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The weapons of modern warfare are as varied as they

are complex and sophisticated. Unlike previous centuries and millennia, during which warriors generally

knew that they would need a shield to counter the

sword or arrow, steel plate

to counter the ball round

from small arms, or an

ironclad warship to counter

the cannon, today’s armor

must be designed to counter any number of threats.

Present day warfighters must be equipped with protection against adversaries that use sophisticated

weapon systems and tactics as well as those that use

improvised weapons and tactics.

As vastly important as armor is to the protection of

soldiers and vehicles, the development and fielding of

better armor materials is an exceedingly difficult,

long-term task. This is evident in the infrequency

with which new materials are introduced for both

body and vehicle armor. The military is always seeking lighter, better performing material systems.

Lighter materials improve mobility, while better performing materials improve protection. Applying

these improvements force-wide can greatly augment

the capability of our military to accomplish mission

goals. However, since armor material systems must

confront innumerable types of weapons and threats,

endure myriad environments and external stimuli,

and sometimes perform secondary functions (e.g.,

multifunctional armor), a material’s ability to withstand a simple projectile impact is clearly not the only

consideration.

While the tools of warfare have been evolving for

millenia, most major advancements in armor (like

many other technologies) have occurred within the

last century. Metallic materials were and still are in

many cases the most effective armor materials.

Although more recently most of the effort has been

focused on developing advanced polymer, ceramic,

transparent, composite and hybrid systems, there is

still interest in improving the mature metallic armor

segment. Even so, advancements in metallic armor

are relatively few and far between.

This issue of the AMMTIAC Quarterly focuses

especially on armor materials and presents a look at

some of the recent armor innovations. The first article introduces a new high hardness, metallic armor.

This is a rather significant development since the last

time a new high hardness armor steel was introduced

to the military, American forces were engaged in

Vietnam. The TechSolutions article provides a summary of some recent advancements in novel armor

materials and systems, including liquid armor. The

last article summarizes the novel behavior of seemingly ordinary materials when they are exposed to

extremely high velocity penetrators and forces.

This issue is just a sample of some important

developments in armor technology. There are many

other significant developments ongoing as scientists

and engineers in the Department of Defense and

supporting industrial and academic communities are

working hard to enhance the protection of the

warfighter against the weapons of modern warfare.

Ben Craig, Editor

Armor from

Ancient to

Modern Warfare

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http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 4, Number 4 3

INTRODUCTION

The US armor community is currently

accelerating efforts to deliver lightweight

armor technologies that can defeat armorpiercing (AP) projectiles at reduced areal

weights and that are available across a large

industrial base. While many of these programs involve the application of lowerdensity metals, such as aluminum and titanium, the selection of steel alloys is still

competitive for many ballistic and structural applications; the ability to fabricate

armor components in both commercial and

military operational areas with available

equipment and personnel is a major advantage of steel solutions. To meet these

requirements, the US armor community

has increased the availability of quenched

and tempered armor steels by updating current steel military specifications, the most

important of which has been the updated/revised MIL-DTL-46100E, Armor

Plate, Steel, Wrought, High-Hardness.[1]

This improved specification was necessary

to supply the large steel demands for combat operations in Iraq and Afghanistan.

This high hardness armor (HHA) specification allows modern, continuous processing

technologies to be used efficiently as well as

the introduction of a new class of autotempered high hardness steels.

The US Army Research Laboratory

(ARL) was directed to investigate various

ways to expand current steel armor plate

production as the large military demand

for armor plate exceeded the current production capacity at US steel facilities for

quench and tempered HHA steel armor

plate. The solution was to expand the availability of HHA steels under the current military specification to include a new class of

air-quenched, auto-tempered steels that do

not use existing water quench and temper

facilities. Allegheny Technologies Incorporated (ATI) developed an auto-tempered

ATI 500-MILTM* steel alloy (see Figure 1)

that has physical and mechanical properties

which meet the current HHA specification.

ARL procured sufficient amounts of ATI

500-MILTM plate to allow acceptance testing and subsequent certification of ATI

500-MILTM plate as complying with the

First Article requirements of the newly

revised MIL-DTL-46100E specification.

Currently, the highest-performing USmade steel alloys for armor piercing (AP)

bullet protection are manufactured to

MIL-DTL-46100E HHA with a hardness

range of 477-534 Brinell hardness number

(BHN) and to MIL-A-46099C Dual

Hardness Armor (DHA) that is produced

by roll-bonding a 601-712 BHN front

plate to a 461-534 BHN back plate.[2]

The roll-bonded DHA steels are complex

to produce and have known production

limitations. In the near-term, the US Army

will be releasing a new ultra high hardness

steel specification for plate hardness over

534 BHN that will further expand the

hardness range for ballistic applications.

The improved ballistic resistance of steel as

a function of increasing hardness is well

established in the ballistic community.[3]

HHA steel increases AP bullet defeat,

reduces armor weight, and is less difficult

to manufacture than the DHA. This article

documents the development of ATI 500-

MILTM plate and subsequent ballistic testing and inclusion into the specification as

Class-2 auto-tempered HHA steel.

ATI 500-MILTM PLATE

In June 2008, ATI announced the successful launch of a new class of HHA specialty

steel. This next-generation armor steel,

designated ATI 500-MILTM, was developed

Dwight D. Showalter

William A. Gooch

Matthew S. Burkins

Jonathan S. Montgomery

Richard J. Squillacioti

Weapons and Materials Research Directorate

Army Research Laboratory

Aberdeen Proving Ground, MD

Figure 2. ATI 500-MILTM ballistic resistance

against 7.62x51 mm, M80 ball rounds at

2750 ± 50 ft/s. Front face (top) and back face

(bottom) showing 35 hits on a 0.25”x12”x12”

plate.

Figure 1. ATI 500-MILTM high hardness auto-tempered armor steel plate.

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4 The AMMTIAC Quarterly, Volume 4, Number 4

in response to limited US HHA production and limited performance features of materials in this class. ATI 500-MILTM alloy is

melted, rolled, and finished in the US on fully integrated assets

owned and operated by ATI. This new material is designed to offer

additional features that were not previously available in traditional

quench and temper high hardness armor steels. Product design is

also geared to obtain improvements in ballistic and blast resistance

when compared with other HHA materials. ATI 500-MILTM steel

plate is designed to meet the requirements in MIL-DTL-46100E

while also offering features that address several common challenges

frequently encountered with conventional HHA plates.

The composition of ATI 500-MILTM alloy includes appreciable

amounts of Ni-Cr-Mo, which results in relatively high hardenability and increased toughness compared to other HHA alloys (see

Tables 1 and 2).[4] As a result, the balanced combination of unique

properties and consistent quality allows this alloy to meet the specifications outlined in the MIL-DTL-46100E, which was recently

revised to account for these improvements.

ATI 500-MILTM armor addresses secondary processing difficulties associated with various operations. Specifically, operations

such as forming (cold and hot), cutting or sectioning, and postoperation heat treatments for restorations of ballistic properties

were successfully alleviated.

These post-process improvements are partly due to the fact that

the alloy is auto-tempered upon air cooling, thereby eliminating

the traditional liquid-quenching and temper treatment. The slower air cooling combined with ATI’s proprietary processing results in

significantly higher dimensional armor stability.

Residual stresses in ATI 500-MILTM products are also reduced

compared to traditional liquid-quenched and tempered products.

These improvements result in flatter armor products that exhibit

minimal distortion during fabrication operations, such as hot or

cold cutting. Since the product is auto-tempered, the alloy does not

require any special post-welding operations involving liquid

quenching and temper to restore ballistic properties.

