ballistic impact into fabric and compliant composite laminates

Ballistic impact into fabric and compliant composite laminates

Bryan A. Cheeseman *, Travis A. Bogetti

US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005-5069, USA

Abstract

The development of tough, high-strength, high-modulus fibers has led to the use of fabrics and compliant composite lami-

nates for a number of impact-related applications, such as turbine blade containment, fuselage protection and body armor.

Numerous studies have been conducted to identify material properties and system mechanisms that are important to the per-

formance of these ballistic textiles. The current paper presents a review of the factors that influence ballistic performance; spe-

cifically, the material properties of the yarn, fabric structure, projectile geometry and velocity, far field boundary conditions,

multiple plies and friction. Each physical mechanism is described in detail, and original references are cited to allow further

investigation.

Published by Elsevier Science Ltd.

Keywords: Ballistic impact; Fabric; Armor-grade composite

1. Introduction

From ancient times, mankind has used textiles and

compliant laminates, not only for clothing and protec-

tion against the elements, but for bodily protection.From the use of leather on Grecian shields, layered silk

in ancient Japan, to chain mail and suits of armor in the

Middle Ages, personnel protection has sought to protect

its wearer from the corresponding advances in arma-

ments. However, the advent of firearms relegated these

forms of protection obsolete until the development of

high-strength, high-modulus fibers in the 1960s. These

materials ushered in a new era of body armor that of-fered protection against small arms munitions. The

current state-of-the-art body armor system being fielded

by the US Army is the Interceptor. Consisting of on

outer tactical vest (OTV) that is capable of stopping

high-powered handguns, it can be coupled with hard

ceramic inserts to stop rifle projectiles. However, heavier

inserts are required to protect against armor piercing

rounds, which result in system weights that would affectthe mobility of a soldier in the field. To achieve ad-

vancements in body armor performance levels at a re-

duced weight will not only require further advancements

in materials, but the use of models and simulations to

develop innovative system designs [1].

The current review was performed as part of an in-

vestigation to identify mechanisms that affect the bal-

listic performance of fabrics and compliant compositelaminates. (Unlike traditional structural composites,

compliant laminates, also known as armor-grade com-

posites, only contain �20% weight fraction matrix and

are made to readily delaminate. The authors will utilize

the term �ballistic textiles� to refer to both fabrics andcompliant laminates.) Although a thorough, quanti-

tative understanding of the all the mechanisms that

occur during ballistic impact into fabrics and compliantlaminates does not yet exist, much has been learned

through experimental observations and interpreted

from modeling efforts. (A separate review paper on

the modeling approaches to the impact of fabrics

and compliant laminates is planned.) The impact and

perforation of fabric and compliant laminates are

functions of a number of parameters including the

material properties of the yarns; the fabric structure;the projectile geometry and velocity; the interaction

of multiple plies; the far-field boundary conditions, and

the friction between the yarns themselves and between

the yarns and the projectile. These parameters will be

discussed separately in some detail; however, as a

starting point, a general description of the physical

phenomena of fabric impact deformation will be de-

scribed next.

*Corresponding author.

E-mail address: [email protected] (B.A. Cheeseman).

0263-8223/03/$ - see front matter Published by Elsevier Science Ltd.

doi:10.1016/S0263-8223(03)00029-1

Composite Structures 61 (2003) 161–173

www.elsevier.com/locate/compstruct

mail to: [email protected]

2. Impact into ballistic textiles

2.1. Background

As a starting point for a description of the impact

into fabric, the transverse impact into a single fiber will

be described first. Shown in Fig. 1, when a projectile

strikes a fiber, two waves, longitudinal and transverse,

propagate from the point of impact. The longitudinal

tensile wave travels down the fiber axis at the sound

speed of the material. As the tensile wave propagates

away from the impact point, the material behind thewave front flows toward the impact point, which has

deflected in the direction of motion of the impacting

projectile. This transverse movement of the fiber is the

transverse wave, which is propagated at a velocity lower

than that of the material.

Noting the similarities between the transverse impact

of a single ply of fabric (Fig. 2) with that of a single

fiber, Cunniff [2] notes that when a projectile impacts thefabric, it produces a transverse deflection in the yarns

that are in direct contact with the projectile (defined as

principal yarns) and generates longitudinal strain waves

that propagates at the sound speed of the material down

the axis of the yarns. Additionally, orthogonal yarns,

defined as yarns that intersect the principal yarns, are

then pulled out of the original fabric plane by the

principal yarns. These orthogonal yarns undergo a de-formation and develop a strain wave like those observed

in the principal yarns. Analogously, these orthogonal

yarns then drive yarns with which they intersect. These

yarn–yarn interactions, which are a function of the

friction between them, produce bowing, the misalign-

ment of the orthogonal yarns, toward the impact point.

The transverse deflection proceeds until the strain at the

impact point reaches a breaking strain [2]. Numericalstudies by Roylance [3] have shown that the majority of

the kinetic energy of the projectile is transferred to the

principal yarns as strain and kinetic energy, whereas, the

contribution of the orthogonal yarns to energy absorp-

tion is small. This can be seen in Fig. 2c from [4], which

shows the principal yarns highly stressed, while the or-

thogonal yarns are not. It can also be seen in Fig. 2 that

the orthogonal yarns change the profile of the transverse

wave from the V-shape seen in the single fiber impact

case to more of a parabolic profile.

The influence of the yarn crossovers has been inves-tigated and discussed by a number of researchers [3,5–7].

Freeston and Claus [5] have concluded from their

analysis that longitudinal strain wave transmission and

reflection at the yarn crossover do not considerably

affect the propagation of these strain waves away from

the impact point during a ballistic event. However,

Roylance [3] has shown that the profile of these strain

waves in a fabric differ considerably from those thatdevelop during the impact of a single fiber. A recent

numerical study by Ting et al. [7] has included the effects

of transverse yarn interactions and has found that these

interactions can significantly influence the results from

ballistic response models.

The description of single ply fabric deformation is

given to serve as an illustrative example to point out

some of the fundamental physical mechanisms observedthat influence the ballistic performance of fabrics.

