Novel Flexible Body Armor Novel Flexible Body Armor Utilizing Shear Thickening Fluid Utilizing Shear Thickening Fluid

(STF) Composites(STF) Composites(1513)(1513)

14th International Conference on Composite MaterialsSan Diego, CA

14 July 2003

Army Research LaboratoryComposites and Lightweight Structures Branch

Bldg. 4600, AMSRL-WM-MBAberdeen Proving Ground, MD 21005-5069

Dr. Eric D. Wetzelewetzel@arl.army.mil 410-306-0851

Prof. Norman J. Wagnerwagner@che.udel.edu 302-831-8079

University of DelawareDept. of Chemical Engineering and

Center for Composite MaterialsNewark, DE 19716

Young Sil LeeRon Egres

Keith KirkwoodJohn KirkwoodPhil Matthews

Outline

• Background– Body armor– Shear thickening fluids (STFs)– STF / Kevlar composite

• Experiments– Ballistic tests– Flexibility tests– Stab 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*

• Currently no armor for extremities– Conventional materials (i.e. neat Kevlar) too bulky, stiff– Material requirements

• Flexible• Low bulk• Lightweight• Minimum protective level: frag / shrapnel protection

Interceptor VestKevlar® KM2

PASGT VestKevlar® 29

*Sources: D. Brown. Washington Post. May 4 2003; R. L. Mabry. J. Trauma. v49 n3 2000; F. Reister. Battlefield Casualties and Medical Statistics: U. S. Army Experiences in the Korean War. 1973; M.E. Carey. J. Trauma. v40 n3 1996.

% of soldiers with % of soldiersnon-fatal injuries (NFI) with NFI due to % of NFI due to

Conflict located on extremities frag / shrapnel bulletsWWII 70% 58% 38%Korea 71% - -Somalia 75% 43% 42%Desert Storm 64-87% 95% 5%Iraqi Freedom 73% 32% 32%

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 or 120 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 0.1 0.2 0.3 0.4 0.5 0.6

STF (450 nm EG)STF (120 nm PEG)Dry silicaEthylene glycol

% E

nerg

y di

ssip

ated

Areal density (g/cm2)

All targets 4 layers of Kevlar, various matrix materials:

Comparison of STF Kevlar with Neat Kevlar

• At high fabric loadings, STF-Kevlar composites require lower areal density than comparable neat Kevlar

• At high fluid loadings, STF-Kevlar composites require fewer Kevlar layers than comparable neat Kevlar

75

80

85

90

95

0 0.1 0.2 0.3 0.4 0.5 0.6

STF (450 nm EG) in 4 layers KevlarSTF (120 nm PEG) in 4 layers KevlarSTF (120-nm PEG) in N layers Kevlar, v

f=83%

Neat Kevlar

% E

nerg

y di

ssip

ated

Areal density (g/cm2)

10 layers Kevlar

14 layers Kevlar

6 layers Kevlar

20 layers Kevlar

4 layers Kevlar

20 layers Kevlar

4 layers Kevlar

14 layers Kevlar

8 layers Kevlar

Flexibility / Bulk of STF-Impregnated Kevlar

• STF-impregnated Kevlar targets are lighter, thinner and more flexiblethan neat Kevlar targets with comparable ballistic performance

4-layer Kevlar:Thickness: 1.4 mmWeight: 1.9 gEdiss: 76.7%

10-layer Kevlar:Thickness: 3.0 mmWeight: 4.7 gEdiss: 86.7% 0.25 mL STF (120 nm)

impregnated 4-layer Kevlar:Thickness: 1.4 mmWeight: 2.3 gEdiss: 87.2%

20 g weight

θ=50oθ=13o

θ=50o

High Velocity Performance• All targets reach critical velocity above which ballistic performance

drops off drastically• Increasing the number of fabric layers

increases the high velocity performance

• STF-Kevlar at high fabric loadingsoffers superior high velocity performance to neat Kevlar

40

50

60

70

80

90

100 150 200 250 300 350 400 450

STF (30 nm EG-PEG) in 4 layers Kevlar, v

f = 38% (5.28g)

STF (30 nm PEG) in 6 layers Kevlar, v

f = 57% (5.20g)

STF (120 nm PEG) in 8 layers Kevlar, v

f = 83% (4.61g)

7 layers neat Kevlar (3.29g)11 layers neat Kevlar (5.17g)

% E

nerg

y di

ssip

ated

Velocity (m/s)

Velocity (fps)656 820492 984 1148 1312

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

0.0 0.1 0.2 0.3 0.4 0.5 0.675

80

85

90

95

Ener

gy D

issip

atio

n (%

)

Areal Density (g/cm2)

Neat Kevlar Full STF + 4 layers Kevlar Full STF + 6 layers Kevlar Center STF + 6 layers Kevlar Edge STF + 6 layers Kevlar Stripe STF + 6 layers Kevlar

Effect of STF Patterning (cont’d)• Pattern of STF fundamentally influences the failure pattern /

mechanism in target

striped edge plain

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

STF appears to be “grabbing” yarns, preventing inter-yarn mobility at high strain ratesunimpregnated 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)

STF addition increases pull-out energy

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

• Quasi-static yarn pull-out experiments:

Stab Resistance of STF-Kevlar Composite

• STF-Kevlar is highly stab resistant– Conventional Kevlar fabric is relatively easy to puncture

NIJ Standard-0115.00

15

20

25

30

35

40

45

50

0.05 0.06 0.07 0.08 0.09 0.1 0.11 0.12 0.13

STF (450 nm PEG) in KevlarNeat Kevlar

Pene

trat

ion

dept

h (m

m)

Areal density (g/cm2)

3.153 kg weighted knife blade, 24 J impact

neat Kevlar

STF-Kevlar

Effect of Particle Anisotropy• Anisotropic CaCO3 particles with aspect ratio of 5:1

– Less particle loading required to achieve shear thickening

φ = 0.51

75

80

85

90

95

0 0.1 0.2 0.3 0.4 0.5 0.6

STF (450 nm EG)STF (120 nm PEG)STF (5:1 anisotropic EG)

% E

nerg

y di

ssip

ated

Areal density (g/cm2)

All targets 4 layers of Kevlar, various STF matrices:

• Potential benefits– Lower nominal viscosity → easier

processing and wearability– Shear thickening effect without

particles approaching close-packing → easier to fabricate

10-2 10-1 100 101100

101

102

103

η(P

a s)

.γ (1/s)10-2 10-1 100 101100

101

102

103

10-2 10-1 100 101100

101

102

103

η(P

a s)

η(P

a s)

.γ (1/s).γ (1/s)γ (1/s)

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