AD-A018 958

NEW MATERIALS AND CONSTRUCTION FOR IMPROVED HELMETS

Anthony L. Alesi , et al

Army Materials and Mechanics Research CenterWatertown, Massachusetts

November 1975

DISTRIBUTED BY:

National Technlcal Info mtion SmitecU. S. DEPARTMENT OF COMMERCE

009124

AMMRC MS 75-9 IADI

NEW MATERIALS AND CONSTRUCTION

FOR IMPROVED HELMETS

SANTHONY L. ALESI, RICHARD P. AMES, ROGER A. GAGNE,

ALAN M. LITMAN, and JOSEPH J. PRIFTI

November 1975

Approved for public release; distribution unlimited.

ARMY MATERIALS AND MECHANICS RESEARCH CENTERWatertown, Massachusetts 02172

NAI- 1 \ ( HNICA1I .)N ',[ \"ICE

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I6 SUPPLEMENTARY NOTES

Presented at \rmy Science Conference, West Point, N'', 19-2,J'ir:o fl74.

19 KEY WORDS (Continue on reverse tide if nec..er% ri and identisl, bv block number)

Atitoc 1 avi ng

A\rmnorFabric laminates

Ile linets

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(SEE REVERSE SIDE)

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ABSTRACT

The feasibility of utilizing several new armor materials in the de-velopmen: of combat helmets was demonstrated on the basis of laboratoryte!sts for fragment protection capabilities and for durability. The mate-rials considered were (1) phenolic/polyvinyl butyral bonded fabric ofKevlar, a high-strength, high-modulus aromatic polyam:de fiber; (2) a rigidlaminate of XP, a highly stretched polyolefin film; (3) polyester bondedglass fabric of LMLD, a low-modulus, low-density glass fiber; and (4) acomposite system of XP encapsulated between laminated skins of polyesterbonded glass fabric (GRP) and Kevlar fabric. Molding procedures weredeveloped to obtain durable constructions without undue sacrifice of bal-listic penetration resistance capabilities. The M-I helmet shapes weremolded from all these materials except For thc LMILID, the proce.;sing forwhich is the same as for ordinary fiberglass.

These materials offer substantially improved proteztion capabilitiesagainst the broad weight range of anti-personnel munition fragments. Interms of" V-;0 ballistic limit velocities obtained with four test projec-tiles, these materials surpass that in the standard M-I helmet by up to 79percent. The Keviar system has the most promising combination of superiorperformance and a minimum of problems for combat helmet development.(Aut ho3rs

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CONTENTS

Page

INTROIDIUCTION ................ ........................... .

F PROBLEMtS IN USING NIW AN\TIERIALS FOR ARIIOR

A. X1 Film ......... ........................ .I...

B. Kevlar Fiber ............. ..................... .

t.•IOLDINt; OF TEST LAMINATES AND HELMETS

A. XP .............. ...........................

B. Kevlar ..................... ..................... 3

C. (GRP/XP/Kevlar Composite ..... ................ .S..

TESTING PROCEDURES AND RESULTS

A. Ballistic Penetration Resistance .............. .S... 5

B. Durability (Permanence) Tests ....... ............. 6

C. Peel Resistance.. ........ .................... 8

DISCUSSION OF RESULTS

A. Protective Capabilities. . . . . . . . ........ 8SB. XP Itelmet . . . . . . . . . . . . . . . . . . . . . . . 9

C. Kevlar llelmet .... . .......... 9

D. G;RPlXP/Kcvlar Composite Helmet ............ 9

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . 10

*,ACK 4NUOWL[ENIX;, l'l.. . . . . . . . . . . . . . . . . . . . . . . . . . 11

ti

-1/

The findings in this report are not to be construed asen official Department of the Army poaition, unless sodesignated by other euthorised documents.

Mention of any trode nomes or manufacturers in this reportshell not be construed as advertising nor as en officialindoraeuemt or approval of such products or companies bythe United .tates Government.

DISIMITIO( INS?5UCTIONS

setre thie report wse tit is so oo~ger sei4s.Lo sot retire it to the ortlisetor.

