N AS A / TM- 1998-208739

Thermal Output of WK-Type Strain

Gauges on Various Materials at Cryogenic

and Elewated Temperatures

Matthew K. Kowalkowski, H. Kevin Rivers and Russell W. Smith

Langley Research Center, Hampton, Virginia

National Aeronautics and

Space Administration

Langley Research Center

Hampton, Virginia 23681-2199

October 1998

https://ntrs.nasa.gov/search.jsp?R=19990009541 2018-05-11T16:54:59+00:00Z
JThe use of trademarks or names of manufacturers in the report is for accurate reporting and does not constitute an I

official endorsement, either expressed or implied, of such products or rr anufacturers by the National Aeronautics and l

Space Administration. I

Available from:

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Abstract

Strain gage thermal output (apparent strain) is one of the

largest sources of error associated with the measurement of

strain when temperatures vary. In this paper, thermal output of

WK-type strain gages is experimentally determined for

temperatures ranging from -450°F to 230°F. The gages are

installed on both metal specimens and composite laminates of

various lay-ups and resin systems. Metal specimens tested

include: aluminum-lithium alloy (AI-Li 2195-T87), aluminum

alloy (Al 2219-T87), and titanium alloy. Composite materials

tested include: graphite toughened-epoxy (IM7/977-2), graphitebismaleimide (1M7/5260), and graphite-thermoplastic

(IM7/K3B). For the composite materials thermal output in both

the 0 ° fiber direction and the 90 ° fiber direction is measured.

The experimentally determined thermal output data are curve fit

with a fourth-order polynomial for each of the materials studied.

The thermal output data and the polynomials that are fit to the

data are compared with those produced by the strain gage

manufacturer, and the results and comparisons are presented.

Unacceptably high errors in the manufacturer's data are

observed at temperatures below -270°F.

Background

The X-33 (see Figure 1) is a half scale

Reusable Launch Vehicle (RLV) prototype

rocket, based on the Single Stage to Orbit (SSTO)

concept, whose project goal is to provide industry

with a cost effective vehicle for the next

generation of space flight [1]. The success and

reusability of future launch systems depend

heavily on the reusability of the cryogenic fuel

tanks. Space Shuttle (a current space

transportation system) cryogenic tanks are

designed for only one flight that lasts

approximately eight minutes, making the launch

process costly and wasteful. Figure 1. X-33 vehicle.

In order to build a viable RLV, a reusable

cryogenic fuel tank must be produced, and the

validation of the durability of this cryogenic tank

system, which includes the cryogenic insulation,

tank structure, and adhesives, requires testing at

cryogenic temperatures. This requirement has

prompted the NASA Langley Research Center

(LaRC) to develop cryogenic thermal-mechanical

tests capable of simulating the flight-cycle

thermal and mechanical loads for various

cryogenic tank concepts. In some of these tests, a

series of 1-ft. by 2-ft. rectangular panels are tested

to determine their durability while subjected to

the thermal and mechanical loads experienced by

a tank during flight [2].
Nomenclature

Figure 2. 1-Foot by 2-Foot panel placed in

the hydraulic load machine.

The cryogenic thermal-mechanical tests

developed at LaRC involve either a metallic or

composite sandwich structure (where the

sandwich core also serves as a cryogenic

insulation) or a metallic or composite stiffened

panel (with external cryogenic insulation).

Figures 2 and 3 are photographs of the front and

back views of one such test. Accurate

measurements of strain at temperatures ranging

from -423°F to 230°F are required during the

tests. Because the electrical resistance of a strain

gage varies with both temperature and load, an

accurate correction of temperature-induced

thermal output is critical for determining the true

mechanical strain from strain gage measurements.

OtR

AT =

(AR._O)T/0 =

PG

K t =

.v0

eTm =

e 1

e 2 ----

eTJO IT,)=

F*

F(T,) =

(J

E =

the coefficient of thermal

expansion of the test material.

the coefficient of thermal

expansion of the strain gage

grid.

coefficient of thermal expansion

of the reference material used to

compute the manufacturer's

curves.

the temperature change from

the reference temperature.

the relative resistance change

from the reference resistance.

the thermal coefficient of

resistance of the gage.

the gage factor of the gage.

the transverse sensitivity of the

strain gage.

