Conference Session B12
Paper 2158
KEVLAR: ARMOR FOR OUR MODERN KNIGHTS
Rebecca Arnold (rma37@pitt.edu), I-Hui Lin (ihl4@pitt.edu)
Abstract—The creation of Kevlar○R , a poly-aramid fiber, has
many different applications, but its use in body armor might
possibly be its most crucial and life-saving function.
Kevlar ○R protection vests have saved more than 3,000
documented law enforcement officers alone, and are
continuing to save more and more every day [1]. This paper
will describe and explain the uses of Kevlar ○R in body armor
for members of law enforcement and military. The physical
and chemical properties of Kevlar® will be explained in
detail to clarify why Kevlar® is used as a component body
armor.. The physical and chemical properties of Kevlar ○R
will be discussed; these properties include: thermal
properties, tensile strength, moisture resistance, chemical
structure, and intermolecular strength. Together, both the
physical and the chemical properties will be applied to
Kevlar ○R fibers’ effectiveness as the main element in body
armor. The many components of body armor will be
mentioned and related to Kevlar ○R body armor’s strength,
flexibility, efficiency, and lightweight characteristic, and will
be broken down, analyzed, and examined through the results
of ballistic experimentation. With all of the mechanical
properties of Kevlar ○R explained and described, it can be
observed that body armor infused with Kevlar ○R fibers isn’t
only effective and efficient, but of high standards compared
to body armor composed of other compounds.
into armor different calibers to protect its wearers from
anything form a pistol shot to shrapnel from a land mine.
What makes this fiber so strong, stable, and veritile in body
armor is explored throughout ballistic experimentation,
which leads us to our next section.
BALLISTICS
The purpose for body armor is to protect its wearer from
bullets or puncturing weapons. Body armor achieves this
task by deformation of the Kevlar® fibers. As a small,
puncturing object, such as a bullet, collides with the armor,
the fibers, which are now under great strain, deform and
create tension waves. These tension waves are caused by the
fibers absorbing the objects kinetic energy; thus the armor
decelerates the object to a stop before it completely
punctures the armor, restricting any harm that could have
befallen the wearer [4]. The higher the velocity of the
tension wave, the more effective the body armor will be,
therefore the maximum wave propagation velocity is very
important in manufacturing the perfect body armor.
It can be determined that the most effective body armor
will have a fiber that has a high energy capacity to absorb
the kinetic energy of the puncturing object, but possibly the
most important physical characteristic the fiber should
possess is a high work of fracture [4]. Work of fracture is
work performed when the fibers break, in turn decreasing
the puncturing object’s velocity. This characteristic is
present in fibers that have a strong radial structure, which is
one of Kevlar®’s most sought after traits. This radial
structure will be covered more in the upcoming section.
The general propagation velocity can be found
using a simple formula relating elasticity modulus and the
thread density (1).
Key Words—Kevlar ○R , Aramid Fiber, Body Armor,
Protection Vests, Polymers
KEVLAR: DEFINED
Stephanie Kwolek, a chemist for the company DuPont,
successfully created many synthetic fibers, but in 1965 she
created possibly the most important fiber of them all: polypara-phenylene terephthalamide, better known as Kevlar ®
[2]. Kevlar® is a poly-aramid fiber that is five times stronger
than that of steel on a pound per pound basis [3]. This fiber
isn’t just extraordinary because of it’s strength, however.
Kevlar® has spectacular thermal properties as well; it
displays minimal shrinkage in boiling water, and in air up to
177˚C, and the fiber doesn’t start decomposing until it
reaches temperatures above 400 ˚C.
These physical and thermal properties work together to
make Kevlar® fiber perfect for over 200 applications [2], one
of the most valuable being body armor. It’s intense physical
and chemical strength, along with its superior thermal
properties make it a great life saving material for wearers all
over the world in different climates, ranging from intense
arctic conditions, to high temperature desert conditions, no
matter what the threat may be. Kevlar® fiber can be made
𝐸
𝑐=√
𝜌
(1) [5]
c- propagation velocity of tensions in a single
thread
E- elasticity modulus
ρ- thread density
A conclusion can be drawn from this equation: the higher
the elasticity modulus and the lower the density of the fiber,
the higher the propagation velocity will be. Since Kevlar®
has a low density and 0.052 pounds per cubic inch and a
high elasticity modulus at 10.2 pounds per square inch
compared to several other fibers such as Nylon® and
polyester, it is a notable choice for body armor [6]. These
numbers can be seen and compared in Table I.