;

INTRODUCTION

Three new materials are now simultaneously available for exploitation inpersonnel armor. They are (1) XP, a highly oriented polypropylene film; (2)LMLD - a low-modulus, low-density glass fiber; and (3) Kevlar - a high-modulus,high-strength aromatic polvamide fiber. All three are superior to the older armormaterials in protection capabilities against munition fragments. 1

The label "new" often signifies that a material has not been completelycharacterized and that actual use experience in product form is limited or non-existent. Further, if the new material has unusual properties, properties muchdifferent from prior materials, then its behavior may not be well understood, itmay be difficult to process into desired forms and constructions, and its per-formance in the end product may not be as expected. Both XP and Kevlar fit intothis category of new material. LN4LD, on the other hand, and E glass fiber(crdinary fiberglass now used in armor) do not differ in properties tremendously.Both combine with the same polyester resins to make bonded fabric armor. Pro-cessing and fabrication for both are alike.

This report will consider only the XP and Kevlar materials and some problemsposed by their "newness" and unusual characteristics in their application tohelmets. The preparation of flat laminates and helmet constructions and theirtesting in several ways for "durability"' and for fragment penetration resistancecapabilities will be presented. The s-uitability of these materials and construc-tions thereof for '.,imets will be evaluated. Remaining problems to successfulapplication will be identified.

PROBLEMS IN USING NEW MATERIALS FOR ARMOR

A. XP Film

XP Is the first armor material in the form of a thin 6ilm. It is preparedfrom flat tubalar polypropylene film by hot-stretching to bout V2 times itsoriginal length and to a tube thickness of about 0.0015 inh. 2 Stretching changesthe tensile strength from 4,3010 to about 50,000 psi in the stretch direction andto practically no strength (40 psi) in the transverse direc ion. Thlis Ihighlyorthotropic strength characteristic can be a problem in hand'ing and assemblingseveral hundred plies into armor since It splits easi.ly. Al!.), it require. thatalterntate plies be cross-plied to be effective armor. Cross-plying is accomplishedin a practical manner by the application of filament winding techniques. htight-inch-wide film is wound at 45 degrees in alternate directions and a pad obtainedby slitting the wound material longitudinally. Pads as large as 9x21 feet havebeen made. Pads may be secured by stitching around the perimeter and in horizontaland vertical rows at intervals. Film pad.,s are converted to a rigid laminate by thesimultaneous application of heat and pressure fol lowed by cooling under pressure.No adhesive or external bonding agent is used between the plies.

I., MASIANICA, I . S. H0l11014 1-hmthto.je t is/ htwtkhi Imvw - IV•.. Atlmy Mwaul% And NW ,ltdn , Hk,%,kr ('Onitot, AMMKCVIA IA-47, Novvtnilvlt 197'l , Co•nlldtastul HIx'ift

1. IKILI, S. A, Sctt, I Up andI (e•tuht ;.,oon I Nts ,n4itr '01, fli-,.l,' ,,l"1W .iIhgtletfh VAP ptItjhP 5illlu ilh' ('ttfl l it'tph lle , 0tIe VI04A(46,?2.t404, I renl Rvikitl. A%14k(' 1 ?-,5. Alptil 1174 , (inltdetlul vIj'ttlKI, .51 A. lelii'lt'1,tw 010lih I 1" ths , ) dn Ii s Rigied 4Irnow .ae.itie ' P. PhllleP VA'MI I sk: ('uslketJtl|sciellon. Co.ni

O)•AA(46,?| .('4W1,.•, hmail HquelW~, AIMUR(* ("I R 74.1-1, 1 qtituary' 1 4. 0 1w etual H

is nrm.a 11v made , nolided XP has a tendency to dc laminate partially whenstevert.lv flu.cd or ;ub Jected to large temperature changes. Such behlavior raisesdoulbt>, as to the durabil1ity of an XP helmet in the field. Yet some capabil ity'tol' delaniinat ion under ballistic impact is necessary for a high degree of resis-tance to penetration. Complete fusion into a solid block will produce poor armor.Reconcil1ini, these opposing requirements is the problem.

ll~k litp1ro'0ichve were taken in achieving a stable rigid structure withoutu~ndue loss iii protect ive capability. The first sought to determine whether theproblem could be ameliorated to a satisfactory dlegree by careful selection ofthe moldinig temiperatuLre and pressure. The second encapsulates a pad of XP filmbetween rigid skins of resin-bonded fabric laminates molded together at the edgesto achieve a rigid, structure that would be durable and equivalent to XP in pro-tect i' e c apab iIi tieus .

