ballistic resistant body armor: contemporary and prospective materials and related protection...

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N. V. David

X.-L. Gao1

e-mail: [email protected]

Department of Mechanical Engineering,Texas A&M University,

3123 TAMU,College Station, TX 77843-3123

J. Q. ZhengProgram Executive Office – Soldier,

U.S. Army,Haymarket, VA 20169

Ballistic Resistant Body Armor:Contemporary and ProspectiveMaterials and Related ProtectionMechanismsModern military operations, technology-driven war tactics, and current on-street weap-ons and ammunition necessitate the development of advanced ballistic protection bodyarmor systems that are damage-resistant, flexible, lightweight, and of great energy ab-sorbing capacity. A number of studies related to new concepts and designs of body armormaterials (including those derived from or inspired by nature) have been conducted in thepast two decades to meet the new demands. Ballistic fabrics, ceramics, and laminatedcomposites are among the leading materials used in modern body armor designs, andnano-particle and natural fiber filled composites are candidate materials for new-generation body armor systems. Properties and ballistic resistance mechanisms of suchmaterials have been extensively investigated. Based on a comprehensive and criticalreview of the advances and findings resulting from these investigations, a comparativestudy on design, protection mechanisms, and performance evaluation of various types ofanti-ballistic body armor is presented in this paper. Body armor systems made fromdifferent materials and exhibiting distinct ballistic energy absorption mechanisms arediscussed, and key factors that influence the ballistic performance and energy absorbingmechanisms of the body armor systems are identified. �DOI: 10.1115/1.3124644�

Keywords: body armor, ballistic material, fabrics, ballistic limit, energy absorbing,impact, ceramics, polymer, composite, nanotube, natural fiber

IntroductionBody armor has been used in military and combat actions for

enturies since the Roman era �ca. 145 B.C.�. During the medievalimes �from A.D. 400 to ca. 1500�, full steel plate harness was usedo protect the torso, which was the most vulnerable part in the

edieval knight style of combat �1�. To date, the primary func-ions of body armor remain the same: to impede weapon androjectile penetration into the human body and to diffuse the im-act energy. However, modern military operations, technology-riven war tactics, and current on-street weapons and ammunitionemand a flexible �wearability and mobility of the wearer�,amage-resistant, and lightweight ballistic protection garmentith superior energy absorbing capacity �2�. Body armor intended

or law enforcement and corrections personnel also requires simi-ar traits �3�. As a result, demand continues to increase for moreeliable and enhanced anti-ballistic body armor systems.

A number of studies related to new concepts and designs ofallistic protection body �soft� armor systems have been con-ucted in the past two decades to meet the new demands men-ioned above. Material properties and damping mechanisms ofnti-ballistic high performance fabrics have been investigated ex-erimentally and analytically �e.g., Refs. �4,5��. A variety of otheraterials including ceramics and laminated composites for ballis-

ic protection have also been developed around the world. How-ver, a detailed and critical comparison of existing ballistic pro-ection body armor systems is still lacking.

This paper aims to provide a comparative study on design, char-cterization and performance evaluation of various types of anti-allistic body armor made from different material systems andxhibiting distinct ballistic energy absorption mechanisms. It is

1Corresponding author.
Published online July 9, 2009. Transmitted by Victor Birman.
pplied Mechanics Reviews Copyright © 20

om: http://appliedmechanicsreviews.asmedigitalcollection.asme.org/ on 03

not the intention of this article to assess or rate the scientific ortechnological merits of each body armor material system. Instead,the objective of the current study is to present a comparativeanalysis of various material systems and their performance in pro-viding ballistic protection to the wearer. Several key factors thatinfluence the performance of ballistic protection armor, includingmaterial systems, structure and properties, design of fabric plying,and energy absorbing mechanisms, will be comparatively studiedby analyzing research articles, patents, and test standards currentlyavailable.

2 Ballistic FabricsBallistic fabrics are typically made from woven yarns, which

consist of interlocked natural or synthetic fibers. The basic mecha-nism of ballistic energy dissipation, the major characteristics ofballistic fabrics, and the quantification of ballistic performance arebriefly summarized below, which will be followed by a discussionon the design and performance of various fabric material systemsused for ballistic protection.

2.1 Mechanism of Ballistic Energy Absorption. Most bal-listic fabrics exhibit a two-dimensional �2D� plain weave patternthat is formed by interlacing warp and fill �or weft� yarns in twoorthogonal directions �e.g., Ref. �6��. Stress waves, which are gen-erated at the point of impact, travel along the yarns toward thefabric edges where they are reflected. These waves are also par-tially transmitted and partly reflected at the warp-fill yarn cross-over points. The speed at which the stress waves travel along theyarns depends on the density and stiffness of the yarns �and thusof the fibers�.

When a projectile strikes a body armor fabric at a certain speed,it is caught in a web of the fibers to which its kinetic energy istransferred. This kinetic energy, carried by the stress waves as
described above, is dissipated through fiber deformations and
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nter-fiber friction when they slip or slide against each other. It haseen found that the structure and frictional characteristics of thesebers significantly affect the overall performance of the fabric �7�.he undissipated energy is absorbed by each successive layer �ifresent� of the material until the bullet has been stopped. Other-ise, the remaining energy may cause material damage such as

abric tear or slit. In the case of multilayer fabrics, the presence ofhysical interaction between layers makes the stress waves traveln both the planar and transverse directions, thereby further in-reasing energy dissipation. The bonding/connecting condition ofabric layers influences the wave propagation in the transverseirection and thus the energy absorption mechanism of the fabricrmor.

2.2 Fabric Characteristics Related to Ballisticerformance. The factors affecting the performance of a ballistic

abric include the fabric weave types and fiber properties �e.g.,ef. �8��, far-field boundary conditions �e.g., Ref. �9��, inter-yarn

riction �e.g., Refs. �10–13��, fabric-projectile friction �e.g., Ref.11��, interaction between layers �upon impact� �e.g., Ref. �4��,nd projectile geometry �albeit an external factor� �e.g., Refs.14,15��.

Among these factors, the inter-yarn friction plays a critical role.t was shown in Ref. �16� that the interfacial frictional work dis-ipated at yarn-yarn junctions affects the stiffnesses of the yarntensile� and the fabric �transverse�, which in turn influences theallistic performance of the fabric. A mesoscopic discrete modelor fabric deformations was developed in Ref. �17�, which ac-ounts for the yarn-yarn interaction occurring at yarn cross-overoints. Their results indicated that fabrics with high yarn-yarnriction dissipate more energy than fabrics with low inter-yarnriction.

The inter-yarn sliding friction can be characterized using thetatic coefficient of friction �FSs�. This parameter is usually deter-ined experimentally �e.g., using a quasi-static yarn pull-out test;

ee Ref. �8�� and is assumed to be independent of the slidingelocity for the sake of simplicity. The yarn-projectile friction cane described by using the kinetic coefficient of friction �FSk�. TheSk was determined by a simple experiment involving the pullingf a block made from the projectile material across a flat sheet ofhe fabric �target� being tested �18�. A study on the ranges of FSsnd FSk that would break four yarns to perforate a fabric waserformed in Ref. �4�, which provided the following results: 0.1FSs and FSk�0.2, with 0.1�FSs+FSk�0.3.Also, some ballistic fabrics exhibit stress-strain relations that

re strongly strain-rate dependent. For instance, it was experimen-ally determined that Twaron® fabric is a strain-rate dependentiscoelastic material �19�. It was found that the failure strain ofhe fabric material decreases with increasing strain-rate. This char-cteristic of the fabric limits its deformation at high strain ratesnd causes the fabric to fail in a brittle mode, both of which leado a reduction in impact energy absorption by the fabric.

In order to obtain enhanced anti-ballistic performance, high te-acity yarns may have to be used on the anterior and/or posteriorf the fabric faces. As described in Ref. �20�, this would dependn the degree of hazard and the type of ammunition. If the tenac-ty of a fabric yarn is greater than 15 g/denier2 with a yarn modu-us of 44–176.5 GPa, the fabric is designated as a high perfor-

ance fabric suitable for ballistic protection �21�. Dimensionalnd thermal stability also counts toward this designation. Somedditional information about the material properties that influencehe performance of ballistic fabrics can be found in Ref. �14�.

2Denier is a measure of linear density �mass per unit length� of a yarn. Thendustrial Fabrics Association International defines this measurement based on gramser 9000 m of fiber or yarn. Another measure of linear density of fabric fibers isnown as Tex and is measured in g/km. Tenacity is the amount of force �in grams�equired to break a yarn, normalized with respect to the denier, and hence the ‘g/
enier’ quantity. Tenacity thus reflects the strength of a fabric.
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2.3 Performance Measures of Ballistic Fabrics. Ballisticperformance of a fabric is often characterized by its ballistic limitand its specific ballistic energy, which represents the mass effi-ciency of the fabric �8�. The ballistic limit, Vbl �in m/s�, is ameasure of velocity at which the projectile �with mass m, in kilo-grams� has just started to penetrate the armor with areal density A�

�in kg /m2�. The specific ballistic energy, Es �in J·m2 /kg�, definedby

Es =mVbl

2

2A�

�1�

refers to the kinetic energy of the projectile at the ballistic limitper unit areal density �e.g., Ref. �8��. The areal density of an armormaterial comprising n fabric layers is defined as

A� = �i=1

n

ti�i �2�

where ti and �i are, respectively, the thickness and density of theith layer.

The dissipated projectile kinetic energy may be written as �e.g.,Ref. �22��

E = 12m�Vi

2 − Vr2� �3�

where E is the dissipated energy �in joules�, Vi is the initial pro-jectile velocity �in m/s� �which can be Vbl�, and Vr is the residualvelocity of the projectile after penetration �in m/s�. This quantityE is usually normalized with respect to the initial kinetic energy ofthe projectile. In the event that only partial or no penetration couldbe achieved, Vr=0 and Eq. �3� would reduce to

E = 12mVi

2 �4�

As per the NIJ Standard 0101.04-2001 �23�, the baseline ballis-tic limit, which refers to the impact velocity at which a projectilecompletely penetrates an armor component 50% of the time �or inother words, 50% probability for full penetration, denoted asV50�, needs to be experimentally determined and statistically ob-tained �23,24�. Note that V50 defined in Ref. �23� differs from theballistic limit �Vbl� mentioned above.

2.4 Degradation of Ballistic Fabrics. Several trademarkedfabric materials fulfill the high performance criterion for ballisticfabrics mentioned at the end of Sec. 2.2. These include Twaron®�by Teijin Twaron BV� and Kevlar® �by DuPont�, which are twopara-aramid types of ballistic fabrics that have been chemicallyand/or physically modified in order to enhance their ballistic per-formance. Other high performance ballistic fabrics include Spec-tra® �by AlliedSignal/Honeywell� and Technora® �by TeijinTwaron BV�, both made from ultrahigh molecular weight polyeth-ylene groups, Zylon® �by Toyobo� and Vectran® �by Hoechst-Celanese�, both manufactured using liquid crystal polymer fibers,and Sentinel® �by Barrday�, which is a quasi-unidirectional com-posite fabric.

However, there are still challenging technical issues on environ-mental degradation associated with some of these high perfor-mance fabrics �25�. Among others, the mechanical properties ofthe Zylon® and the Spectra® fabrics made, respectively, frompoly�p-phenylene benzobisoxazole� �PBO� and ultrahigh molecu-lar weight polyethylene �UHMWPE� fibers have been found todegrade due to moisture, temperature, the UV light, and gammaradiation.

Yarns fabricated using PBO fibers lost about 40% of their ten-sile strength after being exposed to elevated temperatures �up to60°C� and up to 60% relative humidity �RH� �26�. It was ob-served that the reduction in tensile strength, due to environmentalaging �at 50°C, 60% RH� for more than 150 days, is more pro-
nounced for the PBO yarns than for a single PBO fiber. It was also
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ound that the elastic modulus of the yarns is the least affectedechanical property by aging and remains around 145 GPa

hroughout the exposure.Thermogravimetric analysis of PBO fibers revealed that the fi-

ers decompose in a single step between 700°C and 720°C �27�.o conformational change of the fiber microstructure was ob-

erved below these decomposition temperatures. Thermal decom-osition of the PBO fibers and yarns is thus expected to occurnly at temperatures well above room/field temperatures. Thehermal stability of PBO fibers was also observed in another study28�. Immersion of PBO fibers to full moisture saturation �i.e., 100H� at 20�2°C �a room temperature� up to about 100 days ofxposure resulted in the debonding of the fibers and led to con-inuous increase in the midfiber strain �29�. It was also found thathe swelling �quantified by the midfiber strain� of the PBO fibersaused by moisture uptake resembles that of the Twaron® fibers.

The degradation of the PBO and polyethylene fibers due to UVadiation was studied in Ref. �30�. Up to 80% loss of tensiletrength was reported for PBO fibers as a result of exposing thebers to UV radiation for more than 120 hours.In another study �31�, it was determined that the back face

ignature limit, i.e., the deformation of a backing material �usuallyade of clay� caused by the impactor and measured by the depth,

iameter, and volume of the resulting crater in the backing mate-ial, of the Zylon®-containing vests exceeds the threshold speci-ed in the NIJ Standard 0101.04-2001 �23�.The effects of gamma radiation on the tensile properties of

pectra 1000® �UHMWPE� fibers were investigated in Ref. �32�.t was found that aging in air for 160 days reduces the failuretrain of the irradiated fibers by 30%. The decrease in the failuretrain was attributed to the increase in the crystallinity of theicrostructure of the fibers and in cross-linking of the polymer

hains due to the radiation. These effects made the UHMWPEbers more brittle, as evidenced by the scanning electron micros-opy �SEM� micrographs of the fractured surfaces.

The influences of environmental �natural weather� conditionsi.e., different temperatures and precipitation levels� and gammaadiation on the mechanical properties and the ballistic perfor-ance of cross-plied Spectra Shield® fabrics �made from the UH-WPE fibers� embedded in a resin matrix were studied in Ref.

33�. It was reported that a high radiation dose of 250 kGy reduceshe toughness and increases the brittleness of the as-received com-osite plate. However, the high radiation dosage was observed toncrease the flexural strength and the elastic modulus of the plate.ging due to the natural weather conditions and the gamma ra-iation altered the molecular structure of the UHMWPE fibers,hich was determined through the Fourier transform infrared

pectroscopy. The effect of the gamma radiation on the mechani-al properties of the composite plates was found to be more sig-ificant than that of the weather exposure. A similar observationas also made in Ref. �33� pertaining to the ballistic performancef the composite plates, where no perforation was seen for thelates exposed to weathering but a perforation of 50% and 100%ccurred for the plates exposed to 25 kGy and 250 kGy doses ofadiation, respectively.

It should be mentioned that most publications cited in this paperre related to Twaron® and Kevlar® fabrics, which appear to haveeen more extensively studied and utilized.

2.5 Enhancement of Ballistic Performance. As indicated inec. 2.2, the inter-yarn friction of a woven fabric has an importantffect on its ballistic performance. This leads to the novel conceptf modifying commercially available anti-ballistic fabrics withhear-thickening fluids to increase the interyarn friction of theabrics during impact. Another innovative way to improve theallistic performance of the fabrics is to enhance their bendingtiffness via rubber or resin coating. These approaches are dis-ussed next.

The concept of shear-thickening fluid �STF� was studied in
efs. �34,35�. STF is a liquid filled with high concentrations of
pplied Mechanics Reviews


rigid colloidal particles, whose viscosity increases with the shearstress rate �34�. When impacted by a penetrating projectile, thehigh shear stress rate enables hydrodynamic forces to overcomerepulsive inter-particle forces. This affects the formation of hydro-clusters of particles. Clusters of particles are formed via aggrega-tion of particle groups. The short range lubrication effect by theprojectile increases the viscosity of the STF. Clusters will thenface more resistance to move against each other. This affects thefrictional interaction between the yarns and thereby improves theballistic properties. Collisions of the hydroclusters transform theflexible fabric into a macroscopically rigid armor.

