/

UNCLASSIFED

PROJECTILE AND) FRAGMENTPENETRATION IN SNOW AND FROZEN SOIL

George W . itkit. Geoge K. ,Awinzowi/ M~d Dennis arlU.S. Army Cold. Regions Research and-EnglE6-irT LaorN76ry

c3'~ Hanover, N.H. 03755

INTRODUCTION: The work d-eD;1i1d. b---4 '-was accomplished as part ofan investigation of terminal ba~llistic~s in snow, ice and frozen soil,conducted for the Field,-E gi-n~detn-g-']3iv4-iýen;,-Di-rectorate-e-f--Faei44i'----Ities Engineering, OCiý).The objectives "tho g' are to

cz develop design criteria for effective utilization of indigenous coldregions materials in field fortifications, to develop methods forestimating the terminal effectiveness of remotely emplaced munitionsand sensor systems, and to evaluate foreign expertise in these areas.To accomplish these objectives, a number of laboratory and field in-vee~tigations Uae-4&&bO conducted to quantify the effectiveness ofvarious projectiles fired into snow,, ice and frozen soil targets.The performance of , ..lsimuljate typi ~al*,"j'"'g"'anen't's 'from mortar and _rocket rounds have also

bee stdie.:ýeneratondata from these tests were analyzed usinga theory developed for use with unfrozen soil targets and were foundto be in reasonable agreement with predicted penetrations in bothsnow and frozen soil.

BALLISTIC TEST LABORATORY: The laboratory penetration tests wereconducted in the CRREL ballistics laboratory in Hanover, N.H. Thelaboratory was constructed to permit investigation of terminal bal-listics in snow, ice and frozen soil. It consists of a 16 x 28-ftwood frame building containing a controlled temperature target room,a weapons room, and instrumentation and projectile preparation areas(Fig. 1). A detailed description of this facility was presented byFarrell (1975).

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AITKEN, SWINZOW, FARRELL

TEST SOIL DESCRIPTION: Four different soils were used in the labora-tory tests: a sand, a sandy clay, a marine clay and a silt. The firstthree matched, as closely as practicable, unfrozen soils tested bythe U. S. Army Engineer Waterways Experiment Station (USAEWES). Be-cause the strengths of these frozen soils were estimated, only anapproximate correlation between measured and predicted penetrationswas obtained. The silt soil was added to the program because arelatively large quantity of soil was needed to develop the compre-hensive data package necessary to correlate measured with pred.ictedpenetrations and this soil was available locally in sufficient amounts.

TARGET PREPARATION: The soil samples were molded in 12-in.-square,12-in.-high boxes constructed of 3/4-in.-thick plywood. The bkxeswere linedwith polyethylene to minimize loss of moisture. Thesamples were compacted in 1-in.-thick layers using approximately 40blows from a 10-lb hammer with an 18-in. drop height. The soil wastempered overnight at 40OF prior to molding. After each layer wasmolded, the sample was placed in a coldroom at -5'F for freezing.This one-layer-at-a-time preparation method minimized moisturemigration during freezing of the test specimens.

The snow targets were prepared by sifting snow through a no. 4sieve into 20-in.-square, 12-in.-high plywood boxes. The snow sin-tered in these boxes quite quickly, allowing the ends of the boxesto be removed and a sufficient number of boxes aligned to assureprojectile retention in the snow.

PROJECTILES: Two different projectiles were used to obtain labora-tory data: 5.56-mm cubes and 7.62-mm NATO ball ammunition (Fig. 2).The cubes were designed as fragment-simulating projectiles, asdescribed by Kakel (1971), while the 7.62-mm round represents approxi-mately the mid-energy level for small arms projectiles.

PENETRATION TEST RESULTS: Impact velocity vs penetration data for the5.56-mm steel FSP's fired into Hanover silt are given in Figure 3.These data show that penetration into the frozen silt was roughlyhalf that into the unfrozen scil. Small temperature changes of thefrozen soil had a relatively small influence on penetration; but re-ducing the temperature from -3 to -25 0 C noticeably reduced penetration.

