AD—A039 319 ARMY MATERIALS AND MECHANICS RESEARCH CENTER WATERTO—ETC F.’G 11/SONO CESTRUCTIVE TESTING OF RUBBER PRODUCTS USED flY THE ARMY. (U)

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NONDESTRUCTIVE TESTING OF

~ RUBBER PRODUCTS USEDBY THE ARMY

WALTER R. KIAPPERTMATERIALS TESTING TECHNOLOGY DIVISION * ~

January 1977t ,iO DC

\ ~~~~~ Approved for public release, distr ibution unlimited

~~ ARMY MATERIALS AND MECHANICS RESEARCH CENTER~~~~~~~~~

.~~~~~~~~ Watertown, Massachusetts 02172

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1’This project has been accomplished as pert of the U. S. ArmyMaterials Testing Technology Program. which has for its objectivethe timely establishment of testing techniques, procedures orprototype equipment (in mechanical, chemical , or nondestructivetesting) to insure efficient inspection methods for materiel/material procured or maintained by DARCOM.

The flndln~ in this report are not to be construed as an officialDepartment of the Army position, unless so designated by otherauthorized documents.

Mention of any trade neenss or manufacturers hi th is reportdviii not be construed as advertising nor as in othciaIindorssfflsnt or approval of such products or companies bythe United States Government

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C TITLE (and Sub UI.) YPE OF REPORT I PERIOD COVERED

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~~~~~ ~.N ON DE STRUCTIVE ~ E ST ING OF RUBBER PRO DUCTS

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__________________________and Readiness CommandAlexandria,_ Virginia__22333 __________________________

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IS. SUPPLEMENTARY NOTES This project has been accom~ .~hed as part of the U.S.Army Materials Testing Technology Program, which has for its obj ective thetimely establishment of testing techniques, procedures or prototype equipineni(in mechanical, chemical , or nondestructive testing) to insure efficient in-spection m~thod~s for n~~teri ~ 1/ina.t~ riiii prnr t i r~ d or u1A4nt~ Lin od by flkRCt ~ LIC. KEY CORDS (ConlMi,. on tIVa,aa aIdi It n.c.sa ~~y and ddant l ly~brv block numb.t)

E last omersRubberTest methodsNondestructive testing

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ABSTRACT

This paper is a survey of the field of rubber and rubber testing lead-ing to a discussion of the nondestructive testing of rubber products usedby the Army. It includes a primer on rubber itself, a review of physicaltests, and concludes with a survey of rubber-related nondestructive test-ing research .

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INTRODUCTION

At this time, research and development is underway to nondestructivelytest, rubber products. This MD is motivated by specific problems encounteredby the users of rubber products. The Army, as a user of such products, has aneed for the results of such research to solve problems in testing productsranging from pneumatic tires to rocket motor parts. This monograph has beenprepared after a survey of the field bya member of the Nondestruêtive TestingIndustrial Applications Branch of the Army Materials and Mechanics Research

-‘ Center. It is an attempt at objectively reviewing the nondestructive testing• of rubber problems. It begins with a primer on rubber itself, includes a re-

view of physical tests, and concludes with a survey of rubber-related nonde-structive testing research.

To this writer ’s knowledge, this is the only work of its kind and scope.Therefore, the reader is advised that this monograph is an original percep-tion of a very complex field, and that parts of this document m a y be most use-ful as a source for productive debate. That is, several subjects in the fieldof rubber testing are controversial especially when vested interests are af-fected. However, other parts of work are easily traced back to informationin well-accepted publications and can be accepted as fact. As a whole, thisdocument should be useful as an introduction to the problems associated withthe testing of rubber for those beginning to study the problem and as a goodreview and survey for those already involved.

