advanced layered personnel armour

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POINTER, JOURNAL OF THE SINGAPORE ARMED FORCES VOL.37 NO.3-4

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Advanced Layered Personnel Armor1

by MAJ Ong Choon Wei, Roy2

Abstract:

Using shock compression physics and explicit numerical techniques, a method has been developed to designcomposite personnel armor by optimizing the role each layer plays during projectile deeat. The initial designconsists o a very hard frst layer to deorm and racture the projectile, an orthotropic second layer to slowdown the shock wave propagation in the through-thickness direction, whilst allowing rapid propagation in thetransverse directions, a porous third layer to absorb the shock wave energy through PV-work, and a ourth layerto provide confnement or the porous medium. Based on the above armor protect ion concept, composite platescomprising o Alumina (Al

2O

3) Ceramic, Dyneema HB25 and porous Polyurethane (PU) oam were constructed to

test against baseline armor in the orm o AISI 4140 steel plate. An integral experiment was conducted to validatethis composite armor against numerical simulations. Through this study, the composite armor has been shownexperimentally to be more eect ive in resisting penetration than a high strength steel plate o equivalent (and

slightly greater) areal density, and that the material layering sequence is undamentally correct. The results othis research provides a platorm to rethink into the way we design armor by breaking down the ballistic impactprocess into stages, and then designing specifc materials to stop projectile penetrat ion. This has the potentialor creating very high perormance composite armor at a raction o the weight o current armor solutions.

Keywords: Military Technology; Impact Engineering; Personnel Armor; Terminal Ballistics

INTRODUCTION

In the past, armor was t ypically made o monolithic

high strength steels (with yield strengths in excess

o 1GPa) to protect against conventional projectile

threats. However, the high density o steel makes itundesirable or personnel protection. The need or

materials with high yield strength gradually evolved

to the use o technical ceramics (Aluminum Oxide,

Boron Carbide, Silicon Carbide, Aluminum Nitrates,

etc.) which are o very high strength and relatively

lightweight. Such materials cause signifcant plastic

deormation in projectiles eectively turning kinetic

energy into an increase in internal energy. Even

more advanced armor materials make use o layering

techniques comprised o composite structures.

Examples include Kevlar Fiber-Reinorced Polymers

(KFRP), Carbon Fiber Reinorced Polymers (CFRP), and

Aramid or Polyethylene woven abric composites. Such

evolution o protection technology has had varying

success in the deeat o certain classes o projectiles.

It is possible that the protection level or these

existing technologies may have reached a plateau,

with only marginal improvements rom each spiral o

armor development.

Impetus For Ongoing Research

Given the evolving projectile threats, it is important

that better armor protection schemes be developed to

match up to the challenge o penetration protection.

There has been much interest in the development oarmor protection using layered construction in recent

years, as shown in works by Robbins et al,3 and Gama

et al.4 Gupta et al have shown the eectiveness and

easibility o using a wave spreading layer to dissipate

the compressive orces o an incoming projectile

within microseconds.5 Wilkins et al have shown the

eectiveness o ceramics with aluminum backing

plates in plastically deorming the projectile, thus

deeating it rom the onset, preventing extensive

damage to the lower layers o armor.6 Wilkins noted

that the aluminum backing plate allowed the inevitable

ailure caused by the arrival o release waves to beheld o or a longer time, allowing the ceramic time

to cause plastic deormation in the projectile. Porous

materials have been known to be a useul shock wave

isolator and absorber. Fowles and Curran showed

that this was because o the ability o the porous

material to support appreciable elastic stress beore

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Figure 1: Tantalum projectile striking composite target and AISI 4340 steel.

compaction.7 Herrman has also demonstrated the

eect iveness o porous materials and their associated

equation o states in absorbing energy due to shock

compression causing signifcant P-V work.8 These are

all well-understood and great concepts or armorapplications developed independently. However, work

has been lacking in combining these concepts into a

cohesive armor system.

However, work has been lacking incombining these concepts into acohesive armor system.

Poh has shown the easibility o a composite layered

construction made up o quite dissimilar materials each

with specifc properties to aid penetration resistanceo the composite plate.9 It consists o a hard frst layer

to plastically deeat the projectile, and a multiple wave

spreading layer to laterally dissipate the compressive

shock waves. This is then ollowed by a porous layer

to aid energy absorption. Numerical simulations using

the Autodyn hydrodynamic computer code have

shown the benefts o having this sequence o layers to

arrest the shock propagation rom a projectile impact,

and it was predicted that this type o construction has

the potential to outperorm an AISI 4340 armor grade

high strength steel plate o equivalent thickness.