Poisson's ratio of the standard

test material used in calibrating

the gage for its gage factor.

the thermal output (apparent

strain) in strain units. The

magnitude of strain measured

when the gage (mounted to a

test material) is subjected to a

temperature change.

strain magnitude corrected for

both thermal output and gage

factor variation.

indicated or measured strain.

thermal output at test

temperature Tt.

gage factor setting used while

strain measurement is made.

gage factor at test temperature.

the stress in the test material.

Young's modulus of the

test material.

2
ep -- the thermal output predicted for

a strain gage attached to a test

material.

ea = the thermal output for a

reference material used to

compute the manufacturer's

curves.

evo = experimentally determined

thermal output in microstrain.

T = temperature in °F.

Strain Gage Thermal Output

(Apparent Strain)

Temperature-induced thermal output (also

called apparent strain) is caused by two

algebraically additive components. The first

component is the change in electrical resistivity of

the gage due to temperature change. This

resistance change is independent of any

mechanically induced strain on the object to

which the gage is bonded. The second component

of thermal output is the change in electrical

resistivity of the gage due to mechanical strain

produced by the difference in coefficients of

thermal expansion of the gage material and the

material to which the gage is mounted (and in

which we want to measure strain).

The cause of the second component is the ctAT

or "free" thermal expansion of the material to

which the gage is bonded. The change in

temperature (AT) causes an expansion or

contraction of the specimen. Because the strain

gage is strongly attached to the surface of the

material and has a low stiffness, the gage is forced

to undergo the same expansion or contraction as

the material. This strain of the gage will be

registered by an indicator as strain of the

specimen, even if no mechanical strain is induced

in the specimen. Thus, in order to get an accurate

reading of the mechanical strain being placed on

the specimen in an environment in which the

temperature is changing, it is essential to correct

for thermal output due to changes in temperature.

Figure 3. Hydraulic load machine and

cryogenic chambers.

Further information about thermal output can

be found in reference [3], where equation (la) is

derived. This equation shows how thermal output

is the algebraic sum of both components in that

the relative change in resistance of the gage is

given as:

--_ r,o-- [flc + Fc l1-voK'

The thermal output is proportional to the

relative resistance change as indicated in the

following equation:

eT/o =

FG

(lb)

3
One method of experimentally determining

thermal output is to take small material specimens

(having homogeneous properties), gage them in

exactly the same fashion as the test specimen

(which locally has the same properties) and then

subject these specimens to only thermal loads.

Raw strain data collected from this thermal

loading of the small specimens can then be

utilized to generate thermal output data and the

polynomial equations describing those data.

These polynomial equations can then be used to

determine the true mechanical strain to induce in

the test specimen and correct the measured strain

data.

In experiments where test specimens are

subjected to thermal loads, measured strain data

must be corrected for both thermal output and

gage factor variations with temperature [3]. The

thermal output is dependent on the test specimen

material and must be determined experimentally.

The gage factor varies linearly with temperature

and is not dependent on the test specimen material

so data provided by the gage manufacturer is

used. Provided that the gage factor and thermal

output at temperature are known for the material

being tested, Equation 2 below can be used to

correct the strain measurement for both thermal

output and gage factor variations with

temperature.

e =[e2_er,o(T_)]{ F* (2a)

0.075 inches

0.125 inches

Stra n

Measurement I H

Axis. '__ (_,ocouple

Strain Gage

_,_ 2 inches ="-

2Tinches

1

Figure 4. Specimen layout, (dimensions

are approximate).

Test Specimens

For this study, values of thermal output were

measured for specimens of seven different

materi__ls at temperatures ranging from

approximately -423 °F to 230 *F. Those

specimens include both composite and metallic

materials. The specimens were all instrumented

with one strain gage mounted in the center of each

specimen and one Type-T thermocouple placed to

one side of the strain gage as shown in Figure 4.

All composite specimens were gaged with WK00-250BG-350 strain gages, which have a gage

factor of 2.02 at 75°F. Three specimens of each

material had strain gages oriented along the

The total strain is a combination of the

mechanical strain and the thermal strain and is as

follows:

e = el + ctAT (2b)

For uniaxial loads, the mechanical strain is:

a

e, = -- (2c)

E

Figure 5. Graphite-K3B co-cured

specimens.
Figure 6.Aluminum-lithium (AI-Li 2195-

T87) specimens.

0 ° fiber direction, and the other three specimens

of each material had strain gages oriented along

the 90 ° fiber direction. This was done because

composite materials have different coefficients of

thermal expansion in orthogonal directions.

