Prev | TOC | Home | Next

 

Executive Summary

 

ABSTRACT

 

To reduce human casualties associated with demining, a wide range of protective wear has been designed to shield against accidental detonation of antipersonnel (AP) landmines.  Injury protection offered by personal protective equipment (PPE) may include, but is not limited to, head/face protection and thorax protection that may offer the potential for substantial defense against fragments, blunt force trauma, burns, and other consequences of mine blasts.  In this study, five commercially available PPEs were evaluated.  These suits represent a wide range of materials and armor masses.  In addition, the PPEs offer varied areas of head, neck, thorax and extremity coverage.

 

This study utilized the Hybrid III dummy, an instrumented biofidelic surrogate that is anthropometrically similar to the human body.  The primary dummy was a 50th percentile male, anthropometrically scaled to the average North American adult male. Tests were conducted with both an unprotected dummy and a dummy clothed with one of the five commercially available PPEs.  Based on recorded dummy values, injury risk assessments were made using human or animal injury models.  The PPEs were evaluated against two levels of simulated mines containing 100 g and 200 g of C-4 explosive against a widely fielded antipersonnel mine, the PMN containing 240 g of TNT.  The test matrix consisted of 102 tests to confirm repeatability and robustness of the dummies, as well as to evaluate the five PPEs, two size dummies, and two positions (kneeling and prone).

 

The goal of this study was to determine the level of protection offered to the head, neck, and thorax by the protective equipment.  Correlations were drawn between injury risk and various parameters such as PPE mass, projected area, and dummy coverage area.  The effect of certain PPE design features was significant.  For example, higher mass PPE helmets resulted in lower head accelerations and lower neck moments.  This was due to the increased inertia of the dummy by the added mass of the protective equipment.  However, those PPEs that presented a larger projected frontal area to the blast wave resulted in higher total momentum transfer, and increased peak load, moment, and acceleration. Two of the PPEs that were evaluated did not include a helmet.  The lack of helmet reduced the projected area and thus the loading area.  However, this significantly increased the risk of injury by reducing the head/neck inertia and increased susceptibility to fragments and blunt trauma.

 

Introduction

 

The human toll from antipersonnel mines is large.  The United Nations estimates that there are over 100 million antipersonnel mines deployed worldwide [UN-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 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 demining ensemble may include head/face protection, thorax protection, and extremity protection including gloves and boots as shown in Figure 5. This ensemble offers the potential for substantial protection against fragments, blunt force trauma, burns, and other consequences of mine blasts.  However, there is no established standard for testing demining personal protective equipment (PPE).  Without some objective procedure to evaluate the risk of injury while wearing protective gear, the design of such demining equipment is guesswork and may produce additional risk of unforeseen injury.

 

The principal objective of this study is to develop and test an objective methodology for humanitarian demining PPEs that can evaluate the risk of human injuries from mine blasts.  These injuries include blast injuries to the head and thorax, blunt trauma to the head, neck and thorax, and burns.

Essential elements in the development of this procedure for evaluating the risk of injury while wearing demining PPEs are:

 

 

Each of these elements is satisfied by the procedure developed in this study and 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.

 

Methodology and Results

 

Blast testing was performed using Hybrid III dummies as shown in Figure 1. Five styles of PPE suits were tested in 102 blast tests against two simulated mines and one actual mine.  These suits were identified as PPE 1 � PPE 5.  Baseline tests were performed on unprotected dummies for each position and each of the simulated mines.  The same tests were then repeated with the dummies dressed with each PPE.  The threats used in this test series were simulated mines that contain 50 g, 100 g, and 200 g of C-4.  The Soviet PMN antipersonnel mine was used on 10 shots for comparison explosive yield using two of the PPE styles.  The test dummies were placed in two common demining positions, kneeling (k) and prone (p) as shown in Figure 2.  To enhance the statistical significance of the test data, three shots were performed for each combination of position, threat and PPE.  The 50 g simulated mine was found not to cause injurious loads against the unprotected dummies and was therefore dropped from any of the protected testing.

