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1. Introduction and Background

 

1.1 Introduction

 

The human toll from antipersonnel mines is large.  Though estimates vary on the number of mines deployed worldwide [UN-2000], an estimated 20,000 civilians die each year from landmine explosions.  Thousands more are wounded or maimed.  As there is still no inexpensive and reliable mechanical technique for detecting and removing antipersonnel mines, human deminers will be used for 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 personal protective equipment (PPE) demining equipment may include head/face protection, thorax protection, and extremity protection including gloves and boots as shown in Figure 5. This suit 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, design changes in 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].

 

Figure 5: Demining PPE (Photo Courtesy Med-Eng, Inc.)

 

1.2 Objective Test Methodology

 

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 will result in an objective test criterion for the evaluation of the injury risk of a human wearing a PPE.  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 surrogate 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. Elements of this technique include:

 

• Biofidelic surrogate � a dummy that is robust, gives a repeatable physical response, and produces a response that is appropriately human-like.  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.  An injury risk model is without value without successful validation using 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 6.  Development of a relationship between a robust surrogate for injury and a validated injury model is crucial in the success of this approach.

 

 

Figure 6:  Development of Surrogate Injury Model

 

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, not applicable to real world conditions.

 

Widespread use of this technique has saved thousands of lives per year in the automobile industry.  Indeed, all automobiles and safety restraints, including air bags, are evaluated using dummy surrogates.  As there are similarities in human blunt trauma in an automobile crash and in a blast event, aspects of this technique may be adapted for use in determining injury from mine blasts. The current study builds on several previous test series using Hybrid III dummies and simulated mines to evaluate the performance of demining PPEs.  These test series include work performed under the auspices of the Canadian Center for Mine Action Technologies (CCMAT) [c.f. Bergeron-2000] and the U.S. Army � Communications-Electronics Command (CECOM) Countermine [c.f. Chichester-2000].

 

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 timescales.  Automobile crashes have injury timescales of approximately 5-100 milliseconds, but injuries in mine blasts may occur 10 to 100 times faster.  These timescales have an effect on dummy surrogate 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 risk of injury while demining wearing protective PPEs.

 

1.3 Epidemiology

 

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 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 also led to an increase in the occurrence 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 7.   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, injuries from these body regions, especially blunt trauma that may arise while protected, must be included in the injury risk assessment.

 

Figure 7: Injuries from AP Landmines Sustained in Demining Incidents [Landmine-2000]

 

1.4 Dummy and Instrumentation

 

Simulation of a realistic test condition is especially important in mine blast testing.  A high-speed photograph of a simulated mine blast with a dummy surrogate is shown in Figure 9.  The force on a human chest or head is related to the pressure from the blast wave and streaming flow from the blast ejecta.   Since pressure falls rapidly from the blast and the streaming flow is highly directional, the dummy surrogate position in the blast is vitally important in a realistic simulation.  A field survey found that 91% of demining blast incidents occur with the victim within 1 meter of the mine [Landmine-2000].  It is clear, however, that close enough to a large mine blast there may be substantial injury using any personal protective equipment.  So, a balance must be maintained between the desire for test realism and the desire to evaluate the worst case in mine blast injuries.

 

The Hybrid III dummy, widely used in the automobile industry for blunt impact, was selected for this test series.  The reason for this selection was twofold.  First, the dummy has validated frontal blunt impact injury criteria that may be useful for characterizing demining injuries.  Second, it is relatively inexpensive, robust, and widely available.  Full dummy surrogate development can be expensive.

 

A number of different Hybrid III dummies exist that are scaled for different size test subjects.  As changes in anthropometry may change risk of injury, for accurate response, the dummy selected should be most representative of the population modeled. Indeed, the effect of anthropometry may be large.  Worldwide anthropometry of the average male is shown in Figure 8 [Jurgens-1990].  To see the effect of body anthropometry, 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 may substantially increase the risk of head or thorax injury in demining for the average Southeast Asian male.  Further, there are large numbers of mines in West Africa and Southeast Asia, where populations have relatively short arms and/or stature.   So, it seems essential that the small Hybrid III dummy be incorporated into mine protective equipment testing.

Figure 8:  Selected Worldwide 50th Percentile Male Stature and Reach

 

Two pedestrian version 50^th percentile male Hybrid III anthropomorphic dummies, denoted (A) and (B), were used in this test series.  One is shown in Figure 9.  These dummies, used in automobile crash testing, are particularly useful in estimating the risk of frontal blunt trauma and are validated for frontal blunt impacts to both the head and the chest. In addition, a Hybrid III 5^th percentile female dummy was used in selected shots to represent deminers with smaller statures [Bass-2000].  The dummies were placed in each of two positions, kneeling and prone, as discussed in the following section.   Tests were performed using unprotected dummies and dummies in each of five humanitarian demining PPEs. 

Figure 9: Simulated Antipersonnel Mine Blast with Hybrid III Surrogate

 

The Hybrid III dummies were instrumented with acceleration-sensing transducers, force-sensing transducers, displacement transducers, and pressure transducers to evaluate head, neck, and thoracic trauma as shown in Table 1. The data from these transducers may be used with accepted injury thresholds and risk functions to determine the risk of injury in a given test condition as reported below.  Instrumentation data was sampled at 200 kHz with a 40 kHz antialiasing hardware filter.

 

Table1: Instrumentation and Trauma Evaluation

 

To summarize, essential elements in the development of a procedure for evaluating the risk of injury while wearing demining protective equipment are:

 

• Robust dummy surrogate with established and applicable injury criteria � positioned in a realistic manner in positions representative of demining (i.e. kneeling and prone). 
• Robust instrumentation � data handling consistent with the response.
• Accurate positioning � distance to mine must be consistent and quantifiable. 
• Repeatable, quantifiable threat (mine) � with fixed burial and soil characteristics.

 

Each of these elements acts to provide an objective criterion for injury and injury performance while ensuring that the resulting criterion is as applicable as possible to the conditions experienced in the real world.

 

In subsequent sections, the test methodology, dummy, positioning instrumentation, and test results are discussed.  These are followed by conclusions on the suitability of this test methodology to repeatably characterize demining trauma with and without PPEs.

 

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