Issue 4.2 | June 2000
The human toll from AP mines is large. The United Nations estimates that there are over 100 million AP mines deployed worldwide (U.N. 2000). An estimated 20,000 civilians die each year from landmine explosions. Thousands more are wounded and maimed. As there is still no inexpensive and reliable mechanical technique for removing AP mines, human deminers will be used in the foreseeable future to protect the general population from the menace of landmines.
To decrease the human toll from demining, protective equipment should be used. For comprehensive protection, the demining ensemble may include head/face protection, thorax protection and extremity protection, including gloves and boots as shown in Figure 1. This ensemble offers the potential for substantial protection against fragments, blunt force trauma, burns and other consequences of mine blasts. However, without some objective procedure to evaluate the risk of injury while wearing protective gear, the design of such demining equipment is guesswork. Indeed, without an effective injury evaluation technique, protective equipment may exacerbate certain types of injury. For example, the introduction of body armor in Northern Ireland for protection against blast fragments may have increased the potential for blast lung injuries (Mellor 1989).
One technique that has been shown to be effective in the automobile industry is the use of an instrumented surrogate (dummy) to 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 only represent a part of a human. For example, an instrumented beam has been used successfully to represent an arm (Bass 1997). However, dummies may 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 that 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 A correlation between the injury risk evaluation and a physical model of injury. 1) Epidemiology or physical reconstruction of an actual injury event; 2) An animal injury model; or 3) A cadaveric human injury model as shown in Figure 2. Development of a relationship between a robust surrogate for injury and a validated injury model is crucial in the success of this approach.
Two other important elements of injury simulation may be adapted from those used in automobile testing: use of injury epidemiology to direct testing and injury modeling and use of realistic test conditions. Both limit the risk that an injury simulation is an academic exercise and is not applicable to real world conditions.
Widespread use of this technique has saved thousands of lives per year in the automobile industry. As there are similarities in human blunt trauma in an automobile crash and in a blast event, this technique may be adapted to evaluate injury from mine blasts.
Development of Procedure
The goal in the current study is to develop a procedure to evaluate injuries from mine blasts, borrowing tools from existing techniques when appropriate. This approach will result in an objective test criterion for the evaluation of the injury risk of a human wearing a protective demining ensemble. It will allow this injury risk evaluation for protected or unprotected subjects and will indicate the relative levels of protection for subjects wearing different protective equipment.
For decades, work has been performed on human injury from blunt trauma in the automobile field. Simulated automobile crashes are performed, and the response of the dummy is taken to represent the response of a human in that crash scenario. This dummy response may be used in an injury model to assess the risk of injury for that crash scenario.
The tools used in the automobile industry, however, may not be directly applicable to mine blasts for two reasons. First, automobile crashes and mine blasts are substantially different physical phenomena. While both automobile crashes and mine blasts may involve blunt head and chest trauma, mine blasts may have substantial shock wave effects, burns and other blast phenomena. Second, the events may occur on significantly different time scales. Injuries in mine blasts may occur 10 to 100 times faster than those in automobile crashes. These timescales have an effect on dummy response, and the timescale of mine blast injuries may be outside the validity of the injury models used in the automobile industry. So, tools used in the automobile industry must be adapted for use in mine blast testing to effectively assess the of risk injury while wearing protective ensembles.
Another important element in the effective design and evaluation of protection from injury is the epidemiology of the occurrence of those injuries in the field. Initial efforts to categorize injuries from humanitarian deminers (Landmine 2000) have identified the most significant injuries from mine blasts. Epidemiology, however, is a moving target, and future efforts to categorize ongoing injuries and their causes are crucial. For instance, the use of protective features may change the types of injuries experienced and could warrant changes in the focus of injury protection. A clear example of this case came with the widespread use of automobile driver-side air bag restraints. Use of such systems resulted in a substantial decrease in fatal head and thorax trauma, but it also led to an increase in the importance of debilitating leg injuries.
The types of injuries encountered in a number of demining incidents have been summarized in a groundbreaking report (Landmine 2000) as shown in Figure 3. Fatal injuries include blunt trauma to the head and chest, including blast lung, shock and multi-system trauma. Blast injuries may also include blast-induced trauma to hearing, burns and trauma from whole body translations with injury patterns similar to falls. To provide a realistic assessment of injury from mine blasts, all of these injuries must be included in the injury risk model.