Material properties, fabric structure, projectile geome-

try, impact velocity, multiple ply interaction, far field

boundary conditions and friction all play a role. Al-

though the authors attempt to describe these mecha-

nisms individually below, it should be noted that many

of the individual mechanisms have been reported in

a coupled manner (i.e. multiple ply ballistic panelsimpacted by different geometry projectiles at varying

velocities). As such, it is difficult to isolate each mech-

anism; therefore, what follows is a somewhat detailed

description to help elucidate the topic and give reference

to the original work if further detail is sought.

2.2. Mechanisms influencing ballistic performance

2.2.1. Material properties

The development of high-strength, high-modulus fi-

bers, which are subsequently bundled into yarns, has

allowed the development of current bullet resistant

fabrics and compliant laminates. When impacted, theyarn experiences a sharp increase in stress, the magni-

tude of which is related to the impact velocity. At a

sufficiently low velocity, below what is termed the �crit-ical velocity�, this initial stress increase is insufficient torupture the fibers; thus allowing transverse deflection

and resultant yarn extension time to propagate, result-

ing in the absorption of energy by the fabric [8]. Clearly,

fibers possessing high-tensile strengths and large failurestrains can absorb considerable amounts of energy. In

their study comparing the impact performance of dry

Spectra fabrics and their corresponding armor-grade

laminates, Lee et al. [9] have correlated the number of

yarns broken to the levels of impact energy absorbed,

which the researchers state is a clear indication that

fiber straining is the primary mechanism of the energy

TransverseWave Front

LongitudinalWave Front

Fiber V

Projectile

TransverseWave Front

LongitudinalWave Front

Fiber V

Projectile

Fig. 1. Projectile impact into body armor.

162 B.A. Cheeseman, T.A. Bogetti / Composite Structures 61 (2003) 161–173

absorption in the penetration failure of ballistic textiles.

These results reinforce those reported by Shim et al. [8]

and Cunniff [2] who note that at impact velocities

greater than the V50––the velocity at which 50% of the

projectiles perforate the target––fabrics are perforated

during the initial stress rise. As a result, no time is al-lowed for transverse deflection to propagate, which re-

duces fiber straining; therefore, the energy absorbed

above the V50 is smaller [8].

Using a model developed to analyze the impact of

fabric, Roylance and Wang [10] have shown that ma-

terials possessing a high modulus, E, and low density, q,(thus, a high-wave velocity) disperse the strain wave

rapidly away from the impact point, which distributesthe energy over a wider area and prevents large strains

from developing at the impact point. This can be seen in

the high-speed photographic study conducted by Field

and Sun [11], who examined the transverse wave speeds

of a number of different fibers, Kevlar fabrics and

Spectra laminates impacted with steel balls fired at ve-

locities of up to 1000 m/s. They showed that materials

having high-wave velocities were advantageous since thestresses and strains could propagate more quickly to

neighboring fibers and layers, thus involving more ma-

terial in the ballistic event [10].

Numerous modeling studies have been conducted to

study the influence of material tensile properties on the

ballistic performance. However, most of the studies note

the lack of high-rate properties and are conducted using

the static properties of the material. Limited studieshave been reported on the determination of the high-

rate properties of Kevlar 29 [12], and Kevlar 49 yarns

[12–15], Twaron filaments [16] and fabrics [17] and ar-

amid and polyethylene fiber composites [18]. Recently,

Lim et al. [19] have developed a three-element visco-

elastic representation for the rate dependent modulus of

the aramid fabric Twaron. Using the dynamic finite el-

ement analysis code LS-DYNA, they modeled the fabricas a stain rate dependent isotropic elastic–plastic mate-

rial and analyzed the response of a single ply of Twaron

impacted by a rigid sphere. They noted good qualitative

agreement when their numerical results were compared

with photographs of experimental backface deforma-

tions [19].

Although tensile strength, modulus and strain-

to-failure of a yarn play a large role in ballistic perfor-mance, each property individually does not control it.

Prosser et al. [20] note that if ballistic performance were

based on solely on yarn toughness, nylon would be a

better performer than Kevlar (which it is not). Also,

when the performance of high-strength polypropylene

was compared to that of nylon having two-thirds the

strength, the nylon was a better performer [21]. Re-

cently, Cunniff [22] has derived a dimensionless fiberproperty U � defined as the product of the specific fiber

toughness multiplied by its strain wave velocity.

U � ¼ re2q

ffiffiffiffiEq

sð1Þ

where r is the fiber ultimate tensile strength, e is the fiberultimate tensile strain. Developed as a first-level

screening tool to assess the performance of fibers, he

noted that armor performance is not defined by these

Fig. 2. Sphere impacting single ply of fabric (a) side view, (b) top view of z displacement contours and (c) bottom view showing principal yarns underhigh stress (from [4]).

B.A. Cheeseman, T.A. Bogetti / Composite Structures 61 (2003) 161–173 163

properties, but it is coupled to them. Laible [21] perhaps

sums it up best when he writes ‘‘the relationship between

the mechanical properties of a yarn and the ballistic

resistance of a plied fabric from such yarn has neverbeen established.’’ Clearly, there are other factors that

influence ballistic performance.

2.2.2. Fabric structure

While bullet resistant fabrics would not be possible if

it were not for existence of high-strength, high-modulus

fibers, Roylance et al. [23] note that the response of these

fabrics cannot be determined from the properties of the

fibers alone, but that ‘‘the material properties and the

fabric geometry combine to produce a structural re-sponse.’’ It has been observed that loosely woven fabrics

and fabrics with unbalanced weaves result in inferior

ballistic performance [2]. Weave patterns typically used

for ballistic applications are plain and basket weaves,

although other patterns have been investigated. The

density of the weave, known as the ‘‘cover factor,’’ is

determined from the width and pitch of the warp and

weft yarns and gives an indication of the percentage ofgross area covered by the fabric. Chitrangad [24] notes

that fabrics should possess cover factors from 0.6 to 0.95

to be effective when utilized in ballistic applications.