A PRIME R ON ELASTOMERS , THE RUBBER COMPOUND

“Elastomer” is used as a generic term for rubber and rubber-like materials.In this report, the terms “rubber ” and “elastomer” wi l l be used interchangeably,and both will refer to the rubber compound. The polymer which is the chiefingredient in the compound will be called the “elastomeric polymer” to avoidconfusion. This section briefly covers the elastomeric polymer, the filler,the curative and the other components which make up the rubber compound.

The Elastomeric Polymer

The basis of any rubber compound is the elastomeric polymer. (This poly-mer may be called the “elastomer” in other literature.) These polymers arelong chains of molecules which vary in structure and elemental makeup accord-ing to the class of elastomeric polymer. The classes include: natural rubber,Thiokols, neoprenes, nitrile rubbers, butyl rubbers and many others. Eventhese classes may be divided into specific polymers and each of these polymersvary in physical properties. That is, each polymer may contribute differentlyto the ultimate compound with regard to inherent strength, temperature resist-ance, vulcanization properties and other physical and manufacturing properties.For this reason, rubber can be considered to represent a large spectrum ofmaterials having various properties. The only common property is the abilityof all rubber compounds to recover after large deformation. Finally, notethat the elastomer is typically only 60 percent of the compound and there isconsiderable variance in this percentage from recipe to recipe.

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The Fi ller

The filler in the rubber compound does much more than simply occupy space.The filler affects the properties of the compound. Its functions include im-provement of the strength characteristics and adjustment of the cure times.Like the polymer, there is great variance in the materials used for fillers.Fillers may be pigments, fabrics, or even fibers or glass. Fabrics, fibersand other non-pigment fillers results in what may be defined as “compositematerials” and this subject is considered separately in a later section.

Pigments include carbon blacks, zinc oxides, clay, calcium carbonates,and various other substances. Each of the substances is used to change theproperties of the ultimate compound. Carbon blacks are used extensively inArmy applications, because, in addition to their other attributes, they absorbultraviolet light. This absorption of UV retards attack from oxygen. Fillersvary greatly in percent of compound makeup, but a typical percentage is about30 percent of the mix.

The Curative

Unvulcanized rubber has some very poor characteristics. It is sticky whenwarm and rigid when cold. It is vulcanization that makes the material usefulin products. For this reason, the vulcanizing agent, or curative, is a vitalpart of the compound. This component is required to cross-link the polymersin the vulcanization process. The cross-linked polymers cause the materialto exhibit useful properties in the final product. Usually, th~ curative issulfur or a peroxide type which is not used as extensively as sulfur; however,several other substances are good curatives, especially for specific applica-tions. For example, the thiuram disulfides may be used to obtain better heatresistance. Also, certain curatives work best with certain rubbers. Forexample, metallic oxides work best with neoprene. In spite of the curative’simportance, it is only typically 5 percent of the compound, and again, thispercentage varies greatly.

The Additives

Additives comprise a relatively smal l percentage of the compound, usuallyless than 5 percent, but they are the ingredients which determine the dura-bility and processability of the rubber material. In this report , the term“addi tives” includes: antioxidants, antiozonants, accelerators, activators,and all substances included under the sub-class called “softeners.” As thenames imply, these substances do everything from improving the rubber’s resist-ance to the environment to making the rubber more distortable and retentivefor processing. These additives are mostly organic compounds; and, thoughthey are trace substances, it is critical that they be spread throughout abatch of rubber when it is mixed.

Mixing and Curing

The ingredients of the rubber compound are usually mixed in a two-rollrubber mill or in a Banbury internal mixer. Though both methods are considered