At velocities above about 700 m/sec deformation of the FSP's wasnoted in both the frozen and unfrozen silt (Fig. 4). The decrease inpenetration obtained in the frozen soil at the higher velocities isattributed to the increase in frontal area of the projectile thatresulted from this deformation. The magnitude of this area change isshown by the data in Figure 5 where a coefficient of deformation, CD

2

• @ Cold Room@Firing Room

SInstrumentation Room

@D Loading Room

Figure 1. CRREL terminal ballistics facility (TBF).

in cm4 10 1 1 ,

0

0 Unfrozen 0

S-30C8 - -10%

3 & -25"C

PNAIO 00BALL AMMO = 6-

0 .a AK

0AC 0

," o• •&& &&. 4 -

AA&

STEEL 2 7 S!2 -AO•CUBE

w 0 200 400 600 800 1000 1200

20.5 GRAINS 150 GRAINS I r I n I - I I m/sec(STEEL CORE) 0 1000 2000 3000Irrmpoct Velocity ft/sec

Figure 2. Projectiles.used Figure 3. Impact velocity vs penetration forin ballistic test program. 5.56-mm steel cubes in Hanover silt.

3

2.0 -_

Frontal Area Doubled 7

. 0 -100C0 Unfrozen

u • ~-25"C *

"1.6 C

2 1.4

£ -25

1.2- •

O A00

".. .. 1.0 - o & d e 6 40 0~b 0

S0 200 400 600 800 1000 1200,U I I I m/sec0 0000 2000 3000 0

Impact Velocity ft/sec

Figure 4. Deformed 5.56-mm Figure 5. Deformation coefficient vs impactsteel cube. velocity for 5.56-mm steel cube fragment sim-

ulating projectiles in Hanover silt.

in cm20 50 I '

o Sond* Sandy Cloy

4015

g 30-

2,10 St

20-

0 00

5- •102- o9

5 0 a 010- * o °-

00-

O - 0 'I, I .. ' I I I0 200 400 600 800m/seeSI I I I I

0 500 1000 1500 2000 2500Impact Velocity ft/sec

Figure 6. Impact velocity vs penetration for7.62-mm NATO ball ammunition fired into frozen

soil targets at -lO0 C.

49

AITKEN, SWINZOW, FARRELL

(deformed FSP frontal area divided by original area), is plottedversus impact velocity. For a given impact velocity, the temperatureof the frozen soil appears to have a strong influence on the magnitudeof projectile deformation.

Figure 6 ('ntains velocity vs penetration data for 7.62-mm NATOball ammunition f..L'ed into frozen sand and sandy clay soils. Thesetests, conducted at a temperature of -10 0 C and for a given impactvelocity, show significantly higher penetrations for this projectilethan were previously observed for the FSP's into frozen silt. It issuggested that this increased penetration results from the higherenergy of the 7. 6 2-mm projectile due to its increased mass, ratherthan a difference in soil target properties. At velocities higherthan about 600 m/sec the jackets of many 7.62-mm projectiles failed.Several of these rounds were also observed to tumble at impact velo-cities between 570 and 730 m/pec. This tumbling resulted in signi-ficantly reduced penetrations as shown in Figure 7.

Typical impact velocity vs penetration data for FSP's into snoware given in Figure 8. Compared to similar data in frozen silt(Fig. 3) penetration of these FSP's in snow appears to be relativelyinsensitive to impact velocity. The data also indicate that snowtemperature does not affect projectile penetration. As with soil,projectile deformation is suggested as a factor influencing pene-tration into snow at velocities above about 600 m/sec.

PENETRATION PREDICTION TECHNIQUES: There are two methods fxequentlyused to analyze projectile penetration data. One of the most widelyaccepted, described by Young (1972), utilizas penetration test resultsto prepare empirical equations relating impact velocity to penetrationdepth. Young's equations contain a projectile nose-shape factor andrepresent target properties with a soil constant ranging from 0.2 to50. These equations have been verified for projectile weights from0.9 to 2613 kg and impact velocities fror. 33 to 843 m/sec. Equation1 was proposed by Young for impact velocities greater than 66 m/secand produces a linear relationship between penetration and impactvelocity:

D = 0.0117 KSN WA (V-30.5) (l)

where

D = depth of penetration, m

K = mass scaling factor, dimensionless

S = soil constant, dimensionless (1 to 2 for frozen silt or clay)

5.

