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.
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).
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
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).
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]
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
increase for targets having the same body weight and for projectiles of the same diameter.
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.
11Gelatin 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
|Ease of use||Clay constituents have changed considerably since original study|
Relatively low cost
|Clay variability (handling, thixotropy, temperature 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