UNCLASSIFIEDSecuvit, Classification

DOCUMENT CONTROL DATA • R & D(SeC9itry eleaalnllcation of tlle, body of abstract and Indexing annotfal•lon mumf be enfered when the oy•v oll raport Is clasaliled)

1. ORIOINATING ACTIVITY (Ctpoel• , , u'ho.) 2s. REPORT SECURITY CLASSIFICATION

U. S. Army Natick Laboratories UNCLASSIFIEDSNatick, MA 01760 12b, GROUP

3. REPORT TITLE

The Mechanical Properties of Textile Materials as Influenced by Structutre an• StrainRate.

4. DESCRIPTIVE NOTES (Type of lepWa end Inclusive date@)

S. AU THOR(S) (FPlsf nIiae, middle Initial, lasf name)

Frank Figucia, Jr., Louis I. Weiner and Roy C. Laible.

S. REPORT OATE 7E. TOTAL NO. OF PAGES 7Tb. NO. Or REFS

A&r=l 1971 11 8C, CONTRACT OR GRANT NO. 9a. ORIGINATOR'S REPORT NUWBCERS)

71-51-CE (TS-l76)b. PROJECT NO. 1T062105A329

a. ob. OTHER REPORT NOMS) (Any oter numbers •Utma y be 414616nd(his report)

10. OISTRIDUTION STATEMENT

Approved for public release; distribution unlimited.

11. SUPPLEMENTARY MOTUS 12. SPONSORING MILITARY ACTIVITY

U. S. Army Natick LaboratoriesNatick, MA 01760

-0. ASSTRACT

Fibrous structures of increasing complexity, as introduced by ply and twist, have beensubjected to high and low speed tensile tests. The ratio of the strength at highspeed to that at low speed is always greater than one but decreases with increasingcomplexity. The ratio of the elongation to break at high speed to that at low speedis always less than one and also decreases with increasing complexity of structure.The ability to abl'orb energy markedly increases with increasing complexity ofgeometry at the low strain rates characteristic of the Instron. The reasons for thi2additional energy absorbtion are discussed. Tests at high rates of straining pointout the inability of many complex textile structures to translate their superiorenergy absorbtion characteristics *to high strain rates. The results obtained and theprinciples demonstrated are applied to the development of improved materials for usein aerial delivery and ballistic applications.

D7 @orne"WLS o**"LSTS? @ARMYU5 UIa. UNCIASS IFIEDDiDcurlty Classification

UNCLAS•SIFIEDSecurtty CIOssITuiuc-

1.9VWRSLINK A LINK 0 t'

itY *-IO ACeW¶ SLU WN ROLs wif

Measur ement 8.

Strain rate 9 8

Textiles 9

Tensile stress 98

Low speed tests 1,10 10

}{igh speed tests 1,10 10

Tension tests 1,10 10

Eiergy absorption 9 7

Aerial delirvery 4

Ra.llistics 4 4

Ply twist (textiles) 6

TvIst (textiles) 6

Tensile strength 7

M ongation 7

Breaking 7

UNCLASSIFIEDSecurity Classifcatim

Approved for public release; AD7 distribution imlimited.

TECHIMCAL REPORT

"71-51-CE

The Mechanical Properties of Textile Materials a.s

Influenced by Structure azd Strain Rate

by

Frank Figucia, Jr.Louis I. WeinerBoy C. Laible

April 1971

Project Reference: Series: TS-1761TO621e5A329

Clothing and Personal Life Support Equipment LaboratoryU. S. ARMY HATICK IABORATORIESNatick, Massachusetts 01760

Ia m~ iH d•• ai l a o ie g d U~ mIUID m lal e•a

ofabroad~~1 spcrmo dORD

.he design of textile materials for the miit4 'y requlres consider-ation of a broad spectrum of uses antedreloe pergy rmsnce requirements.Among the most severe of •he k of ta terials such those ed for highspeed deceleration of aircraft, 4hich undergo Farge ic Stshrcueck hoahdiegs,or body- armor materials which must resishti high energy te .aiverse mpatIsContinatous improvement in material properties is necessary t no meet theconstantly increasing performance demands in these areas. In constiderratinof these needs, advances in related technology must also keep pace.

