This chapter discusses thoracic ballistic test methodologies, blast injury criteria and blastlike mechanisms, injury scales, potential adverse effects of body armor in blast exposures, possible injury to body organs remote from blunt trauma, and instrumented simulators for research.
As discussed in Chapter 3, modern body armor can defeat incoming pistol and rifle rounds, trading energy and momentum of the round for deformation of the armor. This deformation, however, has the potential for creating injuries in the thorax behind the armor, as well as injuries to remote organs, that may generally be characterized as blunt trauma.
Injuries to the thorax due to deformations of the armor are often termed behind- armor blunt trauma (BABT). Backface deformation (BFD) of the armor can cause local and distant fractures, contusions and hemorrhage in the thorax, as has been demonstrated in numerous animal studies (e.g., Prather et al., 1977; Clare et al., 1975; Cooper et al. 1982; Suneson et al., 1987; Lidén et al., 1988; Knudsen and Gøtze, 1997; Sarron et al., 2000; Gryth et al., 2007; Mayorga, 2010). These injuries are the result of physical deformation of the back face of the armor and associated stress waves that propagate through the thorax. While BABT is a known phenomenon, what is not known is the extent to which there may be significant injury to organs more distant from the point of impact, such as the brain, heart, spinal cord, and gut due to significant pressure waves transmitted through the body armor that result in pressure waves or shear waves in the body.
The injury risk for BABT will generally depend on the type and configuration of armor, the round, and the delivered energy of the round that results in an impact displacement and profile. This impact displacement and profile also depend on the physical characteristics of the person wearing the body armor. For both the body armor and the thorax, the impact location is important, and for the thorax, the rate sensitivity of the impact may be large. Many comprehensive discussions of penetrating ballistic trauma exist (e.g., Ryan et al., 1997), but there are relatively few such discussions on the topic of nonpenetrating
ballistic trauma, especially those that might be relevant to injuries associated with bullets and shrapnel striking body armor. This section presents the physics, biophysics, and methods of studying nonpenetrating blunt trauma with the goal of optimizing the design and testing of manufactured body armor.
A bullet is a localized source of energy that can cause high local compression and shear forces, penetrating protective layers. The most effective bullets deposit energy, shear, and momentum rapidly in the target. One general strategy for protection is to blunt the penetration of the incoming round, picking up as much mass as possible in the body armor while decreasing the round energy and increasing the contact area. Thus, the protective effect of any ballistic protective vest is provided by increasing the area of impact, thus transferring energy and momentum to the vest. However, effective transfer of large amounts of energy and momentum from the incoming round into the body armor generally implies some deformation of the rear, or back face, of the body armor.
The BABT deformation is generally larger under soft body armor for a given incoming round. An interesting comparison of energy and momentum scales may be seen by comparing characteristics of various rounds, as shown in Table 8-1. Energy varies by a factor of over 30 between the relatively slow 9 mm handgun round and the .50 caliber (12.7 mm) rifle round due to the differences in mass and velocity.
|Device||Muzzle Velocity (m/sec)||Round Mass (g)||Energy (kJ)||Momentum (kg m/sec)|
|5.56 x 45 M193 ball||991||3.6||1.7||3.57|
|7.62 x 51 NATO ball||838||9.6||3.4||8.13|
|12.7 mm 50 M2||890||42||16.6||37.4|
SOURCE: Ness, 2011.
A further elucidative comparison may be made between the impact energy and momentum scales of low-rate blunt trauma events such as automobile impacts and high- rate impact events such as BABT. The energy and momentum for various potential blunt trauma situations are shown in Table 8-2 and are plotted in Figure 8-1. It is apparent from Figure 8-1 that a nonpenetrating ballistic impact involves much lower total momentum transfer than typical low-rate blunt impacts. However, the energy transfer is comparable, depending on the round and impact velocity. This implies increased localization of energy transfer and shorter interaction time and likely increased localization of injury.
|U.S. Football Block||5||100,000||2.5||500|
|Automobile Thoracic Dash Impact||5||50,000||0.6||250|
|Automobile Head Impact||5||5,000||0.06||25|
6.7 * 10-6
aBased on assumed total lung volume of 3,000 mL.
bBased on applied lung volume of 300 mL.
SOURCE: Cameron Bass, Duke University
FIGURE 8-1 Initial energy and momentum for ballistics and other blunt impacts. SOURCE: Bass, unpublished.
Blast injury can occur with very small impact momentum and energy over very short time scales. In the limit, as the duration of such impacts becomes very short, an interesting comparison may be made with damage using ultrasonic energy. At high rate (~4 MHz), less than 20 cycles of acoustic energy delivered to lung tissue with a peak pressure of approximately 1 megapascal (MPa) will cause tissue damage (see Raeman, 1996). It is uncertain if these high frequency effects occur with BABT.
Figure 8-2 shows a high speed X-ray of deformation of hard body armor after rifle round impact. The chest deformation shown here may lead to trauma to ribs, lungs, heart, liver, and other organs. Data are needed to determine the optimum vest design that provides protection to the body, potentially including organs remote from the site of impact while minimizing weight that the soldier must carry.
FIGURE 8-2 Superimposed high-speed X-rays of the initial shock wave and deformation of the thorax during a 7.62 mm projectile live-fire test in a pig protected by hard body armor. SOURCE: Mayorga et al., 2010.
This section summarizes and evaluates the current body of evidence for behind- armor effects and whether the current standards for body armor provide sufficient protection to soldiers and law enforcement personnel. As will be seen, attempts to document the effects in animal models have been impaired owing to the inadequate numbers of test subjects studied and inadequacies in test design (e.g., incomplete pressure sampling, instrumentation deficiencies, limited measurement methods, short duration of follow-up). Much is still unknown about the injury mechanisms of BABT. The information provided can be used so that more informed recommendations can be made about the types of further studies needed, such as studies in large animals, physical models, and computer-based
simulations. It is important to conduct experiments to define the thresholds of energy transfer and the biophysical mechanisms that produce significant injury, as well as to evaluate if and how body armor can be improved to prevent remote injury and/or incapacitation.
Until the eighteenth century, combat infantry soldiers are estimated to have carried 15-30 kg (33-66 lb), with the remaining equipment and supplies being transported in a baggage train (Knapik et al., 1996). The weight burden in recent times has increased substantially. Negative consequences of substantial load carriage include potential heat stress (Barwood et al., 2009), decrements in psychomotor performance (Bensel, 1975), and ergonomic factors that may limit mobility (e.g., Harman et al., 1999). Beekley (2007) found also that significant increases in oxygen consumption, respiration, and heart rate for loads to 70 percent of lean body mass in male U.S. Army personnel.
Current body armor basic system mass is 7.1 kg (15.7 lb), accounting for 15 percent of typical maximum load carriage, but can be as high as 15.1 kg (33.3 lb), or 31 percent of maximum load carriage. Clearly, reductions in body armor mass are a potential method of reducing total load carriage and increasing mobility on the battlefield.
Finding: Carried mass, such as that associated with body armor, may decrease a soldier’s mobility and lead to fatigue.
Additional studies of the relationships between injury and the energy and momentum transferred to a body protected by armor innovations could lead to lighter weight armor that provides survivability equivalent to that of current military issue. However, the potential benefits of reducing the load carriage of body armor must be carefully weighed against the advantages of enhanced deformation that may arise from reduced areal density. The U.S. government and the North Atlantic Treaty Organization (NATO) are currently pursuing these goals.58
Beyond the development of techniques to identify injuries from BABT, it is important to develop a technique for assessing the risk of injury to humans behind body armor. One technique that has been shown to be effective in many fields of injury biomechanics is the use of an instrumented surrogate (dummy) to
58See, for example, the U.S. Army Research Institute of Environmental Medicine (USARIEM). Available online at www.usariem.army.mil/pages/download/LoadCarriagePDF.pdf and http://www.nwguardian.com/2010/12/09/9152/jblm-soldiers-provide-feedback.html.
evaluate the risk of injury from blunt trauma in automobile crashes. Elements of this technique include the following:
- Biofidelic surrogate. A dummy that is robust, gives a repeatable physical response, and responds in a human manner. A dummy may be physically very simple and may represent only a part of a human body. For example, an instrumented beam has been used successfully to represent an arm (Bass et al., 1997), and clay is currently used to represent the human thorax (Prather et al., 1977). However, dummies might be very complex, such as the anthropomorphically correct dummies being developed for the automobile industry. Generally, a surrogate should be as simple as possible while still representing the relevant human response.
- Engineering measurement. A physical parameter such as force or acceleration may be used to quantify the physical response of the dummy. Dummies may be instrumented to produce accepted or proposed injury criteria.
- Injury risk evaluation. A correlation between an engineering measurement and some injury model. For example, in frontal thoracic blunt impacts, an injury threshold of 60 times the force of gravity is used in the automobile industry.
- Validation by injury model. The injury risk evaluation is correlated to a physical model of injury. An injury risk model is without value if it has not been successfully validated using (1) epidemiology or physical reconstruction of an actual injury event, (2) an animal injury model, or (3) a human cadaveric injury model, as shown in Figure 8-3. Development of a relationship between a robust surrogate for injury and a validated injury model is crucial to the success of this approach.
FIGURE 8-3 Development of surrogate injury model. SOURCE: Bass, 2000. Reprinted with permission of the Center for International Stabilization and Recovery.
As discussed above, there are only three ways of obtaining direct injury data. Each technique has its strengths and weaknesses as follows:
• Cadaver experiments. A human cadaveric specimen is substituted for a living human body and tested in a realistic manner. The strong advantage of cadaveric experimentation is that the anatomy closely matches that of a living human. Both skeletal injury and tissue damage may be assessed using a cadaveric model. In addition, body kinematics and kinetics may be accurately determined. The principal weakness of cadaveric models is the lack of human physiology. It is not possible to assess certain injuries (e.g. commotio cordis,59 adult respiratory distress syndrome (ARDS), or diffuse axonal injury) using cadavers, as these pathologies require life processes to develop, and some pathological manifestations do not appear until weeks or years after the trauma.
59Commotio cordis is a disruption of heart rhythm that occurs as a result of a blow to the area directly over the heart at a critical time during the cycle of a heart beat. It frequently results in sudden death.
• Animal experiments. A living animal is substituted for a living human and tested in a realistic manner. The strong advantage of animal experimentation is that living physiology is available. Thus, animal experimentation can be used to assess injuries that require life processes before they manifest. The principal weakness of animal models is the limited correspondence of animal anatomy with human anatomy. Typical models in current use include porcine, caprine, and bovine models. As livestock is typically quadrupedal, there are substantial differences in cranial and thoracic anatomy between the experimental model and humans. Ethical considerations may present practical difficulties in testing with animals. Often protocols are restricted to following animals for a short period of time (e.g., 2 hr). This may significantly limit the usefulness of animal experimentation for certain types of injuries. As an example, ARDS from traumatic insult requires multiple hours or days to develop. When justified, however, it is ethically permissible to investigate the course of injuries for extended times. The types of experimentation needed are outlined in Appendix J.
• Human epidemiology. Observations from injuries suffered by humans in field situations similar to those for which testing is desired comprise the data of epidemiology. The advantage of epidemiology is that it often applies directly to the injury being investigated. For instance, epidemiologic data on injuries and conditions from automobile crashes are collected by the U.S. Department of Transportation for all fatalities and large numbers of nonfatal injuries each year. These data may be used to develop injury models and to focus the development of countermeasures. There are several limitations with epidemiological data. First, there is often limited information on the circumstances under which the injury occurred since the injuries do not occur in a controlled laboratory environment. Second, the data are always retrospective. Epidemiology does not have information on future systems or systems that are not in use. Third, in military environments, the collection of data may be quite difficult, and the information may be sensitive.