The ballistic impact characteristics of Kevlar KM-2® �600 de-nier with an areal density of 180 g /m2� woven fabrics impreg-nated with a colloidal STF consisting of silica particles �with anaveraged diameter of 450 nm� in ethylene glycol was reported inRef. �35�. The Kevlar® fabric layers were impregnated with 2 ml,4 ml, and 8 ml of the STF per layer, respectively. Their resultsindicated that the energy absorption of four layers of the Kevlar®fabric is proportional to the amount of the STF. For example, fourlayers of the Kevlar® fabric impregnated with 8 ml of the STFdissipated about 93% of impact energy, which is comparable tothat ��90%� of 14 layers of the neat Kevlar® fabric albeit thesample weight of the former is more than twice of the latter. Thisobservation signifies the trade-off between the improved perfor-mance and the fabric weight. The performance enhancement pro-vided by the STF was thought to be due to the increased frictionalinteraction between the yarns, which is yet to be furtherinvestigated.

The ballistic performance of a Twaron CT615® plain weavefabric �500 denier with areal density of 150 g /m2� impregnatedwith a silica-water suspension �SWS� consisting of silica colloidswith concentrations of 0 wt %, 20 wt %, 40 wt %, and50 wt %, respectively, was studied in Ref. �8�. SWS is a class ofSTF, but silica colloids in water were used in Ref. �8� as opposedto a suspension of silica colloids in ethylene glycol employed inRef. �22�. Their results showed that a 40 wt % SWS particle con-centration yields the highest ballistic limit for single, double, andquadruple ply fabric systems, with the double ply system showingthe greatest improvement. The ballistic limit and the specific bal-listic energy of the double ply system with the 40 wt % SWSparticle concentration were, respectively, 65% and 90% higherthan those of the neat double ply system. The improvement in theballistic resistance was attributed to the increase in the projectile-fabric friction and interyarn friction, arising from the addition ofthe silica particles �with a nominal size of 100 nm in diameter�and the formation of silica clusters. This is similar to what wasobserved in Ref. �22�, as indicated earlier.

Very recently, another innovative approach to modifying theTwaron CT709® microfilament fabric �840 denier with an arealdensity of 202 g /m2� was undertaken by coating the fabric withdifferent grades �in terms of tensile modulus� of natural rubber�NR�, namely, high modulus �15.5 MPa�, medium modulus �12.5MPa�, and low modulus �9.30 MPa� �36�. The four-layer fabricsystems consisting of alternating neat and coated fabric layersresulted in a higher ballistic limit than the all-neat four-layer fab-ric system, with the fabric systems containing layers coated bynatural rubber with a higher modulus dissipating more energy. A21–26% increase in the ballistic limit for the three different com-binations of the neat and coated fabrics �four layers� in compari-son to the all-neat system was observed. The energy absorbed atthe ballistic limit by the four-layer fabric systems is 45–59%higher than that by the all-neat system. Observations of the yarnpull-out of damaged samples revealed that the natural rubbercoated fabric layer suffers less yarn damage with smaller slit size.The natural rubber layer acted as a stiff membrane to deflect andabsorb more impact energy, and hence the improvement. In addi-tion, their study also indicated that the enhancement of ballistic
performance was related to the higher interyarn friction and re-
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tricted yarn movement due to the coating material that penetratednd held the yarns tightly. This agrees with the earlier findings byhe other two groups �see Refs. �8� and �35�� mentioned above.

Table 1 provides the ballistic limits of four different enhancedody armor materials and the improvements achieved. The ballis-ic limit for the Kevlar® system was determined at a fixed pen-tration velocity �244 m/s�, and, as a result, no comparison coulde made with the neat system. Figure 1 shows the performancenhancement of the ballistic energy absorption and dampingechanism of these systems.However, thermal analysis was not performed on the as-

eceived and post-impact fabric materials to determine their ther-al stability in the investigations reviewed here. Therefore, the

ffects of different types of suspension fluids on the thermal sta-ility of the modified fabrics remain to be explored.

Alteration of the bending properties of fabric materials by resin

Table 1 Ballistic limits of different bo

System and configuration

Kevlar® + silica colloidal �in ethylene glycol�;4 layers + 8 ml STFTwaron CT615® + silica colloidal �in water�;double ply + 40 wt % SWSTwaron CT709® + NR;2 neat + 2 high modulus �alternating layers�Woven �Sentinel®� + Nonwoven layers;inventive �needle punched woven/nonwoven layers�

Fig. 1 Comparison of the energy absorp

50802-4 / Vol. 62, SEPTEMBER 2009


coating has also been investigated. The effect of a resin �un-disclosed� coating on Kevlar 29® fabric sheets was analyticallystudied in Ref. �37�. The ballistic performance, V50, of a numberof sheets coated with the resin was compared with that of theuncoated Kevlar 29® sheets for the same relative areal density. Itwas found in Ref. �37� that the fabric sheets without resin coatingoutperform the resin coated fabrics with an equivalent relativeareal density up to a certain point, after which the converse wasobserved. Similar results were experimentally demonstrated inRef. �38�.

The effect of resin coating is mainly on the bending strength ofthe fabric. The uncoated fabric can only support tensile membranestresses. In the presence of resin, the resin/fabric system turns intoa hard and stiff panel �39,40�. This increases the bending resis-tance of the fabric and enhances the resin/fabric panel’s resistanceto inward deformation, thereby improving the ballistic perfor-

armor material systems and designs

istic limit,50 �m/s�

Ballistic limit ofneat system �m/s�

Improvement�%�

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223 135 65

252 200 26

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ance. Similar findings were reported by other groups3,4 �41,42�.As mentioned in Sec. 2.4, in spite of their high strength, most

allistic fibers are susceptible to UV light induced degradationfter a sufficiently long exposure time. Four methods of enhancinghe UV resistance of some ballistic fibers are outlined in Ref. �30�.n addition, an approach to combating the UV degradation of aevlar® fabric was studied in Ref. �43�, where it was reported

hat using nanostructured polymer coatings helps the Kevlar®abric to retain 95% of its tensile strength after 7 days of UVxposure.

2.6 Effects of woven architecture. As mentioned in Sec. 2.1,ost ballistic fabrics have 2D plain weave patterns. A three-

imensional �3D� woven architecture is an integrated structureomprising the conventional 2D weaves and transverse yarns thatonnect the in-plane yarns along the thickness direction. Theransverse yarns thus provide through-thickness reinforcementz-reinforcement�. The 3D woven architecture contributes to thetructural integrity of a fabric or a composite made from the 3Dabric �44�. The relevant anti-ballistic benefits of the intermingledD architecture include improved stiffness and strength in theransverse direction and increased resistance to the disentangle-

ent of in-plane weaves. The impact load on a 3D woven fabricets resolved in three orthogonal directions rather than only in twon-plane directions, thereby enhancing the energy absorption ca-ability of the fabric.

Various weaving techniques have resulted in a wide range oftructural configurations for woven fabrics. The architectural fea-ures of a woven fabric, such as warp-weft crossing over, influ-nce the stress wave propagation in the fabric, which in turn af-ects the energy absorption of the fabric. Weave patterns thus havesignificant effect on the ballistic performance of woven fabrics.The ballistic resistance of 3D woven fabric composites was

tudied in Ref. �45�, where the benefit of using 3D fabrics inontrolling and localizing the delamination upon impact wasoted. It was observed that the 3D woven structures give feweromplete penetrations than 2D woven fabric composites due tohe z-reinforcement, which was pivotal in arresting inter-laminaracks. It was also found that absorption of kinetic energy isainly achieved through intra-lamina delamination. The sizes of

he delamination in the 3D woven composites were found to bemaller than those in the 2D woven composites. A comprehensiveiscussion of various energy absorbing mechanisms of woven fab-ic composites was provided in Ref. �46�, where energy absorptiony the primary yarns �i.e., yarns directly below the projectile�,eformation of the secondary yarns �i.e., yarns within the conicalmpact zone�, delamination and matrix cracking, and projectile-

3Coating Spectra 900® with vinylester and polyurethane resins couples the yarnsnd prevents yarn mobility. This produces a more uniform stress state in the resinoated fabric via stiffening effect of the resin. The inward deflection of the compositeanel is therefore restricted, which makes the penetrator engage and break morearns, resulting in enhanced energy absorption.

4Kevlar 29® plates, made from prepreg fabrics impregnated with a vinyl esteresin �a copolymer of polyethylene�, at a curing temperature of 125°C, with eleven

(a)

Fig. 2 „a… Compound body armor defollowed by the deflection of its traje

lies each having a thickness of 1.8 mm, showed an increased bending stiffness.



fabric friction were studied.The effect of fiber arrangements on low-velocity impact behav-

ior of 3D woven composites using the basalt and the para-aramid�Kevlar®� fibers was investigated in Ref. �47�. Two types of fiberarrangements, i.e., alternating plain-weave layers of para-aramidyarns and of basalt yarns �i.e., interply hybrid� and sequentiallayers each comprising plain weaves of para-aramid and basaltyarns placed next to each other �i.e., intraply hybrid�, were exam-ined. The Kevlar® yarn was used as the z-reinforcement to verti-cally weave six warp yarn layers and seven weft yarn layers. Thefabrics were woven using a 3D weaving machine. The impactenergy absorption was found to depend on the failure mode,which in turn was a function of the fiber arrangements. It was alsoobserved that the interply hybrid failed in a layer-by-layer modegiving larger energy absorption, while intraply hybrid failed in abrittle mode absorbing very little energy.

2.7 Innovations: Patented Ballistic Fabric Systems. A briefcollection of patented body armor designs is presented here. Mostof the patents are related to innovative utilization of commerciallyavailable ballistic fabrics and to novel designs of physical con-figurations of various fibers in combination with ballistic fabrics.

A low-weight compound body armor design that maintains thearmor’s flexibility is described in Ref. �48�. The design concept isa type of flexible body armor compounded with interspersed hardinserts of polymer, metal, or ceramic materials �see Fig. 2�. Thearmor consists of multiple protective layers of fabrics. Two flex-ible metallic layers with appropriately distributed hard nonplanarpyramid-shaped protrusions designed to turn, to redirect, or torotate the incoming projectiles are sandwiched between the pro-tective fabric layers. The pyramids present an oblique angle to theimpacting projectile. As shown in Fig. 2�b�, the projectile wouldimpact the protective layers with its side rather than its tip. Thisincreases the area of impact and enables the distribution of forceover a larger area. Thus, the damping mechanism is controlled bythe diffusion of the kinetic energy of the projectile over an ex-panded area. Momentum, angle, and speed of the projectile affectthe performance of this kind of body armor.

A collection of enhanced energy absorbing material systemswere developed in Ref. �49�. The material system comprises atleast one flexible but strong woven fabric layer and at least onenon-woven fabric layer, which are entangled with the woven orunidirectional layer by needle felting. As shown in Fig. 3�a�, layer1 consists of multiple layers of the Sentinel® fabric with at leasttwo layers cross laid at 90 deg angles relative to each other. Layer2 is made up of non-woven ballistic grade batting layers �whichcan be natural fibers �cotton, wool, sisal, jute, and silk� and/orsynthetic fibers �aramid, rayon, polyester, and rubber��. The bal-listic grade fibers should possess a tenacity of at least 15 g/denierand a tensile modulus of 35.3 GPa. The needle felting is requiredfor consolidating layers 1 and 2. As is evident from the perfor-mance chart �Fig. 3�b��, the needle punched consolidation of thelayers is better than other conventional consolidation methods.

(b)

n and „b… penetration of a projectilery „after Ref. †48‡…

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A blunt trauma reduction fabric system was designed and pat-

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nted �20�. This particular design falls into a class of laminatedomposite armor termed integral armor and will be treated accord-ngly in Sec. 4.1.

Ceramic ArmorIn general, ceramics, such as aluminum oxide �Al2O3; also

nown as alumina�, boron carbide �B4C�, titanium diborideTiB2�, and silicon carbide �SiC� and its variants like SiC–B �sili-on carbon boride� and SiC–N �silicon carbon nitride�, are low-ensity high-hardness materials. In addition to these features, ce-amics usually possess high compressive strength, which is anttractive property for ballistic applications �50�. Nonetheless, ce-amics are brittle materials with low fracture toughness and haveow tensile strength upon damage �51�. A projectile impacting aody armor system usually carries a huge amount of kinetic en-rgy that needs to be either absorbed or dissipated. In the case ofntegral or hybrid armor, this requires a facing material with highardness and high toughness. The hardness is needed for deflect-ng and deforming the projectile, while the toughness will deter-

ine the amount of energy that can be absorbed �to deform therojectile� before the facing material fractures or fails.

The ballistic resistance mechanism of ceramic armor will beiscussed next, which will be followed by discussions on the fail-re mechanisms of the ceramic frontal component of the hybridrmor and the correlation between toughness and hardness withracture types.

3.1 Failure Mechanism of Ceramic Armor Systems. An ac-urate understanding of the failure mechanism of a ceramic armorystem will provide the insight into the ballistic performance mea-ures of this class of armor materials. The failure process of aeramic armor system depends on the geometry, confinement andnterfacial conditions, and material properties. A ceramic armorystem may consist of either a thin ceramic tile supported by aacking ductile layer or a thick ceramic layer confined by a me-allic ring or jacket. A description of these two configurations andheir damage mechanisms was provided in Ref. �52�, which, to-ether with some experimental evidence, is summarized below.

3.1.1 Failure of Thin Ceramic Armor. A thin ceramic armorystem comprises a frontal ceramic tile bonded to a ductile metalr fabric-based backing plate by a thin adhesive layer. The colli-ion of a projectile onto the surface of the ceramic tile sets upigh-amplitude compressive stress pulses that travel across the

(a)

(b)

Fig. 3 „a… Material configuration and „b… performance comptems „after Ref. †49‡…

hickness of the ceramic tile. Due to the mechanical impedance

50802-6 / Vol. 62, SEPTEMBER 2009


mismatch of the ceramic tile and the backing plate, the stresswaves are partially reflected at the ceramic/backing plate interfaceand partially transmitted into the backing plate. The reflectedstress waves propagate with reduced speeds and are of the tensilemode, since the mechanical impedance of the ceramic is higherthan that of the backing plate. Ceramics are stronger in compres-sion than in tension, and hence an upper limit of tensile load existsthat will affect failure if the load due to the tensile stress wavespropagating through the ceramic surpasses this threshold. Thephysical damage formation and propagation within the ceramictarget are thereby controlled by the magnitude and direction ofpropagation of the tensile stress waves. The cracked zone devel-ops from the back surface of the ceramic tile and extends to theimpact surface in an upward tapering shape, as illustrated in Fig.4, since the �reflected� tensile stress waves ascend toward thefrontal surface of the ceramic tile. The projectile is unable topenetrate the ceramic tile before the conical damage zone fullypropagates and extends to the ceramic frontal surface and theprojectile ogive. The time duration pertinent to this non-penetration period is termed dwell. In Ref. �53�, a dwell durationof about 4 �s for Al2O3 tiles restrained �or wrapped� in E-glass/epoxy membrane layers was reported.

For a short projectile or an impact velocity below the ballisticlimit of the ceramic target, the dwell period can be long enough topermit build-up of stress that may exceed the yield strength of theprojectile material. This leads to plastic deformation of the pro-jectile, which, in turn, prolongs the time required for upwardpropagation of the damage zone to the free surface and therebylimits the ability of the projectile to penetrate the ceramic target.The non-penetration phenomenon with a complete erosion of theprojectile is called the interface defeat, even though this term wasspecifically coined for the case of a long rod projectile �54�.

