NCAR-TN-19 RARY

A.L 1u2 1966

Low Modulus Strain Gages forStress Analysis of Balloon Structures

A research program conducted for theNational Center for Atmospheric Researchby the Hauser Research and Engineering Company2965 Peak Avenue, Boulder, Colorado

NCAR Technical NoteNational Center for Atmospheric Research

Boulder, ColoradoJuly, 1966

NCAR Library

i 5 0583 01005823 2

The National Center for Atmospheric Research (NCAR) is dedicatedto the advancement of the atmospheric sciences for the benefit ofmankind. It is operated by the University Corporation for AtmosphericResearch (UCAR), a private, university-controlled, non-profit organiza-tion, and is sponsored and principally funded by the National ScienceFoundation.

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* To ascertain the feasibility of controlling weather and climate,to develop the techniques for control, and to bring about thebeneficial application of this knowledge;

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* To bring about improved description and prediction of atmosphericprocesses and the forecasting of weather and climate;

* To improve our understanding of the sources of air contaminationand to bring about the application of better practices of airconservation.

The research and facilities operations of NCAR are conducted infour organizational entities:

The Laboratory of Atmospheric Sciences

The High Altitude Observatory

The Facilities Laboratory

The Advanced Study Program

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Cornell University New York University University of Washington

University of Denver University of Wisconsin

PREFACE

This report is one of a series prepared for the Materials Research Project

of the NCAR Scientific Balloon Facility. The Project is one of several related

efforts undertaken by the National Center for Atmospheric Research (NCAR) to

increase the reliability, and to extend the capabilities, of scientific ballooning.

NCAR was founded in 1960, to conduct basic research in the atmospheric

sciences, and to foster such research on the part of the universities and other

research groups in the U. S. and abroad. NCAR is operated by the University

Corporation for Atmospheric Research (UCAR) and sponsored by the National

Science Foundation.

The present report covers an investigation of the feasibility of using low

modulus strain gages for stress analysis of balloon structures performed by Hauser

Research and Engineering Company, Boulder, Colorado, under subcontract with

UCAR.

ABSTRACT

An exploratory study has shown that instrumentation can be developed to

permit experimental stress analysis of scientific balloons. Special strain gages

have been made and applied to polyethylene balloon films, providing measures of

strain up to 50% at +25 and -80°C. Gages were made of silver or aluminum deposited

chemically or by vacuum metalizing onto plastic films. Feasibility of low modulus

strain gages for this application has now been demonstrated, and optimization of

materials, processes and instrumentation is anticipated as the next phase of the

development.

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CONTENTS

Preface..................................................... i

Abstract ............................................ .......

List of Tables ....................................... ,...... .i

List of Figures .................... ............... iv

List of Symbols .................................... v

1. Introduction............................ 1

2. Materials .............. 32.1 Conductors .......... 32.2 Substrates ............................................ 42.3 Adhesives ......................................... 5

3. Gage Fabrication.63. Gage Fabrication ,,,...... . .. ............................. ....... ,,,,,, 6

4. Test Methods ......................................... 9

5. Test Results ........................ 105.1 Chemical Silver ............. ............................ 105.2 Aluminized Polypropylene .. ........ ............... 115.3 Silver Metalized Polyethylene .............. .......... .. 12

6. Conclusions .... .......... ............................ 14

7. Operational Prospects ................... 15

Tables. .... .................. ........ 16

Figures ..................................................... 24

References . .............................................. 30

iii

TABLES

1. Mechanical and electrical properties of metallic straingage sensing materials .1.7....... .,......................., 17

2. Gage factors at 25 C of chemical silver on Visqueen A film.. .... 18

3. Gage factors at -80 C of chemical silver on 0.7-milStratoFilm .*•.•.. ..... . ............................ 20

0

4. Gage factors at 25°C of Hy-Sil aluminized polypropylene film ....... 21

5. Gage factors at -80 C of Hy-Sil aluminized polypropylene film ...... 22

6. Gage factors at 25 C of silver vacuum metalized on 1/7-milSeaSpace polyethylene film .... ................................ 23

iv

FIGURES

1. Calibration of chemically silvered gages, electrical resistancevs. strain .......... ..... ... .. ........ . ..... ............... 24

2. Calibration of chemically silvered gages, logarithms ofelectrical resistance vs. strain ................................. 25

3. Calibration at 25 C of gages 5 through 14, Hy-Sil aluminizedpolypropylene film ............. ........................ 26

4. Calibration at -80 C of gages 15 through 22, Hy-Sil aluminizedpolypropylene film ............ .................. 27

5. Calibration at 25°C of gages 23 and 24, silver vacuum metalizedon 1/7-mil SeaSpace polyethylene film bonded to 1.5-milVisqueen A film .S.... .... .............................. ..... 28

6. Proposed format for gage calibration and stress analysis forbal loon materials ..... .. .. * g...... .......................... 29

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v

SYMBOLS

G Gage factor

L Length of strain gage, inch

L Original length of strain gage, inch

R Resistance, ohms

R Original resistance, ohmso

R Extrapolated value of resistance at L , ohmsx 0

m

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1. INTRODUCTION

Opportunities for failure analysis of scientific balloons have been lacking

for many years. The need for evaluation of film stresses in large scientific balloons

has been expressed many times, but appropriate instruments have not been available

for either ground or flight testing.