B. Keviar Fiber

Kev lar aromatic polyamide at present is made as three fibers having differ-ent, streng~th properties that go up to 400,000 psi in tensile strength and 19x10 6

psi inl m1odlu11s.L Fakbrics made from these fibers differ a little in ballistic re-sistancev; th%, better of these, IKevlar 29, is usually selected for armor applications.

ot Nrlaboratordies resins for Kevlar fabric armor have been examined Iy .\N1N'i andot~wrlaboatores. he consensus has selected thle same phenol ic/polyvvinyl

butyral I es inl USed for the nylon fabric helmet in~zr . Ilowever , the interply buind;t ren,,th tis inp this resin is not always adequate. Molding temperaturc and pres-sure andý. provuring of the prepregged (resin impregnated) fabric were briefly in-

a-tst ael ui'41 ig flalt panels before h~elmet-; were molded. Peel strength was usedasthe %criterion fur selecting the precure and molding conditions.

MOLDING OF TEST IAMINATES AND HELMETS

A. XP

'ra or i~oi* ind icated that temperature alont, of the molding cycle variablescould approeial ly alter ballistic limits oif laminated P.a w Iith increasingmolditl ri tenpcrauresr beyond about 32S F~, balli st ic limit decrea-sed . However Itotv:;t, had Iwvti ruit to determine the .ýffVect of' molding temperaturv kill thle sliscup-tihi litý. tit delamiwite. A scries of ,Xl 2-mic test panlels weighling .10 o/qft

wrcmade in a Ito Id ait temperatures 6etween 515 V~ and 3(41 F (all molded for 20tni nutv it I00Il 11:ki1 sih tile Phtillips 1;cintifi c Corp~orat ton And teýsted at AX'1RCfor hall i .t iv limit ve eloctieos. A dupili cte set was cut into 0'M, iiwh panel%~ andtestcd f'ol de lainiaxuat ion by temperature and humidi ty cycl ing and by water immers ion.

\A kofid *e~ sof test panels , 0. inches , weighing, % oz!stq ft , were moldedAt ANP-11W .~ thle sarie tomipe rature range uising Polished mletal pl ate vsWith C).01) -waal 1"t I on beevt N lictuevil the X11 and tile metal p hutes to prevent adhesion. A

4 \I I \. ~ I *I~ Jý , '1 0tt. lijpttlaon tit . At Ia#Ij I oAkin tit *eeno *'eb Nino fl, % \1111 %oa~loW, ,114 Mt. h *o,,1,W tl

thermocouple was inserted at mid-thickness in a corner to measure the molding tem-perature accurately inside the laminate rather than relying on a platen temperature.Molding at temperature was conducted for 30 minutes-sufficient to provide a minimumof 10 minutes at the indicated temperature for the center of the panel. Panels werecooled under pressure to an internal temperature of 100 F. The molding pressurewas raised to about 2500 psi since higher pressures of this magnitude were neededfor helmet molding and since partial internal delamination was observed on thermalstressing of the first series.

With one exception in each series, the A*O1C panels were translucent as .Fhownin Figure 1 while the Phillips panels were striated, white, and opaque. Whenmolded at the highest temperatures (356 to 360 F). both were yellow and hazy.The difference in appearance between the two sets of panels is attributed to thebetter removal of air between film plies at the higher pressure. Excessive flowof XP material occurred at the 356 F molding temperature as indicated by the flashthat squeezed out around the edges.

Figure 1. Relative translucency of XP laminates.Left to right: Phillips panels molded at 1000 psifor 20 minutes at 360 F and 315 F; A.MMRCpanel molded at 2250 psi for 30 minutes at 347 F.

XP helmets were made using an NI-i helmet-shape, production-quality compressionmold in a 300-ton press at the U.S. Arny Natick Laboratories. This t.old produced ahelmet with a wall thickness of 0.2 inch that weighed about one pound. Although a3-pound helmet was desired, this was the only compression mold available for demon-strating the molding process, Full press tonnage and the maximum temperature of320 F z'btainable from the steam supply was used. XP helmet proforms were putinto the hot mold and pressed for 20 minutes, then cooled to room temperatureu,4er pressure. The size of the preform had t& be made smaller than was indicatedby helmet dimensions. The shear cut-off on the mold that operated satisfactorilywhen nylon fabric helmets were molded was unable to cut off any excess XP so thatthe mold did not fully close. The removal of about 1 to 1-1/2 inches from tieperimeter of the preform corrected this. The XP flowed to fill the mold and pro-duced a helmet with a finished rim.

