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BT- 1040.25Recom mendations for a Revision of the

Balloon Specifications MIL-P-4640A (USAF)A D. Kerr, H. AlexanderJun 1970


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cwltwltr25. 1 Introduction 31625.2 Recom mended Tests 31625. 3 The Qualification Tests 31725.4 The Acceptance Tests 32225. 5 Trial Period of Revised Specifications 322

25. Rec om m en d a t ions for a Rev ision o f theBa l lo on Sp ec i fica t ions M IL-P-4640A (USAF)

A.D. Kerr and H. Alexander

Abs t rac t

A study of the deformation and strength properties of balloons and balloon ma-terials was performed by the authors for Air Force Cambridge Research Labora-tories over the past six years. Based on the findings of that study, as well asupon the findings of relevant studies conducted before by others. a revised testprogram to be used in the qualification and acceptance testing of balloon films isrecomm ended and discussed. This new set of tests retains those tests of the oldSpecifications MIL-P-4640A (USAF) which are felt to be relevant to the determina-tion of the suitability of films for balloon use. Tests that are not considered rele-vant to this determination have been discarded and new testing methods have beenadded.

These recomm endations do not necessarily represent the opinion of the U. S.Air Force. Their main aim is to initiate a discussion among film producers, bal-loon manu facturers, balloon users, mechan ics and materials scientists, and theAFCRL and, thus, create a rational basis for the planned revisions.


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25. I IYTRODUCTION

The present balloon specifications of the U. S. Air Force MIL - P - 4640A(USAF) are based on studies conducted by General Mills, Inc. for the U. S. AirForce during the early 1950s. These specifications were written essentially fora specific film, DFD 5500, which at that time was the predominant material usedin the fabrication of balloons.

The recent development and use of new balloon films, the large number ofballoon failures in the early 1960s (all of whose barrier materials has passed theprescribed tests), and the ever increasing demand for larger balloons, necessi-tate the complete revision of the part of the military balloon specifications whichdeals with the strength of the balloon films and seals. During the past six years,the authors studied the deformation and strength properties of balloons and balloonmaterials for the AFCRL. The results are summ arized in Kerr (1969). Based onthese findings, as well as upon the findings of relevant studies conducted before byothers, a revised test program is recommended and discussed. It is expected thatthese recommendations will initiate a discussion among film producers, balloonmanufacturers, balloon users, mechanics and materials scientists, and the AFCRLand, thus, create a rational basis for the planned revisions.

25.2 RECOMMENDED TESTS

It appears reasonab le to prescribe two sets of tests, to be called the qualifi-cation tests and the acceptance tests.

A new film submitted by a manufacturer to USAF for consideration as possibleballoon mat erial will hav e to first pass the qualification tests. I If the subm ittedfilm samples satisfy the prescribed requirements and the USAF places an order,then samples taken from the beginning and the end of the production run should besubjected to the acceptance tests. For very large orders, film samples shouldalso be tested at specified intervals to insure quality through out the entire produc-tion run.

Because the seal is essential for the production of large balloons, and becausethe seal or its vicinity is usually weaker than the film, the tests proposed in thefollowing are designed to test the strength of the seal as well as the proposed film.In this connection it should be noted that a strong film is useless as a balloon ma-terial until a seal is found whose strength matches the strength of the film.

In order to qualify as a balloon material, it is recommended that a submittedfilm and proposed seal be subjected, at least, to the following tests:

(11 Uni-axial tensile tests at room temperature


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Pr

It + 1lOF(2) Bi-axial tes ts at cold temperat ure(3) Mu tilation an d fatigue test at cold t e m p e r a t u r e(4) Molecular weight determ inat ion test sThe qualification test s ar e to be condu cted by th e AFCRL or an AFCRL ap-

*oved Laboratory.