The aluminum alloy (AI 2219-T87) and the

aluminum lithium (A1-Li 2195-T87) specimens

were gaged with WK-13-250BG-350 strain gages

which have a gage factor of 2.08 at 75°F, and the

titanium alloy specimens were gaged with WK06-250BG-350 gages which have a gage factor of

2.02 at 75°F. The strain gages were applied using

standard installation methods [5].

Six IM7/977-2 graphite toughened-epoxy

(GR/EP) and six IM7/5260 graphite-bismaleimide

(GR/BMI) specimens were tested. The lay-up of

the GR/EP and the GR/BMI specimens was

[+45/0/90/0]s for all twelve of the specimens.

A total of twelve IM7/K3B graphitethermoplastic (GR/K3B) specimens (see Figure 5)

were tested, six of which had a [0/60/90/-60/0]s

lay-up and originated from a facesheet of a cocured titanium honeycomb sandwich panel which

had previously been tested within its elastic limit.

The other six specimens were fabricated from a

cured laminate with a [0/45/90/-45] s lay-up.

Three of each of the co-cured and consolidated

GR/K3B specimens were instrumented in the 0 °

fiber direction and the other three were

instrumented in the 90° fiber direction.

Three specimens of aluminum alloy (AI 2219-

T87) and three titanium alloy (Ti) specimens were

also instrumented and tested. All of these

specimens were 2-in. by 2-in. square specimens

with a tolerance of approximately !-0.125 inches.

Three aluminum-lithium alloy (AI-Li 2195-

T87) specimens were tested. They were only lin. by 2-in. with the gages measuring strain in the

rolled direction (see Figure 6). Although AI-Li is

an anisotropic material, the load is introduced in

the rolled direction in planned uniaxial tests, so

measurements of strain gage thermal output were

made in this direction only.

The 2-in. by 2-in. specimen size was picked to

avoid "end effects" in the strain measurements,

and since the gages measured strain in the rolled

direction, the 1-in. by 2-in. AI-Li specimens were

sufficient for measurement of thermal output data.

Figure 7. Dewar used for tests with

spraybar and lid attached.
Experimental Apparatus

Due to the extreme temperatures required for

this experiment, two different test apparatii were

required. To obtain temperatures below -300°F,

an insulated Dewar was used (see Figure 7).

Specimens were placed in a wire basket (see

Figure 8) and the basket was placed inside the

Dewar. A spraybar was then inserted above the

wire basket that allowed either liquid nitrogen or

liquid helium to be admitted to the Dewar. This

device setup allowed the measurement of strain

readings at -450°F (liquid helium temperature) as

well as -320°F (liquid nitrogen temperature).

In order to obtain test temperatures ranging

from -300°F to 230°F, a large, insulated, forced

convection chamber was used. This chamber,

shown in Figure 9, was heated with resistive

elements and cooled with a circulating liquid

nitrogen sprayed "mist". The circulation of the

liquid nitrogen mist helped assure uniform

thermal conditions and prevented the liquid from

directly contacting the specimens. All of the

specimens were placed on a wire tray suspended

approximately 2 in. above the bottom of the

chamber to insure proper circulation around the

specimens for temperature uniformity. Access

ports and gaps on the top and bottom of the forced

convection chamber were sealed to minimize

leakage.

For data acquisition, a Neff data acquisition

system (DAS) and the Autonet data acquisition

software were used; both of which are available

commercially. A photograph of the system is

shown in Figure 10.

The Neff DAS utilized Wheatstone bridge [6]

cards to measure strain (balanced to zero the

strain gage readings at room temperature), and

thermocouple cards to measure temperature. The

signals were received, processed, and displayed

on a computer using the Autonet software

package.

Figure 8. Wire basket used for Dewar tests.

Figure 9. Convection chamber with

specimens on tray.

6
Figure 10. Neff/Autonet data acquisition

system.

Although the Type-T thermocouples attached to

the samples could be used to measure

temperatures down to -450°F, the Autonet data

conversion tables were only programmed for

temperatures down to approximately -340°F, and

when temperatures dropped below this value, the

values were marked "out-of-range". For this

reason, two actions were taken: first, a millivolt

reading was taken from the thermocouples for

later data reduction and second, Type-E

thermocouples, capable of being processed by

Autonet to temperatures of -450°F, were used to

evaluate the temperature inside both the Dewar

and the convection chamber. In the forced

convection chamber, these Type-E thermocouples

were placed in a diagonal pattern across the tray

that held the specimens. For the Dewar tests, the

specimen basket was lined with three Type-E

thermocouples in a spiraling fashion with one at

the top, one in the middle, and one on the bottom.

