MEMORANDUM REPORT NO. 2788

(-Supsedes-WMRNo. 477)

SPALL RING DIAMETER AND PATHS OF A

PERIPHERAL SPALL PARTICLES FROM SHAPED

CHARGE JET PERFORATION OY: ARMOR PLATE •, 4

Eugene T. Roecker 0

September 1977 -

I >.. Approved for publhr release; distribution unlimited.

CD)

USA ARMAMENT RESEARCH AND DEVELOPMENT COMMANDSUSA BALLISTIC RESEARCH LABORATORY

ABERDEEN PROVING GROUND, MARYLAND

I

Y V U U V U U U U U U.U U U

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1. MKPORT NUMBER GOVT ACCESSION NO I. RECIPIENT'S CATALOG NUMBER

BRL MEMORANDUM REPORT NO. 2788

4. TI E d ...l.......-. . ..-.--. ....- ... L s. rYPE oF REPORT A PERIOD COVERED

(~ PALL RING DIAMETER AND P.ATHS OF PERIPHERAL.SPALLSARTICLES FROM SHAPED CHARGE JET PERFORATION OF (2I inal

ARMOR PLATE / _

7. AUTHOR(.) S. CONTRACT OR GRANT NuMBER(e)

Eugene T./ Roecker

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LISA Ballistic Research Laboratory V"7

-.oerdeen Proving Ground, MD 21005 !lE7627ýA!90,l8..-

It CONTROLLING OFFICE NAME AND ADOPESS SEP X Ai.7

US Army Materials 4l Mechanics Research Center SE-low 7:

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-,,DIS TRIBUTIONSTATEMENT,-of t."--Repo""Approved for public release; distribution unlimited.

17. OISTRIBUTION STATEMENT (oa te , abetie ea ¢aee• In , Block 20. II dlttforon tro Repori)

I,. SUPPLEMENTTARY MOTE

This memorandum report supersedes interim memorandum report 477, February 1976.

to. KEY WO1ROS (Coaltau. Bo feve"e old* it fec*4*WV 6-44 140dly by block nuabee)--.

_.•=• '"Spalle 1 ooRing•C.-- --- ,. -- •- •" .•' '" .. Z

Shaped Charge Jet

*• Armor Perforation'fail z~Super Spat *.

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.2 The 3.2-inch BRL precision shaped charge warhead was statically fired into

amor plate of foreign and domestic production. The standoff distances varied ,

from 2 to 12 cone diameters; the plate thicknesses, from I to 30 centimeters; ,%-

and the Brinell hardness numbers of the armor from 100 to 450. Measurements %

of the diameter of the resulting spall scab on the back surface of the target A.

correlate well with the rate of transfer of energy from jet to target.

Analysis suggests that "super spall" may be produced by nearness of the

WO I 1413 CWYOITO Of I NeOYe 64 Ijt OVS UNCLASSIFIED ... . • 1 -S6CIJBITY CLASIFIICATION OF THIS PAGE9 f~aft 9eta liMiff)

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20. ABSTRACT (CONTINUED)__ fatter, leading portion of the jet to the back surface of thin plates.

Some meager information on the trajectories of veripheral spell rarticles

is also presented.

AN.

UNC LAS S I H E D N•SECURITY CL.ASSIFICATION• OF THIS PAGEMI'l~ n DOS* IWOMO•)

x, I

TABLE OF CONTENTS

Page

1. INTRODUCTION ............ ......................... 5..

2. EXPERIMENT .......... .............................. 5

3. THE PARAMETERS OF THE EXPERIMENT ........ ............ 7

3.1 Why jet characteristics instead of warheadparameters? ............. ................... 7

3.2 Shaped charge jet in flight ........ ............. 7

3.3 Resulting penetrator-target parameters .... ....... 12

3.4 Simplification by use of rate of energy transfer. 12

4. RESULTS AND DISCUSSION ............ ................. 14

4.1 Diameter of spall scab ............ ............. 14

4.1.1 Targets with BHN from 250 to 450 .... .......... ... 15

4.1.2 Targets with BHN of 100 .... ............... ... 1is

4.1.3 "Super Spal I" . . . . . . . . . . . . . . . . . . . . is

4.2 Peripheral Spall Trajectories ..... ............ ... 18

4.2.1 Target plates of BHN from 250 to 450 .. ........ ... 19

4.2.2 Target Plates of BHN 100 ...................... 19

S. CONCLUSIONS .............. ...................... 19

DISTRIBUTION LIST • . . . . . . . . . . . . . . . . . . 23

S.... Sect*I...