Two styles of preforms were tried, an oval-shaped one and a "pinwheel" typeshown in Figure 2 with either overlapping or butted "vanes." Sound helmets, thatis, helmets bonded throughout, were obtained with both preform styles. One of theXP helmets is shown in Figure 3 together with helmets molded from the othermaterials.

B. KevIar

Flat laminates consisting of 23 plies of Kevlar 29, 11 oz/sq yd plain weavefabric, preimprognated with phenolic/polyvinyl butyral resin (221) were prepared

Sanne • • mam • •

- no , UI d n~udr aton:, a t 31

:i ~w~te it I l lewcd hy cot;l Ing priorIt rtý;w'a I from the prc-ýs T est papeis,

i!1cht-, Vexe moldcd for hallIist ic!n, iald OX(' inIch panels were cut from

I t' 2 jfln'I paniel -~ fo ther tests, Xli1'.me l, had a .;(, eo " sj ft areal1 densi tv.

le imi t coti',;truct iens of* the standardhelmet shape were prepared 11v conven-

t ma 1 ag m dinrg p rocedure nannt0eIiv ae t 33( 1.* and 25S0 ps i pre' sure forI'P111 inteVS. The moiW %%as at s ii cone

e:'oxv' meidd thait v'a,ý fabric;'ted using -ia t e cat rkade of the mair el halif of the

VP helmect c-prssnmold. The mold %%asFigyirt 2 ''PiT ~~hs'I' pifrrmade so sice 1 x' could 11v in'.erted

o' +t a in it %s i red waIlI th tticns st;s Ti,,oKevaz'~ ~us r~~ (s v0 I vd. O uth '11 23 :, I ivso'S 11 yivJ :,Ia in tweav'e ta-bric,

and thev other of 1I' it. v t ,' 11 it, Sk'?)%, rev I ng wevave tahri: c obtn u were'%%nrgntedwth rheniol i, col pe , I h1;t i' I rv' i n and -'.5 vvrcent, rcespec -

tively). Pi Iiwheel I'r''~-~ tind Iit- an ovi. vwetlap pat te? V1.1 t 11ui hle m1old,the mold1 putitift nto I -tit)v "i 1 1 -'~ I~ C,. hag,. andk t hu i'dig c~~ted I sh1own. i 11F i g tre ,,1 ro r t o cui' , ity 01V the t 1 'lit-fi. I m. t. mat he helme t 1Nad vst 23 -

It i red t h I.Le coni Ve It I '.' Iu It : I u nie t ach ivet it 'filot h, w r i k Iv.tive maodcIt% w tIth v, Io,? v, rP 1 1O tv ' v4-t .h~' IJ Iokewt,'vv r v1 p s cIwovell re v ti, it a I' , C.1, Ih * ý' it% t ,% ' nk Id iii a ~ agle operaIt I n dule totile fabricv ilt't te P dr~w i it,. iti t .Arile Iv-'r total bulK tit thu lay -up,This; he I net *in't .1 1. t 'aeI o ''rox v, -w it -. 'v fr a o vv I rv s t ain e t v ~t IwvreMol~ded aIt u I* to *b'~w Ivk ! k't 111, *,4 V ri I' 1'3 t, In li Iiht th %t V tn : Ie ad W1I%xt~ol ofthe %' I I it Tth It TI U% t 10 letI 4,.'i Ji'd .1boit rond

