Technology Brief
In Pursuit of the
Ultimate Body Armor
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About the Cover:
A soldier wearing protective vest made from
carbon nanotube (CNT) based fibers.
Body armor incorporated with CNT fibers has
the ability to protect a person from the most
aggressive ballistic threats of the future.
Image Courtesy:
Adopted from U.S. Navy photo by
John F. Williams
In Pursuit of the Ultimate
Body Armor
With the ever-increasing multiple threats like traditional
warfare, counterinsurgency and terrorism in border
areas/urban centres/airports/public places etc.
looming, there is an urgent need for providing reliable
armor protection to personnel from the armed forces
and law enforcement agencies. While today’s new
generation bulletproof vests can provide protection
from most low- and medium-energy handgun
threats, the current and future combat scenarios
demand protection against more lethal ammunition,
multiple bullet strikes, IEDs, small fragments from
explosives such as hand grenades etc. In recognition
of this, worldwide efforts are being directed towards
development of improved lightweight body armor.
For this purpose, high performance fibers are being
increasingly exploited as soft, flexible fiber mats for
the manufacture of body armor or bulletproof vests
or as reinforcements in the form of polymer-matrixcomposites (PMCs).
High performance fibers and yarns commonly
used in practice today for ballistic protection are
S-glass, aramids (e.g., Kevlar 29, Kevlar 49, Kevlar
129, Kevlar KM2, Twaron), highly oriented ultra high
molecular weight polyethylene (e.g., Dyneema,
Spectra), PBO (e.g., Zylon) which is a p-phenylene2-6-benzobisoxazole, new polymeric fibers such as
Polypyridobisimidazole (PIPD) (referred to as M5) etc1,2.
These fibers are characterized by low density, high
tensile and compressive strength, high modulus, high
rupture strain, resistance to thermal degradation and
high-energy absorption capacity.
Currently, the bullet-proof vests (soft or flexible
composite armor used for body protection) are
basically made from high stiffness and toughness,
woven or laminated, polymeric fibers stacked in a
number of layers3. Upon impact of the striking bullet,
the fabric material absorbs the energy by stretching
of the fibers and the stiff fibers ensure that the load is
dispersed over a large area throughout the material.
This process slows down the bullet and ultimately
stops it from penetrating the body. In case of polymer
matrix composites (PMCs), the ability of a fiber to
deform is severely restricted due to the presence of
surrounding resin, and therefore, the energy absorption
capacity is reduced. The main failure mechanisms in
PMCs under ballistic impact are straining of fiber and
its fracture, delamination and shear deformation in the
resin matrix.
To provide greater protection against blunt trauma and
higher velocity ammunition than can be provided by
a stand-alone soft ballistic vest, hard body armor has
been developed. It includes a rigid facing comprising
ceramic inserts, steel or titanium panels and a ballistic
fabric backing. In hard armor with ceramic inserts,
the kinetic energy of the projectile is absorbed and
dissipated in localized shattering of this ceramic tile
and blunting of the bullet material during its impact on
the hard ceramic.
Energy Absorption Mechanisms in Body
Armor
Modern body armors provide protection by mitigating
projectile energy in different ways. When a projectile
strikes on woven fiber mats and fiber reinforced
PMCs, it just bounces off or is deflected, provided the
fibers are capable of storing a large amount of elastic
energy. Fibers possessing high tensile strengths and
high failure strains can absorb considerable amounts
of energy via plastic deformation and stretching the
energy absorbed by the fiber at the point of fracture is
given by the equation 11:
where eF is the fracture strain, r the density of fiber and
sF the fracture stress of the fiber
The ballistic performance of a material depends on its
capability to absorb the energy, in an efficient manner,
across the network of fibers. When the projectile strikes
a vest, it is caught in a web of very strong fibers.
These fibers absorb and disperse the impact energy
that is transmitted to the vest from the bullet, causing
the bullet to deform or to “mushroom”. The ability to
spread out energy is dependent on the sonic velocity,
which in turn, is related to the fiber’s elastic modulus as
shown in equation (2) below1, 2:
where VS is the sonic velocity and E the elastic modulus
The higher the strain wave velocity (sonic velocity), the
faster is the distribution of energy over a wide area of
the material, and therefore, more material gets involved
during the impact process.