One effort has been made toward measuring tape stresses in a hangar-

inflated balloon (Ref. #1). Inflation was accomplished with mixtures of air and

helium to simulate various conditions of ascent inflation volume and net lift. The

test results from those experiments were not analyzed by the contractor and the tests

might be termed inconclusive.

The magnitude and direction of stresses in balloon films have been of major

concern, since rupture of the film is always one mechanism of failure. The

operational and environmental effects on the balloon material are a significant

part of the problem. Unfolding and deployment of the large balloons is perhaps

the greatest area of unknown in balloon performance, and most failures occur

during this operation.

To learn the mechanical stress or strain response of a balloon material, the

instrumentation must not cause undue restraints on the original structure. A cast

iron girdle on a plastic balloon would significantly change the normal performance

of the structure being tested. The instrumentation should not be stiff in relation to

the plastic balloon film, and it should be capable of elongation at least to the yield

point of the film, if not to its ultimate elongation. In addition, the strain instru-

mentation should be capable of electrical measurement and telemetering in order

to permit hangar inflation or flight tests of the structures.

Conventional strain gages have high stiffness and low ultimate elongation.

They are not appropriate for balloon film instrumentation.

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In'early 1965, Hauser Research & Engineering Company did some investigation

of low modulus strain gages on a NASA subcontract. These were directed toward

methods of measuring heart beat and respiration rate. Several approaches were

tried -- the mercury-filled rubber tube invention of Gates Rubber Company;

conductive rubber and plastic films; and plastic films coated with a thin layer of

chemically deposited silver. The latter materials were bonded to the rib cage and

successfully indicated the body strains associated with heart and lung operation.

The technology developed on the NASA subcontract was applied to the

problems of balloon material evaluation in late summer, 1965, and results of this

feasibility study are discussed in the present report.

The objectives of this study were to obtain a strain gage that could be

attached to scientific balloon film to provide measures of film strain in hangar

inflation or flight tests. The gage must have low stiffness (product of thickness

times elastic modulus) in relation to the stiffness of the balloon film. It must

have electrical resistance change within measurable ranges. It must have constant

gage factor, or an easily identified and repeatable calibration of strain vs.

resistance change. It should be easily adaptable for electrical connections.

Mechanical and electrical attachments should be capable of operating over the

temperature range of +120 to -1120 F. The gage must be operable up to at

least 10% elongation, and continuity up to 40 or 50% is desirable. Change of

resistivity and of gage factor with temperature should be small, or should be

amenable to reliable calibration. Alteration of solar and convective heating

effects should be minimum. The strain gage and its attachments should be

mechanically durable to withstand wind buffeting and deployment within the

capability of the balloon film to which it may be attached.

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3

2. MATERIALS

The components of a low modulus strain gage require an electrical conductor

as the sensing element, a substrate for the conductor, and adhesives for attaching

the gage to the balloon and for attaching electrical leads to the sensing element.

2.1 CONDUCTORS

The conductive, sensing element might be a thin metal film, or a semi-

conductive plastic or elastomeric material. Experiments were made with both

types of materials, and with mono-films of finely divided graphite which were

claimed to have extremely high gage factors (Ref. #2). Constant gage factors

and high elongations were obtained only with thin metal films, deposited

chemically or by vacuum evaporation.

Some of the candidate materials for metallic strain elements are listed in

Table 1. Electrical resistance and mechanical stiffness are the properties compared

in this table for several elemental metals and one alloy. Ohms Law and the thermal

coefficient of resistivity are the same as for bulk materials if films are at least

5 atoms thick (Ref. #3).

The stiffness factors of these materials may be related to the stiffness of

1-mil Visqueen A polyethylene, which is 10 lb/in. at 250 C and 250 lb/in. at

-80°C. Stiffness is the force per width per unit strain and is the product of

initial modulus (psi) times thickness of the film (in.). The strain gage (conductor

plus substrate and adhesive) should not be stiffer than the balloon film to which

it may be attached.

Among these materials, silver has the lowest stiffness of the common

elements, and it is most easily deposited in thin films by chemical or vacuum

techniques. The ultimate elongation of pure silver in tensile coupons is about 50%.

Indium is a competitive material for this application. It has a very low

modulus and gives the lowest stiffness at equal resistance for any of the materials

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4

of Table 1. In the bulk, it is limited to 22% elongation, but metalized films might

have greater elongation.

Silver and aluminum conductive elements were used on the strain gages

reported on part 5 below.

2.2 SUBSTRATES

If the metallic strain element could be attached directly to the balloon

film, minimum gage stiffness could be accomplished. However, this would

necessitate chemical or vacuum deposition of a thin, long line of conductor

across a.balloon gore during or following manufacture of the balloon. This

would be a time-consuming and tedious operation with high risk of non-uniformity

in gage thickness and resistance.

For this reason, a plastic film substrate has been considered most appropriate

as a carrier for the metallic element. Minimum stiffness of this substrate film is

one of the objectives.

For operation near 25 C, thin films of polyethylene-ethyl acrylate copolymer

or of polyurethane are good candidates. These materials have modulus values from

30 to 50% the modulus of Visqueen A at this temperature (Ref. #4). At -800 C,

these materials are comparable in modulus to Visqueen A and are slightly stiffer

than Visqueen X-124 and Winzen StratoFilm.