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1. INTRODUCTION

To determine the vulnerability of military targets protected byarmor, one must assess the interior damage resulting from spall off theback surface of such armor as well as the damage caused by the residualpenetrator and by the target-penetrator ej ecta. (Ejecta spewed in frontof the ormor is ignored.) The Ballistic Research Laboratory (BRL) isbuilding a data bank to provide such an assessment for spall and ejecta*produced by the shaped charge jet penetration of steel armor plate.I

A characterization of debris (spall plus ejecta) was made by

Systems, Science, and Software, Incorporated (S3 ) under BRL contractl.In reviewing the S3 effort, it was decided that data associated withthe periphery of the spall scab could be correlated with penetrationparameters. This report presents the result of such a correlation.

2. EXPERIMENT

The details of the experimental set-up can be found in References2 and 3. A schematic of the firings is shown in Figure 1.

lR. T. Sedgwick, et al, "Characterization of Behind-the-Armor DebrisResulting from Shaped Charge Jet Formation," (sic), Systems, Science,

and Software, Incorporated, Ballistic Research Laboratory ContractReport No. 249, July 1975 (AD #B006721L).

2Barry H. Rodin, "Documentation of Spall Produced by a 2.3 InchPrecision Shaped Charge," Ballistic Research Laboratory MemorandumReport No. 2407, August 1974 (AD #923S73L).

3C. J. Brow and R. Karpp, "Comparative Effectiveness of Foreign andDomestic Armors Against Shaped Charge Attack," Ballistic ResearchLaboratory Report No. 1902, August 1976. (AD 0B013219L)

*The author interprets spall to mean material released from the armorbecause of the tensile wave reflected from the back surface, oftencalled scabbing to distinguish from front surface spall. Targetmaterial, along with penetrator material, that is residue of thepenetration process and that is pushed out by the remaining penetratoris thought of as ejecta.

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The shaped charge used was the 3.2-in. BRL precision copper linerloaded with Composition B explosive. A description of the relationshipbetween jet penetration and jet characteristics can be found in Reference4. The jet in this experiment has the following measured characteristics:

Virtual origin is 7.5 cm uprange of the base of the liner.

Velocity of the tip of the jet is 7.8 km/s.

Breakup time is llS microsec.

Length of leading pellet after breakup time is 3.2 cm.

Diameter of leading pellet after breakup time is 0.4 cm.

Diameter of main jet after breakup time is 0.2 cm.

The standoff distance varied from two to twelve cone diameters. Thetarget plate thicknesses varied from 2.22 cm (7/8 inch) to 30.48 cm(12 inches). The hardness of the armor varied from a Brinell hardness number(BHN) of 100 to 450 with the armor being of both foreign and domesticproduction. Some an~alysis of the foreign and domestic armor data wasalso done by Brown and Karpp 3 . Table i lists the standoff, target thick-ness, and target properties for each round. (The above jet characteristicsare assumed the same for each round.)

"-" 3. THE PARAMETERS OF THE EXPERIMENT

3.1 Why Jet Characteristics Instead of Warhead Parameters?

Since one objective is to provide spall release criteria for hdro-code simulation, we choose to correlate target effects (spall) with para-

- meters available during such simulation, namely, parameters describingthe on-going penetration process. Warhead parameters, so popular forcharacterizing target effects for vulnerability calculations, do notfulfill this request since thoy do not lend themselves to correlation NY-with the dynamic history of the penetration process. Mhen the warheadis a shaped charge, however, the associated jet characteristics provideparameters quite amenable to such correlation. And, where vulnerabili.ycalculations still require "curves" showing target effects vs warheadparameters, they can be inferred from the jet characteristics.