Complete failure �or complete perforation� of the ceramic tileoccurs once the conical damage zone �or shatter cones� fully un-furls onto the impact surface, and the damaged area becomescomparable to the projectile cross section. This depends on theprojectile length and its velocity, which can be predicted fromstudying the material response and failure under impact loading.The severity of the damage of SiC–N augmented with increasingimpact velocity �from 50 m/s to 500 m/s� was experimentallydetermined in Ref. �55�. The extent of damage of the ceramicsample was quantified by the size of the comminuted region, thedensity of micro-cracks, the number and length of ring, and thecone cracks. A critical velocity range for B4C, i.e., 860–895 m/s,

son „V50… of Sentinel® woven fabric/non-woven fabric sys-
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ism of the ceramic specimen, was reported in Ref. �56�. Theub-critical impact �at 793 m/s� resulted in a damaged zone with aicrostructure occupied by stacking faults and microtwins pro-

uced by plastic deformations during the impact. The hypercriticalmpact �at 907 m/s�, signified by a high pressure and a high de-ormation rate, resulted in a solid-state transformation, observedn a nanoscale region, of the microstructure of the ceramic matrixrom crystalline to amorphous.

The backing plate also experiences compressive stresses, buthe damaged �cracked� zone of the ceramic frontal plate, which

ounds conically, aids the distribution of the impact load over anxpanded area of the backing plate. If the impact velocity or theenetrator length is large enough to overcome the interface defeat,hen a complete penetration will take place and the backing plateill also be subjected to deformation or failure. The extent of theacking plate damage can be controlled by modifying the backingaterial. For instance, in Ref. �53�, it was reported that the initia-

ion of the rear face �of the backing plate� deformation occurs atbout 10 �s after impact on uncovered alumina tiles, but theiresults showed that this duration can be extended to 25 �s byrapping the tiles with three layers of the E-glass/epoxy mem-rane. The back face deformation shall comprise defects such asetalling.

3.1.2 Failure of Thick Ceramic Armor. The projectile defeatechanism and the failure process of a thin ceramic tile have been

escribed above. It was mentioned that if the dwell period is suf-ciently long �such that the strain level surpasses the elastic strain

imit of the projectile material�, then the projectile may deformlastically and erode before overcoming the interface defeat.owever, if the same projectile is axially long enough �i.e., with aigh aspect ratio�, the projectile shall impart its kinetic energyver the same impact area for a longer duration. This impact con-ition demands a thicker ceramic layer than a thin tile to defeathe projectile because it would continue to penetrate the targetlbeit the ogive or some extent of the front end of the projectileay have eroded or fractured during the dwell. A thick ceramic,

owever, will continue to be shattered as a result of continuousenetration by the long projectile, which may exert a compressivetress exceeding the compressive strength of the ceramic. A com-on technique to contain the debris from scattering is to confine

he comminuted ceramic in a metallic ring or cover. For example,he ballistic performance of SiC–N ceramic in a Ti6Al4V confine-ent was tested in Ref. �57�. The confined ceramic target was able

o sustain �or contain� the axial and radial damage �or fragments�,hich developed across these dimensions for an impact velocity

anging from 393 m/s to 1706 m/s. The direct benefit of thisonfiguration is the extended time to total failure and the en-anced capacity of the ceramic target to erode the penetrator for aonger period of time, thereby diffusing its energy. Similar find-

Fig. 4 Damage zones in the ceram„bullet…

ngs about the ability of confined ceramic armor to defeat long



caliber high-energy projectiles were also reported �58�. A confinedfragmented zone can acquire sufficient kinetic energy from a longprojectile that will erode the penetrator by a process of frictionalshear flow if the frictional dissipation is greater than the wearresistance of the projectile material �59�.

3.2 Material Properties and Performance Measures ofCeramics. In the context of integral body armor, the function ofthe ceramic facing is threefold and can be abbreviated as “3D”:decelerate, deform, and distribute. The ceramic frontal has to de-celerate and deform the agave �tip� of the impinging penetrator,and distribute the impact load to a larger area of the backing plate.

The material properties that affect the ballistic performance ofthree different ceramics �i.e., B4C, SiC, and Al2O3� include com-pressive strength, shear strength, bulk, shear and Young’s moduli,hardness, density, and the Hugoniot elastic limit �HEL� �60�. HELis defined as the yield stress of a material under uniaxial dynamicloading. Stress waves are generated in solid materials under shockor impact loading. The elastic waves, set into motion at the instantof impact, may exert high transient pressures at the impact spot.Thus, the dynamic loading that the elastic waves impart on thetarget material may cause loss of yield strength if the loadingexceeds the HEL. A HEL value, normalized with respect to that ofhigh density �99.9%� Al2O3, of 1.5 was obtained for B4C and 1.0for SiC. It should also be noted that the compressive strength of aceramic is determined in a quasi-static manner and does not nec-essarily describe the high strain-rate behavior exhibited by theceramic material under ballistic impact. Thus, this material prop-erty is neither a stand-alone determinant of the ballistic perfor-mance nor can be used in making performance-wise comparisonbetween different ceramics.

It is therefore necessary to relate different physical and me-chanical properties of ceramics to provide one or more parametersthat can describe the material response to ballistic impact. A de-finitive evaluation of the ballistic performance of a ceramic can beobtained through the determination of the depth of penetration andthe ballistic efficiency, with the latter depending on the former�60,61�. The ballistic performance may then be regarded as afunction of the areal density of the ceramic.

Ceramics are susceptible to intrinsic defects such as micro-cracks. An important material property related to the ballistic im-pact response of a ceramic is its fracture toughness, which is ameasure of resistance to fracture in the presence of cracks. Amaterial with a large value of fracture toughness will undergoductile failure �and thus can absorb more energy� as comparedwith brittle failure of a material with a low fracture toughness.SiC, for example, is usually fabricated through sintering and pos-sesses a low fracture toughness. Enhancement of the toughness ofSiC ceramics can be achieved by controlling several manufactur-

tile, backing plate and a projectile
ic
ing parameters such as sintering additives �e.g., aluminum, boron,

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r carbon�, amount of secondary phases, variation in stacking se-uence, lattice parameter of SiC, and grain size �62�. Preparationethods also influence the toughness of SiC ceramics �e.g., Ref.

63��. As will be discussed below, the parameters and variables tochieve an optimum performance are aplenty, and both experi-ental and numerical investigations are needed before a ceramic

or its variant� is deemed suitable for ballistic applications.

3.3 Ballistic Performance of Silicon Carbide and Itsariants. The ballistic performance of high toughness SiC ceram-

cs is investigated in Ref. �64�. The toughness of SiC was en-anced by the addition of aluminum-boron-carbon �Al–B–C� andttrium aluminum garnet �YAG� using the hot-press method. Theigh fracture toughness of 7 MPa·m1/2 in this system is mainlyttributed to the intergranular fracture, which allows crack bridg-ng. The V50 velocity at which the 14.5 mm BS-41 bullets pen-trated through a 12.7 mm-thick ceramic layer was determined toe between 722 m/s and 750 m/s for the toughness ranging from.5 MPa·m1/2 to 8 MPa·m1/2. It was found that higher toughnessid not result in improved ballistic performance, indicating thathe depth of penetration was a poor indicator for the V50 testerformance for low kinetic energy threats �bullets�.

These properties together complicate the design of an integralallistic armor system, using a ceramic as the strike face to resisthe penetration of a projectile. The ballistic performance of a ce-amic is system-dependent and a unique ceramic design �or itsariant� for optimal performance is not often achieved �57�. Fornstance, the SiC performed better than B4C and Al2O3, althoughiC and B4C both have compressive strength approximately 1.3

imes greater than that of Al2O3 �60�.The performance of thick confined SiC ceramics upon impact

y tungsten-carbide �WC� spheres at velocities reaching up to700 m/s is studied in Ref. �57�. Three Cercom® hot-pressederamic variants �constituent-wise�, namely, SiC–N, SiC–B andiC–1R �designated�, and a pressureless sintered SiC called Supe-ior Graphite® were tested. SiC–N is a state-of-the-art ceramicrmor material with a fracture toughness twice as large as that ofolid-state-sintered SiC at the expense of hardness �62�. The ca-ability of SiC–N in absorbing the impact energy was quantifiedy the measure of impact energy per unit mass, expressed in J/g,f the crater. The diameter of the WC projectile was 6.35 mm,hich widened to about 20 mm at the maximum velocity.It was also found in Ref. �62� that the rear surface of the sample

emained smooth with no sign of indentation. The specific veloc-ty regimes differed for the ceramic variants tested, and hence theniqueness of the material design in terms of ballistic perfor-ance. The main ballistic performance measure of the confined

eramic armor is based on energy absorption in tandem withuantification of material damage.

Laminated Composites and Integral ArmorAs mentioned earlier, armor weight is a key factor related to the

earer’s mobility. Although steel has been a popular choice inefense applications, it possesses high density and is not suitableor design of body armor, where light weight is an importantriterion. A suitable alternative to tackle this issue is to considerightweight composites that are competitive to steel in perfor-

ance in terms of ballistic penetration �65�. Laminated compos-tes provide an optimal solution to the design of lightweight bodyrmor. Armor designs, such as the gradient composite armor sys-em composed of layers of a ceramic particles/epoxy composite,ave been found to out-perform solid ceramic tiles in terms ofeight, ballistic limit, and flexibility �66�.A major distinction between conventional engineering materials

nd composites is associated with the deformation and energybsorption characteristics, which are two aspects that have impor-ant relevance to the performance of anti-ballistic materials �67�.
arious studies directed toward demystifying the impact behavior
50802-8 / Vol. 62, SEPTEMBER 2009


and energy absorption characteristics of different composite ma-terial systems have been undertaken recently �68–71�.

Metals undergo plastic deformations during crash or failure,whereby energy is absorbed via rupturing of the structure. Thisdepends on the fracture characteristics �e.g., brittle or ductile� of ametal, which are related to the microstructure of the metal. How-ever, in the case of brittle composites, the energy absorbingmechanism is usually ascribed to the process of multiple crackingor micro-fragmentation, which is the resultant effect of progres-sive fracture of fiber-matrix interfaces.

Ductility is the ability to sustain plastic deformation before fail-ure. The extent of ductile deformation in composites, which givesrise to its energy absorbing capability, depends on the ductility ofthe fibers �measured, for example, by their failure strains� and thefiber-matrix interfacial adhesion. The performance of a fabric�made from Kevlar® or Twaron® fibers� is highly dependent onthe elastic moduli, tensile strengths, and failure strains of the fi-bers �72�.

An outlook of composite materials for ballistic protection ofmoving armor vehicles and body armor for personal protection isprovided in Ref. �67�. Among others, integral armor or compositestructural armor, flexible �fabric-based� composite armor, and theuse of carbon nanotubes �CNTs� were discussed there. Taking cuefrom this line-up of material systems, a review detailing the de-sign and ballistic characteristics of these systems is provided next,which will be followed by a discussion on the performance evalu-ation of selected material systems.

4.1 Integral Armor. Integral armor refers to a class of lami-nated materials consisting of a fascia and a backing plate to over-power an armor-piercing projectile �67�. The purpose of the hard,front-facing layer is to blunt and graze the incoming projectile.The backing layer deforms and absorbs the projectile energy, andit also acts as a flexible support for the facing material during theinitial impact. The façade material is usually made from a ceramicsuch as Al2O3, SiC, B4C, or TiB2. From the material point ofview, ceramic is the prime choice for the front-facing layer owingto its high hardness and low density. When the projectile passesthrough the front-facing layer, it tends to break the layer intomicro-fragments. Backing plate can be made of lightweight mate-rials such as aluminum and fiber reinforced composites�70,73,74�. The mechanics of penetration of ceramic integral ar-mor was originally studied in Refs. �75,76�. The issues of laminatebonding strength and interfacial delamination of integral armormaterial systems will be presented later. Relevant studies on usingSiC as a façade material have been examined in Secs. 3.2 and 3.3.The use of Al2O3 as the façade material is discussed next.

4.1.1 Alumina/Aluminum Laminated Composite Structure.The potential of laminated ceramic composites in providing bal-listic protection is comparable to that of steel but at lower weight.A laminated composite armor system having a ceramic �alumina�front layer and a backing layer of either a metallic or compositematerial was examined in Ref. �74�. The idea behind this design isas follows. The ceramic layer, which faces the projectile, erodesthe ogive and probably breaks it up. This increases the contactarea between the projectile and the backing plate, thereby reduc-ing the local pressure. The metallic or composite backing layerabsorbs the kinetic energy of the projectile besides supporting theceramic fragments. The ballistic resistance mechanism of thin ce-ramic armor has been discussed in Sec. 3.1.1.

In Ref. �74�, the ballistic performance of high-strength low al-loy steel �chromium-vanadium� specimens and alumina �frontlayer�/aluminum �backing layer� laminated composite specimenswere experimentally compared against 7.62 mm armor-piercingprojectiles impacting the target at 805�15 m /s. Their resultsshowed that the ballistic resistance and deformation characteris-tics of each steel sample rely on its areal density �which is pro-
portional to thickness, see Eq. �2�� and surface hardness. Increase


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n hardness augmented the erosion and fracturing of the projectile,esulting in reduced penetration capability of the projectile, as waslaborated in Sec. 3.1.

Sample S1 with a lower hardness value of 40 HRC underwentuctile perforation as compared with brittle fracture of anotherample �S3� with a hardness value of 60 HRC. This is expectedue to the effect of hardness on the type of failure at full penetra-ion. Furthermore, friction between the projectile and steel plateuring penetration was converted to heat, thereby causing localelting of the projectile. This was confirmed by using SEM,hich revealed that the bright zones on sample S1 are melting

races of lead content of the projectile.The laminated composite samples �ceramic façade with Al

acking plate� proved to be weight efficient. The thickness of thelumina front plate ranges from 8 mm to 20 mm and the backingayer varies between 3.17 mm and 7.92 mm. When compared withhe steel specimen, the composite sample provided a weight sav-ng up to 26% for the same ballistic protection �measured as 0%enetration probability�. Another observation was that the façade/acking plate thickness ratio is an important design parameter thatnfluences the ballistic performance. It was determined that theptimum thickness ratio lies between 1 and 3, which gives rise tohe effective areal density of 66–68 kg /m2.

The ballistic failure of the alumina/aluminum armor systemsan occur via different mechanisms: �i� tensile radial crack forma-ion due to generation and propagation of tensile stress wavesrom the frontal/backing plates’ interface to the free boundary ofhe ceramic frontal �see Fig. 4�, or �ii� formation of a conical-haped, stressed zone due to a high compressive stress induced bympact. This compressive stress extends through the direction ofhe projectile and crushes the stressed zone into small fragmentspulverization�. Petalling of the backing aluminum plate shall oc-ur if the projectile manages to pulverize the façade and to perfo-ate the backing plate. This petalling is highly possible when theagnitude of the compressive stress due to projectile impact is

reater than the strength of the aluminum layer.

4.1.2 Fiber Metal Laminates. Similar concepts of body armorystems where two or more separate layers of fibers and metallates are used for complete protection have also been studiedince the 1940s. One such example is the fiber metal laminatesFML�.

An overview of the development of two types of commercialMLs, i.e., Arall® and Glare®, was provided in Ref. �77�. FMLs,ccording to the definition given there, are hybrid laminates con-isting of thin alternating bonded layers of aluminum and fiber/poxy. Arall® �short for aramid reinforced aluminum laminates�as designed and developed in Delft University of Technology,he Netherlands in 1980, and optimizations were continuouslyarried out until the end of the decade. Although the concept ofrall® was initially mooted to cater for the aerospace industry’semand for a lightweight and low-cost material to be attached toxisting aluminum-based structures, Arall® was also consideredor ballistic protection. This was made possible by the develop-ent of a patented variant of Arall® with glass fibers �78�. The

rmor plating variant consisted of frontal ceramic tiles to break upr damage the bullet, thereby absorbing part of the bullet’s kinetic

Fig. 5 Composite structure

nergy, and a backing layer of the patented glass fiber Arall® that



would absorb remaining kinetic energy and eventually stop theprojectile. However, the Dutch military found the special Arall®variant for ballistic protection not to be a viable material probablydue to cost, application �off-axis loading� and manufacturing is-sues. Glare® �short for glass reinforced FML� is another FMLvariant developed and patented in Ref. �79�.