Kýv lar- :fari c'ý ank, liininaitV~-ý could 110the c,,,t c ,leanl ,Y'' ar,%. ot thc u-tia 1 meaiis-scis-or, oi, eor kniv,-' for arc and

han s-, r - i rcul I sat- for lair inates

C . GRP,'XP!Kevlar Composite

sulat 1h2lg It) o.:, ft \P Cli r l , t'.( inc~ws,'tt~eL'f -1 nliL of" t'sx10-inch convvnt jonai

n-c WIINR and. t, "I i eS Ot SX1 0nc :M& l '10 et.er~it~iqrciited k.cvlar 21) sat in %%vavL' fibri c(5 s: 'q yd .This co?-Ipos itL i v~ urve inl

tile auitoclave at 2WIT, tui~der, 7ý ;1,ssl Figure 4. Vacuum-bagged helmetfor 90 mi nutk-si Io n cr 111, v'actlui preform in mold.bag molidi ng ttchn tiqes , tindrn~ thý. vondi~ j- 9E 4ACtions tht X \f'ti in rl ics , a'ý .'x' -cte, 1'cmt!1.ýd

ii s tintlar inaliler. M-1 h m t- ha Cd 11110'.ltS 1 '.V T. M&t i tilt S~aF,.no 1 .. ntit i led t'0r t he keylav heii' In.. . ieCor e M, wa t am-n.: 1)" 111'0 pi ov reS '%

mA Itiý thv rtnnvr 1 iL' es na I iet, r tit the latvge'.t p1) vd.ikhg an i 110 ftor thlehi, oe mt edge so that t hit ("Al av-d Kteý I at' honded t ogk-thi. t o fortt 1 1 .1 iU %doubi e -w I Ithel I I va led it tht: ri im. \Jmin .I liand laiy-Lp pitnwheel ovk-rla;prcde tor'asseilhl I fg ?til Mat crkai s 41 thitIn th 1wMold tkas ump 10yed. 'The hvlnvtý, 'khledaout

3powtulj each.

TESTING PROCEDUR~ES AND RESULTS

A. e Ba al It ic Penetrdti4to V Rithne C lee rtrieia .i'.koitI

i 1 . 'irwka.' uth i fotti re ~e!t tit dift'ereiit mlsvii rvproenoftai e wýt0k, r flvnt litliý initli!Ii n thrvat (under 1011 gra ins.) to coV.t'at i'toilfd troops, t hetv'. t vr' u' It;t i I v1- hkerelk otli~vnt Io , I \'tyv franvitet -k timila t i n.1 pro ivc t i It- 1 ' o tifk ,ili

'It ir ~ I~irl '~ I~ .e'.2* Iolhý Ii vi 'I c'ý t \kle Ith vi or I'vr% tumel1 \rtrir ý!.La crti a11tv. tvt I '!ernitivi l"11 1 1,4t I I )-t v% 1o,:ities~. \ k ewot itw the vtý *v I It, .vel IlIt" *,*e rv tht " .~ t t~'v ~ lt-ý of, -, -tit, t f ~t iOl 4 0-1 ;4"nd %:II cI 1 tetd 1' .ýyt ~

Vltia nu--'v 4 olt u.'~ I I ilh ili At rmlgiý~ of' 1-", 4 'sl% t hAt rei11T l it rmipa tia I.01A 0t*Ik!v ývfw-ntl Il lo l. alh ii; ti dta itwu4 tw icqtiirtd '..t th rut I~l

i4'i ~li x i~' ~rt' ~tile 'iti* I k!4' re-;smt to 1 pcot rat ion rj~ortvi 1wrhi'v'in

I ~i ~ ~~ I i"~ ~elt in V JIt es f'or t he tivu njl eril M-cIrw 4 t he i .uida rditk- ~tvt i -mih'l 4 all" la-ýt 01.1ted llyloti I'ai'ra it: hci ts' gt~vt't tit

1 1; It~ I . It r%, idl I,, '111a re thi it atIl i'our mAiteri ~ai , exhib it i iii,- tatit 4til I v

higher resistance to penetration than the M-1 helmet system. Kevlar and XP arebest, closely followed by the GRP/XP/Kevlar composite. Their improvement over theM-1 helmet system is in the order of 30 to 80 percent.

2. Variation of V50 Ballistic Limit of Laminated XP With Molding Temperature

Only one intermediate caliber projectile was used. Figure 5 shows the combineddata for the Phillips and AIMRC panels presented in terms of V50 ratios based on V5 0value obtained for the lowest temperature (3lc F). This ratio starts to declinerapidly after 345 F, indicating that the ballistic penetration resistance of lam-inated XP is being degraded at the higher temperatures.