Volunteer models may not generally be used to obtain injury data (Figure 8-4); ethically, researchers must keep impacts in volunteer experiments below injury thresholds.
FIGURE 8-4 Volunteer experimentation. SOURCE: Stapp, 1949.
The most effective models use as many of these three means of obtaining injury data as possible or as are available for a given injury situation. The models must be appropriately biofidelic (lifelike). In other words, the models must adequately represent a human body in the situation analyzed. It is useful to employ several models (e.g., cadavers and animals) using consistent instrumentation and test conditions. Given appropriate injury modeling, the injury risk might be found to be as realistic as possible; otherwise, there is a potential for increasing the injury risk with inappropriate modeling.
There are a number of potential sources of injury from BABT; these include the initial contact shock, the subsequent displacement of the thoracic wall, and, in some cases, the propagation of pressure. The initial shock may occur with substantial high- frequency components and a relatively low resulting displacement. This shock pressure peak occurs because of the transmission of a pressure impulse from the rear of the body armor into the thoracic wall. Later bulk displacement may occur following significant local momentum transfer between the back of the body armor and the body. There has been extensive investigation into the relative effect of the initial shock and resulting displacements. Animal experiments at Oksbøl, discussed below, were designed to investigate this (Sarron, 2000). This issue, however, has not been completely resolved.
Pressure profiles have been measured in tissue simulants for impacts behind body armor. Pressure data from measurements in gelatin simulant material behind 6.4 mm ceramic (24 kg/m2) with aramid composite (10 kg/m2) and a fragment protective vest show an initial pressure pulse of 7.5 MPa less than 0.05 msec wide followed by a second pulse 0.8 msec later (van Bree, 2000). Stress wave propagation and concentration of reflected and refracted waves may
enhance injury (Lui et al., 1996). This is discussed further in the Oksbøl animal section.
The experimental basis for high-rate impact is not well established. Variables that might significantly affect injury potential include delivered energy, thoracic wall displacement, contact area or contact profile, loading rate, and location of impact. It is clear that the more localized the energy, the greater the potential deformation into and through the body armor. Large local displacements may cause destructive local shearing or compression of tissue. Distributed thoracic loading has a different injury pattern than localized loading (Crandall, 1998). Indeed, relatively sharp indent profiles may be assumed by penetrations behind soft body armor (Lewis, 2001) or behind full penetrations of ceramic armor captured in the soft backing material (DeMaio, 2001). The sensitivity of human tissue response to the applied loading rate may be large, and the location of impact is important (e.g., anterior thorax versus lateral thorax). Additional factors may include gender, age, body mass, stature, and other anthropometric parameters. Experimental programs designed to develop standard injury test methodologies usually focus first on a single relevant subject population. This may imply a focus on mid-sized males as being appropriate for a military population. However, it is perhaps necessary to consider gender-related size differences for general applications.
In the next section, injury mechanisms and mechanical correlates with injury are discussed. These sections are followed by a discussion of animal, cadaveric, and epidemiological experimentation for assessment of BABT.
BABT Injury Mechanisms
Thoracic anatomy, as shown in Figure 8-5, emphasizes the importance of the thoracic region for BABT. Indeed, ballistic protective measures have been designed specifically to protect this region and portions of this region. The majority of organ systems necessary for life are located in the thorax. The mediastinal region is particularly important. Notable structures in the mediastinum in addition to the heart include major blood vessels (aorta, pulmonary artery, and vein), branch points of the lungs (trachea), and connections to the gastrointestinal system (esophagus). The thorax includes the lungs, which offer a large impact surface area, and the liver, that are not completely protected by the rib cage. On the posterior region of the thorax, the spine presents an additional impact location that is potentially debilitating or life threatening.
There are a number of potential mechanisms for injury that have been seen in animal, cadaver, or human epidemiological studies. These include physiological mechanisms such as commotio cordis (Link et al., 1999) and ARDS (Miller et al., 2002). Commotio cordis, a disruption of heart rhythm that occurs from blunt trauma directly over the heart, is thought to occur during only a small window of the cardiac cycle. So, the risk of such cardiac rhythm disruptions may be small.
Several types of injuries may be attributed to tissue displacement. These include large displacements that may cause shear or crushing injuries. These injuries may include puncture caused by bony fracture. An example of this is a fractured rib penetrating into a lung and disrupting the plural cavity, which may result in pneumothorax or hemothorax. Fung et al. (1988) suggested that lung injuries might be related to compression of the alveoli under mechanical stress. However, at high rate, for ultrasonic forcing, the influence of local cavitation has been suggested. Indeed, negative pressures have been seen recently in ballistic animal experiments (Sarron, 2001).
Pulsation mechanisms similar to those seen in ultrasound tissue damage may be relevant to BABT tissue damage. For short-duration ultrasound, stress waves and cavitation have been proposed that may be BABT injury mechanisms. Ultrasound impulses involve low displacement with relatively high frequency (50 kHz-1 MHz). The effect of exposure duration on the threshold injury pressure is important (Carstensen et al., 1990) (Figure 8-6). Thresholds for damage in lung tissue in the murine model are significantly lower than those for damage in other tissues, as shown in Figure 8-7, and have been found to be frequency dependent.
FIGURE 8-7 Threshold damage for various tissues. SOURCE: Carstensen, 2000.
Unfortunately, no single-pulse thresholds are currently known; current thresholds involve five cyclic pulses. Of the nonthermal mechanisms, spalling from an interfacial impedance mismatch may be a BABT injury mechanism. Spalling may occur at alveolar surfaces, causing local hemorrhage.
For high rate BABT, there is a potential for ablation injuries caused by the friction of the impacting surface on the body. This has been seen in epidemiological studies (Mirzeabassov et al., 2000) and in cadaver experiments (DeMaio et al., 2001). These ablation injuries may be quite severe. DeMaio reported large deep bilateral chest wounds under certain circumstances, and Bass et al. (2002) reported injury from ablation behind helmets with relatively low-velocity incoming projectiles.
A final class of BABT injuries comprises the potential for injuries remote from the direct backface contact. The earliest observations on the effects of penetrating injuries to the nervous system remote from the site of penetration were case studies from the Civil War of temporary and sometimes long term motor and sensory paralysis (Mitchell et al., 1864). Damage from the transmission of kinetic energy from the point of impact on the torso to remote body organs in humans has been observed in a number of cases (Carroll and Soderstrum, 1978;
Sperry, 1993; Akimov et al., 1993; Cannon et al., 2001; and Krajsa, 2009). These corroborate Civil War case studies (Mitchell et al., 1864). A report on human trauma from BABT in law enforcement personnel emphasized the fact that protection from penetration does not protect from significant thoracic trauma (Wilhelm and Bir, 2008).
The important finding of hemorrhages in the sclera and conjunctiva of the eye in an anterior chest gunshot-wounded subject is evidence for the theory that a ballistic impact can lead to the remote transfer of a large pressure pulse through, in this case, the vena cava and vascular circuits (Sperry, 1993). More recent evidence for remote organ damage is from a histopathological analysis of 33 cases of death by gunshots to the thorax in individuals not wearing protective vests and without head wounds or a history of head trauma (Krajsa, 2009). In all cases, microscopic hemorrhages were observed on histological examination of tissue slices taken throughout the brain. These pathological observations, to be discussed further below, were also seen in pig studies of stress waves transmitted to the brain from high-velocity projectiles impacting the thigh (Suneson et al., 1987, 1990).
A second mechanism of injury to organs remote from the impact site is strokelike ischemia caused by air embolism, whether from a blast wave or a BABT impact. The importance of arterial air embolism was discovered by a German investigator, who reported that it is the cause of immediate death from blast injuries (Rossle, 1950). Others reported air embolism as an important mechanism for organ trauma in humans (Clemedson and Hultman, 1954; Weller-Ravell et al., 1975; Mayorga, 1997). Air embolism might be expected from the modeling studies that show alveolar rupture from lung compression (Fung et al., 1988), although the Fung hypothesis might be more relevant to posttraumatic lung edema.
Mechanical Correlates with Injury
An injury criterion might be used to study and categorize blunt trauma if developed with a physical measure (e.g., acceleration) and correlated with a surrogate test device to evaluate injury. The earliest such criteria were based on global acceleration, but these may not be appropriate for injuries with localization to specific parts of the body.
Several mechanical correlates have been proposed for high-rate impingement on the thorax. Cooper and Jonsson (1997) propose a correlation between lung injury and peak acceleration of the lung for short-duration waves. The threshold injury is found at approximately 300 g (ca. 3000m/s2) for the right lateral thorax of porcine test subjects and decreases as approximately the logarithm of the acceleration. Here, the damage mechanism might be based on propagating stress waves. Gross chest wall velocity is unlikely to provide the injury mechanism for high-amplitude pressure application of very short duration. It is also be an unlikely correlate for severe crushing injuries with large displacements. Between these limits, however, the chest wall velocity coupled with rate effects, might be strongly related to injury.
A mechanism for injury from blast loads that is based on chest wall velocity has been proposed (Axelsson and Yelverton, 1996). This mechanism is based on a simple lumped mass thorax model and animal experiments with sheep. They correlated the model results with the injuries in the animal experiments and defined an injury scale called the Adjusted Severity of Injury Index. This index is composed of a sum of injuries as a percentage of the maximum graded score for lung, pharynx/larynx, trachea, gastrointestinal tract, and the intraabdominal space. Results from this assessment are shown in Figure 8-8 for 177 sheep exposed to complex blast waves inside three enclosures with 11 m3, 18 m3, and 36 m3 volumes. The advantage of this model is that it provides a good correlation for complex blast waves. Its drawback is that it is a simple global model that is unlikely to represent the local interactions that actually cause injury and so may be misleading in certain circumstances, such as blunt trauma from small projectiles.
FIGURE 8-8 The Adjusted Severity of Injury Index values versus peak inward chest wall velocity. SOURCE: Axelsson and Yelverton, 1996. Copyright Williams & Wilkins, 1996.
Blast and blastlike injury criteria were extensively studied by the Lovelace Foundation supported by the Defense Atomic Support Agency and the U.S. Army Medical Research and Development Command (Martinez, 1999). These studies generally focused on free field application of pressure to the whole body (Bowen et al., 1968). Updated injury risk assessments have been recently published by Bass et al. (2008). The classic blast pressure thresholds, including the 1 percent fatal and 50 percent fatal free field curves, were developed from data on blast overpressure vs. duration compiled from numerous animal experiments (Bowen et
al., 1968). These are shown in Figure 8-9. These thresholds may be a benchmark for injuries that occur from very sharp initial peak pressures owing to differences in acoustic impedance between the back of the body armor and the thorax. This is discussed further in the context of the Oksbøl animal experiments below.
FIGURE 8-9 Overpressure/duration blast injury criteria. SOURCE: Derived from Desaga, 1950; Bowen et al., 1968; Bass, 2008.
Two common injury mechanisms may also be appropriate for blastlike BABT injury mechanisms (Maynard et al., 1997): These are
- Damage to the epithelial surfaces within the lungs owing to a stress wave passing through the parenchyma. As the wave passes, it encounters surfaces of varying density leading to impedance mismatches and local damage.