University of Pittsburgh
Swanson School of Engineering
February 08, 2012
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I-Hui Lin
However, Kevlar® doesn’t just have one constant
elongation modulus. The modulus depends on several
factors, but most importantly the geometric structure of the
fabric. The structure of the fabric is dependent on the weft
and weave of the fibers. The density of wefts and weaves of
the fabric affects the propagation wave noticeably. The
more wefts and weaves per unit area the fabric has, the
lower the elongation modulus will be, and thus, the lower
the propagation velocity for that fabric will be. This fact is
proved by the Roylance Dependence. The Roylance
Dependence suggests that the fiber in itself has an elongation
modulus that is √2 times greater than that of the fabric
when it is weaved together. So, the more stable the Kevlar®
fiber is made, the less chance it has protecting its wearer
against a bullet.
To maximize the efficiency on the armor then, it
This type of production, however, had a downfall; it is
much more expensive than traditional weaved Kevlar® fiber.
Therefore, some armor is better suited for different jobs. For
instance, a military member who is exposed to shrapnel from
hand grenades and land mines will benefit more from the
more expensive and more efficient form of body armor of
the non-interlaced variety. However, a police officer of a
small town might not need to invest as much money into
armor when traditional weaved armor will protect him all the
same from common pistol-type ammunitions [6].
There are a vast amount of synthetic fibers used in
industry today. Kevlar® fiber now has competition for use in
body armor and protective vests because of other discoveries
in the engineering of synthetic fibers. One of Kevlar ®
cloth’s main threats is cloth formed of SVM fibers. It is
proven that SVM cloth has a much higher energy capacity
FIGURE 1
DIAGRAM OF FORMING NON-INTERLACED STRUCTURES [5]
than Kevlar® for its thickness, but SVM cloth is more than
two times the cost of cloth made of Kevlar® [7]. SVM
material, then, can be put to better use in large industry,
heavy lifting cables and other applications that require more
strength and durability than body armor would require.
SVM would therefore not be used to its full potential in body
armor, making mass production of SVM body armor a sort
of economic blunder.
only makes sense not to weave the fabric. However, it is
impossible to make a fabric without weaving the fibers
together. Engineers have found a way to get around this
barrier, however, by a method using thermowelding. Since
fabric cannot be made (the traditional way) without weave
and weft, Kevlar® is made by layering perpendicular sheets
of parallel fibers between 3 sheets of thermowelding. These
three layers consist of two outer structural layers and one
inner structural layer. To get a better visualization of this
structure, refer to Figure 1.
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TABLE I
COMPARATIVE PROPERTIES OF KEVLAR® VS. OTHER FIBERS [6]
key elements that makes Kevlar® fiber so strong by giving
Kevlar® tensile strength in all directions. [8].
Hydrogen bonding and radial orientation both give
Kevlar® fiber crystal-like properties due to the intense
amount of symmetry and repetitiveness through the fibers.
Kevlar® fiber adopts these crystalline properties during
manufacture- more specifically the spinning process. When
the solution is spun, the polymer chains align themselves
parallel to the fiber’s axis, creating the repetitive and
symmetrical crystalline characteristics [8]. Bright
synchrotron radiation used with X-ray Absorption Near
Edge Structure (XANES) images demonstrates these
tendencies in Figure 2. The more crystalline the compound
is, the stronger the intermolecular bonds are, and thus, the
stronger the fiber is. In fact, the fiber’s crystalline properties
are the leading contributors to Kevlar®’s intense strength.
MECHANICS OF KEVLAR®
Strength
The strength of Kevlar® can be attributed to several of its
unique chemical properties. Kevlar® is a polymer in which
each of its monomers is made of C14H10N2O2, which makes
it a polyaromatic amide [8]. These chains of aramids and
amides are manufactured by tightly spinning high
concentrations of poly-para-phenylene terephthalamide
solution through a spinneret to form straight thin strands of
Kevlar® fiber [9].
This fiber is so strong because the aromatic groups exist
with radial orientation. This accounts for the symmetry and
ordered structure of Kevlar® fiber’s repetitive spine. This
repetitiveness gives the fiber very few flaws and then, in turn,
very few weak spots. The radial structure is uncommon in
poly-aramid fibers, as most are instead transversely isotropic
giving those types of fibers kinks and bends. These kinks
and bends thus in turn make the fiber weaker inch per inch
[10].