The 1/7-mil film manufactured by SeaSpace Systems was thought to be a

candidate substrate for low modulus strain gages, but the results were not favorable.

This material, and also the Visqueen A and polypropylene films have been evaluated

as strain gage substrates, as discussed below in part 5.

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2.3 ADHESIVES

Two types of adhesives are useful for bonding the strain gages. One is for

attachment of the gage to the balloon, and a continuous bond is needed. The

other is a small area, electrically conductive adhesive for attachment of lead wires

to the sensing element of the gage.

Polyethylene is difficult to bond to with any adhesive, but the pressure-

sensitive types bond with sufficient strength to hold patch tapes in place. This

strength should be adequate for bonding strain gages onto the balloons.

Ketone, aromatic and halogenated solvents cause swelling of polyethylene,

but this usually is a reversible effect. Where these solvents might be components

of adhesives, a condiderable puckering of balloon film is usually experienced. The

solvent must evaporate completely in order for the film to return to original dimensions.

This can be accomplished with pressure-sensitive adhesives applied in solution and

then dried to equilibrium.

The electrically conductive adhesive should have low stiffness and it

should be applied in a thin film where wires are to be attached. In addition, it

should have a contact resistance low in relation to the resistance of the sensing

element. Electrical conduction is normally obtained by adding a silver pigment

to the organic binder, in approximately 50% concentration by volume of solids.

Solvent swelling effects in the conductive adhesive are to be avoided, if

possible, lest they cause the conductor to lift from the substrate.

6

3. GAGE FABRICATION

Early formulations of pigmented coatings and elastomers did not provide

useful low modulus strain gages. The methods of their fabrication are not discussed

here.

Chemical deposition of silver onto Visqueen A film has been used for many

of the strain gages. Two different formulations have been used for deposition: the

Brashear's process for mirror silver and a proprietary formulation from Enthone Corp-

oration.

The Brashear's process involved two steps and several chemical formulations.

First, the clean polyethylene was sensitized with a fresh solution of 1% HCI and

6% SnC12. After contact with this solution, the polyethylene was rinsed with

distilled water. A silvering solution was then poured between masking tapes or

into a rubber-faced mask to delineate the area to be silvered. The silvering

solution was made up using:

Part A Silver solution, freshly mixedSilver nitrate 6.84 gm in 150 cc waterAmmonium nitrate 20.6 gm in 150 cc waterPotassium hydroxide 66.0 gm in 150 cc water

Part B Sugar reducing solution, freshDextrose, 2.25 gmSucrose, 6. 3 7 gmWater, 450 cc

Parts A and B were mixed quickly and then poured into the boat-shaped mask to

silver the polyethylene. Approximately 30 cc of mixed solution was used per

square inch of polyethylene to obtain resistances of 2 to 6 ohms per square. After

5 minutes of contact, the silvering solution was rinsed off'and new solution was

added. This process was repeated until the desired resistance was achieved,

The Enthone system was somewhat easier to use and it gave more uniform

deposits. Sensitizer #430 was used on the clean polyethylene, and it was rinsed

7

off after brief contact. Then equal parts of Enthone 420A and 420B were mixed

and poured onto the masked area to be silvered. Approximately 20 cc of mixed

solution was used per square inch to obtain resistances of 1 to 3 ohms per square.

The Enthone system had the advantage of providing pre-mixed, stable solutions

at room temperature.

Perino (Ref. #5) has reported success with vacuum metalized strain gages,

and two types of metalized films were evaluated in this program. One was a

commercial aluminized polypropylene film made by Hy-Sil Manufacturing Company,

Revere, Mass. This material is made in large quantity as a roll film and it is

highly uniform in quality. This film had been obtained for another application,

and it was evaluated to compare processes even though the 1-mil polypropylene

was stiffer than desired. The other metalized film was custom prepared by Rocky

Mountain Instruments and consisted of silver deposited onto SeaSpace 1/7-mil

film. One sample was metalized as received and the other sample was slightly

oxidized for 10 minutes with the following solution:

Sulfuric Acid 100 ccSodium Dichromate 8 gmWater 100 cc

The adhesion of metalized silver to the oxidized film was better than to

SeaSpace film which had not been oxidized. A slightly stronger solution than the

above could probably be used satisfactorily, but concentrated sulfuric acid-sodium

dichromate solution was not appropriate for batch oxidation of this thin polyethylene

film. The exotherm of water rinsing caused melting of the polymer when concentrated

solutions were used. Corona treatment should be as good as the wet chemical

oxidation, and this is much more common for film treatment preparation for bonding.

Several solution and latex adhesives were tried for the two bonding

applications. The preferred adhesive for attaching the strain gage to balloon film

was a silicone pressure-sensitive formulation, Dow-Corning C-271. This solution

was brush-coated onto the balloon film, allowed to dry and de-swell and then

the strain gage was pressed against the tacky adhesive. The bond of this system

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8

was durable at -80°C despite the considerable buffeting caused by the liquid

carbon dioxide jet in the cold test chember. This technique was used only on

gages #23 and #24 of this report.

For attaching electrical leads the preferred adhesive was a natural rubber

latex containing silver flake MD755 in approximately 50% concentration by

volume of solids. This adhesive provided contact resistance in the range of 10 to 30

ohms. Total elongation of this adhesive at -80 0 C was about 6%. Lower contact

resistance and higher elongation at low temperature are still to be desired.

