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Advanced Body Armor Utilizing Advanced Body Armor Utilizing Shear Thickening FluidsShear Thickening Fluids
(AO01)(AO01)
23rd Army Science ConferenceOrlando, FL
3 December 2002
Army Research LaboratoryComposites and Lightweight Structures Branch
Bldg. 4600, AMSRL-WM-MBAberdeen Proving Ground, MD 21005-5069
Dr. Eric D. [email protected] 410-306-0851
Prof. Norman J. [email protected] 302-831-8079
University of DelawareDept. of Chemical Engineering and
Center for Composite MaterialsNewark, DE 19716
Young Sil LeeRon Egres
Keith KirkwoodJohn Kirkwood
Outline
• Background– Body armor– Shear thickening fluids (STFs)– STF / Kevlar composite
• Experiments– Ballistic results– Flexibility tests
• Mechanisms of energy absorption in STF composite
• Continuing work
Body Armor
• Conventional body armor– 20-40 layers of neat Kevlar
• Rigid ceramic inserts for high threat situations– Torso protection only
• Extremities protection– Extremities: arms, legs, neck– Battlefield statistics from WWII, Korea (Reister, 1973)
• ~ 16% of deaths due to trauma to extremities• ~ 70% of non-fatal injures to extremities
• Currently no armor for extremities– Conventional materials (i.e. neat Kevlar) too bulky, stiff– Material requirements
• Flexible• Low bulk• Lightweight• Protective
– Minimum level: frag / shrapnel protection
Interceptor VestKevlar® KM2
PASGT VestKevlar® 29
Shear Thickening Fluid (STF)• Liquid phase highly filled with
rigid, colloidal particles• At high shear rates, hydro-
dynamic forces overcome repulsive interparticles forces, and hydroclusters form
• Particles collide, material becomes macroscopically rigid
equilibrium shear thinning
increasing shear rate
shear thickening
10-5 10-4 10-3 10-2 10-1 100 101 102 103 104
10-1
100
101
102
103
104
105
106
.
Rheology of ethylene glycol based STF
η (P
a s)
γ (1/s)
φ=0.62 φ=0.57
shear rate
visc
osity
200 nm
Application to Body Armor
• Impregnate Kevlar fabric with shear thickening fluid• At low shear rates (normal motion)
– STF behaves like a liquid– High flexibility, little or no impediment to motion
• At high shear rates (ballistic impact)– Relative motion of yarns / fibers within fabric deforms STF at
high rate– STF transitions to rigid phase, enhances ballistic protection of
fabric
STF
Kevlar fabric
before impact during impact
Materials• Shear thickening fluid
– Colloidal silica particles (avg particle size: ~450 nm)
– Ethylene glycol (EG) or polyethylene glycol (PEG) carrier fluid
• Advantages over water carrier fluid:– Wets Kevlar moderately– Environmentally stable
– Final particle concentration: 55-65 vol%• Kevlar
– KM-2 Kevlar® fabric– Style 706, 600 denier (180 g/m2)
• Composite preparation– Dilute STF with ethanol– Wet diluted STF into Kevlar– Evaporate ethanol in oven (80°C for 20 min)
200 nm
colloidal silica particles
10 µm
STF-impregnated Kevlar fabric
• Targets– Impregnate Kevlar with varying amounts, patterns, types of STF– Encapsulate impregnated Kevlar in polyethylene film – Sandwich target between aluminum foil faces– 2”x2” in size
• Ballistic tests– 0.22 cal FSP– Velocity ~ 825 fps– Target set in frame,
not clamped– Clay witness
• Quantify ballistic performance in terms of depth of penetration• Use clay ballistic curves to relate penetration depth to energy
absorbed by target
Ballistic Experiments
adhesivetape
target
clay witness
mountingframe
10-5 10-4 10-3 10-2 10-1 100 101 102 103 104
10-1
100
101
102
103
104
105
106
.