Experimental Procedure

The ballistic performance of ATI 500-MILTM steel plates (see

Figure 2) was determined by obtaining the V50 ballistic limit for

each thickness of plate against the corresponding specified test

projectile. The test methodology is described in detail in the MILSTD-662F.[5] The V50 ballistic limit is the velocity at which an

equal number of fair impact complete penetration (target is defeated) and partial penetration (target is not defeated) velocities are

attained using the up-and-down firing method. Fair impact is

defined as occurring when a projectile with an acceptable yaw

strikes the target at a distance of at least two projectile diameters

from a previously damaged impact area or edge of plate. A complete penetration is determined by placing a 0.020-in. (0.5-mm)

2024 T3 aluminum witness plate 6-in. (152.6-mm) behind and

parallel to the target. If any penetrator or target fragment strikes

this witness plate with sufficient energy to create a hole through

which light passes, the result is considered a complete penetration.

A partial penetration is any impact that is not a complete penetration. For the MIL-DTL-46100E specification, the V50 ballistic

limit is defined as the average of six fair impact velocities comprising the three lowest velocities resulting in complete penetration

and the three highest velocities resulting in partial penetration. A

maximum spread of 150 ft/s (45.7 m/s) is permitted between the

lowest and highest velocities employed in determining ballistic

limits. The resulting data for the ATI 500-MILTM steels were

compared to the baseline ballistic acceptance data of MIL-DTL46100E.

Test Projectiles

The eight ATI 500-MILTM plates tested for First Article certification

ranged in thickness (nominal) from 0.1875-in. (4.8-mm) up to 1-in.

(25.4-mm). The corresponding test projectiles and plate obliquities

required for each thickness under MIL-DTL-46100E are listed in

Table 3. The weights and sizes of the projectiles are shown in Table 4.

These projectiles are shown in Figures 3 and 4, with the 14.5-mm

BS41 being a tungsten carbide core and the rest hardened steel. In

Table 4. Geometries and weights of projectiles utilized in ATI 500-MILTM plate testing.

Projectile Core

Projectile Type Length Diameter Weight Length Diameter Weight

in. (mm) in. (mm) grains (g) in. (mm) in. (mm) grains (g)

0.30-cal. APM2 1.39 (35.3) 0.31 (7.85) 166 (10.8) 1.08 (27.4) 0.24 (6.2) 81 (5.3)

0.30-cal. APM2 2.31 (58.7) 0.51 (12.98) 708 (45.9) 1.87 (47.5) 0.43 (10.9) 400 (25.9)

14.5-mm B32 2.61 (66.3) 0.59 (14.86) 990 (64.1) 2.09 (53.1) 0.49 (12.4) 633 (41.0)

14.5-mm BS41 2.07 (52.6) 0.59 (14.94) 975 (63.2) 1.27 (32.3) 0.43 (10.9) 585 (37.9)

Table 3. Thickness ranges and corresponding test projectiles

for First Article testing (30° obliquity).

Nominal Thickness Range Test Projectile

in. (mm)

0.118 (3.00) to 0.300 (7.62) incl. 0.30-cal. APM2

0.301 (7.62) to 0.590 (15.0) incl. 0.30-cal. APM2

0.591 (15.0) to 0.765 (19.4) incl. 14.5-mm B32

0.766 (19.4) to 1.130 (28.7) incl. 14.5-mm BS41

Table 1. Chemical composition of ATI 500-MILTM plate.

Alloy % C (max) % Si % Mn (max) % P (max) % S (max) % Cr (max) % Ni (max) % Mo (max)

ATI 500-MILTM 0.22-0.32 0.25-0.45 0.80-1.20 0.020 0.005 1.60-2.00 3.50-4.00 0.22-0.37

Table 2. Mechanical properties of ATI 500-MILTM plate.

Alloy Hardness BHN Charpy-V Yield Strength Tensile Strength Elongation (%)

-40°C, ft/lbf (J) ksi (MPa) ksi (MPa)

ATI 500-MILTM 477-534 20 (27) 150 (1034) 260 (1792) 13

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http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 4, Number 4 5

some cases, additional testing was conducted outside

this range to allow the data

to be graphed. This is noted

for the nominal 0.315-in.

(8-mm) thickness.

RESULTS AND

DISCUSSION

The V50 ballistic limits

and standard deviation (σ)

for each plate thickness

were determined experimentally for the ATI 500-

MILTM plates; the data is

shown in Table 5 for each

test projectile. Figure 5

plots the V50 velocities

against the ATI 500-

MILTM plate thickness as

well as the acceptance velocity specification curve for HHA steel

(MIL-DTL-46100E). The ballistic advantage of increased alloying

can be seen in Figure 5

where all the plates exceeded the minimum velocity

acceptance velocities of the

specification. The differences were significant for

the thinner plates and approached the acceptance

line as the thickness

increased. The solid lines of

the acceptance curves for

MIL-DTL-46100E incorporate approximately two

standard deviations reduction below typical performance, which provides an

acceptable variance to allow

the high hardness plate to

meet the specification.

The 0.625-in. thick

plate that did not meet the velocity requirement for the first ballistic plate was retested in accordance with the procedures in the

Figure 3. The 0.30-cal. APM2 and 0.50-cal. APM2 test projectiles. Figure 4. The 14.5-mm BS41 (top) and B32 test projectiles (bottom).

Figure 5. ATI 500-MILTM plate thickness vs. V50 velocity for various rounds (all

tests at 30° obliquity).

0.000 0.163 0.326 0.489 0.652 0.815 0.978 1.141 1.304

Thickness (in.)

Velocity (ft/s)

3500

3000

2500

2000

1500

1000

500

0

Minimum Ballistic Limit for 0.30 caliber APM2

ATI 500-MILTM vs 0.30-cal. APM2

Minimum Ballistic Limit for 0.50 caliber APM2

ATI 500-MILTM vs 0.50-cal. APM2

Minimum Ballistic Limit for 14.5-mm B32

ATI 500-MILTM vs 14.5-mm B32

ATI 500-MILTM vs 14.5-mm B32 retest

Minimum Ballistic Limit for 14.5-mm BS41

ATI 500-MILTM vs 14.5-mm BS41

35.6mm

57.5mm

12.9mm

7.84mm

Steel Core

Lead Filler

Lead Filler

Copper Jacket

Steel Core

Lead Filler

Copper Jacket

Table 5. V50 plate acceptance results. (All results obtained at a 30° angle of obliquity)

Nominal Thickness Projectile Actual Thickness V50 Standard Deviation

in. (mm) in. (mm) ft/s (m/s) ft/s (m/s)

0.1875 (4.8) 0.30-cal. APM2 0.202 (5.1) 2174 (663) 43 (13)

0.250 (6.35) 0.30-cal. APM2 0.272 (6.9) 2688 (819) 36 (11)

0.3125 (7.94) 0.30-cal. APM2 0.305 (7.7) 2672 (814) 40 (12)

0.3125 (7.94) 0.50-cal. APM2 0.305 (7.7) 2058 (627) 47 (14)

0.375 (9.53) 0.50-cal. APM2 0.381 (9.7) 2373 (723) 43 (13)

0.500 (12.70) 0.50-cal. APM2 0.517 (3.1) 2582 (787) 56 (17)

0.625 (15.88) 14.5-mm B32 0.614 (15.6) 2396 (730) 43 (13)

0.625 (15.88)* 14.5-mm B32 0.607 (15.4) 2424 (739) 32 (9)

0.750 (19.05) 14.5-mm B32 0.742 (18.8) 2760 (841) 43 (13)

1.000 (25.40) 14.5-mm BS41 0.966 (24.5) 2851 (869) 56 (17)

*Retest

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6 The AMMTIAC Quarterly, Volume 4, Number 4

specification. The second plate passed by 35 ft/s. The effect of the

projectile diameter to the plate thickness may be a contributing

factor on possible plug formation for this projectile thickness. At

1-in. thickness, the ability to harden the plates by air quenching

may be reaching a limit, resulting in the V50 velocity approaching

the requirement. The armor applications for HHA plates over

0.750-in. are limited, and the most important observation is the

response of the thinner plates to the ballistic test projectiles. This

significant performance is a direct result of the alloying of ATI

500-MILTM steel. The first ordered thickness of MIL-DTL46100E starts at 0.118-in. (3-mm); ATI Defense is expected to

eventually produce plates between 0.118- and 0.1875-in.