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to give a good mix, there is no guarantee of complete dispersion of the ingre-dients. The m ix is sampled and physical tests are performed on the sample todetermine the quality of the mix. A nondestructive, complete test of the mixwould be superior, but no such test exists at this time. Therefore, it ispossible that failure could occur in products made from the parts of a rubberbatch where components were not properly dispersed. It is also notable thata rubber product may be cured inhomogeneously. Curing takes place in the pres-ence of heat. In fact, a rubber compound begins to cure as soon as it is mixeddue to ambient heat. For this reason, there is a limited amount of time thatrubber material can be stored before it is processed into a product. However,Army work has showed that peroxide-based calendered stocks can be stored atroom temperature for several days or longer prior to cure without adverselyaffecting the properties of the vulcanizate. At some point during the produc-tion process, the temperature is increased and the rubber is vulcanized. Ifthe rubber is vulcanized in a poorly designed mold, the heat may be spreadunevenly and the material will cure irregularly. Irregular curing may alsooccur if the rubber product is thick because the rubber itself is a poor con-ductor of heat .

Another problem occurs in the curing of test pieces. Since the test pieceis to represent a batch of material, it should be cured to the extent thatthe final product is cured. This is also difficult to guarantee.

Finally, after the rubber is vulcanized, it may continue to form crosslinks in the presence of ambient heat. This curing after production contrib-utes to the aging of the end item.

THE PHYSICAL TESTING OF RUBBER

Presently, rubber products are usually tested and accepted on the basisof performance in physical tests. This is true of raw rubber materials aswell as final products. Specifications for rubber products are usually writtenwith regard to physical test performance, and even the composition of the rub-ber compound is left up to the manufacturer. It is admitted that the physicaltests for rubber have limitations and that the rubber field is in need of un-proved tests. However, the parameters which could be considered in acceptingrubber for a given application have caused problems to the specifiers. Oneresult of this was a rubber developed in the 1950’s which was thought to havegood characteristics especially with regard to oil resistance. However, whenthe rubber was put into products in the field, it was found to degrade whenexposed to moisture for a period of time.

In an attempt to list the problems present in the physical testing of rub-ber, the following list of standard tests is presented. The list is intendedto survey the common tests and does not claim to include all tests, devicesor standards .

The tests just listed are tests used on raw rubber materials and on samplesof rubber end items. Nondestructive testing could have an advantage over thesetests because it can allow inspection of 100 percent of the material or product

H in question. All of the physical tests listed, with the possible exception

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STNIDARO Ph YSI CAL TESTSTEST DEVICES *5TH STAHOARDS PURPOSE LIMITAT IURS

Plastitity Rotational: Meoiiey Visco.eter *5TH 01646. 1)926 . Find processing character— ‘Results can be disappointinglyI Compreesfonal: wil Ih ~~~ 1)2704 . 02705 , ls ti cs of raw rubber corn- variable. especially between

Plist imata r 02706 pounds to determi ne scord i diffe rent laborato ries ’ .~Reciprocating motion : and cure rate .Monsanto m,e~~ ter.Agfa Vulco ter, etc .

~pecifhc Grevi ty Jol ly Rilence To find If any gross Not extr ~~~1y sensit i ve to small- error has been made in changes in compounding. Does

coeeoundi ng. Cot exami ne entire batch of rawrubber c~~ ound. only a samp le.

Stress-Strain Tension: Scott Tester , *5TH 0412. 0515. To determine the strength These, along w i th hardeess , areAlbertoni Tester 03196 tharacteristics under the mos t widely used tests . How-C~~ ression: (See 0575) Static load. Can be used ever , the stress on the testShear: I.C.I. slow creep to determine rate Of cure, piece of rubber may be a poortest aPparatus , Instron opti~~~ cure. process con— si mu lati on of cond ition s in

trol , acceptance testing, service .agi ng. and research .

Hardness Shore Ourcmeter ASTM 01415 To find hardness, the This tes t is only mildly destruc—e4astjc modulus under t ive . However , devices ire dif—~ma1i Strain. - fic ul t to calibrate and thickness

of rubber and other parameterswill affec t reading .