3.2 Shaped Charge Jet in Flight. L

All elements of the jet can be visualized as emanating from ý. Ningle *point itn space on the axis of the •onlical linger 3tt the s c.•n iInstwilt in

X . tVi Persio. et al, "Characteristics of Jets. froa Small Caliber Shaped

Charges with Copper and Aluminum Linors." ýAllistic Research LaboratoriesMot~orandum Report No. 186 September 19t,71. (AL) OS23839) -~

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Table 1. Spall Ring Diameter and Path Angle

00 U dE/dt

joules Round(cm) (C.D.) nanosec BHN No. (cm) (deg)

2.2 2 9.5 260** 1166 S.7 45.6

260* 67 5.7 47.S

260** 68 5.7 42.7

12 11.2* 260** 69 5.7 44.1

260** 70 S.7 44.7

260** 71 S.7 43.3

2.5 2 9.0 260 96 S.7 48.4

260 97 S.7 42.9

260 98 5.7 39.9

3S0 1084 5.7 40.2

350 85 5.7 48.9

S 20.4* 350 86 5.7 47.7

350 87 5.7 43.5 4

350 88 5.7 46.0

8 14.4* 350 89 5.7 44.7

350 90 5.7 43.2

350 91 5.7 41.7 GA

,•12 1 ,-350 9i 5.7 40. 1

350 93 44.A

SO0 94 5.7 42.0

350 95 5.7 42.5

200 1199 5.7 43.7

" 1200 5.7 41.9

200 01 5.7 46.0

4.4 2 o.4 440" 1172 5.4 44.4

440" _Y. 44.9 4

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TABLE I. Spall Ring Diameter and Path Angle (Continued)

S• •• dE/dt c • -

joule Round(cm) (C.D.) nanosec BMIN No. (cmn) (dog)

4.4 26.4 440** 1174 5. 7 43.6

450 78 6.4 41.6 •

450 79 6.4 46.1i, •

450 80 6.4 40.812 * 450 81 S.l1 44.5450 82 -. 1 42.2

450 83 6.4 40.9

440** 75 4.1 45.3440** 76 3.2 8.4

440n 77 iS.e Ron 40.3 W C6

.4•7.6 2 3.8 350 1096 4.4 42.2

50 97 46.4 44.8.SO 98 4.4 46.1

3.2 5099 3.8 47.6

1500 3 S. 50.6

450 83 3.8 40.8

40* 7602 3.2 40.4

350 03 3.2• 37.7v•X

350 04 3.4 8.42

3. 350 99 3.2 47.6

350 100 3.2 SO.6 %

350 07 3. 44.0.

9.a 2 2.7 270" 90 4.4 45.470° 91 4.21 43.-

2700° 90 4.4 43.4 - ,.2 .07000 9133

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Sr odE/dt -" I"jotules Round

(cm) (C.D.) nanosec BHN No. (cm) (dog)

9.8 12 2.0 270** 1194 3.2 30.2

270** 95 3.8 34.3

10.2 2 2.6 270 84 4.4 38.6

270 85 4.4 47.5

270 86 4.4 40.7

12 2.0 270 87 2.5 30.2

270 88 3.5 34.5

270 89 3.2 45.4

15.2 2 1.3 350 08 3.5 43.4

350 09 3.5 36.5

350 10 3.5 40.2

350 11 3.5 42.7

5 1.8 350 12 2.9 38.7

350 13 2.9 42.7

350 15 2.9 39.2

8 1.8 350 16 2.5 39.3

350 17 2.53.

350 1s 2.15 32.0

12 1.o 350 19 2.5 43.9

350 20 2.5 43.3

350 21 2.$ 32.7

30.5 2 O.28 350 20 .6 1.350 "17 ! .6 7.5

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joules Round

(cm) (C.D.) nanosec BHN No. (cm) (dog)

30.5 8 0.76 350 1130 1.9 28.5

350 31 NO SPALL

2.5 2 9.0 100 39 7.6 38.4100 40 8.9 41.1100 41 8.3 41.3

12 11.2- 100 42 8.3 36.t

100 43 7.6 44.7

100 44 7.6 34.3

7.6 2 3.8 100 45 8.9 38.0

100 46 8.9 38.7

100 47 NO SPALL

12 2.2 100 48 NO SPALL

100 49 5.1 31.7

100 so 3.s 35.9

22.9 - .55 100 si . 1.