25.3 THE QUALIFICATION ESTS

X.3.1 U n i a x i a l Tests at Room TemperatureVarious studies indicate tha t the response of a f ilm to loads i s strongly influ-

enced by its orientat ion induced dur ing extr usion. For example, films which ar estrongly oriented in the machine direction are usually weaker in the tra nsversedirection. Genera l Mills, Inc. conducted an extensive stu dy durin g which th eywere able to obtain, from t he extru der, deta iled inform at ion regarding the manu-factur ing of th eir test ed films. They foun d th at film extru ded with near ly equalelongation in both machine and tra nsverse directions showed that the strength ofthe film was 30 to 75 percent stronger and tougher tha n previous stan dard films(Freeman. 1968).

The findings of Gener al Mills, In c., as well as results of more recent studiesby the aut hors, seem t o indicate tha t a nearly equal orientation in the machine andtr an sverse direction is preferable for balloon films.

In general, extru der compa nies ar e unwilling to supply any deta ils of themanufacturing process. To ensu re th at a balloon film responds similarly in bothdirections an d possesses th e required str ength , th e following uniaxial tensile testis recommended: Cut two film str ips; one par allel to th e ma chine direction, th eother par allel to th e tr an sverse direction. The samples are then tested accordingto ASTM Test Method D882-67, Method A at room tem pera tur e. The speed oftest ing should be 10 in/min on a 3-inch by l-inch sam ple. The test result shouldbe presen ted in a graph a s shown in Figure 25.1. For easy identificat ion of th eneeded characteristics, it is suggested th at th e scale for E (elongat ion) be suchth at th e slopes of th e obta ined c u r v e s f or sm a l l v a l u e s o f E be about 45O, asi n d i c a t e d i n Figure 25. 1.

The slope of th e obtain ed curves sh ould be everywh ere Positive. (cur Ve A).For th e tra nsverse direction, th e obta ined values should not deviate by more tha n20 percent from th e corr esponding values in the ma chine direction.

The curves for both str ips should be similar in chara cter. If th e 4-e curvein th e ma chine direction is of sha pe A a n d t h e o n e i n the transverse irection e


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t

u(W)

Figure 25.1. Method of RecordingUniaxial Tensile Test Results

of shape B, as shown in Figure 25.1.and if curve B deviates strongly fromcurve A, then this indicates strong orien-tation in the machine direction and thesubmitted film m ay not be suitable forballoon purposes. It should be noted thatthe labor and equipment involved in theseproposed tests are essentially the sameas in the uni-axial tensile test prescribedpresently.

The above tests should be repeatedafter 24 hours at a different level of rela-tive humidity in order to determine if theeffect of hum idity and aging is excessive.

25.3.2 Biaxid TestsIn these tests, the sample consists

of a cylinder produced by placing two long film strips on top of each other and thensealing them along the edges, parallel to the machine direction. One of the stripsshould contain a crease. The method of sealing should be identical with the oneto be used in the actual balloon. The sample is closed off at both ends by end fit-tings. The sample may be stressed by pressurizing it with a gas through the up-per end fitting. This type of loading in-duces the fixed stress ratio

circumferential stress = 2axial stress 1

Different stress ratios may be obtainedby additionally subjecting the lower endfitting to a load F . as shown in Figure25. 2.

In order to test the effect of creep

c -L4

P -L

upon the strength of the film and theseal at the high temperature encounteredat launch and at very high altitudes, thefollowing tests to be conducted at+l 1OF are recommended:

Three samples are to be tested. Figure 25. 2. Method of Loading TestThe first sample is subjected only to an Sample


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axial force F. At specified time inter-vals, the axial stra in is measur ed andrecorded as shown in Figure 25.3. Theload F is determ ined as follows:

F = (Au aa ) for tapeless balloons

F= (0.8Ao aa) for ta ped balloons

where A is th e initial ar ea of sam plecross s ection (including seal ma terial)and u a is the ant icipated axial st ressin th e actua l balloon to be built from th e

