Composite Armour—A Review

16 Pages·2015·1.1 MB·English

Journal of the Indian Institute of Science
A Multidisciplinary Reviews Journal
ISSN: 0970-4140 Coden-JIISAD
© Indian Institute of Science s
w
Composite Armour—A Review
e
i
Kiran Akella1 and Niranjan K. Naik2 v
e
Abstract | Primary constituent materials of composite armour can be
categorized as ceramics and fiber reinforced polymer (FRP) composites. R
In this paper, we first examine a range of these primary constituent
materials. We study their individual response to ballistic impact, and then
compare their performance with other constituent materials. We review
various combinations of these constituent materials and compare their
characteristics. Subsequently, we study different configurations of armour.
We also review studies conducted on the effect of bullet shapes and bullet
materials. Based on these studies, we arrived at design considerations.
Furthermore, we have reviewed the probable future directions for
composite armour.
Keywords: ceramic, fiber reinforced polymer, ballistic impact, performance comparison, configuration
1 Introduction or ceramic-FRP composite armour. Such composite
Efficient armour requires hard, tough and armour consists of a ceramic layer backed by a
lightweight materials with significant penetration composite or metal layer. The ceramic layer provides
resistance and energy absorbing capabilities. primary ballistic impact resistance. The inner Ballistic impact: It is a high
High hardness steel was used as one of the composite or metal layer is for secondary energy velocity impact by a projectile
substantially smaller than the
first engineered armour materials. However, absorption. It also serves as the backing for brittle
target mass. It is a localised
with increasing demands on reducing weight, ceramics. Due to the high specific strength and phenomenon and response
researchers and designers explored other materials. specific stiffness of FRP composites used as backing of materials in the vicinity
of impact area is of primary
Aluminum alloys, other lightweight metals such layer, ceramic-composite armour is one of the
importance for mitigation.
as titanium and their alloys, were hence used as lightest alternatives to monolithic metallic armour.
armour materials. There are other forms of armour using only
Ceramics are stronger than metals under heavy FRP composites such as fabric or textile armour.
compressive loads acting in the vicinity of areas These are suitable for lower threats such as
subjected to impact. Their much lower density handgun bullets. They cannot sustain more lethal
makes them attractive lightweight alternatives threats. Hence, we did not include a discussion
to metallic armour. But processing of ceramics on fabric armour. We focus on only ceramic-
requires high temperature and pressure. Hence, composite armour.
they are more expensive than metals. Another In this paper, we first review various constituent
drawback of ceramics is their brittle behaviour materials used for armour. Subsequently, a detailed
1Scientist, Research
causing heavy degradation on impact. They, discussion of some of the widely used ceramics, and Development
therefore, have lower capability to withstand their performance comparison, studies for their Establishment (Engineers),
Defence Research
multiple hits than metallic armour. property enhancement and unconventional
and Development
Attempts were made to improve the ductility processing methods are reviewed. We then review Organisation, Alandi Road,
of ceramics by embedding ceramic fibers inside FRP composites including different fiber materials, Kalas, Pune 411015,
Maharashtra, India.
bulk ceramics. Such materials are popularly matrix materials and prepregs.
[email protected]
known as ceramic matrix composites (CMCs). Furthermore, we discuss the response of
2Adjunct Professor,
However, CMCs are difficult to process and even composites to ballistic impact. We compare the
Aerospace Engineering
more expensive than conventional ceramics. Their performance of various composites and effects Department, Indian
use is hence, restricted to specialized applications. of different weave pattern, followed by a review Institute of Technology
Bombay, Mumbai 400076,
Another way attempted to improve the energy of the work done on hybrid composites and
Maharashtra, India.
absorption of ceramics was by using ceramic-metal 3-dimensional (3D) composites. [email protected]
Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in
Kiran Akella and Niranjan K. Naik
Subsequently, we briefly study two material Pressing can be uniaxial and isostatic. It can
combinations for armour: ceramic-metal and be at room temperature or under heated
ceramic-composites and their performance conditions.
comparison. We then discuss different armour
configurations based on the parameters affecting 2.2.1 Performance comparison of ceramics:
armour design, such as confinement, impedance The performance of different ceramics was
matching, tile edge geometry, ceramic/backing evaluated by multiple researchers and compared
thickness ratio, wrapping of tiles, shape of with the performance of RHA. Medvedoski32
ceramics, stacking sequence and coating ceramics reported that AlO ceramics are the most cost-
2 3
directly on the fabric. effective armour materials. Ceramics with varying
We then review different projectile shapes and AlO content from 97 to 99.6% by weight were
2 3
materials. Based on the aforementioned studies studied. Ballistic energy dissipated and hardness
and reviews, we present design considerations increased with the increase in alumina content.