This work was conducted iznder Project !TO62105A329, Organic miaterials

Research for Army Matevel. The bulk of the study however cis conductedunerTan~tin f ibr pa~~rties into Fabric Structures, which deals

with the mechanical behavior of high performance textile systems, par-tic-ulaly from the point of view of t~he response of component substructuresto impact forces, It aims at characterization of fiber and yarn elements

S~by laboratory methods and combinations of these into higher structure at

or near end-item levels. The ult.ýmate objective is effective correlation

between actual use and laboratory performance under simulated end useconditions.

ot

ii

List oi. Figures iii

Abstract iv

T. Introduction 1

11,. Experimental 2

A. Materials 2

B. Methods 3

1MI. Results and Discussion

A. Complexity by Ply and wist 4

B. Complexity by Knots 7

IV. Conclusions 10

V. References ii

LIST OF FIGURDS

1. Generalized Stress-Strain Behavior for Most Polymeric Materials 5

2. Impact Performance Ratio vs. Twist for Breaking Strength of 5Nylo-a 66 Yarns at High and Low Strain Rates

3. Depact Performance Ratio vs. Complexity for Breakirg Strength 5of Nylon 66 Yarns at High and Low Strait Rates

4. Impact Performance Ratio vs. Twist for Elongation of Nylon 566 Yarns at High and Low Strain Rates

S. T'pct Performance Ratio vs. Complexi: ,, for Elongation of 8SNylon 66 Yarns at Htigh and Lcw Strain Rates

6. Impact Performance Ratio vs. Twist for Emergy Capacity of 8itylv.n 66 Yarns at High and Low Strain Rates

7. Impact Perfoimance Ratio vs. Complexity for Energy Capacity 8

of 1yloxn 66 "fars at High and Low Strain Rates

8. Strength Losea with Increased Twi st fo-r Nylon 6 and Y-500 8S~Yarns

&9. Hig Speed Knot Ste:,gth vs. High Speed Elorgatio:, for a

Variety of Yarn l-ypes

aiia

S~ ABSTRACT

Fibrous structures of increasing complexity, as introduced bý' p3•rand twist, have been subjected to high and low speed tensile tee;. Theratio of the strength at high speed to that at low speed is always greaterthan one but decreases with increasing complexity. The ratio of theelongation to break at higb speed to that at low speed is always less thanone and also decreases witb increasing complexity of structure. The abilityto absorb energy markedly increases with increasing complexity of geometryat the low strain rates characteristic of the Instron. The reasons forthis additional energy absorbtion are discussed. Tests at high rates ofstraining point out the inability of many ccmplex textile structures totranslate their superior energy absorbtion characteristics to high strainrates. The results obtained and the principles demonstrated are appliedto the development of improved materials for use in aerial delivery andballistic applications.

iv

THE MEC-NICAL PROPERTF•1 - -IF TCILE MATFRIALS A5IMUIFLJFNCE BY STRUJC!7JRE AND STRAIN RATE

F. Introduction

Textile items used by industry, the military, and the individualconstmer are subjected to various 'mpp ex stresses. It has been recog-nized for some time that the speed at v4.ich stresses are applied -mist beconsidered in evaluating such textile items, Eraluation of a textileitem under Inst. in conditions of 50-200 w/minute way not give accuirateinformation concerning the breaking strength of an auto seat belt subjectedto strain rates several decades greater. For this reason, scientificpersonnel in industrial, university and government laboratories have

*devsed and utilized various instix-ments such as the falling weight,pneumatic or hydraulic piston devices, and even devices utilizing impactdirectly or indirectly with a missile to attain the higher strain ratescharacteristic of the end use applications. At the U. S. Army NatickLaboratories the strain rates used most frequently have been 100 %/minuteand 288,000 %/minute.

Generalizations may be made concerning the changes in the mechanicalproperties of a textile yarn accompanying this three decade rise in strainrate. The breaking strength rises, the breaking elongation decreases, themodulus increases and the work to rupvure may increase, decrease or staythe same with increased rate of testing. However, problems arise, someof which temd to invalidate this generall•y accepted behavior. For e ample,the strain rate of 288,000 %/minute may still be too low to duplicate thecondit2ons operating at strain rates characteristic of ballistic impact.Or the 6train history may not be at a simple constant rate as obtained-ith a pneumtic tester but may be a more complex strain history as isoperating in the opening of P =a!achute. These two problems, althoughbeiAg addressed, axe far from solved. This report is concerned with athird problem; that of complexity of structure as it interacts with strainrate.