These steps were taken to assess the temperatures

and temperature variations very near the

specimens as well as to provide a temperature

measurement for temperatures below -340°F.

Experimental Procedure

Prior to a test, the strain gages and

thermocouples were connected to the DAS. The

strain gages were zeroed at room temperature by

balancing the Wheatstone bridges in the DAS

with a tolerance of +5 microstrain. The

thermocouple readings were then checked for

proper function. The reference temperature at

which the gages are zeroed is 70°F (room

temperature).

Chamber Tests

The specimens were placed on a wire tray in

the test chamber and the instrumentation wires

were taped to the tray to keep the specimens

stationary during the test (see Figure 11).

After the specimens were placed on the tray,

three Type-E thermocouples were positioned on

the wire tray with the thermocouple junctures

placed as close to the specimens as possible in

order to get an accurate reading of the ambient

temperature in the chamber at the level of the

specimens.

Figure 11. Specimens on platform within

chamber.
•\luminum tape ::nd Cryolite" insulation were

used to seal the pe_ ,neter of the closed chamber

(located at the jut: _ure of the two doors), the

bundle of instrumentation wires exiting the test

chamber, and the access ports of the chamber.

The first step in the testing procedure was to

cool down the specimens to cryogenic

temperature. In this chamber -300°F was the

desired minimum temperature, but due to a

degraded blower motor, it proved very difficult to

hold this test temperature consistently so the

minimum test temperature was raised to -270°F.

Once the measured temperatures reached the

desired test temperature of -270°F, the specimens

soaked at temperature allowing them to reach a

steady-state condition (approximately 5 min.). A

variance of up to 5°F was allowed due to the

temperature control limitations of the forced

convection chamber. Data was logged on the

computer for this 5-min. interval at a rate of 1

scan per second to insure minimal fluctuation and

a steady state condition. After the 5-min. hold,

the data for the particular test temperature were

recorded in the logbook for each specimen. Data

were taken at increments of 50°F with readings

taken for test temperatures between -270°F and

220°F. After obtaining data at all the test

temperatures, the specimens and gages were

allowed to return to room temperature, were

inspected for damage. The test was repeated two

more times, and then the specimens and gages

were again inspected for damage and repaired if

necessary. After this final inspection, the

specimens were removed and prepared for the

Dewar tests.

Dewar Tests

The next portion of the test involved placing

the specimens in a Dewar that was then cooled

with either liquid nitrogen or liquid helium. The

specimens were removed from the convection

chamber and separated according to material type.

It was determined that the complete specimen set

could not fit into the Dewar, so the GR/EP

specimens, the GR/BMI specimens, the

aluminum-lithium specimens, and the aluminum

alloy specimens were tested together. The

remai_,ing specimens (GR/K3B and Ti) were

testeo : 'parately.

The specimens were placed in the wire basket

shown in Figure 8 and inspected to insure they

could hang freely in the wire basket while making

minimal contact with one another. The basket

was then prepared for the test by placing the three

Type-E thermocouples in the basket, and then the

basket was lowered into the Dewar. The spraybar

was then placed inside the Dewar approximately 2

ft. aboxe the specimens, and the lid was loosely

placed _n top of the Dewar. This was done to

avoid spraybar contact with the specimens and to

allow venting from the Dewar due to the

vaporization of the cryogen. The nitrogen or

helium supply was then attached to the port on the

spraybar.

As with the tests in the chamber, the

specimens were cooled to the test temperature and

allowed to soak at that temperature for 5 min. as

data was logged by the computer. After 5 min. a

strain reading was recorded in the logbook. The

liquid nitrogen flow into the Dewar was manually

controlled and a test temperature of -315°F was

achieved. The measurement using liquid helium

to cool the specimens followed the same

procedure and test temperatures of -454°F were

achieved. In order to obtain a statistically

signific mt sample of the data, all tests were

repeated two more times.