When cover factors are greater than 0.95, the yarns are

typically degraded by the weaving process and when

cover factors fall below 0.6, the fabric may be too

‘‘loose.’’

Loosely woven fabrics are more susceptible to havinga projectile �wedge through� the yarn mesh. As depictedin Fig. 3, when a projectile strikes a layer of fabric, the

fabric deflects transversely and the mesh of yarns is

distended, resulting in the enlargement of the spaces

between the yarns. Other factors, in addition to a loose

weave, can contribute to this phenomenon. Specifically,

if the projectile is relatively small and/or impacts at an

angle and/or a few yarns ahead of the projectile break,the projectile can slip through the opening or �wedge

through� by pushing yarns aside instead of breaking

them. This �wedge through� phenomena has been ob-served by a number of researchers including Mont-

gomery et al. [25], Kirkland et al. [26], Shim et al. [8],Prosser et al. [20], Lee et al. [9,27] and Lim et al. [19].

Evidence of this phenomenon is that the hole formed in

the perforated fabric is smaller in diameter than the

projectile [8] and that the number of yarns broken is less

than the number of yarns that intersect the projectile [9].

Prosser et al. [20] has observed (and gives a clear picture

of) the yarn spacing enlargement in his work investi-

gating the impact of chisel pointed 0.22 cal fragmentsimulating projectiles (FSPs) into nylon and Spectra

fabric. They define the hole formed by the yarn spacing

enlargement as a �trap door�.Trap door formation is a function of not only the

fabric structure, but also the mobility of the yarns and

the projectile geometry [26]. Yarn mobility can be in-

fluenced by the frictional behavior of the yarns with

themselves and with the projectile and minimized by theactual physical restraint of the lateral motion of the

yarns through the introduction of a matrix (i.e., making

an armor-grade composite). In their work investigating

Spectra fabric and their corresponding armor-grade

composite lamina, Walsh et al. [28] and Lee et al. [9]

have experimentally observed the matrix restricting the

lateral motion of the yarns. This restraint forces the

projectile to engage and break more yarns in the com-posite than the corresponding fabric, resulting in more

energy being absorbed by the composite [9]. It has also

been reported that Spectrashield, which is not a woven

fabric but consists of unidirectional cross-ply laminates

in a polymeric matrix, has also exhibited more resistance

to wedge through phenomena [29].

Another fabric structural property that has been

noted to influence ballistic performance is crimp [24].Crimp is the undulation of the yarns due to their in-

terlacing in the woven structure. In a plain weave, the

degree of crimp is unbalanced––the warp yarns are

typically more crimped than the weft. Chitrangad [24]

has proposed using weft yarns having a larger elonga-

tion to break. He reasoned that because the weft yarns

possess less crimp, they would break before the warp

yarns, because warp yarns need more time to decrimpand then elongate to failure. To mitigate this pheno-

menon, he proposed introducing weft yarns possessing

larger elongations to failure. To demonstrate the effec-

tiveness of the hybrizided weaves, three sets of V50 tests

were conducted using 17 grain FSPs. The first set of

experiments used plain weave fabrics, one comprised

entirely of 1500 denier Kevlar 29 having a tenacity of

23.0 g/denier, an elongation to break of 3.6% and amodulus of 565 g/denier, a second fabric comprised

entirely of 1500 denier Kevlar 119 having a tenacity of

24.4 g/denier, an elongation to break of 4.4% and a

modulus of 448 g/denier and a third fabric where theFig. 3. A depiction of the ‘‘wedge through’’ phenomenon associated

with fabric impact (figure courtesy of Duan [62]).


warp yarns consisted of the 1500 denier Kevlar 29 yarns

and the weft yarns, the 1500 denier Kevlar 119 yarns.

Six-ply fabric packs were tested and the results are

shown in Table 1. It was seen that V50 from the hy-

bridized fabric was 3% and 5.5% greater than the fabrics

comprised entirely of Kevlar 29 or Kevlar 119 yarns,

respectively. Additionally, basket weave fabrics were

made of the Kevlar 29 yarns and the hybridized con-figuration were tested. Panels of 12 plies were impacted

and the results are shown in Table 1. The hybridized

basket weave yielded a V50 7% greater than that made

exclusively from the Kevlar 29 yarns.

2.2.3. Projectile geometry

The geometry of a projectile influences its ability toperforate a fabric. Montgomery et al. [25] investigated

four different 0.22 projectile geometries on the ballistic

performance of one, two and three layers of Kevlar 49.

The researchers found that pointed bullets had the

ability to wedge through fabric and were not decelerated

as quickly as blunt bullets. Similar results have been

reported by Bazhenov [30] who investigated the impact

of multiple layers of Armos fabric (an aramid fabricproduced in Russia) by a pointed 7.62 mm TT bullet and

a more rounded nose 9 mm bullet. Recently Tan et al.

[31] have studied the perforation of a single ply of

Twaron CT 716 plain weave fabric by projectiles having

a flat, hemispherical, ogival and conical head. They

found fabric creasing and perforation mechanisms

highly dependent on the shape of the projectile. The

conical and ogival projectiles perforated the fabric withleast amount of yarn pull-out, indicating these projec-

tiles were able to slip through the weave. These projec-

tiles resulted in the lowest V50s, 58 and 76 m/s,

respectively. The flat head projectile, with a V50 of 100

m/s, sheared the yarns across the thickness, while the

hemispherical projectile produced the most yarn

pull-out and had the highest V50 of 159 m/s. [31].

Montgomery et al. [25] also report that effect of bulletgeometry decreases as the number of plies increases,

which has also been observed by Lim et al. [19], who

investigated one and two plies of Twaron (also an ar-

amid) fabrics struck by four different geometry projec-

tiles and by Prosser et al. [20].

�Wedge through� perforation and tensile yarn failureare not the only mechanisms observed in fabric perfo-

ration. Sharp edged projectiles, or projectiles traveling

at high velocity, can penetrate fabric targets by shearing

yarns across their thickness [2,19,20,27,32]. Prosser et al.