Tear Test pieces — cres cent . *5TH 0624 To determine tear ‘esis- Tear resistance may have littleangle. Delft test pieces tince of rubber, to do with failure of product

even if failure is due to ab a-- sine wear , Mechanized tests are

not always in agreement withhand tests

Abrasion Cont i , Pico , and many more ASTM 01630. To si mu late s~~ of the Useful only in ‘indicating broad2228 field conditions that the differences between compotatds,

product will bp subject cannot be used to discriminateto with respect to abra- closely between similarsine resistance , compounds’,2

Flex Cracking DuPont. DeMet t i a . Flipper , ASTh 0430-57, To eva l uate rubber which Poor reproducibility. Ozone mayVegt . etc. 813. 430-51 will be used in high-flex cause mu ltiple cracking .

applications , for es~~ le:tire sidewalls.

Fatigue Firestone, Goodrich *5TH 0623 To maasure hysteresis de— It is notable that temperatureFlexaw teru fect by subjectin g s~~ la rise is the data obtained from

to complex nodes of this test. ‘Practic ally, thisdeformation. weasuremant (hysteresis] is en—

tremely difficult to acco mp lishwithout complications, owing tovariations in the heat capacity,conductivity, and radiation ofmaterials being tested’.3 Oneadvantage is that comples forcesare presumably closer to servicecondi tions than bending tests .

Set . Creep, and 1.C.l. shear c reep *5TH 01390, To measure the tie. and Presently, tests have poor repro-Stres s Relaxation test apparatus 395, 4l2-51T, temperature dependent ducibility. ‘The results hive(Flow Properties) 1226-521 flow properties of rub- little absolute weaning and are

bar compound. useful only for comparative orcontrol tests’ . ’

Resilience Dunlop Pendulum, *5TH 01054 To measure a temperature- ‘Although the ... (resilience)Lupha Pendulum , dependent property which tests give useful results, i tGoodyear-Healey is avaloqoub to the co- has been pointed out that theirPa ndlon efficient of restitution, value is limi tad by the fact

that ,‘esi lie ce is a f i ct ionnot only of the internal fric-tian or hystereats but a’so ofthe ~ynamic endulus~.

2Low Temperature Geliman apparatus, A,STM 0832, 797, To determine the proper- Results ar, extremely var iable.

king apparatus 746 , 1229 , 1329, ties of rubber as it Properties at low temperature1053 stiffens at low tempera- can dep.nd on previous history

tue.. For .x~~ le , to of rubber, time , and method off i nd the temperature at cooling as well as.temperatare.wh ich rubber becomesbrittle.

Aging Greer oven *5TH 0673, 572 , To detee.ie. the resin- In many tests, speci mens may not454, 1149 , 865, t.nce to deterioration be aged in the s~~ oven without518. 1171 , 1672 , in an accelerated aging affecti ng the test. It is also2309 test by use of an air difficult in s~~ cases to assess

oven, oxygen pressure, the degree of deterioration der-ozone , light, or liquids . tug the test.

Resistance to *5TH 0257, 150, Tg determine the electri- Electri ca l properties change ifElectricity, 149. 0471, 1400 cal properties or the there is external or internalLiquids, and resistance to fluids of stress. Therefore, care mast beGases the rubber, taken to assure that test piece

• is stressed si m i larly to thestre ss encountered in end item. - -

Performance Tire .I eel. Road test, To deter mine the reaction Tests are gi erelly costly andTests Aircraft tire land ing of rubber to performance destructive . Obviously, omly a

siumilator , other service enviro wmnt. s~~ le of the end it .. cam besimulators , tested. Furtherwere, simulators

rarely replicate all of th, per-formince variables needed forcomplet, evaluation.

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of hardness, destroy or seriously degrade the part of the rubber being tested.Therefore, these tests are limited to the testing of samples or coupons. Grant-ed, it is generally difficult to conceive of a nondestructive test which wouldgive the same information as a particular physical test. However, the creation

• of such nondestructive tests is a goal which needs to be considered.