Kevlar® fibers vertical stable structure isn’t its only
source of strength, however. These strands have strength
horizontally, as well. Kevlar® fibers form strong bonds with
each other through hydrogen bonding that occur between the
hydrogen atoms on the amide groups. These hydrogen
bonds pull the strands close together, and give the fiber
strength in structure. Since there is a hydrogen bond for
every monomer of the chain, hydrogen bonding is one of the
FIGURE 2
XANES IMAGE OF KEVLAR® FIBER’S RADIAL STRUCTURE [8]
Although Kevlar® fiber generally has a comparatively
large tensile strength, high tenacity, and high modulus
compared to other yarns (see Table 1), it lacks in its percent
elongation-to-break. Kevlar® fiber is flexible; however it
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does not stretch very easily, as compared to other fibers.
This characteristic can be modified, however, with the
addition of other materials in the body armor. Therefore,
even though this may be one of Kevlar®’s weak points, it
can easily be optimized. Its other qualities such as its high
tensile strength, high intermolecular strength, and stable
structure are great redeeming qualities that over-ride its low
percent elongation-to-to break.
The strength of Kevlar® fiber is crucial when it is used as
a component of body armor. Its great strength is the key
element in being able to stop any high velocity piercing
projectile, and without its chemical stability, the armor
would be useless. Kevlar® fiber’s strength is what makes
body armor, armor.
the leading body armor fiber, as it was flame and heat
resistant, but it was eventually found that it degraded at such
a fast rate when exposed to heat for long periods of time, it
was discarded as a body armor fiber [12].
Kevlar○R fiber, like Zylon®, is a good heat resistant fiber.
Not only does Kevlar○R fiber resist high temperature, but it
also can improve the fire resistance of the body armor [11].
If a flame is applied to body armor infused with Kevlar ®, the
body armor will degrade. However, once the flame source is
removed, the fiber will stop burning. This is mainly due to
Thermal Properties
Kevlar®’s strength isn’t its only strong suit; Kevlar® fiber
also has optimal thermal properties. Kevlar ○R fiber’s thermal
properties allow it to resist the heat released by the friction
between a bullet and the body armor. This is especially
important, because the deformation of the wave propagation
will not be affected negatively by this heat. The reason
Kevlar® doesn’t melt, is because it is actually classified as a
thermoplastic. Since thermoplastics don’t melt prior to
decomposition, Kevlar® fiber will not melt down [11].
Hydrogen bonds between the Kevlar molecules and the
crystalline structure play a key function in Kevlar®’s thermal
properties. Breaking the crystalline structure requires all the
hydrogen bonds between two molecules to be severed at
once. It takes an immense amount of energy in the form of
heat to break these hydrogen bonds, so that hydrolysis, used
to break down polymers, can occur [11].
The thermal degradation of Kevlar ○R fiber was measured
by a TG 209 F1 Iris device. This device helped calculate
the thermal degradation by means of chemical observation.
When Kevlar ○R fiber was heated, it would release HCN, NO2,
CO, CO2, and H2O. The absorption of those gases could be
observed to determine when the decomposition started [11].
Even though exposure to elevated temperatures can
degrade the properties of Kevlar®, Kevlar ○R fiber still has
relative high thermal stability as we can see from Figure 3.
This graph shows that Kevlar○R fiber starts to lose significant
amount of weight at 548°C and has terminal temperature of
decomposition at 643.7°C.
These temperatures are
extremely high, so they do not affect its use in body armor
that will only be worn in atmospheric temperatures.
However, Kevlar® fiber does lose measurable amounts of
strength in between temperatures of 100°C to 548°C due to
slow oxidation of the polymer. This reaction is extremely
slow, and the strength lost is minimal, but it is still a factor
that needs to be taken into consideration when using Kevlar®
in body armor [11].
Kevlar® may lose strength at high temperatures over
extended periods of time, but it is still the slowest degrating
body armor material made. Zylon®, for instance was once
FIGURE 3
DEGRADATION OF KEVLAR AT HIGH TEMPERATURES [11]
the fact that Kevlar® has a low thermal conductivity.