[20] has reported that the cutting action of a projectilepossessing sharp edges is a prime mode of penetration of

fabric layers in their experiments conducted on multiple

layers of nylon or Spectrashield panels. In panels con-

sisting of 20 layers of fabric, the first few layers were

punched out in the shape of the leading surface of the

projectile [20]. Additionally, Lim et al. [19] have noted

that the reinforcement factor for two plies of fabric is

not observed for flat headed projectiles, due to the cut-ting action of their sharp edges.

2.2.4. Impact velocity

Clearly, the impact velocity of a projectile will affect

the performance of fabrics and compliant laminates.However, the mechanisms associated with the different

velocities needs to be quantified. As briefly mentioned

earlier, it has been observed that higher velocities and

sharper projectiles tend to fail fabrics and compliant

laminates by shearing across the yarns, rather than ex-

tending them to failure. (It should be noted that when

yarns are struck at a sufficiently high velocity, they can

rupture instantly at what has come to be known as thecritical velocity. For a detailed description of critical

velocity, the authors refer to Lyons [33].) In their work

impacting Twaron fabric with steel spheres, Shim et al.

[8] has described the differences observed between low-

and high-velocity impact. With low-impact velocities,

the yarns do not fail during the initial stress rise;

therefore, the transverse deflection of the fabric has time

to propagate to the edges of the panel, which allows thefabric to absorb more energy. Panels struck with a low-

velocity projectile are characterized by extensive creas-

ing and stretching, which may contribute to energy

dissipation. With a high-velocity impact, the damage is

localized and the yarns fail before significant transverse

deflection can develop. Similar descriptions have also

been reported by Tan et al. [31].

However, other potentially important mechanismsare observed for high-velocity impact. Recent studies by

Carr [34] on single yarn impact of Kevlar and ultra-

high-molecular weight polyethylene (UHMWPE) has

similarly found that at higher impact velocities, the

yarns fail in shear; moreover, with UHMWPE yarns,

melt damage of the filaments was also noted. Shear

failure and small amounts fiber melt damage has also

been observed in studies conducted with poly(para-phenylene benzobizoxazole) (PBO) fabric impacted by

chisel point FSPs [35] and in Dyneema panels impacted

by 5.56 mm bullets [36]. Such heat degradation of fibers

has been observed since the 1950s, when it was reported

that filaments were damaged by softening, melting, de-

composition, burning and fibrillation during ballistic

impact experiments of nylon panels [37]. Similar melt

Table 1

Ballistic performance of fabric (from [24])

V50 ballistic performance (ft/s)

Plain weave

six plies

2� 2 Basket weave12 plies

Kevlar 29 1266 1645

Kevlar 119 1235 N/A

Warp K-29/Weft K-119 1304 1761


damage such as fiber fusion, bridging and contraction

has also been observed in impacted panels of

UHMWPE [20,36,38] along with crystallinity changes in

the UHMWPE filaments [20,39]. Using an infraredcamera focused on the rear surface of nylon ballistic

panels, temperatures as high as 76.6 �C have been re-

corded just after the panels were perforated by 0.22 cal

right circular cylinder projectiles [20]. However, using

finite element and finite difference analysis, Prevorsek

et al. [38] determined that the heat generation and cor-

responding temperature rise was due to friction and

concluded that due, in part, to the brief time frame ofthe impact event, that the temperature rise has a minimal

influence on the ballistic performance of UHMWPE.

This conclusion is supported by work on UHMWPE

panels that were shot in heated ovens with only a 5%

decrease in ballistic performance when the panel was

heated to 110 �C [26,40].In their investigation of bumper shields required for

the protection of space vehicles from hypervelocity im-pact, Hiermaier et al. [41] have investigated the behavior

of Nextel and Kevlar fabric and Kevlar-epoxy com-

posite under a range of strain rates. Using inverse flyer

plate experiments, the researchers observed that when

Kevlar-epoxy composite plates were impacted at 788 m/

s, all the epoxy vaporized. Evidence of epoxy vapor-

ization has been reported at plate impact velocities of

388 m/s [42]. At an impact velocity of 1015 m/s, theKevlar-epoxy plate underwent a phase change, ther-

mally decomposed and turned into a mass of fine Kevlar

particles.

2.2.5. Multiple plies

A great number of experiments have been performed

impacting multiple plies of ballistic textiles. The major-

ity of the information acquired is presented in the formof the residual velocity (VR) of a projectile as a functionof its of striking velocity (VS) (see, for example, [43–45]),which gives a measure of the ballistic performance of the

material against a particular threat. Cunniff [2] and Lim

et al. [19] have investigated the ballistic impact of multi-

ply systems to characterize the reinforcement effect of

multiple layers. These investigators compared the per-

formance of two ply spaced armor systems, where thetotal energy absorbed is the summation of the energy

absorbed by each ply, with two ply layered systems.

Cunniff [2] impacted panels of Kevlar, Spectra

(UHMWPE) and nylon with chisel-pointed FSPs, and

found that theoretically, the energy absorbed by spaced

single plies was greater than that absorbed by layered

systems. Lim et al. [19] impacted panels of Twaron with

various projectile geometries and found that the ab-sorbed energy for the layered systems was greater than

that of the spaced systems for certain impact velocities

for certain projectile shapes. Clearly, the issue of layered

or spaced multiple ply systems needs further investiga-

tion.

Detailed descriptions of the ballistic impact into

multiple ply compliant composite laminates have been

given in [9,27,46,47]. Damage mechanisms are depen-dent on the projectile geometry and velocity, the prop-

erties of the matrix and fibers and the fiber-matrix

adhesion. (It should be noted that for ballistic applica-

tions, weak fiber-matrix adhesion is wanted. This results

in the ready delamination of the compliant lami-

nate, which allows the fibers to extend to failure. How-

ever, depending on the application, a certain degree

of structural stiffness may be warranted, thus increasedfiber-matrix adhesion may be used.)

When an armor-grade composite is impacted, if the

projectile possess sharp edges and/or the composite

properties are somewhat brittle and/or there is an in-

creased level of fiber/matrix adhesion, the first few plies

may be sheared out, forming a plug. Depicted in Fig.