- Compression and re-expansion of alveoli owing to the passage of a shock wave.
These mechanisms may be implicated in the primary pressure wave seen in BABT impacts.
A systematic treatment of the extensive blast injury literature has been provided by Bass et al. (2008). Care must be taken in directly relating these data
to blunt impact injuries, as the area of impact and rates of loading can be different.
A large body of research has been performed on thoracic blunt trauma and thoracic injury mechanisms (Kent et al., 2006). Such blunt injury mechanisms may be relevant for trauma from larger, slower projectiles defeated by the body armor. The most widely used criteria for thoracic injury are the compression criterion and the viscous criterion for frontal impacts and the thoracic trauma index for side impacts. The compression criterion relates the relative chest deformation with respect to the chest depth to the level of injury. According to Kroell and coauthors (1971, 1974), 30 percent and 40 percent chest compressions cause abbreviated injury scale (AIS) level 2 injuries and AIS level 4 injuries, respectively. (The AIS injury scale of 0 to 6 and other injury scales are described in the next section.)
In the viscous criterion, the rate dependency of soft tissue injury is taken into account and VCmax, the maximum product of velocity of deformation and relative compression, is proposed as an effective predictor of injury risk. In an analysis of 39 unembalmed cadaver sternal impacts, a VCmax of 1.3 m/sec was associated with a 50 percent probability of AIS 3 (Viano and Lau, 1985). Eppinger et al. (1984) analyzed a large number of side impact test results and proposed the thoracic trauma index that is proportional to age, mass, and the average of the peak values of fourth struck-side rib and twelfth thoracic vertebra accelerations.
There is a strong need for the investigation of epidemiology appropriate for behind-armor effects. Indeed, the sparsity of data on high-rate incidents limits evaluation of the performance of current and future ballistic protection. The value of such epidemiology is seen in many fields. Use of such studies in the field of anesthesia substantially lowered the incidence of death from adverse events over the last 20 years (Hawkins et al., 1997). Epidemiological and retrospective studies are used in aircraft accidents and automobile crashes worldwide. Such critical data gathering has been useful in designing countermeasures to injury.
There have been several efforts to gather military injury data; these include the current Army program Joint Trauma Analysis and Prevention of Injury in Combat, the Navy-Marine Corps Combat Trauma Registry, the national injury center at Fort Rucker, Alabama, and past efforts such as the Vietnam-era Wound Data Munitions Effectiveness Team database for combat injuries. There is, however, a great need for a centralized repository of data from military trauma incidents.
An organized and robust injury scale is necessary for the evaluation of injuries using a common basis for animal experiments, cadaveric experiments, and epidemiology. There are several extant injury scores, but none is completely adequate for use in scoring BABT injury. An accepted standard for the assessment of thoracic injury is the AIS of the Association for the Advancement of Automotive Medicine. The numerical rating system ranges from 0 (no injury) to 6 (maximum, virtually not survivable). According to the 1990 revision, a flail chest with four or more rib fractures and/or bilateral lung laceration is AIS 4. The most serious thoracic injury is aortic lacerations, which is ranked from AIS 4 to 6 (Cavanaugh, 1993). As discussed below, however, this criterion is not generally sufficiently discriminative for BABT research.
The Injury Severity Score, or ISS (Baker, 1974) is used as an overall score for multiple injuries. Each body region (head, face, chest, abdomen, extremities, external) is assigned an AIS score. The highest AIS score for each region is selected, and the three highest of these are squared to produce the ISS score as:
ISS = AISmax12 + AISmax22 + AISmax32
The ISS score value ranges from 0 to 75. This score has been correlated with outcome for thoracic trauma, potential chest, abdomen, and external injuries.
A scale for combat-specific BABT trauma was developed from data obtained during the Soviet conflict in Afghanistan (Mirzeabassov, 2000). The scale shown in Table 8-3 and the associated injury scale in Table 8-4 are based on levels of damage suffered from BABT.
|Level of Trauma||Nature of Injuries|
|I (slight)||Scratches on the skin, ecchymoses, and restricted subcutaneous haematomas. Isolated focal subpleural haemorrhages.|
|II (medium-gravity)||Contused cutaneous wounds. Focal intramuscular haemorrhages. Plural focal subpleural haemorrhages. Isolated focal haemorrhages into the intestinal mesentery.|
|III (grievous)||Closed and open rib fractures. Lacerations of the pleura, haemorrhages into the pulmonary parenchyma. Subepi- or subendocardial haemorrhages. Subcapsular haematomas of parenchymal organs of the abdominal cavity and retroperitoneal space. Subserous haemorrhages into the intestines, ruptures of the mesentery. Restricted haemopneumothorax, haemoperitomeum. Vertebral fractures without injury to the spinal cord.|
|IV (extremely grievous, lethal)||Ruptures and crushing of internals. Closed trauma of the vertebral column followed by an injury to the spinal cord.|
SOURCE: Mirzeabassov et al., 2000.
|Gravity Level of the Trauma||Characteristic of the Loss of Fighting Efficiency||Probability of Rehabilitation
|Class of Losses|
|Loss of fighting efficiency for 1 to 3 min. Limited fighting efficiency during 15 min. Complete restoration within 24 hr.||99||Left in action|
|Loss of fighting efficiency for 3 to 5 min. Limited fighting efficiency up to 10 days. Complete restoration within 15 to 20 days.||85||Combat sanitary (recoverable) losses|
|Complete loss of fighting efficiency, limited fighting efficiency within 15 to 20 days, complete restoration within 30 to 60 days. Possible fatal outcome.||25||Combat sanitary (recoverable) losses|
|Immediate death. Death caused by complications. Invalidism and complete loss of fighting efficiency in surviving persons.||0||Unrecoverable losses|
|SOURCE: Mirzeabassov et al., 2000.|
Mirzeabassov et al. (2000) report the most extensive epidemiology available. It includes data from 17 subjects hit in the thoracic region wearing body armor with either 1.25 mm titanium (6B2) or 6.5 mm titanium (6B3TM) plates. The data were acquired during the Soviet experience in Afghanistan. Data include location of impact, injury description, long-term consequences of the impact, and age of the patient. The ballistics data include the type of weapon fired (either 7.71 mm Enfield or 7.62 AKM), firing distance, and impact kinetic energy.
Bullet kinetic energy was plotted against injury severity for both human epidemiology and animal experiments, as shown in Figure 8-10. The lighter body armor (6B2) had a significantly lower threshold for the onset of severe injuries. The most serious injury reported was hemopneumothora (i.e., accumulation of blood and air in the pleural space) in two patients that progressed to abscesses. In addition, an impact in the left rib cage was reported to have resulted in a large ecchymoses60 that extended from the groin to the knee.
While coverage of the plates is not reported, substantial injuries occurred to the back from relatively low-energy impacts, implying that the impacts were occurring in an area without plates. The researchers developed an injury level
60Ecchymoses is more commonly known as a “bruise.”
scale that relates the initial bullet kinetic energy with the severity of injury in both the human epidemiology and animal experiments, as shown in Figure 8-10.
FIGURE 8-10 Kinetic energy vs. injury severity. SOURCE: Mirzeabasov et al., 2000.
To understand the Russian epidemiology data, it is important to consider details of the Soviet military medical service. Extensive data analysis is available for mine trauma victims from the Soviet experience in Afghanistan (Nechaev et al., 1995). Over 90 percent of the patients were evacuated by air, but only 4 percent were delivered to the central military hospital (CMH) within 6 hours of the mine blast injury (Figure 8-11). This was the initial treatment received for more than 80 percent of the wounded. The severity of the wounds was distributed as shown in Figure 8-12. It is interesting to speculate that the distribution of the severity of injuries may have changed with the delivery time. Increased delivery times would exacerbate the severity of the injuries but tend to decrease the number of fatal injuries.
FIGURE 8-11 Time of delivery of wounded to the CMH (average 1983-1984). SOURCE: Nechaev et al., 1995.
FIGURE 8-12 Severity of wounds for patients delivered to the CMH (average 1983-1984). SOURCE: Nechaev et al., 1995.
This work is almost the only instance of comprehensive epidemiology for BABT trauma. As with all epidemiological studies, there are limitations. The ranges were estimated, and battlefield trauma care may have been significantly different from care by Western militaries. This difference could influence the distribution of injuries. To correlate this study with Western body armor, it might be useful to acquire Russian body armor for calibration with Western vest designs.
Both the U.S. Army and the Office of the Director, Operational Test and Evaluation personnel emphasize to the committee that there have been no known U.S. soldier deaths due to small arms and shrapnel that were attributable to a failure of the issued ceramic body armor for threats for which the armor was designed (Rickey, 2010). Based on this, the U.S. military has fielded hard body armor with adequate survivability characteristics to soldiers in combat. Further, since the National Institute of Justice (NIJ) undertook the responsibility to standardize personal body armor for law enforcement personnel in 1973, over 2,900 lives have been saved.61
The tragic failure of soft zylon body armors in a small number of well-publicized cases in law enforcement further emphasizes the rarity of either soft or hard armor failures and the success of the testing programs based on clay. What is unknown, however, is the link to human injury for the existing hard armor assessment methodology. It is necessary to determine whether the standard is overly conservative and how to assess trade-offs of weight and mobility against protection from ballistic threats.
Animal experiments may be used for the development of injury criteria for blunt trauma, including BABT. However, animals used in such experimentation, typically livestock, have significant differences in anatomy from humans. So, scaling data from animals to humans must be performed with techniques that may be uncertain or nonexistent.
There have been a number of experiments that investigated thoracic penetration and BABT with soft body armor (Prather et al., 1977; Carroll and Soderstrom, 1978; Lidén, 1988), but the committee concentrated on studies that are applicable to BABT with hard body armor. This includes several direct impact studies from high velocity projectiles..
Large animal studies specifically designed to assess injury resulting from nonpenetrating ballistic impact on body armor have been done by U.S., Canadian, Swedish, Danish, Dutch, and French teams, mostly in anesthetized pigs and using various models of body armor and threats ranging from .38 cal to .30 cal (7.62
61Available online http://www.dupont.com/kevlar/lifeprotection/survivors.html?NF=1.
mm) and .50 cal (12.7 mm). Many of these studies are reported by the NATO Task Group on Thoracic Response to Undefeated Body Armor (Mayorga et al., 2010) and a series of international symposia called Personal Armor Systems Symposia.
Bowen et al. (1966) report experiments on dogs with the lateral thorax impacted by nonpenetrating missiles as shown in Figure 8-13. Aluminum missiles with variable masses were used, and the impact end was a flat cylinder with a diameter of 7 cm. The impactor masses varied from 63 g to 381 g, impact velocities varied from 18.9 m/sec to 91.4 m/sec, and dog mass varied from 12.2 kg to 23.1 kg. The dogs were positioned so that impacts were produced at the right lateral chest wall near the midthorax. Ribs fractured include the fourth rib through the eighth rib, implying impact locations near the fifth or sixth rib. Animals that survived the immediate postimpact time were sacrificed 30-40 minutes after the impact time.