Kevlar®’s thermal conductivity is realatively low to other
fibers that are similar to Kevlar®, making it another great
choice of body armor material [11].
Since Kevlar® fiber has an extremely high degradation
temperature, oxidizes very slowly when exposed to high
temperatures over extended periods of time, and can resist
heat and flames, Kevlar® fiber is a prime choice for body
armor.
Weaknesses
Although Kevlar® has outstanding physical and chemical
properties making it one of the strongest synthetic fibers
ever engineered, it does have its downfalls. Kevlar® fiber
can stand an intense amount of heat, has a very high tensile
strength for its density, and is an extremely stable chemical
polymer, but it has a few commonplace threats.
One of biggest issues with Kevlar® fiber is its sensitivity
to ultraviolet light. If Kevlar® is exposed to certain
wavelengths of the solar spectrum (between 300nm to
450nm), degradation may occur. The energy given off by
these wavelengths are at just the right level to excite the
electrons in the bonds of the fiber to break the bonds holding
the polymer together. If this occurs, the result will be
significant loss of the mechanical properties of the fiber [6].
However, Kevlar® body armor can easily be covered up
when wearing outside. Along with that seemingly simple fix,
other components can be added to the armor to increase its
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resistance to UV light, such as Soft Armor Panels, which
will be talked more about in the components section.
Another one of Kevlar® fibers faults is its weakness
against strong acids and bases. When Kevlar® is exposed to
very strong acids and bases, its strength deteriorates
considerably. This can be a serious fault if the wearer is
going to be present among concentrated chemicals that have
a potential of being splashed onto the armor. The good side
of this is it still takes several hours for Kevlar® to degrade
completely, so its wearer will still be protected long after the
spill occurs. As of now, there is no immediate fix to
Kevlar®’s weakness to strong acids and bases, as most fibers
have the same, if not worse, effect.
Sensitivity to UV light and strong acids and bases are
Kevlar®’s only downfall. The issue of UV light can easily
be avoided, and strong acids and bases aren’t very likely to
be splashed onto most wearers, and if they do get splashed
onto the vest, the wearer still has several hours of protection
remaining before any significant loss of strength develops.
standard weights of body armor are 6.31 kg and 6.62 kg for
a standard and large size vest, respectively. These weight
maximums are hard not to surpass. As we know, Kevlar®
fiber is the main component of body armor. It has high
strength to weight ratio, but it still takes between 20 and 40
layers of Kevlar to stop a bullet [4]. The stacking of layers
increases the weight and the stiffness considerably, which is
a large issue. Thanks to materials engineering, there is a
solution to this problem. There are two fluids that can
greatly optimize body armor in terms of lightness and
flexibility. These fluids increase Kevlar®’s efficiency by
soaking into Kevlar® and strengthening the fiber [14].
The first liquid is a shear-thickening fluid (STF). When
external forces exert pressure on STF soaked Kevlar®, the
surface of it will abruptly solidify. Thus, when a bullet
strikes on the STF soaked body armor, the fibers will harden
in a few milliseconds, stopping the bullet before it penetrates
the armor. This strengthening occurs because the fluid is a
colloid, made up of tiny suspended particles repelling each
other. When an external force exert on this fluid, the
suspended particles form masses called hydroclusters.
These hydroclusters act as a wall that stops the bullet within
milliseconds. Then, once the force is done being exerted,
the masses disperse back into suspended particles, leaving
the vest almost as it was before the bullet was shot at it.
This is an important factor when considering body armor.
The wearer should be able to be protected against multiple
shots, and this colloid gives the body armor a much greater
chance of protecting the wearer multiple times before the
vest is no longer stable [14].
STF soaked Kevlar® is much more efficient than plain
Kevlar® vests, as shown in laboratory tests. Four layers of
STF-treated Kevlar○R fibers can dispel the same amount of
energy as fourteen layers of Kevlar○R fibers do. Soaking the
Kevlar® fibers in STF thus optimizes Kevlar® fiber’s
efficiency and also decreases the weight considerably,
making the weight maximums easier to stay under [14].