4(a), Lee et al. [27] describe this type of failure observed

when Spectra fiber reinforced composites were impactedby 0.22 cal FSPs. After the plug is formed, sequential

delamination was noted, along with fiber pull-out and

fiber tensile failure in the back layers of the laminate.

Lateral movement of fibers was observed for unidirec-

(b)

Formation of transverse shear cracks

Delamination crackscoalesce to form a‘wedge’ (shear plug)

Plug displaces,results in increaseddelamination, backsurface bulging

Tensile failure

Formation of transverse shear cracks

Delamination crackscoalesce to form a‘wedge’ (shear plug)

Plug displaces,results in increaseddelamination, backsurface bulging

Tensile failure

(a)

Fig. 4. Penetration into compliant laminates (a) with shear plug formation and (b) with compaction and spring-back (after Scott [47]).


tional cross-ply laminates, but not seen in fabric lami-

nates [27].

Iremonger and Went [46] detail the deformation and

failure that is observed when a chisel-point FSP impactsa nylon panel. They postulate that when the projectile

initially impacts that panel, it produces a compressive

wave that propagates through the thickness of the

laminate and reflects off the rear surface as a tensile

wave, which may produce delamination. Iremonger and

Went [46] have observed that the sharp edges of the FSP

cause intense shear, which cut through the fibers, while

the oblique faces stretch the fibers, which eventually failin tension [46].

Depicted in Fig. 4(b), Scott [47] remarks that with

compliant laminates, there is considerable evidence of

fiber stretching, even in the first few layers. Fibers are

driven into the underlying layers before they fail, re-

bound and form a reverse pyramid on the impact sur-

face. Beneath the projectile, the material is compressed

and the remaining layers form a membrane, which ab-sorbs the remaining energy through fiber elongation and

fiber pull out [46].

When multiple ply armor systems are impacted by

sharp-edged projectiles, the first few layers are punched

out in the shape of the impacting surface [20]. The re-

maining layers behave as a membrane. In other words,

the layers close to the impact surface behave inelasti-

cally, whereas the layers toward the back behave elas-tically. Cunniff [43] has studied this decoupled response

through the thickness of multi-ply systems. He notes

that the material of the first few plies behaves as though

it were unbacked by the remaining plies of material.

When struck at sufficiently high velocities, the armor

system response is dominated by the inelastic behavior

of the material. Cunniff [43] has investigated hybridiza-

tion of armor systems by replacing the material at thestrike face with a different (less expensive) material.

Experimental results have shown that the ballistic per-

formance of the armor system can be maintained by

replacing the original material with a surrogate that has

similar inelastic properties and that the surrogate ma-

terial could be chosen to improve the other properties,

such as stiffness [43].

Hybridization of the armor through its thickness hasbeen done for a number of years and can be seen by the

number of patents and commercially available soft ar-

mor systems consisting of multiple materials. However,

few reports exist in the literature detailing the effect of

the hybridization on ballistic performance. Recently,

Larsson and Svenson [48] conducted a comprehensive

investigation of hybridized compliant armor systems for

improved ballistic performance using various combina-tions of carbon, Dyneema and PBO. Using a 5.46 mm

FSP, the performance of panels composed entirely of

either Dyneema or PBO was obtained and then com-

pared against the performance of panels where different

percentages of Dyneema or PBO fibers were replaced

with corresponding amounts of carbon fibers. It was

found that by using approximately 50% carbon fibers on

the impact face of the panels (i.e. replacing 50% of theDyneema or PBO), the ballistic limits were essentially

the same as the corresponding panels containing 100%

Dyneema or PBO. Ballistic limits were improved when

25% carbon fiber was used on the impact surface. Such

hybridized systems have also been investigated by

Thomas [49]. Thomas found that using a non-woven

facing on a woven fabric provided enhanced protection

against handgun threats rather than just Spectrashieldalone. Further improvement was found by using a

Spectrashield facing on a non-woven, backed by fabric.

The use of layers of woven and nonwoven aramid tex-

tiles has also been studied by Chitrangad [50].

2.2.6. Far-field boundary conditions

When testing fabric or compliant armor systems for

ballistic impact, the size of the specimen and the means

of fixturing it during the impact event are important.

Cunniff [2] tested single plies of Kevlar and Spectra

clamped between aluminum plates having 1-, 2-, 4-, and

8-in. apertures and observed that the ballistic limit ofthe fabric was strongly dependent on the aperture

size. Smaller apertures decreased the ballistic limit. It

was surmised that the smaller openings constrained

the amount of transverse deflection (therefore, the

amount of tensile elongation) and longitudinal deflec-

tion. However, the effectiveness of longitudinal con-

straint was questionable as it was observed that the

fabric slipped between the clamped plates for all veloc-ities tested. Above the ballistic limit, the size effect was

negligible, as the fabric is perforated without significant

transverse deflection for all cases [2]. However, using

two different aperture sizes, 50 and 200 mm, Lee et al.

[27] reported minor differences in the ballistic limit in

their experiments with Spectra fiber reinforced com-

posite laminates impacted by 0.22 caliber FSPs. Re-

cently, Lee et al. [9] have investigated the failuremechanisms for compliant laminates over a range of

testing rates using quasi-static punch, dynamic drop

tower and ballistic experiments. Using fixtures typical

for drop tower testing of rigid composite panels was

found to be inadequate because fabric and compliant

laminates slipped from the clamping apparatus due to

inadequate clamping pressure [9,28]. The absorbed im-

pact energy was found to be a function of the clampingpressure, therefore, specialized clamping plates were

employed and clamping pressures increased until the

absorbed impact energy was found to be independent of

the clamping pressure. It is interesting to note that when

insufficient clamping pressure was used and the speci-

men slipped from the clamps, the energy absorbed was

4.5 times greater than the no-slip cases [9].


