Research shows injury risks to deminers can vary depending on their body positioning. Here, the authors present the preliminary results of a study testing the effects of body position on deminer injury using mannequins. They hope to refine further their methodology and continue to learn information that will benefit the demining community.
Unlike the spherically expanding blast wave caused by the detonation of unconfined, bare high explosives, the blast resulting from the detonation of an anti-personnel blast mine buried in the soil is strongly directional and non-uniform. The confining soil prevents uniform expansion of the detonation products, which are preferentially vented upwards and entrain soil ejecta. This results in a conical multi-phase "jet," the shape, associated blast pressure and jet velocity of which depend on many factors such as charge size and shape, charge casing, depth of burial, soil type and particle-size distribution, soil moisture, soil compaction, etc.1 Therefore, the loading on a deminer accidentally exposed to the detonation of a mine buried in the ground should depend on the position of the body with respect to the blast cone. A "standard" test methodology using anthropomorphic mannequins was developed to evaluate upper-body protective equipment for deminers.2–4 As expected, small changes in body position resulted in large differences in the loading of the Hybrid III mannequins used.4
The preliminary results of the current study were conducted by Defense R&D Canada-Suffield, a research center that develops defensive countermeasures against the threat of chemical and biological weapons. Their findings have been published in "Effects of Body Position on Injury Risk Against AP Blast Mines."5 The work is aimed at generating information to further refine the test methodology. Toward that end, the effects of nose-to-charge standoff distance and body orientation with respect to the blast cone on the forces recorded on the unprotected kneeling mannequin for different explosive charges have been studied. Precise placement of a standard AP mine simulant, combined with the use of a standard test soil and precise positioning of the mannequin, ensured minimum variability in the results.
The mannequin-positioning rig used to hold the mannequin in an accurate position, which was mounted on a metal platform, consisted of a base and two vertical arms that can be tilted forward to support the mannequin in various positions. Chains anchored on the back of the mannequin are attached to a metal bar that runs across the two vertical posts of the positioning rig and rests on brackets in such a way that the mannequin can fall without being restrained by the rig when the blast is strong enough. A measurement fixture, consisting of a vertical column with a ruler and two sliders that can be moved along the vertical axis, was bolted to the platform for self-alignment. It allowed the mannequin and the explosive charge to be accurately positioned (the fixture created the charge hole in the sand).
The 50th percentile male and the fifth percentile female Hybrid III mannequins were selected to represent people of two statures found among deminers and because such differences impact on the standoff distance that deminers can realistically achieve during mine clearance. The mannequins were positioned in three body orientations with respect to the blast cone, defined by the angle between the vertical and a straight line joining the nose and the mine. The three angles (θ), denoted A1, A2 and A3, were 35, 27.5 and 20 degrees, respectively. For each mannequin, three realistic nose-to-mine standoff distances were tested (50, 60, and 70 centimetres for the fifth percentile; 60, 70 and 80 centimetres6 for the 50th percentile). The standoff distances were shorter for the fifth percentile to account for the shorter reach of smaller individuals. Each scenario was repeated at least three times to determine the extent of the standard deviation of the results.
Dry, coarse sand was placed without compaction in a sandbox (a cube measuring 600 millimetres [24 inches] on each side, large enough to mitigate the effect of shock reflections) built into the test platform. Given that soil moisture is an important variable in a blast event, the sand was dried to less than 1-percent moisture content prior to shipment and kept under cover until immediately before the trial setup. Following each trial, fractured sand7 was removed and replaced with fresh sand.
The explosive charges used (50 and 100 grams, ±1 percent) were prepared by packing C4 plastic explosive into standard containers. Each charge was pre-armed with an RP87 detonator, boosted with 2 grams of Detasheet® and buried at "ground zero" (in the centre of the sandbox), with 20 millimetres (0.8 inches) of soil overburden. Detonation was initiated from the bottom centre of the charge.
Lightweight and thin body-conforming armour was designed to protect the rubber neck and skin of the mannequin from ejected sand. This "protection" (668 grams and 992 grams for the fifth and 50th percentile mannequins respectively) did not significantly alter the mass distribution or the nominal profile exposed to the blast.