for choice of ceramics, composites and ceramic- High level of bullet erosion was observed for
composite armour. The future directions for ceramics with higher hardness.
composite armour are discussed followed by the Ernst et al.9 compared the performance of four
concluding remarks. ceramics, namely, AlO, BC, SiC and TiB. They
2 3 4 2
compared volume gain and mass gain due to each
2 Armour Constituent Materials ceramics with RHA as the base. The volume gain
The constituent materials used for armour are V is calculated using the formula
g
discussed in this section. We briefly review metals.
P −P
We then present detailed review of ceramics, V = RHA res , (1)
g T
composites and their performance. cer
where P is the penetration of the projectile in
RHA
2.1 Metals plain RHA target, P the penetration in RHA when
res
Monolithic metal armour systems primarily ceramic tile of thickness T is placed before it. These
cer
comprise steel, aluminum and titanium alloys. parameters in Equation 1 are graphically illustrated
Meyers34 observes that steel has been the principal in Figure 1. Mass gain M was calculated using
g
armour material for heavy armour in tanks due
ρ
to its low cost, ease of fabrication and structural M =V RHA , (2)
g g
ρ
efficiency. Rolled homogenous armour (RHA) cer
is the standard armour material. The author where ρ is the density of RHA and ρ the
RHA cer
mentions high hardness steel and electroslag- density of ceramics.
refined steel as the more advanced materials. The authors reported a nominal volume
Aluminum alloys and titanium alloys are also used gain for AlO and BC. The highest volume gain
2 3 4
popularly for metallic armour. for TiB is ∼1.7. Therefore, volume gain due to
2
ceramics is not substantial. On the contrary,
2.2 Ceramics when mass gain is compared, ceramics are found
Ceramics were used for armour applications to be more attractive. The lowest mass gain for
during the late 1960s and early 1970s. Lanz28 alumina is two-fold, which is higher than the
mentions that early experiences on the behavior highest volume gain of 1.7 for TiB. For all other
2
of AlO ceramics subjected to ballistic impact ceramics, the mass gain is three-fold and BC is
2 3 4
were documented in 1979. Some of the widely most promising.
used ceramics for armour applications are AlO, James25 estimated the mass and thickness
2 3
AlN, BC, SiC, TiB and WC. required to defeat 14.5 mm, 30 mm fin stabilized
4 2
FSAPDS: These projectiles Armour ceramics can be classified on the armour piercing discarding sabot (FSAPDS) and
are supported inside the basis of density.17 Ceramics with density greater
gun barrel by a sabot. The
than RHA are high density ceramics. Those with
sabot is released once the
projectile is out of the barrel. density lower than RHA are low density ceramics.
These projectiles have fins to Typical high density ceramic is WC, whereas low
stabilize their flight.
density ceramics include AlO, AlN, BC, SiC and
2 3 4
TiB.
2
Another method to classify ceramics is based
on the manufacturing process.7 Some widely
Figure 1: Schematic illustration of the test
used processes for ceramic armour are sintered,
specimen after projectile penetration experiments.
pressed-sintered, reaction-bonded3 and gelcast.
298 Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in
Composite Armour—A Review
40 mm FSAPDS rounds. In this study, the cost of flow of fragmented material govern the penetration
ceramics is also included. The author reports that resistance.
the thickness and mass is the lowest with TiB.
2
Novel-AlO, reaction-bonded SiC (RB-SiC), BC, 2.2.2 Property enhancement of ceramics: Reaction bonded: Reaction
2 3 4
SiC and AlN also have mass very close to TiB. Properties of ceramics can be enhanced by mixing bonded silicon carbide is
2 manufactured by a chemical
However, the cost of TiB is much higher than all two or more ceramics (heterogenous ceramics)
2 reaction between porous
other ceramics. or making ceramic-composites. Galanov et al.13 carbon with molten silicon.
For easier comparison, we normalized the studied various ceramic composites such as BC-
4
volume gain and mass gain with RHA and TiB, BC-ZrB, BC-WB and BC-TiB-WB
2 4 2 4 2 5 4 2 2 5
compared between two different studies as listed systems. By altering the percentages of different
in Table 1. Both these studies show similar trends. phases, they observed a substantial increase of
TiB shows highest volume gain. TiB, BC and SiC mechanical properties; they also reported that the
2 2 4
have relatively similar mass gain. flexural strength of these composites increased
Peron48 studied high density ceramics for from 450 MPa of BC to 700 MPa.