This steel was chosen as a baseline material becauseits properties are very well understood, and itis a close match in dynamic properties to steelarmor materials used in many armor applications.Figure 1 shows simulation results or a 15mm

length, 8mm diameter Tantalum cylinder penetratingcompletely through a 16mm thick AISI 4340 16mmSteel Plate at an impact velocity o 1000m/s. Thesame projectile is stopped by a composite plate o

the same thickness (16mm).

The motivation or this study was thus to investigatea layered concept in armor plate technology basedon undamental shock physics to stop a projectilepenetration in a series o stages:

Stage 1: Projectile Deormation Using highyield strength, high impedance materials to resist

penetration rom compressive orces as much aspossible causing signifcant plastic deormation in

the projectile. This layer also helps to decrease the

impulse delivered into subsequent layers.

Stage 2: Wave Spreading Using special orthotropiccomposites with as high a lateral speed o sound as possible

to spread shock waves laterally away rom incident axis.

This causes signifcant stress wave attenuation.

Stage 3: Energy Absorption Using porous materialsto convert kinetic energy into heat and work done in

compressing the pores o the material (PV-work)



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Stage 4: Penetration Prevention Final stopping layer

to prevent penetration o the projectile with nominal

velocity. This layer also serves as containment and

provides spall prevention or the porous layer. Figure

2 illustrates the above idea.

Research Approach

The approach was to frst develop material models

based on literature research. This was ollowed by

conducting proo o concept experiments using

suitable materials or each layer which satisy the

Figure 2: Graphical Illustration o New Armor Layering Concept.

Test Matrix

TestSample

Material Total Thickness Average Density (gm/cm2) Areal (gm/cm2) Density Purpose

1 AISI 4140 Steel 4.76mm 7.85 3.737 Baseline comparison o armor plants

2 Ceramic + DyneemaHB25

11mm 2.52 2.771 Basic composite plateto replace armor steel

3 Ceramic + DyneemaHB25 + P1 + AI plate

17.5mm 1.86 3.256 Full - up CompositePlate

Material Notes: All target plates were 100mm x 100mm squares.

AISI 4140 Steel (S) 4.76mm thick AISI 4140 Steel Plate as a control sample (Desity 7.85gm/cm3)Precision ground in accordance to ASTM A322, Rockwell C30, with a yield strength o 95,000 psi(655 MPa).

Cer amic (C) 6mm thick Corbit 98 alumina ceramic pl ates rom Indust rie Botossi were used. Alumina 98% with aYoung's Modulus o 384 GPa, and hardness H o 16.3 GPa, and Density o 3.81 gm/cm3

Dyneema HB25 (D) 5mm thick Dyneema HB251 rom DSM. Density o 0.97 gm/cm3, Fiber Tensile strength o Approximately

2 GPa.

P1 5mm thick Polyurethance, P1 - PR6710 Aircart Foam (Density 0.16 gm/cm3)

A1 1.5mm Thick Aluminum 6061-T6 as an inertial backing plate to provide confnement or the porousoam. (Density 2.70 gm/cm3)

Adhesive Plates are glued together using low viscosit y (500 cps) Angstrom Bond@ AB9110LV. Glued andambient air-cured over at least 24hrs.

Table 1: Test Matrix

requirements at each stage to arrest the incoming

projectile. The conditions o the experiment (i.e.

impact velocity) are then simulated using the

numerical model developed so as to make sense o the

experimental results. In this paper, the experimentalresults are presented frst, ollowed by the numerical

modeling and simulations.

EXPERIMENT AND LIVE FIRING

A test matrix was set up to systematically test the

eects o each layer. Our baseline material was AISI

4140 high strength steel, rom which our composite

plates were tested against and compared to. Table 1

shows this test matrix. Table 2 shows the properties

o the projectile used in this experiment while Figure

3 shows a picture o the actual projectile.

Live Firing Set Up

The live fring experiment was conducted at a

helium pressure gun acility at University o Caliornia,

Santa Barbara (UCSB). With a gas gun pressure o up

to 2000 pounds per square inch (psi), the gun has a

maximum projectile velocity o about 475 m/s based



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Figure 3: A2 Tool Steel Cylinder projectile that was used in the experiment.