Each mannequin was instrumented with accelerometer triads mounted in the centre of gravity of the head (Endevco 7270A) and in the spine (Endevco 7264B), a load cell (Denton 1716A) for the upper neck forces and moments, as well as two "flat pack" pressure transducers (Kulite LQ-125) near the ear location. The sampling frequency for all channels was 1 MHz, which was sufficiently high to capture the full frequency spectrum of the acceleration, force and pressure signals. The head acceleration signals, along with the neck force and moment signals, were digitally filtered (low-pass) using a four-pole Butterworth digital filter with cutoff frequency of 1650 Hz, while a 300-Hz cutoff frequency was used for spine acceleration, in accordance with known standards used in the automotive industry (Society of Automotive Engineers J2118). Finally, the pressure signals were digitally filtered using a two-pole Butterworth digital filter (low-pass) with a 10-kHz cutoff to remove spurious noise.
Injury criteria used in the automotive industry and/or blast community were used to estimate the severity of injury that would result for the different test scenarios. It was understood that these criteria may not provide meaningful injury predictions, since they were not developed based on mine-injury data. Nevertheless, they were used to rank scenarios from the least to most injurious, since they take into account the parameters (e.g., peak values, duration and integral under the curves, etc.) that relate to injury, as well as their relative importance.
Neck injuries were evaluated using the Nij neck injury criterion based on human cadaver, volunteer and animal data.9 The Nij can be related to various levels of injury severity defined by the standard 1985 SAE Abbreviated Injury Scale (AIS 0 being non-injurious, AIS 6 being lethal). The Nij criterion consists of a linear combination of neck forces and moments.
The head injury criterion, developed and used by the automotive industry to correlate the head acceleration experienced by crash victims to injury,9, 10 was used in combination with the curves of Prasad and Mertz11 to predict the probability of different injury severity levels defined by the AIS scale. For head injuries, this scale ranges from AIS 0 (no injury) to AIS 6 (lethal injury), with various levels of unconsciousness (AIS 2 through AIS 5).
Figure 1: Probabilities of neck injury levels versus body orientation, fifth-percentile mannequin, 100 g C4, 60-cm standoff.
The ear is the part of the body most susceptible to blast overpressure injury. For the blast durations obtained in this test series, the threshold of eardrum perforation lies at a mere 0.35 bar, while an overpressure of 1 bar will yield 50-percent probability of eardrum perforation.12
Lastly, chest injuries resulting from the acceleration measured on the spine were evaluated using the FMVSS 208 Standard, which specifies as acceptable any acceleration pulse of the spine that "shall not exceed 60 g’s13 except for intervals whose cumulative duration is not more than 3 milliseconds."
Results and Discussion
Figure 2: Probabilities of neck injury levels versus standoff distance, 50th-percentile mannequin, 100 g C4, orientation A3.
Sample results of the neck injury analysis are presented in Figures 1 through 4. The points represent the mean values, while the bars indicate the range of the data. Body orientation was found to have an effect on injury probabilities (see Figure 1 for example) with the risk increasing when the head approached the center of the blast cone (i.e., A3 is more injurious than A2, which is more injurious than A1). The effect of body orientation was more pronounced against the 100-g charge than against the smaller 50-g charge for both mannequins. Similarly, the standoff distance was found to have a moderate effect against the larger charge, with the risks being greater when the standoff was shortest (see Figure 2 for an example).
Figure 3: Probabilities of neck injury levels versus charge size, 50th-percentile mannequin, 70-cm standoff, orientation A2.
Figure 4: Probabilities of neck injury levels versus mannequin size, 100 g C4, orientation A2, 70-cm standoff.
Predicted neck injuries were slightly more severe when facing blasts from the 100-g charge compared to the 50-g charge, although the difference was usually not very great (see Figure 3 for example). It was also observed that the injury risks were very comparable between the two mannequin sizes (fifth and 50th percentile Hybrid III), for an identical charge and standoff distance (see Figure 4).