100 S2 3.2 32.7"

100 53 3.2 7.4

12 1.1 I00 54 3.8 34.9

100 ;5 3.8 - 5

100 sc, 3.2 40.1

"et'nergy is traisf-rrod by loading portion uf tho )Jt (twizt thudiam ter of the rcmaaiivler of the jet)- .'

"'Forignarmor ,,,

"1"Valu unc rtain sinki position in tar~gt near tho abru,3', chulg_

in ict dtamoter bctw•net thek Iit ing and trailing p.,rtiunv of tdh)cte. d1/dt wies bwt•n L ?.. d 10.4 Nu/nto . Note- I•'vg

scatter in spall dizmter.

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time (t = 0). Any cross section of the jet at some time t, however,has a different velocity than any other cross section. The velocitydecreases linearly with position as one moves to the rear of the jet.We have, therefore, a stretching of the jet with time. Since we maythink of the jet material as maintaining uniform density during flight,the elongation of the jet must be accompanied by that shrinking of thediameter that preserves the volume of any element of the jet. Thisstretching continues until such a time (t = t ) that the jet breaks upinto discrete elements as shown in Figure 2. 4Thereafter, the discreteparticles continue in flight, each with its own velocity (mean value att = tl), no longer elongating. There is, of course, some retardationdue to air resistance. This effect has been measured 5 and could be in-cluded in this presentation. The effect on spall is felt to be negligibleand is avoided for simplicity.

3.3 Resulting Penetrator-Target Parameters.

Since we are addressing here only steel targets and copper jets,each of uniform material density, we restrict our penetrator-targetparameters to p.-netrator diameter, penetrator velocity, target thickness,and target hardness. (Penetrator length is always sufficient to perforate

:• .• the target.),

The effect of standoff distance can, therefore, be handled by pre-scribing the known velocity and diameter of each cross section of thejet at t-ime of impact and at any time during penetration.

The penetration process is clearly described in Reference S. Thetheory, well supported by experimental evidence, identifies at any instantof time what portion of the jet remains to do further penetration. Theeffect of target thickness can, therefore, be handled by permitting thistheory to identify what portion of the jet is doing the penetrating atany desired time. The diameter of that portion is calculated accordingto the description above.

Thus, we are able to calculate the velocity and diameter of thecross section of the jet at the target-penetrator interface for all pene-tration time; and these jet parameters will be used to supplant two con-trol variables for our experiment, standoff distance and tar et thickness.The remaining two variables, target hardness and foreign vs dmestic pro-duction, will be handled by searching for differences in the data. _

3.4 Simplification by Use of Rate of Energy Transfer.

Our problem is simplified if we are able to find a functional thatmaps the velocity and diameter of the jet into a single number that

*R. Di Persio, J. Simon, and A. B. Merendino, "Penetration of Shaped-ChargeJets into Metallic Targets," Ballistic Research Laboratories Report No.1296, September 1965. (AD 0476717)

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correlates with the spall measurements. One good candidate for such afunctional is the rate at which energy is transferred, dE/dt, from thepenetrator to the target when the interface reaches a specific locationat or near the back surface of the target. The back surface was thechoice of Eichelberger and Regan 6 and later authors when the number ofdebris particles was che target response variable. These writers sep-arated the rate of energy transfer into (dE/dP) x (dP/dt) where P ispenetration. Unlike dE/dt these terms can be evaluated without knowingjet diameter per se. The penetration velocity, dP/dt, can be calculatedusing the ideal jet penetration theory in Reference S. The term dE dPis proportional to the cross-sectional area of the penetration hole *

* and, hence, measurable to within a constant of proportionality.

In keeping with the idea, however, to provide spall release criteriafor use in hydrocode simulation, we do not want to correlate spall withany effects (area of hole) that are not known until the event is allover. Therefore, we will deal directly with dE/dt which can be calcu-lated from the known jet characteristics. 7

In sum, te have reduced the target, warhead, and standoff parametersof the experiment to three: rate of transfer of energy from jet to target,target hardness, and foreign vs domestic armor production.