Most gages were made with conductors 1/4 in. wide and 4 in. long.

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4. TEST METHODS

At first, a Wheatstone bridge type of circuit was used for testing the low

modulus strain gages. One experiment was sufficient to indicate that the resistance

change was large and that the bridge circuit was appropriate only for infinitesimal

changes.

In order to obtain a voltage linearly proportional to the resistance, a

constant-current power supply is needed. The simplest measuring source was

an ordinary volt-ohm-ammeter. The resistance-measuring circuit of an Eico

vacuum tube voltmeter was used for calibrating the experimental strain gages.

The meter voltage was tapped off for plotting on the X-Y recorder (recorder

input impedance was high in relation to meter impedance).

Strain gage calibration was accomplished using the Dillon testing machine

adapted with electrical read-out of cross-head position. Thus the resistance out-

put from the VTVM was plotted on the Y-axis and deflection was plotted on the

X-axis. Division of the X-axis calibration by original strain gage length provided

the resistance/strain relationship for the particular gage.

This relationship was approximately logarithmic and followed the equation:

log (R/R) Glog (L/L)

The VTVM circuit provided a voltage output approximately proportional to the

measured strain. The calibration was accomplished merely by reading the meter

and marking intervals of resistance while output voltage was being plotted on

the chart.

The earlier strain gages were calibrated using testing machine grips as

electrical contacts. Later tests used the conductive adhesives for contact. When

calibration was attempted at -800 C, the carbon dioxide cold chamber was used

on the testing machine.

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5. TEST RESULTS

5.1 CHEMICAL SILVER

Low modulus strain gages were deposited with Brashear's process using

masking tape to define gage areas 1/4 in. x 4 in. on 1.5-mil Visqueen A strips

1 in. x 6 in. Removal of the masking tape after silvering caused some edge

discontinuities which altered strain gage characteristics. Calibration curves

for gages 1 through 4 are presented in Figures 1 and 2. The dog-leg effects in

the region of 0 to 5% strain were apparently caused by edge peeling of the silver

during removal of the masking tape.

Over the range of 5 to 50% strain, three of these four gages were very nearly

linear in logarithm of resistance related to strain. The semi-logarithmic plot of

Figure 1 was helpful in extrapolating resistance to zero strain (correction for

torn edge effects) and this is a convenient graph for stress analysis purposes.

The gage factor calculations of Tables 2 and 3 represent slopes of the log (R/R)

vs. log (L/L ) curves of Figure 2.o

Non-uniformity of thickness was the most probable cause of anomalies in

gage #1. Gages with thinner conductors (#1, and #2) had higher gage factors

in these tests.

Based on the extrapolated resistances, these gages had silver coatings in-7

the range of 0.76 to 5.1 x 10 in. (19 to 130 A ) and tensile stiffness at room

temperature 0.83 to 5.6 lb/in.

Three chemical silver gages were deposited on 0.7-mil Winzen StratoFilm

(Ref. #6); these were #25, #26, and #27 in test sequence. These gages were

tested only at -800 C, and their data are included in Table 3 and Figures 1 and 2.

Conductive latex adhesive was used for lead wire attachment in these gages.

Initial resistances include contact and lead resistance, and were measured to be:

11

Gage R at 250 C R at -800 CNumber (ohms) o (ohms)

25 15 12

26 15 13

27 24 13

In gages #25 and #27 the plastic films failed at surprisingly low elongations,

11.2%. In gage #26 a fairly constant gage factor was measured up to 28.8%

elongation, and the lead wire was pulled off by the testing machine at about 30%

elongation.

The gage factor uniformity of these three strain tests was particularly

noteworthy.

5.2 ALUMINIZED POLYPROPYLENE

The next series of strain gages, #5 through #22, were of the commercial

Hy-Sil aluminized 1-mil polypropylene. These gages were calibrated at room

temperature with electrical contact at the grips. Gages #5 through #9 were cut

from the machine direction of the film, the next five were cut from the transverse

direction; significant differences were observed in linearity and apparent gage

factor with direction.

Test results for the ten Hy-Sil gages at 250 C are shown in Figure 3. The

aluminum/polypropylene gages ruptured in the range of 12 to 20% elongation

by failure of the plastic film. Gage factors were higher and more uniform than

the silvered Visqueen, in the region of 16.5 to 21.4. The differences in machine

and transverse directions might be due to aluminum thickness variations, but they

are more probably due to differences in Poisson ratio behavior of the film.

Test results from these gages are presented below in Table 4. In three of

the machine direction gages,anomalies were observed in the low-strain region

where initial resistance was 10 ohms, but gages were linear for R equal to 5.6~or~~ ~6.80 ohms.or 6.8 ohms.

12

The Hy-Sil gages cut from the transverse direction were particularly

uniform in initial resistance and gage factor related to the measured initial

resistance.

Eight of the Hy-Sil gages were tested at -80 C, where uniformity was not

nearly so good as at room temperature. Resistance of the 1/4 in. x 4 in. strips was

virtually unchanged from the 10 ohms at 250 C, which was a surprise; the resistivity

of aluminum drops from 2.83 ohm cm at 20 C to 1.53 ohm cm at -100 0C.