Figure 5: Schematic o how the targets or the experiment were set up. A 50mm x 50mm square area was exposed to the incoming

project ile while the 100mm x 100mm target is held up by simple ric tion.

Figure 4: Schematic o Gas Gun Facility.

Specifcation/Shape Dimension Density (gm/cm3) Mass (gm) Hardness Yield Strength

AISI A2/Cylinder 25.4mm, 7.49mm diameter 7.75 8.70 RHC - 55-56 1.8 GPa

Table 2: Properties o steel projectile used.

AISI A2 Cylinders were rom UCSB, manuactured in-house

on the mass o 25.4mm long steel cylinders o 7.49mm

diameter. Figures 4 and 5 illustrate the experimental

set up. A high speed camera, an IMACON Model 200,

captures images at up to 200 million rames per second

(ps). Through imaging sotware, the velocity o the

projectile can be est imated to within 1% accuracy.

Live Firing Results

Table 3 summarizes the results o experiments

using the test samples shown.

AISI 4140 Armor Steel

It can be seen that a rough estimation o the

Kinetic Energy (KE) necessary to penetrate the AISI



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Sample/ ProjectileShape

Impact Velocity (m/s) KE (J) Relative KE Ratio Penetrate

AISI 4140

Rod(1")

484 1018.61 1.000 YES

C-DRod (1")

483 1014.41 0.996 NO

C-D-P1-Al

Rod (1")

481 1006.02 0.988 NO

Table 3: Live Firing Results

Figure 6: Target samples and high speed camera rame photo-

graphs o the AISI 4140 Steel Plate Impacted with the 1 long

projectile at 484m/s. Complete penetration is observed.

4140 armor steel plate was in the region o about

1000 Joules. High speed photography showed that the

rod penetrator was completely shattered and residual

velocity o the ragments were about 118m/s. These

photos, shown in Figure 6, were taken at 30s interrame

time (33,333 ps).

Composite Plates

The composite plates were tested in a progressivemanner to evaluate the role o each material inpenetration resistance. The ull-up test plate has analuminum backing plate, which as shown by Boey,10 isnecessary to provide rear support and confnement tothe oam material to prevent it rom spalling.

Both composite plates were able to resist the 1 A2projectile. This was a direct indication o the betterperormance aorded by the composite constructionover that o the AISI 4140 armor steel o equivalentareal density. Figures 7 and 8 shows the actual sample

and damage results.

Figure 7: Sample C-D showing complete racture o the ceramic

frst layer but no penetration.

Figure 8: Sample o C-D-P1-Al showing complete racture o

the ceramic frst layer but no penetration.

Post Test Measurements

Deormation measurements o the damaged samples

were done in order to compare the relative perormance o

the various target plates. The dimensions are as defned

in Figure 9. Table 4 summarizes the damage results.

Discussion I (Result From The Live Firing Experiment)

Perormance O Composite C-D

The Ceramic/Dyneema composite plate o lower

areal density outperormed that o the AISI 4140

armor steel. This provides the proo o concept or

armor protection that this investigation had initially



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Figure 9: Dimensions that were measured using a 1/100 th mm

accuracy vernier calipers.

Description Projectile Speed(m/s)

Overall Depth(mm)

Hole Depth(mm)

Hole Crater(mm)

Hole Diameter(mm)

Designation a b c d

AISI 4140 484 9.25 9.25 12.47 8.44

C-D 483 45.97 39.36 25.51 25.51

C-D-P1-Al 481 35.12 21.27 31.76 31.76

All Projectiles were 25.4mm long rod penetrators with a diameter o 7.49mm, made o A2 AISI Tool Steel

Table 4: Post Test Measurements o Specimen Damage

set out to achieve and showed the benefts o using

wave spreading and energy absorption materials

as discussed above. We note here that the ceramic

layer is expected to be even more eective against

penetrators that are less hard than those used in this

study. This is because, or soter projectiles, there will

be a greater amount o plastic deormation when they

strike the very high compressive strength ceramicmaterial. This eectively converts projectile kinetic

energy into heat through the plastic ow process.