Comple-ity itself has been studied in the past. notably by Tre•loarand Riding( 1 ), who derived relationships for the stress-strain propertiesof yarns based on their structural geomet•-and the known stress-strainbehavrior of component filaments. Treloax also used a similar approachto predict plied yarn behavior, and Rlding(3) checked this theory experi-mentally, obtaining satisfactory agreement.

SPlatt(4), Kilby(5), Symes( 6 ), and others hav%'e also analyzed the manystractural factors involved in yarn construction. L- the p-esent study,the influence of ply and twist are exmmined with the empha-is on commonlyused amounts of these.

.n Ecrmen-ta

A. Materials

A variety of yarns were used to make various comparisons in thisstudy. The identification of these is as follows:

Fiber Designation Denier Producer

:iigh Modulus X-500, Type 1 1120 MonsantoPolyamide

High Modulus X-500, Type 3 1125 MonsantoPo3yamide

Glass E Glass 2400 United Merchants &Manufacturers

Polyamide Nylon 6, Bal-lO 1050 Allied

Polyamide Nylon 66, Type 728 840 DuPont

Polyamide Nylon 66, Type 330 210 DuPont

Polypeptide Silk - Size F 1170 A. H. Rice

Polypeptide JXB-I 156 Gulf South ResearchInstitute

Polypeptide JES-2 157 Gulf South ResearchInstitute

The nylon 66 yarns obtained from E. I. duPont deNemours & Co., were210/34 single ply. They were plied to various levels (2, 3, 5 and 7),with various amounts of twist (2, 5 and 9 t.p.i.). This provided a yarnseries with gradually increasing complexity which could be examined foreffects of either plying, twisting or a combination of both. The processingfor this series was performed at the Lowell Technological Institute, Lowell,Massachusetts.

The silk is a three-ply sewing yarn made from mulberry silk by the. H. Rice Co. It was tested both in its final form and in implied single

st-rands.

The two polypeptides from Gulf South, the nylon 6 and the X-500 areexperimental developments currently under investigation at the U. S. ArmyNatick laboratoriks.

2

The fibrousglass yarr. was extracted from 20, 0C0 denier roving, whichhad been woven into an expe-rimental fabric by United MIerchaL-ts and

* Maniufacturers.

X-500 is a high modulus aromatic pclyamide available in th•-ee basic* types, with variations of each type. Those used in this sltudy were Type-

1, which has the highest modulus, highest stre gth eand lowest elongation,and Type-3, with the lowest moduilus, lowest strength and highestelongation.

B. Methods

Tensile properties, (breaking strength, elongation, work-to-break,knot strength, and modulus), were measured at conventional test rates onan Instron Tester, and at impact rates with a -PTJS* impact Tester. TheLnstron Tester speed was 5 inches/minute which res-.;lted in a strain rateon the test specimen of 100 %/minute. High speed tests were performedat 20 feet/second for a strain rate of 288,000 %/minute.

The FRITS Tester is a pneumatic piston type instrument which utilizescompressed nitrogen as a source of power. A piezoelectric crystal loadcell transmits forces to an oscilloscope readout. A displacement-timesignal is also provided by means of a pre-marked magnetic tape. Mhoto-graphs of the oscilloscope screen are made to permanently record each test.

Twisting of X-500 and nylon 6 yarns was performed at NIABS with aSuter Twister under constant tension of 30 grams.

SIII. Results and Discussion

Translational effects in manufactured textile materials have beenrecognized for many years. The tensile strength of the finished item israrely equal to the sum of its component yarn or fiber strengths. Theelongation, on the other hand, is usually greater than that of thecomponents due to additional plying, twisting, or weaving, which intro-duced additional "structiiral" elongation.

Though the efficiency writh -Which an item trarxlates mechanicalproperties may be dependent upon the type of fiber used, it is alsodependent upon structural geometry. "When a material is stressed, eiiher

axially or transversely, there is a natural tendency for the componentmembers to orient in the direction of load, and to reinforce one another

in resisting the load. The extent to which this reorientation takes

*Made by Fabric Research Laboratories, Dedham, Massachusetts

3

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anll its yarns oriented in the direction of load, will translate strengthbet.ter than a braided cord, whose component yarns i estrain each other froma perfectly axial orientation; and (2) The speed with which the load isapplied. Internal readjustment is a time dependent process which can onlyreach the optimum point if loading takes place very slowly. As loads are•Lpplied more and more rapidly, there is less time for total adjustment!o take place, mnd efficiency losses are noted.