CONCLUSIONS

This report has documented the ballistic performance of the first

Class-2 auto-tempered HHA steel under MIL-DTL-46100E. The

increased alloying of ATI 500-MILTM steel has resulted in a very

tough high hardness steel for both blast and ballistic applications,

such as those shown in Figures 6-8. The development and availability of an air-quenched, auto-tempered HHA steel increases the

availability of high hardness plate, as traditional water or oil

quench and temper facilities are not required. This new class of

tough HHA steel plates will increase the metallic armor solutions

for armor designers.

ACKNOWLEDGMENTS

This article is based on the information documented in US Army

Research Laboratory Technical Report ARL-TR-4997 [6].

The authors would like to acknowledge Andrew Nichols, Larry

Martin, Ronald Bailey, and Glenn Swiatek from ATI Defense for

their contributions with regard to material composition and production specifications. Photos courtesy of Allegheny Technologies

Incorporated and Carapace Armor Technology.

NOTE & REFERENCES

*ATI 500-MIL is a trademark of ATI Properties, Inc.

[1] “Armor Plate, Steel, Wrought, High-Hardness 2008,” MIL-DTL46100E, October 2008.

[2] “Armor Plate, Steel, Roll-Bonded, Dual Hardness (0.187 Inches to

0.700 Inches Inclusive),” MIL-A-46099C, 1987.

[3] Rapacki, E., K. Frank, B. Leavy, M. Keele, and J. Prifi, “Armor Steel

Hardness Influence on Kinetic Energy Penetration,” Proceedings of the 15th

International Symposium on Ballistics, May 1995.

[4] “ATI 500-MILTM High Hard Specialty Steel Armor,” ATI Defense

Data Sheet, 3 September 2008.

[5] “V50 Ballistic Test for Armor,” Department of Defense Test Standard,

MIL-STD-662F, December 1997.

[6] Showalter, D., W. Gooch, M. Burkins, J. Montgomery, and R.

Squillacioti, “Development and Ballistic Testing of a New Class of AutoTempered High Hard Steels Under Military Specification MIL-DTL46100E,” Army Research Laboratory, ARL-TR-4997, September 2009.

Figure 6. Machined Component made from ATI 500-MILTM high hardness armor steel.

Figure 8.

StrykShield

situational

awareness kit

by Carapace

Armor.

Figure 7. StrykShield

situational awareness kit

by Carapace Armor

Technology made using

ATI 500-MILTM high hardness

armor steel.

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Stephanie L. Knoeller

AMMTIAC

Rome, NY techsolutions 14

Protecting the Warfighter – Recent Armor Innovations

INTRODUCTION

Global military conflicts have historically exposed the deficiencies in

soldier and weapon system protection and conversely, the effectiveness of weapons. Thus, rapid advancements in

armor technology are typically associated with

these global conflicts. For instance, World War I

marked the beginning of a rapid evolution in

armor systems. The armor during this period was

restrictive, bulky, and heavy, and was not commonly used at the time because the materials

technology was not sufficiently advanced.

However, by World War II, the Army began providing its forces with body armor for protection

against shrapnel and munitions fragments, but it

failed to protect soldiers from pistol, rifle, and

knife threats. As the war continued, soldiers were

equipped with improved protection systems; and

by the end of the war, they were being suited

with armor containing aluminum plates.[1]

Operation Iraqi Freedom (OIF) and

Operation Enduring Freedom (OEF) have also

demonstrated the need for advancements in

armor. This article covers some of the armor

technologies for soldiers and vehicles that have

evolved since the engagement of forces in OIF

and OEF as well as those that are in development and use.

LIQUID BODY ARMOR

Although significant improvements have been

made, conventional body armor is still heavy and

bulky and only provides protection for the head

and chest.[2] The current armor design for chest

protection consists of several Kevlar®* layers

with ceramic tile inserts. This armor design is

much too restrictive to provide protection to

other body parts without inhibiting soldier agility, and thus soldier extremities

often remain unprotected.[2] The current body armor is designed to resist penetration from small arms ammunition

(SAA) and typically is inadequate against

large weapons. Projectiles from ever

improving weapons continue to be one

of the most serious threats faced by the

individual soldier during daily deployment.[3]

The introduction of urban conflict in OIF and OEF has demonstrated the additional need for stab and puncture resistant

armors.[4] The close quarters that often accompany urban conflict

have exposed soldiers to direct stab assaults and to more frequent

encounters with sharp objects, such as razor wire, broken glass, and

other debris that can be used as weapons.[4] In an effort to satisfy

the need for a body armor that provides stab and extremity protection, the US Army Research Laboratory’s (ARL)

Weapons and Materials Directorate collaborated

with the University of Delaware’s Center for

Composite Materials to create a lighter, more

flexible armor solution.[2] This effort resulted in

the development of liquid body armor: Kevlar®

impregnated with a shear thickening fluid

(STF).[2, 4]

Background

A shear thickening fluid is a colloidal suspension

that behaves in a non-Newtonian manner, which

means that shear stress increases nonlinearly with

the increase in the rate of shear strain.[5] More

importantly, because these fluids are shear thickening they exhibit an increase in viscosity† with

an increasing shear rate.[5]

A colloidal suspension is a mixture that contains nano-sized particles larger than those of

normal solutes yet small enough to remain suspended in the dispersing medium.[6] Liquid

body armor consists of silica nanobits suspended in ethylene glycol as shown in Figure 1.[7]

When concentrated in solution, the silica

nanobits form hydroclusters‡ under applied

stress.[2, 8] The hydroclusters make a solid barrier and prevent the penetration of sharp objects

and SAA upon impact. Above the critical shear

rate§, shear thickening fluids often exhibit large,

sometimes discontinuous increases in the viscosity of the system due to particle dynamics.[9]

This thickening process is reversible, allowing

the STF-Kevlar® to return to its normal state

following impact.[2] Figure 2 shows the

mechanics of a shear thickening fluid,

while Figure 3 demonstrates how shear

thickening fluids enhance the protection

of armor systems by forming a rigid surface.[10]

Production

Ethanol solvent and the STF are combined to create the STF impregnation

fluid. Ethanol enhances the ability of the Kevlar® fibers to uptake the

STF suspension. Once combined, Kevlar® or another fiber is submerged in the STF/ethanol mixture. The fabric’s fibers spontaneously absorb the solution. The ethanol solvent is then removed using a

hot convection oven.[2]

Figure 1. Colloidal suspension of silica

and ethylene glycol magnified

40,000 times.[7]

Figure 2. STFs are filled with rigid, colloidal particles. At high shear rates,

shear thickening occurs as the hydrodynamic forces overcome the repulsive, inter-particle forces, and hydrodynamic clusters form. As the particles

collide, the material becomes macroscopically more rigid and resistant to

penetration by incoming projectiles.[10]

Figure 3. When Kevlar® is impregnated with STFs,

under normal conditions the material has a high

flexibility; however, under the high shear rates

imposed during impact, the STF becomes rigid and

enhances the ballistic protection of the fabric.[10]