1
Table 1: Mechanical Properties of High Performance Polymeric Fibers
Specific
Strain to
Modulus
Energy
Fracture
(E),
Absorption
e,
(GPa)
Capacity
(%)
(m2/s2)
Sonic
Velocity
VS,
(m/s)
Ref.
57017
1622
(4)
1.8-5.4
48600
5865
(5)
6-11
6-7
15806
2663
(4)
0.5
5
15
28409
1946
(6)
1320
1.3
22
40
196969
4082
(7)
M 5 Fiber (Goal)
1700
9.5
450
2.0-2.5
69852
16269
(8)
M 5 Fiber (Conservative)
1700
8.5
300
2.5
62500
13284
(8)
Zylon HM
1560
5.8
270
2.5
46474
13155
(9)
Spectra 1000
970
2.57
120
3.5
46365
11122
(10)
Dyneema SK 76
970
3.6
116
3.8
70515
10935
(11)
Kevlar 29 (1500 denier)
1440
2.90
74.4
3.38
34034
7187
(10)
Kevlar 49 (1140 denier)
1440
3.04
120
2.3
24277
9128
(10)
Kevlar 129 (840 denier)
1440
3.24
99.1
3.25
36562
8295
(10)
Kevlar KM2 (850 denier)
1440
3.34
73.7
3.80
44���
069
7154
(10)
T–1000 (Toray)
1820
7.06
294
2.4
46549
12709
(12)
P–120
2190
2.24
827
0.2
1022
19432
(13)
SWCNT-a
1400
13
1000
16
742857
26726
(14)
SWCNT-b
1400
53
1000
16
3028571
26726
(14)
Density
(r),
(kg/m3)
Strength
(s),
(GPa)
Nylon 6
1140
0.5
3.0
18-26
S2 Glass
2500
4.5
86
Cotton
1550
0.3-0.7
Silkworm Silk (Bombyx mori cocoons)
1320
Spider Silk (Dragline of Nephila)
Fiber
The mechanical properties of various high performance
polymeric fibers relevant to ballistic armor applications
are summarized in Table 1. The ballistic potential of
some promising fibers is shown in Fig. 1, wherein the
sonic velocity is plotted against the energy absorption
capability (toughness) of several polymeric fibers.
M5, which is a new high performance fiber based on
polypyridobisimidazole (PIPD), exhibits the highest
specific energy absorption capacity and sonic
velocity among the all high performance fibers and,
therefore, shows great promise for armor applications.
Armor panels based on M5 fibers have also shown
exceptional performance during actual ballistic testing
trials8. The superior ballistic properties of M5 fibers
could be attributed to the unique hydrogen bonded
network in lateral dimensions and its peculiar fracture
mode. In Fig. 1, Zylon, Dyneema, Spectra 1000, M5
(conservative) and T1000 fibers are shown in the
group depicted as armor grade fibers, while all aramid
fibers are grouped under the Kevlar family. P 120,
a ultra-high modulus carbon fiber, has the highest
strain wave velocity of 19432.59 m/s among the all
fibers. However, it is of interest to note that the P 120
carbon fiber is not a preferred material for ballistic
applications, because it exhibits a very low specific
2
energy absorption capacity (~1022.83 J/kg). On the
other hand, Spider silk fiber shows outstanding energy
absorption capacity (Table 1 and Fig. 1), which is a key
fiber property controlling the ballistic performance, but
also shows very high elongation of about 160%, which
may not be desirable because the large deformations
associated with low modulus can result in extreme
blunt trauma. Therefore, an ideal fiber should also
have an adequate stiffness along with high strain to
failure to prevent blunt trauma. One can achieve high
ballistic penetration resistance and an acceptable blunt
trauma by proper designing and optimization of the
Fig. 1: Specific energy absorption capacity as a function of
sonic velocity for selected high performance fibers
In Pursuit of the Ultimate Body Armor
vest. For example, combinations of woven fabric-UD
“hybrid” vest designs can provide adequate ballistic
protection with acceptable blunt trauma. For the
sake of comparison, Fig. 1 also includes the data for
pristine single wall carbon nanotubes (SWCNTs), which
show extraordinary promise as next generation armor
material.