A EXTENSION, x

TIME t hours)

tested mat erial (presently about 800 Figure 25.3. Method of Recording CreepTest Result s for Sam ples 1. 2. an d 3psi). The second sam ple is subjectedonly to an internal pressur e p. Thecorr esponding axial an d circumferen tialstra ins are measur ed at specific timeintervals and they are also plotted as shown in Figure 25.3. The pressur e p is tobe of such a magnitude tha t it creates the stress oaa in the circumferential direc-tion of the sample. In the third test, the sample is subjected to F, a nd p/2 a ndthe axial and circumferential stra ins are plotted in Figure 25. 3.

When a film is test ed for a balloon with un usu al flight requir ement s such asa long stay at very high altitudes, tha t is long exposure to high temperat ures, thenth e obtained grap hs in Figure 25. 3 should be checked for th is additiona l requ ire-men t. Assuming th at such a time period does not exceed, let us say, five hour s,th en if none of th e plotted curves indicates failur e within this time per iod (that is,if th ey behave as indicated by curves 1 an d 2), then the film an d seal h ave passedthis test.

It should be noted th at th ese experiment s also test th e effect of creases, dielines, pinh oles, an d other imper fections when th e film is subjected to high temp er-at ur es an d loads of long dur at ion. For some additional background inform ation onth e above test, th e rea der is referred to Alexander an d Mur th y (1968).

In order to test th e effect of creep dur ing la un ch, upon th e str ength of th efilm a nd th e seal (to be used) at th e low temp erat ur e of th e tr opopau se, th e follow-ing test s ar e recomm ended: Several sam ples, as shown in Figure 25.2 ar e to betested.

In th e first test , th e sam ple is brought into a cold cha mber , cooled for 30minutes, then pressurized unt il rup tur e. The burst pressur e p is then recorded


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BURST PRESSURE, p (psi )

(1)PO - -------------__

I I \0 500 IO00PRELOAD STRESS (psi 1

Figure 25. 4. Method of RecordingBurst Pressure

as the upper dashed line in Figureand the failure pattern is noted.

25. 4,

In the next test, the sample is sub-jected to an axial force F for two hoursat a temperature of 1 OoF. Then theloaded sample is placed in a cold cham -ber and cooled for half an hour. At theend of this cooling period, the sampleis pressurized until rupture. The burstpressure is then recorded as shown inFigure 25. 4 and the failure pattern isnoted. For each consecutive test, F isincreased until a sharp drop in the burst-ing pressure occurs, as indicated inFigure 25 .4. The temperature of thecold chamber should be lOoF below the

lowest temperature anticipated du ring ascent.For background information on this test, the reader is referred to Kerr and

Alexander (1968).The purpose of the above biaxial tests is to determine if the film can satisfy

design requirements for a specific balloon under consideration. In cases wheretwo films seem to have equally acceptable strength and creep properties from thepoint of


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25.3.1 M olecular Zeight Determination TestsTh e s e t e s t s c h e c k a n i m p o r t a n t p r o p e r t y o f t h e b a s i c r e s i n ( n o t o f t h e e x t r u d e d

fi lm ). I t a p p e a r s t h a t m o l e cu l a r w e i g h t i s r e l a t e d t o t h e c ol d b r i t t l e n e s s proper-t i e s of t h e f il m ; h i g h m o l e cu l a r w e i g h t b e i n g a s s o ci a t e d w i t h g o od co ld b r i t t l e n e s sp r o p e r t i e s .