 

 

Figure 1: Simulated Antipersonnel Mine Blast withHybrid III Surrogate

 

Figure 2: Nominal Kneeling and Prone Positions Relative to the Center of the Mine - Radial Lines at 300 and 600

 

Two blast resistant positioning fixtures were used to support and position the dummies and were placed at least 4 meters from the wall and each other to prevent blast interference. These positioning fixtures were developed by a U.S.-Canadian collaboration including U.S. Army CECOM, Canadian Center for Mine Action Technologies (CCMAT), U.S. Army Aberdeen Test Center (ATC), and the University of Virginia.  They allow accurate positioning for each shot to within � 3 mm of reference locations in each spatial axis.

 

 

 

Figure 3: HIC Values for Mine Blast into Kneeling Dummy, All Charge Sizes, All PPEs

For the Head Impact Criterion (HIC), a widely used injury measure for the Hybrid III dummies, the facial protection with PPE 1 did not reduce the risk of head blunt trauma when compared to the unprotected case as shown in Figure 3.   This unexpected result may be explained by the physical features of the head protection gear, including the projected frontal area and the helmet mass.  First, the heavier helmet/visor sets produced lower HIC values.  The two heaviest helmets, those from PPE 4 and PPE 5, performed better than those from the other PPE for dummies in the kneeling position because the larger mass decreases the acceleration of the head, resulting in a smaller HIC value.  The mass of the standard Hybrid III head/neck complex is 5.8 kg.  So, the 2.6 kg mass of the helmet/visor set from PPE 5 adds approximately 45% more weight to the structure, and probably explains the significant drop in HIC when the helmet/visor from PPE 5 is added to an unprotected dummy. 

 

The peak external pressures for the protected and unprotected dummies at the 100 g and 200 g charge level from the upper left thorax gauges are shown in Figure 4.   Approximate durations of these pressure time histories are 0.7 ms. These are compared with the threshold lung damage free field values taken from classic work by Bowen et. al. [Bowen-1968]. Both the unprotected and protected dummies show much larger peak pressures for the 200 g charge size than the 100 g charge size.  In addition, all of the dummies with PPEs show decreased peak pressures relative to the unprotected dummies except PPE 2 for the 200 g charge size.  Complex wave interactions behind the PPEs may be the explanation for the large spread in thorax peak pressures for certain PPEs.  However, for both the 100 g and 200 g charge sizes, the peak thorax pressure does not exceed the threshold for blast lung injuries. The complexities of evaluating injury criteria for near field blasts with complex pressure waves suggest the strong need for an experimental effort to evaluate such waves in an injury model.

Figure 4: Peak Thorax Pressure for Kneeling Hybrid III 50th % Male Dummies

 

Conclusions

 

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 and injury performance while ensuring that the resulting criterion is as applicable as possible to the conditions experienced in the real world. 

 

Each of these elements was satisfied in this proposed test methodology.  The simulated mines show repeatable pressure time histories, and the largest simulated mine is comparable to an actual mine of the same threat level.  Mine burial can be controlled very precisely, and soil characteristics have been fixed.

 

The Hybrid III dummy has been found to be a robust and repeatable surrogate.  None of the dummies used suffered a significant mechanical failure during the testing.  The dummies are available in sizes that are anthropometrically similar to a human mid-sized male and similar to a small female.  Positioning was accomplished to within �3 mm relative to the center of the mine with an inexpensive measurement device.  Both the kneeling and the prone positions were specified to produce a significant risk of blunt head trauma to an unprotected dummy.

 

At first glance, it appears that the prone position has a higher risk of neck injury than does the kneeling position.  However, it is important to realize the significant difference in nose-to-mine distance for the two positions.  For the kneeling position, the dummy�s nose-to-mine distance is 65 cm, whereas for the prone position, the distance is reduced to 45 cm.  The two positions were not selected so that the injury risks for the head, neck, and thorax were nearly equivalent, but to directly compare risk of injury between the kneeling and prone positions.

