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3
Historical Basis for Current Body Armor Testing
This chapter discusses the foundational basis for the current body armor
testing methodologies as practiced by military and law enforcement agencies.
In the 1970s developments in fiber technology and protective vests along with
injury biomechanics investigations led to the study performed by Prather et al.
(1977) (called “the Prather study” in subsequent text). That study provides the
basis for the current clay-based test methodology for the assessment of blunt
trauma risk from backface deformation (BFD). The work focused on protection
from low-velocity handgun rounds using soft body armors. It included an injury
assessment methodology developed using animal tests and the correlation of
animal chest deformation response with the response of simulant materials at
velocities that are typical of rounds used to test soft body armors. A diagram of
this process is shown in Figure 3-1.
The process can conceptually be separated into two stages. As shown in
Figure 3-1, the first stage is a soft body armor evaluation using paired (goat and
simulant) tests to look at realistic deformation responses and fatalities behind soft
body armor on goats. Typical penetration depths were derived using gelatin as a
tissue stimulant for comparison. The second stage is an injury risk assessment
using a hard cylindrical impactor into goat chests and correlated depth of
penetrations using the same impactor into gelatin and clay. The development of
this process is discussed below.
BACKGROUND
In January 1973, the Law Enforcement Assistance Administration, a
branch of the U.S. Department of Justice (DOJ), tasked the U.S. Army Land
Warfare Laboratory at Aberdeen Proving Ground, Maryland, to develop
lightweight, inconspicuous protective garments for public officials in response to
an increasing number of armed assaults on public officials. The U.S. Army Land
Warfare Laboratory contacted the Biophysics Division of the U.S. Army
Biomedical Laboratory for assistance in developing a research program to
accomplish this task. Other players included the U.S. Army Natick Laboratories
and Aerospace Corporation. In March 1973, the program was expanded to
include protection for law enforcement personnel (NIJ, 2001).
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Stage 1
Stage 2
FIGURE 3-1 Overview of development of Prather clay methodology. Stage 1: depth
correlation of goats with gelatin in soft body armor. Stage 2: injury assessment with rigid
impactor onto goats, gelatin, and clay. SOURCE: Prather et al., 1977.
In the 1970s, 80 percent of the civilian handgun threat comprised .38-
caliber and smaller handgun rounds. The primary rounds chosen for this program
were the .38-cal, 158-grain lead round nose (LRN) bullet with initial velocity of
244 m/sec (800 ft/sec) and the .22-cal, 40-grain long rifle high-velocity bullet
with an initial velocity of 305 m/sec (1,000 ft/sec). The garments developed
under this program had to be lightweight, inconspicuous, and wearable.
Additional requirements included protection from bullet penetration, blunt trauma
mortality risk of less than 10 percent, and sufficient protection to allow the wearer
to walk away from any shooting incident. Note that these last two requirements
are not necessarily contradictory, because overall mortality risk might involve
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delayed hemorrhage and other sequelae. So, a major assumption was that medical
attention would be available within 1 hr of being shot.
In a nonpenetrating impact, the kinetic energy must be dissipated by the
deformation of the armor, fragmentation of the bullet, and deformation of the
underlying body wall. This energy transfer to the body has the potential to cause
serious injury or death. This nonpenetrating impact injury is termed behind-armor
blunt trauma (BABT).
The first step in the development of the new body armor was to determine
which materials could satisfy the requirements. Materials investigated included
high-tenacity nylon, nylon felts, high-tenacity rayon, graphite yarns, XP (an
experimental plastic developed by Phillips Petroleum), Monsanto fibers, and
DuPont Kevlar 29 and 49. Selection factors included weight to strength ratio,
flexibility, cost, availability, ballistic qualities (ballistic limit and behind-armor
deformation), and tailorability. Kevlar 29 (K29) was ranked as the best candidate
for further development: specifically, seven plies of 400/2 denier K29.
To assess BABT, the biophysics researchers selected the 40-50 kg angora
goat as a model for a typical 70-kg man. Goldfarb et al. (1975) used a waterjet
stream to evaluate the mechanical response of the lung, liver, kidney and spleen in
both the goat and human organs. They concluded that the collapsed lung and
spleen of a goat and of a human exhibited similar mechanical responses, and that
the goat kidney and liver were less resistant to trauma than the counterpart organs
of a human. Thus the goat was assumed to be a conservative model for BABT
testing.
