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

Stephanie L. Knoelle

AMMTIAC

Rome, NYtechsolutions14Protecting the Warfighter Recent Armor Innovations

INTRODUCTIONGlobal military conflicts have historically exposed the deficiencies insoldier and weapon system protection and conversely, the effective-ness of weapons. Thus, rapid advancements inarmor technology are typically associated withthese global conflicts. For instance, World War Imarked the beginning of a rapid evolution inarmor systems. The armor during this period wasrestrictive, bulky, and heavy, and was not com-monly used at the time because the materialstechnology was not sufficiently advanced.However, by World War II, the Army began pro-viding its forces with body armor for protection

against shrapnel and munitions fragments, but itfailed to protect soldiers from pistol, rifle, andknife threats. As the war continued, soldiers wereequipped with improved protection systems; andby 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 alsodemonstrated the need for advancements inarmor. This article covers some of the armortechnologies for soldiers and vehicles that haveevolved since the engagement of forces in OIFand OEF as well as those that are in develop-ment and use.

LIQUID BODY ARMORAlthough significant improvements have beenmade, conventional body armor is still heavy andbulky and only provides protection for the headand chest.[2] The current armor design for chestprotection consists of several Kevlar* layers

with ceramic tile inserts. This armor design ismuch too restrictive to provide protection toother body parts without inhibiting sol-dier agility, and thus soldier extremitiesoften remain unprotected.[2] The cur-

rent body armor is designed to resist pen-etration from small arms ammunition(SAA) and typically is inadequate againstlarge weapons. Projectiles from everimproving weapons continue to be oneof the most serious threats faced by theindividual soldier during daily deploy-ment.[3]

The introduction of urban conflict in OIF and OEF has demon-strated the additional need for stab and puncture resistantarmors.[4] The close quarters that often accompany urban conflicthave exposed soldiers to direct stab assaults and to more frequent

encounters with sharp objects, such as razor wire, broken glass, andother debris that can be used as weapons.[4] In an effort to satisfythe need for a body armor that provides stab and extremity protec-

tion, the US Army Research Laboratorys (ARL)Weapons and Materials Directorate collaboratedwith the University of Delawares Center forComposite Materials to create a lighter, moreflexible armor solution.[2]This effort resulted inthe development of liquid body armor: Kevlar

impregnated with a shear thickening fluid(STF).[2, 4]

Background

A shear thickening fluid is a colloidal suspensionthat behaves in a non-Newtonian manner, whichmeans that shear stress increases nonlinearly withthe increase in the rate of shear strain.[5] Moreimportantly, because these fluids are shear thick-ening they exhibit an increase in viscosity withan increasing shear rate.[5]

A colloidal suspension is a mixture that con-tains nano-sized particles larger than those ofnormal solutes yet small enough to remain sus-pended in the dispersing medium.[6] Liquidbody armor consists of silica nanobits suspended in ethylene glycol as shown in Figure 1.[7]

When concentrated in solution, the silicananobits form hydroclusters under appliedstress.[2, 8] The hydroclusters make a solid bar-rier and prevent the penetration of sharp objectsand SAA upon impact. Above the critical shearate, shear thickening fluids often exhibit largesometimes discontinuous increases in the viscos-ity of the system due to particle dynamics.[9]This thickening process is reversible, allowingthe STF-Kevlar to return to its normal state

following impact.[2] Figure 2 shows themechanics of a shear thickening fluid

while Figure 3 demonstrates how shearthickening fluids enhance the protectionof armor systems by forming a rigid surface.[10]

ProductionEthanol solvent and the STF are com-bined to create the STF impregnation

fluid. Ethanol enhances the ability of the Kevlar fibers to uptake theSTF suspension. Once combined, Kevlar or another fiber is sub-merged in the STF/ethanol mixture. The fabrics fibers spontaneously absorb the solution. The ethanol solvent is then removed using ahot convection oven.[2]

Figure 1.Colloidal suspension of silica

and ethylene glycol magnified40,000 times.[7]

Figure 2.STFs are filled with rigid, col-loidal particles. At high shear rates,shear thickening occurs as the hydro-dynamic forces overcome the repul-sive, inter-particle forces, and hydro-dynamic clusters form. As the particlescollide, the material becomes macro-scopically more rigid and resistant topenetration by incoming projec-tiles.[10]

Figure 3.When Kevlar is impregnated with STFs,

under normal conditions the material has a high

flexibility; however, under the high shear ratesimposed during impact, the STF becomes rigid and

enhances the ballistic protection of the fabric.[10]

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The AMMTIAC Quarterly, Volume 4, Number 48

techsolutions14TestingShear thickening, fluid-impregnated fabrics have performed wellin several tests. These fabrics increase the quality and level of pro-tection offered by neat** fabrics without compromising weight,

flexibility, or comfort.[11] One test, using a smooth bore heliumgun, demonstrated that four layers of STF-impregnated Kevlar

provided the same level of protection as ten layers of the neatKevlar with no weight increase.[2]