 

Most of the instrumentation proved robust.  For the head and chest accelerometers, the only failures arose from inadvertent wire separation. The head accelerations experienced by the dummies showed a substantial risk of serious head injury from blunt trauma for the larger mines.  However, questions remain about the applicability of typical acceleration based injury criteria to mine blasts.  It is recommended that a limited test series be performed with an injury model under blast loading to determine the boundaries of applicability of the currently used injury criteria.

 

The neck sensors performed well.  The neck showed forcing similar to that seen in automobile impacts for which the sensors were developed. The sensor data showed good differentiation between the level of mine, and was repeatable within a test dummy. The loosening of the neck of Dummy B compromised the comparison of Dummy A to Dummy B for neck loading.  This indicates the large vibration loads in blast shock loading, not seen in the usual automotive application.  For future tests, it is strongly recommended that the dummy neck tensioning be checked regularly during the test series.

 

The thoracic instrumentation proved generally robust.  However, neither the chest displacement nor the Viscous Criterion showed injurious values, even for an unprotected dummy.  The sternal accelerometers performed poorly, likely owing to high frequency oscillations in the sternum under blast loading.  In future testing, the accelerometer should be mounted on the top of the sternum to avoid some of these oscillations.  The upper thoracic pressure sensors proved robust, while the lower pressure sensors failed repeatedly.  This may be the result of the greater compliance of the Hybrid III dummy in the lower thorax.  All PPEs but one reduced the peak thoracic pressure for both the 100 g and 200 g charge size.

 

The ear pressure sensors proved relatively robust.  Surprisingly, two PPEs with the largest helmets showed increased ear peak pressures relative to the unprotected dummy.  This may be attributed to the helmets capturing the pressure wave.

 

Burn sensors used on the dummy hand and chin in this testing showed a very small risk of serious burns for the mines and depth of burial used. As the sensors are exceedingly delicate for blast testing, it is recommended that no burn sensors be used in subsequent testing.

 

Finally, this testing showed the strong effect of the blast cone induced by the geometry of the mines and simulated mines. This conical blast pattern limited the risk of injury to the thorax in both the kneeling and the prone positions.   To provide the most comprehensive understanding of this effect, a small test series should be performed to quantify dummy response as a function of position in the blast cone.

 

Design of personal protective equipment against fragment and blast damage when demining involves numerous tradeoffs between protection of various types and ease of use.  Such tradeoffs underscore the value of a complete assessment of PPE function that includes ergonomics, protection against fragments and protection against blunt trauma.

 

Future Work

 

Several detailed recommendations for future work were developed from this study:

 

  1. This study focused on several �typical� demining positions.  However, there is a strong potential for large changes in dummy response with small changes in position.  A limited test series should be performed to investigate the force/response of the dummy from changes in local position and orientation.  Such a study will define the necessary precision for dummy positioning which may be crucial in verification of the performance of demining PPEs.

 

  1. The force time histories seen in this study may be outside the range of validity of usual automotive models for which the dummy was developed.  It is strongly recommended that a limited test series be performed with a human injury model in several typical test conditions that will verify the use of the dummy surrogates under mine blast conditions. 

 

  1. Ear pressures were obtained in this study using a planar pressure sensor mounted to the surface of the Hybrid III dummy head.  The human ear acts to amplify incoming pressure waves.  So, in concert with an additional dummy test series, it may be prudent to investigate the potential for significant ear damage using a realistic ear form with a pressure sensor located at a distance representative of an eardrum.

 

  1. Finally, the impact of complex blast waves behind body armor is a relatively unexplored area.  It is recommended that a limited test series of blast shocks behind body armor be performed with enhanced instrumentation and a human injury model.

 

Top | Prev | TOC | Home | Next