Scaling techniques were developed for transferring dog values to equivalent human values. These scaling relations, however, are uncertain, because the impact area of the missile was not scaled in the experiments with the body mass of the dog (Bowen et al., 1966). The mass of the impacted lung was compared with the mass of the lung on the contralateral side. This ratio is generally correlated with severity of injury; the threshold value of the ratio of the right mass to the left for fatalities in the watched period is approximately 2.3. In addition, the scaled energy of impact (scaled to a 75 kg man) is well correlated with fatalities. In Figure 8-14, the increased impact lung mass is plotted against the scaled impact energy. The increased lung mass of the animals measured postmortem has some limitations as an injury measure, because bleeding into the lung occurs most effectively while the animal is living. However, over the limited time that the study followed the test animals, the correlation is relatively good.
FIGURE 8-14 Impact energy (scaled to a 75 kg man) vs. increased lung mass. SOURCE: Bowen et al., 1966.
The impactor mass of 63 g may represent behind-armor impact in some regimes. For body armor with 24 kg/m2 areal density, the effective impact diameter of approximately 37 mm reported by Mirzeabassov et al. (2000) implies an additional 84 g of mass owing to the induced motion of the body armor. In addition, the time scale is similar to BABT forcing. Tam et al. (2000) report approximately 2 msec to full displacement of approximately 4 cm with a peak acceleration of approximately 28,000 g.
Danish Army Combat School
Animal experiments were performed to assess the potential for thoracic injury from BABT behind undefeated body armor. As reported by Knudsen and Gøtze (1997), projectiles included a 5.56 mm NATO ball at 921 m/sec, a 7.62 mm NATO ball at 848 m/sec, and 12.7 mm AP rounds with reduced load at 463 m/sec to 595 m/sec. Tests were performed using 20 swine weighing 60 kg each, with one used as a control. The swine were supported in a standing posture, and lateral impacts were performed at the level of the xiphoid cartilage. Physiological monitoring by electrocardiogram (ECG), spirometer, and pulse oximeter was performed for an hour-long observation period prior to sacrifice.
Two of the four animals at the largest kinetic energy were sacrificed for ethical reasons before the end of the 60-minute period posttrauma. Injuries were assigned as shown in Table 8-5. Minimal injuries were seen at kinetic energies below 1.7 kJ, while fatal injuries were expected above 8 kJ for the body armor selected for testing. Cardiac lesions were seen in many of the test animals as well as in the controls, suggesting this damage was an experimental artifact. There was no consistent correlation seen between skin damage and lung damage. No specific injury criterion was developed that is independent of the experimental setup.
|Round||Impact Velocity (m/sec)||Kinetic Energy (kJ)||Injuries|
|5.56 mm NATO ball||920-922||1.693-1.700||Minimal|
|7.62 mm NATO ball||838-861||3.248-3.429||Minimal to moderate|
|12.7 mm M2||463-595||4.839-7.992||Severe to fatal|
SOURCE: Knudsen and Gøtze, 1996.
As a more extensive follow-up to the Danish Army Combat School tests, trials were performed using extensive instrumentation in Oksbøl, Denmark, in
1999 under the aegis of NATO. The Oksbøl trials used porcine specimens similar to those used earlier in the Danish Army Combat School tests. The protective equipment tested was provided by Denmark, France, the United Kingdom, and the United States. The Danish and French developed body armor systems that defeat the 7.62 × 51 mm threats while the U.K. and U.S. systems defeat the more energetic 12.7 mm (.50 cal) sniper rifle threat. Three designs of body armor were used with each design tested in groups of eight pigs each. There were three control animals. The impact site was the right lateral thorax in the middle of the eighth rib. Instrumentation included pressure and acceleration measurements in the thoracic wall and physiological measurements for half an hour before euthanasia, as stipulated by the animal welfare oversight committee. In retrospect, this was too short a time in which to develop traumatic sequelae.
Observations included recordings from pressure transducers, accelerometers, ECG, blood oxygen saturation, respiratory rate observations, and postexperiment autopsy examinations. The gross and microscopic studies included lungs, liver, and, in some studies, the heart and kidneys (Mayorga et al., 2010). Unfortunately, no intestine, brain, spleen, aorta, or spinal cord studies were reported. Each of four separate experiments (Danish, British, and two French studies) followed more or less the same protocol with respect to animal anesthesia and physiological observations, but the selection of protective armor varied widely, with an attempt in each group to compare one type of armor to another.
The extensive test series was designed to discern the cause of wounding, by separating the effects of the initial large, short-duration pressure peak from the effects of a secondary displacement pressure peak, presumably caused by the deformation of the body wall behind the body armor. The three body armor configurations are shown in Figure 8-15. The first type is a typical body armor with a steep first pressure peak and a more extended later displacement peak (G1). The second type should have a second peak only (G2), and the third should have a first peak only (G3).
FIGURE 8-15 Body armor for Oksbøl trials. SOURCE: Sarron et al., 2000.
The armor did not perform as expected. As shown in Figure 8-16, the first-peak-only armor (G1) decreased the second peak but also reduced the first peak pressure. Second-peak-only (G2) armor significantly reduced both the first peak and the second peak but never so much that the first peak was less than the second peak. The first-peak-only armor is a difficult engineering problem, implying a need for infinite rigidity in the armor system and thereby significantly limiting resulting BFD. The second-peak-only design also presents a very difficult problem insomuch as it requires a match in impedance between the rear of the armor and the tissue.
FIGURE 8-16 Average first and second peak pressure, Oksbøl trials. SOURCE: Sarron et al., 2000.
All animals tested were injured; however, injury scaling is difficult. The typical injury criterion used in automobile blunt trauma, AIS, is not specific enough to delineate wounding behavior in the Oksbøl series. All subjects had AIS 2-3 level injury. However, postmortem lung mass, as shown in Figure 8-17 appears to be a more specific injury measure in this experiment as all the animals were sacrificed within 60 min. Different times of expiration and the dynamic effects of hemorrhage may confound lung mass measurements.
FIGURE 8-17 Average postmortem lung mass, Oksbøl trials. SOURCE: Sarron et al., 2000.
The effect of the first peak only may be best studied by comparison with shock impingement. To get a qualitative idea of the effect of this pressure peak, we can compare the Oksbøl peak uncorrected for the location of the pressure transducer in the tissue with the Bowen curves as shown in Figure 8-18. While the use of this internal pressure is not appropriate for the Bowen curves, it is likely conservative for the assessment of an injury threshold. The Oksbøl experiments had pressure peaks in excess of 30 MPa with durations of 0.3 to 0.4 msec. The 100 percent blast lethality threshold (Bowen et al., 1966) is below this value. This suggests that the local lung damage may be due to transmission of a high-amplitude pressure wave. Indeed, lung mass injury from the Oksbøl tests scales directly with the first peak only.
FIGURE 8-18 Oksbøl first peak on Bowen curve. SOURCE: Sarron et al., 2000.
The results showed significant injury and a high mortality for most of the study groups, along with surprisingly significant thorax and lung injuries from hard armor having areal densities up to 24 kg/m2 with foam backing. Pressure and accelerometer recordings were incomplete as the majority of studies experienced saturation of the instruments with pressures exceeding 34 MPa. Nevertheless, some very important observations and correlations were made for high-velocity ballistic impacts and the subsequent deformations of the back face of body armor.
French Délégation Générale pour L’Armement (DGA)
An extensive test series using porcine subjects was performed by DGA in the 1990s and early 2000s. (Sendowski et al., 1994; Sarron et al., 2001) The model selected was a female swine of mass 60 kg (± 5 kg).
Four impact areas were selected:
• Pulmonary near the seventh right dorsal vertebra. The animals were observed for 2 hr after trauma.
• Lateral cardiac area at the fifth left thoracic vertebra. The animals were observed for 2 hr after the trauma. This condition was chosen to investigate cardiac contusion.
• Mediastinum opposite the apex of the heart. The animals were observed for 15 min prior to euthanasia. This condition was chosen to investigate induced ventricular fibrillation.
• Liver. The animals were observed for 15 min.
As with most NATO-supported animal experiments, the DGA animals were not observed for a substantial time posttest. This prevents the investigation of such injuries as ARDS and others that require long-term physiological monitoring for symptoms to develop.
All shots occurred at the end of the inhale cycle except for the lateral cardiac tests, in which shots occur at the end of the exhale cycle. Body armor used included ultra-high molecular weight polyethylene (UHMWPE). Test rounds included a 7.62-mm round at 829 m/sec and a 5.56-mm round at 989 m/sec. Five tests were performed for each of the rounds for each test condition (40 tests).
Instrumentation included an accelerometer on the rib and a balloon gauge inside the thoracic esophagus to measure impact pressure. The accelerometer, however, did not function for most of the tests, and the systemic pulmonary pressures were very low (~1.8 mm Hg). Physiological measurements included respiration and cardioactivity (ECG), pulmonary artery pressure, abdominal aortic artery pressure, and vena cava pressure. Blood gases and cellular enzymes CPK and LDH were also measured.
Posttest measurements included extensive grading of locations of skin damage. The cutaneous wound was statistically significantly greater for the 7.62 series than for the 5.56 series. The average diameter of the cutaneous wound was found to be about 14 cm.
Pulmonary wounds were assessed from the right thoracic shots. A bruise developed under the pleura surrounded by a region with inflammation. In some tests, emphysema developed at the center of the impact. The percentage of injured lung for the 7.62 mm (13 percent) was statistically different from that for the 5.56 mm series (7.4 percent). Cardiac wounds were assessed using the lateral cardiac area shots and the mediastinal shots. The tests were highly variable, probably due to the shape of the sternum.
Initial apnea duration in 30 tests averaged 15 sec for the 7.62 mm tests and 8 sec for the 5.56 mm tests. This included all shots in the sternal area. Secondary apnea occurred in several of the tests. Deaths seen in these tests are shown in Figure 8-19.
FIGURE 8-19 Animal fatalities during monitoring period. SOURCE: Adapted from Sendowski et al., 1994.
Experiments at the DGA using 7.62 mm test rounds into laminated UHMWPE body armor concentrated on investigating physical measurements of the thoracic wall using a flash X-ray technique with lead markers, as shown in Figure 8-20 (Sarron et al., 2001). This technique provides a good representation of the motion of the chest wall up to a maximum velocity of 30 m/sec. These studies showed a large negative pressure (~3 MPa) in preliminary data, suggesting that a cavitation injury mechanism might be involved. A shock wave arrives before significant displacements, and pressures are not correlated with the local displacements.
From the extensive studies in anesthetized pigs by the French investigators reported in a NATO summary of international studies (Mayorga et al., 2010), a synopsis of the relationships between lung contusion areas, recorded pressures, and deformation measurements can be made. These results are summarized in Figure 8-21 and Figure 8-22. Since Figure 8-20 presents a superimposition of a series of cine-radiographs, the timing of the chest deformation relative to the BFD cannot be related to the maximum deformation or pressure measurements shown in Figures 8-21 and Figure 8-22.
FIGURE 8-20 BABT flash X-ray. The red lines estimate the skin surface and the square and circle enclose reference markers. The left view is before impact. The right view superimposes four time images 0, 1.5, 2, and 2.5 msec after impact. The impact disperses the single marker inside the square and the circle showing the deformation of the thoracic wall with time. SOURCE: Sarron et al., 2001.
FIGURE 8-21 Relationship between area of lung surface contusion and maximum back-face deformation of body armor. SOURCE: Sarron et al., 2001.
FIGURE 8-22 Relationship between area of lung surface contusion and pressure 6 cm from point of impact. SOURCE: Sarron et al., 2001.