These rate-depend-nt effects make the study of translational efficiencyunder rapid loading rates particularly important when characterizingmaterials for impact applications.

A. Complexity by Ply :md Twist

Figure 1, shows generalized stress-strain curves for a typicalfiber loaded at a conventional (slow) speed and a speed approximately3000 times faster. E&pressing the response of this material as the ratioof high speed to low speed output, it can be seen that the stress ratio isapproximately one. Computing rat- in this manner gives an index of thepercent ouitput for a particular narameter at high speed compared to thatat low. For convenience, in the. following discussion these indices willbe referred to as "impact performance ratios." Impact performance ratioswere examined for the strength, elongation, and energy outputs of a seriesof nylon yarns. A gradual build-up in complexity of structure within theseries was obtained through the addition of ply and twist. Thus, trendsin impact performance could be observed and related to Vhe variouscomplexity levels. Figure 2, shows st ergth imct ."rm tioplotted against twist for vario~u plied jarns, It is seen that the ratiosare all greater than one which reflects the inherent characteristics ofthe polymer to exhibit greater strength at high speed. However, thedownward trends with both ply and twist indicate theat strength translationI at high speed becomes increasingly poor with additional complexity. Theeffect of twist is more severe than that of ply especially at the higherply levels. The combined effects of ply and twist are illustrated inFigure 3, with the impact performance ratio plotted against (ply x twist)eas a complexity factor. This further depicts an overall downward trendin strength impact performance with increasing complexity.

The inherent tendency of the basic polymer to exhibit decreased' 2Lo.gation at high strain rate is observed in Figure 4 where impact.lezrformance ratios for elongation are all less than one. Here again,Aecreases are noted in impact performance vith increases in both ply andtwist. A downward trend with complexity is also noted in Figure 5. Thisindicates that although structural elongation is increasing with the

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e

addition of ply and twist, it is not reflected in the ability of the yarnsto elongate at high st~rain rate. Internal stictlzral reinforcement when

loads are applied instantaneously canot progresL to the same levels thatcan be attained at conventional speeds.

The combined losses for strength and elongation are shown in Figures6 and 7 where energy impact performance ratios are plotted. It is seenthat at the highest complexity level, the material at high speed trans-lates just over 50% of the energy capacity e.hibited at low speed (Figure7).

Thes'i effects may apply to problems in the field of aerial uelivery.Due to the complexity of suspension cords, tie down straps, knotted joints,ard splices one gets an impression frcm low and quasi static test data ofgreater energy absorption, strength, and elongation than is actuallyavailable under actual use conditions. Thus, materials of this typefurnish an excellent example of the interaction of comp.exity and speedof testing. Qgestions then arise as to the application of this cMlexity-rate of testing interaction to other areas. One of these areas, ballisticimpact, has previously been investigated from the viewpoint of rate oftesting, but never with the added consideration of complexity.

Yu general, it is known that textile yarns and fibers with goodmechanical properties, namely high strength and high modulus and withsufficient ductility even under static iesting conditions, are primecandidates for the preparation of felts and fabrics with optImm ballisticproperties. However, occasionally a fiber or yarn with good utatleproperties performs poorly ballictically, or one with fair or poormechanical properties under static conditions performs better thanexpected. The simplest explanation would appear to be the rheologicalbehavior of the material; the fact that processes of relaxation which canoccur under a slow speed test are no longer able to occur at the veryhigh speeds of test characteristic of a ballistic impact. In some cases,this explanation suffices and it may only be necessary to raise thetesting rate a few decades to see the improved or degraded prcperties,

However, observation of the c-mplexity of the structure. governingthe behavior of felted or woven fibers 6nows that a simple tensile testmay be insufficient to predict or elucidate the bballistic performanceof a new material. As a specific case, a new series of high mod-luspo-yamide fibers with the designation X-500 bec-me available frmMonsanto Chemical Compemy. wo types examined had the following properties:

Breaking Stress (gpd) Breaking ELongation () o us

Type I 14 2.5-4.0 550

9jpe lii 6.5 20 150

6

ballistic limit value with the 17-s:-&in fage simulator Of 1070 f~t/secas compared to greater than 1225 ft/sec fo. nylon, 66 falbric plied to the

sam aral enity T'e bllsti liit.Nfl-,e can be described as theZ- vlociy wih wich misile oul 'hae a 50~ chance of penetreting the

targt mterall Tofurherinvstiatethis poor brallist-ic behavior, ayar copleityanaysi wa pefo~medonthe L-500. 11"pe I material in