The other kind of fluid is magnetorheological (MR) fluid,
a type of oil filled with tiny iron particles. Armor soaked in
MR is hooked up to a portable power supply. This portable
power supply can produce a magnetic field around the armor,
causing the iron in the fluid to line up and harden
significantly, as shown in Figure 4. This hardening process
takes only around twenty thousandths of a second, making it
a very efficient and quick way to protect its wearer. The
armor becomes so hard, that it can stop a bullet or any type
of shrapnel that may be projected at the wearer, potentially
saving the wearer’s life. When the magnetic field is
suppressed, the armor becomes fluid again, allowing the
wearer to move about freely once again [14].
MR fluid has several drawbacks, nonetheless. MR fluid
infused armor relies on the wearer to activate the magnetic
field. This is especially a large issue if the wearer is not
expecting to be hit with an explosion, shrapnel, or a bullet.
If this happens, the wearer could be severely injured, and on
top of that, the armor could be damaged. If the armor
COMPONENTS OF BODY ARMOR:
When body armor is produced, Kevlar ® fiber is not the only
component present. Other fibers and materials are needed to
make body armor “user friendly” and also secure. One of
the main and probably most simple components of body
armor is Velcro. Velcro straps allow the vest to be taken off
easily and quickly, which can be vital in cases where the
wearer is wounded [13]. Even though the main function of
the Velcro fasteners is to improve the easiness of wearing,
they still need to provide certain level of strength to ensure
the stability of the body armor. The peel and sheer strength
of the Velcro used in body armor cannot be below certain
regulations made for each vest [13].
In order to fit human bodies comfortably and snugly,
body armor must have a certain level of softness and
flexibility to keep up with the range and speed of the wearers
body movements. This is extremely important, because
body armor must protect the largest possible area on the
wearer, including the neck and collar. The neck has to be
totally free to move around as it needs to, so the restriction
on it has to be minimal from the armor. Other areas need to
have minimal restriction as well, such as the shoulders, the
groin, and the back. This softness is accomplished by
adding Soft Armor Panels [13]. These Soft Armor Panels
are made of two flexible polyethylene plates that allow the
wearer to move more freely. If these areas of the body were
restricted too much by body armor made just of Kevlar ®
fiber, the body armor could end up putting the wearer at
more risk, so these Soft Armor Panels are definitely an
important component of Kevlar® body armor.
Flexibility of the body armor is not the only element that
can limit the movement and speed of the wearer; the weight
of the body armor can as well. In order to avoid any severe
hampering of the wearer by the weight of the body armor,
there is a standard maximum weight the armor can be. The
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infused with MR gets damaged, there is a high chance the
power supply will short circuit, leaving the body armor
almost useless [14].
The iron particles in MR fluid may eventually clump
together, settle in the armor, or precipitate out of the solution,
thus leaving the armor much less effective. There is
research being done on how to make these particles less
likely to leave the solution or become dormant, but other
factors need to be taken into consideration, such as the other
large downfalls of this type of armor [14].
WHY KEVLAR?
Ever since the day Kevlar® was engineered in 1965, more
and more applications of Kevlar® fiber have been added to
its already lengthy list, which makes it one of the most
versatile synthetic fibers we have today. With all of the new
improvements and optimization of Kevlar® fiber with
regards to how it is put to use in different cloth, it is no
wonder why Kevlar® is the prime choice for body armor.
When its characteristics are compared with other synthetic
fibers of its type, few are even comparable to its superb
ballistics, strength, and stability. Even with the few flaws
Kevlar® has, it is still one of the most incredible synthetic
fiber innovations ever to be engineered.
Kevlar®’s ballistic properties can easily be maximized
with the use of non-interlacing structures settled together
with thermowelding, and multi-layering of fabric. This
makes Kevlar® cloth versatile and usable in several different
threat-level environments, anywhere from a small
community, to overseas in a war zone.
These outstanding ballistics properties come from the
Kevlar® fiber’s chemical composition. Kevlar® fiber is
composed of long chains of poly-aramid compounds, which
allow for a straight, “train-like” polymer. This is one of
Kevlar®’s most unique properties, because the polymer
doesn’t have kinks or bends. Therefore, weak spots in the
fiber are nonexistent. On top of having a uniform structure
vertically down the length of the fiber, horizontally it
possesses hydrogen bonds. These strong intermolecular
bonds keep the fibers closely held together adding even
more strength to the fiber altogether.
