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    PHOTOGRAYH THIS SHEET

    _____I

    LEVEL INVENTORLn. 0 A,/L - 7/ o/ - 3DOCUMENT IDENTIFICATION

    ""DITRIBUTION STATEMENT AApproved or public releasel

    Distribution UnlimitedDISTRIBUTION STATEMENT

    ,CCESSION FORTris GRA&Ir)IC TAB Q OTIONANNOUNCEDANNOUUSTIFICATION

    8OCT 24 1984

    sy DI E{ 9N p 'RTAI LABILITy CODI yp-ff AVAIL AND/ORt SPECIAL. /

    ~' DATE ACCESSIONED

    DISTRIBU7ION STAMP

    UNANNOUNCED

    84 10 23 3319DATE RECEIVED INDTIC REGISTERED OR CERTIFIED NO

    PHOTOGRAPH THIS SHEET AND RETURN TO DTIC-DDAC

    DOCUMENT PROCESSING SHEET PRZVIOU SKOITION MAY Or USED)TC~ 70A "'"' ' TOCK 15rXHA1JTr'0.

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    II

    (I for pitfl , j-i.

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    I, IAL /mar

    Laboratory (QZ)

    *1bj0ott Watertown Asrorno Lboratory Iszperientsl "eozt '-o. "10/07-3Tot chief of OrdnaneAwy seryle. TorePontago% BiildingWashington 25, D, C.

    Attn: POTB - Teohnioal Reports1. Inoloepd *re six (6) opies of eubject roportentitled OPrlnoiplte of Aror P-otection, Yourth T'V,tkeport".2. ':he follovin dlstribtin Is sueeteod

    I - Crdnance ?eObnioel Library. Index cardsare Inclosed,for Its nue.I - SPOTD Technical ReportsI - SPOTI - - /ttn Col. 1. A. 1,0-e~3. . . HTail. l.eeroh Laborr.tory

    Since it Is believed tho~t tho British Army Steff wo,4d indthis report of interest, three (j) opies are beinc retainedin seordase with directive letter 0.0. 4O0.112/7TOW - tm.35,0/607, 23 Way 1944, pending further Lnstroatione frn his

    3. IX MUOU. GOpe Of the $%%Jost report Wll %0 **Atto each of the folalvingi 8p0mfield Armory, Watervliet Aresenal,Pleatiuw AreeudA WAmUe* (3) opiee to te Ordanmeo Roeerehceter. Iwo eote abis will *so oa& to Offieo Cbiet of Lnmno-Deoroit.

    14. In this reoert a study to *m&A of the ofvtetagoe to beSgatd b? the additton of e thin (1/Ith caliber) 4 apI plate

    4I)4.

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    Su~bea.tt Vaterto,' Arsenal TAobvratn1r )JpertmontalkoptrJ4O. 710/607-3Ohef of Ordumiwo, AIW, Washin~gton 01J D'c'Attnt SMUl7 - Toobaial )Ioporte

    i front of tkie min armor. it is fouad *tAatMxmrw&ae Intectlon thereby tafl'oded is abouat five tagtT ~~~would reonl% mserely from an equivalent Incrattoiii tfa ?hidiknessof the vain itroor.Vor thos QommAnding COfficeii

    Colonel, Ord. Dept.,

    2 InolsWAL Hpt. T10/607-3 (6 00,&0.)Index *aria

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    REPORT NO. WAL U VTOTAL NO. OF COP I ES - -DATE DISTRIBUTED EXTRA COPIESAuthor 04Laboratory Report Fe IeOffice, Chief of Ordnance 9'.

    TECHNICAL DIVISIONOrdnance Technical Library ISPOTS - Tech. Reports ISPOTB - For Iritish Army Staff 3SPOTMR I.SPOTTSPOTC /. , .SPOTSOt hers

    INDUSTRIAL DIVISIONSPOIRSPOIM C'S.A -~~-~~-/POS / ,.,

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    Watertown Arsenal! La'oatoryReport Number WAL 710/607-3Problem Number J-l.2 28 June 1944

    PRINCIPLES OF ARMOR PROTECTIONFourth Partial Reort

    OBJECTTo investigate the increase in protection fnich isafforded by a thin decapping plate.