Calibrations for gages #15 through #22 are shown in Figure 4, and calcula-

tions are in Table 5. Gage factors ranged from 16 to 85 for the eight Hy-Sil gages

at-80 C.

The vacuum metalized conductor of the Hy-Sil gages was very promising,

but the polypropylene substrate was capable of only 10 to 15% elongation.

Additionally, the 1-mil polypropylene was stiff in relation to conventional

balloon films.

5.3 SILVER METALIZED POLYETHYLENE

Thus the next series of gages was made using 1/7-mil SeaSpace polyethylene

film vacuum metalized with silver. Two sets of film were custom made by Rocky

Mountain Instruments, Inc. The first had the thin polyethylene film spread inside

a spherical dome (24 in. radius) with only surface tension and static forces holding

it in place. Not all wrinkles were squeezed out of the film, and vacuum caused

the film to be lifted from the plate in some areas. The second set was prepared

with the film stretched over windows in the spherical dome. In both cases, plating

was continued until an isolated test strip 4 in. x 1/4 in. had a resistance of 25 ohms.

Both sets of metalized 1/7-mil film had discontinuities in the silver coating.

Lines were quite prominent and the surface imperfections in the plastic film were

made visibly evident by the metalizing. Ridges or valleys 107 in. thick were

significant in relation to silver coatings of the same thickness.

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

13

A few satisfactory strain gages were made out of the SeaSpace material.

Two were calibrated at 250 C and results are presented in Figure 5 and Table 6.

In both cases the gages were bonded to 1.5-mil Visqueen A with pres-sure-sensitive

silicone adhesive. Gage #23 used the testing machine grips for electrical contact.

Conductive latex adhesives was used to attach #36 copper wires to the gage #24.

The latter gage operated continuously to 80% elongation, but its resistance went

off-scale just above 50% elongation.

The second set of silver metalized 1/7-mil films were of very poor quality

and of high resistance. Gages of this material were bonded to Visqueen A film

with the C-271 adhesive in two ways: silvered side away from the adhesive; and

silvered side in contact with the adhesive. Both methods were satisfactory. The

latter had the advantage of protecting the thin silver from abrasion, but the gage

could not be repositioned after initial contact with the adhesive, lest silver would

pull off the film.

None of the second set of silver metalized films had gage continuity for

over 5% elongations. Their initial resistances were more than 200 ohms. Bonds

of these gages were satisfactory at -800 C to 28% elongation.

The 1/7-mil film might have been smooth enough on one side and rougher

on the other. This possibility was not checked during preparation of these gages.

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6. CONCLUSIONS

Low modulus strain gages are now feasible for balloon film stress analysis.

They are not yet optimized in terms of fabrication, application or calibration.

Gages made of metalized plastic film can have gage factors in the range

of 8 to 20 at room temperature and up to 50 at -80 C . They can provide electri-

cal strain measurement up to at least 40% elongation, if the test material is capable

of such elongation. Their stiffness can be comparable to the stiffness of I-mil

polyethylene at room temperature, and local stiffening effects in a balloon should

be less than the stiffening by adhesive tape repair of balloons.

To date Dow-Corning C-271 is the preferred adhesive for bonding low

modulus strain gages to balloon film. The solvent of this adhesive must be completely

evaporated before the gage is applied. Gages can be bonded with the conductor

side adjacent to the balloon film, if care is used in placement.

Electrically conductive adhesives of natural rubber latex plus silver powder

are marginal for strain gage electrical attachments. Formulations to date have beentoo viscous and too thick when applied to the film. To date, minor effort has been

made toward this material, and further study should be fruitful.

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7. OPERATIONAL PROSPECTS

There is need for further research and evaluation of low modulus strain gages

before they can be made operational on balloon systems.

Identification of preferred substrate is a basic requirement. Visqueen

X-124 and Winzen StratoFilm in 0.5-mil thickness should be appropriate. New

urethane films should be considered as alternates, depending upon results of

mechanical property tests to be made in the immediate future.

Indium should be evaluated as the conductive element, as alternative to

silver and aluminum. Vacuum metalizing appears to be the appropriate method

for deposition.

The handling ease of electrically conductive adhesives should be improved,

and lower modulus at low temperatures is further to be desired. Silicone and

urethane polymers are probably better prospects for such adhesives.

Where strain gages may be used across complete gores, mechanical clamps

would be of value in addition to conductive adhesives. But where meridian or

diagonal stresses are to be measured, the conductive adhesive must be fully

reliable by itself.

The Hy-Sil Manufacturing Company has done aluminum deposition on 0.75-

mil polyethylene and anticipates that it can be deposited on 0.5-mil film. A roll

of film up to 72 in. wide can be aluminized for $250 plus film cost. They question

whether silver can be put on 0.5-mil polyethylene because of condensation heat

dissipation. On the other hand, a slit lay-flat tube of 0.5-mil materials should

metalize on one side much like a 1-mil film.

Confirmation of thermal radiation effects should be accomplished, preferably

by instrumented flight tests using patches of metalized film. This need not wait

for operational, calibrated strain gages.

16

A hangar inflation test of a strain gage instrumented balloon should precede

flight testing. A hangar test has the advantages of control, communication, and

observation and calibration. Telemetering problems can be checked out and fold

patterns can be observed in the hangar test.