AITKEN, SWINZOW, FARRELL

N = nose performance coefficient, dimensionless (0.56 for flatnose)

W = projectile weight, kg2

A = projectile area, cm

V = velocity, m/sec

This approach has the advantages of relative mathematical simpli-city together with the inclusion of a projectile nose-shape factor.It has also been adapted for predicting penetrations through layeredmaterials. Its primary disadvantages are that penetration tests mustbe conducted on all target materials of interest to develop appro-priate material constants and that a mass scaling factor must bedeterminfci for projectiles weighing less than 27 kg.

Another common approach to penetration analysis is to develop amathematical model for predicting penetration that considers per-tinent projectile characteristics and target strength properties.

One such model, based on dynamic cavity expansion theory, wasdeveloped by Ross and Hanagud (1969). It was used by Rohani (1973)to analyze penetration data from unfrozen soils. This model, equation2, describes a spherical nose projectile penetrating a homogeneousisotropic material. The projectile is further characterized by itsweight and radius. The target material is idealized as a locked-elastic, locked-plastic medium (Fig. 9) and described in terms of itsmass density, yield strength, plastic and elastic moduli and com-pressibility.

BIR ( 2

3W + 1 in i +_ V2

4Ag pp B2 2 B2 3B(3 (2)

whereV = velocity, ft/secP = penetration, ftW = projectile weight, lbA = projectile area, ft 2

g = acceleration of gravity, ft/secR = projectile radius, ft

PP = locked plastic density of target material, slugs/ft 3

and

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T w- r -- r-- -'---0 0 >'

S , 0,,,d 0 4-) 4.,T T # °.

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ATTKEN, SWINZOW, FARRELL

S= P0 :xp (E ) (3)Ppp

where

P = initial density of target material, slugs/ft 3

E = plastic volumetric strain, %p

and

B = _ (4)2E 3

wherey = yield strength of target material, psfE = Young's modulus of target material, psf

Si = elastic volumetric strain, %

1O

P =1 P- (5)p

6 = exp (-3B) (6)

1/3

B2 = 3/2 - l+ap ) 61/3 + 1/2 6 4/ (8)

B3 = 4/9E (1- exp(-3B) - 2/3 y 1n 6 + 2/27 2 Et -4/9 Et n (9)

where

Et = plastic modulus of deformation, psf

and2

(10)

n1

The advantage of this method is that it does not require any empiricalconstants. Its disadvantages are: that the target yield strength isassumed to be independent of projectile velocity and penetration depth;projectile mass and/or caliber area change during penetration are notaccounted for; and it is strictly applicable only to projectiles witha spherical nose-shape. Rigorous use of this approach also requiresthat the constitutive properties of the target material be obtained at

AITKEN, SWINZOW, FARRELL

strain rates equal to those occurring under actual projectile pene-tration.

TARGET STPENhGTH DATA: Frozen soil strength data were obtained fromunconfined cumpression tests. These tests were conducted at a strainrate of 4 444%/min on 1.4-in.-diam, 4.5-in.-high cylindrical samples.The samples were compacted in 1-in. layers using 60 blows of a Harvardminiature compactor per layer (40-lb spring with 1/2-in.-diam compac-tion head). After compaction the specimens were tempered at 33'F forone week to assure uniform moisture distribution and then placed in a50F cold chamber for freezing. Ends of the samples were squared bylapping prior to testitAg.

Typical stress/strain curves from these tests are shown in Figure10. The yield strength, elastic and plastic moduli were obtained byfitting idealized curves (dashed lines on Fig. 10) of the form shownin Figure 9 to these stress/strain curves. The compressibility wasestimated by assuming that the volumetric strain Ep is equivalent tothe volume of air in the soil sample. Data obtained from the com-pression tests on Hanover silt are summarized below.