    S MYSMflARY

    One-twelfth caliber plates remove the caps of APCprojectiles at service velocities, irrespective of hard-ness of cap, closeness of fitting, or, within wide limits,of cap design.

    The use of such a decapping plate would result in amarked lowering of the shatter velocity of the enemy'sprojectiles, and therefore in thetr effectiveness, par-ticularly in the important obliquity range 300 - 450. Asan example, the lowering of the shatter velocity would beenough to exclude the possibility that 75 mm. APQ pro-jectiles could penetrate, without shattering, 320 BHN

    0plate of 2.5" thickness at obliquities of 35 and over.

    C. Zener- _,- Senicr Physicist

    F. SullivanAssistant EngineerAPPROVED:

    H.rZORN MI04o9el, idance Dept.~'LDirector of Laboratory IIT

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    CON i LYA inL -==CONTENTS1

    Introduction... 0PlReults and Dission...... 5

    1. Remova ocap . .. . .. .. . ... . .. 52. Effects of removal of cap. ..... 6

    _ CO~iD81u~iAL

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    -:f

    INTRODUCTION

    In armor design primary consideration is given toa high resistance to penetration consistent with satis-factory shock properties. The concept of resistance topenetration is not, however, without considerable ambi-guity,

    Firstly there is an ambiguity as to the conditionsof attack. These conditions may be roughly representedby a velocity - e/d diagram, as in Figure 1. In armordesign two conditions need not be considered, high ve-locity impacts against considerably undermatching plate,a condition under which there is no hope fcr protection,and low velocity impacts against overmatching plate, acondition under which protection is certain. In designone need consider only the regions in the velocity - e/ddiagram corresponding to low velocity impacts againstundermatching plate, and to high velocity impacts againstplates which either overmatch, or only slightly under-match, the projectile. Those properties of armor whichgive best protection in one region do not necessarilygive the best protection in the other region. Under thecombat conditions prevalent in the present war, impactsare much more likely to occur in the high velocity (above2000 f/s) region than in the low velocity region.

    Secondly there is ambiguity as to the quality of the "NMattacking projectile. If the enemy's projectiles could

    ='-Q.~ Pig-3-

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    not be deformed, then design would have to be concernedexclusively either with absorbing the kinetic energy ofthe projectile, or with deflecting the projectile. Actu-

    ally all projectiles may be deformed, and consequentlytheir kinetic energy dissipated, if the conditions ofattack are sufficiently severe. Changes in plate designwhich increase the ability of a plate to deform andfracture projectiles frequently lower the plates re-sistance to penetration by projectiles which are not sodeformed or fractured. Thus face hardened armor Is betterable to break -oS than 4 s h^^oenec rcr,but if the projectiles are not broken up, the facehardened armor offers less resistance to penetration thandoes the homogeneous armor, Armor design must thereforeinvolve a compromise between the attempt to defeat theenemy's projectiles-by absorbing or deflecting theirkinetic energy, and the attempt to defeat th projectilesby breaking them up. To maintain an optimum compromiserequires constant vigilance in observing and in antici-pating any improvements in the enemyts projectiles whichmight render its defeat by the second method more unlikely.

    One design feature which favors the defeat of aprojectile by satter is the apportionment of part of thearmor in a front thin plate, the purpose of which is toremove the cap of the attackizrojectiles. Unless thecap is removed, however, such a design feature would lower

    5

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    the resistance of the armor to penetration. This may beseen from the fact that the surface layers of armor offerless resistance to penetrationI per unit depth than doesthe interior. Since any spaced armor must necessarily havea greater portion of the armor in surface layers, it wil.Abe less resistant to penetration unless the first layeralters the projectile in some manner other than in the lossof some kinetic energy, The present report presents aninvestigation of the advantages which may accrue from theintroduction of a thin plate in front of the main armor,the purpose of which is to decap the attacking projectiles.