In order to find a correlation between intrathoracic pressure, BABT, and high- vs low-velocity bullets, 20 pigs, protected by a NIJ Level 3 or 4 bulletproof vest, were shot with 7.62 mm NATO bullets (2.4 kJ and 3.2 kJ), and 10 unprotected pigs were shot by air gun with 40 mm rubber projectiles (0.07 to 0.2 kJ) (Prat et al., 2010). Rib fractures occurred in 21 of the 30 animals with no correlation to the projectile kinetics; however, intrathoracic peak pressures showed a good correlation with the volume of lung contusions.
U.S. Army Aeromedical Research Laboratory
Tests were performed by the U.S. Army Aeromedical Research Laboratory at Fort Rucker, Alabama, to evaluate the effect of polyvinyl chloride foam backing at standoffs of 14 mm, 21 mm, and 27 mm (Haley et al., 1996). Seventeen pigs (91 kg) were tested behind ceramic body armor with a 12.7 mm test round. Three control pigs were used, and the animals were monitored for 3 hr after the tests. The subjects were instrumented with accelerometers on the front and back faces of the armor plate, and load cells were placed behind the pig to evaluate the global force. Heart rate and respiration rate were monitored. The standoff foam was found to be rate sensitive, transmitting large pressures to the
thorax. Based on the force measurements, the researchers concluded that a 25 mm standoff is necessary for effective protection of the thorax.
Researchers at Anter Corporation of St. Petersburg, Russia, investigated BABT on small dogs with various vest types, including ceramic plates with two thicknesses of titanium plates (Mirzeabassov, 2000). Rounds included three bullet types: a 7.62-mm round with two different impact energies, a .45-cal M11911A1, and a 9-mm round. They reported impact kinetic energies ranging from approximately 0.3 kJ to 3.2 kJ on 21 canine subjects. Tests were monitored using high-speed flash X-ray. Injuries received included superficial wounds, rib fractures, hemorrhages, and deep lacerations. The tests were correlated to both cadaveric tests and a limited epidemiology as discussed in the section on cadaveric experiments for BABT below. Unfortunately, information on instrumentation used in the animal testing is not reported.
The researchers found that BFD with soft body armor has a maximum depth of penetration (H) that is positively correlated to the diameter (L) of the maximum area of contact (S). In contrast, the action of ceramic plates tends to lower the depth of penetration relative to the contact diameter (L). For these experiments, the depth of deformation and area of contact were measured using a high-speed flash X-ray, and the volume of the deformation was inferred assuming an ellipsoid of revolution.
Swedish studies conducted 32 years after NIJ protocol development demonstrated that the U.S. criterion allowing a 40 mm impression behind the vest was not protective for higher velocity projectiles (Gryth et al., 2007). The tests involved 22 pigs protected by armor and 7.62 × 51 mm (800 m/sec) rifle bullets. The anesthesized animals were monitored for brain, circulatory, respiratory, and blood chemistry changes during the acute period after the ballistic exposure. Extensive anatomical examinations were performed. The principal conclusion was that 50 percent of the animals died in the group that had vests protective to 40 mm. Indeed, 25 percent of animals died in armor that protected to 34 mm impressions. It is important to note that neither the type of clay nor the temperature conditions, well known to be important variables for the clay surrogate, were stated for the Swedish studies. Another important point is that the velocity of the projectiles used was more than twice that of the earlier U.S. handgun studies using .38 cal, 128-grain bullets on goats.
Other Swedish studies involving animals and body armor used larger caliber projectiles in soft-armor-protected pigs and compared the pathological results to depth of penetration in a tissue surrogate consisting of soap to emulate the U.S. clay surrogate. In addition to using 9 mm projectiles, they also used 44 magnum and 12 gauge solid shotgun projectiles. The soft armor protection
consisting of various layers of Kevlar with and without foam backing was minimally protective for most of the nine experiments (Lidén et al., 1988). One of the most important studies with hard armor was an electroencephalographic study on pigs impacted with 7.62 mm (800 m/sec) bullets. Although the armor was not pierced, five of the eight pigs showed temporary electroencephalogram changes, and all pigs showed lung injuries (Drobin et al., 2007).
The most recent Swedish studies evaluated the efficacy of adding lightweight material to hard body armor to attenuate the transmission of pressure waves (Sondén et al., 2009). This material is now generally called trauma-attenuating backing (TAB). Twenty-four pigs protected by a ceramid/aramid body armor without (n = 12) or with TAB (n = 12) were shot with a standard 7.62 mm assault rifle. The TAB significantly decreased the size of the lung contusion, decreased hemoptysis, and reduced peak pressures by 91 percent.
Cardiac Trauma Thresholds
The most extensive live-animal, live-fire tests using body armor were conducted in the 1980s in the United Kingdom. Forty-eight anesthetized pigs were instrumented with pressure transducers and accelerometers. High-speed photography as well as cineradiography observations were used to define the kinetics of deformations in the armor and in the chest. In addition, temporal and spatial pressures were measured. Animals were studied for the consequences of 64 J to 363 J delivered mostly to the anterior sternum using an air gun with projectiles of 0.14 and 0.38 kg and velocities between 20 and 74 m/sec (Cooper et al., 1982). The principal findings were that the degree of heart damage was related to the ratio of chest wall displacement to anterior-posterior chest diameter and that this was proportional to the energy of the impact divided by the product of the diameter of the impactor and the body mass. The diameters of the impactors (3.7 and 10 cm) are much larger than those of the live-fire projectiles but are in the range of the diameters of the backface deformations.
In another study, small steel ball fiduciaries were implanted in and around the heart to measure displacements using cineradiography (Cooper et al., 1984). These studies give important basic information regarding the thresholds for cardiac damage from energy delivered to the chest wall and the diameter of the BFD. This information can be included as criteria for body armor design, not only to defeat a high-velocity, high-energy projectile (e.g., 3 kJ to 13 kJ), but also to evaluate risks to personnel from BABT.
Other Animal Ballistic and Blast Experiments
There have been a multitude of both large- and small-animal studies on the effects of the overpressure associated with explosive discharges from nearby bomb explosions, improvised explosive devices, and artillery weapon discharges. In addition, there have been studies of the dispersion of trauma from penetrating wounds in both small and large animals using histopathology to evaluate trauma
to organs remote from the projectile wound trajectory. Observations date from the writings of Aristotle, who noted intestinal trauma in deer after blunt injury to the body (Vance, 1923). Notable among experimental studies is one that showed pressure changes and damage to the peripheral and central nervous systems in pigs after a high velocity (1500 m/sec) projectile wound of the thigh (Suneson et al., 1987, 1990). There were permeability increases in small vessels of the brain and sciatic nerve after impacts with energy of 700 kJ associated with pressures of 125 kilopascal (kPa) in the brain and 270 kPa in the abdomen. The speed recorded for the shock wave was 1,400 m/sec, about what would be expected for sound velocity in tissues. Another important live-animal study of blunt trauma used impactors with velocities from 16 to 94 m/sec in anesthetized pigs (Cripps, 1996). Cripps produced small bowel injury at a threshold deformation speed of 40 m/sec. Injury to the colon occurred at all speeds. Microscopic studies of the brain of 15 kg dogs after bullet impacts to the thighs showed clear signs of damage from transmitted pressures (Wang et al., 2004). Other studies in pigs without protective vests focused on the consequences of blast trauma (Axelsson et al., 2000).
Finding: U.S. body armor prevents high-velocity bullets from penetrating the body but may not protect personnel from the shock wave resulting from the initial projectile impact and the trauma induced by the backface deformation. Tests in Europe have shown that adding trauma attenuated backing material to body armor vests may provide some degree of protection by attenuating the transmission of pressure waves.
Finding: Details surrounding the force that is transmitted from the body armor to the person wearing the armor, including the amount, the timing, and the immediate and long-term consequences of this force, are unknown.
An important missing link between the design of body armor and thresholds of injury and death is a lack of knowledge of the kinetic energy thresholds. Animal and human cadaver research experiments in this area are vital to establish these thresholds. These thresholds will guide the development of more effective and possibly lighter protective body armor. Such data are needed to validate blunt trauma prediction models and to provide guidelines for developing a physical surrogate for testing manufactured armor adherence to specifications. A general plan for the conduct of the needed studies is given in Appendix J.
While body armor protects against ballistic penetrating missiles, it might lead to adverse effects from blast exposures. British Army studies showed a higher incidence of primary blast injury in fatally injured soldiers, 90 percent of whom were wearing body armor, than in civilian bystanders (Mellor and Cooper, 1989). Human experiments involved exposure of vest-wearing volunteers to a blast wave from a chemical explosion to simulate a muzzle blast. Ten test subjects
were exposed to the blast wearing different clothing configurations, including Kevlar and ceramic vests. Explosive charges were detonated 3 m from the subjects, who were instrumented with a strain-gauge pressure transducer in the esophagus for intrathoracic pressure measurements. Shock wave pressures outside the body were about 17 kPa, and those on the esophagus were 7-8 kPa (Young et al., 1985). These experiments did not evaluate the relationship of the blast wave frequency and the resonance frequency of the thorax covered by a particular vest. However, the maximum energy transfer occurs when the predominant frequency of the incident shock wave most closely matches the resonance of the thorax (Cooper 1996). The resonance of the human thorax is between 40 and 50 Hz (Von Gierke, 1968). But the stress wave, whether from an air blast or from pressure transduced from a ballistic hit, will have different frequency spectra and a different energy coupling relationship depending on the projectile velocity in the case of a ballistic hit. Higher blast loadings can have an energy spectrum whose components are close to the natural resonance of the human thorax, thereby causing greater injury.
Another mechanism that can account for adverse effects in subjects wearing body armor is the reflection of incoming power at interfaces associated with the vest-air-thorax space. An analysis of the physical phenomenon can be made based on the physics of reflection and transmission of power through or from surfaces of differing density and bulk compressive moduli (i.e., acoustic impedance). The governing equation is the same that applies for propagating electromagnetic fields and acoustic pressure waves. The well-known Fresnel equation for sound gives the ratio of pressure reflected to incident pressure based on the difference in acoustic impedance. The impedance is proportional to the square root of the compression modulus times the density of the material. The power transmitted to the thorax is greatest when there is a match between the medium of the vest and that of the thorax. But if the vest impedance for a given frequency is less than that of the thorax (ribs and associated muscle and skin), then the reflected power will be less than that for a high impedance vest next to a lower impedance thorax.
However, the vests used in the previous studies are no longer used. Recent results (Wood et al., 2010) show that NIJ Level 2 and Level 4 police-issue ballistic vests substantially reduce the peak overpressure of primary blast waves. Attenuation ratios of peak reflected pressure were observed to increase with increased input pressure. The NIJ Level 2 vest showed overpressure attenuation ratios ranging from 3.4 at low input pressure levels to 14.2 at maximum input pressure levels of this study. Similarly, the NIJ Level 4 vests showed an ability to attenuate the peak reflected pressure seen behind the armor vest. Attenuation ratios for the Level 4 vests varied from 9.5 at low input levels to 56.8 at the maximum peak pressure input used in this study. The vests used in the human studies wherein a definite increase in intrathoracic pressure was observed did not have a NIJ level classification. They were described as (1) military field jacket (control), (2) woven ballistic armor (Kevlar vest), (3) ceramic flak vest (6.4 kg), and (4) ceramic flak vest over the Kevlar. Further study is needed using vests that
are consistent between studies and that are manufactured according to the NIJ specifications.