Another study demonstrated that both Kevlar and nylon whenimpregnated with an STF were superior to their neat versions (ofequivalent areal densities) in stab and cut resistance. The STFrestricted fiber motion and thereby prevented the object from pass-ing through.[8] Further testing demonstrated that the shape andhardness of the particles used in the colloidal suspension do affectthe level of protection offered.[12] Platelet-shaped clay particlesdemonstrated a marked increase in needle resistance and yarn pull-out when compared to the spherical shaped silica particles, but

were inferior in ballistic tests. Thissuggests that the platelet particleSTFs become more effective as thesize of the threat decreases. Softerparticles were shown to be effec-tive against spike penetration

while worse in ballistic protection,suggesting that particle hardnessplays a role in resisting higherenergy ballistic impact.[13]

STF fabrics have demonstratedpotential for several alternativeapplications. These include bomb

blankets, suspicious package cov-ers, and as ankle supports for

jump boots that stif fen onimpact.[14] Future efforts havebeen planned to investigate theimpact of STFs on different fab-rics varying weave, type, anddenier. Further efforts will alsoinvestigate the layer sequencingand STF-to-fabric ratio in addi-tion to increasing the target sizesand projectile velocity duringtesting.[10]

REACTIVE ARMORReactive armor was originally developed to enhance the protectionof ground vehicles (see Figure 4) while minimizing weight andcost. They have proved to be effective against shaped-charges andlong rod penetrators. Passive armors typically employ hard, frac-ture-resistant materials to defeat energetic and projectile threats,

whereas active armors counter these threats using combinations ofpassive and energetic materials.[15, 16]

Originally developed to counter the ever-changing munitions

threats reactive armor hassince evolved. Several newertypes of reactive armor havebeen created including

explosive reactive armor(ERA), self-limiting explo-sive reactive armor (SLERA),non-explosive reactive armor(NxRA), and Non-EnergeticReactive Armor (NERA).

Explosive Reactive ArmorExplosive reactive armor typically consists of two metal sheetsarranged in a sandwich configuration with a layer of explosivematerial in the middle.[17, 18] ERA is used to defeat or deflectshaped-charge anti-tank weapons including hollow charges, as wellas kinetic projectiles, small arms ammunition, and shrapnel.[18]Upon impact by a projectile, the ERA explodes causing the metalplates to separate, which can subsequently deflect or defeat thethreat at a small standoff distance from the vehicle the armor isprotecting. Once the armor explodes, however, that portion of thevehicle becomes vulnerable to other attacks.[17, 19] ERA is a com-mon add-on armor as it provides increased protection and is com-bat proven.[15] In terms of protecting against shaped charges, theERA explosion disrupts the plasma jet that is created from theshaped-charge warhead and decreases its penetrating power.[17]One disadvantage to ERA is that the high explosive detonates out-side the protected vehicle and subjects the surrounding area tosmall blast loads.[19]

Self-Limiting Explosive Reactive Armor

Self-limiting explosive reactive armor is considered a passive formof ERA because of the low mass of explosives used.[15, 16, 18]Theexplosive materials of SLERA are placed in a specific configurationto confine the energy released to a controlled area.[16] This con-figuration 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 whenapplied in a modular configuration. While SLERA is not as effec-tive as ERA, it may prove to be a more practical solution due to itssurvivability characteristics.[18]

Non-Explosive and Non-Energetic Reactive ArmorNon-explosive reactive armor uses gas-generating material and

other non-explosive systems to dissipate the impact energy ofhollow-charged warheads.[16, 18] Since it is non-explosive, it isrelatively easy to integrate onto vehicles. In addition, because it doesnot explode on impact, NxRA is less damaging to vehicle structuresand relatively inexpensive.[16] NxRA is similar to another form ofreactive armor, non-energetic reactive armor, in that neither formuses energetic components nor are they consumed when hit. Bothcan be applied to lightweight vehicles because they weigh less thanconventional reactive armor types and provide multi-hit protectionagainst chemical energy munitions.[15, 16, 18] Non-energetic reac-

A shaped charge is defined as acylinder of explosive with a hollowcavity at the end opposite the initia-tion train. If this cavity does notcontain a liner, it is referred to as ahollow charge or an unlined-cavitycharge. If the cavity contains a linermade from a metal, an alloy, glass,ceramic, wood, or another material,the device is termed a shapedcharge. The liner geometry may be

conical, hemispherical, parabolic, orany arcuate device. If the liner isbow shaped it is called an explo-sively formed penetrator (EFP).Shaped jets with conical, hemi-spherical, or bow shaped linerscollapse to form jets and/or slugsin different manners.[1]

[1] Walters, W., A Brief History of

Shaped Charges, ARL-RP-232,

December 2008.