Strain gage calibration should be accomplished using the same film as the

balloon is made of. The combined Poisson ratio effects of balloon film, strain

gage substrate and conductor will probably determine gage factor for each direction

and for each temperature. Calibration can be accomplished concurrent with balloon

fabrication and instrumentation.

Change of gage factor and of stress-strain properties with temperature will

be of concern for flight tests. The calibration data for such tests will be more

complex, and might best take the form of Figure 6.

Instrumentation for hangar and flight tests should be based upon resistance

measuring circuits, and these might be adapted from small ohm meters. Strain

gages of one ohm/square resistance, 30 in. long and 0.25 in. wide would be 120

ohms, R , and resistance might increase by a factor of 100. If voltage outputo

can be made a function of R/R instead of R, data can be reduced with one less

step.

As long as the voltage across the strain gage is less than 5 volts, the energy

dissipation in the gage will be less than 0.1 watt/sq.in. This plus solar heating

should lead to not more than 2°C change from balloon film temperature during

flight. Wagner's work on heat transfer to and from thermistors are the basis- for

the estimation (Ref. #7).

Table 1

MECHANICAL AND ELECTRICAL PROPERTIES OF METALLIC STRAIN GAGE SENSING MATERIALS

Gage Resistance

Metal Resitivity Modulus Ultimate 1 ohm/square 2 ohm/square10 ohm 10 psi Elongation Thickness Stiffness Thickness Stiffness

cm % A Ib/in. A Ib/in,

Aluminum 2.828 10.0 43 283 11.0 141 5.5

Chromium 13.0 36.0 1 1300 184.0 650 92.0

Constantan 44.1 22.0 32 4400 381.0 2200 190.0

Indium 8.37 1.57 22 837 5.4 418 2.7

Lead 20.6 2.0 50 2060 16.2 1030 8.1

Nickel 6.14 30.0 60 614 72.5 312 36.0

Silver 1.629 11.0 48 163 77.06 81.5 3.5

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Table 2

GAGE FACTORS AT 250C, CHEMICAL SILVER ON VISQJEEN A FILM

Ln(R/R )Gage No. Strain L/L R R/R Ln(L/L ) Ln(R/R ) G LLL) Lx o(/L Ln(L/L)

0 1 134 1.045 1.045 200 1.492 .0440 0.400 9.09.072 1.072 500 3.730 .0695 1.317 18.90.213 1.213 1000 7.460 .1930 2.010 10.40.326 1.326 2000 14.920 .2820 2.700 9.58.436 1.436 5000 37.300 .3620 3.620 10.00.503 1.503 10,000 74.600 .4070 4.310 10.60.560 1.560 20,000 149.000 .4450 5.000 11.20.610 1.610 50,000 373.000 .4760 5.920 12.40.630 1.630 100,000 746.000 .4890 6.600 13.50

2 0 1 80.017 1.017 100 1.250 .0168 .224 13.30.104 1.104 200 2.500 .0990 .916 9.25.204 1.204 500 6.250 .1860 1.830 9.83.284 1.284 1000 12.500 .2500 2.520 10.10.390 1.390 2000 25.000 .3290 3.220 9.80.475 1.475 5000 62.500 .3880 4.140 10.70.523 1.523 10,000 125.000 .4200 4.830 11.50.646 1.646 50,000 625.000 .4990 6.450 12.90.660 1.660 100,000 1250.000 .5070 7.140 14.00

Table 2, continued

Ln(R/R )Gage No. Strain L/L R R/R Ln(L/L) Ln(R/R ) G = L L 0 x 0 x Ln(L/L)

3 0 1 30 1.066 1.066 50 1.667 .0639 .511 8.00.185 1.185 100 3.333 .1700 1.203 7.06.307 1.307 200 6.670 .2680 1.900 7.10.454 1.454 500 16.700 .3740 2.810 7.52.528 1.528 1000 33.333 .4240 3.500 8.25.644 1.644 2000 66.700 .4960 4.200 8.48.768 1.768 5000 166.700 .5700 5.110 8.97.832 1.832 10,000 333.000 .6060 5.800 9.57 ,.861 1.861 15,000 500.000 .6210 6.210 10.00

4 0 1 27.5 1.083 1.083 50 1.818 .0796 .597 7.50.171 1.171 100 3.640 .1580 1.290 8.16.303 1.303 200 7.270 .2650 1.980 7.48.395 1.395 500 18.200 .3333 2.900 8.70.495 1.495 1000 36.400 .4020 3.600 8.95.606 1.606 2000 72.700 .4740 4.290 9.04.656 1.656 5000 182.000 .5400 5.200 10.30.676 1.676 10,000 364.000 .5160 5.900 11.40.698 1.698 100,000 3640.000 .5300 8.200 15.50

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Table 3

GAGE FACTORS AT -800C, CHEMICAL SILVER ON 0.7-mil STRATOFILM

Ln(R/R)Gage No. Strain L/L R R/R Ln(L/L) Ln(R/R ) G=

o o o o Ln(L/L)

25 0 1.0000 12.0625 1.0625 22 1.83 .0605 .605 10.00.1120 1.1120 32 2.67 .1130 .980 8.65