Perormance O Composite C-D-P-Al

Porous oam as a third layer has proven to be a

good shock attenuator. Addition o a porous oam

layer decreased the amount o target deormationsignifcantly, by almost 24%. By absorbing the impact

kinetic energy through cell wall collapse and porecompression, and turning it into waste heat through

PV-work, the penetration process is made lesseective, and the total deormation decreases.

Generally speaking, the greater the porosity,subjected to a limit, the greater amount o energyabsorption due to the combination eects o elastic

buckling o cell walls, plastic deormation o collapsing

cell walls, and volume compression.

NUMERICAL SIMULATIONS USING AUTODYN

It is useul to develop a computer model that

reproduces the conditions o the experiment so as to

allow a comparison with the experimental results. This

will also help guide uture experiments. The approach

was to use Autodyn and defne the properties o

the material used in the experiment based on either

literature research or reasonable estimates. Some

assumptions are necessary due to the lack o concrete

values in the available literature as well as the lack o

resources and time to do material testing to obtain

the actual mechanical properties. Material models

that are used in the simulations are discussed frst,

and the simulations are presented thereater.

Ceramic Modeling Polynomial Equation O State(Johnson-Holmquist Constitutive Model)

Ceramics are unique in their response to impact

loading. To capture the response o such brittle and

sophisticated damage mechanisms, Autodyn uses the

Johnson-Holmquist (JH-2) Constitutive Model which

captures the progressive damage o ceramic materials

subjected to impact loading.11 The various properties o

Alumina (Al2O

3) have also been derived experimentally

by Anderson et al and are shown in Table 5.12 These

values have been adopted and incorporated into

the Autodyn Material Library.

Orthotropic Material Modeling

To capture the somewhat complex dynamicresponse o the wave-spreading layer we must use a

non-isotropic material model. Below we describe how

we have constrained such a model or the Dyneema

material used, in the absence o measured values or

many o the relevant properties.



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EOS Polynomial Strength Johnson

Holmquist

Failure Johnson

Holmquist

Re Density 3.89 gm/cm3 ShearModulus,G

152 GPa Hydro Tensile Limit -0.262 GPa

A1, Bulk Modu-lus

231 GPa Model Continuous JH-2 Model Continuous JH-2

A2 -160 GPa HEL 6.57 GPa D1 0.01

A3 2774 A 0.88 D2 0.7

B0 0 N 0.64 Bulkingconstant,

1.0

B1 0 C 0.007 Damage Gradual JH-2

T1 231 GPa B 0.28 Tensile Failure Hydro Pmin

T2 0 M 0.60 Erosion Geometric Strain

Erosion Strain 2.0

Type Instantaneous

Table 5: Material Properties o Technical Ceramic, Alumina 99.5%.

Conventions Used In This Section

The ij-subscript o each normal stress or shear

component represents the respective orce in the

i-direction acting on the j-plane. In the case o

Poisson ratios (e.g. v12), the ij-subscript represents

contraction in the j-direction, when subjected to

extension in the i-direction. The simulations in

this research were done using axial symmetry about

the x-axis. This would mean that the 11-directionwould reer to the through thickness direction

(x-axis), while the 22-direction would reer to the

transverse direction (y-axis) o the material samples.

A hypothetical orthotropic material, D1, is defned to

closely resemble the behavior o the actual Dyneema.

Equation O State

Dyneema fbers are available in layers with

identical 0/90 fber or ientation, and can be assumed

to be transversely isotropic or added simplicity.

Strictly speaking though, Dyneema will have slightly

dierent properties in any directions other than the0/90 orientation, but this increases the number o

unknowns in the Stiness Matrix. This assumption

o Dyneema being transversely isotropic reduces

the Compliance Tensors Matrix to 5 unknowns rom

the original 36. Equations (1) and (2) have been

documented by Jones.13

The Compliance Matrix or a Transversely Isotropic

material is given as:

Where {Sij} elements are defned as ollows:

(1)

(2)



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To model the porous material using the P-

equation o the state compaction model option in

Autodyn, several critical parameters must frst be

specifed. The parameters are as ollows:

1. Initial density o the porous material0.

2. Bulk sound speed in the elastic compaction region ce.

3. Maximum elastic pressure (pressure at yield) Pe.

4. Solid compaction pressure Ps.

5. Solid material Hugoniot parameters C and S.

The initial density o the porous material can

be determined experimentally using the immersion

density technique or, more requently, it is a parameter

provided as a material specifcation by the material

manuacturer.