As changes in anthropometry may change risk of injury, for an accurate response, the dummy selected should be most representative of the population modeled. Worldwide anthropometry of the average male is shown in Figure 6 (Jurgens 1990). If the distance of the body to the mine when demining is taken to be roughly proportional to the mean reach (arm length), the average Southeast Asian male is approximately 70 mm closer to the blast than the average North American male. This distance may substantially increase the risk of head or thorax injury in demining for the average Southeast Asian male deminer. As there are large numbers of mines in West Africa and Southeast Asia where populations have relatively short arms and/or stature, the small Hybrid III dummy should be incorporated into mine protective equipment testing.
To summarize, essential elements in the development of a procedure for evaluating the risk of injury while wearing demining protective equipment are
Each of these elements acts to provide an objective criterion for injury risk while ensuring that the resulting criterion is as applicable as possible to the conditions experienced in the real world.
Existing Human Injury Criteria
Preliminary tests were performed using dummies with protective ensembles and simulated mines. The mines were plastic explosive with 200 g C-4, 100 g C-4 and 50 g C-4. These devices were found to be comparable in blast energies to a wide range of existing mine types (Bergeron 2000). The dummy used was a Hybrid III 50th percentile male dummy or equivalent.
From the database of existing injuries, the types of injuries evaluated should be blunt head trauma, blunt neck trauma, blunt thorax trauma, blast lung, blast-induced hearing damage and burns. Blunt injuries can also evaluate the potential for "fall" type injuries caused by whole body displacement from blasts. All of these injury types except burns were evaluated in the preliminary test series.
Blunt Trauma Head Injuries
As discussed above, fatalities from head injuries are significant in mine blasts. These injuries may be caused by direct blast impingement on the head. One simple surrogate for the risk of head injury from force experienced by the dummy head is the peak acceleration at the center of the dummy head. This surrogate has the advantage of being easily measured, and existing injury criteria use this measurement.
One injury criterion commonly used with the Hybrid III dummy head/neck complex in frontal impacts is the Head Impact Criterion (HIC) for concussive head injury (Versace 1971). HIC includes the effect of head acceleration and duration; a HIC value of 1000 is specified as the level for onset of severe head injury. Physically, HIC predicts that large accelerations may be tolerated for short times. HIC is based on human cadaver and animal impact data with durations that are usually one millisecond or greater.
HIC values obtained in mine blast testing are shown in Figure 7 for mine blast strengths of 50 g C-4, 100 g C-4 and 200 g C-4. Several tests with 200 g C-4 showed potentially injurious levels of HIC, one near a value of 10,000, presumably a fatal injury. For several tests in this series, however, the duration of the acceleration peak was substantially shorter than the usual value of HIC duration (> one millisecond). This result suggests that the data on which HIC is based must be reevaluated for use with mine blasts and that the resulting injury model must be validated with a physical injury model.
Blunt Trauma Neck Injuries
Neck injuries from blast trauma are possible owing to different rates of acceleration of the head and chest under blast loading. Physical trauma to the neck may be evaluated using the neck force transducers that may be incorporated into the Hybrid III dummy. Barring local damage of the neck itself, the dynamic impulse in the neck must be transmitted through the relative motion of head and chest. It is likely that this transmission of force is relatively slow when compared to the impact of the blast wave. This type of injury likely is similar in rateto impact neck injuries that have been studied in automobile safety.
One available injury criterion is based on the bending moment necessary to flex or extend the human neck. For extension, the human injury tolerance is 135 N-m bending moment. In Figure 8, the neck extension moment injury for several tests in the preliminary series is shown. Several tests in this series with protective helmets and suits showed potentially injurious levels of neck bending. All but one test that exceeded the injury threshold had the largest simulated mine (200 g). The 50 g test that showed injurious neck moments may be attributed to a loose Hybrid III neck for that test. Paradoxically, the use of a protective suit and helmet resulted in generally higher neck moments than when no protective equipment was used. This tendency is likely the result of the increase of surface area exposed to the blast when using the protective gear.