Cunniff and Auerbach16 have developed a criterion
using a dimensionless analysis model for assessing
the ballistic figure of merit of fibers by combining the
energy storage capacity of the fiber per unit mass
(sF eF/2r, fiber specific toughness) and strain wave
velocity (
) which is expressed as
another fascinating armor material, which has evoked
great interest among the military community because
of its outstanding mechanical properties and energy
absorption characteristics. Nexia Biotechnologies Inc.,
Canada and the US Army Research Laboratory have
demonstrated for the first time in history that man has
found a way to replicate the manufacturing process
of spider silk filaments which, as shown in Fig. 2,
show significant potential for armor application. Other
potential candidates for armor applications belong
to the Dyneema, Zylon, Spectra and Kevlar family.
The ballistic figure of merit for pristine SWCNT is also
plotted in Fig. 2, which once again shows its immense
potential as a future armor material.
Potential of CNTs for Ballistic Armor
(U*)1/3 is a theoretical parameter that estimates the
maximum velocity of a bullet that the fibers of a vest
can stop and is independent of vest construction. They
have further shown that the V50 velocity (the V50 ballistic
limit is the velocity required for a bullet to penetrate
or perforate the armor 50 % of the time) of an armor
system scales with (U*)1/3 for a fixed target density, and
a fixed projectile, the ratio of V50 ballistic limits for two
armor systems may be computed by the ratio of the
(U*)1/3 numbers.
The theoretical ballistic figure of merit (U*)1/3 has been
calculated for several fiber materials based on the data
given in Table 1 and the results are plotted in Fig. 2.
As can be seen, the M5 fiber-based armor shows the
highest ballistic impact potential. It was estimated by
Cunniff and Auerbach8 that fragmentation protective
armor based on M5 has the potential to reduce the
weight by approximately 40-60% over Kevlar KM2
fabric at the same level of protection. Spider silk is
Carbon nanotube is an ideal candidate material for
bulletproof vests due to its unique combination of
exceptionally high elastic modulus and high yield
strain. A Young’s modulus of about 1000 GPa, strength
ranging between 13-53 GPa, and strain at tensile
failure predicted to be as high as ~16% typically
characterize SWCNTs14. Assuming that the specific
gravity of SWCNT is about 1.4 g/cm3, one can estimate
the ballistic performance parameter, (U*)1/3, to range
between 2708 m/s and 4326 m/s. These values are in
agreement with the previously reported value of 3000
m/s by Alan Windle17 for the ballistic performance
parameter of carbon nanotubes. If one compares
these values with those given in Fig. 2, the enormous
potential of CNTs as a candidate material for
bullet-proof armor system is quite evident.
There are three different approaches for utilizing carbon
nanotubes to enhance the ballistic performance of a
body armor. These are:
1) Incorporation of CNTs into
PMCs, metals or ceramics
to enhance their hardness
or toughness and erosion
resistance
2) Use of neat or composite
fibers of CNTs in the form of
woven or non-woven fabric, for
achieving exceptional ballistic
performance
3) Reinforcing the armor grade
fibers like Kevlar, UHMWPE
or PBO with CNTs to improve
their elastic modulus
and energy absorption
capacity. These methods are
schematically shown in Fig. 3.
Fig. 2: Ballistic figure of merit, (U*)1/3 (m/s) for various fibers suitable for ballistic
applications
Carbon nanotubes possess very
high hardness. In fact, superhard
3
Fig. 3: Methods of employing CNTs for ballistic armor applications
materials synthesized by compressing SWCNTs at 24
GPa exhibit hardness of up to 152 GPa, which is even
greater than that of a diamond sample14. Therefore,
incorporation of CNTs as one of the components in a
polymer matrix composite armor tile is likely to deform/
erode/ fracture the projectile when it is attacked,
because of its extreme hardness18.
Carbon nanotubes, due to their unique combination
of high elastic modulus and high strain to failure are
capable of elastically storing an extreme amount of
energy, which can cause the bullet to bounce off or
be deflected. This attribute of carbon nanotubes can
also provide the armor improved protection against
blunt trauma effects19. Based on their computational
modeling studies, Mylvaganam and Zhang19 have
shown that body armor comprising six layers of carbon
nanotube yarns, each of 100 μm thickness, would have
the capability of bouncing off a bullet with a muzzle
energy of 320 J.
