Ballistic-Resistant Body Armor Selected Research Initiatives

Ballistic-Resistant Body Armor
Selected Research Initiatives
Body Armor Workshop
NIST, Gaithersburg, Maryland
Kirk Rice
NIST
November 29, 2011
Outline
Bullet/Ammunition Threat Study
Dynamic Materials Research: Bullet, Clay
Thermal Characterization and Modeling of Clay
High-Strength Fiber Research: aging studies,
mechanical damage, theoretical framework
Future Work—contoured armor, hard armor testing
Molecule
Fibers
Yarn
Fabrics and Panels
Molecules
↓
Fibers
↓
Yarns
↓
Fabrics
↓
Panels
↓
Vests
Vests
Ammunition Performance
Comparison
• NIJ Body Armor TWG expressed interest in
comparison of 9 mm round specified in standard
with some others (A and B)
• Methods
– Velocity characterization (handguns and 10” barrel
used in universal receiver at lab)
– V50 ballistic limit tests against “Type II” shoot packs
– Vstd (mfr) ballistic limit tests against field-return Type
II armor
– Metallurgical tests (hardness)
A typical V50
ballistic test…
• Not perfect, but very
useful
• More shots are
better
• Keen interest in
lower tail region
Velocity (ft/s)
Analysis
leads to
risk model…
2000
1900
1800
1700
1600
1500
1400
1300
1200
1100
All Shots
All Partials
All Completes
V50 Experimental
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Shot Number
V50 Results with Logistic Fit
Armor Model: Combined Panels (NIJ .04 Level 2)
1.0
Probability of Penetration
• Up-Down method
• Simple controls
V50 Ballistic Limit Test Series
Armor Model: Combined Panels (NIJ .04 Level 2)
0.8
0.6
0.4
0.2
0.0
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
Velocity (ft/sec)
Test Data
Logistic Fit
Experimental V50
Logistic V50
NIJ Ref Vel Max
Estimate potential for vest penetration
Penetration and Velocity Probability Curves
357 Sig 125gr FMJ / Generic Test (V50 = 1563 ft/s, rate = 0.0167)
Combined Probability of Street
Threat Penetrating Vest: 2.8%
1.0
0.027
0.9
0.024
0.8
0.021
0.7
0.018
0.6
0.015
0.5
0.012
0.4
0.009
0.3
0.006
0.2
0.003
0.1
0.000
1000
1100
1200
1300
1400
1500
1600
1700
Penetration Probability
Velocity Probability
0.030
0.0
1800
Velocity (ft/s)
Velocity Prob.
Penetration Prob.
Mean Velocity
V50
NIJ Ref Vel
What if material degradation occurs?
Logistic regression of
V50 data and velocity
characterization
Brand A, 115 gr
SIG P229 Ruger P95DC Universal Receiver
V1
Mean
1159.2
1165.2
1263.5
Standard Deviation
26.9
26.3
9.6
Maximum
1189.9
1191.5
1273.6
95 % Confidence
1213.0
1217.8
1282.8
V2
Mean
Standard Deviation
Maximum
95 % Confidence
1160.6
27.4
1190.0
1214.3
1173.6
20.9
1192.5
1226.2
1256.1
12.5
1271.4
1275.4
Brand B, 124gr
V1
Mean
Standard Deviation
Maximum
95 % Confidence
1059.9
21.0
1103.1
1101.9
1087.3
17.3
1109.6
1121.8
1219.0
9.1
1229.5
1237.2
1060.3
21.2
1103.6
1102.6
1087.7
17.2
1109.9
1122.2
1214.4
6.3
1223.4
1227.0
V2
Mean
Standard Deviation
Maximum
95 % Confidence
Remington
Brand B
Brand A
Probability of Perforation
Consolidated plot of logistic
regression curves
Projectile Velocity (ft/s)
Penetration and Velocity Probability Curves
Brand A, 115 gr / Generic Test (V50 = 1472 ft/s, rate = 0.0336)
1.0
0.030
Combined Probability of Street
Threat Penetrating Vest: 0.0%
0.9
0.024
0.8
0.021
0.7
0.018
0.6
0.015
0.5
0.012
0.4
0.009
0.3
0.006
0.2
0.003
0.1
0.000
0.0
1000
1100
1200
1300
1400
1500
1600
1700
1800
Velocity (ft/s)
Velocity Prob.
Penetration Prob.