26 0 1.0000 13 .0625 1.0625 24 1.85 .0605 .615 10.20.1250 1.1250 50 3.85 .1180 1.350 11.40.1875 1.1875 120 9.25 .1720 2.220 12.70.2500 1.2500 220 16.90 .2230 2.820 12.60.2880 1.2880 250 26.90 .2530 3.290 13.00

27 0 1.0000 13.0625 1.0625 23 1.77 .0695 .570 9.42.1120 1.1120 44 3.39 .1130 1.220 10.90

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21

Table 4

GAGE FACTORS AT 25 C, HY-SIL ALUMINIZED POLYPROPYLENE FILM

Gage No. Strain L/L R R/R Ln(L/L ) Ln(R/R ) G

5-M 0 1 6.8 1.05 1.05 15.0 2.2 .0486 .788 16.2.10 1.10 34.0 5.0 .0950 1.610 17.0

6-M 0 1 10.5 1.05 1.05 23.0 2.19 .0486 .785 16.2.10 1.10 62.0 5.90 .0950 1.774 18.7.15 1.15 110.0 10.50 .1400 2.350 16.8

7-M 0 1 10.5 1.05 1.05 26.0 2.48 .0486 .908 18.7.10 1.10 62.0 5.90 .0950 1.774 18.7.15 1.15 123.0 11.70 .1400 2.460 17.6

8-M 0 1 5.6 1.05 1.05 13.0 2.32 .0486 .841 17.3.10 1.10 30.0 5.35 .0950 1.678 17.7.15 1.15 66.0 11.80 .1400 2.465 17.6

9-M 0 1 5.6 1.05 1.05 15.0 2.68 .0486 .985 21.5.10 1.10 39.0 6.95 .0950 1.940 20.4.15 1.15 86.0 15.40 .1400 2.735 19.5

10-T 0 1 10.0 1.05 1.05 32.0 3.2 .0486 1.162 23.9.10 1.10 75.0 7.5 .0950 2.018 21.2.15 1.15 140.0 14.0 .1400 2.640 18.8

11-T 0 1 11.5 1.05 1.05 35.0 3.2 .0486 1.162 23.9.10 1.10 75.0 6.52 .0950 1.878 19.8

12-T 0 1 10.5 1.05 1.05 32.0 3.05 .0486 1.113 22.9.10 1.10 80.0 7.72 .0950 2.140 22.5.15 1.15 150.0 14.30 .1400 2.660 19.0

13-T 0 1 10.0 1.05 1.05 30.0 3.0 .0486 1.098 22.6.10 1.10 68.0 6.8 .0950 1.920 20.2

14-T 0 1 10.5 1.05 1.05 32.0 3.05 .0486 1.113 22.9.10 1.10 82.0 7.08 .0950 2.150 22.6.15 1.15 160.0 13.60 .1400 2.610 18.6

22

Table 5

GAGE FACTORS AT -80 C, HY-SIL ALUMINIZED POLYPROPYLENE FILM

Gage No. Strain L/L R R /R Ln(L/L) Ln(R/R) G

15-M 0 1 10 1.025 1.025 80 8 .0246 2.08 84.5.050 1.050 240 24 .0487 3.18 65.4.075 1.075 350 35 .0722 3.55 49.3

16-M 0 1 10 1.025 1.025 30 3 .0246 1.1 44.6.050 1.050 120 12 .0487 2.48 51.0.075 1.075 180 18 .0722 2.89 40.0.10 1.10 240 24 .0951 3.18 33.4

17-M 0 1 10 1.025 1.025 15 1.5 .0246 .405 16.4.050 1.050 50 5 .0487 1.61 33.0.075 1.075 130 13 .0722 2.56 35.5.10 1.10 200 20 .0951 3.14 33.0

18-T 0 1 10 1.025 1.025 16 1.6 .0246 .47 19.1.050 1.050 130 13 .0487 2.56 52.5.075 1.075 260 26 .0722 3.25 45.0.10 1.10 350 35 .0951 3.55 36.8

19-T 0 1 10.025 1.025 15 1.5 .0246 4.05 16.4.050 1.050 44 4.4 .0487 1.48 30.4.075 1.075 135 13.5 .0722 2.60 36.0.10 1.10 240 24.0 .0951 3.18 34.4

20-T 0 1 10 1.025 1.025 35 3.5 .0246 1.25 50.1.050 1.050 130 13.0 .0487 2.56 52.5.075 1.075 200 20.0 .0722 3.0 41.5.10 1.10 270 27.0 .0951 3.3 34.8

21-T 0 1 10 1.025 1.025 25 2.5 .0246 .915 37.2.050 1.050 86 8.6 .0487 2.15 44.2.075 1.075 170 17 .0722 2.84 39.4.10 1.10 240 24 .0951 3.18 33.5

22-T 0 1 11 1.025 1.025 36 3.3 .0246 1.19 48.5.050 1.050 120 11.0 .0487 2,40 47.3.075 1.075 190 17.3 .0722 2.90 40.2.10 1.10 260 23.6 .0951 3.16 30.0

23

Table 6

GAGE FACTORS AT 25 C, SILVER VACUUM METALIZED ON

1/7-mil SEASPACE POLYETHYLENE FILM

Gage No. Strain L/L R R/R Ln(L/L ) Ln(R/R ) G

23a 0 1 65 1

.125 1.125 150 2.31 .118 .835 7.07

.250 1.250 260 4.00 .223 1.387 6.20

.375 1.375 390 6.00 .318 1.791 5.63

24b 0 1 45 1

.1 1.1 100 2.22 .095 .798 8.38

.2 1.2 173 3.84 .183 1.345 7.34

.3 1.3 250 5.56 .262 1.716 6.53

.4 1.4 310 6.89 .336 1.93 5.72

.5 1.5 400 8.89 .405 2.18 5.38

a) #23, gage bonded to 1.5-mil Visqueen A with pressure-sensitive siliconeadhesive, electrical contacts at grips.

b) #24, gage bonded to 1.5-mil Visqueen A with pressure-sensitive siliconeadhesive, conductive adhesive contacts.