Chitrangad [51] has observed that improved ballistic

performance can be realized if the aramid filaments can

be maintained under tension. Ballistic tests using 17

grain FSPs were performed on fabrics that were eithertensioned or not tensioned. Improvements over in the

V50 performance of 7% and 8% were noted for two and

three plies of Kevlar 29 yarns in a basket weave when

the warp and weft tows were tensioned to an extent of

0.018 and 0.012 g/dtex, respectively. Utilizing five plies

of a hybrid plain weave with the Kevlar 129 yarns in the

warp direction and Kevlar 29 in the weft, both pre-

tensioned to 0.018 g/dtex, improved the V50 23% overthe no tension sample.

Investigating the application of PBO fabric to protect

airplane fuselages against turbine engine fragments,

Shockey et al. [52] have performed a number of quasi-

static and impact experiments to study the effect of the

boundary conditions on absorbed energy. Using 25 g

blunt and 26 g sharp fragment simulators, fabrics were

impacted at velocities between 52 and 113 m/s. It wasshown that when the targets are gripped on two edges

rather than four edges, more energy was absorbed

for both impactors. These results spurred an extensive

quasi-static penetration investigation of PBO, Kevlar

and Spectra fabric gripped on all four edges or gripped

on two edges and having the other two edges free. A

typical load-stroke (penetrator displacement) curve for a

quasi-static punch test into a single ply of PBO is shownin Fig. 5. Different loading rates and penetrator geom-

etries were studied, the failure mechanisms recorded and

it was seen that when gripped on four edges, the yarns

typically failed locally at the sharpest edge of the pene-

trator (Shockey et al. [52] remark that this type of failure

is also characteristic of that seen in high-velocity im-

pact). This local yarn failure corresponds to the peaks

on the curves shown in Fig. 5. However, the post-peak

behavior of the fabric clamped on four edges is quitedifferent than that clamped on two. For the four edge

case, the penetrator perforates the fabric and the load

drops abruptly to zero. In the case of the fabric clamped

on two edges, a number of different mechanisms were

observed. In addition to the local yarn failure, yarns

away from the impact site were observed to break.

Through frictional interaction with other yarns, re-

motely failed yarns still exerted a significant load on thepenetrator. Yarn pull-out, where the yarns do not break

but are pulled out of the fabric, was observed and

contributed to the energy absorption. This can be seen

as the post peak region in Fig. 5 [52].

Although the results of Shockey et al. [52] are for

quasi-static penetration of fabrics, qualitatively similar

curves have been experimentally obtained for ballistic

impact by Starratt et al. [53,54]. Square panels of eightand 16 plies of Kevlar 129 fabric were mounted in a

fixture that clamped the top and bottom while the sides

remained free. These panels were impacted with 5.38

mm diameter blunt projectiles at velocities ranging from

267 to 459 m/s. High-speed photography was used to

capture the deformation of the fabric, while a novel

technique, the enhanced laser velocity system (ELVS),

was used to continuously measure the projectile dis-placement before and during impact. From these

measurements, the velocity-time, force-projectile dis-

placement history and energy absorbed by the target

during the impact event was calculated. The force on the

projectile as a function of projectile displacement for the

Fig. 5. Load as a function of penetrator displacement for quasi-static penetration of a single ply of PBO fabric gripped on two or four edges (figure

from Shockey et al. [52], used with permission).


impact into eight layers of Kevlar is shown in Fig. 6.

For the case where projectile impacts at 428 m/s, the

projectile perforates the layers of fabric. Here, the

loading increases to a peak of 12 kN, remains constant,

then drops rapidly as the projectile makes a hole in the

fabric layers. For the 267 m/s case, the projectile is

stopped by the fabric. As shown in Fig. 5, the load in-

creases to �12 kN, then gradually decreases. The in-vestigators report that this target had minimal broken

yarns and that the decrease in the force may be attrib-

uted, in part, to yarn pull-out [53].

When observing the curves in Figs. 5 and 6, one can

see similarities. Recall that Shockey et al. [52] noted the

failure of the fabric when gripped on four edges is

similar to that seen in high-velocity impact. This ob-

servation is supported by similarities in the force–dis-placement curves shown for the quasi-static push test

case where the fabric is gripped on four edges shown in

Fig. 5 and the ballistic perforation case shown in Fig. 6.

Moreover, the force–displacement curve for the case

where the projectile was arrested in the fabric is very

similar to the force–displacement curve of the fabric

push tests where the fabric was gripped on two edges.

Recalling that the ballistic experiments were done withthe fabric gripped on two edges, these similarities may

be the result of the yarn pull out mechanism, which both

Starrat et al. [53] and Shockey et al. [52] partially at-

tribute to the post-peak behavior of these cases. As the

yarn pull-out is responsible for some of the energy ab-

sorbed during the impact event, the frictional interaction

between yarns directly plays a role in the absorption of

energy during an impact event. The importance of yarn–yarn friction has also been noted in a numerical study of

fabric impact conducted by Parga-Landa and Hernan-

dez-Olivares [12]. The next section will review some of

the work concerning the role of friction, both directly

and indirectly, on impact performance.

2.2.7. Friction

Friction has been shown to play a role, both directly

and indirectly, on the impact performance of textiles. In

the previous section, it was seen that yarn pull-out maybe directly responsible for absorbing energy during a

non-perforating impact event. However, friction be-

tween the projectile and the yarns and the yarns them-

selves may also be responsible for how much energy is

absorbed during an impact event. The work of Lee et al.

[9] has shown that by restricting the ability of the yarn to

move laterally out of the path of the projectile during

impact (by using small amounts of resin) increases theamount of energy the fabric can absorb. Or, more gen-

erally, increasing the friction between the projectile and

the fabric and the yarns themselves will hinder the mo-

bility of the yarn and require the projectile to engage

and break more yarns, which would result in greater

energy absorption. The result of this type of interaction

can be considered an indirect mechanism of increased

energy absorption due to friction. The importance offriction for ballistic impact has been studied by a num-

ber of researchers. Some of this work is summarized

next.

Briscoe and Motamedi [55] published a quantitative

study examining the role of yarn friction on the ballistic

performance of Kevlar 29 and Kevlar 49. The investi-

gators studies a plain and a satin weave of Kevlar 29 and

a ‘‘crows foot’’ weave of Kevlar 49 under three states ofyarn–yarn lubrication: ‘‘as-received,’’ Soxhlet extracted

(scoured) and coated with a 5% solution of poly-

dimethysiloxane (PDMS). The ‘‘as-received’’ fabrics

were left with their proprietary lubrication aids intact.