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8 The AMMTIAC Quarterly, Volume 4, Number 4

techsolutions 14

Testing

Shear thickening, fluid-impregnated fabrics have performed well

in several tests. These fabrics increase the quality and level of protection offered by neat** fabrics without compromising weight,

flexibility, or comfort.[11] One test, using a smooth bore helium

gun, demonstrated that four layers of STF-impregnated Kevlar®

provided the same level of protection as ten layers of the neat

Kevlar® with no weight increase.[2]

Another study demonstrated that both Kevlar® and nylon when

impregnated with an STF were superior to their neat versions (of

equivalent areal densities) in stab and cut resistance. The STF

restricted fiber motion and thereby prevented the object from passing through.[8] Further testing demonstrated that the shape and

hardness of the particles used in the colloidal suspension do affect

the level of protection offered.[12] Platelet-shaped clay particles

demonstrated a marked increase in needle resistance and yarn pullout when compared to the spherical shaped silica particles, but

were inferior in ballistic tests. This

suggests that the platelet particle

STFs become more effective as the

size of the threat decreases. Softer

particles were shown to be effective against spike penetration

while worse in ballistic protection,

suggesting that particle hardness

plays a role in resisting higher

energy ballistic impact.[13]

STF fabrics have demonstrated

potential for several alternative

applications. These include bomb

blankets, suspicious package covers, and as ankle supports for

jump boots that stiffen on

impact.[14] Future efforts have

been planned to investigate the

impact of STFs on different fabrics varying weave, type, and

denier. Further efforts will also

investigate the layer sequencing

and STF-to-fabric ratio in addition to increasing the target sizes

and projectile velocity during

testing.[10]

REACTIVE ARMOR

Reactive armor was originally developed to enhance the protection

of ground vehicles (see Figure 4) while minimizing weight and

cost. They have proved to be effective against shaped-charges and

long rod penetrators. Passive armors typically employ hard, fracture-resistant materials to defeat energetic and projectile threats,

whereas active armors counter these threats using combinations of

passive and energetic materials.[15, 16]

Originally developed to counter the ever-changing munitions

threats reactive armor has

since evolved. Several newer

types of reactive armor have

been created including

explosive reactive armor

(ERA), self-limiting explosive reactive armor (SLERA),

non-explosive reactive armor

(NxRA), and Non-Energetic

Reactive Armor (NERA).

Explosive Reactive Armor

Explosive reactive armor typically consists of two metal sheets

arranged in a sandwich configuration with a layer of explosive

material in the middle.[17, 18] ERA is used to defeat or deflect

shaped-charge anti-tank weapons including hollow charges, as well

as kinetic projectiles, small arms ammunition, and shrapnel.[18]

Upon impact by a projectile, the ERA explodes causing the metal

plates to separate, which can subsequently deflect or defeat the

threat at a small standoff distance from the vehicle the armor is

protecting. Once the armor explodes, however, that portion of the

vehicle becomes vulnerable to other attacks.[17, 19] ERA is a common add-on armor as it provides increased protection and is combat proven.[15] In terms of protecting against shaped charges, the

ERA explosion disrupts the plasma jet that is created from the

shaped-charge warhead and decreases its penetrating power.[17]

One disadvantage to ERA is that the high explosive detonates outside the protected vehicle and subjects the surrounding area to

small blast loads.[19]

Self-Limiting Explosive Reactive Armor

Self-limiting explosive reactive armor is considered a passive form

of ERA because of the low mass of explosives used.[15, 16, 18] The

explosive materials of SLERA are placed in a specific configuration

to confine the energy released to a controlled area.[16] This configuration results in decreased performance compared to ERA.

However, because SLERA has a limited and controlled explosion,

it is potentially capable of providing multi-hit protection when

applied in a modular configuration. While SLERA is not as effective as ERA, it may prove to be a more practical solution due to its

survivability characteristics.[18]

Non-Explosive and Non-Energetic Reactive Armor

Non-explosive reactive armor uses gas-generating material and

other non-explosive systems to dissipate the impact energy of

hollow-charged warheads.[16, 18] Since it is non-explosive, it is

relatively easy to integrate onto vehicles. In addition, because it does

not explode on impact, NxRA is less damaging to vehicle structures

and relatively inexpensive.[16] NxRA is similar to another form of

reactive armor, non-energetic reactive armor, in that neither form

uses energetic components nor are they consumed when hit. Both

can be applied to lightweight vehicles because they weigh less than

conventional reactive armor types and provide multi-hit protection

against chemical energy munitions.[15, 16, 18] Non-energetic reac-

“A shaped charge is defined as a

cylinder of explosive with a hollow

cavity at the end opposite the initiation train. If this cavity does not

contain a liner, it is referred to as a

hollow charge or an unlined-cavity

charge. If the cavity contains a liner

made from a metal, an alloy, glass,

ceramic, wood, or another material,

the device is termed a shaped

charge. The liner geometry may be

conical, hemispherical, parabolic, or

any arcuate device. If the liner is

‘bow shaped’ it is called an explosively formed penetrator (EFP).

Shaped jets with conical, hemispherical, or bow shaped liners

collapse to form jets and/or slugs

in different manners.”[1]

[1] Walters, W., “A Brief History of

Shaped Charges,” ARL-RP-232,

December 2008.

Figure 4. An M2A2 Bradley

Fighting Vehicle with reactive

armor. (Photo by Staff Sgt. Shane

A. Cuomo, US Air Force)

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AMMTIAC A D V A N C E D M A T E R I A L S , M A N U F A C T U R I N G A N D T E S T I N G

tive armor provides excellent survivability and greater multiple-hit

capability when compared to NxRA. These armors commonly contain an inert layer between two metal sheets that dissipates the energy from the incoming projectile.[18]

Lightweight Enhanced Reactive Armor

Lightweight enhanced reactive armor (LERA), developed for light

and medium weight combat vehicles, incorporates state-of-the-art

technologies in reactive armor along with an insensitive, high-energy explosive. LERA has been subjected to extensive qualification

testing with more than 1,500 LERA armor tiles tested. LERA has

proven to be an excellent lightweight solution as it can defeat the

shaped-charge threats associated with an urban battlefield, while

having an incident-free safety record in field operation and combat. It is currently available for integration on light and medium

combat and tactical wheeled vehicles.[20]

Electric Reactive Armor

Electric reactive armor uses an electrical current to convert a

shaped-charge or rod penetrator into plasma, vapor, and a harmless mixture of melted and pulverized debris that disperses around

the vehicle.[21] The electric current is able to convert the shapedcharge jets because it enhances the hydrodynamic instabilities

within the jet. This causes it to break into a string of particles that

cannot penetrate the armor as a continuous jet would. A similar

effect is observed when a jet is heated to its melting point.

However, when it reaches its melting point, the jet breaks apart

and expands into a series of rings that have little penetration power.