The other input parameters or the computation

model are determined using the methodology discussed

by Grady and Winree.15 First, to determine the bulk

sound speed, in Eq. (3) is the relationship o how bulk

sound speed cevaries with initial density

0:

where is the bulk modulus,

For isotropic materials, the bulk modulus is

related to the Youngs modulus, E, and Poissons ratio

v by Eq. (4):

Table 6: Material Properties o Orthotropic Material D1.

(3)

(4)



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Using the scaling relat ion in Eq. (5) developed by

Gibson and Ashby, the expectation is or the Youngs

modulus E to vary with the square o density.16 The

scaling relations or porous materials are derived

mostly through empirical ftt ing o experimental data

rom extensive test ing on polymeric oams.

Assuming material isotropy during deormation,

the bulk wave speed o the porous material can be

related to the properties o the ully dense solid by

Eq. (6):

The Hugoniot Elastic Limit, HEL

is a measure o

strength in a shock loading process that can be ound

rom the yield strength. Assuming linear elastic

behavior to yield point and a von Mises yield condition,

the HEL

can be determined rom the uniaxial stress

loading yield strength yby Eq. (7):

Using the y

data provided by the oam

manuacturer, the value o HEL

is calculated. The

maximum elastic pressure, Pe, at which yielding begins

is then determined rom Eq. (8). The Poisson ratio vo

the material is obtained rom published literature and

manuacturer inormation.17 The hydrostatic pressure

at which yielding occurs is rom Grady and Winree:

(8)

Where HEL

is taken to vary with oam initial density

according to Eq. (9):

This model predicts that HEL

increases with

density. The values oHEL

or dierent oam densities

o the same material are calculated using the ydata

provided by the manuacturer. The constant Cy

can

then be determined rom a least-squares ft. Finally, to

determine the pressure or complete compaction, Ps,

the theoretical relation oHEL

as a unction o density

is used. The solid compaction pressure is thereore

the elastic pressure Peat which

0=

s.

The solid material Hugoniot parameters, namely

solid bulk sound speed, C and the Hugoniot shock

velocity slope S, are rom Grady and Winree.

AISI 4140 Steel Plate

The preceding section has defned the material

properties which are needed to perorm simulations

o our experiments. The next stage o the simulation

study was to establish a numerical model rom which

the perormance o uture AISI 4140 target samples

subjected to high velocity impact rom A2 material

projectiles can be predicted. The material properties

were taken rom the Autodyn Material Library o 4340

steel, with slight modifcations to its yield strength.

A 0.295 diameter, 1 length A2 cylinder was modeled

as the projectile.

This simulation, shown in Figure 11, correctly

predicted that the 1 long A2 steel rod striking at

484m/s would punch through the 5mm high strength

Figure 10: P- Model.

(5)

(6)

(7)

(9)



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Table 7: A summary o the material parameters or the polyurethane oam, PR 6710 GR

armor steel plate easily. Penetration o the AISI 4140

steel plate was achieved with a projectile residual

velocity o 137m/s, which was higher than the

experimental result o 118m/s. This is due to the act

that the projectile was completely ractured in the

penetration process during the experiment, and this

reduces its kinetic energy, resulting in a slightly lower

residual velocity observed.

Composite Plate Numerical Simulations

Composite Plate C-D1

A composite target consisting o 6mm Ceramic

layered over 5mm D1 material was modeled (Figure

12). The same A2 Tool Steel, 1 long cylinder, was made

to hit at 483m/s. Gages were set up in the through

thickness direction, as well as the 22-directions. It

can be seen that the ailure mechanism observed in

the simulations agrees well with what was observed in

the experiment. The presence o the ceramic dissipates

much o the initial impact energy o the projectilethrough brittle racture. There is delamination o

the D1 material in all the layers, without shearing

ailure,and a large deormation o the D1 material is

observed.18 The projectile was stopped about 0.32ms

ater impact.

Composite Plate C-D1-P1-AL

Composite plates were modeled with 6mm Alumina

ceramic, 5mm Dyneema HB25, 5mm PU Foam and

a thin 1.5mm Aluminum 6061-T6 backing inertial

backing plate to provide confnement or the porous

oam. Problem set up and fnal deormation shape are

shown in Figure 13. Complete crushing o the PU oam,

delamination o the D1 layer, and brittle racture o

the ceramic is observed. This is comparable to theactual test sample. Time taken to arrest the projectile

was 0.20ms.