Temp,p PO t slugs/ft 3 E,_pSf Y, psf

-3 3.7 17.6xi06 29.8xi04

-10 3.7 30.9xlO6 46.3xlO4

-25 3.,7 57.6x10 6 86.14xl0 4

Snow strength properties were obtained by relating the snow tar-gets' density to yield strength, compressibility and elastic modulususing information presented by Mellor (1964). Typical values aretabulated below.

Po, slugs/ft 3 (g/cc) E, Psf

0.58 (0.3) 4.3xlO6 o.14x104

0.78 (0.' ) 13.2xlO6 0.22xi0 4

0.97 (0.5) 20.7xi0 6 2.16xi04

COMPARISON OF MEASUJED WITH PREDICTED PENETRATION: Predicted pene-tration of the 5.56-mm FSP's into frozen silt, computed using Equation2, is compared with test results in Figure 11. There appears to be atendency to underpredict penetration at velocities between 600 and1000 m/sec. This could have resulted, in part, from differences insoil properties between the ballistic targets and the unconfined com-pression test specimens. The average dry unit weight of the soil

4 _ __ __ __ __ . 4 1 1TT • -3*CY' 2070 psi (29 8 x10 4 psi) /E. 122,000 pS (17.6 • IO Wpsf)Po *3.7 lug$/ft s

2 --

o 2

22

! I -

To -I09C T" -25*CY Y 3220 psi (46.31 104 pt ) Y" 6000 psi (86.4 x 104 pSf)E-, 215,000 psi (30.9,it 1 psf ) E,400,000psi (57.6 KIOpsf)

EtO EtcOPI 3.7 slugs/fts PO 3.7s$ugs/ft 3

00 I 2 0 I 2 3Strain, % Strain, % Strain, %

Figure 10. Stress vs strain curves from unconfined uniaxial compressiontests on frozen Hanover silt.

in cm4 -I10

8-3-

,. 60,

4--/0°

A -250C

C -0 . . . .II I I 1 ,C 00 200 400 600 800 1000 1200

1m/sec

0 1000 2000 3000ImpOct Velocity ft/sec

Figure 11. Comparison of test data with pre-dicted penet."-tion curves for 5.56-mm steel

cubes into Hanover silt.

I0

AITKEN, SWINZOW, FARRELL

targets was about 10 lb/ft3 (0.31 slug/ft 3 ) less than that of theunconfined test samples. This lower soil target density resulted be-cause the size and shape of available soil compaction equipment wasnot compatible with the size and shape of the target samples. Thepenetration computations were thus made using parameters biased on thehigh strength/low penetration side which should result in measuredpenetrations being somewhat larger than predicted.

A similar comparison between measured and predicted penetrationin snow is presented in Figure 12. These computed penetrations are inexcellent agreement with the test data. These data are of particularimportance because they show the effect of projectile mass and verifythat mass is correctly represented in Equation 2.

FIELD TEST PROGRAM: A field test program was conducted in Alaska to

expand the scope of the laboratory experiments. A complete descrip-tion of this program was presented by Johnson (1975). The programincluded extensive tests to evaluate the ability of snow structures toresist penetration by 5.56-mm, 7.62-mm and 50-cal ammunition. Pro-jectile penetration vs snow density data obtained during these testsare given in Figure 13. As expected, the smallest and lightest pro-jectile (5.56-mm) had the least penetration. The small increase inpenetration of the 50-cal round relative to the 7.62-mm was not ex-pected. It had been estimated, using the Ross Hanagud equation, thatthe 50-cal round would penetrate about twice as deep as the 7.62-mm.The relatively low observed penetration of the 50-cal rouxd is attri-buted to increased resistance generated by case rupture and deforma-tion (Fig. 14).

Data from these tests and the laboratory experiments, which em-phasized the influence of snow density on penetration, suggested theconcept of a hardened snow trench for pasty expedient protection oftroops against small arms fire. A trench in the snow can be exca-vated very rapidly. Even when the snoi is so light that it appears itwould offer little or no resistance to small arms fire, tests haveshown that such a trench (Fig. 15a) offers a surprising amount ofprotection. An important reason for the effectiveness of this trenchis that fire against it normally strikes the snow at a shallow angle,resulting in ricocheting and broaching of the rounds. Increasing thedensity of the snow ahead of the trench by rodding and packing (Fig.15b) greatly increases the probability for ricocheting as well as

reducing penetration of bullets that do not ricochet or broach. In

tests where approximately one hundred 5.56- and 7.62-mm rounds were

fired at these trenches from close range, only two 5.56-mm and three

7.62-mm bullets came through the snow into the simple trench and no

penetrations were observed into the hardened trench. Forty rounds of

ii

in cm

30

Steel 120.5 groins)

60-

c 20-

00

=. 40

10 - Aluminum (6.71)20.

