING NUMBERS 



WU-523-21- 
C-86401-D 



13 



7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

Hitec Products, Inc. 

100 Park Street 

Ayer, Massachusetts 01432 



8. PERFORMING ORGANIZATION 
REPORT NUMBER 



E-10617 



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 



10. SPONSORING/MONrrORING 
AGENCY REPORT NUMBER 



NASACR-202316 



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 

14 



16. PRICE CODE 

A03 



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OF REPORT 

Unclassified 



18. SECURfFY CLASSIFICATION 
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Unclassified 



19. SECURITY CLASSIFICATION 
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Unclassified 



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