Th e p r e s e n t Sp e c if ic a t i on s M I L- P- 4 6 4 0 A p r e s c r i b e a m o l e cu l a r w e i g h t m e a s -u r e in t e r m s o f a m e l t i n d e x , d e t e r m i n e d b y m e a s u r i n g f lo w r a t e s w i t h a n e x t r u -s i on p l a s t o m e t e r i n a c c or d a n c e w i t h A STM t e s t m e t h o d D 1 2 3 8 . A c co r d i n g t o t h eF i n a l R e p o r t o n Ev a l u a t i o n o f Ba l l oo n M a t e r i a l s ( p . 9 ) b y G e n e r a l M i l ls , I n c .( F r e e m a n , 1 9 5 2 ), m e l t i n d e x i s a m e a s u r e o f a v e r a g e m o le c u l a r w e i g h t a n d i sa f fe c t e d b y c h a i n b r a n c h i n g .

I n F r e e m a n ( 19 5 2 ), i t i s a l s o p oi n t e d o u t t h a t e v e n w h e n t h e a v e r a g e m o l e cu l a rw e i g h t i s h i g h , r e l a t i v e l y s m a l l p o r t i on s o f l ow m o le c u l a r w e i g h t p o ly m e r i n ap o l ye t h y l e n e r e s i n w i l l ca u s e t h e r e s i n o r f il m t o h a v e a p o o r c o l d b r it t l e n e s st e m p e r a t u r e . Fo r t h i s r e a s o n G e n e r a l M i l ls co n s i d e r e d a c h l or o f or m e x t r a c t i o nt e s t , w h i c h w a s f el t t o b e a b e t t e r i n d i ca t o r o f m o le c u l a r w e i g h t d i s t r i b u t i o n . Th i st e s t w a s d i s co n t i n u e d b e c a u s e a t t h a t t i m e i t w a s d i ff ic u l t t o i s ol a t e t h e e f fe c t o fa l l o f t h e v a r i a b l e s i n t h e t e s t i n g p r o c e d u r e .

R e ce n t l y , n e w m e a s u r i n g i n s t r u m e n t s h a v e b e e n d e v e lo p ed t h a t c a n b e u s e d t od e t e r m i n e m o l e cu l a r w e i g h t s . I n p a r t i cu l a r , t h e g e l p e r m e a t i o n c h r o m a t o g r a p hy i e ld s a c om p l e t e m o l e cu l a r w e i g h t d e n s i t y d i s t r i b u t i on p l ot o f n u m b e r o f m o le c u l e sv e r s u s s i z e .

25.3.3 .!a N o t e On Resin Characterization TestsI t h a s b e en t h e e x p e r i e n c e o f t h e A i r Fo r c e , a n d h a s b ee n v e r i fi e d b y t h e

a u t h o r s , t h a t t h e b a s i c r e s i n s a r e s o m e t i m e s m o d if ie d b y t h e r e s i n p r o d u c e r w i t h -o u t t h e k n o w l e d g e o f e i t h e r t h e b a l l oo n m a n u f a ct u r e r o r t h e A i r Fo r c e . T h e s em o d i fi ca t i o n s h a v e c on t r i b u t e d t o a t l e a s t o n e s e r i e s o f b a l l oo n f a il u r e s a n d p r o b -a b l y t o o t h e r s n o t y e t i n v e s t i ga t e d .

I t t h e r e f or e s e e m s p r u d e n t t o c on s i d e r t h e i n c lu s i o n o f a n u m b e r o f r e s i nc h a r a c t e r i za t i o n t e s t s , i n a d d i t i o n t o t h e m o le c u l a r w e i g h t d e t e r m i n a t i o n t e s t ,w i t h i n t h e q u a l i fi ca t i o n t e s t s . Th e r e s u l t s o f t h e s e t e s t s w o u l d b e l a t e r c om p a r e dw i t h t h e r e s u l t s o b t a i n e d d u r i n g a c c e p t a n c e t e s t i n g . Th i s c om p a r i s o n s h o u l d i n -d i ca t e i f a n y r e s i n m o d if ic a t i on h a s t a k e n p l a c e .