4. RESULTS AND DISCUSSION

Since other authors1 ,2 have already reported other results from this-experinent, we restrict our discussion to spall scab diameter, "superspall,"and peripheral spall trajectories.

4.1 Diameter of SpalI Scab.

Table I lists the measured diameter of the spall ring (scab) foreach firing. We desired first to correlate these measurements with therate of energy trznsfer at the instant the penetrator-target interfacehas reached some location near the back surface of the target. Aftertrying a few locations between the back surface and a position 7.5 cmahead of the back surface, we decided that the best choice was onecentimeter ahead of the back surface. The other positions appeared tobe only sl•ghtly inferior choices.

6J.3 M. Regan and R. J. Eichelberger. "Prediction of Effectiveness ofShaped Charge Warhead Designs," Ballistic Research Laboratories Tech-

" nical Note No. 1296. January 1960. (AD #574409) 1 N

A. Mtertudino, R. Di Persio, and J. Simon. "A Moethod for Predicting Final. -lation from Targets Perforated by Shaped Charge Jets," Ballistic Research

A Laboratories Report No. 1308, January 196b. -AD 371925)

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4.1.1 Targets with BHN from 250 to 450. Figure 3 is a plot of the spalldiameter from armor plate with BHN of 250 to 450 vs dE/dt, the rate ofenergy transfer when penetration has reached a position one centimeterahead of the back surface of the plate. The data seem to correlate wellwith this parameter. The levels of hardness are symbolized by 2, 3,and 4 for nominal BHN of 240, 350, and 450, respectively. There is nodiscernible effect due to hardness in this range. Nor are there any

• clear differences between the foreign and domestic armor. Figure 3shows the spall diameter increasing linearly (?) with dE/dt from holediameter at low dE/dt to a diameter of 5.7 cm (2 1/4 in.) at dE/dt5 joules/nanosec.

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spall ring diameter = 1.5 + 0.84 dE/dt""'(cm) (joules/nanosec)

For further increase in dE/dt, the diameter remains constant at 5.7 cm.

4 4.1.2 Targets with BHN of 100. When we change from the harder armor toarmor with BHN of 100, we have a distinct increase in spall ring diam-

A, eter. Figure 4 shows this increase. The plotted points are the BHN 100data, and the curve is that of the harder armor of Figure 3."4.1.3 "Super Spall."

Figure 3 suggests a division of the data into two parts: dE/dt < 5joules/nanosec with varying spall ring diameter; dE/dt > 5 joules/nanosec with constant (maximum) spall ring diameter. Nearly the samepartitioning of the rounds occurred in Reference 7/where debris measure-Ments were charucLerized. li that report the rounds were separated by

a cricical target thickness, eight centimeters. The data support thisvalue only approximately. A critical target thickness of 7.5 cm wouldproduce a partitioning of the rounds idertical to that provided heroby a c•itical dE/dt of 5 joulets/nanosec. AkL-

Reference 1 refers to tht spall of the thinner targets as "superSp,.l" ' "a label coinA-d at BRL to identify this large-diameter,

,*S~rictly sneaking we cannot equate the termi:nation of penetration to a

targe',s actunlly ceases when the penetration velocity decreases to 0.1 cm/

microsec.6 In this experiment all targets were sufficiently thin to permitperforation. The l'west value for our rate of energy transfer parameter,

8O. joules/nanosec, occurred for a 30.5 cm thick target at two conediwoters sta.,doff. A tcrget about ten centimeters Ahicker would prevent .•;perforition. In that case our rate of energy transfer parameter would be0.09 joules/nanosec rather than 0.

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donut-shaped, spall scab which fragments into large pieces. In view ofFigure 3 we feel this super spall ring to be of fixed diameter, 5.7 cm.From References 1 and 2 we learn that the weight of this ring increaseswith increasing dE/dt. We conclude that the thickness of the super spallscab, and not its diameter, increases with increasing dE/dt.