4
armour applications. Reported results show AlO-TiB composites were studied by Adams
2 3 2
significant volume gain of more than 2-fold for et al.2 DOP studies showed that the improvement
high density ceramics (WC) than RHA, therefore, with composites is within the experimental
these ceramics are considered suitable where there scatter shown by commercial armour ceramics.
is a severe constraint of space. However, they show Medvedoski33 studied SiC-AlO-SiN
2 3 3 4
an increase in mass for certain types of WC (0.67) compositions and found them to have superior
and negligible mass gain (1.01) for a specific type ballistic performance. TiC-steel composites were
of WC. Gooch and Burkins17 also studied high made by Zaretsky et al.67.
density ceramics subjected to depth of penetration Strassburger and Lexow56 studied the effect of
(DOP) tests. They found a thickness dependency grain size on ballistic resistance using DOP tests.
for mass gain and volume gain. The gains increase The authors observed better performance of AP steel projectiles: They
as the thickness increases. submicron alumina in DOP than the commercially are designed to penetrate
armour. They are made
Orphal44 studied three ceramics—BC, SiC and available alumina. However, no significant
4 of high strength materials
AlN, in a confined state by performing difference in ballistic resistance was observed to withstand the shock of
reverse ballistic experiments with long rod when 14.5 armour piercing (AP) steel projectiles punching through armour.
penetrators. He reported that normalized were used against ceramic-aluminum targets.
penetration versus impact velocity curve is almost Reverse ballistic
independent of the material. Shockey and 2.2.3 Unconventional processing: Lillo et al.29 experiments: In these
experiments, a ceramic target
Marchand53 carried out impact experiments on studied pressureless sintered SiC and SiC whisker
is accelerated towards the
confined ceramics to study their failure reinforced SiC matrix composites for lightweight stationary projectile.
phenomenon. They observed that friction and armour applications. Pressureless SiC showed
similar ballistic limit as in conventional SiC, but at Pressureless sintering: It is
the sintering of a powder at
a reduced manufacturing cost. Similarly,
Table 1: Volume gain and mass gain reported by high temperatures without
pressureless densification of BC was studied by applied pressure. This avoids
two different studies. 4
Speyer and Lee.55 density variations in the final
Volume gain Mass gain Aghajanian et al.3 developed SiC and BC component which occurs with
4 more traditional hot pressing
Ernst Ernst reaction bonded ceramics. The authors could methods.
et al. James et al. James
match the performance of the same hot pressed
Ceramic (2001) (2001) (2001) (2001)
ceramics at lower processing costs. Whiskers: Whiskers are
RHA 1 1 1 1 filament-like crystalline
materials. They have
AlO sintered 1.01 0.7 2.14 1.67 2.3 Composites properties representing the
2 3
95% purity Composites are gaining in popularity due to their crystal anisotropy and an
AlO sintered – 0.78 – 1.67 high specific strength and stiffness. Researchers almost defect-free structure.
2 3 Their strength can be close
98% purity have extensively studied them for armour
to the theoretical ultimate
Novel-alumina – 0.88 – 2 applications. Long fiber reinforced composites are strength value of a given
sintered widely used for such applications. Different fiber material. As a result, whiskers
can be several dozen times
RB-SiC – 0.88 – 3.33 materials, matrix materials and prepregs have been
stronger than regular crystals.
TiB 1.67 1.75 2.96 3.33 studied. Woven and non-woven, 3D and hybrid
2
fabrics with different types of fibers or fabrics in a
BC 1 0.78 3.13 2.5
4 single laminate are some of architectures evaluated
SiC 1.25 1 3.05 2.5
by researchers. Such studies are reviewed in the
AlN – 1 – 2.5
subsequent subsections.
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Kiran Akella and Niranjan K. Naik
Table 2: Physical properties of fibers.