A f t e r a p r e l i m i n a r y i n v e s t i ga t i o n o f a v a i l a b le t e s t i n g p r o c e d u r e s , i t i s r e c o m -m e n d e d t h a t i t b e c on s i d e r e d t h a t r e s i n ch a r a c t e r i z a t i o n b e a c co m p l i s h e d t h r o u g ht h e u s e o f ( 1 ) g e l p e r m e a t i o n c h r o m a t o g r a p h y t o e s t a b l i s h m o l e cu l a r w e i g h t d e n -s i ty d i s t r ibu t ion , (2 ) in f r a - r ed abso rp t ion t e s t ing to e s t a b l i sh m o lecu la r com pos i -t i o n , a n d ( 3) d i ff er e n t i a l t h e r m a l a n a l y si s t o o b t a i n m e l t i n g p o in t i n f or m a t i o n .


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25.3.6 A Note on the Presently Used Cold Brittleness and Toughness TestsThe present cold brittleness tests essentially consist of a steel ball puncturing

a film sam ple at cold temperature, and an examination of the failure pattern inorder to determine if the failure is ductile or brittle. The inconclusiveness ofthis failure-pattern criterion is very aptly described in Hauser (1966). However,the inconclusiveness of the criterion was recognized long before by the investiga-tors of General Mills, Aeronautical Research Lab oratories (Freeman, 1952).They suggested that as a criterion for passing this test, a prescribed minimu mamou nt of energy should be used up during rupture at cold temperatures. (Thisis essentially a combination of the cold brittleness and toughness tests. )

At a much later date, the testing apparatus to accomplish this was built andtested (Parsons, 1967). It was found that due to surface effects and other mechan i-cal difficulties, this new criterion was no more accurate than the previous one.In fact, the authors of the present report have been informed (Dwyer, privatecomm unication) that many of these same operating difficulties appear in the roomtemperature falling-ball tests, making toughness criteria bases on these testsquite questionable.

It is therefore recommend ed, for the present time, to eliminate all falling-ball type tests from the revised specifications.

25.4 THE ACCEPTANCE TESTS

Only the tests in Sections 25. 3.1, 25. 3. 2, and 25. 3.4 are recommen ded.

25.5 TRIAL PERIOD OF REVISED SPECIFICATIONS

It is recommend ed that the trial period should be one year and that the revisedspecifications should not be binding during this time interva l.

A c k n o w l e d g m e n t s

The authors wish to thank Mr. J. F. Dwyer AFCRL for a careful review ofthe manu script and for his comm ents on various aspects of the proposed testingprogram.


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References

Alexander, H. and Murthy, G. K. N. (1968) A Failure Stress Criterion for aPolyethylene Balloon Film, N. Y. U. Report No. AA-68-18.Dwye r. J. F., Private Communication.Freeman , A. J. (1952) Final Report on Evaluation of Balloon Materials ProjectGopher, Contract AF33 (600)-6298, General M ills, Inc., Aeronautical ResearchLaboratories.Freeman , A. J. (1968) Progress Report on Evaluation of Materials for BalloonFabrication, General Mills, Inc. , Report No. c/R 5159, p. 3.Hauser, R. L. (1966) Round-Robin Cold Brittleness Tests o f Balloon F ilms,Proceedings 4th AFCRL Scientific B alloon Sympo sium, AFCRL-67-0075.Kerr, A. D. (1966) Experimental Study of Balloon Material Failures, Proceedings,4th AFCRL Scientific Balloon Sympos ium, AFCRL-67-0075.Kerr, A. D. and Alexander, H. (1968) A Cause of Failure of High Altitude PlasticBalloons, N. Y. U. Report No. AA-68-28.Kerr, A. D. (1969) On the Strength of High Altitude Balloons, Journal of theFacilities for Atmosphe ric Research, National Center for AtmosphericResearch, No. 9, pp. 8-11.Parsons, W. B. (1967) Relationship Between Toughne ss, Tensile, and ColdBrittleness for Unreinforced Balloon bil r-n, Applied Science Division, LittonSystems, Inc., Report No. 3116.

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