B. Durability (Permanence) Tests

1. Temperature Cycling

All test panels were prepared with two adjacent edges cut with a hand saw toobtain unsealed edges as well as molded edges. With XP1 this procedure producedpartial delamination along the edges and some fusing of the cut surfaces.

TABLE 1. V. BALLISTIC LIMITS fr NEW HELM!" "-"LRIAL%

COMPARED TO THE STALDARO M-i '4LMET

Percent Iffurovement*

material A a C 0

EEVLAR 79 4? 6? 5FXP' 66 ]A 69 '7GRP/tP'b4VLAD 64 33 5-" 7

) ..... 1 ,it , , j,0 o t. . %I .... in

All'it it, 4k l~ 40~f Vt : ý

N + h l i i j I. P

+t

+ +L+

Figure 5 Effect of uviclamg tclflikti~tuiQ Ut V5wIsihuac laniai of tamsneledl XI'.

The panels in an environmental test chamber were cycled frc'm room temperatureto -76 F, then to 176 F and back to room temperature within 8 hours. Two suchcycles were run. Panels were inspected visually for changes after each cycle.

There here no changes in the appearance of the Kevlar and GRP/XP/Kevlarcomposite. The Phillips XP panels molded at 31S to 340 F delaminated around the cutedges to a depth of 1/8 to 3/8 inch. All panels molded at 315 to 350 F maintainedtheir integrity and rigidity but were whicer in appearance than originally and feltslightly spongy when squeezed, indicating partial delamination. The 360 F panelwarped and split partially along a curved surface of discontinuity that was visiblebefore the test started.

AVMNRC XP panels molded at 316 and 324 F turned white and split along the cutedges. The former puffed up to about 1-1/2" in thickness. The panels molded at337, 147, and 356 F showed no change.

2. Humidity Cycling

This test was ceouducted in similar fashion to the temperature cycling test.The first part of the cycle was at 95 percent relative htnnidity and 176 F; thesecond part was at the same temperature but without the high humidity.

The behavior of the Phillips XP specimens submitted to humidity cycling wassimilar to those subjected to temperature cycling. It should be noted that humid-ity cycling also involves temperature cycling (room temperature to 176 F to roomtemperature). The panels molded at 315 F ani 360 F split as before. All but thelatter felt slightly spongy when squeezed, The AMtRC panels molded at 316, 326and 338 F split with the frequency and extent of splitting decreasing with in-

X creasing temperature. The panels molded at 346 and 356 F did not change inappearance.

The Kcvlar and GRP/XP/Kevlar composite panels did not change in appearanceexcept that for the composite the Kevlar skin had puffed up away trom the XP to aheight of about 1/4 inch at the center.

3. Water Absorption

Water absorption tor all the materials except the composite was conducted inaccordance with procedures outlined in ASTM D570-63: Standard Method of Test forWater Absorption of Plastics The composite specimens consisted of Wxb-illch X1'encapsulated within 8x10-inch GRP and Kevlar skins. Cuts to expose unkealvtd edeQtswere made out3ide the X1P are.

The 24-hour water absorption values determined were:

XP NoneKevlar 2.5 PercentGRP/X!/Kev lar composite 5.8 Percent

Water could he forced out of the composite specimens at the cut edges 1yshaking or tapping. Instrction of all edges showed that the Kevlar plies could hepeeled from one) anot1,,:r with very little eff.,'t . However, the Keovlar ply adJacentto the tRw qas fis'mly bonded.

7

4. Impact Damage Rtc:;istance

To determine the relative impact resistance of M-1 shape helmet constructions,a 4-inch-diameter steel ball weighing 8 pounds was dropped from a height of S feetonto the crown of the helmet placed on a concrete floor. The amount of permanentdeformation was measured and each helmet was closely examined for delamination orother damage.

No impact damage could be detected for either Kevlar helmet construction.The GRP/XP/Kevlar composite helmet suffered no permanent deformation but carefulexamination of the inside of the helmet revealed an area below the impact point,about 2 inches in diameter, that had a slightly altered appearance resulting pre-sumably from the transient deformation of the helmet structure. Impact of the M-1Hatfield steel helmet and nylon liner system produced a dent in the metal shell7/16" deep and 1-7/8" in diameter. The X1' hclmet was also tested even though itwas only 1/3 the weight of the others and it had been wolded at a temperature toolow for adequate bonding. Accordingly, it was not surprising that upon impact adent 1-1/4 inches deep and 3-1/4 inches in diameter formed. The dented materialhad delaminated.