Glass-fiber layers/metal laminate systems have not been fullyexplored for ballistic armor, and there is still a need for furtherstudies in this area. For instance, the impact properties ofALGLA-2S42 �as in Ref. �79�� consisting of a prepreg core �con-tinuous glass filament-synthetic layers� sandwiched between twometal layers �Fig. 5� have not been studied yet.

4.1.3 Aluminum Foam Ballistic Protection Armor. The designconcepts and structures of ballistic armor systems evolve aroundthe success of new material developments. This is becoming moreevident with the advent of composite armor. Metallic foams, suchas closed-cell aluminum foams, have been investigated for theirrole as an effective intermediate layer between a hard façade ma-terial �such as ceramic� and a fiber reinforced polymer backingplate �or layer�. The low-density intermediate layer acts to absorbor attenuate stress waves resulting from projectile impact. It alsohelps to reduce the intrusion depth �by the deformed projectile� inthe backing plate, thus providing an opportunity for the design ofan effective anti-ballistic armor system with minimal thicknessand reduced inward deformations.

The application of Alulight® closed-cell aluminum foams inballistic protection armor was mentioned in Ref. �80�. This is aclass of composite armor panels called integral armor with anintermediate layer of low-density closed-cell aluminum foams. Aqualitative description of the ballistic protection mechanism ofthis type of armor is provided in Refs. �81,82�, and a brief expla-nation is given below.

During ballistic impact, the foam exhibited significant nonlineardeformations and stress wave attenuation. The experimental bal-listic results were compared with plane strain finite element analy-ses of stress wave propagation. These numerical analyses corre-lated well with the experimental observation that the aluminumfoam can delay and attenuate stress waves associated with theballistic event.

The experimental results of the ballistic tests, together with thefinite element simulations, indicated that the aluminum foam ex-periences a dynamic nonlinear deformation that propagatedthrough the thickness until complete densification. Stress wavesgenerated due to this deformation are propagated and attenuatedwithin the cellular microstructure of the aluminum foam, whichfunctions as microscopic wave-guides that impede the propaga-tion of the elastic waves. Effective propagation of stress waves inthe foam is directly influenced by the amount of densificationachieved after the impact. Partial foam densification acts as awave filter to delay and attenuate the stress waves, as was foundin Ref. �81�. The time lapse for complete densification corre-sponds to the time delay in stress transfer to the backing layer,which was found to be a linear function of the foam thickness. Itwas reported that compared with a baseline armor design of thesame areal density without the foam, the armor with the foamlayer performed better with the advantage of insignificant increase

LGLA-2S42 „after Ref. †79‡…
of A
in the total mass of the armor, thus satisfying the lightweight

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riterion. Among others, the aluminum foam improved the ce-amic fracture characteristics, provided less separation of theolymer layer covering the entire armor lay-up, induced localizedoam deformations, reduced the dynamic deflection of the backinglate, controlled the volumetric delamination of the backing plate,nd prevented ceramic-foam interface debonding.

4.2 Flexible Composite Armor. Flexible composite armor oroft armor is mainly used for body protection and for the sake ofearer’s mobility. The armor materials are primarily woven fab-

ics composed of high-stiffness densely stacked organic fibers.bility to absorb impact energy without extensive deformation so

s to protect the wearer from being struck by the ammunition ishe main design consideration of flexible composite armor �67�.y and large, flexible composite armor materials are closely asso-iated with ballistic fabrics.

4.2.1 Fabric Systems. A host of commercial fabric materialystems such as those made from Kevlar®, Twaron® and Spec-ra® fibers has been used in the design of vests and helmets forhe United States military troops involved in the on-going battle inraq and Afghanistan. Discussions on the design of new materialystems based on these commercial fabrics have been provided inec. 2. Besides the para-aramid fibers used to produce the Kev-

ar® and Twaron® fabrics, other high performance fibers that areeing utilized to manufacture ballistic fabrics include the UHM-PE, PBO, and S-2 glass fibers. The degradation aspects of the

BO and UHMWPE fibers and fabrics have been discussed inec. 2.4. An overview of the chemistry and ballistic properties andjuxtaposition of the advantages and disadvantages of these fibersere provided in Ref. �83�.The polypyridobisimidazole organic fiber, also known as the5® fiber, is worth mentioning here. This fiber was developed byagellan Systems, LLC �a subsidiary of DuPont Advanced Fiber

ystems� and has been tested for the Future Force Warrior projecty the United States Army Soldier Systems Center. The M5®ber is said to be stronger and lighter than the para-aramid fibersnd the UHMWPE fiber �84�. For instance, the M5® fiber pro-ides at least 35% weight saving for the same level of protectionffered by the Kevlar® fiber �85�. The M5® fiber has been de-cribed as a rigid rod polymer featuring bidirectional hydrogenonding that creates a three-dimensional honeycomb network86�. Even though the M5® fiber possesses strength and weightdvantages, some issues related to its manufacturability and itstability at higher temperatures still need to be addressed �29�.

4.2.2 Polymer Composites. Polymer matrix composites haveeen utilized for protection against ballistic impact. Epoxy andolyester are two types of polymers that have been widely studiedor body armor applications. Carbon and glass fibers, usually inabric preforms �mats�, are commonly employed to enhance thetrength and toughness of the epoxy and polyester matrices. Otherigh-strength polymeric fibers �such as the para-aramid and UH-WPE fibers� have also been investigated �87�. Polymer matrix

omposite plates can be used as protective inserts in modular bodyrmor vests, e.g., the Interceptor Body Armor �84�.

The ballistic performance of plates made from polyester rein-orced by E-glass mats and filled with coarse sand was studied inef. �88�. The areal density of the three-phase lightweight com-osite panels varies between 0.61 g /cm2 and 1.65 g /cm2, de-ending on the thickness of the laminated plates and the volumeraction of the filler. It was found that the V50 ballistic limit andhe corresponding specific ballistic energy �see Eq. �1�� of theomposite plates increase with the volume fraction of the micro-ized �600–700 �m� sand filler. High velocity impact tests withhe impact velocity ranging between 70 m/s and 200 m/s wereonducted on the plates. For unfilled composite specimens, it wasound that the specific ballistic energy tends to increase with thehickness �i.e., number of layers� of the plates. A similar finding
as reported in Ref. �87�. The V50 ballistic limit generally in-
50802-10 / Vol. 62, SEPTEMBER 2009


creases with the thickness of the composite plates and the fillervolume fraction. Adding sand into the composite improves thestiffness of the composite. For thin plates �i.e., four layers about 4mm thick�, it was observed that the sand-filled composite platesperform better than the unfilled composite plates in terms of V50due to the stiffening effect of the filler. The thick specimens �i.e.,12 plies at about 10 mm thick� absorbed 400% more energy perunit areal density than the thin ones mainly by delamination.Thus, the overall protection mechanism against penetration andperforation involves the E-glass fiber straining and the delamina-tion of the plies, even though the later may dominate in energyabsorption under high-velocity impact loading, as elaborated inRef. �89�.

The ballistic properties of Kevlar 29®/Polivnyl Butyral™�KPB� woven fabric composites and UHMWPE laminated platesfor lightweight armor design were investigated in Ref. �5�. Thethickness of the KPB and UHMWPE specimens was 4 mm and2.9 mm, respectively. The plates were made from 20 layers ofindividual composite laminas. The areal density of the KPB speci-men was 9.28 kg /m2 and that of the UHMWPE was 5.04 kg /m2.The back face deformation was determined by measuring the cor-responding deformation endured by a paste that was affixed to therear of each composite plate. The ballistic performance of thecomposites was tested against a 9 mm full-metal jacket bullet. Itwas determined that the V50 of the KPB specimen is 680 m/s,whereas the UHMWPE plate has a V50 equaling 480 m/s. Theballistic performance of each composite was quantified in terms ofV50 and the back face deformation normalized by its areal den-sity. For the same areal density, the UHMWPE plate yielded aV50 that is 30% higher and a back face deformation that is 100%smaller than those of the KPB composite. It was found that thehigher elastic modulus and strength of the UHMWPE compositeenhance its ballistic performance. Also, a higher projectile masswas observed to reduce the V50.

4.2.3 Design of a Blunt Trauma Reduction Armor. The mainfunction of the anti-ballistic body armor is to provide protection tothe wearer by resisting projectile penetration and reducing its ki-netic energy. An effective anti-ballistic body armor system is ex-pected to perform this function in order to shield the wearer fromexperiencing excessive transfer of kinetic energy, which maycause injuries to the bone structure and internal organs due toshock. This is known as blunt trauma. The degree of blunt traumaexperienced by the wearer depends on the extent of inward defor-mation of the armor after the impact.

A patented blunt trauma reduction fabric system for body armorapplications was proposed in Ref. �20�. The fabric system is de-signed to reduce the inward deformation with consideration of airpermeability for comfort of wearing. The latter criterion is men-tioned in Ref. �3�. The material concept is a three-dimensionalfabric structure known as spacer fabric and made from warp in-terweaves. As shown in Fig. 6, the spacer fabric consists of threelayers: the frontal and backing layers made from fabrics of highperformance yarns, and an interconnecting layer �12–30 mm� ofhigh performance monofilament yarns. The back face fabric layeris a porous open mesh design that facilitates air circulation and isof hydrophilic nature to absorb body moisture. A polyester fabricwould fit this purpose.

The high performance feature of the front and back faces refersto yarns with high tenacity �15 g/denier�, which means that yarnsmade from Kevlar®, Twaron® or Spectra® fibers can be applied.For example, the linear mass density of the Kevlar 29® yarn is1000 denier, which spells its opaqueness and thereby the resis-tance to the motion of the projectile. In the context of blunttrauma, it is the interconnecting yarns that act to resist the inwarddeflection of the fabric system due to impact. The interconnectingmonofilament layers are made of fabrics with a thread density ofup to 3000 threads / in2 and yarns with a tenacity of 15 g/denier.This provides a resilient yet strong layer which is able to increase,
by 5% to 10%, the deformability of the fabric system exposed to


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he common handgun bullets. The patent provides no quantitativenformation on the efficiency of the fabric system in reducinglunt trauma. However, it provides the conceptual facts and ma-erial property requirements for the design of a blunt trauma re-ucing armor system. A variant of this configuration was patentedarlier �90�.

Nanomaterial Systems and Futuristic Design Con-epts for Body Armor

5.1 Nanocomposites. Nanocomposites are fabricated usinganometer-sized fillers, such as carbon nanotubes �CNTs�, as re-nforcements in a matrix material. Polymer matrix nanocompos-tes have been gaining a growing attention due to their superiorroperties �91�. The ballistic impact characteristics of polycarbon-te layered-silicate nanocomposites were investigated in Ref. �92�.he ballistic performance of polycarbonate and polymethyl-ethacrylate layered-silicate nanocomposites was studied in Refs.

93,94�. An approach to manufacturing high performance poly-eric nanofibers suitable for body armor components was re-

orted in Ref. �95�. Laminated hybrid composites consisting ofonventional composite plies and intercalated layers of nanocom-osites have also been explored for body armor applications �96�,n example of which will be presented in Sec. 5.1.2.

CNTs are regarded as a lightweight, flexible, and high-strengthaterial with exceptional energy absorption capability �97�. The

otential use of CNTs as a ballistic textile material is mentioned inef. �98�. There has been a growing interest in extending the usef CNTs in military applications. Within the context of body ar-or, the ballistic resistance of CNTs has been investigated. At-

empts are being made to produce CNT fabrics using innovativeanufacturing methods. These developments will be elaborated inecs. 5.1.1–5.1.3, where the mechanical properties and ballisticerformance of CNTs in the form of fibers and fabrics will beighlighted.

In addition, CNT reinforced composites �hereafter called CNTomposites� are also making their way into ballistic protectionpplications. The mechanical and thermal properties of CNT rein-orced polymer matrix composites have been extensively studied99–101�. CNTs as reinforcing elements provide significant ad-antages over other types of reinforcements due to their excellentechanical properties. Considering the superior mechanical and

hermal properties of CNTs and CNT composites, investigationsf the effect of inserting CNT composite panels into the structural

Fig. 6 Composite armor for blun

rmor or even flexible composite armor would certainly be inter-



esting, provided that the overall production cost is not too high. Adetailed discussion on harnessing the advantages of CNTs to pro-duce body armor systems is presented next.

5.1.1 Ballistic Performance of CNTs. The concept of utilizingonly CNTs in the form of nanotube fibers/yarns for ballistic fab-rics has been explored in Refs. �102,103�, the latter of which waspartially conducted at the United States Army Natick Soldier Cen-ter. The research outcome of these groups is summarized here.

The ballistic resistance dynamics and capacity of CNTs via mo-lecular dynamics simulations and the potential use of CNT-basedfabrics for body armor systems were examined in Ref. �102�. Theballistic performance of single-walled CNTs with constant length��75 Å� but various radii �10.576 Å, 8.608 Å, and 7.051 Å� wasstudied. The “bullet” was a piece of diamond with dimensions of35.6�35.6�7.1 Å3. This implies that the tip width of the bulletwould be larger than a flattened CNT �upon impact with no bondbreakage� by several orders of magnitude. The bullet was releasedfrom a distance of 15 Å from the central axis of the CNT atdifferent heights. The results were presented as a function of theCNT radius and the relative impinging location of the bullet onthe CNT.

The simulated tests involved a range of initial bullet speeds,which would not break the carbon-carbon bond, and the bulletwas expected to bounce back after impact. An independent simu-lation was performed to determine the initial bullet speed forbounce-back, which indicated that a speed varying between 1000m/s and 3500 m/s complies with this requirement. The principle ofconservation of energy was used to recalculate the bullet speed�from the time of its release� for every time step of 0.5�10−12 s. This facilitated the determination of the kinetic energyof the bullet by subtracting the energy absorbed by the CNT in thepreceding time step. No energy loss was assumed, since impactdeformation occurred in a very short duration of time and there-fore heat dissipation through the thermostat atoms bordering thephysical ends of the CNT would be negligible.

Molecular dynamics simulations were employed to study thebullet impact mechanism and the impact energy absorption capac-ity of CNTs with different radii. Atomic interactions in the CNTwere described by a three-body Tersoff–Brenner potential. Theinteraction between the bullet and CNT was of a no-bonding typeand was represented by a two-body Morse potential. Details of thesimulation parameters can be found in Ref. �102�.

Their results indicated that a CNT with a larger radius absorbsmore energy and withstands a greater bullet speed. However, the

rauma reduction „after Ref. †20‡…

variation in the relative absorption energy �defined as the maxi-

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um energy absorbed normalized by the total energy of the CNTefore impact� for different values of the nanotube radius wasound to be flimsy, i.e., the energy absorbed is weakly related tohe variation in the radius. In contrast, a strong linear relationshipetween the maximum absorption energy and the CNT length wasbserved. The authors of Ref. �102� also provided a case study ofhe prospective use of CNTs as a body armor material. They sug-ested that a possible CNT body armor system would consist ofeveral layers of woven CNT fabrics.