Body orientation also had an effect on predicted head concussive injury severity, as illustrated in the sample graph of Figure 5, which shows the injury severity predictions for the 50th percentile mannequins against the 100-g charge for a 70-cm standoff distance. Again, position A3 would result in the greatest risks. Standoff distance and charge size were also found to have a strong influence on the injury severity, as illustrated in Figures 6 and 7. Finally, the injuries predicted for the 50th percentile male mannequin were more severe than those of the fifth percentile female mannequin (see Figure 8); this observation was made for all body orientations. This difference between the two surrogates became more evident against large charges.
Figure 5: Probabilities of concussive injury levels versus body orientation, 50th-percentile mannequin, 100 g C4, 70-cm standoff.
Figure 6: Probabilities of concussive injury levels versus standoff distance, fifth-percentile mannequin, 100 g C4, orientation A2.
Figure 7: Probabilities of concussive injury levels versus charge size, fifth-percentile mannequin, 60-cm standoff, orientation A2.
Figure 8: Probabilities of neck injury levels versus mannequin size, 100 g C4, orientation A2, 70-cm standoff.
Body orientation had no effect on peak overpressures measured at the location of the ears. This can be seen in Figures 13 and 14, which present the peak ear overpressure measured on both ears of the fifth and 50th percentile mannequins, respectively, when facing the 100-g charge blast with a 70-cm standoff distance. Another observation was that increasing or decreasing the standoff distance did not have an evident effect on peak ear overpressure, whereas increasing the charge size had the effect of strongly increasing ear overpressure. Moreover, ear pressures on the fifth percentile were higher than on the 50th percentile (comparing Figure 9 to Figure 10), likely due to slight differences in head position/dimension between the two mannequins.
Figure 9: Peak ear overpressure versus body orientation, fifth-percentile female mannequin (100g C4 at 70 cm). The dotted lines indicate the 0.35 bar and 1 bar thresholds.
Figure 10: Peak ear overpressure versus body orientation, 50th-percentile male mannequin (100 g C4 at 70 cm). The dotted lines indicate the 0.35 bar and 1 bar thresholds.
Finally, no chest injuries were predicted based on spine acceleration measurements for the conditions tested.
A test series was conducted using Hybrid III mannequins to assess the effects of AP mine detonations against the upper body of a deminer who is positioned at various standoff distances from the mine and who is oriented in different positions relative to the blast cone produced by the detonation of the charge. Two Hybrid III anthropomorphic mannequins of different sizes were accurately positioned using a specially built test platform/positioning rig to face blasts from simulated mines made of C4 explosive. Based on transducers located in the mannequins, injury assessments were made using the latest injury criteria used in the fields of automobile accidents and blast scenarios.
It was found that changes in body orientation with respect to the blast cone had an effect in the harshest test conditions (short standoff and/or large charge) on neck and head injuries only. Injuries to the neck, ear and head were affected slightly by the standoff distance and charge mass used, but again, this effect was only evident in the harshest test conditions. When comparing between the two mannequin sizes, it was found that for the same blast, head injuries were more severe for the 50th-percentile mannequin, whereas ear injuries were more severe for the fifth percentile mannequin. Lastly, chest injuries resulting from spine accelerations were found to be unlikely in this study.
*All graphics by Med-Eng Systems/MAIC
Executive Assistant to the CEO of Defence R&D Canada Dr. Denis Bergeron has a Ph.D. in aerospace science and engineering from the University of Toronto. At DRDC-Suffield, he worked on projects that include aerospace systems and weapon effects. Recently he chaired NATO Task Group 24 to define how personal protective equipment should be tested against AP landmines.
Matt Ceh worked as a defence scientist at DRDC-Suffield after graduating from the University of Calgary with a mechanical engineering degree. He worked in several demining-related projects but has focused mainly on studying injury to the human body based on its position in mine blast events. Mr. Ceh is also working on explosive ordnance disposal projects.