The Soxhlet extracted, or ‘‘scoured,’’ fabrics were

soaked in acetone for two days to clean the yarns of the

as received processing aids and the PDMS fabrics were

first scoured and then immersed in a 5% solution ofpetroleum ether and dried. Yarn–yarn friction was

measured using the hanging fiber friction experiment

configuration, and the frictional coefficient was shown

to be a function of the applied normal load. The high-

load limit coefficients of friction for Kevlar 49 yarns are

shown in Table 2 [55].

From Table 2, it can be observed that the PDMS

treated fabric has more lubrication and the scouredfabric has less lubrication than the ‘‘as-received’’ fabric.

A previous investigation by the researchers on the

Fig. 6. Force–displacement results for impact into eight plies of Kevlar

129 fabric obtained from the ELVS (figure from Starratt et al. [54],

used with permission).

Table 2

High-load limit coefficients of friction and apparent yarn modulus for

Kevlar 49 (from [55,56])

Fabric treatment l E (N)

As-received 0.22� 0.03 2132

Soxhlet extracted

(scoured)

0.25� 0.03 2773

PDMS treated 0.18� 0.03 1964


Kevlar 49 ‘‘crow�s foot’’ weave fabric examined the

amount of force required to pull out the yarns from

these fabrics and it was shown that the scoured yarns

required the most force, while the PDMS treated yarnsrequired the least [56]. Additionally, yarns were ex-

tracted from the Kevlar 49 fabric and tested in tension.

The apparent modulus, E (force/strain), from these tests

is given in Table 2. The increased modulus of the

scoured yarn and the decreased modulus of the PDMS

treated yarn is attributed to changes in the interfilament

friction in the yarn itself caused by the surface treat-

ments [56].Ballistic impact experiments were conducted along

with high-speed photography on single layers of fabric

to examine the effect of friction on the ballistic perfor-

mance. Steel ball bearings, 6.35 mm diameter, were fired

from a gas gun into 100 mm circular fabric specimens

that were pretensioned and clamped between two steel

rings. The researchers noted the transverse wave veloc-

ities (TWV) were different, with the scoured fabrichaving a much larger TWV. Also, the impacted fabrics

exhibited observable differences, with the scoured fabric

having more significant disruptions of the fabric struc-

ture and the ‘‘as-received’’ fabric having more fiber pull-

out. Generally, for each weave, the velocity required to

perforate increased with decreasing levels of lubrication

whereas the residual velocity increased with increasing

levels of lubrication. Alternatively, these results could berestated as more energy was absorbed in the fabric with

higher levels of friction. The researchers conclude that

even modest changes in yarn–yarn and inter-filament

friction can produce changes in the ballistic performance

of a fabric. The authors of this paper note that this may

be true, though the frictional interaction between the

steel sphere and the fabric was not considered and the

frictional properties under high pressures and velocitiesare not known.

The use of finishes on aramid yarns has been inves-

tigated since finishes are required to weave them into

fabric without damaging and degrading the constituent

yarns. Chitrangad and Rodriquez-Parada [57] have

noted that low coefficients of friction are needed between

the aramid yarn and metals and/or ceramics used in

processing, but high-fiber–fiber coefficients of frictionare thought to improve ballistic properties. Before the

disclosure of a novel finish developed by the researchers,

they noted that to achieve these conflicting frictional

goals, a finish would first be applied for processing, re-

moved and then a second finish applied to improve the

ballistic performance. Chitrangad and Rodriguez-

Parada [57] developed a fluorinated finish for aramids

that increased the fiber–fiber friction as compared tothat of the standard finish used in processing aramids

while keeping the frictional coefficient between the yarns

and metal approximately the same. Additional research

on aramid finishes can be found in Rebouillat [58] who

noted that a processing finish was still required before

coating the aramid fibers with a fluoro-containing finish,

which served as a water repellent agent. (If not treated

with water repellant, the performance of Kevlar, whenwet, is known to decrease). Rebouillat [58] reports the

development of a ketene dimer surface treatment for the

aramid fibers that can either be applied ‘‘in-line’’ during

the spinning of the fibers or ‘‘off-line’’ when the fibers

are on bobbins, when the fibers are ‘‘never-dried’’ (still

swollen with water) or dried. The ketene dimer serves as

a hydrophobic coating that changes the frictional

properties of the yarns and modifies the fiber-resin ad-hesion in aramid composites. Frictional measurements

between yarns and the yarns and metal were conducted

with a Rothschild friction meter for yarns coated when

‘‘never dried,’’ dried and a comparison yarn not coated

with the ketene dimer. The results are given in Table 3.

They show that when the ketene dimer is applied to the

fibers in a ‘‘never dried’’ state, they decrease the fric-

tional coefficients. Additional tests show that fabricswoven with the ‘‘never-dried’’ fibers were 100% hydro-

phobic (the dried yarns could not be woven). Ballistic

tests were done using fabrics produced with the ketene

dimer surface treated yarns and ‘‘comparison’’ yarns.