    RESULTS AND DISCUSSIONmI-

    1. Removal of cap.It has recently been found2 that a 1/4" plate, either

    face hardened or homogeneous, will remove the cap from75 mm. APC 161 projectiles fired through it at velocitiesover 1400 f/s. Confirmatory experiments have been reported3on the removal of the cap from 37 mm. APC YI51 projectilesby plates of the same e/d ratio of 1/12, namely by 1/S" plate.

    It was suspected that the cap removal was associatedwith the brittleness of the hard caps. This suspicion was

    1. 0. Zener: "Principles of Armor Protection, ThirdPartial Reportn, Report No. WAL 7lo/6o7-2.2. Aberdeen Proving Ground Projectile Test Report No. AD-P27.3. C. Zener and J. Sullivan, "Principles of ProjectileDesign for Penetration, Second Partial Report", Report No.

    -5- __

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    pow

    removed by the firing of 1.151 projectiles whose caps hadbeen previously softened. These were also removed bythe l/S" plate.

    It was then suspected that the removal of the capmight be due to a failure which starts at the threadswhereby the ballistic windshield is attached to thecap. Accordingly 37 mm. APC 1M59 projectiles, in whicathese threads are absent, were fired through 1/9" plate.The caps of these projectiles were likewise removed.

    It was further suspected that the cap removal mightbe attributable to a relatively weak bond between capand core due to an imperfect fitting. Accordingly thecaps of several M59's were removed, a lap fitting wasmade, and they were then resoldered on. These caps werelikewise removed by the 1/8" plate.

    The lack of success in the above attempts to makecaps resist the penetration of 1/8" plate, together withthe reports that thin nlates also decap German pro-jectiles , make it seem likely that if our armored vbhl-cles ave equippei with 1/12th caliber decapping plate.the enemy will not, in the near future, perfect his pro.jectiles so as to defeat the purpose of such plates.

    S2. Effects of removal of cap.Previous experiments2 have shown that the decapping

    1, Aberdeen roving GrouM, lo' cit.2. 0. Zener amd J. Sullivan, lo. cit.

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    plate need be placed only one projectile length in frontof the main plate in order for the projectile to behaveas if the cap were entirely absent. Hence in thepresent experiments the effect of cap removal was in-vestigated Vy comparing the behavior of projectileswrith and without caps. The projectiles used in thisstudy were cal. .30 artillery type projectiles modelledafter the German 75 mm. APC Pak 40 projectiles. Adescription of these projectiles is given in a current1report.

    The primary difference between the performance ofa capped and an uncapped projectile against homogeneousarmor lies in their respective shatter velocities. Theshatter velocity for the capped projectiles, in attackagainst plate of 321 BHN, lies above 2700 f/s underall circumstances. The capped projectiles are thereforenot subject to shatter under the usual conditions ofcombat. In order to find the conditions under which theuncapped projectiles shattered, they were fired againstmatching plate (0.30n) of the same hardness (321 BAN),at various obliquities. The observations are presentedas Figure 2. From this figu.e it is seen that theshatter velocity falls from above combat velocity at

    .150 obliquity, to a minimum below 2200 f/s in the

    1. D. Van Winklc: "Principles of Projectile Design forPenetration, Third Partial Report," Report No.WAL 762/231-3.2. B. C. Ward: "Principles of Armor Protection, SecondPartial Report," Report No. WAL 710/607-1.7.-

    --=-=-=~rr~~ ~_ _ - ~--

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    range 350 -400 obliquity, and rises again with a furtherincrease in obliquity.

    The initial drop in shatter velocity with obliquitycan be readily understood from the mechanism of shatter.This mechanism has previously been discussed, and willbe briefly reviewed. A projectile experiences twodistinct types of forces when it strikes a plate. Oneforce arises froa the resistance of the plate material toplastic deformatipn. The second force arises from theresistance rf the plate material to acceleration, whichacceleration it must h-:,e in order to get out of the wayof the projectile. .i.ib second force is called the"inertia force". It is the force which is responsiblefor the shatter of projectiles. The primary function ofa cap is to distribute the acceleration of the platematerial over a longer time, and hence to reduce its peakvalue, and consequently the peak value of the inertiaforce. In the absence of a cap, the acceleration, andhence the inertia force, will be greater both the higherthe velocity of the projectile and, at normal incidence,the blunter the ogive, or, more generally, the blunterthe apex the ogive presents to the plate. In the par-ticular projectile used in the present experiments, the