Mean Velocity
V50
NIJ Ref Vel
Baseline Rem 124 gr
Penetration Probability
Velocity Probability
0.027
Bullet Micrographs and Vickers Hardness
NIJ Standard (Remington)
Brand A
Comparison of average Vickers hardness
HV100
STDEV
Remington/Nose
97.2
8.2
Remington/Side
157.8
9.6
Brand A/Nose
182.6
11.8
Brand A/Side
206
2.3
Bullet/Region
Motivation
Dynamic Material Properties are Key to
Understanding the Ballistic Resistance of Soft
Body Armor
•
•
•
Projectile materials & structures (whole bullets)
Ballistic test clay (Roma Plastilina #1)
Armor materials (fibers, yarns and weaves)
Measurements Are Needed For:
•Developing physically-based models of ballistic
impact
•These models will inform next-generation body
armor test standards
Modeling & Simulation
11
Standards Example 1:
Projectile Specification
Why are some bullets more
penetrative than others?
These are considered different threat
levels according to the standard…
Is it materials? Or
construction? Or both?
Should test round
specifications be more
specialized?
How?
0.44 JHP
… but these are not
Dynamic Materials Testing
• Kolsky Bar Test
• The most widely used method for measuring
dynamic material behavior
• Measures materials at 102 to 104 strain/second
– 6 orders-of-magnitude higher than conventional
materials tests
– Designed for testing metals under uni-axial load
NIST Kolsky Bar
13
4000
1200
3500
3000
1000
2500
800
2000
600
1500
400
1000
200
30000 fps movie
-1
True Stress [MPa]
Wave Direction
1400
True Strain Rate [s ]
Compression Kolsky Bar Test
500
0
0
0
0.1
0.2
0.3
Limited to uni-axial deformation
only! (cylinder must remain
This data is used to understand how bullet
cylindrical)
materials deform during impact, but not
True Strain
whole structures
14
Dynamic Testing of Structures
(Bullets) Using DIC
High Speed Digital
Image Correlation
(DIC)
– Measures Detailed 3D
Shape During Dynamic
Testing
– Allows us to validate
models of structures
(bullets) with
unprecedented
accuracy
15
DIC – How it Works
High Speed Camera Pair
Two images from different angles are used to
construct 3D object shape using speckled subsets
Bullet Impact Test: Simulation
• Kolsky Bar
Direct Impact of
0.40 S&W FMJ
• Impact velocity:
15.3 m/s
• Striker bar KE:
62.5 J
17
Example: Bullet Model Validation
Measured vs. Modeled Shape Evolution
8
t = 0 µs
6
5
3
3
15
y [mm]
20
8
7
7
6
5
4
10000
10
15
t = 880 µs
3
Experiment
Baseline Sim
Improved Sim
6
10
15
20
8000
6000
4000
2000
5
0
4
t = 1120 µs
3
5
20
y [mm]
8
Baseline Sim
Experiment
Improved Sim
12000
5
z [mm]
z [mm]
5
4
10
14000
6
4
5
Measured vs. Modeled Force
t = 416 µs
7
z [mm]
z [mm]
7
Force [N]
8
0
500
1000
1500
Time [µs]
5
10
y [mm]
15
y [mm]
DIC
Measurement
20
Quantitative comparisons of
shape and force data obtained
during a Kolsky Bar test with
model results enabling
accurate model validation
18
Standards Example 2:
Roma Plastilina #1
• Evolving Test Protocols
– RP#1 Conditioning Protocol
• Thermo-mechanical response
– Replacement for RP#1?
• Need to maintain links to legacy
test data
• Useful for establishing
requirements in standard and
understanding factors that
influence results.
Back face signatures
19
Clay Characterization
• Significant activity underway within DoD to
develop special-purpose clay intended for
ballistic-resistant body armor testing.
• Comparisons based on
– Ball drop tests
– Backface deformations resulting from ballistic
tests on armor
– Analytical tests (rheology, density, etc.)
RP#1: Ball Drop Model
Purpose: Obtain validated
model of RP#1
Conditions: 63.5 mm
diameter steel ball
dropped from 2 m onto a
140 mm thick clay block
Initial Model Parameters:
from literature and
preliminary dynamic
measurement data
21
Ball Drop Model: Sensitivity
Analysis
-0.006
-0.007
Indentation Depth [m]
• Model sensitivity
analysis reveals what
properties of RP#1
determine
indentation depth
• This tells us what are
the most important
properties of RP#1
to measure
-0.008
-0.009
-0.01
Shear Angle
Poisson's Ratio
Modulus
Yield Stress
-0.011
-0.012
-0.013
0.50
1.00
1.50
2.00
2.50
Variable Value (Normalized)
22
Ball Drop Model: Lessons So
Far
Indentation depth is
sensitive to modulus
and yield strength
Indentation depth is
NOT sensitive to
hyrdostatic pressure
Indentation
Depth
Greatly simplifies
property measurements
23
Thermal Studies:
Roma Plastilina #1
Develop guidance on how long the clay box
test set up can be used after removal from
oven:
– Develop database of thermophysical properties of
currently employed ballistic clay (Roma Plastilina
#1)
– Model thermal performance (heating and cooling)
of the clay box test set up employed in body armor
testing
Material Thermal Properties
Material
Thermal
conductivity
[W/(m·K)]
Heat
capacity
[J/(kg·K)]
Density
(kg/m3)
Emissivity
Plastilina #1
0.6
1280
1570
0.9
PlywoodB
0.15
1410
540
0.9
Stainless
steelA
16
480
7920
≈0.3
AluminumA
215
910
2700
≈0.1
PolystyreneA
Insulation
0.03
1340
20
ATaken
from the literature.