K ~ ~ ~ 0 tnepand two pl8yrn t wpas Figr-le testows at raiia twst levels from110 t~oi.to"81 t~~i Fi-jae slhow amarked decre-iase i.n the strength

of the plied and unplied yarns as a Pznctlon of t-wis-t e-en at the staticrate of testing (100%/min). Ltn contrast, a nyl-on 6 yarn shows a barelyperceptible drop in strength with increasing ttwist. The nylon 6 in fabricform is knomn to give a higher ba-llist-ic -Tritac . Thact loading of'the X.-500 further reflects the eomple-dty eff-ct" disc-issed e-rir hrthe losses due to increased twist a--e accentujated at th-e high3 speed. T-heloss observied (Figure 8), is pairticularly severe, with onl~y 25% of' itsoriginal strength zetained at 8 turns per incSh. This behavicr illustratesthe extreme sensitivity of --his material to structural influences, whichhelps to explain its poor ballistiic performance.

Amaterial this structur-e-sensitive woud most likely be verysusceptible to failure at stress conc~entration points such as the yarncrossovers in a woven fabric. Observation of experimental ballis-ticpanels supports this idea. The hole formed by penetration of' th missileinto the kabric is square in many cases for X-5C/0, indicaatinig that theyarns were cleanly sheared off' along the c-rosso-vers on t e o.er edge,,of' the failure, This is in contrast tc! more irandom failure patte-ras -nxnyon ballistic fabrics. Clearly the energy at. the Point of impact ofthe missile is not being t'ranslated beyond the nearest yarnn crossovers.

B. Complexity by Xnots

The distribution of stresses abot themsi nablitcimpact is a complex subject, rr.Nt to be treated in depth in this repoz-t.Considering it briefly., however, exainetion of layered fabric parnels-v-ich hwve been tested at their V-50 tal2.istic himit speed, indicates thata combination of fiber,, yarn. and weave properties c-ontribute to theover-all resistance of the complete layered azseiibly. 1It is not impliedthat the same effect to be discussed will curat, impact speeds con-.siderably greater or less than the V-50 speed. Initial penetration of thefirst layer is normally a concen-trated raptewihnl veary localizeresistance from the contacted yarns. -FaiULres at this p-oint, avpeazr to bein shear,, with thie level of' failure highl'y dependen~t apo modi~LL-s, orstiffness, as well as conctntrations of be-nding str-esses at. crossovrerpoints. As the missile proceeds throagh thce laysered panel, a de-partuare

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FIGeS IMPACT PERFORMANCE RATIO VS COMPLEXITY FOR FIGS IMPACT PERFORMANCE RATIO VS TWIST FOR ENERGYELONGATION OF NYLON 66 YARMSAT HIGH oDLtOW CAPACITY OF NYLON QS YARNS Af HIGH AND LOW

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FIGe? IPACT RATIO VS COMPEXITY FOR rig II STRENGTH LOSSES W,1TH INCREASED TWISTENERGY CAPACITY OF NYLON 46TARNS AT Hl"4 AND FOR NYLON 6 AND X-500 YARNS

LOW STIIN RATIS

8

fr om this initial shattering effect takes plac-. in the l--t layer-yig. of the yarns seemser

renetrated, a drawirg of the yarns seems to take place, which indicatesa more longitudinal or tenuile Y rresponse to the applisd load. Muchof the resistance at this point is provided by the tenacity, elasticity,and toughness characteristics of the material. The stress is also dis-persed over a wider area, allowing for 'abric constructional features tocontribute to the resistance of forces. At present, the combined effectsof shear, elasticity, weave, etc., can only be evmluated by actual missilefirings on layered panel specimens. However, the isolation of these andmany other effects in the laboratory contributes greatly toward theevaluation of candidate materials for ballistic applications. Standardlaboratory tensile tests at high speeds provide information related to thequasi-longitudinal responses in ballistic resistance. The shear-likeresponses related to the initial missile penetration, however, have beenmore difficult to assess in the laboratory. Methods based on theevaluation of tensile moduli or uave velocity, such as the Pulse Propo-gation Meter, have at times proved useful in understanding impact orballistic behavior. In other instances (i.e., X-500) the high modulusand high wave velocity are inconsistent with the poor ballistic perform-ance exhibited and one is inclined to search for another method ofevaluation. One method which combines material stiffness characteristics,stress concentration, and complexity effects is the simple knot test."This test provides a means for evaluating a wide variety of materialseasily aad quickly. Figure 9 shows the percent of normal strength retainedby knots under impact loading, as a function of high speed elongation.It is seen that the X-500 yarn, Type I, which displayed such poorballistic resistance, had a strength retention of only 12% while nylon 6and nylon 66 which possess relatively good ballistic resistance, retain