    1. . zener: "Mechanism of Armor Penetration, ThirdPartial Report," Report No. WAL 710/492-1.Partalp.t,

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    ogive tip presents a tangent surface to the plate at anobliquity angle of 300. The apex of the ogive thereforepresents an increasingly blunter apex to the plate as

    0the obliquity angle increases from 0 to 30 . The velocityneeded for shatter therefore decreases in this obliquityrange.

    The final rise in the velocity needed for shatterwith obliquity is due to the fact that when the ogivepresents a tangent surface to the plate, essentially onlythe normal component of the projectile's velocity isresponsible for the acceleration of the plate material,and therefore for the inertia force. In order for thisnormal component of velocity to remain constant, thecritical velocity for shatter, V, must vary with obliquityeas

    V-Vtl/cos e (I)This theoretical relation is consistent with the obser-vations in Figure 2 beyond 40.

    Equation (1) also gives the relation in the case ofcapped projectiles between ballistic limit and obliquityfor obliquities over 30 . From the manner in which thecritical shatter velocity and the ballistic limit for noshatter vary with obliquity, it may be deduced that ifa projectile shatters at one obliquity below the ballistic

    1, B. C. Ward: oo. cit.,-.

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    limit for no shatter, the same weill be true for all higherobliquities.

    The above conclusion mr-y bu used to make a roughsurvey of tha consequences of decapping. Tt was found that ,Hslplates with an e/d of 0.83 (2,5" plate for 3" projectiles)or overAshattered before pene~ration at 35 . Thereforedecapped pxoje i' ! cnnot penetrata, without shattering,plates thicker than O.6 d unless the obliquity is less

    0than 35 . On the other hand, it was found that at 30obliquity projectiles could penetrate, without shattering,plates as thick as 1,2 d (3.5" plate for 3" projectiles).Such penetration is possible, however, only for a restrictedvelocity range, shatter again occurring at the upper limitof the range.

    The influence of a 1/12th caliber decapping plate maybe most vividly presented by comparing the obliquity rangeover which penetration without shatter is excluded by thedecapping plate, with the decrease in critical angle forpenetration which would be introduced if the thickness ofthe main armor were increased by 1/12th caliber, Supposethat a projectile will just penetrate a plate of thicknasse at an obliquity 8 and velocity V. if the thickness isnow increased to e + Ae and the velocity maintainedconstant, the projectile will then just penetrate at somereduced obliquity 0 AO, From the formula

    o.63V-s (e/d) cos eB. C. Ward: oc. cit.

    ".10-

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    for the ballistic limit with respect to the projectilesused in the present study, it may be seen that}'0.63

    cos 9.s) eSAe

    When Ae/e: 1., this equation reduces to

    &o o.63 cot 9 radians (2)Upon tking Ae/e - Ae/d = 1/12, one finds

    A 0 30

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    T A BLE I~EFFECT OF ADDITION OF ONE TWELFTH CALIBER PLATE THICKNESS

    SADDED AS3 INCREMSED5PLATE 1 THICKNESS OF 14AIN ADDED AS DECAPPINOTHICKNESS PLATE PLATEtDecreases critical angle Decreases criticalfor penetration (without angle for penetrationshatter) (0>p300) by without shatter from0 0

    3.0"1 30 500 to 350A

    00ina3 natre0odiin0o nle faako5 tor35greater 36" 3 5 to

    decapiigyeededthesamrpuldnotpenerat0-.~

    in an -nhte odto o nlso tako 5o

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    FIGURE I*AKSCHEMATIC REPRESENTATION OF COND1IONS

    OF ATTACK

    IVI7//INw

    F-

    >

    !e/d 4

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    - 77 -1 -7 7 "TT 77TT~AA

    321M PLj TEI

    0-J6

    2600w

    . ix

    X, HTTER