Discussion II (Experimental vs Autodyn Results)

Having completed the simulations and the live

fring experiments, it is possible to compare the

results o each in order to determine the quality o the

material modeling and the relative perormance o the

composites. Table 8 summarizes the results.

Overall Penetration Depth

It is to be expected that Autodyn produces results

which show larger overall depth and bulge width

compared to those observed in the experiment because

o the confnement eects due to the experimental

set up. It can be seen that the simulations have

approximated the deormation in the 11-directionairly accurately, with a maximum dierence o 11.65%.

Eos - P-Alpha

Re Density 1.265 gm/cm3SolidCompactionPressure

112.54 MPa Parameter C1 2.490km/s

Porous Density 0.1602 gm/cm3 Solid EOS Shock Parameter S1 1.56

Porous Soundspeed

669.44 m/sGruneisenCoefcient

1.55 Specifc Heat 86J/kgk

Initial CompactionPressure

2.6 MPa Re Temp 300KCompactionCurve

Standard

CompactionExponent

3.0

Strength Model - von Mises

Shear Modulus 68.58 MPa Yield Stress 6.64 MPa

Failure Model - Hydro(Pmin

) Erosion

Hydro Tensile limit - 2 GPa Erosion Strain 2.0 Geometric Strain Instantaneous



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However, as there were many assumptions made in theAutodyn Simulations, we can only conclude that themodeling provides a general guidance to the behavior

o the system.

Time Taken To Stop Projectile

C-D1 perormed the worst in this experiment

as it took the longest time, 0.32ms, to stop the

projectile, probably due to the ailure mechanism o

delamination and plastic deormation o the Dyneema

fbers. Addition o the PU oam (C-D1-P1-Al) cut down

the time by about one-third (~0.12ms) due to energy

dissipation through volume compression, and hence

less work done on the Dyneema to cause stretching

and delamination.

CONCLUSIONS FROM THIS STUDY

Our initial design, which was arrived at by

considering how shock (stress) waves propagate

through several classes o materials, has been shown

to perorm dynamically in a way that is close to

what was predicted rom simulations beore doing

the integral experiments. It has been shown that a

composite plate consisting o a very hard frst layer,

a wave spreading second layer, and a shock absorbing

third layer will perorm better than conventional

armor steel o equivalent areal density.

Ceramic has proven, as expected based on prior

results in the literature, to be important as a frst layer

to meet the incoming projectile. However, ceramics

NIJ Level Mass (gm) Velocity (m/s) KE (J) Remarks

I 2.6 320 133.12 .22 Cal

I 6.2 312 301.77 380 ACP

IIA 11.7 312 569.46 .40 Cal

II 10.2 425 921.19 9mm

IIIA 15.55 426 1410.98 .44 Cal

III 9.7 838 3405.88 7.62mmIV 10.8 868 4068.49 7.62mm AP

This Experiment

Material Mass (gm) Velocity (m/s) KE (J) Remarks

A2 Tool Steel 8.7 484 1019.01 1" Cylinder

Table 9: NIJ Ballistic Protection Kinetic Energy Conversion Chart

Target

Conguration

Projectile

Shape/ Material

Type Overall

Depth

(mm)

Hole

Depth

(mm)

Hole

Crater

(mm)

Hole

Dia

(mm)

Time

(ms)

C-D1 Rod (1") E 45.97 39.36 25.51 25.51

Impact Velocity.483m/s

A2 - RHC 55-56 A 49.90 43.20 16.85 12.87 0.320

D 9.14% 9.76% -33.94% -49.6%

C-D1-P1-A1 Rod(1") E 35.12 21.27 31.76 31.76

Impact Velocity.481m/s

A2 - RHC 55-56 A 39.21 32.91 17.32 13.04 0.200

D 11.65% 54.72% -45.47% -58.94%Legend:C - Ceramic 6mm think E - Experimental

D1 - Replicate closely Dyneema HB25 5mm thick A - Autodyn

P1 - PU oam 5mm thick, = 0.16gm/cm3 D - Deviation

Table 8: Comparison o Experimental and Autodyn Results



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alone are not good armor materials because o theirbrittle racture behavior. They convert a great deal o

kinetic energy into racturing their own matrix, but

once ailed, provide very litt le penetration resistance.