Of course, the concept of CNT fabric body armor will not beealized without identifying a feasible practical means of fabricat-ng the woven CNT fabrics. Fabrication of CNT woven fabricsecessitates the production of CNT fibers. Even though the keyechnique of producing CNT fibers is spinning from a pool ofNT-rich substance, there is more than one method to implement

his �103,104�. One of the processing techniques is to spin CNTbers from a liquid crystalline suspension of nanotubes. This pro-ess is similar to that used for producing polymeric fibers. An-ther method involves formation of a gas phase aerogel of single-alled CNTs �SWNTs� and double-walled CNTs �DWNTs� via a

hemical vapor deposition �CVD� process. CNT fibers are thenirectly spun from the SWNT and DWNT aerogel. An alternativeethod, which produces strong and tough CNT fibers, involves

imultaneous spinning and twisting of multiwalled CNTsMWNTs�. The electrospinning method is also emerging as anfficient method to fabricate nanofibers. Electrospun nanofibersan be produced in several thousands of meters long �105�. How-ver, the length and mechanical properties of electrospun nanofi-ers depend on process parameters �e.g., Ref. �106��. The fabrica-ion methods and the properties of the resulting CNT fabrics haveeen studied by various research groups, which will be brieflyummarized in the following snippets.

The solution spinning process, which involves an intermediateel-state, was employed to fabricate strong CNT fibers that areougher than either spider silk or any other fiber used for mechani-al reinforcement �107�. The energy needed to break the CNTbers �normalized against weight� is about 4 times greater than

hat for spider dragline silk fiber and 20 times higher than that forteel wires. The roles of nanotube length and structure, fiber den-ity, and nanotube orientation in achieving optimum mechanicalroperties were explored in Ref. �103�, where CNT fibers wereeveloped using the aerogel method. Axial orientation of theanotubes �i.e., alignment of bonds during mechanical extension�as naturally achieved during the drawing of the aerogel out of

he reaction zone, which also condensed the nanotubes into a fiberf low specific gravity ��0.01�. It has been found that windingates can be varied between 5 m/min and 20 m/min to control theNT orientation, fiber density, and mechanical properties. Theighest measured values of the specific strength and specific stiff-ess of the CNT fibers were about 10 GPa/SG �with SG denotingpecific gravity� and 400 GPa/SG, respectively, though the statis-ical mod of the latter averaged 200 GPa/SG. In comparison, theverage specific strength and specific stiffness of the Kevlar 49®ber were 3 GPa/SG and 90 GPa/SG, respectively.The mechanical properties of pure CNT fibers directly spun

rom a gas phase aerogel formed through CVD were reported inef. �108�. The continuous spinning of CNT fibers from the gashase benefits the process and the product by providing strongbers �1.46 GPa� with a low nominal density �2 g /cm3� and byabricating CNT fibers in relatively large quantities. The CNTbers were produced using iron nano-particles as the catalyst toissociate the carbon stock. It has been experimentally determinedhat decreasing the iron concentration by 0.010% significantly en-ances the strength and stiffness by 300% and 400%, respectively.ynthesis of continuous CNT fibers via direct spinning from aapor phase ethanol feedstock was elaborated in Ref. �109�. Theffect of process parameters on the continuous spinning of CNTbers was investigated in depth in another study �110�.
By introducing twist during spinning of MWNTs from nano-
50802-12 / Vol. 62, SEPTEMBER 2009


tube forests to make multiply torque-stabilized yarns, a yarnstrength greater than 460 MPa was achieved �104�. The deforma-tion of the yarns followed a reversible hysteretic pattern over largestrain ranges, providing a maximum of 48% damping. This studyalso showed that the CNT yarns do not degrade in strength byoverhand knotting, unlike ordinary fibers and yarns. The CNTyarns also retain their strength and flexibility upon heating in airat 450°C for one hour and subsequent cooling by immersing themin liquid nitrogen.

5.1.2 CNT Hybrid Composite Armor. The concept of hybridCNT-polymer composites for ballistic protection was explored inRef. �96�. The armor is made from E-glass continuous fiber rein-forced poly-vinyl-ester-epoxy �PVEE� matrix composite laminasinterlaced with MWNT reinforced PVEE mats.

Their experiments showed that the location and thickness of theMWCNT reinforced PVEE composite mats influence the ballisticperformance of the hybrid composite system. The total thicknessof the laminate is 12.7 mm. The 0.30 caliber fragment simulatingprojectile at the penetrating speed of 610 m/s was used for theballistic test. Of all the configurations �location and number of theMWCNT-PVEE mats� tested, it was determined that the one withone layer of the MWCNT-PVEE mat positioned at the top �nearestthe frontal surface� and another layer placed in the middle of thehybrid composite gives the optimum ballistic performance interms of the temporal evolution of the projectile velocity. Thevelocity profile does not differ significantly compared with a100% E-glass/epoxy composite lamina owing to the fact that thedifference in the acoustic impedance of the composite lamina andthe MWCNT-PVEE mat is between 5% and 6%. It was also ob-served that the use of a double-layer mat �100 �m� reduced moreenergy due to the larger amount of energy needed to penetrate thestrong MWCNT-PVEE mat.

As a closing note, although CNT-based polymer composites arepromising with several functional benefits, its viability, especiallyin the manufacturing aspects and cost effectiveness, must be givenequal importance in the design process. The emergence of appro-priate techniques that have an impact on the feasibility of fabri-cating CNT fibers and CNT fabrics at a reasonable cost will ad-vocate the development of CNT-based body armor systems. Thecost factor will be the fulcrum that would determine if CNTsindeed will coup d’etat other material systems currently beingdeveloped into termination or otherwise.

5.1.3 Kevlar®/Nylon and CNT Fibers/Nylon Composites. Bal-listic nylon was the leading candidate for body armor fabrics dur-ing and after the World War II until the 1960s, when it was re-placed by the para-aramid fibers including Kevlar® fibers �3�.Even though nylon fibers are about 20% less dense than Kevlar®fibers, the outcast of nylon fibers as the prime choice for ballisticfabrics could be attributed to its lower strength and toughness thanthose of Kevlar® fibers �84�. Enhancement of mechanical proper-ties of a composite material that are essential to ballistic protec-tion requires efficient stress transfer from the matrix to the rein-forcing fibers. An attempt to reinforce nylon 6,6 with surfacetreated Kevlar 29® fibers was made in Ref. �111�. The nylon/chopped Kevlar 29® bulk composite was processed via the meltblending technique. The elastic modulus and the tensile strengthof the composite were found to increase by about 10%. Nylonfibers can be made stronger and tougher by grafting the Kevlar®or CNT fibers onto the surface of the nylon polymer chain. Recentadvances in the processing techniques of polymer nanocompositeshave made it possible to enhance the mechanical properties ofnylon in several ways.

For example, it was found in Ref. �112� that nylon 6,6 can begrafted onto the surface of plasma treated Kevlar® fibers througha method called interfacial step polymerization to improve theKevlar®/nylon interface adhesion. An interfacial polymerizationmethod to improve the dispersion of SWNTs in the nylon 6,6
matrix was provided in Ref. �113�. This method was later em-


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loyed in Ref. �114� to boost the SWNT-nylon 6,10 interfacialdhesion at the molecular scale by using alkyl segments to bondhe SWNT and the nylon chains. The SWNT-reinforced nylonbers showed an increase of 140% in toughness, while both thelastic modulus and tensile strength increased by 160%. Thesemprovements were achieved by using only 1 wt % of SWNTs. Itas also observed that the addition of SWNTs did not affect theolecular weight of the nylon. The melt-mixing extrusion process

o produce MWNT reinforced nylon 6 filaments was employed inef. �115�. Single filament tensile tests of the SWNT-reinforcedylon indicated that the elastic modulus increased from 1.1 GPaneat� to 3.6 GPa �reinforced�, while the ultimate tensile strengthncreased by 164%. These developments in reinforcing the nylonber using CNTs were made possible by the emerging nanotech-ology. Lightweight ballistic fabrics can therefore be made fromow density but tougher and stronger fibers by utilizing the inno-ations in the processing techniques for fibers that are nanometersn diameter and several meters in length.

5.1.4 Inorganic Fullerene Nanotubes. Fullerene is a new formf carbon �besides graphite and diamond� with a molecular struc-ure of plane, tubular, or hollow spherical shape. CNTs are organicullerenes that are cylindrical in shape, with carbon atoms ar-anged hexagonally �116�. Generic MX2 inorganic compoundswhere M represents, for example, tungsten, W, or molybdenum,

o, and X represents sulfide, S� naturally occur as flat layeredtructures. Recent advances in fullerene technology enabled inor-anic layered compounds such as the tungsten disulfide, WS2, andolybdenum disulfide, MoS2, to be produced in the form of nano-

ubes or nanospheres with high reproducibility �117,118�. Inor-anic fullerenes �IFs� are tough and possess good shock absorbingroperties, both of which are relevant to applications where dy-amic pressures are involved such as in ballistic protection bodyrmor systems �119,120�. Fullerene-like WS2 and MoS2 nanopar-icles, for example, can resist shock waves with a peak pressurep to 25 GPa and 30 GPa, respectively, before disintegration121�. The impact energy absorption mechanism of the IFs can bexplained as follows. Due to impact, a series of shock waves areenerated and pass through the IFs. The hollow cage-like structuref the IF nanotubes undergoes continuous compression and de-ompression like a spring, through which the shock energy isissipated. The progression of shock waves causes two types ofeformations to the IFs, i.e., elastic deformation in the spring-likeeaction of the nanotubes, and plastic deformation that reduces theattice �crystallographic plane� separation distance. The hightrength and impact resistant characteristics of the IF nanopar-icles can be exploited for body armor applications by combininghem with materials that possess good elasticity �e.g., natural rub-er� to produce lightweight, strong, and flexible composites.

5.2 Futuristic Design Concepts. As discussed in Sec. 2, aandful of application-oriented research endeavors, which areostly related to enhancing existing body armor materials or sys-

ems, are undertaken to ensure personnel safety in the war field.n-field requirements for timely delivery of enhanced body armor

all for research efforts in improving existing systems rather thaneveloping totally new body armor systems which require addi-ional time for performance and reliability tests. Nevertheless, re-earch and development are constantly being pursued to producebers, yarns or material systems in the interest of reducing theody armor weight and increasing its impact energy absorptionapacity in addition to identifying feasible ways of manufacturinghe materials in large quantities. Features that are supplementaryut crucial for soft body armor, such as heat dissipation, humidityontrol, and resistance to chemical or biological attacks, are beingiven due attention in order to provide the wearer with an opti-um level of comfort and protection. The resulting innovations

an be either infused into currently available body armor systems
e.g., composite plates as protective inserts for the Interceptor


Body Armor� or synergized into a brand new body armor concept.A few examples of such innovations are presented in Secs.5.2.1–5.2.3.

5.2.1 Micro-truss Armor. A technique to produce 3D sub-micrometer truss frames using a photo sensitive epoxy resin andoverlapping laser beams was reported in Ref. �122�. The densityof the resulting lightweight material is three times smaller thanthat of the conventional epoxy resin. Due to its high toughness, itis anticipated that several layers of the polymeric micro-framescould absorb a high amount of impact energy and undergo exten-sive deformations, thereby making it suitable for lightweight bodyarmor applications.

5.2.2 Biomimetic Material Systems. Some new concepts ofmaterial design for body armor systems are inspired by naturallyoccurring biomaterials. As an example, spider silk fibers havedrawn a lot of attention due to its exceptional stiffness, strength,and toughness. A 3-�m-diameter spider silk fiber was reported tohave a secant modulus of 16 GPa, an ultimate tensile strength of1.7 GPa, and an ultimate tensile strain of 10.7% �123�. Large peakstress and failure strain enabled the spider silk fiber to absorb ahigh amount of impact energy. The ballistic limit of the spider silkfiber was found to be 426 m/s �123�, whereas that of the Kevlar29® fiber is known to be 500 m/s. The morphology of the spidersilk fiber consists of alternating elastic and crystalline regions,with the latter forming up to 50% of the fiber volume fraction. Theballistic energy absorption of the spider silk fiber occurs mainlythrough heat dissipation �124�. Efforts have been made at MIT’sInstitute of Soldier Nanotechnology to replicate the chemicalstructure of the spider silk fiber by fabricating a series of seg-mented polymer chains �125�.

In another study, the dermal armor of the polypterus senegalusfish �also known as the dinosaur eel� was investigated for its anti-penetrative capability �126�, where the mechanisms of penetrationresistance of the scales of the dinosaur eel were studied. It wasfound that the scales of the fish are made up of four organic-inorganic composite layers measuring between 10 �m and300 �m in thickness. Nanoindentation tests revealed that eachlayer of the scale possesses different mechanical properties �andneeds to be represented by a different constitutive model� thatvary according to a unique spatial function, thereby giving eachlayer exclusive deformation mechanisms to mitigate penetrationand to absorb impact energy. The indentation modulus �e.g., Ref.�127��, derived from load versus indentation depth curves for a500 �N maximum-load, decreases from 62 GPa for the outer-most layer to 17 GPa for the innermost one. The effective micro-indentation hardness reduces from 4.5 GPa for the outmost layerto 0.54 GPa for the innermost layer. Thus, the stiffness and hard-ness of the scales showed a negative gradation from the outmostlayer. All layers undergo mechanical hysteresis with energy dissi-pation, as seen from the loading-unloading nanoindentationcurves. These observations indicated that the stiff and hard inor-ganic outmost layer shields penetration, while the underlyingsofter layers absorb impact energy �via plastic deformation�.Hence, a novel material design for ballistic protection based onfunctional gradation of mechanical properties can be achieved byvarying the structure-property relationship along the thickness di-rection of a body armor system.

5.2.3 Natural Fiber Composites. The advantages of naturalfibers �NFs� include availability, cost effectiveness, environmentalsafety, manufacturability, and energy absorbing capacity �e.g.,Refs. �70� and �128–132��. NFs sourced from hemp, flax, sisal,and coir possess high elastic modulus, high specific tensilestrength, large failure strain, good formability, low density, andfibrous structures, all of which are in favor of their use for pro-ducing anti-ballistic fabrics. A review of various NFs and theirproperties is provided in Ref. �133�. Figure 7 shows the physical
and mechanical properties of selected NFs, glass fibers �GFs�, and
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ther high performance fibers. The hydrophilic nature �i.e., affin-ty to water� of NFs may lead to high moisture absorption, dimen-ional instability, and chemical incompatibility, which need to beell studied before NFs can be successfully used. In addition, the

ffects of environmental aging on the mechanical properties of NFomposites are also important and will be discussed next.

Blending of a biodegradable polymer, polylactic acid, and a NF,enaf �or rice husk�, results in a fully-degradable composite �134�.he temporal variations of the properties of each resulting com-osite due to moisture absorption and UV light were tested in anging machine. It was found that the infiltration of moisture intohe fiber/matrix interface over 1500 hours degrades the interfacialonding, thereby diminishing the composite flexural strength. Forach composite, the modulus of elasticity was also found to de-rease with aging, which was accompanied by a reduction in theuctility.

The effect of water absorption on the tensile properties ofemp/polypropylene �PP� composites and hemp-glass fiber/PP hy-rid composites was investigated in Ref. �135�. The hydrophiliceature of the surface of the hemp fibers led to an 8% increase inhe moisture absorption of the PP matrix. It was determined thathe incorporation of 40 wt % of glass fibers reduced the waterbsorption of the hemp/PP composites by 40%. The durability ofhe hemp/PP composites thus can be improved by adding glassbers, as also observed in the case of the bamboo-glass fiber/PPybrid composites in Ref. �136�. However, moisture absorptioneduced the elastic modulus and tensile strength of the hemp/PPomposites by 57% and 35%, respectively. The reduction islightly improved by the addition of glass fibers. The tensiletrength of redried aged hemp/PP composites is about 77% of thatf the wet composites. It was observed that absorption of moisturey the hemp/PP composites weakens the fiber/matrix bonding,hich possibly increases stress concentrations at the interface,

hereby decreasing the tensile strength of the composites. Similar

Fig. 7 Elongation at failure and tensile

bservations were made for the sisal and jute composites in Refs.

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�137,138�.However, contrasting experimental evidence was reported in

Ref. �139� for sisal/PP composites, whereby the elastic modulusand the tensile strength of the aged composites were found to behigher than those of the fresh composites. It was proposed thatgreater interfacial adhesion in the aged composites due to reducedmoisture absorption of aged fibers enhances the stress transfer,which results in improved mechanical properties. As observed inRef. �140�, aging does not significantly affect the impact strengthof the NF composites studied.