Blunt Trauma Thorax Injuries
The blast pressure wave and following pressure wave from the detonation of a mine have the potential to produce severe blunt trauma to a human thorax in proximity to the blast. Acceleration injury criteria for blunt trauma to the human thorax were developed by Mertz and Gadd (1971). This injury tolerance is 60 g limit over a three ms duration. As with the head, acceleration may be taken as a proxy for the global force experienced by a thorax.
Representative chest accelerations from the preliminary test series are shown in Figure 9. As expected, the most severe chest accelerations occurred with no protective suit while the least severe occurred at the lowest level of mine blast (50 g) with the protective equipment. This injury criterion does not include other possibly significant effects, such as chest compression injuries or blast lung injuries. However, these factors may be included using other measurements.
Blast Overpressure Injuries
There is a substantial risk of blast overpressure injuries, either blast lung or blast-induced hearing injuries, when at a close distance to AP mine blasts. However, the usual instrumentation of the Hybrid III dummy does not include any assessment of the effects of blast overpressure, either in the head or the chest. So, in preliminary testing, a pressure sensor was mounted on the surface of the chest to evaluate the potential for blast lung injuries, and another pressure sensor was mounted in the head at the location of the ear to evaluate the potential for hearing damage. The evaluation of blast wave injuries is important since addition of protective equipment for the thorax may exacerbate blast overpressure injuries.
Pressure profiles seen in the preliminary testing are similar to a typical ideal shock wave with a nearly instantaneous rise to peak pressure with exponential decay. Peak external pressure vs. duration for the thorax are shown in Figure 10. This data is compared with the classic threshold, one percent fatal, and 50 percent fatal free field curves taken from classic work by Bowen et al. (1968). While several of the tests with 50 g and 100 g C-4 show potential lung injuries at the threshold level, only the 200 g C-4 simulated mines show greater than one percent fatalities on this scale.
Future tests should incorporate additional pressure sensors in the thorax and head to minimize the potential for spurious pressure results. In addition, as the injury criterion used is a free field blast value for plane waves, an assessment should be made of the effects of differences between the ideal blast wave and local blast shock for AP mines.
As mine blasts involve explosive deflagration, there is a significant potential for burns close to mine blasts. The mechanism for this injury is rapid radiant and convective heat transfer to the skin. Predictive criteria exist for such flash burns; in 1960, the Naval Materials Laboratory developed a skin simulant for evaluating injuries caused by thermal insults (Derkson 1960). The technique uses a thick plastic resin with an embedded thermocouple. The temperature output of the thermocouple was correlated with human injury 120 mm below a living skin surface. A temperature of 44 degrees C was derived as the threshold for such transepidermal injury.
In future tests, thermocouple sensors should be embedded in the dummy skin at the thorax, head and extremities to determine the risk of thermal injuries. This method is especially useful in the unprotected reference tests to provide a baseline for the comparison of the effectiveness of the protective ensemble for burn prevention.
Mine blasts are forceful events for which there is significant risk of injury, even while wearing protective equipment. To provide the most effective protection that is practical, a procedure must be developed to objectively and systematically evaluate protective demining equipment. Such a procedure is proposed in this study.
To avoid long development and high costs associated with the development of a completely new test procedure, this procedure builds on techniques currently used in the automobile industry to evaluate risk of injury from blunt force trauma in automobile crashes. The technique includes the use of Hybrid III dummies in testing with injury criteria adapted from accepted injury risk tolerances. Mine blasts are simulated in repeatable conditions for the rapid and inexpensive evaluation of a wide range of blast conditions.
Injuries modeled in this procedure include blast trauma to the head and chest, neck injuries, blast lung and blast-induced hearing damage and thermal injuries. For these injuries, existing injury criteria may be used with the Hybrid III dummy or injury criteria can be adapted for use with the Hybrid III dummy.
Preliminary tests have been performed that suggest the need for augmented instrumentation and validation using an appropriate physical injury model. These tests suggest that AP mine blasts may be injurious or fatal even with protective headgear and body armor. Further work is needed to characterize the robustness, repeatability and applicability of this promising technique for the evaluation of personal protective systems for demining.