RUBBER IN CONJUNCTION WITH OTHER MATERIAL S

In many items, rubber is used with other materials. For example, textilesand fibers are used as fillers in the rubber compound to produce materialsused in tent materials, belt materials and in other items where strength and!or liquid impermeability are required. Rubber may also be bonded (usually“vulcanize” bonded) to other materials, typically to metal or textiles. Thismay be used to reduce noise or avoid destruction by the metal item, or to pro-tect the metal or textile from the environment Applications include Armytank treads and roadwheels as well as rubber coatings for handweapons. Thebonding of rubber to textiles also becomes important in tire manufacture.However, the tire is such a complex rubber product that it might be best clas-sified as a composite of rubber and non-rubber items rather than a compoundproduct. Tire testing is a full study in itself.

The tests on products which contain rubber and other items are chieflyperformance tests. However, the greatest amount of nondestructive testingresearch has been done on these compound items and a few physical tests areused to determine the strength-of-bond. Two of these physical tests follow:

BONDING TESTS

TEST DEVICE ASTM STDS PURPOSE LIMITAT IONS

Rubber-to- Peel devices D413 Determine Stress on item doesTextile dead load, D2138 Adhesion not replicate stressAdhesion H-block in operation.

Rubber-to- Peel devices D429 Determine Stress on item doesMetal plate test D2229 Adhesion not replicate stress

specimens, in operation.conical testspecimens,shear tests

The tests listed above are static tests. That is, the bond is testedunder a static load. Dynamic tests are being developed at this time.

NDT OF RUBBER ITEMS

Most work done in nondestructively testing rubber has been work done ininspecting end items rather than raw materials . In many cases , the NDT is ap-plied without full information on the failure modes of the item being inspected.There may simply be a hope that a nondestructive test will be found that

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correlates with performance of the item. Though this may not be a rigorousapproach, the NDT test, once proven, may be superior to a destructive physicaltest. Physical tests may be based on more basic material characteristics, butthe destructive measurement of these characteristics may not correlate wellwith performance.

• The principal approaches which have been applied to the nondestructivetesting of rubber are: ultrasonics, radiography, optical holography and ther-mal methods. Generally, all of these approaches can be tried in the attemptto solve a rubber problem, but the approaches will yield different informationwhich will be more or less relevant to the problem. For example, when inspect-ing an automobile tire for separations between ply layers, the inspector willfind that both optical holography and ultrasonic methods will be useful. Butradiography will probably be useless. On the other hand, if one is inspectingfor broken and misplaced cords (particularly steel cords), then radiographicmethods are most useful and the other methods become comparatively useless.

:1 In general, if the anomaly which leads to failure is known, an NDT approachmay be able to detect it. However, the anomaly which leads to failure usuallyis not known or is the subject of debate. Therefore, research has tended toexamine tires (and other rubber products) by some NDT approach and look forcorrelation between results and field tests. It has become common to examinefor anomalies called by some generic name like “degradation” or “structuralintegrity.”

The lack of information on failure modes extends beyond examination of thepneumatic tires. Tank treads and roadwheels, though simpler than tires, alsohave debatable failure modes. The anomaly usually inspected for in the tanktreads and roadwheels is an unbonded area between the steel and rubber partsof the product. Certainly such a debond could lead to failure, but it isargued that the items also fail due to environmental attack and misuse. Again,the recourse has been to test NDT results against field test results. NDT has

S also been applied to the inspection of elastomeric rocket motor case liners inan attempt to find separations. However, from the information available tothis writer, the work on case liners is still highly experimental.

.4 In summary, many standard NDT approaches (such as radiography and ultra-sonics) can be applied to rubber product problems. Also, newer approaches(such as thermal methods and optical holography) can be developed to enhanceon standard approach results or to obtain new information altogether. Thegreat gap lies in that the NDT inspector usually does not know what anomaly heshould try to find. But, in spite of this unknown, some NDT work in rubbergoes on in the hope of finding an inspection method from statistical correla-tion rather than a rigorous approach.