On the other hand, the effect of moisture absorption on thetensile and flexural properties of wood sawdust/polyvinyl chloride�PVC� composites was seen to be rather fluctuating �140�. Thetensile strength of the wood sawdust/PVC composites �containing37.5 wt % of sawdust fibers� was found to decrease with increas-ing moisture content up to 2% but subsequently increase by12.5% for moisture content up to 4%. Similarly, the elastic modu-lus of the composites was observed to decrease with increasingmoisture content up to 1.5% but subsequently increase by 6% formoisture content up to 3%. While the decrease in the tensilestrength is ascribed to the hydrophilic nature of the sawdust fibers,the increase in the elastic modulus and in the tensile strength athigher moisture contents is attributed to the effect of moisture inswelling of the fibers and to the reduced interfacial debonding.The impact strength was found to be independent of the moisturecontent at two higher sawdust fiber contents �i.e., 28.6 wt % and37.5 wt %� than at a fiber content of 16.7 wt %.

The foregoing discussion highlights a few positive effects ofaging on the mechanical properties of NF composites that arerelevant to body armor applications. In terms of degradation offibers due to moisture absorption and the UV light, some NFcomposites possess better mechanical properties than compositesreinforced by synthetic fibers.

rength of NFs, GFs, and ballistic fibers

Studies on NFs as an anti-ballistic body armor material are



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imited to laminated NF/polymer composites and/or for low-elocity impact applications. The damage of plain weave jute fab-ic reinforced polyester composites due to low-velocity impactas characterized in Ref. �141�. In Ref. �142�, tetraethyl orthosili-

ate was incorporated into the micro-tunnels of jute fibers, whichere pyrolyzed at elevated temperatures to fabricate �-SiC / juteber mats. The mats were intended for high temperature thermallanket applications. The surface treatment of epoxidized naturalubber �ENR� using a thermosetting resin and its effects on thempact strength of a Twaron®/ENR composite were studied inef. �143�. It was found that the surface modification results in a

light improvement of the impact strength partly due to the im-roved fiber/matrix interfacial adhesion. It was observed in Ref.144� that the toughness of coir fiber based composites can bemproved by alkaline treatment of the coir fiber, which increaseshe cellulose crystallinity. The low-velocity impact response ofon-woven hemp fiber reinforced unsaturated polyester compos-tes was investigated in Ref. �145�.

The use of NFs in a woven architecture for body armor appli-ations was made in Ref. �146�, where the anti-ballistic perfor-ance of NF �flax, hemp, and jute fibers� woven fabric reinforced

olypropylene sheets with and without mild steel backing or fac-ng was investigated. Flax composites exhibited a better energybsorption capacity than hemp and jute composites. The ballisticerformance of a Twaron CT716®/coconut shell powder-epoxyomposite �COEX� as an integral armor system was reported inef. �147�. Their results indicated that using the COEX composite

iles as the frontal component of the integral system reduces theequired number of Twaron® layers from 15 �in a Twaron® onlyanel� to 5, with the latter showing a 170% increase in energybsorption.

Figure 8 shows the V50 performance of the selected fabricrmor systems discussed above. It can be seen that the perfor-

Fig. 8 V50 performance of diffe

ance of flax and hemp composites was competitive to that of



ballistic fabric systems based on high performance para-aramidfibers. This was obtained at the expense of high areal density ofthe NF composites �11–14 kg /m2�, since the material thicknessrequired to achieve these performance levels was high eventhough the areal density of the base NF fabrics is lower. Forcomparison, the areal density of the three anti-ballistic fabric sys-tems shown in Fig. 8 ranges from 0.5 kg /m2 to 1 kg /m2.

6 Comparative Analysis of Composite and Hybrid Ar-mor Systems

A multitude of material systems for anti-ballistic body armorhas been discussed in Secs. 4 and 5. The ballistic limits of severallaminated composite and hybrid armor material systems, forwhich the performance measure and other physical features canpossibly be compared, are graphically shown in Fig. 9. The designand performance of these selected material systems are juxtaposedin Table 2. It is not the intention here to weigh one materialsystem against another because almost every key parameter variesfrom one system to another. For instance, the material thickness,areal density, bullet type, and striking speed are inherently differ-ent for these distinct composite systems. Therefore, the compara-tive analysis presented in Table 2 only serves to provide an im-mediate insight of how different composite systems influence theballistic limits and energy absorption mechanisms.

7 Concluding RemarksThe design, protection mechanisms, and performance evalua-

tion of ballistic fabrics, ceramics, laminated composites, and hy-brid materials normally used for body armor systems are dis-cussed. The ballistic performance of these materials is evaluatedon different but corresponding bases. In general, factors that di-rectly influence the ballistic limits of body armor systems are areal

t ballistic fabric armor systems

density �and thickness�, type and velocity of projectile, and mate-

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Fig. 9 Ballistic limits of selected laminate and hybrid composite systems „areal density shown inside bar…

Table 2 Comparative analysis of the design and performance of selected composite and hybrid armor material systems

aterial designThickness

�mm�Areal density

�kg /m2�Ballistic limit

�m/s�

Note�energy absorption mechanism,advantages and/or disadvantages�

l2O3 /Al laminatedomposite �74�

Al2O3=8–20Al=3.17–7.92

66–68 �optimal�805

�armor-piercingtype, 7.62 mm�

Energy dissipation via conversion of kinetic energyinto tensile stress waves or a compressive stress withinthe ceramic façade, which affects the material’s partial orcomplete failure by cracking. Performance comparablewith steel was achieved at a weight savings of 26%, andhence the mass efficiency of this system. However, theweight of this system is still high compared with theCNT and polymer composite hybrid armor systems.

NT hybridomposite �E-glass/PVEEnd MWNT/PVEE� �96�

12.7 N/A

610�fragment simulating

projectile type,6.35 mm�

Projectile defeat mechanism is the kinetic energyabsorption. This is enhanced by interlacing the 50 �mthick CNT/PVEE composite mats between thick layersof E-glass/PVEE.

evlar 29®/Polivnylutyral™ composite aspposed to UHMWPE �5�

4.0 �2.9�a 9.28 �5.04�a680 �480�a

�9 mm, full-metal jacketed�

The plate absorbs the kinetic energy of the projectile byconverting it into strain energy, which affects thedeformation �damage� of the back face.

NT �18,0� �102�Length�75 Å,

radius=10.576 Å�optimal�

N/A 2600�no penetration�

Energy conversion through elastic deflection andrecovery of CNT that bounces the bullet back. The bulletused is a 35.6�35.6�7.1 Å3 diamond piece. Theballistic limit is determined for a single CNT �withoutbond disintegration�.

omposites plates maderom E-glass choppedtrand mat reinforcedolyester containing coarseand �88�

4.0 and 10.0 6.1 and 16.5 73 and 156

The V50 ballistic limit generally increases with thethickness of the composite plates and the filler loading�both at the expense of weight�. The thick specimensabsorb greater ballistic energy than the thin ones by400% mainly through delamination.

Values in parenthesis are for the UHMWPE specimens.

50802-16 / Vol. 62, SEPTEMBER 2009 Transactions of the ASME

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ial configuration. In addition, elastic modulus, strength, andoughness are important properties that affect the projectile defeat

echanisms and influence the impact energy absorption of bodyrmor. Several innovative modifications of existing ballistic fabricaterials to cater for the needs of improved safety and reducedeight were outlined. The development of high performance fab-

ics for the next generation body armor will depend heavily onigh tenacity yarns made from fibers with high modulus, hightrength, and excellent anti-degradation traits. It has been estab-ished that ceramic tiles made from SiC and Al2O3 and polymeromposites �including nanocomposites� will, respectively, be thexcellent candidates for the frontal and back face components ofhe laminated integral and/or the hybrid armor systems.

Body armor design has evolved into the present form over thou-ands of years of improvement and with the help of advances inaterials science. Though still at its embryonic stage in develop-ent, simulations and experiments have demonstrated the poten-

iality of CNTs and CNT composites as new body armor materialshat possess superior anti-ballistic properties. More research stilleeds to be conducted in order to verify and validate a few hypo-hetical claims and ideas related to the ballistic resistant capabilitynd functions of CNT fibers and fabrics. Design concepts moti-ated by nature are being explored for possible body armor appli-ations. Utilization of natural fiber composites for body armoresign is a viable option, and attempts that have been made tonvestigate the ballistic performance of woven fabrics based onatural fibers provide the testimony for an encouraging future ofhis class of materials. These natural fiber based woven fabricsossess specific properties on a par with the high performanceallistic fabrics currently in use, and, in addition, they havenique advantages in the context of biodegradability, environmen-al safety, and manufacturing cost.

cknowledgment/DisclaimerThe work reported here is partially funded by the U.S. Army –

oldier Equipment Program. This support is gratefully acknowl-dged. The authors also wish to thank Prof. Victor Birman andwo anonymous reviewers for their encouragement and helpfulomments on an earlier version of the paper. The views and con-lusions contained herein are those of the authors and should note interpreted as necessarily representing the official policies orndorsements, either expressed or implied, of the U.S. Army.

eferences�1� Starley, D., 1999, “Determining the Technological Origins of Iron and Steel,”

J. Archeol. Sci., 26, pp. 1127–1133.�2� Scales, R. H., 2006, “Clausewitz and World War IV,” Armed Forces J., re-

trieved Feb. 11, 2008, http://www.armedforcesjournal.com/2006/07/1866019.�3� NIJ, 2001, “Selection and Application Guide to Personal Body Armor,” NIJ

Guide 100–01, U.S. Department of Justice.�4� Barauskas, R., and Abraitiene, A., 2007, “Computational Analysis of Impact of

a Bullet Against the Multilayer Fabrics in LS-DYNA,” Int. J. Impact Eng., 34,pp. 1286–1305.

�5� Colakoglu, M., Soykasap, O., and Özek, T., 2007, “Experimental and Numeri-cal Investigations on the Ballistic Performance of Polymer Matrix CompositesUsed in Armor Design,” Appl. Compos. Mater., 14, pp. 47–58.

�6� Gao, X.-L., and Mall, S., 2000, “A Two-Dimensional Rule-of-Mixtures Micro-mechanics Model for Woven Fabric Composites,” J. Compos. Technol. Res.,22, pp. 60–70.

�7� Barauskas, R., 2005, “Combining Mezzo- and Macro-Mechanical Approachesin a Computational Model of a Ballistic Impact Upon Textile Targets,” Pro-ceedings of the Fifth WSEAS International Conference on Simulation, Model-ing and Optimization, Corfu, Greece, Aug. 17–19, pp. 427–432.

�8� Tan, V. B. C., Tay, T. E., and Teo, W. K., 2005, “Strengthening Fabric ArmourWith Silica Colloidal Suspensions,” Int. J. Solids Struct., 42, pp. 1561–1576.

�9� Zeng, X. S., Shim, V. P. W., and Tan, V. B. C., 2005, “Influence of BoundaryConditions on the Ballistic Performance of High-Strength Fabric Targets,” Int.J. Impact Eng., 32, pp. 631–642.

�10� Nadler, B., and Steigmann, D. J., 2003, “A Model for Frictional Slip in WovenFabrics,” C. R. Mec., 331, pp. 794–804.

�11� Duan, Y., Keefe, M., Bogetti, T. A., and Cheeseman, B. A., 2005, “Modelingthe Role of Friction During Ballistic Impact of a High-Strength Plain-WeaveFabric,” Compos. Struct., 68, pp. 331–337.

�12� Duan, Y., Keefe, M., Bogetti, T. A., Cheeseman, B. A., and Powers, B., 2006,
“A Numerical Investigation of the Influence of Friction on Energy Absorption


by a High-Strength Fabric Subjected to Ballistic Impact,” Int. J. Impact Eng.,32, pp. 1299–1312.

�13� Zeng, X. S., Tan, V. B. C., and Shim, V. P. W., 2006, “Modeling Inter-YarnFriction in Woven Fabric Armour,” Int. J. Numer. Methods Eng., 66, pp.1309–1330.

�14� Cheeseman, B. A., and Bogetti, T. A., 2003, “Ballistic Impact Into Fabric andCompliant Composite Laminates,” Compos. Struct., 61, pp. 161–173.

�15� Ulven, C., Vaidya, U. K., and Hosur, M. V., 2003, “Effect of Projectile ShapeDuring Ballistic Perforation of VARTM Carbon/Epoxy Composite Panels,”Compos. Struct., 61, pp. 143–150.

�16� Briscoe, B. J., and Motamedi, F., 1992, “The Ballistic Impact Characteristicsof Aramid Fabrics: The Influence of Interface Friction,” Wear, 158, pp. 229–247.

�17� Boubaker, B. B., Haussy, B., and Ganghoffer, J.-F., 2007, “Consideration ofthe Yarn–Yarn Interactions in Meso/Macro Discrete Model of Fabric Part II:Woven Fabric Under Uniaxial and Biaxial Extension,” Mech. Res. Commun.,34, pp. 371–378.

�18� Lim, C. T., Shim, V. P. W., and Ng, Y. H., 2003, “Finite-Element Modeling ofthe Ballistic Impact of Fabric Armor,” Int. J. Impact Eng., 28, pp. 13–31.

�19� Shim, V. P. W., Lim, C. T., and Foo, K. J., 2001, “Dynamic MechanicalProperties of Fabric Armor,” Int. J. Impact Eng., 25, pp. 1–15.

�20� Gehring, G. G., Jr., 2000, “Blunt Trauma Reduction Fabric for Body Armor,”U.S. Patent No. 6,103,641, Aug. 15.

�21� Magat, E. E., 1980, “Fibres From Extended Chain Aromatic Polyamides,”Philos. Trans. R. Soc. London, Ser. A, 294, pp. 463–472.

�22� Lee, Y. S., Wetzel, E. D., and Wagner, N. J., 2003, “The Ballistic ImpactCharacteristics of Kevlar Woven Fabrics Impregnated With Colloidal ShearThickening Fluid,” J. Mater. Sci., 38, pp. 2825–2833.

�23� NIST, 2001, “Ballistic Resistance of Personal Body Armor �Revision A�,” NIJStandard–0101.04, coordinated by for NIJ, U.S. Department of Justice.

�24� 1997, V50 Ballistic Test for Armor, MIL-STD-662F Standard, Department ofDefense Test Method Standard.

�25� Holmes, G. A., Rice, K., and Snyder, C. R., 2006, “Ballistic Fibers: A Reviewof the Thermal, Ultraviolet and Hydrolytic Stability of the Benzoxazole RingStructure,” J. Mater. Sci., 41, pp. 4105–4116.

�26� Chin, J., Forster, A., Clerici, C., Sung, L., Oudina, M., and Rice, K., 2007,“Temperature and Humidity Aging of Poly�P-Phenylene-2,6-Benzobisoxazole�Fibers: Chemical and Physical Characterization,” Polym. Degrad. Stab., 92,pp. 1234–1246.

�27� Tamargo-Martinez, K., Villar-Rodil, S., Paredes, J. I., Montes-Moran, M. A.,Martinez-Alonso, A., and Tascon, J. M. D., 2004, “Thermal Decomposition ofPoly�P-Phenylene Benzobisoxazole� Fibres: Monitoring the Chemical andNanostructural Changes by Raman Spectroscopy and Scanning Probe Micros-copy,” Polym. Degrad. Stab., 86, pp. 263–268.

�28� Bourbigot, S., Flambard, X., and Poutch, F., 2001, “Study of the ThermalDegradation of High Performance Fibres—Application to Polybenzazole andP-Aramid Fibres,” Polym. Degrad. Stab., 74, pp. 283–290.