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9

62"11 of thieir strength when knot~ted. Th-,e silk, which also retains strengt~hwe.',- in -the knotted form, possesses good ballistic resistance. Thep -otein fibers shown are experimental and have not been evaluated

ballistically. Another e3mVple is that of glass. The knot strengthret;etion was 30%. It is known that glass yarns in fabric form exhibit

very low ballistic resistance and, therefore, glass with a reasonablyhigh strength (10 g/d), and a high modulus (300 g/d), is limited by itslack of ductility. This plot suggests the simple expedient of determining-lorgation and forgetting the knot strength test. However, it is inter-esting to note that Ldiy 10-15% elonge~tion is needed to insure goodperfcrmance in the knot test, and that additional elongation to break is"Lnnecessary. This feature of the curve may explain the good ballisticperformances of a low elongation materiEl (polyvinyl alcohol)(7) and therelatively, or performance of a very high elongation material (poly-propylene )(9.

X-500 has a good potential for impact resistance because of its highmodulus and accompanying high wave propogation velocity. It is the lackof ductility and translational ability that nullifies its usefulness inballistic applications, In Figure 9, an X-500 yarn with greater ductilityis represented. This yarn, Type III, was able to retain 57% of itsbreaking strength as compared to only 12% for Type I. Type III alsoexhibited mrch improved ballistic resistance (1244 ft/sec).

IV. Conclusions

Translational efficiency at high speed mrast be carefully consideredin tensile or ballistic applications. The usual tensile strength andelongation changes with strain rate are altered in the case of complexstructures. The strength Impact Performance Ratio, which is normallyexpected to be greater than one, is shown to steadily decrease withcomplexity. This trend can be carried to the point where complex braidedcords used in parachutes actually have strength I.P.R.'s less than one,and #,nergy I.P.R.'s as low as 0.5.

Increased stiffness and elasticity are advantageous only to certainlimits in ballistic resistance. High modulus materials respond betterballistically, but only to the point where materials are too stiff totranslate shear energy effectively. More extensible materials seem totranslate more efficiently, but beyond a fairly low elongation level,extensibility has no further effect on traaslation. The proper balance-if modulus, extensibility, and strength in a fiber seems to be the keyz improved impact performance.

10

V. References

1. Treloar, L. R. G. and G. Riding, A Theory of the Stre-SrainProperties. of Continuous Filament Yarns. Journal cf the Textile lnstit"te54, 156 -170, 1963.

2. Treloar, L. R. G., The Stress-Strain Properties of Mu4lti-plyCords: Part I - Theory. Journal of the Textile Institute 56, h770.:3,1965.

3. Riding, G., The Stress-ctrain Properties of 14:lti-piy Cords:Part II - Experimental. Joarnal of zhe Text±£pe Lnstitute 56, 489-497,1965.

4. Platt, M. M., W, G. Klein and W. J. Hamdburger. Mechanics ofElastic Performance of Textile Materials. Part IX - Factors AffecttingTranslation of Certain Mechanical Properties of Cordage Fibers intcCordage Yarns. Textile Research Journal 22, 641-667. Supplement 827-829, 1952.

5. Kilby, W. F., The Retraction of Twisted Continuous FilamentYarns. Journal of the Textile Institute 50, T673-687, 1939.

6. Symes. W. S., The Prediction of Cord Properties. Journal of theTextile Institute 50, 241-248, 1959.

7. laible, R. and H. Morgan, The Viscoelastic Behavior of Orie-itLQPolyvinyl Alcohol Fibers at large Strains. J. Polymer Science 54, 563, 1961.

8. Iaible, R., The Mechanical Properties of Polypropylenre as Relatedto Ballistic Applications. Symposium on Polypropylene Fibers, Birmingham,Alabama. September 17, 1964.

11@

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