To assess BABT, seven-ply K29 armor samples were mounted on
anesthetized angora goats and tested with the .38-caliber LRN bullet at a velocity
of 244 m/sec (800 ft/sec). Targeted organs included the lung, liver, heart, spine,
gut, and spleen. Concurrently, tests with the same body armor and bullet rounds
were performed using 20 percent gelatin as the backing material to develop a
profile of the behind-armor deformation. High-speed motion pictures were taken
of the impacts on gelatin in order to derive the rate of deformation, as well as the
deformation depth, volume, and area. These measures were then correlated to the
damage seen in liver, lung, spleen, and heart injuries to goats. The average
deformation recorded from the gelatin profiles was 44 mm, as shown in Figure 3-
2, and 44 mm was therefore selected as the BABT standard injury reference
value. It is important to note that no deaths were seen from the back-face effects
in goats with the .22- or .38-cal rounds that corresponded to the 44-mm
deformation in gelatin (Goldfarb et al., 1975).
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FIGURE 3-2 Blunt deformation profiles into gelatin using seven-ply K29 armor samples
mounted on gelatin and tested with the .38-cal LRN bullet at 213 m/sec (800 ft/sec).
Comparison experiments with anesthetized angora goats with the same armor and round
showed no fatalities. The deformation envelopes shown in this figure were obtained as
fits to respective indentation profiles read from the high-speed video film frame exposed
at the time of maximum penetration. It was noted by the author that the penetration
profiles were not necessarily parabolic and that, in some cases, the fitted curve was not as
deep as the deepest part of the uneven surface it approximated. SOURCE: Prather et al.,
1977.
Concurrent with the BABT testing, Clare et al.(1975) were developing
blunt trauma correlation models formulated from experimental data sets obtained
from tests on unarmored animals, where the physical characteristics of the
impacting projectile (mass, velocity, diameter) were known. High mass (50-200
g), low-velocity impacts were involved. The first model, a four-parameter
discriminant model, accomplishes its discrimination in a plane whose axes, x1 and
x2, are defined as follows:
x1 = ln [MV2]
and
x2 = ln [WD]
where M is projectile mass in grams, V is projectile impact velocity in meters per
second, W is experimental animal body weight in kilograms, and D is projectile
diameter in centimeters.
The discriminant lines establish three zones—low, medium, and high
lethality. As the impact dose increases, the probability of lethality should also
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increase for targets having the same body weight and for projectiles of the same
diameter.
EVOLUTION OF CLAY USAGE
Before the mid-1970s, behind-armor deformation testing used 20 percent
ballistic gelatin as a backing material; the ballistic gelatin required the use of
high-speed photography to record the BFD because the medium was elastic and
returned to its original shape after the projectile firing. An alternative material
was sought that would retain its deformed state to avoid the use of expensive
high-speed photography. The Law Enforcement Assistance Administration
requested a backing material that was inexpensive and reusable, that exhibited
little material recovery, and that was easy to use so that law enforcement agencies
could conduct testing at their own facilities. This material should exhibit a
penetration and deformation response similar to gelatin. Data already existed for
impacts on the goat thorax using a 200-g, 80-mm noncompliant hemispherical
impactor with an impact velocity of 55 m/sec. These deformation-time histories
were used to compare the response of various materials under similar impact
conditions.
Figure 3-3 shows the response of the most promising materials tested.
Although none of the materials duplicated the thoracic response, Roma Plastilina
#1 clay had a deformation depth response similar to that of gelatin and was
considered to be a suitable tissue simulant that was easy to use, inexpensive, and
repeatable and that required no high-speed photography.11 The clay and the
ballistic gelatin were generally softer and less resistant than the goat thorax to the
impactor at the testing velocity of 55 m/sec.
Blunt impactor data on goats were used to link this deformation response
with fatality using a logistic regression model (Clare et al., 1975). This
relationship was derived between deformation and the probability of lethality as
shown in Figure 3-4. The displacement levels for goat survival and death are
indicated in the figure. The figure shows that a 44-mm deformation in the goats is
correlated with ~10 percent probability of death from the impactor, similar to the
initial program requirement of less than or equal to 10 percent. Further, because
this deformation depth in the goats is less than the depth at which any of the goats
died in the impactor tests, it was selected as the injury reference value in the clay.
11
Gelatin was recommended for consideration as a possible mid -range alternative to clay
by the Phase II committee (NRC, 2010), in part because the marginal costs for high-speed
photography imaging technologies are now much lower. As discussed in Chapter 4, advances in
sensor technology can also provide such time-resolved information.