C. Peel Resistance

This test was conducted on Kevlar laminates only. The procedure followed wasthat of ASTNI method D1876, Peel Resistance of Adhesives (T-Peel Test), except thebonded part of the specimens was 5 instead of 0 inches. P,,-0 resistance of thelaminates made with 11 oz/sq yd plain weave fabric and molded at 330 F and with looz/sq yd woven fabric molded at 350 F was about 7 pounds per inch of width whenpressed at 250 psi but only about 5-1/2 pounds at 22'5I psi. Precuring the firstfabric (prepregged) for 8 minutes at 235 F did not significantly change the peelstrength. The peel strengths shown in Table 2 are considered adequate but nothigh for well-bonded durable laminates.

TABLL ?, PCE, P RES!STANCr Or q" VIA R LAMI:NATlS I / in)

MoldinQ

(rF) ("'50 t'cI) (p%0O 11,Ji)

Not Prtu•wev 7, 3

Yd :11 W00i4VO fhfIt, r , phuin, Ui

o0or r glfutet , i.t

DISCUSSION OFr RESULTS

A. Protective Capabilities

Thc 04 to 2") percent increase in Vo hal Ilisti e limit of1 the Ky vlar and XPlaminates and ot the comptositte over th:. '-t';ta dida " M-I helmet syste,,i when tested it fl-.the projecutile oft I oat mass iA espevc tlly significant. A mujor threat to ground

troops are detonating munitions emitting small fragments traveling at high veloc-ities. Kevlar and XP (or composite constructions having XP as the major compo-nent) helmets weighing 3 pounds (the same as the M-1 helmet system) are consideredcapable of stopping most and possibly all fragments at relatively short distancesfrom burst. The M-1 helmet system is considered ineffective under these conditions.

B. XP Helmet

Molding of XP helmets, on the basis of experience with flat laminates, requiresthe selection of molding temperature of 340 to 350 F and a pressure of at least2000 psi to insure removal of air between film plies, effect adequate bonding, andrestrict the reduction in V50 ballistic limit to under 10 percent of the maximumattainable. The temperatures measured must be that of the XP, not that of the mold.Air removal appears to be a problem, and more positive measures are required.There appears to be wide latitude in preform design for XP helmets provided pre-forms are undersized (but not underweight) and of minimum bulk to allow the moldto close since any excess XP cannot be cut by the mold. Although molded XP helmetswill require no cutting of the rim, piercing techniques with heated penetratorsmay be needed to make holes for mounting helmet suspensions without incurring localdelamination of the X11.

l.aminated X11 when properly molded will not delaminate on thermal stressing,is dimensionally stable, and does not absorb water. The ability of 3-pound XPhelmets to withstand low velocity impact as exemplified by the ball impact testremains to be determined.

Thermal stressing and exposure to varying humidity can distinguish betweenhelmets molded wider different conditions of temperature and pressure. As few astwo cycles may suffice as a test. Translucency and color can also serve as visualgu.des to adequate molding, A yellowish tinge and good translucency indicate asatisfactory molding. Opaqueness with whiteness indicates too low molding tempera-tures ind/or pressures. Very hazy yellowness indicates too high a temperature.

C. Kevlar Helmet

kevlar fabric laminates u,,ing phenolic/polyvinyl butyral resin withstandthermol stressing 'ad have low water absorption (2.5 percent in 24 hours). Amolding pressure of 250 psi and temperature of 350 F are adequate. Weavy fabricsof good drapiing characterist is make molding of helmet shapes easier. A tovenfablric ot 17 o:/sq yd. and possibly up to 25 o/"sq yd, is satisfactory, and rep-resents a lower cost fabric construction.

KvIar fabrics and laminates of low resin content, 20 to 30 percent, were notable to he cut cleanly. ltven helmets compression nnilded in imtched steel dies (asthey would lit, ii production) will require some treatment of the as-twolded fibrousedgte to olbtain a tinishod rim.