We expect that the ceramic layer will be even more

eective against projectiles made o soter materials,

where signifcant plastic deormation in the projectile

will occur.

Our primary scientifc result is thatthe idea o creating a composite

armor by design, based on eachmaterial providing a unique dynamicproperty or properties to the systemresponse, has merit.

In this study, a wave spreading second layer such

as an advanced fber composite, Dyneema, has proven

to be an important asset in penetration resistance.

By delaying the shock wave propagation in the

11-direction, time is allowed or the shock energy to

be spread and dissipated through the target in the

22/33-directions.

Porous oam as a third layer has proven to be a

good shock attenuator by widening the shock rise time

to delay the shock wave propagation. By absorbing

the impact kinetic energy through compaction o

the porous material, and turning it into waste heat

through PV-work, it reduces the amount o kineticenergy to be dissipated by the Dyneema, thus reducing

total deormation as well as total t ime taken to arrest

the projectile.

Through observations o the ailure mechanism

o each layer (racture, energy absorption, energy

dissipation), it is concluded that the sequence o the

layering armor concept is undamentally correct, and

that the next stage o work would be to optimize the

thickness and perormance o each layer, as well as

improving our ability to more accurately model these

materials.

POTENTIAL OF THIS METHOD FOR ARMOR

DESIGN

Our primary scientifc result is that the idea

o creating a composite armor by design, based on

each material providing a unique dynamic property

or properties to the system response, has merit. We

believe that this approach, along with undamental

physics inormation and insights can lead to higher

perormance armor concepts. This is in contrast to a

trial-and-error experimental approach. As mentionedearlier, the kinetic energy o the projectile (ignoring

projectile geometry) is about 1000 joules. From

Table 9, this can be roughly classifed as an American

National Institute o Justice (NIJ) Level II ballistic

threat. On this basis, we may examine the potential o

this research or uture armor designs.

Composite (NIJ Level III) For Reerence

- Kevlar 29 / Aramid + Al 2O

3Ceramic 19.00 1.86 3.53

* Water has reerence density o 1.0 gm/cm3

Table 10: Comparison o this study with Commercial Armor.

Rank Composite(NIJ Level II Equivalent)

Total Thickness(mm)

Average Density*(gm/cm3)

Areal Density(gm/cm2)

1 Kevlar / Aramid Ballistic Panel 5.00 0.84 0.42

2 Ballistic Steel Plate

(1.45GPa Yield Strength)

3.20 7.78 2.49

3 Ceramic + Dyneema 11.00 2.52 2.77

4 Ceramic + Dyneema + Porous Polyure-thane + Aluminum

17.50 1.91 3.34

5 AISI 4140 Steel (655MPa Yield Strength)

4.76 7.85 3.74



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71tech edge

2. Other co-authors o the original paper were:

Mr. Boey Chung Wai, Principal Engineer, Singapore

Technologies Kinetics P te. Ltd.

Proessor Robert S. Hixson, Department o Physics, NavalPostgraduate School.

Proessor Jose O. Sinibaldi, Department o Physics, Naval

Postgraduate School.

3. J. R. Robbins, Ding J.L., Y. M. Gupta, Load Spreadingand Penetration Resistance o Layered Structures A

Numerical Study, International Journal o Impact

Engineering30, no. 6 (2004): 593615.

4. B. A. Gama, T. A. Bogetti, B. K. Fink, Yu C. J., T. D. Claar,

H. H. Eiert, J. W. Gillespie Jr., Aluminum Foam IntegralArmor: A New Dimension in Armor Design, CompositeStructures 52 (2001): 381-395.

5. Y. M. Gupta, Ding J. L., Impact Load Spreading in Layered

Materials and Structures: Concept and QuantitativeMeasure,International Journal o Impact Engineer ing27,

no. 3 (2002): 277291.

6. M. L. Wilkins, Mechanics o Penetration and Peroration,

International Journal o Engineering Science 16, no. 11(1978): 793807.

7. G. R. Fowles and D. R. Curran, Experimental Testing o

Shock Attenuating Materials, AFSWC-TDR-6222 (March

1962).