Figure 4.An M2A2 BradleyFighting Vehicle with reactivearmor. (Photo by Staff Sgt. ShaneA. Cuomo, US Air Force)




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

AMMTIACA 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-hitcapability when compared to NxRA. These armors commonly con-tain an inert layer between two metal sheets that dissipates the ener-gy from the incoming projectile.[18]

Lightweight Enhanced Reactive ArmorLightweight enhanced reactive armor (LERA), developed for light

and medium weight combat vehicles, incorporates state-of-the-arttechnologies in reactive armor along with an insensitive, high-ener-gy explosive. LERA has been subjected to extensive qualificationtesting with more than 1,500 LERA armor tiles tested. LERA hasproven to be an excellent lightweight solution as it can defeat theshaped-charge threats associated with an urban battlefield, whilehaving an incident-free safety record in field operation and com-bat. It is currently available for integration on light and mediumcombat and tactical wheeled vehicles.[20]

Electric Reactive ArmorElectric reactive armor uses an electrical current to convert ashaped-charge or rod penetrator into plasma, vapor, and a harm-

less mixture of melted and pulverized debris that disperses aroundthe vehicle.[21] The electric current is able to convert the shaped-charge jets because it enhances the hydrodynamic instabilities

within the jet. This causes it to break into a string of particles thatcannot penetrate the armor as a continuous jet would. A similareffect is observed when a jet is heated to its melting point.However, when it reaches its melting point, the jet breaks apartand expands into a series of rings that have little penetration power.Importantly, it takes less current to melt a jet than to vaporizeit.[22] The British military has tested electric reactive armoragainst rocket-propelled grenades (RPGs). The armor wasobserved to provide complete protection except for a few scratchesand dents. Electric reactive armor is advantageous because it

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

RECENT ARMOR TECHNOLOGY DEVELOPMENTSSensors for Crack Detection in Ceramic Armor TilesTodays military uses ceramic body armor plates because they pro-vide good ballistic protection at a relatively low weight. However,once the plates are damaged they offer limited protection. Platesare not only damaged as a result of use, but sometimes they can bedamaged during manufacturing and shipping.[23]

Research is currently underway to improve the protection oflightweight body armors and helmets for individuals by investi-gating several different sensors that could be used for detectingdamage in the ceramic inserts.[24-26] Under investigation arefiber optic strain sensors and conductive paint.[25, 26]

Testing has also been performed to determine a method forusing piezoelectric lead zirconate titanate (PZT) transducers tocharacterize the vibrational modes of ceramic vehicle/body armorsupport system (VBASS) plates as a way of detecting cracks. It wasfound that when damaged, the plates produce a different signalthan when undamaged. This research resulted in the developmentof a handheld device for crack detection. However, future workneeds to be done to develop a more robust system.[23]

Carbon NanotubesCarbon nanotubes (CNTs)have excellent mechanical,electrical, and magneticproperties.[27, 28] Theycan be woven like cloth intobody armors that are sever-

al times stronger, tougher,and stiffer than those cur-rently in use.[27] CNTs can

withstand multiple ballistic impacts, although research does suggesthat a minor interval is required between hits for full recovery.[28]

Mosaic Transparent ArmorTraditional transparent armors are made of layers of laminatetransparent materials such as SiC, boron carbide (B4C), or alumi-na. These traditional armors impose significant costs when damaged, as the entire armor surface must be replaced. In an effort toreduce 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 anadhesive with the same refractive index as the tiles.[29]

It is thought that the mosaic tiles will restrict the damage froma ballistic impact to a portion of the armor surface. Therefore, onlythe damaged tiles need to be replaced.[29] Not only does mosaictransparent armor localize damage, it is also lower in weight andeasier to fabricate than conventional transparent armor. Weightsavings is provided in the fact that less transparent material isrequired, as the tiles are smaller and the spaces filled with an almostnegligible weight adhesive. A prototype window made of thismosaic transparent armor is currently being developed using trans-parent glass and ceramic tiles.[29]

Aerogel CompositesResearchers have developed a technology that uses inorganic aero-gels, such as silicon dioxide and carbon, to absorb kinetic energyThis silica aerogel system is lightweight and low in density

When impacted, the network collapses relatively slowly. Thecrushable aerogel layer was designed for placement between twoelastomeric fabric layers. This technology has the potential forbody armor applications, as it is envisioned that the aerogel layers

would be thin and flexible with a protective composite consistingof multiple layers.[30]

Magnesium Alloys for Lightweight Vehicle ArmorMagnesium (Mg) is the lightest structural metal; it has a densitylower than that of iron, titanium, and aluminum. Mg alloys aretherefore of high interest for lightweight armor applicationsHowever, before Mg alloys are employed for ground vehiclearmor, commercial scale manufacturing capability and capacitymust be demonstrated.[31]