Using 12 layers of fabric, V50 tests were performed with

a FSP. These test showed the ballistic performance of

the fabrics to be the same, each having an average V50

of 452 m/s. Additional tests were conducted using 22layers of each fabric struck with a 124 grain 9 mm full

metal jacket projectile. The V50 performance for the

ketene dimer treated fabric was improved by 8% when

dry and 10% when wet when compared to the perfor-

mance of the ‘‘comparison’’ fabric. Composite plates

made from 24 layers of ketene dimer treated fabric and

18% phenol resin exhibited 20% higher ballistic resis-

tance against the 17 grain FSP than panels made fromthe ‘‘comparison.’’ This may be a result of the fiber-resin

interface in the composite. Recalling that delamination

is preferred in armor-grade composites, short beam

shear tests showed that the ketene dimer treatment re-

duced the adhesion of the matrix resin to the yarns. The

measured short beam shear strength was 33 MPa,

whereas the composite made from the ‘‘comparison’’

fabric was measured to be 54 MPa [58].Additional studies using a pin-on-disc tribometer in

either an alternating sliding mode or a continuous slid-

Table 3

Frictional coefficients determined using a Rothschild friction meter

(from [58])

Coefficients of friction

Never dried

fibers

Dried fibers Comparison

Fiber–fiber 0.09 0.18 0.15

Fiber-to-metal 0.20 0.40 0.30


ing mode were reported by Rebouillat [59]. Here, he

determined the frictional properties of Kevlar yarns and

fabrics sliding on a polyvinyl chloride (PVC) disc as a

function of their surface treatment. (Rebouillat notesthat Lavielle [60] has reported that the friction coeffi-

cients measured using a PVC disc face are good ap-

proximations of those done with steel.) The friction

properties were determined for Kevlar 29 yarns of 930

dtex and 3300 dtex, and plain weave fabrics made from

them. For fabrics woven from the 3300 dtex yarns and

930 dtex yarns, the yarn count was 7� 7 yarns and12� 12 yarn per cm, respectively. However, the studywas tribological and not concerned with the ballistic

performance.

Recently, Dischler [61] has developed a coating �2lm thick that, when applied to aramid fibers, increases

the yarn–yarn frictional coefficients. He reports when

fiber-to-fiber bonding is minimized, the coated fibers

result in substantial performance increases when tested

against flechettes.Having observed the degradation of yarns, the high

temperatures generated due to friction and the plastic

deformation of the projectile caused, in part, by friction

at the projectile-fabric interface, the effect of friction

between adjacent Kevlar fabric layers and between the

fabric and various metals has been studied by Martinez

et al. [32]. Following ASTM standard D4917, the in-

vestigators determined the static friction coefficientsbetween two plies of same kind of fabric. Three different

fabrics were investigated: a 1100 dtex Kevlar HT with

8.5 yarn/cm, a 1100 dtex Kevlar 29 with 12.2 yarn/cm

and a 1270 dtex Kevlar 49 with 6.7 yarn/cm. Results are

given in Table 4 and show that the Kevlar 29 fabric,

which had the tightest weave, had the highest static

frictional coefficient. Yarn pull-out tests were also per-

formed and the frictional force per yarn crossover andfrictional force per unit length was also computed. The

Kevlar 29, which had the tightest weave, also took the

most force to extract the yarn. Dynamic wear tests for

mutual fabric plies between the fabric and steel, brass,

aluminum and lead were performed to determine the

dynamic frictional coefficients as a function of pressure.

Noting that the friction and wear properties were

characterized in the low-pressure regime, 1 kPa forfabric-to-fabric contact and 1 Mpa for aramid-metal

friction, the researcher noted that higher pressure data

are required [32].

Bazhenov [30] has investigated the mechanisms that

occur when multiple layers of an aramid fabric (Armos)

on a plasticine foundation are impacted by 9-mm bullets

(although not explicitly stated in the paper, it appearsthat the fabrics were not fixtured). In addition, the effect

of the moisture on the impact behavior of the multiple

fabric layers was studied. By studying the number of

yarns pulled out in each layer during the impact and the

width of the pull-out zones, Bazhenov concludes that

the pull-out zone gives some measure of the energy

transferred to a fabric layer and that friction between

the yarns leads to a larger pull-out zone and therefore,improved energy dissipation. When impacting 20 layers

of Armos fabric in a dry or wet state with a 9-mm bullet,

Bazhenov [30] observed that the width of the pull-out

zones were quite different. In the case of the dry fabric,

the pull-out zones were greater than the diameter of the

bullet. In the first layer, the width of the pull-out zone

was �14 mm, and this increased until the 15th layerwhere the pull-out zone width was �19 mm. In the lastfew layers, the pull-out zone width decreased, which he

attributed to the bullet stopping. For the case of the wet

fabric, the bullet perforates the system. Post-failure in-

spection revealed that the width of the pull-out zones

were much smaller than those observed from corre-

sponding layers in the dry system. In addition, in the

first fabric layer, impacted wet yarns were not broken,

but perforation was caused by the sliding of the bulletbetween slightly pulled out yarns. Bazhenov [30] sur-

mises that the water served to lubricate the interface of

the spherical bullet nose and the yarn (i.e., friction was

reduced).

3. Concluding remarks

The current paper has reviewed a number of mech-

anisms that influence the ballistic performance of bal-

listic textiles. Although it is clear a priori that some, such

as the material properties, projectile geometry, impact

velocity and multiple plies, have a profound influence onperformance, others, such as fabric structure, far field

boundary conditions and friction are not as apparent.

Indeed, although much has been learned, much more

could be investigated.

It appears that studies are ongoing to optimize the

performance of ballistic textiles through hybridization

of fabric and compliant composite laminate systems by

the layering of different materials and even differenttextiles, such as felts. However, it seems that most of

these research results are proprietary because they do

not appear in the open literature. Moreover, the effect of

fiber sizings and coatings and their influence on fric-

tional properties has generated a number of research

efforts. Though a quantitative understanding of how

these sizings affect the ballistic performance has not yet

Table 4

Ply–ply frictional coefficients and forces required during yarn pull-out

experiments (from [32])

Kevlar HT Kevlar 29 Kevlar 49

lstatic 0.47 0.51 0.41

Force per yarn crossover (N) 0.043 0.103 0.044

Force per cm of fabric (N) 0.365 1.257 0.295


been gained, this may be an area future research (It is

interesting to note, that the ketene dimer surface treat-

ment, which decreased the yarn–yarn and yarn-metal

friction as evaluated alongside the ‘‘comparison’’ sizing,performed as well or better ballistically. A possible ex-

planation is that the frictional coefficients were deter-

mined for low pressures and slow sliding velocities).

Indeed, the work of Briscoe and Mohamedi [55,56]

seems to indicate that surface treatments not only affect

the inter-filament friction, as possibly indicated by the

differences in the apparent yarn modulus in Table 2, but

also the yarn pull-out results [56] and ballistic results[55].

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