8. W. Herrman, Constitutive Equation or the DynamicCompaction o Ductile Porous Materials, Sandia

Laboratories, Albuquerque, New Mexico, 12 December

1968. Reprinted romJournal o Applied Physics 40, no. 6(1969): 2490-2499.

9. Poh C. W., Investigation o New Materials and Methods

o Construction o Personnel Armor, MSc thesis, Naval

Postgraduate School, Monterey, Caliornia, December 2008.

10. Boey C. W., Investigation o Shock Wave AttenuationIn Porous Materials, MSc thesis, Naval Postgraduate

School, Monterey, Caliornia, December 2009.

11. T. J. Holmquist, G. R. Johnson, Response o Boron

Carbide Subjected To High Velocity Impact,International

Journal o Impact Engineer ing35, no. 8 (2008): 742-752

12. C. E. Anderson Jr, G. R. Johnson and T. J. Holmquist,

Ballistic Experiments and Computations o Confned

99.5% Al2O3 Ceramic Tiles, in Proceedings o the 15th

International Symposium on Ballistics, M. Mayseless, S. R.Bodner, eds., Vol. 2, Jerusalem, Israel, 1995, 6572.

13. Robert M. Jones, Mechanics o Composite Materials, 2nd

Edition (London: Taylor and Francis, 1999), 68-70.

14. J. W. S. Hearle, High Perormance Fibers (Cambridge:

Woodhead Publishing, 2001), 78.

15. D. E. Grady and N. A. Winree, A Computat ional

Model or Polyurethane Foam, in Fundamental Issues

and Applications o Shock-Wave and High Strain Rate

By optimizing the layers urther,it is possible that this approachwill eventually surpass current NIJ

Level III solutions using Aramidcomposites.

Table 10 provides a rough ranking o the test samples

against well-established ballistic protection solutions

using ballistic steel and Kevlar. It should be noted

that the test samples were not tested to ailure

(penetration) as it was not the objective o this

research to ascertain the NIJ equivalent o the test

samples, and we do not claim that the test samples

are ready or feld applications. However, it can be

observed that this approach to armor design is ableto provide immediate success with an areal density

(weight) that is comparable to ballistic steel. By

optimizing the layers urther, it is possible that this

approach will eventually surpass current NIJ Level III

solutions using Aramid composites. We also envision

this approach leading to a single composite material

that is created using a mater ial-by-design process that

could combine several benefcial dynamic properties

or armor usage. This process can be readily extended

to higher perormance, design concepts guided by

both calculations and experiments.

ACKNOWLEDGMENTS

The author would like to thank his supervisor and

mentor, Proessor Robert Hixson or his guidance

and enthusiasm during his studies at the Naval

Postgraduate School, without which this research

would not have been possible. His encouragement

and confdence were instrumental in the publication

o this research in theInternational Journal o Impact

Engineering.

ENDNOTES

1. This is a condensed version tailored to the interests o SAF

POINTER readers. The original paper was frst published

in the International Journal o Impact Engineering 38,

no. 5 (May 2011): 369-383, http://dx.doi.org/10.1016/j.

ijimpeng.2010.12.003.



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72tech edge

MAJ Ong is a Commando Ofcer by vocation and is currently a Weapon StaOfcer in GS (Development) Systems Integration Ofce. He is a recipient

o the SAF Academic Training Award and SAF Postgraduate Scholarship. He

holds a BEng (First Class Honours) in Civil Engineering and MSc in Deence

Technology & Systems rom NUS, and a MSc o Science (Distinction) in CombatSystems Technology rom the Naval Postgraduate School. MAJ Ong is also

concurrently pursuing a PhD in Protective Technology rom the Department

o Civil Engineering, NUS.

Phenomena, By K. P. Staudhammer, L. E. Murr, M. A.Meyers, eds. (Oxord: Elsevier Sc ience, 2001), 485-491.

16. M. F. Ashby, et al, Metal Foams: A Design Guide, (Oxord:

Butterworth-Heinemann, 2000); L. J. Gibson and M.

F. Ashby, Cellular Solids: Structure and Properties, 2nd

Edition (New York: Cambridge University Press, 1997).

17. General Plastics Manuacturing Company, LAST-A-

FOAM FR-6700 Aircrat Foam, June 2009, http://

www.generalplastics.com/products/product_detail.

php?pid=15.

18. Ong C. W., Investigation O Advanced Personnel

Armor Using Layered Construction, M.S. thesis,Naval Postgraduate School, Monterey, Caliornia,

December 2009.

advanced layered personnel armour

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