�29� Cervenka, A. J., Young, R. J., and Kueseng, K., 2005, “MicromechanicalPhenomena During Hygrothermal Ageing of Model Composites Investigatedby Raman Spectroscopy. Part II: Comparison of the Behaviour of PBO and M5Fibres Compared With Twaron,” Composites, Part A, 36, pp. 1020–1026.

�30� Said, M. A., Dingwall, B., Gupta, A., Seyam, A. M., Mock, G., and Theyson,T., 2006, “Investigation of Ultra Violet �UV� Resistance for High StrengthFibers,” Adv. Space Res., 37, pp. 2052–2058.

�31� Wilhelm, M., and Bir, C., 2008, “Injuries to Law Enforcement Officers: TheBackface Signature Injury,” Forensic Sci. Int., 174, pp. 6–11.

�32� Deng, M., Latour, R. A., Drews, M. J., and Shalaby, S. W., 1996, “Effects ofGamma Irradiation, Irradiation Environment, and Postirradiation Aging onThermal and Tensile Properties of Ultrahigh Molecular Weight PolyethyleneFibers,” J. Appl. Polym. Sci., 61, pp. 2075–2084.

�33� dos Santos Alves, A. L., Cassiano Nascimento, L. F., and Miguez Suarez, J. C.,2005, “Influence of Weathering and Gamma Irradiation on the Mechanical andBallistic Behavior of UHMWPE Composite Armor,” Polym. Test., 24, pp.104–113.

�34� Bender, J., and Wagner, N. J., 1996, “Reversible Shear Thickening in Mono-disperse and Bidisperse Colloidal Dispersions,” J. Rheol., 40, pp. 899–916.

�35� Lee, Y. S., and Wagner, N. J., 2003, “Dynamic Properties of Shear ThickeningColloidal Suspensions,” Rheol. Acta, 42, pp. 199–208.

�36� Ahmad, M. R., Wan Yunus, W. A., Salleh, J., and Samsuri, A., 2007, “Perfor-mance of Natural Rubber Coated Fabrics Under Ballistic Impact,” MalaysianPolymer Journal, 24, pp. 39–51.

�37� Walker, J. D., 2001, “Ballistic Limit of Fabrics With Resin,” Proceedings ofthe 19th International Symposium on Ballistics, I. R. Crewther, ed., RUAGLand Systems, Thun, Switzerland, Vol. 3, pp. 1409–1414.

�38� Cunniff, P. M., 1999, “Decoupled Response of Textile Body Armor,” Proceed-ings of the 18th International Symposium of Ballistics, W. G. Reinecke, ed.,CRC, Boca Raton, FL, Vol. 1, pp. 814–821.

�39� Egres, R. G., Jr., Decker, M. J., Halbach, C. J., Lee, Y. S., Kirkwood, J. E.,Kirkwood, K. M., Wagner, N. J., and Wetzel, E. D., 2004, “Stab Resistance ofShear Thickening Fluid �STF�–Kevlar Composites for Body Armor Applica-tions,” Proceedings of the 24th Army Science Conference, Orlando, FL, Nov.29–Dec. 2.

�40� Roylance, D., Chammas, P., Ting, J., Chi, H., and Scott, B., 1995, “NumericalModeling of Fabric Impact,” Proceedings of the National Meeting of theAmerican Society of Mechanical Engineers, San Francisco, CA, Oct.

�41� Lee, B. L., Walsh, T. F., Won, S. T., Patts, H. M., Song, J. W., and Mayer, A.
H., 2001, “Penetration Failure Mechanisms of Armor-Grade Fiber Composites
SEPTEMBER 2009, Vol. 62 / 050802-17


0

Downloaded Fr

Under Impact,” J. Compos. Mater., 35, pp. 1605–1633.�42� Silva, M. A. G., Cismasiu, C., and Chiorean, C. G., 2003, “Low Velocity

Impact on Laminates Reinforced With Polyethylene and Aramidic Fibres,”Proceedings of the Ninth International Conference on Enhancement and Pro-motion of Computational Methods in Engineering and Science, V. P. Lu, L. N.Lamas, Y.-P. Li, and K. M. Mok, eds., Taylor & Francis, London, UK, pp.843–851.

�43� Katangur, P., Patra, P. K., and Warner, S. B., 2006, “Nanostructured UltravioletResistant Polymer Coatings,” Polym. Degrad. Stab., 91, pp. 2437–2442.

�44� Miravete, A., 1999, 3-D Textile Reinforcements in Composites Materials,Woodhead Publishing Limited, Cambridge, UK, p. 2.

�45� Grogan, J., Tekalur, S. A., Shukla, A., Bogdanovich, A., and Coffelt, R. A.,2007, “Ballistic Resistance of 2D and 3D Woven Sandwich Composites,” J.Sandwich Struct. Mater., 9, pp. 283–302.

�46� Naik, N. K., Shrirao, P., and Reddy, B. C. K., 2006, “Ballistic Impact Behav-iour of Woven Fabric Composites: Formulation,” Int. J. Impact Eng., 32, pp.1521–1552.

�47� Wang, X., Hu, B., Feng, Y., Liang, F., Mo, J., Xiong, J., and Qiu, Y., 2008,“Low Velocity Impact Properties of 3D Woven Basalt/Aramid Hybrid Com-posites,” Compos. Sci. Tech., 68, pp. 444–450.

�48� Wynne, R. C., 1998, “Flexible, Lightweight, Compound Body Armor,” U.S.Patent No. 5,804,757, Sept. 8.

�49� Price, A. L., Erb, D. F., Jr., and Ritter, E. D., 2006, “B2: Enhanced EnergyAbsorbing Materials,” U.S. Patent No. 7,101,818, Sept. 5.

�50� Shockey, D. A., Marchand, A. H., Skaggs, S. R., Cort, G. E., Burkett, M. W.,and Parker, R., 1990, “Failure Phenomenology of Confined Ceramic Targetsand Impacting Rods,” Int. J. Impact Eng., 9, pp. 263–275.

�51� Zhang, X., and Mai, Y.-W., 1997, “Damage Wave Propagation in Elastic-Brittle Materials,” Proceedings of the IUTAM Symposium on Rheology of Bod-ies With Defects, R. Wang, ed., Springer, New York, Vol. 64, pp. 179–190.

�52� Chen, W. W., Rajendran, A. M., Song, B., and Nie, X., 2007, “DynamicFracture of Ceramics in Armor Applications,” J. Am. Ceram. Soc., 90, pp.1005–1018.

�53� Sarva, S., Nemat-Nasser, S., McGee, J., and Isaacs, J., 2007, “The Effect ofThin Membrane Restraint on the Ballistic Performance of Armor Grade Ce-ramic Tiles,” Int. J. Impact Eng., 34, pp. 277–302.

�54� LaSalvia, J. C., Horwath, E. J., Rapacki, E. J., James Shih, C., and Meyers, M.A., 2001, “Microstructural and Micromechanical Aspects of Ceramic/Long-Rod Projectile Interactions: Dwell/Penetration Transitions,” Proceedings of theExplomet 2000, K. P. Staudhammer, L. E. Murr, and M. A. Meyers, eds.,Elsevier Science, New York, pp. 437–446.

�55� LaSalvia, J. C., Normandia, M. J., Miller, H. T., and MacKenzie, D. E., 2005,“Sphere Impact Induced Damage in Ceramics: I. Armor-Grade SiC And TiB2,”Ceram. Eng. Sci. Proc., 26, pp. 171–181.

�56� Chen, M., McCauley, J. W., and Hemker, K. J., 2003, “Shock-Induced Local-ized Amorphization in Boron Carbide,” Science, 299, pp. 1563–1566.

�57� Normandia, M. J., 2004, “Impact Response and Analysis of Several SiliconCarbides,” Int. J. Appl. Ceram. Technol., 1, pp. 226–234.

�58� Hauver, G.E., Rapacki, E.J., Netherwood, P.H., and Benck, R.F., 2005, “Inter-face Defeat of Long-Rod Projectiles by Ceramic Armor,” U.S. Army ResearchLaboratory, Report No. ARL-TR-3950.

�59� Doyoyo, M., 2003, “Experiments on the Penetration of Thin Long-Rod Pro-jectiles Into Thick Long-Cylindrical Borosilicate Targets Under Pressure-FreePolycarbonate, Aluminum and Steel Confinements,” Int. J. Solids Struct., 40,pp. 5455–5475.

�60� Kaufmann, C., Cronin, D., Worswick, M., Pageau, G., and Beth, A., 2003,“Influence of Material Properties on the Ballistic Performance of Ceramics forPersonal Body Armor,” Shock Vib., 10, pp. 51–58.

�61� Moynihan, T. J., Chou, S.-C., and Mihalcin, A. L., 2000, “Application of theDepth-of-Penetration Test Methodology to Characterize Ceramics for Person-nel Protection,” U.S. Army Research Laboratory, Report No. ARL-TR-2219.

�62� Ray, R. D., Flinders, M., Anderson, A., and Cutler, R. A., 2003, “Hardness/Toughness Relationship for SiC Armor,” J. Am. Ceram. Soc., 24�3�, pp. 401–410.

�63� Zhang, J., Huang, R., Gu, H., Jiang, D., Lin, Q., and Huang, Z., 2005, “HighToughness in Laminated SiC Ceramics From Aqueous Tape Casting,” Scr.Mater., 52, pp. 381–385.

�64� Flinders, M., Ray, R. D., Anderson, A., and Cutler, R. A., 2005, “High-Toughness Silicon Carbide as Armor,” J. Am. Ceram. Soc., 88, pp. 2217–2226.

�65� Marsh, G., 2005, “Composites Fight for Share of Military Applications,” J.Reinf. Plast. Compos., 49�5�, pp. 18–22.

�66� Jovicic, J., Zavaliangos, A., and Ko, F., 2000, “Modeling of the Ballistic Be-havior of Gradient Design Composite Armors,” Composites, Part A, 31, pp.773–784.

�67� Hogg, P. J., 2006, “Composites in Armor,” Science, 314, pp. 1100–1101.�68� Arias, A., Zaera, R., Lopez-Puente, J., and Navarro, C., 2003, “Numerical

Modeling of the Impact Behavior of New Particulate-Loaded Composite Ma-terials,” Compos. Struct., 61, pp. 151–159.

�69� Clegg, R. A., White, D. M., Hayhurst, C., Ride, W., Harwick, W., and Hier-maierl, S., 2003, “Advanced Numerical Models and Material CharacterizationTechniques for Composite Materials Subject to Impact and Shock Wave Load-ing,” J. Phys. France, 110, pp. 311–316.

�70� Mahdi, E., Hamouda, A. S. M., Mokhtar, A. S., and Majid, D. L., 2005, “ManyAspects to Improve Damage Tolerance of Collapsible Composite Energy Ab-sorber Devices,” Compos. Struct., 67, pp. 175–187.

�71� Khalid, A. A., 2006, “The Effect of Testing Temperature and Volume Fraction

50802-18 / Vol. 62, SEPTEMBER 2009


on Impact Energy of Composites,” Mater. Des., 27, pp. 499–506.�72� Ivanov, I., and Tabiei, A., 2004, “Loosely Woven Fabric Model With Vis-

coelastic Crimped Fibres for Ballistic Impact Simulations,” Int. J. Numer.Methods Eng., 61, pp. 1565–1583.

�73� Grujicic, M., Pandurangan, B., Zecevic, U., Koudela, K. L., and Cheeseman,B. A., 2007, “Ballistic Performance of Alumina/S-2 Glass ReinforcedPolymer-Matrix Composite Hybrid Lightweight Armor Against Armor Pierc-ing �AP� and Non-AP Projectiles,” Multidiscipline Modeling in Materials andStructures, 3, pp. 287–312.

�74� Mustafa Ubeyli, R., Yıldırım, O., and Ogel, B., 2007, “On the Comparison ofthe Ballistic Performance of Steel and Laminated Composite Armors,” Mater.Des., 28, pp. 1257–1262.

�75� Wilkins, M. L., 1978, “Mechanics of Penetration and Perforation,” Int. J. Eng.Sci., 16, pp. 793–807.

�76� Woodward, R. L., 1990, “A Simple One-Dimensional Approach to ModelingCeramic Composite Armor Defeat,” Int. J. Impact Eng., 9, pp. 455–474.

�77� Vermeeren, C. A. J. R., 2003, “An Historic Overview of the Development ofFibre Metal Laminates,” Appl. Compos. Mater., 10, pp. 189–205.

�78� Vogelesang, L. B., Verbruggen, M. L. C. E., and Paalvast, C. G., 1989, “Ar-mour Plate Composite With Ceramic Impact Layer,” U.S. Patent No.4,836,084, Jun. 6.

�79� Vogelesang, L. B., and Roebrocks, G. H. J. J., 1991, “Metal-Resin LaminateReinforced With S2-Glass Fibres,” U.S. Patent No. 5,039,571, Aug. 13.

�80� Schaeffler, P., Rajner, W., Claar, D., Trendelenburg, T., and Nishimura, H.,2005, “Production, Properties, and Applications of Alulight® Closed-Cell Alu-minum Foams,” Proceedings of the Fifth International Workshop on AdvancedManufacturing Technologies, London, Canada, May 16–18, pp. 151–156.

�81� Yu, C.-J., Claar, T. D., Eifert, H. H., Gama, B. A., Gillespie, J. W., Jr., Bogetti,T. A., and Fink, B. K., 2001, “Application of Porous Metal Foams in HybridArmor Systems,” Proceedings of Explomet 2000, K. P. Staudhammer, L. E.Murr, and M. A. Meyers, eds., Elsevier Science, New York, pp. 429–436.

�82� Gama, B. A., Bogetti, T. A., Fink, B. K., Yu, C.-J., Claar, T. D., Eifert, H. H.,and Gillespie, J. W., Jr., 2001, “Aluminum Foam Integral Armor: A NewDimension in Armor Design,” Compos. Struct., 52, pp. 381–395.

�83� Lane, R. A., 2005, “High Performance Fibers for Personnel and Vehicle ArmorSystems: Putting a Stop to Current and Future Threats,” AMPTIAC Quarterly,9�2�, pp. 3–9.

�84� Bonsignore, E., ed., 2006, “Body Armour—Technological Issues,” MilitaryTechnology, 30�4�, pp. 72–79.

�85� 2003, “New Fibers Could Lighten Body Armor,” SSC-Natick Press Release,U.S. Army Soldier Systems Center-Natick, retrieved Feb. 14, 2008, http://www.m5fiber.com/magellan/about_m5.htm.

�86� McConnell, V. P., 2006, “Ballistic Protection Materials: A Moving Target,” J.Reinf. Plast. Compos., 50�11�, pp. 20–25.

�87� Faur-Csukat, G., 2006, “A Study on the Ballistic Performance of Composites,”Macromol. Symp., 239, pp. 217–226.

�88� Sabet, A. R., Beheshty, M. H., and Rahimi, H., 2008, “High Velocity ImpactBehavior of GRP Panels Containing Coarse-Sized Sand Filler,” Polym. Com-pos., 29, pp. 932–938.

�89� Thaumaturgo, C., and Da Costa, A. M., Jr., 1997, “Shock-Waves on PolymerComposites,” J. Mater. Sci. Lett., 16, pp. 1480–1482.

�90� Coppage, E. A., 1997, “Anti-Ballistic Protective Composite Fabric,” U.S.Patent No. 5,660,913, Aug. 26.

�91� Njuguna, J., Pielichowski, K., and Desai, S., 2008, “Nanofiller-ReinforcedPolymer Nanocomposites,” Polym. Adv. Technol., 19, pp. 947–959.

�92� Hsieh, A. J., Song, J. W., Nebo, J., and Singh, A., 2001, “Ballistic ImpactMeasurements of Polycarbonate Layered-Silicate Nanocomposites,” ANTEC2001 Conference Proceedings, Society of Plastic Engineers, CT, pp. 2185–2190.