Density Initial modulus Tensile strength Elongation
Material (kg/m3) (GPa) (MPa) (%) Source
E-Glass 2540 72 3448 4 Gibson (1994)
Carbon (T300) 1760 231 3750 1.4 Gibson (1994)
Aramid 1440 71–97 2900–3350 3.6–4.4 Tam et al. (2006)
HMPE 970 113–124 3210–3610 3.6–4.4 Tam et al. (2006)
Prepreg: It is a term for 2.3.1 Fibers: The primary reinforcement 2.3.4 Prepregs: Bhatnagar et al.6 presented a
pre-impregnated composite materials for composites in armour are fibers. Tam discussion on the use of prepregs for ballistic
fibers where a matrix material
and Bhatnagar57 categorized the high performance composites. They highlighted the increasing use
is already present. This matrix
material includes curing fibers as: (i) classical (glass, carbon); (ii) rigid chain of ballistic prepegs. The authors pointed that
agents. Prepregs can be aromatic (nylon, aramid); (iii) high temperature ballistic prepregs are different from structural
placed on the mould without
(Poly-benzimidazole—PBI, Polyphenylene- prepregs since they are resin starved, the resin
the addition of any more
resin. Subsequently, they are benzobisozazole—PBO); and (iv) thermoplastic content is only 10–20% unlike structural prepregs
heated under pressure for (liquid crystal fiber, high modulus polyethylene— that have are 40–50%. These prepregs are not
complete curing.
HMPE). Comparative physical properties of some tacky and are in ‘A’ stage. Structural prepregs are
Thermoset and of these fibers are listed in Table 2. ‘B’ staged. Both thermoset and thermoplastic
thermoplastic: The primary Three types of glass fibers were reported in resins are used for ballistic prepregs.
physical difference is
literature—Eglass, Rglass and Sglass. Carbon
that thermoplastics can
be remelted back into a fibers can be classified as poly-acrylo-nitrile- 2.3.5 Response of composites subjected to
liquid, whereas thermoset based (PAN) or Pitch-based depending on the ballistic impact: Pandya et al.47 gave a detailed
plastics always remain in a
precursor used. Nylon, nomex and aramid (Trade description of the different stages of penetration
permanent solid state.
names: Kevlar®, Twaron®) are derived from and perforation of a rigid cylindrical projectile
aromatic acids and amines. These are synthetic with a flat end into a 2D woven composite target.
organic fibers known for their high performance. These stages are presented schematically in
PBI and PBO (commercially available as Zylon® Figure 2. When the projectile strikes onto a
from Toyobo™) are high strength fibers with composite target, the planar view can be sub-
excellent stability at higher temperatures. High divided into two regions as shown in Figure 3. The
temperature melting and spinning is used to make region directly below the projectile is referred to as
thermoplastic fibers such as liquid crystal fibers Region 1. The surrounding region up to which the
(commercially available as Vectran®) and HMPE transverse stress wave travels along the in-plane
Yarn: It is a long continuous (commercially available as Spectra®, Dyneema®). directions is referred to as Region 2. The yarns
length of interlocked fibres. that are in contact with the projectile during the
There are two main types of
2.3.2 Fabrics: Fabrics can be woven or non- ballistic impact event are referred to as primary
yarn: spun and filament.
woven. They can be as 2-dimensional (2D) lamina yarns; the primary yarns are along the warp and
or with 3D architecture. Song54 listed the types fill directions. The remaining yarns within the
of 2D fabrics as plain weave, basket weave, twill surrounding region up to which the transverse
weave, crowfoot weave and satin weave. The author stress wave travels along the in-plane directions
specifies that the commonly used fabric structures are referred to as secondary yarns. It may be noted
for ballistic applications are plain weave, basket that only primary yarns are present in Region 1,
weave and unidirectional. Thomas59 identified whereas both primary and secondary yarns are
different types of non-woven fabrics. They are present in Region 2.
parallel filaments with resin reinforcement, stitch Figure 2(a) indicates beginning of the ballistic
bonded, cross-lapped and needle punched. impact event. The impact event can be sub-divided
into three stages. During Stage 1, the compressive
Waves: When ballistic impact 2.3.3 Matrix materials: Hannibal and Weir20 and shear stress waves travel along the thickness
occurs on a target, different mention that matrix resins are either thermoset or direction. The layers of the composite target
types of stress waves are
thermoplastic. Song54 reports that the first matrix undergo compression directly below the projectile
generated. From the point
of impact, they propagate material system qualified for ballistic protective body and also in the surrounding region as shown in
outward. They may be armours was phenolic resin. The authors also report Figure 2(b). The compression of layers in Region 2
reflected at interfaces and
the use of vinyl ester and thermoplastic polyurethane. is because of transverse shear wave propagating in
boundaries are generate a
complex stress patterns Blends of these as well as other resin systems such as the in-plane directions. Compression of layers
in the material. epoxies, polyethylene etc. are also widely used. also produces tension along the in-plane direction
300 Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in
Composite Armour—A Review
the layers could fail under compression, tension,
or shear plugging whenever the induced strains Shear plugging: This type
exceed the corresponding failure strains. of failure occurs when a
projectile punches through
Stage 2 starts when the shear wave reaches the
the material. The residue of
back face of the target (Figure 2(c)). Depending the material comes out along
on the number of layers failed in Stage 1 and the with the projectile is called
the plug.