0. GRP/AP/Kevlar ComIposite HelOmt

Thl': ýonIJIUItIL t onstructiiUsn w.', obviously not water proot. as evidenced ItythIe .ic wmzation of wa,?er htett'eln the skirns. The entry point Ws a3pp3arently lIt-tivel •,cvlar pl ivs Whose, adldvin t~ zs quite poor. 'Polyester is evidently' not a

suitable bonding resin for Kevlar fabrics. Phenolic/polyvinyl butyral would be abetter choice but its use would require some inte-: .iediate bonding medium to effectthe bond with the GRP' since the phenolic resin inhibits the cure of the GRP's poly-ester. If water also enters through the thickness of the skins, surface plies orall plies having somewhat higher resin contents would reduce or prevent this.

CONCLUS IONS

1. The feasibility of utilizing the new materials, Kevlar 29 fabric lami-nated with phenolic/polyvinyl butyral resin, and laminated XP polypropylene film,in the development of combat helmets has been demonstrated on the basis of labo-ratory tests for fragment protection capabilities and for durability. Some prub-lems of secondary importance remain; obvious solutions for all but one areavailable.

2. Processing parameters have been defined for molding helmets weighing 3pounds that are durable and possess at least 90 percent of the fragment protectioncapabilities of the material.

a) XP hielmets are best molded at 345t5 F for a minimum for 30 minutesunder a minimum pressure of 2000 psi. The specified temperature is that of thelaminate and not a temperature measured somewhere in the mold metal. Proceduresshould be adjusted or modified to prevent the occlusion of air between film pliesduring moldi4 'g. The making of holes in XP helmets for mounting suspension systemsmust be conducted with care using properly selected tools and techniques to insurethat the new edges are sealed by fusion and no delamination occurs. Heated pene-trators :-ay be suitable hole-making devices.

b) There are no indications that the molding of laminated Kevlar 29 fabrichelmets has any critical requirements. A molding pressurv of 250 psi and a tem-perature of 310 F produced satisfactory helmvts. Procedures or devices for cleanilycutting the helmet rim, or for removing frayvd fibers and sealing the rim. or forbinding the rim to form a finished edge are needed.

Q) Poletr revsins are not satisfactory bonding imiterials for Kevlar fab-ric in tht GtdRP/Kkvvlar composite helmet. PIhvnolic/polyvinyl butyral resin isrecomundenlvd with th t use of a polvester-compatihie adhesive or resin-coated scrimp1v lyet.een rh, CRI' and Kevlar laminates. Resin contents of 30 percent (greaterfor stir t'a• s ,) , re rvcoilnelid.d to redtuce w•ater absorption. Hlole% through theColmiposrt. for 1mounting suspension hardware are expected to require the developmentof sea 1i ng method; to prevont the entry of watcr and possible separation ofCOMpt(lIVI t •4.

3$. S.uitablvI ty'pes of tests for the juality of molded helmets are:

j V,,. bat llt.stic limit velocity and falling hall impact for the threeconL t rluct io•";

hi 'Thvrmal tress and visual inspection of volor and translucency fur X11

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c) Water absorption for GRP/YP/Kevlar composite helmets.

d) Peel strength for Kevlar 29 helmets.

4. Laminated Kevlar 29 fabric, laminated XP film, GRP/XP/Kevlar composittand LMLD offer substantially improved protection capabilities against the broadrange of fragment masses inherent to anti-personnel munitions. In terms of V50ballistic limit velocities for the range of test projectile masses utilized, theimprovement over the standard M-1 helmet system is 42 to 79 percent for Kevlar,36 to 77 percent for XP, 33 to 75 percent for the GRP/XP/Kevlar composite, and13 to 53 percent for LMLD.

5. The development of a combat helmet can be successfully completed withany of the helmet systems examined. The Kevlar and LMLD helmet developments arethe least difficult, XP is next, and the GRP/XP/Kevlar composite is the mostdiificult. The one having the most promising combination of superior performanceand a minimum of problems in a Kevlar helmet.

ACKNOWLEDGMENT

The authors thank all who participated in the work reported on by this paper:Venie Stimpert and Douglas F. Mitchell for the molding of lmiinates and helmets;John Shippie, Elizabeth Cilley, Salvatore Favuzza, and Frank J. Maloney for thetesting; Jack F. Furrer and Thomas HI. Bresse of the U.S. Army Natick Laboratorie!:made available their helmet mold and press and aided in the molding of the XPhelmets,

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