Kevlar®’s thermal properties are another key to this
fiber’s effectiveness in body armor. Kevlar® won’t lose
significant amount of weight or strength until it reaches its
decomposition temperature of 548°C, which is a temperature
that no human will be exposed to over a long period of time,
and although all aramid fibers oxidize at high temperatures,
Kevlar®’s oxidation is extremely slow and does not affect
the strength significantly. Kevlar® is also a good flame
resistant fiber, which is a useful property of body armor.
These thermal properties then, in turn, work together to help
the body armor resist the heat released by the friction
applied by the bullet, allowing the fibers to absorb the
bullets energy to their best potential, and decelerate it to a
stop before it penetrates the armor, saving the wearers life.
Body armor also has to fit and work with the human body
well. The armor has to move with, not restrict, the wearers
movements to ensure the wearers safety. Although Kevlar ®
itself is not very flexible, additives to the armor help it
become more efficient, one of these being Soft Armor
Panels. They are added to increase flexibility and movement
range. Although Kevlar® is already relatively lightweight, it
can be soaked in liquid body armor such as STF to increase
its strength, lessening the number of layers needed to protect
the wearer. This greatly decreases the weight, and thus
FIGURE 4
MR FLUID INFUSED KEVLAR® WITH MAGNETIC FIELDS [14]
Another extreme drawback of MR fluid soaked Kevlar ®
armor is the fact that the wearers need to carry around a
hefty battery all the time to produce a magnetic field. So
although the body armor infused with MR fluid is much
lighter than plain Kevlar® armor, the weight of the battery
would increase the wearer’s burden dramatically, which
would cancel out the benefit of the lighter armor [14].
However, with the constant improvement in batteries
over the years, a lightweight and efficient battery might not
be too far out of sight. Li-ion batteries are a possible source,
as they are extremely lightweight at about only 0.5g/cc 3, and
it provides a decent amount of voltage per cell (3.4V to 3.7V)
[15]. These cells can then be placed in circuit with one
another, creating a higher voltage. Li-ion batteries are also
rechargeable. There is still much more research to be done
on Li-ion batteries to make them suitable for a battlefield.
Nevertheless, the efficiency of having to carry around a
battery pack is still unfavorable, thus the usage of MR fluid
is still debatable.
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[16]
(2011).
“Safe
Guard
http://www.safeguardarmor.com/
hampers the wearer less, giving them more freedom of
movement.
There are other body armor fibers other than Kevlar ®,
but with all of the positive properties of Kevlar®, it is hard to
beat. For instance, SVM fiber has a higher energy capacity
than Kevlar® fiber, but Kevlar® is much cheaper. It’s not
just a “cheap” way out, however. Body armor infused with
Kevlar® fiber will protect its wearer just as well as any more
expensive fiber by optimizing its efficiency by adding in
other components such as STF, or thermowelding. So, both
economically and dependably speaking, Kevlar® vests are
the smarter choice for use in body armor, and it has been for
over 30 years [16].
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ACKNOWLEDGMENTS
We would like to give special thanks to the writing center,
our Co-Chair Julie Ramone for helping us format our paper
and giving us new ideas. We would also like to thank
Eugene Wagner for giving us helpful information during
class lecture. Lastly, we would especially like to thank our
Chair person, Rob Boback for giving us his time and advice.
[10] S.B. Warner. (1982, December). “On the Radial Structure of Kevlar.”
Macromolecules.
[Online].
Available:
http://pubs.acs.org/doi/pdf/10.1021/ma00243a025
[11] Hong-Ting Zhang. (2011, December). “Comparison and Analysis of
Thermal Degradation Process of Aramid Fibers (Kevlar 49 and Nomex).”
Journal of Fiber Bioengineering and Informatics. [Online].
Available:http://www.jfbi.org/admin/Issue/JEBI%20Vo1%203,%20No.%2
03.%20December%202010_201191185819_paper.pdf
[12] “Zylon ○
R Fiber.” C.S.R. Incorporated. [Online]. Available:
http://csrbraids.com/index.php/zylon-fiber.html
[13] “Specification for Bullet Proof Jackets.” Technical Guide. SJVN.
[Online]. Available: http://sjvn.nic.in/tenders/pcd/ppr1594/jacket-spec.pdf
[14] Tracy V. Wilson. (2007, February). “How Liquid Body Armor
Works.”
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StuffWorks.com.
[Online]
Available:
http://science.howstuffworks.com/liquid-body-armor.htm
[15] E.Wagner. (2012, Feb.28). General Chemistry II. [Lecture].
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