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Time (msec)
FIGURE 3-3 Deformation depth vs. time of candidate materials in a goat thorax using a
blunt impactor at 55 m/sec. SOURCE: Prather et al., 1977.
FIGURE 3-4 Logistic regression model of death vs. deformation for blunt impact into
goat chests. SOURCE: Based on data from Prather et al., 1977.
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It is important to note that this lethality relationship is not derived from
actual body armor testing, which means that the use of the blunt impactor tests to
model the injury behavior from BABT is likely conservative for this impact
velocity range. However, other than for this velocity range, which is typical of
handgun rounds into soft body armor, the relationship between injury and
deformation response in the clay is uncertain.
A further caveat is outlined in Prather’s original study on the injury
regression shown in Figure 3-4:
Attempts have been made using the original blunt impact data to
correlate deformation depth with the probability of lethality. A depth
of penetration greater than 50 mm is associated with a probability of
lethality of approximately 15%. However, the available data is
limited and hence no solid conclusions can be drawn as yet
regarding the effect of deformation depth. (Prather et al., 1977, p.
10, emphasis added)
Thus, the original injury correlation is quite limited, even for soft body
armor back-face effects. For a given impact, the clay and gelatin depth of
penetration were found to be generally greater than the goat depth of penetration.
Figure 3-3 can be used to roughly scale the response of the goats to that of the
clay. It shows that for deformations between 3 and 60 mm, the ratio of clay
deformation to goat deformation is approximately constant:
Dclay/Dgoat = 1.28
This value varies by less than 0.3 percent for goat impactor deformations
between 30 and 60 mm. This implies that 44-mm goat impactor deformation is
similar to 56-mm deformation in clay or gelatin. Conversely, 44-mm deformation
in clay is similar to 34-mm deformation in the goat for a hard impactor, as shown
in Figure 3-5.
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FIGURE 3-5 Logistic regression model of death vs. deformation for blunt impact into
clay using deformation response into goat chests and clay. SOURCE: Based on data from
Prather et al., 1977.
HIGH-ENERGY THREATS
In the late 1970s, the primary civilian ballistic threat changed to more
powerful handgun rounds. Research was initiated to develop body armors to
protect against these higher energy threats. The threats investigated included the
.357-cal, 158-grain semiwad cutter at 396 m/sec (1,300 ft/sec) and the 9-mm, 124-
grain full metal jacket at 350 m/sec (1,150 ft/sec). Investigations determined that
these rounds would not penetrate 16 plies of K29. They also showed that the
behind-armor deformation profiles for this soft body armor were similar to those
derived under the original program. Limited goat studies demonstrated injuries
similar to those incurred in the seven-ply tests. The program was terminated
before sufficient tests could be conducted to verify these preliminary conclusions.
Though there are no existing reports on this work, the studies did not produce
deaths in the animal model in tests with actual soft armor.
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Rifle Threats for Hard Body Armors
In the Prather study, no comprehensive studies were performed on rifle
threats with hard body armors. Rifle threats were evaluated on an ad hoc basis.
For example, .50-cal antipersonnel threats to helicopter pilots were assessed in
animal models, but there were no recommendations generated concerning risk
assessment methodology for generic body armor BABT.
Work Performed after the Prather Study
To assess the risk of injury using clay at rifle round velocities, a series of
tests was performed using human cadavers. The results of these tests were
compared with the clay-based National Institute of Justice (NIJ) Standard 1010.04
at a commercial test laboratory using a ultrahigh molecular weight polyethylene
(UHMWPE) hard body armor system (NIJ, 2000; Bass et al., 2006).
As described by Bass et al.: “. . . the test round was a 7.62 M80 ball
projectile. Tests were performed [on both the cadavers and the clay] at velocities
ranging from ~670 m/sec to ~800 m/sec. The resulting backface deformations
[Figure 3-6] showed a very low correlation of deformation with the range of
velocities [Figure 3-7]. In contrast, the human cadaver, over the same velocity
range, showed a wide range of injury outcomes that generally scaled well with
impact force and velocity.”
FIGURE 3-6 Clay deformation behind hard armor with rifle round threats. SOURCE:
Bass, 2006.