ARL has partnered with industry to develop and establish acommercially scalable direct chill cast Mg alloy. Two Mg alloys

were developed from this partnership: a high strength and an ultrahigh strength alloy, both alloyed with yttrium and zirconiumHigh strength refers to a yield strength of approximately 350 MPa

whereas ultra high strength materials have a yield strength ofapproximately 500 MPa. Yield strength is used to define the point

Figure 5.Undamaged and damagemosaic transparent armors show th

importance of damage localization.(Photo Courtesy of LawrenceLivermore National Laboratory)




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The AMMTIAC Quarterly, Volume 4, Number 410

techsolutions14at which plastic deformation begins and is important because thehigher the strength the less likely a material is to permanentlydeform and eventually fracture, making it useless as an armormaterial. The high strength alloy has demonstrated exceptional

ballistic performance, showing promise for application as an armormaterial.[31] The ballistic properties of the ultra high strengthalloy have yet to be determined.

Composite Lightweight Adaptable Reactive ArmorComposite Lightweight Adaptable Reactive Armor (CLARA) isunder development to provide light armored vehicles protectionagainst shaped charges and kinetic energy penetrators. CLARA is ametal-free, low fragment reactive armor.[32, 33] CLARA is designedto provide protection against ammunition less than 12.7 caliber.[33]

CONCLUSIONFrom personal body armor to vehicle armor, innovation andchange are necessary to protect a well-equipped and agile force

during times of global conflict. This article briefly reviewed sever-al of the advancements that have been made in body armor systemsto produce lighter weight, flexible systems which can provide the

warfighter with superior protection without compromising mobil-ity and strength. Vehicle armor systems have also evolved intomore sophisticated, lighter weight systems. While these advancesare significant, future forces will undoubtedly require new armortechnologies to counteract even more advanced weapons and pen-etration threats.

NOTES & REFERENCES* Kevlar is a registered trademark of the E.I. du Pont de Nemours and Company. Viscosity quantifies a fluids ability to resist flow. For instance, a high viscosityfluid has a strong resistance to flow, while a low viscosity fluid flows readily.

Hydroclusters are transient aggregates that result from hydrodynamic lubricatingforces between the particles in the suspension.[2, 8] Critical shear rate is the point at which the dynamic shear thickening is approx-imately equal to the steady state shear thickening. This point must be achieved inorder 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 replacingparticles in a gel with a gas.

[1] Wittman, R.E. and R.F. Rolsten, Armor Of Men and Aircraft,Advances inStructural Composites, 12th National SAMPE Symposium, October 1967.[2] Lee, Y., E.D. Wetzel, R.G. Egres Jr., and N.J. Wagner, Advanced Body ArmorUtilizing Shear Thickening Fluids, 23rd Army Science Conference, December2002.[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 FabricsUtilizing Shear Thickening Fluids,IFAI 4th International Conference on Safety andProtective 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: TheCentral Science, 10th Edition, Pearson Education, Inc., 2006.

[7] Lee, Y.S., E.D. Wetzel, and N.J. Wagner, The Ballistic Impact Characteristicsof KevlarWoven 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 ThickeningFluid (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 ColloidalSuspensions, Rheologica Acta, Vol. 42, No 3, 2008, pp. 199-208.[10] Wetzel, E.D., and N.J. Wagner, Advanced Body Armor Utilizing ShearThickening Fluids,23rd Army Science Conference, December 2002.[11] Arndt, M., Body Armor Fit For A Superhero, BusinessWeek, The McGraw-Hill Companies, August 2006.[12] Rosen, B., C. NamLaufer, et al., Multi-Threat Performance of Kaolin-BasedShear Thickening Fluid (STF)-Treated Fabrics, SAMPE 2007, June 2007.[13] Kalman, D., J. Schein, J. Houghton, et al., Polymer Dispersions Based ShearThickening Fluid-Fabrics For Protective Applications,SAMPE 2007, June 2007.[14] Johnson, T., Army Scientists, Engineers Develop Liquid Body Armor,Todayin 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 CombatSystems 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-ExplosiveEnergetic 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 ExplosiveLayer 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, ArmyResearch 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 forNondestructive Testing, June 2008, DTIC Doc. ADA493437.[24] Serna, J., Sensing Protection Issues, Daily Pilot, January 2, 2008.[25] Stewart, C., Ceradyne Backs Maria Fengs 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 CarbonNanotubes,Nanotechnology, Vol. 18, 2007.[29] Elder, R., Mosaic Transparent Armor, Lawrence Livermore NationalLaboratory, 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 AppliedPhysics Laboratory, June 1, 2009.[31] Cho, K., et al., Magnesium Technology and Manufacturing for Ultra-Light-

weight Armored Ground Vehicles, AR L-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/, accessed7 August 2009.



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