Experimental Results

and Analysis

The Jeduction of the data from this experiment

required an average of the thermal output for each

materiai type at each test temperature. For

example:, nine data values for thermal output,

obtainec for the three 0 ° GR/EP specimens at a

given temperature from three separate tests were

averaged to produce one value of thermal output

for that material and material direction. A 95%

confider ce interval was calculated for each test

material at each test temperature using the

procedure outlined in Reference [7]. These

intervals ranged from ±0.5 microstrain (±1%) for

the AL-LI specimen at 230°F to ±321.1

8
microstrain (___10%) for the GR/K3B cured

laminate at -421°F, indicating little scatter in the

data. Once the data were obtained for all desired

temperatures and averaged, a plot of the thermal

output vs. temperature was generated. A

commercial spreadsheet package was used to fit a

fourth-order polynomial to the data points,

generating an equation for that curve. A least

squares fit through the data is used to calculate

this curve. A fourth order polynomial was used

because the manufacturer provides a fourth order

polynomial and it accurately captures the

variations in the data.

An initial comparison was made between the

experimental values of thermal output and those

values produced by the gage manufacturer. The

manufacturer's data were obtained from gages

mounted to a titanium silicate reference material.

Because the reference material has a different

coefficient of thermal expansion than the

materials tested, the values of thermal output must

be corrected for differences in coefficients of

thermal expansion. Equation (3) illustrates the

equation suggested by the manufacturer [3] to

correct the thermal output:

eR = ep + (a - a R)AT (3)

Figure 12 contains both the experimentally

determined thermal output and the corrected

manufacturer's data for the titanium alloy (Ti). In

this and subsequent plots of thermal output, the

gage factor is not corrected for temperature. As

shown in the Figure, the manufacturer's thermal

output curve (corrected using Eq. (3)) does not

closely approximate the experimental data

(differing by a maximum of -408 microstrain

(34.3%) at -409°F). For other materials the

corrected manufacturer's data were acceptable at

temperatures above -200°F, but proved to be less

than satisfactory at cryogenic temperatures. For

this reason it is preferable to directly determine

the thermal output of the gages on a test material

or specimen. These data can then be subtracted

from the test data to determine true mechanical

strain using Eq. (2a) [4].

5OO

0

-500

:r,? -I000

IXIn./In.

-1500

-2000

-2500

© Experimental Data

[] Manufacturer's Data

m

T I l I i I i I L

-400 -200 0 200

Temperature, °F

Figure 12. Thermal output for titanium

(experimental data and strain gage

manufacturer's data).

Figure 13 contains the thermal output data for

the aluminum and aluminum-lithium alloys. The

95% confidence intervals are also shown for these

and subsequent data as well as the fourth-order

polynomials that were fitted to the data. The

fourth order polynomial fitted to the data for this

material and those for subsequent materials are

given in Table 1.

400

200

0

-200

-400

ET_ -600

_in.hn.

-800

-1000

-1200

-1400

-1600

© AI 2219-1"87

[] AI-Li 2195-T87

r I i I i I i I i

-400 -200 0 200

Tempemture,°F

Figure 13. Thermal output data for AI and

AL-LI with a gage factor of 2.08.

9
1000 I

500

0 -

-500 -

_T_ -1000

_in._n.

-1500

-2000

-2500

-3000

-3500

0 GR/EP 90 °

[] GR/EP 0 °

-400 -200 0 200

Temperature, °F

Figure 14. Thermal output data for GR/EP

with a gage factor of 2.02.

Figure 14 contains plots of the thermal output

data for the graphite-epoxy (IM7/977-2)

specimens with a [+45/0/90/0]s ply lay-up. Data

in both the 0 ° ply orientation direction and the 90 °

direction are given.

Figure 15 contains plots of the thermal output

data for the graphite-bismaleimide (IM7/5260)

specimens with a [+45/0/90/0]s ply lay-up. Data

are given for both the 0 ° ply orientation direction

and the 90 ° direction. Fourth-order polynomials

for both ply orientation directions were generated.

Thermal output data for the graphitethermoplastic (GR/K3B) specimens are given in

Figure 16. Thermal output data for both the

specimens extracted from the facesheets of a cocured sandwich structure and those fabricated

from a cured laminate are given in both the 0 ° and

90 ° ply orientation direction.

Figure 17 contains the thermal output data in

the 0 ° ply direction for all of the materials tested.

The thermal output data for gages mounted on

metals differ greatly from those mounted on

composite materials, but variations among the

metals and composite materials were relatively

low.

ETIO

ix in./in.