Importantly, it takes less current to melt a jet than to vaporize

it.[22] The British military has tested electric reactive armor

against rocket-propelled grenades (RPGs). The armor was

observed to provide complete protection except for a few scratches

and dents. Electric reactive armor is advantageous because it

weighs one-tenth that of conventional reactive armor while providing the same level of protection; it can thus be more easily integrated onto vehicle structures.[21]

RECENT ARMOR TECHNOLOGY DEVELOPMENTS

Sensors for Crack Detection in Ceramic Armor Tiles

Today’s military uses ceramic body armor plates because they provide good ballistic protection at a relatively low weight. However,

once the plates are damaged they offer limited protection. Plates

are not only damaged as a result of use, but sometimes they can be

damaged during manufacturing and shipping.[23]

Research is currently underway to improve the protection of

lightweight body armors and helmets for individuals by investigating several different sensors that could be used for detecting

damage in the ceramic inserts.[24-26] Under investigation are

fiber optic strain sensors and conductive paint.[25, 26]

Testing has also been performed to determine a method for

using piezoelectric lead zirconate titanate (PZT) transducers to

characterize the vibrational modes of ceramic vehicle/body armor

support system (VBASS) plates as a way of detecting cracks. It was

found that when damaged, the plates produce a different signal

than when undamaged. This research resulted in the development

of a handheld device for crack detection. However, future work

needs to be done to develop a more robust system.[23]

Carbon Nanotubes

Carbon nanotubes (CNTs)

have excellent mechanical,

electrical, and magnetic

properties.[27, 28] They

can be woven like cloth into

body armors that are several times stronger, tougher,

and stiffer than those currently in use.[27] CNTs can

withstand multiple ballistic impacts, although research does suggest

that a minor interval is required between hits for full recovery.[28]

Mosaic Transparent Armor

Traditional transparent armors are made of layers of laminate,

transparent materials such as SiC, boron carbide (B4C), or alumina. These traditional armors impose significant costs when damaged, as the entire armor surface must be replaced. In an effort to

reduce costs, researchers have developed mosaic transparent armor

(MTA). MTA is comprised of small transparent tiles, made from

traditional transparent armor materials and held together by an

adhesive with the same refractive index as the tiles.[29]

It is thought that the mosaic tiles will restrict the damage from

a ballistic impact to a portion of the armor surface. Therefore, only

the damaged tiles need to be replaced.[29] Not only does mosaic

transparent armor localize damage, it is also lower in weight and

easier to fabricate than conventional transparent armor. Weight

savings is provided in the fact that less transparent material is

required, as the tiles are smaller and the spaces filled with an almost

negligible weight adhesive. A prototype window made of this

mosaic transparent armor is currently being developed using transparent glass and ceramic tiles.[29]

Aerogel Composites

Researchers have developed a technology that uses inorganic aerogels, such as silicon dioxide and carbon, to absorb kinetic energy.

This silica aerogel†† system is lightweight and low in density.

When impacted, the network collapses relatively slowly. The

crushable aerogel layer was designed for placement between two

elastomeric fabric layers. This technology has the potential for

body armor applications, as it is envisioned that the aerogel layers

would be thin and flexible with a protective composite consisting

of multiple layers.[30]

Magnesium Alloys for Lightweight Vehicle Armor

Magnesium (Mg) is the lightest structural metal; it has a density

lower than that of iron, titanium, and aluminum. Mg alloys are

therefore of high interest for lightweight armor applications.

However, before Mg alloys are employed for ground vehicle

armor, commercial scale manufacturing capability and capacity

must be demonstrated.[31]

ARL has partnered with industry to develop and establish a

commercially scalable direct chill cast Mg alloy. Two Mg alloys

were developed from this partnership: a high strength and an ultra

high strength alloy, both alloyed with yttrium and zirconium.

High strength refers to a yield strength of approximately 350 MPa,

whereas ultra high strength materials have a yield strength of

approximately 500 MPa. Yield strength is used to define the point

Figure 5. Undamaged and damaged

mosaic transparent armors show the

importance of damage localization.

(Photo Courtesy of Lawrence

Livermore National Laboratory)

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10 The AMMTIAC Quarterly, Volume 4, Number 4

techsolutions 14

at which plastic deformation begins and is important because the

higher the strength the less likely a material is to permanently

deform and eventually fracture, making it useless as an armor

material. The high strength alloy has demonstrated exceptional

ballistic performance, showing promise for application as an armor

material.[31] The ballistic properties of the ultra high strength

alloy have yet to be determined.

Composite Lightweight Adaptable Reactive Armor

Composite Lightweight Adaptable Reactive Armor (CLARA) is

under development to provide light armored vehicles protection

against shaped charges and kinetic energy penetrators. CLARA is a

metal-free, low fragment reactive armor.[32, 33] CLARA is designed

to provide protection against ammunition less than 12.7 caliber.[33]

CONCLUSION

From personal body armor to vehicle armor, innovation and

change are necessary to protect a well-equipped and agile force

during times of global conflict. This article briefly reviewed several of the advancements that have been made in body armor systems

to produce lighter weight, flexible systems which can provide the

warfighter with superior protection without compromising mobility and strength. Vehicle armor systems have also evolved into

more sophisticated, lighter weight systems. While these advances

are significant, future forces will undoubtedly require new armor

technologies to counteract even more advanced weapons and penetration threats.

NOTES & REFERENCES

* Kevlar is a registered trademark of the E.I. du Pont de Nemours and Company.

† Viscosity quantifies a fluid’s ability to resist flow. For instance, a high viscosity

fluid has a strong resistance to flow, while a low viscosity fluid flows readily.

‡ Hydroclusters are transient aggregates that result from hydrodynamic lubricating

forces between the particles in the suspension.[2, 8]

§ Critical shear rate is the point at which the dynamic shear thickening is approximately equal to the steady state shear thickening. This point must be achieved in

order for the suspension to thicken.[9]

** The term “neat” refers to fabrics in their conventional, unaltered form.

†† Aerogels are highly porous, extremely lightweight solids formed by replacing

particles in a gel with a gas.

[1] Wittman, R.E. and R.F. Rolsten, “Armor – Of Men and Aircraft,” Advances in

Structural Composites, 12th National SAMPE Symposium, October 1967.

[2] Lee, Y., E.D. Wetzel, R.G. Egres Jr., and N.J. Wagner, “Advanced Body Armor

Utilizing Shear Thickening Fluids,” 23rd Army Science Conference, December

2002.

[3] Poh, C.W., “Investigations of New Materials and Methods of Construction of

Personal Armor,” Naval Postgraduate School Thesis, December 2008, DTIC Doc.

ADA493730.

[4] Wetzel, E., R. Egres, Jr., Y. Lee, et al., “‘Liquid Armor’: Protective Fabrics

Utilizing Shear Thickening Fluids,” IFAI 4th International Conference on Safety and

Protective Fabrics, October 2004.

[5] Munson, B., D. Young, and T. Okiishi, “Fundamentals of Fluid Mechanics,”

5th Edition, John Wiley & Sons, 2006.

[6] Brown, T., H. LeMay, Jr., B. Bursten, and C. Murphy, “Chemistry: The

Central Science,” 10th Edition, Pearson Education, Inc., 2006.

[7] Lee, Y.S., E.D. Wetzel, and N.J. Wagner, “The Ballistic Impact Characteristics

of Kevlar® Woven Fabrics Impregnated with a Colloidal Shear Thickening Fluid,”

Journal of Material Science, Vol. 38, 2003, pp. 2825-2833.

[8] Wetzel, E., R. Egres Jr., M. Decker, et al., “Stab Resistance of Shear Thickening

Fluid (STF)-Kevlar Composites for Body Armor Applications,” 24th Army Science

Conference, 2005, DTIC Doc. ADA433286.

[9] Lee, S.Y., N. J. Wagner, “Dynamic Properties of Shear Thickening Colloidal

Suspensions,” Rheologica Acta, Vol. 42, No 3, 2008, pp. 199-208.

[10] Wetzel, E.D., and N.J. Wagner, “Advanced Body Armor Utilizing Shear

Thickening Fluids,” 23rd Army Science Conference, December 2002.

[11] Arndt, M., “Body Armor Fit For A Superhero,” BusinessWeek, The McGrawHill Companies, August 2006.

[12] Rosen, B., C. NamLaufer, et al., “Multi-Threat Performance of Kaolin-Based

Shear Thickening Fluid (STF)-Treated Fabrics,” SAMPE 2007, June 2007.

[13] Kalman, D., J. Schein, J. Houghton, et al., “Polymer Dispersions Based Shear

Thickening Fluid-Fabrics For Protective Applications,” SAMPE 2007, June 2007.

[14] Johnson, T., “Army Scientists, Engineers Develop Liquid Body Armor,” Today

in the Military, April 2004.