Dr. Jean-Philippe Dionne, P.Eng., holds a Ph.D. in mechanical engineering from McGill University and has nine years’ experience in the fields of detonations, blast waves and combustion. He has been involved in explosive tests on demining and bomb disposal protective equipment. During the 2004 Personal Armour Systems Symposium, Dr. Dionne received the ESM Dyneema Young Talent Award, recognizing his contribution in personal protection against blast.
Currently enrolled in a Mechanical Engineering Ph.D. program at McGill University, Montréal, Canada, François-Xavier Jetté, M.Eng., was a research scientist at Med-Eng Systems until June 2005, involved in the data and injury analysis related to explosive blast protection.
Ismail El Maach is currently a senior research engineer at Med-Eng Systems. He holds a master’s degree in biomedical engineering from the école Polytechnique de Montréal and has six years’ experience in the field of realistic testing and simulation of impact trauma and the effects on personal protective equipment.
Dr. Aris Makris holds a Ph.D. in mechanical engineering from McGill University. He has 20 years of experience in the fields of shock waves, detonation, combustion and protection technologies. Dr. Makris has managed numerous R&D programs geared towards the development of highly advanced personal protective equipment and related tools.
- Bergeron, D.M., R.A. Walker and C.G. Coffey. June 1998. "Mine Blast Characterization—00-Gram C4 Charges in Sand," Report 668, DRDC-Suffield, Ralston, Alberta, Canada.
- Bass, C.R., B. Boggess, M. Davis, C. Chichester, D.M. Bergeron, E. Sanderson and G. Di Marco. 2001. "A Methodology for Evaluating Personal Protective Equipment for AP Landmines," presented at The 2001 UXO/Countermine Forum, New Orleans, USA, April 2001.
- Chichester, C., C.R. Bass, B. Boggess, M. Davis, D.M. Bergeron, E. Sanderson and G. Di Marco. 2001. "Effectiveness of Personal Protective Equipment for Use in Demining AP Landmines," presented at The 2001 UXO/Countermine Forum, New Orleans, USA, April 2001.
- Nerenberg, J., A. Makris, J.P. Dionne, R. James, C. Chichester and D.M. Bergeron. 2001. "Enhancing Deminer Safety Through Consideration of Position," presented at The 2001 UXO/Countermine Forum, New Orleans, USA, April 2001.
- Braid, M.P., D.M. Bergeron, R. Fall, M. Ceh. 2003. "Effects of Body Position on Injury Risk Against AP Blast Mines," NATO paper RTO-MP-AVT-097, presented at the joint RTO AVT/HFM Specialists’ Meeting on "Equipment for Personal Protection (AVT-097)" and "Personal Protection: Bio-Mechanical Issues and Associated Physio-Pathological Risks (HFM-102)," Koblenz, Germany, 19–23 May 2003.
- 10 centimetres equals approximately 4 inches.
- Fractured sand is sand that has been pulverized by explosive forces, with silica dust as the main by-product of this process.
- SAE J211 refers to the SAE Recommended Practice J211, Instrumentation for Impact Tests (MAR95). It provides standards for the performance of equipment in impact tests.
- Kleinberger, M., E. Sun, R. Eppinger, S. Kuppa and R. Saul. September 1998. "Development of Improved Injury Criteria for the Assessment of Advanced Automotive Restraint Systems," National Highway Traffic Safety Administration, U.S. Department of Transportation.
- Versace, J. 1971. "A Review of the Severity Index," Proceedings of the Fifteenth Stapp Car Crash Conference, pp. 771–796, 1971.
- Prasad, P., H.J. Mertz. 1985. "The Position of the United States Delegates to the ISO Working Group 6 on the Use of HIC in the Automotive Environment," SAE Paper Number 85-1246, Society of Automobile Engineers, Warrendale, Pa., USA, 1985.
- Garth, R.J.N. 1997. "Blast Injury of the Ear," In: Scientific Foundations of Trauma, GJ Cooper (Ed.), Oxford: Butterworth-Heinemann Publisher, 1997, pp. 225–235.
- 1 g = 9.8 m/s2.
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