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80
70
Depth of Clay Penetration (mm)
y = 0.0307x + 23.637
R2 = 0.0854
60
50
40
NIJ-0101 Injury Reference
30
20
10
0
660 680 700 720 740 760 780 800
Velocity (m/s)
Velocity (m/sec)
FIGURE 3-7 Variation of clay penetration depth with velocity for behind-body armor
deformation (7.62-mm NATO round, UHMWPE body armor). SOURCE: Bass, 2006.
120
y = 0.2508x - 104.58
Area of Clay Penetration (mm )
2
2
R = 0.541
100
80
60
40
20
0
660 680 700 720 740 760 780 800
Velocity (m/sec)
Velocity (m/s)
FIGURE 3-8 Variation of clay penetration area with velocity for behind-body armor
deformation (7.62-mm NATO round, UHMWPE body armor). SOURCE: Bass, 2006.
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Measurements of the cross sectional area or volume of the clay improved
the correlation, but the R2 value was still less than 0.6 (Figure 3-8). The poor
correlation of clay depth with resulting cadaveric injury for rifle round threats
with this body armor type raises concerns about hard armors with low areal
density that may result in high-velocity BFDs.
Further, the NIJ 0101.04 standard procedure relies on the measurement of
the static residual depth of penetration into the clay (NIJ, 2000). Bir (2000)
performed an analysis of dynamic clay deformation for nonlethal baton rounds
and found that there was no guarantee that the residual deformation was equal to
the dynamic deformation. Indeed, individual tests saw as much as 20 percent
greater dynamic deformation than residual deformation after the dynamic test. In
addition, there is no evidence that this dynamic deformation is not sensitive to rate
or contact area.
Finding: Existing research raises concerns regarding the correlation of the
damage measured in the clay with the bodily injury at the very high rates typical
of backface deformations caused by rifle rounds in hard body armor.
CURRENT STANDARD
Strengths and weaknesses of the current Prather methodology are
displayed in Table 3-1 and discussed extensively in Chapter 9. Key concerns
regarding the methodology include the very limited validation basis, especially
with regard to the hard armor plates regularly tested by the Department of
Defense. Since the biomedical basis of the Prather methodology is not current,
the impact of changes in clay composition that have occurred since the Prather
study can only be surmised.
REFERENCES
Bass, C., R. Salzar, S. Lucas, M. Davis, L. Donnellan, B. Folk, E. Sanderson, and
S. Waclawik. 2006. Injury risk in behind armor blunt thoracic trauma.
International Journal of Occupational Safety and Ergonomics 12(4):429-442.
Bir, C. 2000. Dissertation: The Evaluation of Blunt Ballistic Impacts of the
Thorax. Detroit, Mich.: Wayne State University.
Clare, V., J. Lewis, A. Mickiewicz, and L. Sturdivan. 1975. Blunt trauma data
correlation. EB-TR-75016. Aberdeen Proving Ground, Md.: Edgewood
Arsenal.
Goldfarb, M.A., T.F Ciurej, M.A. Weinstein, L. Metker. 1975. A method for soft
body armor evaluation: Medical assessment. EB-TR-74073. Aberdeen
Proving Ground, Md.: Edgewood Arsenal.
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NIJ (National Institute of Justice). 2000. Ballistic resistance of personal body
armor. NIJ Standard 0101.04. Available online at
http://www.ncjrs.gov/pdffiles1/nij/183651.pdf. Last accessed March 10, 2011.
NIJ. 2001. Selection and application guide to personal body armor. NIJ Guide
100-01. Available online at http://www.ncjrs.gov/pdffiles1/nij/189633.pdf.
Last accessed March 10, 2011.
NRC (National Research Council). 2010. Phase II Report on Review of the
Testing of Body Armor Materials for Use by the U.S. Army. Washington,
D.C.: National Academies Press.
Prather, R., C. Swann, and C. Hawkins. 1977. Backface signatures of soft body
armors and the associated trauma effects. ARCSL-TR-77055. Aberdeen
Proving Ground, Md.: Edgewood Arsenal.
TABLE 3-1 Strengths and Weaknesses of the Prather Methodology
Strengths Weaknesses
Ease of use Clay constituents have changed considerably since
original study
Immediate results
Clay variability (handling, thixotropy, temperature
Relatively low cost effects, etc)
Large historical database of results Current methodology requires elevated clay temperatures
Apparent success in field for soft body armor All variability in testing results is assumed to be design
flaws in the armor
Apparent success in field for hard body armor
Method has limited medical validation for soft body
armor
Method has no medical validation for hard body armor
Pass/fail criterion
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