[15] “Add-On-Reactive Armor Suits,” Defense Update, No. 1, 2004.

[16] “Reactive Armor Technologies under Development for Battle Tanks,”

Advanced Materials & Processes, Vol. 159, No. 9, September 2001, pp. 38-39.

[17] “Developing Science and Technologies List, Section 9: Ground Combat

Systems Technology,” Defense Threat Reduction Agency, August 2003,

http://www.dtic.mil/mctl/DSTL/DSTLSec09g.pdf.

[18] Yael, C-A., E. Sokol-Barak, S. Friling, and M. Tzalik, “Non-Explosive

Energetic Material and Reactive Armor Element Using Same,” US Patent No.

7,360,479, April 2008.

[19] Held, M., “Stopping Power of ERA Sandwiches as a Function of Explosive

Layer Thickness or Plate Velocities,” Propellants, Explosives, Pyrotechnics, Vol. 31,

No. 3, 2006.

[20] “LERA: Lightweight Enhanced Reactive Armor,” General Dynamics,

http://www.gdatp.com/files/PDF/A116_LERA.pdf, accessed 7 August 2009.

[21] Shachtman, N., “U.S. Military Uses the Force,” Wired, August 22, 2002.

[22] Hummer, C., “Inductance of Parallel Plates in Electromagnetic Armor,” Army

Research Laboratory, ARL-TR-3788, May 2006.

[23] Meitzler, T., G. Smith, M. Charbeneau, et al., “Crack Detection in Armor

Plates Using Ultrasonic Techniques,” Materials Evaluation, American Society for

Nondestructive Testing, June 2008, DTIC Doc. ADA493437.

[24] Serna, J., “Sensing Protection Issues,” Daily Pilot, January 2, 2008.

[25] Stewart, C., “Ceradyne Backs Maria Feng’s UCI Research on Armor Defects,”

OC Register, January 4, 2008.

[26] “Non-Destructive Damage Detection in Advanced Ceramic,” http://

mfeng.calit2.uci.edu/Maria_Feng/Research_activities/NDE/page1_NDE.htm.

[27] Rincon, P., “Super-Strong Body Armor in Sight,” BBC News, October 23,

2007.

[28] Mylvaganam, K., and L.C. Zhang, “Ballistic Resistance Capacity of Carbon

Nanotubes,” Nanotechnology, Vol. 18, 2007.

[29] Elder, R., “Mosaic Transparent Armor,” Lawrence Livermore National

Laboratory, https://ipo.llnl.gov/?q=technologies-mosaic_transparent_armor,

accessed June 25, 2009.

[30] “Kinetic Energy Absorbing Aerogel Composite Structures for Use in Crash of

Impact Protection and Body or Vehicle Armor,” Johns Hopkins University Applied

Physics Laboratory, June 1, 2009.

[31] Cho, K., et al., “Magnesium Technology and Manufacturing for Ultra-Lightweight Armored Ground Vehicles,” ARL-RP-236, February 2009.

[32] “Protection,” Dynamit Nobel Defence, http://www.dn-defence.com/en/index_

2.html, accessed 7 August 2009.

[33] von Kospoth, N., “Closing the Gaps of Modern Military Requirements,”

Defense.Professionals, July 2009, http://www.defpro.com/daily/details/359/, accessed

7 August 2009.

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INTRODUCTION

The conditions encountered during modern warfare have required

researchers to examine a vast array of available materials to augment

the protection of soldiers and weapon systems. As weapons continue to advance, it is important to determine the behaviors of materials under extreme conditions ranging from explosions to hypervelocity* penetrator impact. Such conditions often exist only over

the period of ten to 30 nanoseconds. Surprisingly, relatively common materials, such as plate glass, poly(methyl methacrylate)

(PMMA, more commonly known as Plexiglas®†), and polyurea/

polyurethane, are of great interest as they exhibit quite unexpected

capabilities under these extreme conditions.

Weapons in modern warfare, such as shaped charges and explosively formed projectiles (EFPs), pose great danger to military

forces. EFPs are formed by the explosive compression of shallow

copper cones; resultant fragments are then expelled at rates reaching

9,000 ft/s. These large, lethal fragment trains can be accelerated to

speeds nearly three times that of an assault rifle bullet. Even higher

velocities are achieved by shaped charge jets, which can reach speeds

between 15,000 and 30,000 ft/s. Shaped charge jets are typically

produced from lower-angle copper cones and approach the highest

velocities generated by human activity, outside of particle accelerators. Shaped charge jets are used in a number of weapons including

rocket-propelled grenade (RPG) warheads. These deadly hypervelocity penetrators are some of the more challenging threats that

must be countered by armor designers. The force of an explosion is

also an important consideration to armor design, particularly when

a sizable explosive is detonated in direct contact with a target.

This article describes the behaviors of plate glass, PMMA, and

polyurea/polyurethane over nanosecond time periods. These materials, which often have a glassy phase, are of emerging importance

to armor development. For very small transient periods, several of

these materials exhibit strength-to-weight values in excess of highstrength steel and titanium, while some demonstrate self-healing

capabilities. These time-scale-dependent properties are significant in

the fields of armor and blast protection.

COMMON MATERIALS WITH UNCOMMON BEHAVIORS

Many of the materials that are useful for armor and blast protection

applications exhibit unique behaviors under dynamic situations,

such as high explosive loadings or hypervelocity impacts. One

feature commonly exhibited by such materials is a glassy state,

which on a molecular scale refers to a period of short-range order or

an amorphous condition. This glassy state precludes the formation

of crystalline fracture planes, often associated with fully crystalline

solids, that are frequently accompanied by imperfections (e.g., point

defects, dislocations, etc.). Plate glass, PMMA, and polyurea/

polyurethane exist in a glassy state and exhibit unique behaviors

under dynamic conditions.

In the first case of glass, under high loading rate conditions the

material is converted from a glassy solid to a compacted volume of

energetic pulverized powder. The pulverized glass phase employs its

energetic, multi-fractured surfaces to pinch the shaped charge jet or

hypervelocity fragment and erode the surface through friction and

chemical interaction.

Poly(methyl methacrylate) employs another method for interfering with a high loading rate penetrator. Impact causes the material

with moderate strength and ductility properties under normal conditions to convert to an ultra-high strength-to-density material with

brittle attributes at high loading conditions. Material failure occurs

by the formation of a conical (Hertzian) fracture front, which

absorbs the kinetic energy of the penetrator through the fragmentation of the PMMA. This material is particularly resilient when constrained by a flexible membrane backing.

In the case of selected polyurethane/polyurea polymers, when

applied to a metal backing the polymer actually changes the

response of the metal in these nanosecond periods and is capable

of self-healing. High-strength steels often fail by adiabatic shear

localization, and computer finite element simulations of shear localization/banding have shown that the polymer delays or suppresses

the shear localization and thus allows the steel to plastically deform

without fracture.

Each of these materials is explored further in the following

sections. Their uncommon behavior at high loading rate conditions is also described in more detail.