In experimental studies with rats and pigs it has been shown that the lung injury increases if foam material is interposed between a blast wave and the thorax. However, if another material with high impedance is placed between the blast and the foam over the thorax, the injury is much less severe (Cooper et al., 1991). These phenomena are explained by the acoustic impedance of the layering of materials of a particular body armor design and the acoustic impedance of the tissues of the thorax and lungs (Cooper et al., 1991). Animal studies confirm that much less pathological damage is sustained in the lungs and intestines from a blast pressure for high impedance body armor than for lower impedance body armor, presumably because less energy coupling occurs (Cooper, 1996; Cripps and Cooper, 1996).
Animal studies confirmed that with higher levels of blast loading, the wearing of cloth ballistic vests resulted in increased lung injury as measured by lung weight increase and death relative to the control group without vests. The experiments on sheep used blast pressures of 115 kPa to 420 kPa (Phillips et al., 1988) and suggest that vests be designed to modulate the blast or ballistic energy spectra so that less coupling to the resonance frequency of the thorax occurs.
Data for current issue military vests are not available. The observations above are relevant to blast effects and also to trauma from ballistic effects as acoustic impedance matching physics is relevant to the transmission of energy to the body. These experimental results from different groups with a variety of armor material and the related physics suggest opportunities to design vests where selected material properties of layers will confine the energy of a projectile to the vest rather than allow transmission to the body.
Finding: The design for future body armor vests should consider blast effects as well as trade-offs between bulk, weight, and protection.
Discrepancies between published measurements of pressure changes in intrathoracic pressure for human subjects exposed to blasts from explosives with and without vests needs to be resolved. In the current threat environment, protection against blasts must be considered at least as important as ballistic impact protection, and the relationship between the two threats needs to be better understood.
Recommendation 8-1: The Army medical and scientific testing communities should adequately fund and expedite the research necessary to experimentally and epidemiologically quantify the physiologic and medical impact of blunt force trauma on the body from both ballistic and blast threats to soldiers.
A very limited number of ballistic behind armor impact studies on cadavers have been performed. For injury association using animal experiments, such cadaveric experimentation is likely crucial as there are significant differences between livestock anatomy and human anatomy, especially in the mediastinal region.
In addition to the animal test described earlier, the Russian Anter Corporation conducted 13 cadaver experiments (Mirzeabassov et al., 2000), The tests used various vest types, which included two thicknesses of titanium plates and ceramic body armor. Rounds included 7.62 mm with two impact energies, a .45-cal M11911A1 and a 9 mm round. The researchers reported impact kinetic energies ranging from approximately 0.3 kJ to 3.2 kJ. Details regarding the preparation of the cadavers and conditions under which the tests were conducted are not reported. The authors state some differences in tolerance between animal and cadaver results for the internal organs but do not quantify them.
Instrumented Cadaveric Specimens
Researchers instrumented 17 cadaveric specimens, including 6 females and 11 males with a mean age of 73 years, as reported by DeMaio et al. (2001). The study investigated various body armors with different velocity regimes. Instrumentation included accelerometers at the sternum, T7, carina, and ligamentum arteriosum, as well as pressure transducers in the right and the left heart ventricle, and the left chest. Measurements of the impact pressure between the armor and chest wall were reported as not reliable. Pressurization of the lungs and cardiovascular system was used to make the cadavers more realistic human surrogates. Posttest evaluations assessed exterior wounds, sternal and rib fractures, cardiac bruising, and other injuries, including pulmonary injuries and spinal fractures.
Three body armor systems were tested: a soft vest with 9 mm test rounds, a light plate with a 7.62 mm round at two representative velocities, and a heavy plate with a 7.62 round at one representative velocity. Three injury levels were defined: survivable (with minimal trauma), immediately survivable (nonlethal if treated within 1 hr), and lethal (fatal even with treatment within 1 hr). Injuries received ranged from light surface friction to deep, extensive bilateral open chest wounds. The most severe injuries arose from complete plate penetration; an estimated three quarters of the cases where the round went fully through the plate were estimated to be lethal.
Use of the data from the instrumentation for this study poses several difficulties. The accelerometers were attached to the sternum using suture
material. However, accelerations were seen in excess of 1,000 g in some tests. The effective mass of a typical shock accelerometer (nominal mass ~ 1 g) at this acceleration level exceeds 1 kg. To ensure repeatable results, a rigid connection is required. Second, the measurement of uniaxial accelerations on viscoelastic components within the thorax such as the carina and ligamentum arteriosum is questionable. Results will vary significantly based on local details of mounting and local viscoelastic behavior of the compliant structures of the carina and ligamentum arteriosum. While it may be possible to use these measurements for qualitative estimation of the gross arrival of local tissue deformation owing to the significantly increased density of the accelerometer compared with the surrounding tissue, intra specimen comparisons are not reliable. So, this work is most useful for qualitative injury performance of the articles tested. No injury risk function has been developed in terms of measured dynamic variables.
BABT Injuries Behind Hard Armor
A highly deforming hard body armor study to estimate the mechanical correlates with BABT injury in nine cadavers and two anthropomorphic test dummies used a range of velocities including low-severity impacts, medium-severity impacts, and high-severity impacts based upon risk of sternal fracture (Bass et al., 2006). Thoracic injuries ranged from minor skin abrasions (AIS 1) to severe sternal fractures (AIS 3+) and were well correlated with impact velocity and bone mineral density. Eight male cadavers were used to develop a criterion for injury risk. A 50 percent risk of AIS 3+ injury corresponded to a peak impact force of 24,900 ± 1,400 N. This study also investigated spinal impacts behind body armor in a single test. Correlation of the injuries to the sternum and spine in the same specimen under the same threat round velocity suggests that sternal impact may not be the worst case for behind-hard-armor impact. Preliminary data from a single specimen with a matched sternal and spinal impact behind the body armor suggest that additional spinal impact research would be of significant value. This study, however, did not assess impacts to the ribs or more general loading conditions with body armors of different characteristics.
Wayne State University
Human cadavers provide realistic models for studying injury biomechanics in blunt ballistic impacts. Past experiments looked at the use of projectiles with masses and velocities much less than those of handgun and rifle threats; nevertheless these experiments do provide data on the effects of behind-armor deflections of given forces and kinetic energy. Bir (2000) reported results of low velocity (20-250 m/s) and high mass (20-200 g) projectiles used to study force–time, deflection–time and force–deflection responses on the chest of 13 human cadavers The chest wall deflection was about 5.5 cm for a kinetic energy of 112 J and a deflection of about 2.5 cm for energy of 28 J for the two conditions of 0.14 kg mass at 40 m/sec and a 0.14 kg mass at 20 m/sec. The measured forces for these two conditions were 10 kN and 3.4 kN, respectively. Whereas the velocity realms differ substantially from those of bullet threats, these data can be
used to evaluate the consequences of forces and kinetic energy behind body armor impacted by high-velocity projectiles if the area or volume of the vest indent is similar to that from the projectiles used in these experiments.
Finding: Although there are several studies using animal and cadaveric experiments to study behind-armor blunt trauma (BABT) injuries for hard body armor, the current work does not allow the development of a thoracic BABT injury criterion. Additional animal and/or cadaveric experimentation are required to develop a BABT injury criterion.
Finding: There is a need for a robust and widely used ballistic trauma injury classification scale. Although there is a number of existing injury scales, including a widely used scale for automobile injuries, the abbreviated injury scale promulgated by the Association for the Advancement of Automotive Medicine, none is well suited to ballistic trauma. Data on which to base a satisfactory injury scale will require the collection of military epidemiological data on a large scale.
Finding: The fidelity of anatomical, physical, and mathematical finite-element models simulating the human thorax, heart, lungs, liver, and kidneys is limited at the present time. Thus, damage to such organs as the intestines, spinal cord, brain, or vascular system from transmitted pressures associated with blunt trauma cannot be predicted.
Recommendation 8-2: The Army should perform high-speed ballistic tests using human cadavers and large animal cadavers to provide responses to deforming hard armor impacted by velocities likely to be encountered in combat. These tests should be extensively instrumented to determine dynamic deformation characteristics in the human and animal torsos to provide data that can be correlated with clay response at the same rates (or with alternative media or other test methodology) and with epidemiology and medical outcomes in the soldier. The studies should ensure that velocity and backface deformation regimes replicate those for current and future desired body armor testing protocols.
The recommended testing should be performed as soon as practical to address the following goals:
- Near term. Determine local three dimensional displacement time histories of animal and human cadavers to correlate with clay or other emerging test methodologies.
- Intermediate term. Determine pathophysiological effects of behind armor injury and correlate with acute injury and potential injury cascades.
- Long term. Incorporate injury outcome and mechanical response into emerging test methodologies and ongoing assessments of pathophysiological behind-armor effects in order to develop protective concepts.
Large animal studies are needed to evaluate damage to organs remote from the site of the blunt trauma for both acute effects and late effects. The experimental evidence for such remote effects comes from battlefield observations, previous large and small animal studies, and medical reports on civilian gunshot accidents.
The earliest observations of the effects of penetrating injuries on the nervous system remote from the site of penetration were case studies from the Civil War of temporary and sometimes long term motor and sensory paralysis (Mitchell et al., 1864). During World War I and World War II, autopsy studies revealed evidence of brain pathologies caused by blast, but more detailed study of soldiers’ symptoms later concluded that most of the disorders were of a psychological rather than a physical nature (Mott, 1919). Livingstone et al. (1945) were among the first to propose that the transfer of kinetic energy from a pressure wave might damage the nervous system. Blast exposure is not the same as a ballistic impact from high-speed projectiles, but internal biophysical phenomena as they relate to the central nervous system trauma may be similar.
Damage caused by the transmission of kinetic energy from the point of impact on the torso to remote body organs in humans has been observed on a number of occasions (Chamberlin, 1966; Carroll and Soderstrum, 1978; Sperry, 1993; Akimov et al., 1993; Cannon, 2001; Krajsa, 2009) and are corroborated by Civil War case studies (Mitchell et al., 1864). A report on human trauma from BABT in law enforcement personnel emphasized that protection from penetration does not preclude significant thoracic trauma (Wilhelm and Bir, 2008; Courtney and Courtney, 2010). When remote organs such as brain and intestines were included in examinations of animals clad in body armor then subjected to live-fire tests, there was evidence of substantial injury. A notable result of the small-animal studies was definite evidence of blood–brain barrier dysfunction subsequent to a high-speed bullet impact distant from the brain. Those studies used Evans Blue dye injected into the blood pool before the test.
Other observations in people exposed to blast waves indicated diminished cognitive caability and long-term encephalographic changes as well as complex neuropsychiatric symptoms (Cernak and Noble-Haeusslein, 2010). It should be noted that a causal connection between pressure waves from explosives or nonpenetrating blunt trauma and cognitive or psychiatric disorders is not the topic of this report, although this subject of blast-induced traumatic brain injury remains an area for intense scrutiny, as exemplified by the conclusions of a recent National Institutes of Health workshop (Hicks et al., 2010). An advanced imaging method study of soldier brains after blast trauma showed neuronal damage from blast exposure, but the cases were complicated by associated nonpenetrating head trauma (MacDonald et al. 2011)
The need to evaluate transmitted pressure waves in a variety of battlefield threats is an important reason for recommending large-animal live-fire experiments.
In addition, far more extensive data are needed than were collected in the past. Long-term testing involving large animals will require extensive use of pressure transducers, cineradiography, metabolic imaging, and neurochemical cerebral spinal fluid and blood assays that are appropriately instrumented, as described in Appendix J.
