2. (2013TT10944)
Introduction:-
A good body armor is generally lightweight and flexible in nature. The
main characteristics a body armor should have is impact energy
absorbance and low penetration depth. Over a long period of times many
types of body armor are prepared from metal, ceramics etc. but all these
had restrictions of weight, flexibility etc. But with the advent of Kevlar in
the industry, revolutionary changes has occurred in body armor
characteristics like flexibility, improvement in weight to performance ratio,
improvement in performance to thickness ratio. But since Kevlar doesn’t
provide better protection against spike threat in compare to bullet threat
hence new methods have been evolved to provide better spike threat by
using some extra material. STF is light weight material and can be used
with Kevlar in form of composite to provide better spike as well as bullet
resistance.
STF is non-Newtonian fluid having low viscosity at low
shear rate but as the shear rate increases it starts showing increase in
viscosity abruptly after a critical shear rate due to which its rigidity
increases but as the shear rate decreases it comes backs to normal form.
The reason behind the increase in viscosity is that in initial stage all the
colloidal solid particles remain in fluid order but as the shear rate
increases they become disordered and forms clusters jamming with each
other and these clusters are known as hydrocluster also. For making STF
silica nanoparticles are used as they are rigid in nature with a fluid like
Polyethylene Glycol for suspension. This STF is uniformly coated over
Kevlar and multiple layer of this fabric is then used to make the armor.
The STF increases energy dissipation capabilities. Shear thickening
fluids coated fabric shows better stab resistance during drop tower
impact test and quasi static tests.
3. MATERIAL USED AND PREPARATION OF STF COATED FABRIC:-
For shear thickening fluid silica nanoparticles are suspended in poly
ethylene glycol (200 MW) and to improve the bonding between the
inorganic and organic silica particle and polyethylene glycol some
coupling agents are used like Silane which helps in forming stable bonds.
To increase the dispersion of liquid on the fabric surface ethanol is mixed
with silica and polyethylene glycol which reduces the surface tension of
liquid which helps in infusion of STF in fabric deeply. Now to make fabric,
layers of STF coated Kevlar are stitched together and placed in oven at
70 degree Celsius for 20 minutes so that all the ethanol get removed
from the fabric.
TESTS PERFORMED TO STUDY CHANGES IN THE FABRIC
COMPARE TO STF COATED FABRIC:-
To study the ballistic and stab resistant properties of fabric mainly two
test are done. The tests are as follows:-
A)Drop Tower Test
B)Quasi static Loading Test
A) Drop Tower Test:-
This test is performed to test the ballistic properties of the material and
the impact energy absorbance of material. The test is performed by drop
tower on which the impactors are rigidly mounted to a crosshead .The
fabric sheet is placed on a multi-layer foam backing which consist of
neoprene sponges, polyethylene foam ,rubber and witness paper.
4. The impactors (knife, spike) are mounted on crosshead of drop tower to
perform test, which is then loaded with weights to a specific mass. The
crosshead is dropped from a fixed height to impact the target. The
velocity of the crosshead at impact is measured using fixed flags and
sensors attached to the frame. Impact loads are measured using a load
cell mounted to the impactor. The depth of penetration into the target is
quantified in terms of the number of witness paper layers penetrated by
the impactor.
Now to sets of experiments are done for both STF coated fabric and neat
Kevlar fabric.
B) Quasi Static Loading Test:-
To study the stab resistant properties of Kevlar fabric Quasi static loading
test is performed Tests were performed by mounting a penetrator into
the crosshead of the load frame, and then displacing the penetrator into
a fabric at the rate of 5 mm/min. The fabric is backed by a multi-layer
foam backing which consist of neoprene sponges, polyethylene foam.
Load versus displacement is recorded for each target and penetrator;
higher loads indicate higher resistance to penetration.
RESULTS AND DISCUSSION-
In the drop tower stab performance of fabric we can observe from
the figure that as the impact energy increase threat the penetration
depth increases. At low impact energy the penetration depth for
STF Kevlar fabric is very less in compare to neat Kevlar fabric. But
as the impact energy increased more the penetration depth
increased more but neat fabric penetration depth was achieved at
less energy in compare to neat Kevlar fabric.
5. The neat Kevlar fabric target becomes penetrated at a small
displacement and load and thereafter provides little resistance to
further penetration. While the functionalized STF-Kevlar targets
shows little damage or penetration in fibre when comparing with
others at small loads and its maximum loading capacity is much
more in compare to the neat fabric.
As the STF disperses uniformly over the fabric it helps in binding
the yarn due to which yarn to yarn friction increases resulting in high
yarn pull out force due to which puncture resistance of fabric
improves.
High padding pressure helps in low add on and deeper penetration
of STF molecules resulting in better impact energy absorbance or
ballistic properties.
6. The biggest advantage of STF is that the thickness and bending
angle almost remain same for STF Kevlar fabric with different layers
of fabric in comparison to neat Kevlar fabric. Flexibility is also
remain more or like same as neat Kevlar fabric.
As we increase concentration of silica particles in the suspension
up to a limit the ballistic property of fabric increases significantly but
silica particles are rigid in nature hence increased concentration
increases the probability of fibre damage increases which will
impact the overall properties of fabric. Hence optimum
concentration of silica particles in the STF is necessary.
CONCLUSION-
Since the silica particles are harder in nature, some other material also
can be used like PMMA (poly methyl methacrylate) instead of silica which
are more softer in compare to silica and PMMA does not damage the
fabric also but results in slight reduction in impact energy absorbance
compare to silica particles. And as we increase the number of layers of
kevlar more ,the fexibility of body armour also reduces.Hence we can’t
increase kevlar layers after a certain point.Hence a new method of using
Carbon nanotubes has been discovered which can increase the strength
of kevlar fabric two fold times. By modifying Kevlar fabric with the addition
of an embedded cross linked network of carbon nanotubes (CNTs), the
mechanical properties of Kevlar are increased such that less layers are
needed to protect with same ballistic and stab resistance property.