Because of the existence of super spall, let us put aside tempo-rarily our dE/dt parameter and examine the primary jet characteristics /involved. As stated earlier, the leading portion of the jet has abouttwice the diameter of the remainder of the jet breakup. The leadingportion, therefore, can be solely responsible for as much as the initial

3.4 cm of penetration. For thin targets, then, the spall effect isdominated by the fatter, leading portion of the jet. This fatness ofthe leading portion magnifies dE/dt by a factor of four over the trailingportion of the jet. The questions arise: Does super spall result fromthin targets per se or from the nearness of the fatter portion to theback surface of the plate? Would a jet, deprived of its leading portion,

produce super spall from thin plates?4.2 Peripheral Spall Trajectories

Having determined the diameter of the spall scab, that is, the ringof incipient spall, we also want to know the direction, speed, and sizeof the spall fragments emanating from this ring. The speed and size have Vbeen characterized in Reference 1. That report also estimated the direc-tion ty postulating that all particles, spall and ejecta, traveled alongconcurrent straight line paths, these lines meeting at a point five cen-timeters ahead of back surface of the plate and lying on the jet axis(normal impact). Presumably this point was selected after scanning alldebris data. Here we will look only at the paths of spall particleslocated at the periphery of the spall scab, and these paths we willdefine by the angle they make with the jet axis.

To determine the path angle of peripheral spall, visualize a frustumof a right circular cone circumscribing the debris fragments trapped in

the collecting medium. The smaller base has a diameter equal to themeasured spall scab diameter and is superimposed on the spall scab. Wecall the semi-vertex angle of this cone the "measured" path angle ofpcripheral spall. We make the usual assumptions, tacit in Reference 1:

Spall particles travel along straight line pathsthrough air and through the collecting medium(no Snell effect).

Spall particles fan out with no possibilityof collision.

"As seen in Table 1, replicate firings were made at all test con-ditions. Ordinarily one would plot all data points individually or plotU

A 18

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averages of replicated points. In this part of the experiment, unfor-tunately, bias was introduced by two forms of fragment culling:

The debris particles trapped in the first 2.5 cmof the Celotex collection medium were discardedbecause they were too numerous and, hopefully,of negligible contribution to vulnerability assess-ment. Consequently, because of the larger pathangle it makes, a peripheral spall fragment couldbe discarded even though it traversed 40 per centmore Celotex than a non-discarded inner-lying debrisparticle.

The test set-up was such that spall particles lyingoutside of a specified spatial cone were retainedonly on a few occasions. It was thought that sig-nificant fragments would lie only within this cone.The spatial cone is defined2 as: (1) axis coincidingwith jet axis; (2) vertex 5.08 cm ahead of the backsurface of the target plate; and ,3) semi-vertexangle of 41.37 degrees. For the data in this reportthis is equivalent to saying that if the path angleof a peripheral spall particle exceeds 43.5 degrees,

it likely was discarded. N

Because of this bias we selected the largest path angle from replicatedfirings as represen*ative of the test condition.

4.2.1 Target Plates of BHN from 250 to 450. Figure 5 plots the peri-pheral spall path angle vs spall ring diameter for armor plate with BuNranging from 250 to 450. We believe the three data points having diam-eters under two centimeters represent data free from the above bias andare, therefore, acceptable data. These points i.re the measurements for '%Vthe thickest target, 30.5 cm, in the experiment. All other data pointslook suspiciously neao, the maximum path angle permitted in the collectionprocess.

"4.2.2 Target Plates of BHN 100. Figure 6 plots the peripheral spallpath angle vs spall ring diamTeter for target plates of BHN 100. Allthese data are similarly questionable.

S. CONCLUSIONS N.- %II

(1) Spall ring diameter correlates well wý.th the rate of energytransfer parameter defined in this report. This empirical informationcan be incorporated into a hydrocode as part of a recipe for spall *-

release simulation. Information on the thickness of the spail ring is o.

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(2) No effects iere observed for variation in target hardness forIBUN from 250 to 450. Plates with BHN of 100, however, gave larger spall

rings. Also, steel with BHN > 500 may prove sufficiently brittle toalter the spall characteristics drastically.

(3) No effects were observed between foreign and domestic armor.

(4) Super spall may depend upon how near the fatter leading portion R

of the jet gets to the target's back surface before being totally con-sumed in the penetration process.

(5) Peripheral spall trajectories are uncertain since the neededdata may have been discarded in the experiment. IN

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