Plate Glass

One of the interesting features of plate glass is its ability to sustain

an alternate physical state precipitated by what are called failure

waves‡. According to some researchers, a material exposed to a failure wave can be rendered almost instantly into a comminuted

state§.[1-3] The sizes of the comminuted particles can range from

that of a typical powder all the way down to the nanoscale. The

failure waves are generated from stress waves, which are initiated at

the glass surface by the impact of the jet or projectile. The stress

waves propagate through the glass and are reflected back on each

Roshdy G.S. Barsoum

Office of Naval Research

Arlington, VA

Philip J. Dudt

Naval Surface Warfare Center, Carderock

West Bethesda, MD

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12 The AMMTIAC Quarterly, Volume 4, Number 4

other when they encounter defects in the glass or material boundaries. These numerous reflection interactions are also points where

fracture initiates ahead of the penetration of the high velocity projectile or jet. In total, these interacting points of fracture generate a

continuous failure wave. Failure wave velocities have been measured around 16,000 ft/s.[4] The fractured/comminuted material

ahead of the projectile would occupy a greater volume than the

unfractured glass because of ruptured and stretched atomic bonds;

however, the surrounding material confines the volume and

induces compressive forces, which creates a pinching effect on the

jet or projectile. The expansion of the glass is also suppressed by

interlocking and friction forces in the comminuted volume. This

results in resistance to the projectile or jet, which has to push

through the compressed, erosive debris field.

The high stopping power of plate glass was a notable finding during World War II studies. Glass was found to be the strongest material for resisting shaped charge jet penetration, and researchers

hypothesized on how glass exerted a pinching effect on the shaped

charge jet.[5] Combining their conceptions with the idea of failure

waves appears to support a strong defeat mechanism as just

described, which combines the idea of pinching with an erosive

comminuted particle field. This would be relevant to high-speed

penetrators and to a shaped charge jet that had been slowed in transiting the glass.

Figure 1 shows a layered glass construction that has been

exposed to a shaped charge jet. The high degree of glass fracture is

obvious as are the ring-like patterns radiating from the strike area,

which are related to internal failure waves generated in the glass.

Another aspect of the comminuted state, with its very fine localized

particles, may enhance chemical reactivity by increasing the

exposed surface area. The highly charged surfaces at the most nanofractured volumes in the glass can perhaps be thought of as a solid

plasma that exists over a very small time interval. As a result of the

highly energetic fracture surfaces created by the fracture wave, the

copper jet forms red glass, which is a copper-silicate material.[6]

Glass, though still very effective at stopping shaped charge jets, is

not as effective against less chemically reactive steel jets.[5]

Poly(Methyl Methacrylate)

Poly(methyl methacrylate) is a transparent glassy polymer under

normal conditions. Under typical loading conditions, PMMA

exhibits moderate strength and ductility, which is true for a wide

range of polymers. However, as seen in Figure 2, there is a marked

increase in the stiffness (modulus) and strength as the rate of loading increases, causing this relatively soft material to become much

more rigid.[7, 8] Dynamic testing rates as high as 15,000 in./in./sec

are noted in the technical literature.[9] While the transient strength

and stiffness increase, the ductility decreases noticeably transforming the polymer into a high-strength-to-density material with a

brittle-like nature. The strength-to-density ratio (called the specific strength) is in excess of high strength steels and titanium alloys

under these conditions.

Unlike plate glass, PMMA does not generate failure waves since

the stress wave propagation is much slower. The type of fracture

favored in the PMMA is classified as Hertzian, which occurs when

a fractured cone is generated at the strike point of a high-speed

projectile. When the cone of fractured material is generated ahead

of the projectile, it can further impede the projectile’s progress. In

addition, energy is transferred by momentum to the fractured

pieces.

PMMA is used in a number of conventional, laminated, transparent armor designs, demonstrating the transitional capabilities for

Hertzian fracture even against shaped charge jets. Figure 3 shows the

results from the impact of a shaped charge jet on a PMMA panel.

The shaped charge penetrated the PMMA and then produced a

wide-angled Hertzian cone of fractured material.

Polyurea/Polyurethane

Certain polyurea and polyurethane polymers also exhibit interesting behavior under dynamic conditions. These materials are distinct, as they contain two phases: a hard domain that has a high

glass transition temperature (with some limited order at normal

temperatures) and a soft domain with a low glass transition (flexible at normal temperature). The hard domains serve as cross-links

and reinforcement for the soft phase. At very high deformation

Figure 1. Comminuted

glass pattern formed

as a result of a shaped

charge impact.

Figure 2. Dynamic rate promotes Hertzian fracture.

Figure 3. Hertzian failure pattern from shaped charge jet entry.

Increasing Rate

Strain

Entrance Point of Shaped Charge Jet

Exit Point with Hertzian Cone

Stress

Hertzian Cracking

(Cone Shape)

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rates, the soft domains are believed to

exhibit a rate-induced transition toward a

glassy state.[10] The polymer as a whole

approaches a glassy state over a very small

transient time period under high loading

rate conditions.

At conventional testing rates, these polymers exhibit a low yield, modulus, and high

elongation-to-failure, 300 to 800 percent or

more. These properties are more typical of

what would be expected from common

rubbery materials. However, as the rate of

loading increases to values on the order of

10,000 in/in/sec, unlike the brittle PMMA,

the polymers exhibit a multi-fold increase

in yield and modulus while retaining their

high elongation properties.[11] This

mechanical property enhancement is extenuated when the material is in a confined

state with surrounding material that

restricts its deformation. Similar to PMMA,

the transient specific strength of polyurea or

polyurethane is in excess of high strength

steel and titanium alloys while retaining

high levels of ductility. However, under

some conditions at extremely high rates, the

polymers can also transform to recoverable

high-shear fluids. It is possible that a larger

volume is occupied by the glassy-transitioned region than the material under more

benign conditions. Consequently, there is a

snap-back effect demonstrated by the polymers after penetration by a projectile. In

this respect, there is a self-healing tendency

in these types of polymers. Figure 4 shows

sealing around a bullet that was stopped in

a thick layer of polyurea polymer and the

“healed” region where a bullet has penetrated through.

Demonstrating other extreme loading

regimes, Figure 5 shows the test setup and

results using contact charges against

polyurea-coated and uncoated 50-in. diameter steel circular panels.

With a contact charge, maximal loads are produced in the underlying material, which is accelerated by the explosive force generated.

The instantaneous explosive force applied tends to be much larger

underwater with a better coupling of the shock loads and the panel;

therefore, these charges were detonated underwater. Furthermore,

there is a large follow-on, pulsating bubble

(see Figure 6) that can produce further

damage to an underwater structure over a

few milliseconds.

The resulting damage to coated and

non-coated panels from the detonated

charges is shown in Figure 7. In the first

photograph, there is no coating of polyurea

polymer, and significant rupture occurs in

the panel. The rupture level would have

been even greater in size had there been a

larger holding fixture. Four large petals

were formed as part of the rupture pattern.

Increasingly thick layers of polyurea polymer are noted in the subsequent photographs. As the polymer thickness increases,

the damage to the panels decreases. For

instance, the number of petals formed

decreases from four petals to three, followed

by two (a central split), and then no apparent rupture for the 5/8-in. thick polymer

coated steel panel. It did not matter significantly in the test whether the polymer coating is on the front or back of the panels.

Similar benefits can also be obtained by

applying the polymer to composite materials, but the mechanisms are different. An

alternate or supporting mechanism has been

suggested, where the polymer undergoes a

near instantaneous transformation to a

higher volume state that markedly increases

its rigidity. This is somewhat analogous to

the water-ice transition, except the transformations all occur within the solid state that

resists the dynamic applied loads.

Contrary to other armor materials,

increasing the thickness of polyurea on a

steel substrate continuously does not translate to a continuous increase in performance (mass efficiency). The reason is its

interaction is a localized phenomenon,

which occurs at nano-time scales on the

surface of the steel. Beyond the optimum thickness, the polyurea

becomes just an added mass burden on the armor. To understand

this phenomenon, which is almost instantaneous, bifurcation instability dynamic analysis was used to find the optimum thickness

of the polyurea. It is clear that this thickness is a function of the

weapon and the thickness of the substrate.[12]

Figure 4. Healing of penetrated area in

polyurea layer.

Figure 5. Test set-up for plates with polyurea

coating. The charge was placed in direct contact as shown.