1000

500

0

-500

-1000

-1500

-2000

-2500

-3000

-3500

© GR/BM190 °

[] GR/BMI 0°

T I I I I I t I

-400 -200 0 200

Temperature,°F

Figure 15. Thermal output for GR/BMI

specimens with a gage factor of 2.02.

For the metals, the largest variation in thermal

output occurs at -273°F and is 281 microstrain.

For the composite materials in the 0 ° ply

direction, the maximum variation is 399

microstrain and occurs at -423°F.

The 95% confidence intervals calculated for

the the'mal output data gathered during the

chamber tests are relatively small and indicate a

1000 -

500-

0-

-500 -

-" 000 -

ET/O

p,in./in. -_ 500 -

-'-_000

-;_500

-" ;000

-" 500

-_000 7

© CO-CURED GR/K3B90*

C1 CO-CUREDGR/K3B 0°

0 GR/K3B 90 °

_7 GR/K3BB 0 °

I , I , I

-400 -200 0

Temperature, °F

Figure 16. Thermal output data for

GR/K3 B with a gage factor of 2.02.

I

200

10
minimum of scatter in the data for all the

materials at test temperatures above -270°F. For

the thermal output data gathered during the Dewar

tests, the 95% confidence intervals calculated

indicate a large scatter in the data at temperatures

below -270°F for all the materials tested. This is

primarily due to the wider test temperature

variations observed in the Dewar test.

All the data collected are summarized in Table

1. Listed are the fourth order polynomial

equations determined for each material, the

minimum and maximum temperature to which

each material was subjected, and the minimum

and maximum measured thermal output. The

maximum error between the manufacturer's

provided thermal output, corrected by equation

(3), and the experimental thermal output is also

given. Note that this difference is the absolute

strain error that would result if the manufacturer's

thermal output data were used to correct for

thermal output, rather than the experimentally

determined thermal output data.

1000

500

0

-500

- 1000

_T:O I

I_ in.fin." 500

-2000

-2500

-3000

-3500

-4000

_ /_ © AL 2219-1"87

_/_ [] AL-LI 2195-T87

_' A TI

///_ Compo,aes

J//.L V GR/EP 0°

T ,_'// <>GR/BMIo°

_/]_/ _ GR/K3B 0 °

, I = I _ l , l

-400 -200 0 200

Temperature, OF

Figure 17. Thermal output data in the 0 °

fiber direction for all materials tested.

Table 1. Summary of Thermal Output Data.

Material

GR/EP 0 °

GR/EP 900

GR/BMI 0°

GR/BMI 900

Co-Cured

GR/K3B 0 °

Co-Cured

GR/K3B 900

GR/K3B 0 °

GR/K3B 900

Temperature, Strain,

p.strain

oF

Min. Max. Min. Max.

-400 230 -3316 291

-406 230 -3023 241

-403 230 -3169 263

-397 230 -3001 227

-409 230 -2976 228

-430 230 -3210 260

-454 230 -3220 201

-421 230 -3577 253

Fourth Order Model of Thermal Output, eT/o,

Ixstrain

-211.42 + 3.9755T - 0.0112T 2+ 7x10_I "3+ 3x10-ST 4

-181.16 + 3.3991T - 0.0096T 2+ 7x106T 3+ 2x I0-8T 4

-200.55 + 3.6989T - 0.0099T 2+ 6x106T 3+ 2xl0-8T 4

-177.80 + 3.3392T - 0.0095T 2+ 7x10"6T 3+ 2xl0ST 4

-167.67 + 3.2353T - 0.0103T 2+ 9xl0+T 3+ 3x10-ST 4

-191.43 + 3.5523T - 0.0105T 2+ 8x10"6T 3+3x10-aT 4

-165.95 + 3.4110T -0.0113T 2 + 8x I0"6T3+ 4x10"ST 4

-192.23 + 4.1037T - 0.0111T 2+ 3x10"6T 3+ 2xl0"aT 4

Max

IA Errorl,

ktstrain (%)

1731 (52)

1461 (48)

1593 (48)

1446 (50)

1381 (46)

1549 (43)

1494 (46)

1943 (54)

AI-Li

2195-T87

AL 2219-T87

Titanium

-411 230 -1408

-411 230 -1173

-409 230 -1188

80 -81.26 + 1.7143T - 0.0069T 2+ 5x106T 3+ 3x10-ST 4

38 -55.31 + 1.4220T - 0.0077T 2+ 6x 10"6"I"3+ 4x 10-gT4

23 12.57 + 0.5643T - 0.0084T 2 + 5x 104"r 3+ 3x10ST 4

254 (43)