�93� Song, J. W., and Hsieh, A. J., 2002, “Ballistic Impact Resistance of Mono-lithic, Hybrid and Nanocomposites of PC and PMMA,” Proceedings of theAmerican Society for Composites, 17th Technical Conference, C. T. Sun andH. Kim, eds., Purdue University, West Lafayette, IN, Oct. 21–23.

�94� Sands, J. M., Patel, P. J., Dehmer, P. G., Hsieh, A. J., and Boyce, M. C., 2004,“Protecting the Future Force: Transparent Materials Safeguard the Army’s Vi-sion,” Advanced Materials and Processes Technology Information AnalysisCenter �AMPTIAC� Quarterly, 8�4�, pp. 28–36.

�95� Liff, S. M., Kumar, N., and McKinley, G. H., 2007, “High-Performance Elas-tomeric Nanocomposites Via Solvent-Exchange Processing,” Nature Mater.,6, pp. 76–83.

�96� Grujicic, M., Pandurangan, B., Angstadt, D. C., Koudela, K. L., and Cheese-man, B. A., 2007, “Ballistic-Performance Optimization of a Hybrid Carbon-Nanotube/E-Glass Reinforced Poly-Vinyl-Ester-Epoxy-Matrix Composite Ar-mor,” J. Mater. Sci., 42, pp. 5347–5359.

�97� Baughman, R. H., Zakhidov, A. A., and de Heer, W. A., 2002, “CarbonNanotubes—The Route Toward Applications,” Science, 297, pp. 787–792.

�98� Qian, L., and Hinestroza, J. P., 2004, “Application of Nanotechnology for HighPerformance Textiles,” Journal of Textile and Apparel, Technology and Man-agement, 4, pp. 1–7.

�99� Harris, P. J. F., 2004, “Carbon Nanotube Composites,” Int. Mater. Rev., 49, pp.31–43.

�100� Thostenson, E. T., Li, C., and Chou, T.-W., 2005, “Nanocomposites in Con-text,” Compos. Sci. Technol., 65, pp. 491–516.

�101� Hiroaki, M., Manjusri, M., and Mohanty, A. K., 2005, “Mechanical Proper-ties of Carbon Nanotubes and Their Polymer Nanocomposites,” J. Nanosci.Nanotechnol., 5, pp. 593–1615.

�102� Mylvaganam, K., and Zhang, L.C., 2007 “Ballistic Resistance Capacity of



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Carbon Nanotubes,” Nanotechnology, 18, p. 475701.103� Koziol, K., Vilatela, J., Moisala, A., Motta, M., Cunniff, P., Sennett, M., and

Windle, A., 2007, “High-Performance Carbon Nanotube Fiber,” Science,318, pp. 1892–1895.

104� Zhang, M., Atkinson, K. R., and Baughman, R. H., 2004, “MultifunctionalCarbon Nanotube Yarns by Downsizing an Ancient Technology,” Science,306, pp. 1358–1361.

105� Reneker, D. H., and Chun, I., 1996, “Nanometre Diameter Fibres of Polymer,Produced by Electrospinning,” Nanotechnology, 7, pp. 216–223.

106� Thompson, C. J., Chase, G. G., Yarin, A. L., and Reneker, D. H., 2007,“Effects of Parameters on Nanofiber Diameter Determined From Electrospin-ning Model,” Polymer, 48, pp. 6913–6922.

107� Dalton, A. B., Collins, S., Razal, J., Munoz, E., Ebron, V. H., Kim, B. G.,Coleman, J. N., Ferraris, J. P., and Baughman, R. H., 2004, “ContinuousCarbon Nanotube Composite Fibers: Properties, Potential Applications, andProblems,” J. Mater. Chem., 14, pp. 1–3.

108� Motta, M., Li, Y.-L., Kinloch, I., and Windle, A., 2005, “Mechanical Prop-erties of Continuously Spun Fibers of Carbon Nanotubes,” Nano Lett., 5, pp.1529–1533.

109� Li, Y.-L., Kinloch, I. A., and Windle, A. H., 2004, “Direct Spinning of Car-bon Nanotube Fibers From Chemical Vapor Deposition Synthesis,” Science,304, pp. 276–278.

110� Motta, M., Kinloch, I., Moisala, A., Premnath, V., Pick, M., and Windle, A.,2007, “The Parameter Space for the Direct Spinning of Fibers and Films ofCarbon Nanotubes,” Physica E �Amsterdam�, 37, pp. 40–43.

111� Yu, Z., Ait-Kadi, A., and Brisson, J., 1991, “Nylon/Kevlar Composites I:Mechanical Properties,” Polym. Eng. Sci., 31, pp. 1222–1227.

112� Salehi-Mobarakeh, H., Nakata, S., Ait-Kadi, A., and Brisson, J., 2007, “Kev-lar and Glass Fiber Treatment for Thermoplastic Composites by Step Poly-condensation,” Polym. Compos., 28, pp. 278–286.

113� Haggenmueller, R., Du, F., Fischer, J. E., and Winey, K. I., 2006, “InterfacialIn Situ Polymerization of Single Wall Carbon Nanotube/Nylon 6,6 Nanocom-posites,” Polymer, 47, pp. 2381–2388.

114� Moniruzzaman, M., Chattopadhyay, J., Billups, W. E., and Winey, K. I.,2007, “Tuning the Mechanical Properties of SWNT/Nylon 6,10 CompositesWith Flexible Spacers at the Interface,” Nano Lett., 7, pp. 1178–1185.

115� Mahfuz, H., Adnan, A., Rangari, V. K., Hasan, M. M., Jeelani, S., Wright, W.J. and DeTeresa, S.J., 2006, “Enhancement of Strength and Stiffness of Ny-lon 6 Filaments through Carbon Nanotubes Reinforcement,” Appl. Phys.Lett., 88, pp. 083119.

116� Terrones, M., 2003, “Science and Technology of the Twenty First Century:Synthesis, Properties and Applications of Carbon Nanotubes,” Annu. Rev.Mater. Res., 33, pp. 419–501.

117� Tenne, R., Remškar, M., Enyashin, A., and Seifert, G., 2008, “InorganicNanotubes and Fullerene-Like Structures �IF�,” Top. Appl. Phys., 111, pp.631–671.

118� Tsirlina, T., Feldman, Y., Homyonfer, M., Sloan, J., Hutchison, J. L., andTenne, R., 1998, “Synthesis and Characterization of Inorganic Fullerene-LikeWSe2 Material,” Fullerenes, Nanotubes, Carbon Nanostruct., 6, pp. 157–165.

119� Rapoport, L., Fleischer, N., and Tenne, R., 2005, “Applications of WS2�MoS2� Inorganic Nanotubes and Fullerene-Like Nanoparticles for Solid Lu-brication and for Structural Nanocomposites,” J. Mater. Chem., 15, pp.1782–1788.

120� Zhu, Y.Q., Sekine, T., Li, Y.H., Wang, W.X., Fay, M.W., Edwards, H.,Brown, P.D., Fleischer, N. and Tenne, R., 2005, “WS2 and MoS2InorganicFullerenes-Super Shock Absorbers at Very High Pressures,” Adv. Mater.�Weinheim, Ger.�, 17, pp. 1500–1503.

121� Zhu, Y. Q., Sekine, T., Li, Y. H., Fay, M. W., Zhao, Y. M., Patrick Poa, C. H.,Wang, W. X., Roe, M. J., Brown, P. D., Fleischer, N., and Tenne, R., 2005,“Shock-Absorbing and Failure Mechanisms of WS2 and MoS2 NanoparticlesWith Fullerene-Like Structures Under Shock Wave Pressure,” J. Am. Chem.Soc., 127, pp. 16263–16272.

122� Jang, J.-H., Ullal, C. K., Choi, T., Lemieux, M. C., Tsukruk, V. V., andThomas, E. L., 2006, “3D Polymer Microframes That Exploit Length-Scale-Dependent Mechanical Behavior,” Adv. Mater. �Weinheim, Ger.�, 18, pp.2123–2127.

123� Cunniff, P. M., Fossey, S. A., Auerbach, M. A., Song, J. W., Kaplan, D. L.,Wade Adams, W., Eby, R. K., Mahoney, D., and Vezie, D. L., 1994, “Me-chanical and Thermal Properties of Dragline Silk From the Spider Nephila



Clavipes,” Polym. Adv. Technol., 5, pp. 401–410.�124� Laible, R. C., 1980, “Fibrous Armour,” Ballistic Materials and Penetration

Mechanics, R. C. Laible, ed., Elsevier Applied Science, New York, pp. 73–115.

�125� Massachusetts Institute of Technology, 2007, “Spider Silk Inspires Strongand Stretchy Nanocomposite Fibers,” retrieved Oct. 20, 2008, http://www.sciencedaily.com /releases/2007/01/070119115103.htm.

�126� Bruet, B. J. F., Song, J., Boyce, M. C., and Ortiz, C., 2008, “Materials DesignPrinciples of Ancient Fish Armour,” Nature Mater., 7, pp. 748–756.

�127� Vlassak, J. J., Ciavarella, M., Barber, J. R., and Wang, X., 2003, “The In-dentation Modulus of Elastically Anisotropic Materials for Indenters of Ar-bitrary Shape,” J. Mech. Phys. Solids, 51, pp. 1701–1721.

�128� Mohanty, A. K., Misra, M., and Drzal, L. T., 2002, “Sustainable Bio-Composites From Renewable Resources: Opportunities and Challenges in theGreen Materials World,” J. Polym. Environ., 10, pp. 19–26.

�129� Netravali, A. N., and Chabba, S., 2003, “Composites Get Greener,” Mater.Today, 6, pp. 22–29.

�130� Pervaiz, M., and Sain, M. M., 2003, “Carbon Storage Potential in NaturalFiber Composites,” Resour. Conserv. Recycl., 39, pp. 325–340.

�131� Joshi, S. V., Drzal, L. T., Mohanty, A. K., and Arora, S., 2004, “Are NaturalFiber Composites Environmentally Superior to Glass Fiber Reinforced Com-posites?,” Composites, Part A, 35, pp. 371–376.

�132� John, M. J., and Thomas, S., 2008, “Biofibers and Biocomposites,” Carbo-hydr. Polym., 71, pp. 343–364.

�133� Taj, S., Munawar, M. A., and Khan, S. U., 2007, “Review: Natural Fiber-Reinforced Polymer Composites,” The Proceedings of the Pakistan Academyof Sciences, 44, pp. 129–144.

�134� Garcıa, M., Garmendia, I., and Garcıa, J., 2008, “Influence of Natural FiberType in Eco-Composites,” J. Appl. Polym. Sci., 107, pp. 2994–3004.

�135� Panthapulakkal, S., and Sain, M., 2007, “Studies on the Water AbsorptionProperties of Short Hemp—Glass Fiber Hybrid Polypropylene Composites,”J. Compos. Mater., 41, pp. 1871–1883.

�136� Thwe, M. M., and Liao, K., 2002, “Effects of Environmental Aging on theMechanical Properties of Bamboo-Glass Fiber Reinforced Polymer MatrixHybrid Composites,” Composites, Part A: Appl. Sci. Manuf., 33, pp. 43–52.

�137� Costa, F. H. M. M., and D’Almeida, J. R. M., 1999, “Effect of Water Ab-sorption on the Mechanical Properties of Sisal and Jute Fiber Composites,”Polym.-Plast. Technol. Eng., 38, pp. 1081–1094.

�138� Joseph, K., Thomas, S., and Pavithran, C., 1995, “Effect of Ageing on thePhysical and Mechanical Properties of Sisal-Fiber-Reinforced PolyethyleneComposites,” Compos. Sci. Tech., 53, pp. 99–110.

�139� Mukhopadhyay, S., and Srikanta, R., 2008, “Effect of Ageing of Sisal Fiberson Properties of Sisal-Polypropylene Composites,” Polym. Degrad. Stab.,93, pp. 2048–2051.

�140� Sombatsompop, N., and Chaochanchaikul, K., 2004, “Effect of MoistureContent on Mechanical Properties, Thermal and Structural Stability and Ex-trudate Texture of Poly�Vinyl Chloride�/Wood Sawdust Composites,” Polym.Int., 53, pp. 1210–1218.

�141� Santulli, C., 2001, “Post-Impact Damage Characterisation on Natural FibreReinforced Composites Using Acoustic Emission,” NDT & E Int., 34, pp.531–536.

�142� Ray, A. K., Das, S. K., and Pathak, L. C., 2003, “Synthesis of Silicon Car-bide Mats Using Natural Fibers,” Mater. Lett., 57, pp. 1120–1123.

�143� Ahmad, I., Chin, T.S., Cheong, C.K., Jalar, A., Abdullah, I., 2005, “Study ofFiber Surface Treatment on Reinforcement/Matrix Interaction in TwaronFiber/ENR Composites,” Am. J. App. Sci. �special issue�, pp. 14–20.

�144� Silva, R. V., Spinelli, D., Bose Filho, W. W., Claro Neto, S., Chierice, G. O.,and Tarpani, J. R., 2006, “Fracture Toughness of Natural Fibers/Castor OilPolyurethane Composites,” Compos. Sci. Tech., 66, pp. 1328–1335.

�145� Dhakal, H. N., Zhang, Z. Y., Richardson, M. O. W., and Errajhi, O. A. Z.,2007, “The Low Velocity Impact Response of Non-Woven Hemp Fibre Re-inforced Unsaturated Polyester Composites,” Compos. Struct., 81, pp. 559–567.

�146� Wambua, P., Vangrimde, B., Lomov, S., and Verpoest, I., 2007, “The Re-sponse of Natural Fiber Composites to Ballistic Impact by Fragment Simu-lating Projectiles,” Compos. Struct., 77, pp. 232–240.

�147� Risby, M. S., Wong, S. V., Hamouda, A. M. S., Khairul, A. R., and Elsadig,M., 2008, “Ballistic Performance of Coconut Shell Powder/Twaron FabricAgainst Non-Armour Piercing Projectiles,” Def. Sci. J., 58, pp. 248–263.

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Natarajan V. David earned a Bachelor of Engineering (Honors) degree in Mechanical Engineering (1999)and a Master of Science degree in Mechanical and Materials Engineering (2001) from the National Uni-versity of Malaysia. He has been a lecturer in the Department of Mechanical Engineering, MARA Univer-sity of Technology (UiTM) in Shah Alam, Malaysia since 2002 and has consulted various private establish-ments related to polymer manufacturing, automotive, and oil and gas industries. His research interestsencompass the areas of composites engineering and structural health monitoring. He is presently a Ful-bright Scholar at Texas A&M University, pursuing a doctoral degree in Materials Science and Engineering.

Xin-Lin Gao is currently an associate professor in Mechanical Engineering and Materials Science andEngineering at Texas A&M University. He received an M.Sc. degree in Engineering Mechanics in May 1997and a Ph.D. degree in Mechanical Engineering (with a minor in Mathematics) in May 1998, both from theUniversity of Wisconsin-Madison. He has conducted research in a variety of areas in mechanics andmaterials and is an author/co-author of 64 published/accepted journal papers and 66 conference publica-tions. He has been a reviewer for 54 international journals, 8 publishers, and 9 funding organizations andhas organized 13 symposia at international conferences. He has been a guest editor of three special issuesfor technical journals. His research has been funded by NSF, AFOSR, AFRL, and the Army.

James Q. Zheng is currently the United States Army’s Chief Scientist, Project Manager–Soldier Equipment,Program Executive Office–Soldier. He obtained his Bachelor’s degree in Analytical Chemistry and Master’sDegree in Solid State Physics from the University of Science and Technology of China. He earned his Ph.D.degree in Physical Chemistry from Purdue University in 1991. He holds three patents and published morethan 30 scientific papers. He was the recipient of the Army’s AMC Greatest Invention Award in 2002 fordeveloping the current DOD standard body armor system—The Interceptor Multiple Threat Body Armor. Hereceived the United States Army “Superior Civilian Service Medal” Award in 2008 for “exceptional meri-torious and superior technical achievement.”

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