-cube deformed

0 200 400 600 800 1000 1200 1400m/secI ii I I I , I0 1000 2000 3000 4000 fl/sec

ImpOcI Velocity

Figure 12. Comparison of test date, with pre-dicted penetration curves for 5.56-mm cubesin snow. Snow temperature -13°C, density 0.8

slug/ft 3 (0.41 g/cc).

PENETRATION(meter)

EFFECT OF BULLETo0- SIZE

M 16

.S . D . T .4 .5 Figure 14. 50-cal projec-tile after impact into snow,Figure 13. Bullet penetration vs snow den- illustrating magnitude ofsity for 5.56-mm, 7.62-mm and 50-cal ammuni- case damage.

tion.

Incoming Fire

L- ,W 7. PockedSnow

Simple Trench Horde-ied Trench

O. b.

Figure 15. Snow trenches for personnel pro-tection.

AITKEN, SWINZOW, FARRELL

50-cal ammunition were also fired against the hardened trench and,again, no penetrations were observed.

CONCLUSIONS: Based on these test data, projectile penetrations intofrozen soil are significantly lower than in unfrozen soil. For the5.56-mm steel FSP's, penetration was reduced by about a factor of 2 infrozen Hanover silt. Temperature of the frozen soil influencedprojectile penetration, with penetration decreasing at lower tempera-tures. But for Hanover silt, temperature changes in excess of 100Cwere required to obtain significant changes in penetration.

For a projectile at a given impact velocity, penetration is afunction of target properties with yield strength, density and com-pressibility probably being the most important. A theoreticaltechnique based on dynamic cavity expansion in a locked-elastic,locked-plastic n.edium can be used to calculate projectile penetrationin both frozen soil and snow with reasonable accuracy.

There are some critical impact velocities above which damage toprojectiles occurred not only in frozen soil, but also in snow. Infrozen soil the 5.56-mm steel FSP's deformed at velocities above about800 m/sec and 7.62-mm NATO rounds started to ttumble and/or strip theirjackets above a velocity of about 600 m/sec. In snow, the aluminumFSP's started to deform at impact velocities above 900 m/sec and steelcubes above 1000 m/sec.

Snow can be used as a construction material for expedient de-fensive poditions and affords protection against small arms fire upto 50 cal. In part this protection was achieved by designing theposition to cause ricocheting and broaching of the rounds fired at it.

13

AITKEN, SWINZOW, FARRELL

LITERATURE CITED

1. Farrell, D., 1975 Terminal Ballistics Testing Procedures,USACRREL Technical NOte, June.

2. Johnson, P.R., 1975 Design and Effectiveness of Snow Forti-fications in the Subarctic, USACRREL Technical Note, June.

3. Kakel, W.W. • 1971 Fragment Defeating Capabilities of PlasticArmor, WES, TR N-71-10, June.

4. Mellor, M.,1964 Properties of Snow, USACRREL Moncgraph III-Al,Dec.

5. Rohani, B., 1973 Fragment and Projectile Penetration Resistanceof Soils. Rpt. 2, High Velocity Frafgent Penetration intoLaboratory-Prepared Soil Targets, WES, 14P S-71-12, June.

6. Ross, B. and Hanagud, S., 1969 Penetration Studies of Icewith Application to Arctic and Subarctic Warfare, Rpt.prepared for ONR by Stanford Research Institute, RPT. NWRC7000-452-4.

7. Young, C.W., 1972 Empirical Equations for Predicting PenetrationPerformance in Layered Earth Materials for Complex Pene-trator Configurations, Sandia Laboratories Rpt. SC-DR-720523, Dec.

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