504(18)

408 (34)

11
Concluding Remarks

From the data gathered and the plots

produced, one can conclude that the testing

procedure produced very repeatable

experimental results. Strain gage manufacturers

produce plots and equations for thermal output

as a function of temperature and also provide

gage factor variations with temperature for

gages mounted to a wide variety of standard

materials. However, the manufacturer's

methodology to correct for nonstandard

materials was found to be inaccurate, especially

at cryogenic temperatures. For the materials,

lay-ups and temperatures of concern, the

absolute errors associated with using the thermal

output data provided by the strain gage

manufacturer are too high and so experimentally

determined thermal output data must be used.

The maximum variation in thermal output

among the composite materials is moderate, and

if data for a different material system is lacking,

any of the composite data presented here could

be used as a first approximation to the thermal

output.

References

. Moore, Thomas C., "Recommended Strain Gage

Application Procedures for Various Langley

Research Center Balances and Test Articles",

NASA TM-110327, March 1997.

6. Daily, James W., and W. F. Riley,

E_perimental Stress Analysis, Third ed.,

M:Graw-Hill, Inc., 1991.

7. Coleman, Hugh W., W. Glenn Steele,

Experimentation and Uncertainty Analysis, pp

17-37, J. Wiley & Sons, 1989.

1. Cook, Stephen A., "The X-33 Advanced

Technology Demonstrator", AIAA-96-1195,

Presented at the AIAA Dynamics Specialists

Conference, April 18-19, 1996.

2. Johnson, Theodore F., R. Natividad, H. K.

Rivers, R. Smith, "Thermal Structures

Technology Development for Reusable Launch

Vehicle Cryogenic Propellant Tanks", Presented

at the Third Space Technology and Application

International Forum Conference on Next

Generation Launch Vehicles, January 25-29,

1998.

. Anonymous, "Strain Gage Thermal Output and

Gage Factor Variation with Temperature",

Micromeasurements Group-TN-504-1, 1993.

4. Anonymous, "Temperature-Induced Apparent

Strain and Gage Factor Variation in Strain

Gages", Micromeasurements Group-TN-504,

1983.

12
REPORT DOCUMENTATION PAGE ForrnApproved

OMB No. 0704-0188

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Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Thermal Output of WK-Type Strain Gauges on Various Materials at

Cryogenic and Elevated Temperatures 242-33-01-08

6. AUTHOR(S)

Matthew K. Kowalkowski, H. Kevin Rivers and Russell W. Smith

7.PERFORMINGORGANIZATIONAME(S)ANDADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

9.SPONSORING/MONITORINGAENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

L-17752

10. SPONSORING/MON_ORING

AGENCY REPORT NUMBER

NASA/TM-1998-208739

11.SUPPLEMENTARY NOTES

1211. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified-Unlimited

Subject Category 15 Distribution: Nonstandard

Availability: NASA CASI (301) 621-0390

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Strain gage apparent strain (thermal output) is one of the largest sources of error associated with the

measurement of strain when temperatures and mechanical loads are varied. In this paper, experimentally

determined apparent strains of WK-type strain gages, installed on both metallic and composite-laminate

materials of various lay-ups and resin systems for temperatures ranging from -450°F to 230°F are presented. For

the composite materials apparent strain in both the 0° ply orientation angle and the 90 ° ply orientation angle

were measured. Metal specimens tested included: aluminum-lithium alloy (AI-LI 2195-T87), aluminum alloy

(AL 2219-T87), and titanium alloy. Composite materials tested include: graphite-toughened-epoxy (IM7/997-

2), graphite-bismaleimide (IM7/5260), and graphite-K3 (IM7/K3B). The experimentally determined apparent

strain data are curve fit with a fourth-order polynomial for each of the materials studied. The apparent strain

data and the polynomials that are fit to the data are compared with those produced by the strain gage

manufacturer, and the results and comparisons are presented. Unacceptably high errrors between the

manufacture's data and the experimentally determined data were observed (especially at temperatures below -

270°F).

14. SUBJECT TERMS

Apparent Strain, Thermal Output, Cryogenic, Temperature, Composite, Aluminum

Aluminum-Lithium, Titanium

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

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17

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A03

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