Recommendation 8-3: The Army should perform live large-animal, live-fire tests to simulate the behavior of current and proposed new body armor against expected threats.
A dummy or surrogate for human response is generally used to provide a reliable and inexpensive test methodology for research. This surrogate allows repeatable characterization of the performance of ballistic protective gear. Existing ballistic impact simulators may be divided into three classes, as shown in Figure 8-23. The first class, “bulk tissue simulants,” is made up of a single layer of material that allows measurement of deformation responses—for example, posttest residual clay penetration depth or dynamic gelatin penetration depth. The second class, “instrumented response elements,” generally includes a simplified thoracic wall that simulates the motion of the surface and may incorporate multiple layers. The third class is “instrumented detailed anatomical surrogates.” These generally include some form of thoracic viscera and are designed to investigate a wide range of blunt trauma kinetic energies and projectile diameters.
To develop BABT test methodologies, all three types may be useful. There are trade-offs as devices run from simpler to more complex, usually less expensive to more expensive, but the more complex devices generally have the potential to assess more detailed injury criteria where appropriate. For instance, it is difficult to evaluate complex BABT interactions, especially for very lightweight body armor systems, without detailed anatomical surrogates. However, it is advisable to use a relatively inexpensive bulk tissue simulant or instrumented response element for production testing, because in multiple tests there is a potential for penetrating events that can destroy a test device worth thousands or tens of thousands of dollars. Each class of device may have an advantage for a given test condition. The three classes of devices are discussed below.
Bulk tissue simulants are characterized by a single ideally isotropic and homogeneous layer. Although a number of bulk tissue simulants have been studied (e.g. Mirzeabassov et al., 2000), the Army has focused on simulants originally proposed in Prather et al. (1977). See Chapter 4 for a discussion of gelatin, plasticine, and clay simulants.
There are at least six instrumented response elements used in current ballistic research environments. These are described below.
DERA BABT Simulator
The U.K. Defense Evaluation and Research Agency (DERA) has developed a test device intended to evaluate the injury effect of behind-armor blunt trauma, called the DERA BABT rig (Tam et al., 2000). The device is similar to a half-cylinder silicone rubber chest wall (GE Silicones RTV 428) developed by Cooper et al. (1996) enclosed in a framework providing for rotation and vertical positioning. The physical model was derived from a finite-element model that included high-rate blast and blunt response of the thorax. The DERA rig instrumented response element also allows for varying the thickness of response element materials.
The assumed BABT injury mechanism for this physical model is that injuries are a function of chest wall motion, including displacement amplitude, velocity, acceleration, and deformation profile. The plausible mechanism for porcine organ damage is the impact pressure wave and subsequent displacement. The test device uses a novel laser deformation sensor array system to measure the time history of displacement, as shown in Figure 8-24. The deformation sensor is kept outside the bullet trajectory and may provide velocity and acceleration response of the back face of the test device.
FIGURE 8-24 DERA BABT simulator displacement sensor system. SOURCE: Adapted from Tam et al., 2000.
The body armor has been tuned to lateral eviscerated pig baton data and has been tested using a 12.7-mm AP round against the U.K. Enhanced Body Armor and a 7.62-mm NATO ball against Improved Northern Ireland Body Armour. Validation studies included both lateral and anterior shots. Peak accelerations were comparable between the BABT rig and lateral pig shots. These accelerations were approximately 20,000 g for the 7.62-mm test round and the Improved Northern Ireland Body Armour. However, the response of the anterior porcine chest wall varied somewhat from the rig behavior. As the rig was developed using lateral impacts, this is not surprising.
Comparative experiments reported by Cannon et al. (2000) involved six eviscerated and six intact pigs using 7.62-mm rounds at 3 kJ behind commercial ceramic body armor. The velocity of the wall deformations of the eviscerated model were similar to that of the physical model (9.6 m/sec for eviscerated vs. 15.2 m/sec for physical). Peak displacement of 12.6 mm was seen in the eviscerated pig, and peak displacement of 7.8 mm was seen in the intact pigs. Local peak acceleration was approximately 13,000 g. Mean time to peak
displacement was found to be approximately 2.7 msec. Viscous criterion of 0.29 m/sec was calculated for both. The DERA BABT device has been used in cooperative testing with Natick Soldier Center. The tests include body armor with a ceramic plate and a UHMWPE laminate. The test round was a 7.62-mm M80 ball round at 838 m/sec nominal velocity.
There are two significant drawbacks to this system. First, the system has been tuned to a specific thoracic wall velocity. Outside this regime, it is unlikely to respond appropriately to the rear face impact. Second, the simulator is shaped like a cylinder, and the laser system relies on this cylindrical shape to operate properly. As the human body is not cylindrical, it is difficult to use to evaluate actual body armor. To enhance the system, a different displacement measuring system could be developed using a multiple-laser time of flight system.
DREV Torso Injury Assessment Rig
The Defense Research Establishment Valcartier (DREV) has developed a thoracic injury assessment rig that is similar to the DERA BABT simulator (Bourget et al., 2002). For the DREV simulator both the material and the geometry have been altered to have a similar mechanical response to nonlethal baton impacts (Bir, 2000). Otherwise, the DREV test rig is similar in character and performance to the DERA BABT rig.
The private Anter Company in St. Petersburg, Russia, has patented a multilayer thorax simulator, as shown in Figure 8-25 (Mirzeabassov, 2000). This simulator is also covered under U.S. Patent 5850033. The simulator is proposed for both penetrating and blunt injuries. It includes an outside skin layer, a layer of muscle-simulating material, and a layer of bone-simulating material. Stiff paper is located on either side of the muscle stimulant, which consists of 10 mm layers of unvulcanized rubber. The brittle, strain-sensitive paper indicates the level of strain inside the simulant to approximate the temporary cavity during penetrating injury and to determine the extent of local deformation from blunt trauma.
There is no electronic instrumentation for this simulator; penetration or injury is indicated by paper between layers. The construction is reusable, and the paper indicator layers may be replaced. To collect additional information, these layers could be augmented with pressure-sensitive film. The basis for this injury model is experimentation using small dog subjects. Validation rounds include 7.62 mm with two impact energies, a 5.56-mm M16A1 and a 5.45-mm AK74. Validation included local thoracic deformation imaged using flash X-ray.
Hybrid III Dummy
The standard automobile frontal crash test dummy, the Hybrid III, has been used by several investigators for high rate impact (Figure 8-26). The Hybrid III dummy has been used for mine blasts and high rate impacts from blasts in structures (Bass et al., 2001a and Bass et al., 2001b). The rib structure of the Hybrid III consists of six ribs constructed of steel overlying a viscoelastic damping material. The ribs are connected in the front to a sternal bib, and a single displacement sensor is standard instrumentation. This single displacement sensor has significant limitations (Butcher et al., 2001). Enhanced instrumentation is available with an array of string potentiometers. Use of the Hybrid III dummy has several advantages: It is widely used and is manufactured in several different sizes representative of various standard anthropometries.
There are substantial drawbacks to the use of the Hybrid III as a ballistic BABT test device. The dummy has been validated only for low-velocity impacts.
Further, the local response of the Hybrid III is likely not biofidelic, even for these low-rate impacts (Kent et al., 2006).
FIGURE 8-26 Hybrid III 50th percentile male dummy. SOURCE: Courtesy Humanetics Innovative Solutions.
Anthropomorphic Test Module
The U.S. Army Medical Research and Materiel Command developed an anthropomorphic test module (ATM) as an instrumented response element for BABT injury assessment, as shown in Figure 8-27.62 The shoulders of the torso are not instrumented but provide an anthropometrically correct platform for mounting body armor as worn by soldiers. The response of the ATM element is measured using a multiple accelerometer array implanted within a polymer with approximately cylindrical form. The initial peak impact response is mitigated using a rubber pad over the surface of the response element as shown in the center of Figure 8-27.
62Michael Leggieri, U.S. Army Medical Research and Materiel Command, briefing to the committee at Aberdeen, Md., on August 11, 2010.
FIGURE 8-27 Left: ATM with mounted body armor. Center: ATM instrumented response element with padding. Right: oblique view of response element within torso. SOURCE: Michael Leggieri, Director, DoD Blast Injury Research Program Coordinating Office, U.S. Army Medical Research and Materiel Command, “Blunt Trauma Research to Support a New Body Armor Blunt Trauma Performance Standard and Testing Method,” presentation to the committee, August 11, 2010.
Essential elements of the ATM methodology include these:
- The development of a detailed anatomical finite-element model of the human torso (Figure 8-28).
- Identification of global mechanical response and injury response in animal and cadaver tests for impact with hard projectiles.
- Development of a simple response element with mechanical response that is correlated with the animal and cadaver tests.
- Validation of the human finite-element model with the animal and cadaver tests.
- Correlation of the response element deformation with finite-element model calculations and hence animal and cadaver injury response.
FIGURE 8-28 Left: human CT scan. Right: finite-element model, ribs and internal viscera. SOURCE: Michael Leggieri, Director, DoD Blast Injury Research Program Coordinating Office, U.S. Army Medical Research and Materiel Command, “Blunt Trauma Research to Support a New Body Armor Blunt Trauma Performance Standard and Testing Method,” presentation to the committee, August 11, 2010.
The model is based on 30 moderate-rate porcine and 12 low-rate blunt impactor cadaver tests. Generally, test impact velocities did not reach those typical of high-rate impact behind hard body armor. A limited model validation was performed using the animal and cadaver tests for the finite-element model. The response included global response of the model and surrogates for a limited number of instrumentation locations.
Though the data provide good correlation with automobile impact corridors (Kroell et al., 1974), the principal limitation is the lack of robust validation data for rates typical of impact behind hard body armor at typical rifle round velocities. Further limitations of the model involve the beam formulation of the ribs, which must be derived from validation data.
Research questions raised include response to penetration; repeatability of measurements, including the potential for material properties changes in the rubber with repeated shots; and model validation. These issues are the subject of ongoing research. 63
The ATM model and associated finite-element models are more complex than the typical response elements considered previously and likely represent a transition device between instrumented response elements and detailed anatomical surrogates.
The final class of instrumented surrogates, detailed anatomical surrogates, includes anatomical features that are similar to humans, generally including internal organs. When validated, these may in principal be appropriate for the
investigation of complex behind-armor phenomena. Owing to the complexities of design and instrumentation, however, it is unlikely that these devices will directly form the basis for large-scale testing of body armor.
AUSMAN is a reusable mechanical surrogate developed by the Australian Department of Defense, Defense Science and Technology Organization. The torso surrogate is shown in Figure 8-29, and body armor is shown mounted on the torso in Figure 8-30. AUSMAN consists of a 21-kg polymeric skeletal system enveloping a simulated cardiopulmonary system and liver and incorporates an anthropomorphic rib structure and a realistic spine. The entire thorax is encased in polymer, with gel coupling between internal viscera and the rib structure. The lungs are simulated by open cell foam as are a heart and liver. In addition, the design includes a mount for a Hybrid III head and neck to allow neck injury assessment.
FIGURE 8-30 AUSMAN thorax with body armor in place, prior to testing. SOURCE: Reprinted with permission of Cameron Bass.
Mechanical response is measured by accelerometers in the sternum and midthoracic spine with pressure transducers in the lungs. AUSMAN has limited validity for blast, although an early version was correlated against sternal ballistic response in cadavers (Bass et al., 2006). The blast version of the dummy surrogate is currently under development.