kinetic energy available with the projectile, conical
deformation of the target could take place on the
back face as shown in Figure 2(d). Thickness of
the laminate and the incident impact velocity
of the projectile influence the number of layers
failed and the kinetic energy available with the
projectile at the end of Stage 1. The layers that
do not fail in Stage 1 undergo tension as a result
of conical deformation, and could fail when the
induced tensile strain exceeds the failure strain
(Figure 2(e)). Stage 2 ends when the material is
completely failed either by shear plugging or by
tension. Even after the complete failure of the
target, there can be friction between the target
and the moving projectile. This stage is referred to
as Stage 3. Some energy can be absorbed because
of friction. Stage 3 ends when the projectile tip
reaches the back face of the target as shown in
Figure 2(f). At this stage, if the projectile is having
some residual kinetic energy, it would exit from
the target with a certain residual velocity.
Pandya et al.47 identified that during the
ballistic impact event, energy lost by the projectile
is absorbed by the target through various damage
Figure 2: Penetration and perforation stages of and energy absorbing mechanisms such as
2D woven fabric composite target during ballistic compression of the target directly below the
impact (Pandya et al., 2014). projectile, compression in the region surrounding
the impacted zone, shear plugging, stretching and
tensile failure of yarns/layers in the region
consisting of primary yarns, tensile deformation
of yarns/layers in the region consisting of
secondary yarns, conical deformation on the back
face of the target, delamination, matrix cracking, Delamination: In laminated
and friction between the projectile and the target. materials, repeated cyclic
stresses or impact can cause
The authors studied the effect of target
layers to separate. This
thickness on ballistic limit and found that ballistic results in a significant loss of
limit increased as the target thickness increased. mechanical toughness.
The energy absorbing mechanisms in two types of
targets, one 6 mm and another 20 mm thick were
studied. The major energy absorbing mechanisms
for target thickness of 6 mm are shear plugging,
stretching and tensile failure in the region consisting
of primary yarns, tensile deformation of yarns/layers
in the region consisting of secondary yarns, and
Figure 3: Schematic arrangement of a typical
energy carried by the moving cone. Energy absorbed
2D woven fabric composite target during ballistic
impact: front view (Pandya et al., 2014). by the other mechanisms is not significant.
The major energy absorbing mechanisms for
in the surrounding region. The shear wave follows target thickness of 20 mm are shear plugging,
the compressive wave. As the compressive and stretching and tensile failure in the region
shear waves travel along the thickness direction, consisting of primary yarns, tensile deformation of
Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in 301
Kiran Akella and Niranjan K. Naik
yarns/layers in the region consisting of secondary epoxy. Consequently, glass-epoxy laminates absorb
yarns, and friction between the projectile and the higher energy than carbon-epoxy, and therefore
target. Energy absorbed by the other mechanisms show a higher ballistic limit velocity.
is not significant. It may be noted that, in this case,
Hybrid composites: It consist energy carried by the moving cone is negligible. 2.3.7 Hybrid composites: Ellis et al.8 studied
of two or more types of fibers The strain rate during ballistic impact event the ballistic impact resistance of Spectra™ hybrid
in a composite part.
was also reported by Pandya et al.47 The authors graphite composites. They observed a significant
observed that in compression, the strain rate was increase in energy absorption on adding Spectra™
high initially. During the event, strain rate of to the back face of the graphite composite. Muhi
4211/s was reported. A peak tensile strain rate of et al.38 compared E-glass fiber reinforced plastics
3141/s was also reported by the authors. and hybrid composites consisting of E-glass and
Experimental studies on ballistic impact Kevlar™-29 fabrics. They observed that penetration
behavior of composites were carried out by several resistance was enhanced by the addition of
groups19,26,36,52,65,68–70. Hazell and Appleby-Thomas21 Kevlar™-29 layers to E-glass layers. Hazell and
and Kasano27 (1999) presented reviews of ballistic Appleby-Thomas22 found significant improvement
impact behavior of composites. in ballistic performance of CFRP-based structures
Pandya et al.46 studied the effect of projectile upon addition of Kevlar™-29 layers to carbon
mass on ballistic limit of glass-epoxy composites. layers.