BMeasured at NIST. Lawrence Berkeley Laboratory database has values of
0.12, 1210, and 540, respectively.
Some ANSYS Observations
Selection of convection coefficient for room cooling
Results are extremely sensitive to convection coefficient, h.
Best value for h in comparison to existing measured data is 5 W/(m2·K).
Some ANSYS Observations
Steel vs. aluminum testing frames
Aluminum is more conductive (heat) than steel resulting in a smaller ΔT for the
metal frame itself during cooling (graph on the left)
However, due to their respective design geometries, aluminum frame actually has a
thermal mass that is 15 % greater than the steel frame; it brings more energy with
it upon removal from the oven.
Overall influence on clay temperatures is minimal, with slightly higher temperatures
predicted when the aluminum frame is employed (graph on the right).
Future Work: Clay
• Refine mechanical and thermal models.
• Linking thermal response with mechanical
response
• Result: Physical basis from which to:
– Compare alternate backing materials
– Explore “what if” scenarios with standard
testing procedures
• Develop guidance for clay block
construction and handling
28
High Strength Fiber
Research Overview
Research Background
• NIST and NIJ published summary findings
that PBO fibers in fielded body armor may
change over time in use, thereby
compromising armor performance.
• NIST developed test methods to evaluate
armor’s resistance to conditions of high heat,
humidity, and mechanical damage that were
incorporated into NIJ Standard--0101.06.
• Emphasizes the importance of understanding
new and current materials used in armor
designs.
High Strength Fiber Research
• Evaluation of degradation properties of new and
lesser-used high strength fibers, such as copolymer
aramid fibers, and a continued investigation into the
properties of field-aged body armor.
• Investigation of link between mechanical properties
and ballistic performance.
• Long-term aging studies for service life prediction of
high-strength fibers.
• Development of recommendations for used/fielded
body armor
Artificial Aging of High Strength
Fibers Used in Ballistic
Applications
Motivation
Ballistic Resistance of Body Armor
NIJ Standard-0101.06: Environmental Conditioning
5.2.3 Test Conditions
5.2.3.1 Air Temperature
Keep the air temperature uniform, both inside the conditioning chamber
and in the storage environment. Verify that the air temperature is uniform
by using verification sensors to ensure that the air temperature is within ±
2 °C (± 3.6 °F) of the required temperature. Storage temperatures are
Based on
given in section 5.2.1.
estimates that for
The test temperature shall be 65 ºC (149 ºF).
every 10 oC
increase in T, the
5.2.3.2 Relative Humidity
reaction
Keep the relative humidity uniform and non-condensing, both inside
therate is
doubled
test chamber and in the storage environment. Verify that the relative
humidity is uniform by using verification sensors to ensure that the relative
humidity is within ± 5 % of the required relative humidity. The storage
relative humidity is given in section 5.2.1.
The test relative humidity shall be 80 %.
Overview of Arrhenius Model
Arrhenius
Equation
k = Ae
− Ea
RT
k = rate constant
T = temperature
Ea = activation energy
A = pre-exponential
factor
R = gas constant
Not known if this relation applies
to solid state reactions such as
those that occur in ballistic fibers.
Temperature
Time
35° C
83 d
45° C
42 d
55° C
21 d
65° C
10 d
75° C
5d
Based on general rule of thumb:
every 10°C, reaction rate doubles.
Objectives
• Establish scientific basis for the environmental
conditioning protocol contained in NIJ 0101.06, and
provide confidence that the conditioning protocol
settings can be extended to other fibers, by
• Systematically measure changes in mechanical and
chemical properties of common high strength fibers
used in body armor applications, as a function of
temperature, relative humidity, and time, and
• Relate changes in fiber properties that occur during
environmental conditioning to an expected period of
performance in the field, via predictive models.