Swedish Anthropometric Dummy
The Swedish National Defense Research Institute (Jonsson et al., 1986) has developed a model thorax to be used for blast, ballistic blunt impact, and missile impact. The thorax is enclosed by a rubber tube with a thickness of 6 mm and an elliptical shape that is 35 cm laterally and 25 mm in the anterior/posterior direction. Internal viscera are simulated by a water filled cavity that includes foam rubber surrogate lungs shaped as cylinders. These lungs have a diameter of 10 cm and a length of approximately 20 cm. They are sealed using a thin shell of rubber to prevent water infiltration. The lungs are positioned in the chest using wire mesh and strings that are rigidly mounted at each end of the thorax model. Each simulated lung includes a pressure transducer located at the center of the lung structure.
Experiments on this model were performed using a shock tube, a pendulum device, and ballistic impact behind body armor. Test rounds included 7.62 mm at 870 m/sec in both soft point and full metal jacket, a 9-mm submachine gun at 430 m/sec, and a 37-mm antiriot baton round at 76 m/sec. Body armor for the rifle rounds was 10 mm thick polyethylene trauma pack, 17-ply Kevlar 29, and a 10-mm ceramic plate. For the 9-mm round, the body armor
used a 2.3-mm steel plate in place of the 10-mm ceramic plate. For the baton round, no body armor was used. Impacts were performed at the mediastinum with energies that range from 0.2 kJ to 5.6 kJ.
Intrathoracic pressure was measured for impact with the various rounds tested and compared with lateral rabbit thorax impact experiments using the pendulum impactor. An injury scale for intra-lung pressure was developed using pendulum impacts with velocities of approximately 5 m/sec that are stroke limited to 10 percent thoracic compression. Researchers found that the blast pressure peaks were far larger in the blast experiments than in the impact experiments for similar levels of lung damage. So, they concluded that dummies validated for blast pressure will not be calibrated for blunt impacts using the same pressure measurements. Assessments using this dummy predict a low risk of lung injury from a 7.62-mm test round at 3.2 kJ behind the body-armor combination of trauma pack, ceramic plate, and Kevlar-29.
The researchers observed small (~10 percent of peak) oscillations in the pressure signal at approximately 3 kHz. They concluded that the oscillations were related to structural forcing. There are significant questions regarding impact injury with mediastinal forcing using injury indices developed in a different velocity regime for lateral impacts. Further development of this surrogate is unknown.
Instrumented Model Human Torso
The Johns Hopkins Applied Physics Laboratory has been developing two research models for assessment of ballistic trauma: a frangible device and a reusable device.
Frangible JHU Model. The physical structure of the proof of concept frangible model includes rib, sternum, and spinal structure. Internal organs are represented as homogeneous solid viscera of urethane material, and the model has a skin and subcutaneous fat layer with an interposed sensor pad between the skin and fat layer. An emphasis was placed on incorporation of biofidelic materials. The bone material was chosen to be frangible with a failure modulus similar to human values. The design is intended to allow rapid replacement of frangible components.64
Instrumentation includes a sensor pad composed of an array of piezoelectric elements that are sensitive to bending. Additional piezoelectric arrays and resistive flexure grids are used inside the rib structure and near the heart.
Data from the anterior piezoelectric sensor array were sampled at 10 MHz and the remaining arrays were sampled at 25 kHz. This lower data rate does not appear to be an intrinsic limitation in data rate of the piezoelectric sensor arrays.
64Personal communication between Matt Bevin, Johns Hopkins University, and committee member Dale Bass circa 2002.
Preliminary testing was performed with soft body armor (Type II). Rounds tested include 9-mm at 330 m/sec and .357-Magnum JSP at 420 m/sec. Analysis of the anterior piezoelectric element only was reported. The researchers draw a correlation between measured peak voltage of the array and entering kinetic energy. There was no reported analysis of additional arrays.
An advantage of this model is that it has the potential for extending sensor instrumentation. The calibration of the sensors might be difficult, however, because of the difficulty integrating bending elements across linear and planar structures without substantial error (Bass et al., 1998). In addition, the piezoelectric materials used for measurement of compressive force are generally extremely sensitive in bending. Further, the model is frangible. It is difficult to produce a cost-effective model with large frangible components.
Reusable Johns Hopkins University Human Surrogate Torso Model. The reusable Human Surrogate Torso Model developed by the Johns Hopkins University has two versions, including 5th and 50th percentile human anthropometry, and has a detailed anthropomorphic skeletal structure with overlying skin and internal organs, including heart, lungs, stomach, and intestinal mass. The model is constructed so that the ribs have the fracture and bending properties of bone, and the organs are constructed of silicone polymers. Material properties of these organs have not yet been compared with human material properties at high rates of deformation. Sensors used in the torso include sternal and spinal accelerometers and pressure sensors at various locations. Additional instrumentation may include a vertebral load cell, surface pressures and load transducers, strain gauges for bone simulants, and possible displacement sensors.
The Human Surrogate Torso Model has been used in NIJ soft body armor tests to characterize mechanical response (Roberts et al., 2007; Merkle et al., 2008). Tests include 9-mm threats at 436 m/sec incoming velocity. The responses have further been correlated with clay response. The device has no validation against an injury metric and is still in development.
In sum, there are several existing test devices that are potentially suitable for use in the development of a test methodology for ballistic BABT. None is currently suitable for use as a test device for BABT with hard body armor without further development or experimentation.
For such developmental testing, two aspects must be considered. The first is the biofidelic response of the surrogate, and the second is the validation of the mechanical correlate from this model with an injury model. Each surrogate class has advantages. For example, many of the instrumented response elements and all of the anatomical surrogates have anthropomorphically appropriate thoracic form, reducing the risk of misleading appliqué response in clay or gelatin for body armor meant to be worn. A substantial drawback to instrumented surrogates is the cost of surrogates and sensor replacement. However, careful design can likely minimize both handling and sensor maintenance costs.
Finding: Instrumented response elements are in a primitive state for the evaluation of ballistic behind-armor blunt trauma for hard body armor against rifle round threats. Although several devices have associated instrument response and injury criteria that have been validated against a small range of loading conditions, there is no test device suitable for use without further development and validation.
Finding: Instrumented anatomical surrogates are not detailed enough to assess ballistic behind-armor blunt trauma for hard body armor with rifle round threats.
Recommendation 8-4: The Army should develop finite-element simulation models of human and live-animal thoracic response to behind-armor blunt impact. The validation of this simulation should be hierarchical from the small scale to the large scale. This includes the dynamic local response of constituent materials such as skin, bone, muscle, lung, liver, and other tissues; the regional response of the tissues under loading; and the global response of the whole torso. It should also include deformations from soft and hard body armor impacted with appropriate threats.
Recommendation 8-5: The Army medical community should enhance the current trauma registries to provide a program of injury epidemiology for ballistic impact, including behind-armor blunt trauma. This should include collection of both injury and noninjury events and should be similar to the federal crash databases used by the Department of Transportation—for example, the Fatality Analysis Reporting System and the National Automotive Sampling System for traffic injuries/fatalities, including injuries induced by both penetrations and backface deformations.
Recommendation 8-6: Using experimentally determined links to injury, response, and epidemiology, the Army should ensure that the clay or other alternative test methodology for hard body armor has humanlike dynamic response and is suitable for the development of behind-armor blunt trauma injury criteria.
Recommendation 8-7: To achieve improvements in behind-armor blunt trauma (BABT) research methodology in the medium term, the Army should develop instrumented thoracic simulators as response elements (sensors). Necessary preludes to this effort include the following:
- Establishing BABT phenomenology and injury criteria using human cadavers, animal models, and field injury epidemiology coupled with well-validated finite-element simulations.
- Establishing human BABT mechanical response for the range of design conditions for personal protective body armor. This should include impact on soft and hard body armor of anticipated threats.
Because of the high kinetic energy imparted to vests from current threats, laboratory testing with surrogates must remain well below ballistic V0 using well-characterized armor systems to avoid extensive damage to structures and instrumentation. On the other hand, complex phenomenology can be investigated using appropriately validated devices that have the potential to reduce the overall cost of assessing BABT. One approach would be to initially use instrumented response elements in parallel with current methodologies focusing on dynamic displacement and/or force-response sensing capabilities.
Recommendation 8-8: In the long term, beyond simple clay torso surrogates and one-layer torso simulants, the Army should use the road map in Figure 8-31 to investigate the use of detailed anatomical surrogates (such as cadavers, instrumented models, etc.) as research devices to evaluate behind-armor blunt trauma.
The principal biomedical issues relating to the development of a test methodology for soft and hard body armor testing with a strong basis in biomedical response and injury include these:
- The response of the clay currently used in the backface impact methodology has limited biomedical basis in human body response. As the human torso responds differently for impacts at different rates, the current clay response methodology has no biomedical basis for hard body armor impacted with high-velocity rifle rounds.
- The link to human injury in the current clay methodology was developed for the behind-armor impact of soft armor and has a limited biomedical basis even for soft body armor. The current methodology has no link to human injury for hard body armor impacted with high-velocity rifle rounds.
- The backface torso response and the effects of BABT on organs remote from the point of trauma are likely dependent on impact rates for both soft and hard body armor.
- There are only very limited links to human epidemiology for injuries from BABT when rifle rounds impact hard body armor worn in combat. This fact combined with the fact provided by DoD that there are no known fatalities from design threats suggests that it is unknown whether body armor is overdesigned against current threats. This has substantial implications for battlefield mobility, thermal loads, and other important issues in combat.
In the face of real-world constraints on dollars and manpower, Figure 8-31 provides a prioritized, time-phased road map for the near-term and medium-term medical research that is needed to reach the long-term goal of developing a test methodology for soft and hard body armor based soundly in biomedical response and injury.
As shown in Figure 8-31, several near-term actions are needed to address the biomedical issues enumerated above and provide a strong biomedical basis for future body armor testing:
- The backface response of clay must be tested for its plastic and viscoelastic characteristics and correlated with relevant drop tests as well as animal and human thoracic response. See Recommendation 4-2.
- Injury risk assessments for structural and physiological injuries must be developed using animal tests and human cadaver tests for a typical range of hard body armor backface velocities. Experience from the
- limited number of previous animal, cadaveric, and surrogate studies should be assessed to help guide these studies. See Recommendations 8-2 and 8-3.
- Tests involving human cadavers or animals must be conducted to determine response behind deforming hard armor for a typical range of hard body armor backface velocities. Experience from the limited number of previous animal, cadaveric, and surrogate studies should be assessed to help guide these studies. See Recommendation 8-2.
- A military medical epidemiology database must be established that focuses on ballistic backface trauma vs. penetrating trauma, including non-injury cases to provide information to assess tradeoffs of protection and actual levels of protection in field, See Recommendations 8-1 and 8-5.
The near-term actions shown in Figure 8-31 should provide input to medium-term medical research to produce a biofidelic test methodology for soft and hard body armor.
Key medium-term actions are an assessment of the biofidelity of the clay test method and development of instrumented response elements. Specifically,
- Assess the biofidelity of the clay or other alternative test method using near-term research results. See Recommendation 8-6.
- Develop an alternative to the clay methodology using digital sensors with a thorax response element. See Recommendation 8-7.
- Develop detailed anatomical surrogates. See Recommendation 8-8.
The long-term goal is to improve body armor by choices of materials and system configurations so the weight and protection are optimized. To reach this goal, a practical biofidelic test methodology for both soft and hard body armor is essential.
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