They found that ballistic limit reduced as Pandya et al.46 studied hybrids of Eglass-epoxy
the projectile mass increased. Gellert et al.14 laminated and carbon-epoxy laminates. Ballistic
conducted tests using hard-steel cylinders of two limit was highest for Eglass-epoxy laminates and
diameters and three nose shapes against glass- least for carbon-epoxy laminates; it was in between
fibre-reinforced (GRP) plastic composite plates for hybrid composites. The authors reported that
of various thicknesses. Bi-linear pattern of energy targets with Eglass layers in the exterior and carbon
absorption was observed. layers in the interior showed higher ballistic limit
Shahkarami et al.51 observe that materials with velocity than placing carbon layers in the exterior
high specific energy absorption characteristics and Eglass layers in the interior.
such as high strength, rupture strain and low
3D composites: They are density are considered ideal. They also observed 2.3.8 3D composites: Flanagan et al.10 studied
made from yarns or tows that transverse properties of the yarns affect the penetration resistance and failure modes of 3D
arranged into complex 3D
interaction between projectile and target. The textile composites under high velocity impact
structures. A resin is applied
to the 3D preform to create authors also presented that an optimally twisted experimentally. They used both 3D woven and
the composite material. yarn structure will maximize strength. The braided composites for their studies. The materials
sensitivity to strain-rate and temperature was used were Spectra, Kevlar and Twaron. They
highlighted. The efforts focused on frictional observed that 3D textile composites have higher
properties of yarns to the behavior in impact zone, penetration resistance. They also observed that
and the global energy absorption mechanisms there was only limited growth of delamination
were also highlighted. because of the reinforcement in through-the-
In addition, the authors highlighted the thickness direction.
effect of fiber configuration. 3-dimensional Udatha et al.60 compared 3D woven composites
weaves were reported to have higher interlaminar with 2D plain weave composites of Eglass-epoxy.
fracture toughness and high damage tolerance The authors reported that limit velocity for
than 2-dimensional weaves. Further, it was noted complete penetration for 3D orthogonal woven
that weaving process degrades the yarns. They composite is higher than that for 2D plain weave
also presented the observations that brittle resin composite.
systems experience instantaneous delamination,
whereas tough systems experience steady and 3 Material Combinations for Armour
controlled delamination growth. Two different material combinations are reviewed,
ceramic-metal and ceramic-composite. The details
2.3.6 Performance comparison: Pandya et al.46 are included in the subsequent sub-sections.
compared the ballistic impact performance of
glass-epoxy versus carbon-epoxy laminates of the 3.1 Ceramic-metal
same thickness. They observed that the ballistic Initially, monolithic or layers of metal sheets
limit velocity (V50) of carbon-epoxy laminates was formed armour. Researchers observed that
17% lower than glass-epoxy. The authors reported ceramics have lower density, high stiffness and high
larger extent of damage in glass-epoxy than carbon- compressive strength. But they are weak in tension.
302 Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in
Composite Armour—A Review
Hence, a metal plate with a ceramic face plate was in Figure 3); compression in the surrounding
studied to arrive at a design of armour lighter region referred as Region 2 (see Figure 3);
than monolithic metals. Such targets observed formation of ring cracks and radial cracks leading
substantial weight savings. Wilkins64 presented to tensile failure; shear plugging; pulverization;
studies on metal and ceramic-metal targets. and heat generation.
AD85™ alumina was bonded to aluminum alloy The major damage and energy-absorbing
6061-T6. The effect of alumina and aluminum mechanisms in the composite backing plate
thickness on ballistic limit was studied. A multi- are—compression of the target directly below
linear response was observed. Increase in ceramic the projectile referred as Region 1 (see Figure 3);
thickness caused a higher increase in ballistic compression in the surrounding region referred
limit than increase in aluminum thickness. as Region 2 (see Figure 3); tension in the yarns;
Futhermore, the effect of projectile shape was also shear plugging; delamination and matrix
evaluated. Blunt projectiles caused more damage cracking; bulge formation on the back face;
and penetration than sharp projectiles. friction between the target and the projectile;
Mayseless35 presented the experimental results and, heat generation.
for ceramic-metal targets. They found that on At higher incident impact velocities, erosion of
the basis of areal density, metal plates prefaced by the tip of the projectile would take place. As the
ceramic plates are ballistically inefficient in the velocity of the projectile decreases, deformation
low velocity range, while the reverse was found of the projectile would take place. Erosion and
at speeds higher than 250 m/s. They also found deformation of the projectile would absorb some
that the energy required to erode the projectile energy during ballistic impact event. This would
was several orders of magnitude more than that lead to reduction in velocity and kinetic energy of
consumed in the process of fracture of the ceramic the projectile.