The above-mentioned characteristic of carbon
nanotubes has been practically utilized by Block
Textiles, Inc., USA. The
company has developed
a light weight impact
deflecting bullet-proof vest
comprising directionally
aligned single-walled carbon
nanotubes in the matrix of
an epoxy resin, that is near
impervious to bullets fired at
close range at all angles of
incidence. Moreover, it also
exhibits improved impact
puncture and penetration
resistance, which provides
the wearer of the vest
enhanced protection against
blunt trauma effects20.
The above armor tiles can be fabricated by curing a
mixture of carbon nanotubes in an epoxy resin under a
controlled temperature and humidity environment and
applying an electric field of sufficient strength to align
the SWCNTs. The typical surface topography of the
armor tile is shown in the following SEM micrograph
(Fig. 4), wherein the rope-like structure formed by the
unidirectionally aligned single walled carbon nanotubes
is quite evident.
Researchers from Lockheed Martin Corp., have
developed a hybrid composite containing fibrous
reinforcement, wherein the polymer matrix is enhanced
by the additions of either SWCNTs or MWCNTs (or
combination of both)21. The incorporation of the CNTs
in the PMC based armor results in improved ballistic
properties and is reflected in significant reduction in
the projectile velocity as determined by the V50 ballistic
test (Table 2). The above ballistic material developed
is promising for applications in personal body armor,
aircrafts, ships, and armored vehicles.
Table 2: Comparison of reduction of projectile velocity
in Kevlar-based armors with and without CNTs
Strike
Face
Average
Velocity
Reduction
(m/s)
Average
Areal
Weight
(kg)
Kevlar without
CNTs
Alumina
361
3.22
Kevlar with CNTs
Alumina
495
3.37
Armor Type
Note: MIL-STD-662 2007 (7.62 APM round at
~ 883.92 m/s)
Fig. 4: SEM micrograph (magnified 1500 times at a distance
30 μm) showing the surface topography of nano-enhanced
impact deflecting material20
4
Nanocomp Technologies Inc., Concord, N.H., USA has
been working with US Army’s Natick Soldier Center
to develop a new generation lightweight, body armor
based on their CNT technology (www.nanocomptech.
com). Their proprietary CVD process is capable of
In Pursuit of the Ultimate Body Armor
producing large quantities of one-millimeter long
CNTs, and CNT-based yarns, sheets and rolls. In April
2009, the company demonstrated that their ~ 5 mm
thick CNT-composite panels can stop a 9 mm bullet in
controlled ballistic testing. Their body armor technology
has now matured beyond early stage of development
and has made considerable progress towards its
commercialization.
investigations to determine the ballistic performance
of CNT fiber reinforced 7017 aluminium alloy25. Their
numerical model analysed the impact of a sharp
nosed projectile on the metal matrix composite plate
by performing computer simulations employing
finite element methods and clearly showed that the
CNT fiber reinforcement plays an important role in
determining the overall ballistic resistance of the
composite plate.
Currently, the hard body armor incorporates a
ceramic tile strike face for providing superior ballistic
performance while being lightweight. Alumina, silicon
carbide and boron carbide are some of the candidate
ceramic materials commonly used in body armor.
Although these ceramics are very hard, they are also
quite brittle and, therefore, hardly able to survive one or
two shots before catastrophic fracture of the ceramic
tile leading to collateral damage. Consequently,
there is a need to improve their multi-hit capability.
This could be achieved by enhancing their fracture
toughness. Carbon nanotubes are being considered
as a reinforcing material to enhance the mechanical
properties of ceramics, particularly by fracture
toughness, which is likely to improve their resistance
against multiple hits by bullets. Recent studies22-24
have shown that incorporation of CNTs in ceramics
like alumina and silicon carbide can have a strong
influence on the microstructure, fracture mode and
mechanical properties. A significant improvement of
up to 94% in fracture toughness was observed when
4 vol. % of CNTs are added to alumina22. Incorporation
of CNTs to reaction bonded SiC24 can also result in
24
increased fracture toughness from 4 to 7
.
CNT-based Fibers for Ballistic Armor
Applications
Macroscopic CNT-based fibers (sometime referred to
as yarn) show a unique combination of extraordinary
mechanical, thermal and electrical properties with
significant promise for futuristic applications such as
next generation body armor, space tethers, space
elevators, super-actuators etc. CNT fibers can be
woven into the form of textile structures or used
as electrical transmission cables because of their
excellent strength, toughness, resilience and electrical
properties. Several approaches for processing CNTs
into the form of fibers have been reported25, 27, 28.
