SUPERMAN SUIT: FUTURISTIC BODY ARMOR
March 31st 2008.
________________________________________________________________________
Emma Lecours, Megan Swain, Jonathan Boulanger, Sarah Xu
Bullet-proof or ballistic vests are a type
of body armor worn on the torso to
absorb the impact of projectile objects.
Originally crudely made from wood or
various types of metal, these became an
ineffective form of protection with the
invention of firearms.
The disadvantage to this type of armor is
that the wearer of the vest is still forced
to absorb all of the energy possessed by
the projectile, resulting in blunt force
trauma, possibly causing injury to the
wearer. The effects of this can vary from
the wind being knocked out of the
wearer, bruising of the skin or even fatal
injuries to the internal organs.
CURRENT BULLET-PROOF
VESTS
A superior bullet-proof vest would
instead deflect both the bullet and the
majority of the energy possessed by the
bullet away from the wearer reducing the
possibility of blunt force trauma.
Current bullet-proof vests are made of
layers of fibrous materials. Upon impact
the material absorbs the energy of the
bullet and disperses it throughout the
material. This slows down the bullet and
stops it from penetrating the body.
BULLET REPELLENT
There are three main materials currently
used to produce bullet-proof vests:
Kevlar, Twaron and Dyneema. Kevlar is
a synthetic polymer fibre with a structure
as seen in Figure 1. It is a useful material
due to its very high strength to weight
ratio. Twaron was later developed and
has a nearly identical chemical structure.
Finally Dyneema is a thermoplastic
material which consists of extremely
long polymer chains that allows the
material to effectively transfer its load,
giving it a high impact strength.
Carbon nanotubes (CNT) as a
bulletproof vest material are ideal due to
the combination of high elastic modulus
and high yield strain. CNT have been
measured to have a Young’s Modulus of
~1 TPa [2]. Furthermore, up to 40%
strain to yield in tensile tests has been
measured.
Consequently, CNT are
capable of elastically storing an extreme
amount of energy. The capacity to
absorb so much energy while not
permanently deforming or failing is the
reason CNT make ideal bulletproof
vests.
The high strength and elasticity of CNT
can be explained by its deformation
modes. CNT can be either single or
multi-walled. In this application, single
walled nanotubes are preferred for their
superior mechanical properties. A single
walled nanotube (SWNT) is composed
of a single rolled graphene sheet. This
sheet may be rolled in several ways,
FIGURE 1: The molecular structure of
Kevlar
1
indicated by an index (n,n). SWNT used
in this experiment were rolled in a zigzag structure (n,0), thus all tubes were
metallic [1,2]. All carbon atoms in the
2D lattice are bonded by sp2 bonding,
which is a critical factor in CNT’s
remarkable thermal, mechanical, and
electrical properties. In particular, these
bonds allow elongation of the hexagonal
structure when under tensile stress up to
extremely high strains. Furthermore,
additional elastic deformation modes
consist of highly mobile defects,
stepwise tube diameter reduction, and
tube collapse/bending [2].
These
deformation modes are fully reversible
after the removal of the applied stress.
The final deformation mode of a CNT is
necking, which is irreversible. In Figure
2 TEM and computer simulated
deformation of SWNT are shown.
simulation a very small diamond tip
(3.56 x 3.56 x 0.71 nm) was used as the
bullet. To replicate the real situation, the
width of the projectile’s tip was much
larger than the width of the flattened
nanotube.
FIGURE 3: Model
diamond ‘bullet’
showing
the
Their simulation used an empirical bond
order potential that incorporates bond
energies, lengths, and force constants for
hydrocarbon molecules, as well as
elastic properties, interstitial defect
energies, and surface energies for
diamond. For the atomic interactions
inside the nanotubes, the three-body
Tersoff-Brenner potential was used
while for the interaction between the
bullet and the nanotube, the two-body
Tersoff-Brenner potential was used.
They performed a simulation that shot
the diamond at 400m/s (arbitrary speed)
at a nanotube with two fixed ends to
determine its maximum absorption
energy, that is to say the energy
difference of the CNT before its
interaction with the bullet and just before
the onset of the fracture. From this
experiment, they estimated the initial
bullet speed that would not break the
bonds of the nanotubes. These speeds
(between 1000 and 3500 m/s were used
to simulate diamond being fired at a
distance of 0.1nm for the nanotube. They
FIGURE 2: Tube collapse and diameter
reduction reversible deformations
EXPERIMENTAL RESULTS
The team from the University of Sydney
investigated the dynamic properties of
carbon nanotubes through the use of a
computer
simulation.
For
their
2
investigated the effects of parameters
such as height of hit, radius of nanotube,
length of nanotube, and initial speed of
bullet. Figure 4 demonstrates that the
maximum energy absorption occurs at
the center of the nanotube. The effect of
radius on the energy absorption is very
small. Figure 5 shows that the maximum
absorption energy varies linearly with
nanotube length. Figure 6 shows that the
bullet started to bounce back almost at
the same time no matter the initial speed
and position of the bullet hitting the
nanotube.
decreases, unless the CNTs are given at
least 12.5ps to recover.
FIGURE 6: Variation of speed during
the bullet impact with the CNT.
EXTENSION TO MACROSCOPIC
APPLICATIONS:
Because the nanotube diameter is
generally defined during the growth
process, the atomic structure of CNT is
primarily determined by its length.
Therefore, CNTs do not go through the
transition from monocrystalline structure
to polycrystalline structure, which
usually happens in metals. The fact that
the
scale-effect
in
CNTs
is
approximately minimal allows us to
apply previous analysis and results to a
macroscopic scenario.
FIGURE 4: Variation of relative
absorption energy with respect to the
position at which the bullet strikes.
Say we weave loose CNT bundles into a
body armor material composed of
several layers of woven CNT yarns. For
a (18, 0) CNT, a 100 μm diameter
bundle usually contains 5 X 109
nanotubes. From previous calculation,
we know that the absorption energy
varies almost linearly with the tube
length. If we assume nanotubes yearns
of 0.9cm length are used to protect a
person from a revolver bullet, which
typically has a damage area of 0.652 cm2
and energy of 320J—as shown in Figure
FIGURE 5: Maximum energy absorbed
with CNT length.
If subsequent impact happens right after
the first one, the maximum load bearing
speed and the absorption energy
3
7—180 nanotubes yearns will be
required. A single nanotube yarn should
be able to absorb 0.344J of energy;
therefore 6 layers of woven fabric should
be sufficient, which corresponds to a
thickness of 600 μm.
REFERENCES
1. Ajayan, P. M. (1999). Nanotubes
from carbon. Chemical Reviews,
99(7), 1787.
2. Mylvaganam, K., & Zhang, L. C.
(2007).
Ballistic
resistance
capacity of carbon nanotubes.
Nanotechnology, 18(47), 475701.
FIGURE 7: A Layer of woven CNT
yarn material.
4