JEC COMPOSITES MAGAZINE - Issue #79 - March 2013 - 90

fabric
Hepper et al [2]. The effect of energy transfer can be minimised
in two general ways. The first is to promote energy absorption
through breaking, stretching, and compressing the protective
materials, or by extending the time over which the impact
energy is applied to the body. The second is to redistribute or
dissipate the impact energy to wider areas of the protective
material so as to reduce the impact energy density over the
protective material.
High-performance fibres are usually constructed into plain woven fabrics as the layering materials for the body armour. Plain
woven structures became natural candidates for ballistic fabrics
because they provide the best structural stability and strongest
holding of the constituent yarns. This has been supported by
numerous investigations on the effects of inter-yarn friction on
ballistic performance. Briscoe and Motamedi [3] experimented
on a plain woven ballistic fabric with three different treatments.
The results showed that the fabric with the highest friction
between the fibres was most energy absorbent and created the
least vertical deformation, and vice versa. Rao et al [4] reported that, as shown by FE modelling, higher initial velocity is
required to achieve near zero residual velocity for fabrics with
inter-yarn friction than for fabrics without inter-yarn friction. In
another numerical study, Duan et al [5] simulated the ballistic
performance of plain woven fabrics under different boundary
conditions with inter-yarn frictional coefficients being 0 and
0.5 respectively. The results suggested that the fabric with high
friction absorbs more energy than the fabric with no friction in
all concerned circumstances. Sun and Chen [6] reported that
when plasma treatment is employed to increase the inter-yarn
friction for an aramid woven fabric, the resistance to the quasistatic yarn pull-out from the fabric quadrupled when compared
to the same fabric without plasma treatment. This research also
showed through an FE study that the fabrics with rougher yarn
surfaces are able to absorb more energy from the impacting
projectile.
Layers of single-ply fabrics are used to form the ballistic panels
for body armour. Considerable effort has been made to investigate 3D woven fabric panel constructions and their effects on
ballistic performance. The stress and strain distribution through
the thickness direction indicates what properties the layering
fabric should have and how the panel should be constructed.
3D fabric structures were also investigated for their protection
effectiveness against ballistic impact.
This paper presents some achievements made on 2D and 3D
fabrics for ballistic protection based on the research carried out
at the University of Manchester.

Stress and strain distribution
A soft armour system defeats the projectile by absorbing the
kinetic energy and spreading it over a larger area to minimise
the effect of the ballistic impact. During a ballistic impact event,
a cone is formed on the exit side of the target just behind the
point of impact due to the transverse wave propagation [7, 8,
9]. The surface radius of the cone increases with time, and the

90 jec composites magazine / No79 March 2013

cone moves along with the projectile, so that the depth of the
cone increases. The projectile displacement at any instant of
time and the cone depth formed would be the same. Naik et al
[7] reported the variation of cone surface radius against time.
Initially, the transverse wave velocity increases significantly and
then remains nearly constant during the remaining period of
ballistic impact event. The rate of cone depth increase decreases
with time, and this depends on the velocity variation of the projectile during the ballistic impact event. The variation of cone
surface radius is nearly linear with respect to time, whereas the
change in cone depth against time is non-linear.
Porwal and Phoenix [10] studied the relationships between
cone wave positions in different layers and found that cone wave
velocity in the th layer can be written as:
(1)
where a0i is the longitudinal strain wave velocity in the ith layer,
which is constant for each layer, consti is constant, and V is
projectile velocity.
The radius of cone wave front, Ψi, normalized by projectile
radius in layers i, is represented by:
(2)
where a0i and a0j are the longitudinal strain wave velocities in the
ith layer and jth layers respectively.
The longitudinal wave velocity for isotropic solids is expressed
as follows according to Lyons [11] :
(3)
where E is Young’s modulus and r is material density.
In an impact situation, faster moving longitudinal waves and
slower moving transverse waves help to dissipate the impact
energy through the yarns involved at the impact site [12]. The
length of the yarns involved in the waves increases with time
and is subjected to various stress levels and strain rates. The propagation speed of the shock waves formed in the protective material during the ballistic impact is related to the fabric’s energy
absorption capability and is important from a ballistic point of
view. The energy absorption and propagation ability of fabric
layers depends on the tensile modulus of the fibres and yarns
forming the fabrics. The tensile modulus and tensile strength of
yarns is the main parameter affecting ballistic performance.
While the mechanical properties of the constituent yarns are
important, fabric parameters such as fabric density, yarn count
and weave structure can also have a significant effect on ballistic

JEC COMPOSITES MAGAZINE - Issue #79 - March 2013

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