These mainly include solid-state spinning, liquid-state
spinning and melt spinning of composite fibers. Solid
state spinning involves mechanical drawing of CNTs
directly from the gaseous reaction zone of the CVD
reactor. The liquid state processes include surfactantbased solution spinning and super-acid based solution
spinning. Apart from these processes, a variety of
other techniques like electrospinning, gel spinning,
hot drawing, twisting, infiltration by polymer solution
and surface tension driven densification are also being
employed for production of CNT fibers.
Researchers from Military University of Technology,
Poland have conducted numerical modeling
Table 3: Processing techniques and mechanical properties for selected CNT-based neat and composite fibers
Type of Fiber
SWCNT neat fiber
(Rice University)
CNT neat fiber
(University of
Cambridge)
CNT neat fiber
(University of
Cambridge)
Processing Technique
• Dispersion of individual
SWCNTs in fuming sulphuric
acid
• Coagulation in water
• Extrusion by solution
spinning
Direct spinning of CNTs from
the CVD synthesis zone of a
furnace using liquid source of
carbon and iron nanocatalyst
Direct spinning CNT of fibers
from aerogel sock
Strength
(s),
GPa
Properties
Strain
Elastic
to
Modulus
Toughness,
Fracture
(E),
J/g
(e),
GPa
%
Density
(r),
kg/m3
Ref.
(33)
0.116
120
-
-
0.87
0.55
22
-
-
0.55
1.0
50
-
-
1.0
(34)
(35)
5
MWCNT neat fiber
(Georgia Tech and
NASA)
SWCNT strands
(RPI)
Carbon nanotube (DWNT)
fiber(University of
Cambridge)
Multifunctional
carbon nano-tube
yarns (UTD)
Solid state spinning of
1mm-long MWCNT forests
synthesized by water-assisted
CVD
Direct synthesis of several
cm–long SWNCT strands by
catalytic pyrolysis of n-hexane
with enhanced vertical floating
technique
Dry spinning of CNT fibers
from CVD Zone
Introducing twist during
spinning of MWCNTs from
nano-tube forests to make
multi-ply, torque-stabilized
yarns
25
0.40
10
0.50
8.0
-
100
– 150
GPa
Solid state spinning of CNT
arrays synthesized by CVD
Process
SWCNT (60 wt%)/
PVA composite
fibers (UTD)
SWCNT (0.3wt%)/
PVA fibers (Shinshu
University ,AIST
and JSTA)
Kevlar/MWCNT
(1–1.75 wt%)
composite fiber
(University of
Dublin)
SWCNT and
MWCNT fibers
(CNRS)
Solution spinning
• Surfactant dispersion
coagulated in water-PVA
Gel spinning and hot drawing
8.8
-
-
-
-
357
6.3
121
1.11
Single yarn
0.15 to
0.30
-
13
(39)
14
0.8
20
0.8
Two – ply yarn
0.25 to
0.45
3.3
1.8
-
13
Single Yarn + PVA Infiltration
11
Maximum Properties
205
8.94
975
Average Properties
195
5.28
309
80
105
570
0.2
(40)
0.2
-
(41)
(42)
2.2
36
4.7
115
5.9
207
Hot drawing of wet-spun fibers
1.8
1.4
UHMWPE/MWCNT Gel spinning process
composite fiber
(HKUST)
-
(38)
1.90
Swelling Kevlar in suspension
of nanotubes in the solvent
N-methyl pyrrolidone
(36)
-
(37)
0.85
Ultrastrong, stiff
and light weight
DWNT fibers
(LANL)
Densified (D)
2.2
5
Twisted (T)
5.0
11
(D) + (T)
11.0
30
0.35
3.51
4.17
10
-
Kevlar
4.7
63
Kevlar + MWCNT
5.4
99
SWCNT fiber
45
430
870
MWCNT fiber
35
340
690
UHMWPE fiber
122.6
4.03
UHMWPE/ 5wt% MWCNT
136.8
4.65
-
(30)
1.44
1.5
(43)
1.5
1.5
( 29)
0.97
0.995
Another option is improvement of the ballistic performance of current armor grade fibers by reinforcing them with
CNTs. The incorporation of CNTs in these fibers is expected to improve their stiffness, strength and toughness. A
number of studies were carried out on UHMWPE (Dyneema and Spectra are commercial versions)29 , Kevlar30, Nylon
631 and PBO (commercially sold as Zylon)32 fibers to enhance their mechanical properties by the incorporation of
CNTs. This nanotech-enabled approach offers significant potential to increase the ballistic properties of armor grade
fibers and represents a major advance in the field of armor technology.