Rheology of ethylene glycol based STF
η (P
a s)
γ (1/s)
φ=0.62 φ=0.57
shear rate (s-1)
visc
osity
(Pa
s)
STF Rheological Properties• Shear thickening transition at shear rate of ~ 101-103 s-1
• Shear rate during ballistic experiments
– Ballistic impact should transition fluid to rigid state
104-105 s-1projectile diameterprojectile velocity
0.56 cm244 m/s
= =
Effect of STF Impregnation • Impregnation of STF into Kevlar is critical to enhance ballistic
performance of neat fabric
0
5
10
15
20
A B C D E F
Pen
etra
tion
dept
h (m
m)
Target geometry
A D
B E
FC
Legend:
STF fluid
single Kevlar layer
4 Kevlar layers impregnated with STF fluid
Effect of Volume of STF• Adding more STF increases energy absorption in target• Adding neat ethylene glycol (EG) or dry silica powder of equal
mass has less effect on energy absorption
Absorbed EnergyEnergy Dissipation (%) = 100Initial Impact Energy
×
65
70
75
80
85
90
95
0 2 4 6 8 10 12 14
STF impregnated 4-KevlarEG impregnated 4-KevlarDry silica impregnated 4-Kevlar
Ener
gy D
issi
patio
n (%
)
Target mass (g)
Comparison of STF Kevlar with Neat Kevlar
• For targets of equal weight, STF-impregnated Kevlar demonstrates similar ballistic performance to neat Kevlar
STF impregnated 4-KevlarEG impregnated 4-KevlarNeat Kevlar
STF-impregnated targets have significantly fewer layers of Kevlar than the comparable neat Kevlar targets
0 2 4 6 8 10 12 1460
65
70
75
80
85
90
95
100
Ener
gy D
issi
patio
n (%
)
Weight of Sample (g)
4 layers of Kevlar
10 layers of Kevlar
14 layers of Kevlar
4 layers of Kevlar
Flexibility / Bulk of STF-Impregnated Kevlar
• STF-impregnated Kevlar targets are thinner and more flexible than neat Kevlar targets with comparable ballistic performance
4-layer Kevlar:Thickness: 1.4 mmWeight: 1.9 g
20 g weight
10-layer Kevlar:Thickness: 3.0 mmWeight: 4.7 g
2mL STF impregnated4-layer Kevlar:Thickness: 1.5 mmWeight: 4.8 g
θ=50oθ=13o
θ=51o
Effect of STF Patterning• Compare fully-impregnated Kevlar with pattern-impregnated Kevlar
– All patterns with 6 layers of Kevlar
center edge stripe
Impregnation pattern has little or no quantitative effect on depth of penetration
0 2 4 6 8 1075
80
85
90
95
Ener
gy D
issip
atio
n (%
)
Weight of Target (g)
Neat Kevlar Center Patterned STF Edge Patterned STF Stripe Patterned STF Full STF
Effect of STF Patterning (cont’d)• Pattern of STF fundamentally influences the failure pattern /
mechanism in target
Effect of Particle Anisotropy• Anisotropic CaCO3 particles with aspect ratio of 5:1
– Less volume of particles required to achieve shear thickening
• Secondary benefit: low cost, readily available particles → applicable to large scale testing
0 10
0
5
10
15
20
25
30
Dis
sipa
ted
Ener
gy (J
)
Weight of Target (g)
Isotropic (Spherical) STF with 4 Kevlar Anisotrpic STF with 4 Kevlar
10-2 10-1 100 101100
101
102
103
.
η (P
a s)
γ (1/s)
φ = 0.51
Mechanism of Ballistic Energy Absorption in STF Composite
• Mechanisms of energy absorption in conventional fabric armors– Yarn pullout– Fiber plastic deformation– Fiber fracture
• Compare impacted targets (4 layers of Kevlar with and without STF)– Less pullout in STF composite– More fiber fracture in STF composite
• Possible sources of increased energy absorption in STF composite– STF restricts yarn motion, allows yarns to
be loaded to failure → energy absorbed by fiber fracture
– STF increases pullout energy, less pullout required to achieve high energy absorption
STF appears to be “grabbing” yarns, preventing inter-yarn mobility at high strain rates
unimpregnated Kevlar
first layer of Kevlar (back three layers show comparable pullout)
STF-impregnated Kevlar
first layer of Kevlar (back three layers show little
pullout, no fracture)
Continuing WorkIsolation and Analysis of Energy Absorption Mechanisms
• Quasistatic fiber pullout test
• High velocity ballistic tests
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 5 10 15 20 25
STFPEG
Nor
mal
ized
Pul
lout
Ene
rgy
% Liquid Impregnation
40
50
60
70
80
90
100
100 150 200 250 300 350 400 450
Energy Dissipation (%) - 7KEnergy Dissipation (%) - 11K
Energy Dissipation (%) - STF
Ener
gy D
issi
patio
n (%
)
Target mass (g)
Continuing WorkMaterial and Target Design
• Materials– STF material
• Particle anisotropy• Particle size
– Possibility for enhanced energy absorption mechanisms at very small particle sizes
• Particle material -> polymeric, rubber particles– Lower density particles for reduced target weight– Softer particles for modification of energy absorption
mechanisms• Particle surface energy
– Fabric • Denier• Weave• Fiber type
• Test configuration– Larger target sizes– Higher velocities
– Architecture• Patterning / STF-to-fabric ratio• Layer sequencing