24 I 2 4A .

105 I V

Symbol Gage Temp. Film — I250C. 1.5 mil Visqueen A

V 2 , ,,i0 3O 4 .... _ A 25 -80°C. 0.7 mil StratoFilm L /____

10'

· I /I/

•UCEIA 27LVERED GAGE / IO // .~~25|

10

l0 _ _ ______

__________ __LECTRICAL_ RESISTANCE__._

.S /_ _____

0 10 20 30 40 50 60 70 80 90STRAIN, PERCENT

I

25

8

Symbol Gage Ro, o ms Facfor Temp. Film0.40%

A I 134 10.0 250C. j.5milI Visqueen A7— V 2 100 10.0 It eA

0 3 30 7.5 ' 0 4 27.5 8.5 ,A 25 12 9.4 -800C. 0.7mil 6 A26 13 12.0 StratoFilm

6 — 27 . 13 10.2 ,,,

C/ /

3

2~0~~/

0 0.1 0.2 0.3 0.4 0.5 0.6

Ln(L/L0 )

FIGURE 2 o CALIBRATION OF CHEMICALLYSILVERED GAGES LOGARITHMS OFELECTRICAL RESISTANCE VS. STRAIN

i yT/ /

iELECTRICAL RESISTANCE VS. STRAIN

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26

Machine Direction Transverse DirectionSymbol Gage Symbol Gage

0 5 0 10o 6 0 II~A 7. A 12

, 0 8 0 13V _ _9 V 14

4 / / /l

025 0.5

/7y ^

5 THROUG.H 14 Y-SIL ALUMINIZED POLY-t

PROPYLE NG E FactorI LM

Maxi nimum'Gage Factor

= 16.5

0.5 o. ///

0 , 05 .10 .15 0 i05 .10 .15

Ln (L/L o ) Ln (L/L o)

FIGURE 3 o CALIBRATION AT 250 C. OF GAGES5 THROUGH 14, HY-SIL ALUMINIZED POLY-PROPYLENE FILM

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27

Machine Direction Transverse DirectionSymbol Gage Symbol Gage

A 1-7 A 20

FIGURE 4 CALIBRATION ATC. OF GAGES21

15 THROUGH 22' Y-SIL ALUMINIZED PO LY-

PROPYLENE F LM

3 — / —

/A

0 - 0 -0 .05 .10 0 .05 .10

Ln (L/L o ) Ln (L/Lo)

FIGURF 4 o CALIBRATION AT 80 C. OF GAGES15 THROUGH 22, HY-SIL ALUMINIZED POLY-PROPYLENE FILM

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28

Symbol Gage

2.5 — 0 23A ___ 24

2.0

.5

~ ; 0

1.0

0.5__ _ _ _

o0 0.1 0.2 0.3 0.4 0.5

Ln (L/Lo)

FIGURE 5 o CALIBRATION AT 25°C. OF GAGES 23AND 24. SILVER VACUUM METALIZEDON /7-MIL SEASPACE POLYETHYLENEFILM BONDED TO 1.5-MIL VISQUEEN A FILM

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29

T3

T2

CALIBRATON AND TRESS ASTRAIN

FI0

I0

I000

T2

10,00

T3

FIGURE 6 o PROPOSED FORMAT FOR GAGECALIBRATION AND STRESS ANALYSIS

FOR BALLOON MATERIALS

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30

REFERENCES

1. Anderson, A.A. and R.W. Barrett: Investigation of Stress Distributions inInflated Balloons, Report No. 2248, Contract AF 19(604)-7212, GeneralMills, Inc., Minneapolis, Minnesota, December 1961.

2. Graphite Strain Transducer, Technical Report RADC-TR-65-38, Rome AirDevelopment Center, Griffis Air Force Base, New York, April 1965.

3. Mayer, H.: in Structure and Properties of Thin Films, Neugebauer, et.al.,editors, 225-252, John Wiley Co., New ork, 1959.

4. Tests of Balloon Materials, NCAR Facilities Report FRB-1-64, NationalCenter for Atmospheric Research, Boulder, Colorado, November 1964.

5. Perino, P.R.: "Thin-Film Strain Gage Transducers," Instruments and ControlSystems, Vol. 38, No. 12, 119-121, December 1965.

6. Material Strength Properties of Winzen StratoFilm, NCAR Technical NoteTN-14, National Center for Atmospheric Research, Boulder, Colorado,April 1966.

7. Wagner, W.C.: "Temperature Measurements from Floating Balloons,"Proceedings, 1964 AFCRL Scientific Balloon Symposium, Report AFCRL-65-486, Air Force Cambridge Research Laboratories, Bedford, Mass-achusetts, July 1965.

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