Figure 6. High speed photographs of bubble

expansion and contraction after detonation.

Figure 7. Resistance to rupture from different-thickness polyurea to equivalent contact charges.

None, 4 petals 0.4-in. front, 3 petals 0.5-in. back, 2 petals 0.75-in. front, no rupture 1-in. back, no rupture

Bullet Trapped in Polyurea

Coating with Sealing

around Base

Holding Fixture

Charge

Test Plate (without coating)

Sealed/Healed Area after

Transit of Bullet

Charge

Detonation

on Polyurea

Coated Plate

1 2

3 4

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14 The AMMTIAC Quarterly, Volume 4, Number 4

MEASUREMENTS AND PREDICTIONS

Performing in-situ measurements in truncated time intervals and

under extreme conditions, such as hypervelocity collisions and

shock extremes, is very challenging. Sensitive equipment can be

damaged from fragments and explosive blast loads. The nano to

microsecond time intervals require special imaging techniques, such

as high-speed photography and flash X-rays, to capture the gross

response. However, much of the detailed micro-structural or molecular-level behavior can only be postulated from conditions produced after the event and from computer predictions. After-the-fact

examination can include non-reversible phase changes produced

under the high dynamic pressures, chemical activity, evidence of

microstructural shear banding in metals, evidence of localized melting on polymer fracture surfaces, and procedures such as low-angle

(LAX) radiography to examine the inter-molecule bonds between

hard and soft molecular phases in polyurea and polyurethane polymers. An overall “holistic” examination approach provides clues for

the level of sophistication and detail required for computational

tools needed to understand the event at these time scales and to postulate the global performance. Current tools use EulerianLagrangian finite element or smooth particle hydrodynamics and

have to be coupled with additional codes for tracing wave propagation. Because of the highly nonlinear behavior of the nano-time

materials, stress wave propagation may have to be considered at a

very small molecular unit level.

Computational simulations (see Figure 8) normally make use of

material constitutive models, which are equations that correlate

material response with applied stress, time, and temperature, assuming behavior is largely path-independent. Extrapolated data is normally used from Split-Hopkinson bar (HPB) and pressure/shear

plate impact tests, but at the extremes encountered, there is considerable guesswork in applying these values. Theoretical work suggests

that thermo-mechanical-high strain (from time-temperature relaxation using the Williams-Landel-Ferry (WLF) equation) behavior

for polymers can be inferred from dielectric constant verses frequency (GHz) determinations under extreme pressure.[13, 14] Based on

this idea, a facility that was used for making artificial diamonds has

been modified for making these measurements. Hopefully, this data

can be used with the constitutive equations to form a firmer basis

for extrapolating and predicting material response.

CONCLUSION

The behavior of a number of off-the-shelf materials in nanoseconds

under extreme conditions is an interesting area of research. The

range of possible materials is, of course, more extensive than

covered here and includes glass-ceramic compositions, variations in

polymer molecular configurations, and inclusions of nanoparticles.

Transient strength-to-weight properties can be developed that

exceed those of high strength steel and titanium while maintaining

high ductility, transparency, and/or self-healing capabilities. To

work in the modern battlefield environment, these materials are

best used together to complement their strengths and weaknesses.

It is also important to incorporate conventional crystalline solids

where applicable. The potential of this class of materials is an

emerging area of great interest for defeating threats in modern

warfare. Use of these concepts in military and civilian applications

can be found in references 15-17 and other pending patents.

NOTES & REFERENCES

* Hypervelocity refers to the muzzle velocities of small arms, artillery,

and tank-launched projectiles in excess of 5000, 3500, and 3350 ft/s,

respectively.

† Plexiglas is a registered trademark of the Rohm & Haas Company.

‡ A failure wave is the rapid propagation of networked cracks upon impact

or compression loading.

§ A comminuted state refers to a highly fractured/powdered condition.

[1] Heine-Geldern, R. and E. Pugh, “The Photography of High-Speed

Metallic Jets,” Meteoritics, Vol. 1, No. 1, 1953.

[2] Bless, S., N. Brar, G. Kanel, and Z. Rosenberg, “Failure Waves in

Glass,” Journal of the American Ceramic Society, Vol. 75, No. 4, 1992.

[3] Bless, S. and N. Brar, “Failure Waves and their Effects on Penetration

Mechanics in Glass and Ceramics,” Shock Wave Science and Technology

Reference Library, Springer, 2007.

[4] Hornemann, U., H. Rothenhausler, and H. Senf, “Experimental

Investigation of Wave and Fracture Propagation in Glass Slabs Loaded by

Steel Cylinders at High Impact Velocities,” Third Conference on Mechanical

Properties at High Rates of Strain, Institute of Physics, 1984.

[5] Pugh, E., “Defeat of Shaped Charge Weapons,” Final Report, Carnegie

Institute of Technology, April 1960, DTIC Doc. AD0316551.

[6] Hauver, G., P. Netherwood, R. Benck, and A. Melani, “Penetration of

Shaped-Charge Jets into Glass and Crystalline Quartz,” Ballistic Research

Laboratory, Report No. BRL-TR-3273, September 1991, DTIC Doc.

ADA243520.

[7] Fleck, N., W. Stronge, and J. Liu, “High Strain-Rate Shear Response of

Polycarbonate and Polymethyl Methacrylate,” Proceedings of the Royal

Society of London, A429, 1990, pp. 459-479.

[8] Moy, P., T. Weerasooriya, W. Chen, and A. Hsieh, “Dynamic StressStrain Response and Failure Behavior of PMMA,” Proceedings of 2003

ASME International Mechanical Conference, November 2003.

[9] Rittel, D., “A Note on the Dynamic Failure of PMMA,” International

Journal of Fracture, Vol. 106, No. 2, November 2000, pp. 3-8.

[10] Pathak, J., J. Twigg, K. Nugent, D. Ho, E. Lin, P. Mott, C. Robertson,

M. Vukmir, T. Epps, and C. Roland, “Structure Evolution in a Polyurea

Segmented Block Copolymer Because of Mechanical Deformation,”

Macromolecules, Vol. 41, No. 20, 2008, pp. 7543-7548.

[11] Amirkhizi, A., J. Isaacs, J. McGee, and S. Nemat-Nasser, “An

Experimentally-Based Viscoelastic Constitutive Model for Polyurea,

Including Pressure and Temperature Effects,” Philosophical Magazine,

Vol. 86, No. 36, December 2006, pp. 5847-5866.

[12] Freund, L.B., “The Influence of a Surface Coating on the High Rate

Fragmentation of a Ductile Material,” ONR Final Report, 2005.

[13] Knauss, W.G. and J. Zhao, “Improved Relaxation Time Coverage in

Ramp-Strain Histories,” Mechanics of Time-Dependent Materials, Vol. 11,

Nos. 3-4, December 2007, pp. 199-216.

[14] Roland, C.M. and R. Casalini, “Effect of Hydrostatic Pressure on the

Viscoelastic Response of Polyurea,” Polymer, Vol. 48, No. 19, September

2007, pp. 5747-5752.

[15] Barsoum, R. and P. Dudt, “Armor Including a Strain Rate Hardening

Elastomer,” US Patent 7,300,893, November 2007.

[16] Barsoum, R.G.S., R.O. Peterson, D.P. Owen, W.C. Jones, and D.E.

Johnson, “Mine and Collision Protection for Passenger Vehicle,” US

Patent 7,114,764, October 2006.

[17] Dudt, P.J. and R.G.S. Barsoum, “Elastomeric Damage-Control

Barrier,” US Patent Application No. 20070093158, Patent pending, April

2007.

Figure 8. Extremely

high pressure and

strain rates

extrapolated models

from HPB, plate

impact, and WLF.

Pressure Range

in Polymer

2-4 GPa

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