plate. Figure 5 shows different damage and energy
absorption mechanisms in ceramic-composite
3.2 Ceramic-composite armors during ballistic impact event. The figures
Typical ceramic-composite armor is shown in show a cylindrical projectile. When the projectile
Figure 4. Naik et al.39 have identified that the strikes the target, the target would offer resistance
major energy absorption is provided by the for the penetration of the projectile into the target.
ceramic. In an ideal situation, the damage should As shown in Figure 5, the material directly below the
not spread to the composite backing plate. This projectile is called Region 1, while the surrounding
is because the composite plate should be able to material offering the resistance for penetration is
provide the structural requirements, and act as a called Region 2. As the projectile strikes the target,
load carrying element during post impact period longitudinal and shear stress waves are generated
even after damages have taken place in ceramic. and travel along all the directions.
The composite plate can also absorb energy, and Only that part of the target up to which these
in such cases, it can be damaged. The rubber layer waves have reached would offer the resistance for
in between the ceramic and the composite delays penetration. The remaining portion of the target
the penetration process, Also it prevents merging does not sense the applied impact load. As the
of damages within the ceramic and the composite. time progresses, the stress wave would propagate
The front composite cover layer prevents the further and larger part of the target would offer
ceramic from micro damages. resistance.
The major damage and energy-absorbing As the projectile strikes the target, the material
mechanisms in ceramic layer are—compression of directly below the projectile would be under
the target directly below the projectile, referred as compression. The material in the surrounding
Region 1 (schematically illustrated for composites region is also under compression along thickness
direction. If the induced compressive stress
exceeds the permissible limit, compressive
failure of ceramic would take place. The induced
compressive stress is calculated based on the
deformation in ceramic and the distance up to
which, through the thickness, the longitudinal
stress wave has traveled.
Additionally, because of the impact force
generated, shear stresses are also generated within
Figure 4: Typical ceramic-composite armour.
the target around the periphery of the projectile.
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Kiran Akella and Niranjan K. Naik
Figure 5: Different stages of penetration of a cylindrical projectile on ceramic-composite armour (Naik
et al., 2012).
The resistance for the shear failure would be As the projectile strikes the ceramic, micro
offered only by that part of the target up to which cracks would be formed in the ceramic. As the
the shear wave has reached. ballistic impact event progresses, the micro cracks
As the compression of the ceramic takes place could become macro cracks. During this phase
during the ballistic impact event directly below the additional micro cracks would be formed. In other
target, the ceramic along radial direction would be words, the ceramic would be broken into granules
under tension. This can lead to the formation of and later into powder. The compressive strength
ring cracks and radial cracks. of the ceramic powder is significantly higher
304 Journal of the Indian Institute of Science VOL 95:3 Jul.–Sep. 2015 journal.iisc.ernet.in
Composite Armour—A Review
than that of the ceramic plate. This indicates that Figure 5(b). The distances traveled by different
as the penetration progresses, the compressive waves are also indicated in the figure. Penetration
resistance offered by the ceramic/ceramic powder of the projectile is shown in Figures 5(c) to (e).
would increase. The process of formation of fine Since compressive waves reach the back face of the
powdery particles from the ceramic plate is called composite backing plate through-the-thickness,
pulverization12. bulging of the composite takes place (Figure 5(f)).
Different damage mechanisms such as Plug formation can be seen in Figure 5(g). The
compression, shear plugging, tension and projectile and the plug start moving further
pulverization would lead to energy absorption. As as shown in Figure 5(h). Figure 5(i) shows the
the energy is absorbed by the target, kinetic energy projectile and the plug just exiting from the back
of the projectile would decrease accordingly. This face of the composite backing plate.
would lead to reduction in velocity of the projectile. Hetherington and Rajagopalan23 carried out
As the projectile strikes the target, resistance experiments on ceramic-composite targets with
would be offered by the target for penetration different target thicknesses. For the same incident
of the projectile onto the target. If the resistance ballistic impact parameters, they measured
offered by the target is more, erosion of the residual velocities of the projectile as a function
projectile could take place. This process also of target thickness. Navarro et al.40 carried out
would absorb some kinetic energy of the experimental studies on ballistic impact behavior
projectile leading to reduction in the velocity of of ceramic-composite armors.
the projectile. As the velocity decreases, erosion Shahkarami et al.51 reviewed the effect of
would stop and deformation of the projectile in-plane dimensions and thickness of target. As the
would take place. impact velocity increases, the effect of boundaries
As the ballistic impact event progresses, diminishes. They reviewed the transition
through the thickness, normal and shear mechanisms for thinner targets and thicker targets.
stress waves would enter into rubber layer and Thinner targets show dishing as a favourable
composite backing plate. With this, the composite penetration mechanism. For thicker targets, plug
backing plate as well as the rubber layer would formation is a favourable penetration mechanism.
offer resistance to penetration and perforation.