SUMMARY

This report has been an introduction to the problems associated with thenondestructive testing of elastomeric products. It has reviewed the basics ofrubber compound ing and physical testing and has briefly surveyed the problemin the Army concerning elastomeric products.

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BIBLIOGRAPHY

ALBERTSON, Application of Neutron Radiograp hy Using a Radioactive NeutronSource, General Electric, 1969.

Automobile Tire Hot-Spo t Detection, Barnes Engineering Company.

BIBBY, Nondestructive Infrared Tire Testing, Monsanto, 1972.

BLOW, Rubber Technology and M~.nuf acture , Butterworth, London , 1971.

BOBO, An Infrared &irp eriment on a Road Wheel During an ~VVSS 109 TypeCorrplianoe Teat, National Highway Traffic Safety Administration, 1972.

BOBO, Isolation of Fla ws by Use of Ther~nal Differential-s on a Tire Underl.f il.d Loading Conditions, DOT, Cambridge, 1972 .

BOITFAINE , A Radiographic Tracer Method for Nil]] of Aircraft Tires, A.T.A.— Nondestructive Testing Forum, 1974.

COLLINS, Solid Rocket Motor Inspection by Acoustical Ho lography Techniques,Air Force Rocket Propulsion Laboratory, 1975.

DEAN, Air-Coupled (Iltrasonic Probe, British Journal of NDT, 1968.

L FRANING, Test Report M140 Gun Mount Seals, ARMCOM .

FRERSON , Hologr aphic Interferometry Techniques and Applications, ARMCOM, 1969.

GARDNER , Investigation of Techniques for the Development of a Seam InspectionMethod fo r Flexible Fue l Storage Container I *zter ial, Southwest Research ,1970 .

HALSEY , The Nondestructive Testing of Passenger Tires, Scientific TestingLaboratory, Materials Evaluation, Indiana, Pennsylvania, 1968.

HENDERSON, Radiographic Techniques for Examining the Fiiament-Wound Tire,Wright-Patterson AFB, 1968.

IVERSON, Holographic-Nondestructive Testing of Rubber-to-Metal Bonds,ARI4~OM, 1969.

JACOBY , Microwave Measurement of Aerospace Material 8, Lockheed Missiles andSpace Company , 1972 .

LAVERY , Nondestructive Tire Testing Studies, DOT, Cambridge, 1972.

LEVINE, Dynamic Infrared Inspection Techniques, GATX, 1963.

LICHODZIEJEWSKI, Nondestructive In8pection of Track Shoe Rubber Pads, GATX,International Journal of NDT , 1972.

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MARTIN , The App lication of a Radi o Influence of Voltage Test to the De tectionof Voids in Eiastomers, Materials Evaluation, 1970.

~)RTON, Introduction to Rubber Technology, 2d ed., Van Nostrand Reinhold,New York, 1973.

NAIJNTON, The Applied Science of Rubber, E. Arnold, London, 1961. -•

PR LJSINSKI , Bond-Line Inspection Probe for Rubber-Steel Composites, GATX, 1972.

Rubber in Engineering, Chemical Publishing Company, Brooklyn, New York , 1946.

- SPERRY , A New Generation of Aircraf t Tire Test Dyncvnometer Systems, DoD Con-

-: ference on NDT, 1970.

VOGEL, Proceedings of the Firs t, Second and Third Symposia on the NDT ofTires, Army Materials and Mechanics Research Center, 1973-1976.

WALTERS, Fai lure Analysis of Wire-Reinforced Flex Hose used in Damping Retra ct!- Reconnect System, NASA/Kennedy Space Center, 1973,

WILLIAMS, Environmenta l Expos ure and Accelerated Testing of Rubber-to-Metal-~~ Vulcanize Bonded Asse7rblies, ARMCOM, 1975.

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