Table 3 shows the representative mechanical properties of various macroscopic CNT based fibers and their
6
In Pursuit of the Ultimate Body Armor
M5 fibers along with SWCNTs
(Fig. 5). As can be seen,
SWCNT neat fiber (from LANL)
shows spectacular properties
in terms of specific energy
absorption capacity and sonic
velocity, which is comparable
to that of SWCNTs (Fig. 5). It
also shows the highest ballistic
figure of merit, which is three
times greater than that of
Dyneema SK76 (Fig. 6). The
remarkable ballistic potential
of this SWCNT based neat
fiber can be attributed to its
exceptional toughness (975
J/g), very high elastic modulus
and ultra low density (~0.2
Fig. 5: Specific energy absorption capacity as a function of sonic velocity for CNT-based
g/cm3). Other CNT based
neat and composite fibers, and pristine SWCNT along with high performance fibers
fibers also show outstanding
potential for ballistic applications on account of their
respective processing routes. These include neat CNT
extraordinary strain to failure (430% elongation for
fibers and yarns, and CNT-based composite fibers.
CNRS fiber) and enormous strength (8.8 GPa in the
Based on the data given in Table 3, the specific energy
case of University of Cambridge fiber). In Fig. 5, the
absorption capacity, sonic velocity and ballistic figure
data for UHMWPE fiber reinforced with MWCNT is
of merit (see eqns. 1-3) for specific CNT-based fibers
also displayed and, shows higher energy absorption
(Los Alamos National Lab (LANL), Centre National
capacity than that of Dyneema. Based on these
de la Recherche Scientifique (CNRS) and University
encouraging reports, it is obvious that the approach
of Cambridge), selected on the basis of the highest
of utilizing CNT-based fibers for the development of
strength, toughness and strain to failure properties, can
bullet-proof vests is very promising on account of its
be calculated. The data has been displayed in Figs. 5
outstanding ballistic properties.
and 6. For the sake of comparison, the data has been
shown for high performance Dyneema, spider silk and
Concluding Remarks
Fig. 6: Elastic modulus, toughness, strain to failure and ballistic figure of merit for CNTenhanced fibers, Dyneema and spider silk
In the current world scenario,
it is well-acknowledged that
soldiers and law enforcement
personnel will be exposed to
increasingly lethal threats in
the future. In order to meet
these “asymmetric” threats,
there is an increased demand
for lightweight, flexible, and
multifunctional body armor that
can provide improved ballistic
protection and mobility. CNTs,
being among the strongest
and the stiffest materials in the
world, will play a key role in
creating ultrastrong, lightweight,
flexible and multifunctional
body armor to meet the above
requirements. The incorporation
of CNTs in polymer matrix
composites would enable
production of armor with
outstanding bullet penetration
7
resistance along with high strength and toughness.
CNT-enhanced ceramic inserts with enhanced fracture
toughness would improve the multi-hit capability of
the body armor. CNT-based neat or composite fibers
are candidates for the strongest, toughest and stiffest
super fibers of the future and exhibit enormous energy
absorption capacity and sonic velocity. Therefore,
its incorporation in the form of woven or non-woven
fabric in the body armor has the potential to protect
a person from the most aggressive ballistic threats of
the future.
References
1. M. J. N. Jacobs, J. L. J. Van Dingenen, “Ballistic
Protection Mechanisms in Personal Armour”,
J. of Mater. Sci., 36 (2001) 3137-3142
2. ammtiac.alionscience.com/pdf/AMPQ9_2ART01.pdf
3. en.wikipedia.org/wiki/Ballistic_vest
4. http://cnst.rice.edu/uploadedFiles/
Resources/Presentations/
694%20Intl%20Fiber%20Conf%20080709.pdf
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