The energy-absorbing mechanisms in the rubber 3.3 Performance comparison of different
layer are compression and shear plugging. As the types of armour
through-the-thickness longitudinal stress wave Adams1 developed a ballistic performance map for
enters into the composite, compression would evaluating the performance of ceramic armour; the
take place in Region 1 as well as in Region 2 of author used a 3-axes plot. Two axes at the base were
the composite. Because of the impact force, shear areal densities ceramic and backing in the target,
stresses are generated in the composite around the vertical axis represented the impact velocity.
the periphery of the projectile. The yarns under A ballistic limit surface was plotted using on the
the projectile would be in tension. If the induced ballistic limit values, while the stochastic behavior is
compressive stress, shear stress or tensile stress visualized as the thickness of the surface. Protection
exceed the permissible limit, failure of composite areal density can be estimated using these plots.
would take place. Delamination, matrix cracking
and bulge formation on the back face of the 4 Armour Configurations
composite leading to possible tensile failure of the Composite armour design involves various
yarns are the other damage and energy-absorbing parameters that contribute to different
mechanisms. During the later part of the ballistic configurations, such as ceramic/backing thickness,
impact event, the velocity of the projectile is very shape of ceramics, geometry of the ceramic
low. During this period, frictional energy would tile edges, confinement, tile wrapping, stacking
be absorbed by the target. All these damage and sequence, impedance matching layers and ceramic
energy-absorbing mechanisms would absorb coated fabrics. The studies conducted to evaluate
some energy leading to further decrease in the the effect of each of these configurational changes
kinetic energy of the projectile. This would lead to are reviewed in the subsequent subsections.
further decrease in the velocity of the projectile.
Different stages of penetration of a cylindrical 4.1 Optimum ceramic/backing ratio
projectile on to the armor are shown in Figure 5. James25 presented a study on optimal ceramic/
Figure 5(a) shows the initial position when the metal thickness ratio. For normal impact of
projectile just hits the armor. Possible erosion 7.62 mm AP rounds, he suggested an optimal
and deformation of the projectile are shown in ratio of 1.28. He included an empirical model
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Kiran Akella and Niranjan K. Naik
to estimate the optimum backing for alumina/ investigated are shown in Figure 6. The author
aluminum armour systems using the relation, showed that an 8.5 mm tile with vertical edges
subjected to impact at joint between two tiles
T v
cer = (90−α), (3) absorbs energy equivalent to a 6 mm tile subjected
T 60000
met to impact at the center. Tiles with 45° inclined edge
showed no degradation at the edges whereas all
where T is the thickness of the ceramic layer, T other configurations illustrated in Figure 6 showed
cer met
the thickness of the metallic layer, v the incident substantial degradation at the edge.
velocity and α the angle of obliquity with the
normal to the armour surface. The results were in 4.4 Effect of confinement
agreement with experiments. This model quantified Ernst et al.9 studied the effect of confinement. Glass
optimal ratio and its dependence on obliquity. It and alumina samples were tested and depth of
was found to be in agreement with experiments. penetration was measured. Unconfined specimens
showed that penetration reduced with higher
4.2 Shape of ceramics lateral dimensions of the specimen. Confined
Salame and Quefelec49 categorized different shapes specimen showed much lower penetration than
of ceramics as flat tiles and shaped ceramics. Flat unconfined specimen. The penetration of confined
tiles could be of square or hexagonal shape. Shaped ceramics could be matched by a three-fold increase
ceramic tiles can be in ball or cylinder form. in lateral dimensions of unconfined ceramics.
Different multi-curved forms are also being used.
4.5 Effect of wrapping
4.3 Effect of tile edge geometry Nemat-Nasser et al.42 studied the effect of
James25 studied the effect of tile edge geometry wrapping ceramic tiles with a thin membrane on
on energy absorption. Different edge profiles armour performance. Four wrapping materials
Figure 6: Edge configurations studied by James (2001).
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