Experimental Approach
• Materials: Aramid (various types), UHMWPE
• Conditioning temperatures (oC):
– 25, 43, 55, 70 for Aramid (43 and 70
completed, 55 IP)
– 40, 65, 90, 115 for UHMWPE (completed)
• Inclusion of moisture for aramid: Moisture
sorption analysis will be carried out to
determine moisture content of fibers as a
function of temperature and RH.
Experimental Approach
• General procedure:
– Age materials at prescribed temperatures
(and for aramid, prescribed moisture levels )
– Measure properties (mechanical and
chemical) at regular intervals until changes in
properties reach a plateau
– Combine data at all temperatures and
moisture levels
• Currently, UHMWPE studies are completed,
and aramid yarns are under test in dry
conditions.
UHMWPE Aging: Tensile Strength
Measure % loss in tensile strength at each temperature:
Percent Loss in Tensile Strength
70
43 deg C
60
65 deg C
90 deg C
50
115 deg C
40
30
20
10
0
100
1000
Aging Time (h)
10000
100000
UHMWPE Aging: Tensile Strength
Shift curves until they superimpose smoothly:
Percent Loss in Tensile Strength
60
43 C
50
40
30
65 C
90 C
115 C
20
10
0
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
aT*aging time (shifted aging time), hours at 43oC
1.E+08
Tensile strength retention
for para-aramid yarn
Ongoing Work
Studies of aramid fibers incorporating moisture:
12
Moisture Content (mass %)
11
10
9
8
7
25 C
6
43 C
5
55 C
4
70 C
3
2
1
0
0
10
20
30
40
50
60
70
80
90
100
Relative Humidity (%)
* To do a valid study incorporating moisture, the amount of moisture in
the fibers must be well-quantified and kept constant *
Ongoing Work
Custom temperature/humidity chambers for fiber research
Mechanical Degradation Due to Folding
Fabric
Constant
tensile load
Close
PPTA and PBO fabrics
Cyclic fold
Folding rod
(d=6.3 mm)
Open
Constant
tensile
load
J.H. Kim et al., J. Appl. Mech. 75(2008) DOI: 10.1115/1.2755131
Mechanical Damage
• Molecular Structure affects hydrolytic and
mechanical stability of ballistic fibers
PBO fiber
PPTA fiber
Sample
Unfolded PPT
5k folded PPT
80k folded PPT
Strength
[GPa]
3.14 ± 0.31
3.05 ± 0.41
3.07 ± 0.43
Modulus Strain to failure
[GPa]
[%]
84.7 ± 6.0 3.61 ± 0.35
82.9 ± 4.6 3.54 ± 0.45
85.0 ± 5.3 3.49 ± 0.43
Sample
Unfolded PBO
5k folded PBO
80k folded PBO
Strength
Modulus Strain to failure
[GPa]
[GPa]
[%]
3.36 ± 0.37 143 ± 10
2.97 ± 0.39
2.90 ± 0.42 146 ± 9
2.50 ± 0.45
1.99 ± 0.30 136 ± 8
1.74 ± 0.32
Schematic graph
Schematic graph
4
3.5
Unfolded PPT
3.5
Unfolded PBO
3
5k folded PPT
3
5k folded PBO
Stress (GPa)
Stress (GPa)
4
2.5
2
1.5
2.5
2
1.5
1
1
0.5
0.5
0
0
0
1
2
Strain (%)
3
4
0
1
2
Strain (%)
3
4
Framework for Assessing Potential
Ballistic Performance
Modulus is the slope of this line
4
3.5
Unfolded PPT
3.5
Unfolded PBO
3
5k folded PPT
3
5k folded PBO
Stress (GPa)
Stress (GPa)
4
2.5
2
1.5
2.5
2
1.5
1
1
0.5
0.5
0
0
0
1
2
Strain (%)
[U ]
* 13
3
4
σ uf ε uf
=
 2 ρ
Specific Energy Absorption
0
1
E1 f 

ρ 
2
Strain (%)
13
Sonic Velocity
3
4
Pristine & Degraded PBO vs. Kevlar-29 Reference
Curve
Underlying the design are the fundamental properties of the fibers – do they change with
time?
Future Work
Contoured armor
– Female designs, other designs producing 3-D
contours
– Test protocol adjustments
Hard armor plate testing
– Size effects
– Curvature effects
– Mounting effects
Acknowledgements
A special thanks to the researchers who
contributed to this overview:
– Steven Mates
– Aaron Forster
– Gale Holmes
– Dale Bentz
– Joannie Chin
– Michael Riley
– Amanda Forster