EOD HELMET PROTECTION IN SMALL BLAST SCENARIOS


By Aris Makris, Ph.D., Ming Cheng, Ph.D., Jean-Philippe Dionne, Ph.D., Jeffrey Levine, M.Eng., R&D and Engineering, Med-Eng

INTRODUCTION
In 2016, with the objective of defining the essential protection and functionality requirements of Personal Protective Equipment (PPE) for bomb technicians, the United States National Institute of Justice released the NIJ 0117.01 Standard for Public Safety Bomb Suits[1], in order to regulate the design and manufacture of such PPE. This standard document includes a wide array of requirements, such as ergonomics, sizing, field of view, visor optics and of course protection from blast threats. In terms of protection from blast overpressure, the NIJ 0117.01 standard includes a qualitative evaluation of bomb suits against a 0.567 kg charge of C4 explosive at a horizontal standoff of 60 cm, tested with a Hybrid III anthropomorphic mannequin placed in a kneeling position donning the suit. This explosive configuration is deemed severe, given that an unprotected individual (not wearing a bomb suit) is not expected to survive from such a blast (even only taking into account the overpressure threat), based on an analysis of the Bowen survivability curves[2]. Facing such an explosive threat, Standard Operating Procedures for Explosive Ordnance Disposal (EOD) obviously call for EOD personnel to don full EOD PPE and utilize remote handling means, including remotely operated vehicles and tools. This involves wearing a purpose made EOD helmet with visor, a jacket, trousers and various protective plates over vulnerable body regions.

Unfortunately, in perceived lower risk situations (involving small amounts of explosive, search, routine disposal procedures), or missions involving higher speed tactical support, it has been noted that some personnel choose to not wear the full EOD PPE for purposes of enhanced comfort, reduced heat stress, and faster response time. This may imply wearing only a subset of the PPE (for instance just a standard combat helmet and protective vest) in deployed theatres of military operations. This choice of protective gear may sometimes be derived from the misconception that when dealing with small amounts of explosive, or known/routine threat devices, minimizing time exposed to the threat might be preferable over being fully protected with EOD PPE. With the objective of addressing this false feeling of safety, the present article describes the benefits and injury reduction capabilities of a bomb suit, even in small blast scenarios, focusing on head injury potential arising from the blast overpressure wave.

BACKGROUND
The threat of relatively low level blast waves has been investigated by various research groups over the last few years. A convenient way to investigate the protective capabilities of helmets in various blast scenarios is to conduct computer simulations of helmets subjected to blast. Using simulations to conduct these types of analyses provides an invaluable tool for characterizing the interaction between the shockwave and the anatomical features of the head. Moreover, head computational models specifically provide spatial and temporally resolved descriptions of parameters such as stress, strain, and acceleration, of relevance to injury, thus enabling a connection between the external blast event and the mechanical tissue response.

One pioneering simulation study by Nyein et al.[3] at MIT in 2010 investigated the blast wave interaction with a military combat helmet, using an advanced model human head. More specifically, they examined protection yielded by the addition of a face shield compared to the case with no head protection at all. The blast scenario investigated consisted of a comparatively small charge of 3.16 g of TNT explosive at an also small standoff distance of 0.12 m. This scenario produced a peak incident overpressure of approximately 1000 kPa. Their results revealed that the main blast wave transmission pathways are along the soft tissues in direct contact with the incident blast wave. They also identified the potential for fluid cavitation in the brain, resulting from negative pressures in areas adjacent to the presence of cerebrospinal fluid (CSF). Finally, strong pressure gradients were observed at the interface between the CSF and the cerebrum, revealing the severity of this blast threat (at such short standoff distance), despite the small amount of explosives involved.

More importantly though, their results showed no significant mitigation of blast effects on brain tissue through wearing a combat helmet alone. This is because the combat helmets do not cover the face and the top part of the head, which were both noted to be major pathways of load transmission into the intracranial cavity. On the contrary, it was found that adding a conceptual face shield with a simple geometry to the combat helmet significantly reduced the magnitude of the stresses propagated inside the brain, thus greatly reducing the injury potential.

These computer simulation results are echoed in some experimental trials conducted by Med-Eng, whereby Hybrid III mannequins were subjected to blasts from 100 g and 250 g of C4 explosive at a standoff distance of 1 meter (Figure 1). Again, the use of a combat helmet alone shows little effect on the peak head accelerations, whereas the addition of a face shield significantly lessens the accelerations subjected to the head.

Figure 1: Average and range of peak head accelerations from a Hybrid III mannequin facing either 100g C4 or 250g C4 explosive at 1.0m standoff distance.

More recently, Valverde-Marcos et al.[4] conducted an extensive study of the protective capability of an EOD helmet for small blasts, also using computational models. They used the HHFEM (Human Head Finite Element Model) model developed by J. Antona-Makoshi[5] and modeled an existing EOD helmet used by the Spanish police. The blast scenarios considered for their study consisted of 0.05 kg, 0.075 kg, and 0.1 kg of TNT at a standoff distance of 0.618 m. By comparing with the unprotected case, it was found that wearing the EOD helmet delays the impact of the shockwave on the wearer’s head and reduces the maximum head acceleration by 80% in all three cases simulated. When compared to relevant published injury thresholds, they concluded that wearing an EOD helmet reduces the severity of injuries from a highly probable death (when unprotected) to a low probability of injury, of a mild and localized nature. It must be emphasized however that these findings were obtained through simulating relatively low explosive charges.

NUMERICAL SIMULATIONS
To complement the computer simulation studies listed above, and with the intent of investigating the role played by different protective components, a set of numerical simulations was undertaken by Med-Eng, using the LS-Dyna hydrodynamic code (LSTC, Livermore, CA). These simulations focused on investigating the effect not only of a face shield, but also the jacket and collar from an EOD ensemble. As such, finite element models of EOD PPE components were developed (Figure 2) to investigate how the response of the head and neck is affected by these protective features. To isolate the contributions of each helmet component, simulations were carried out for four levels of protection: (1) An unprotected head and neck, (2) Head and neck with an EOD helmet shell only, (3) Head and neck with an EOD helmet and EOD face shield, and finally (4) Head and neck with the complete EOD suit. These helmet and suit components were fitted on a Hybrid III 50th percentile head and neck finite element model developed by the University of Virginia Center for Applied Biomechanics[6].

Figure 2: Finite element models of EOD PPE components.

Figure 3 illustrates the blast wave interacting with the Hybrid III mannequin model, in the four protection configurations mentioned earlier. The blast scenario consists of 2.25 kg of C4 explosive at a standoff distance of 3 m, a significantly lesser threat than that listed in the NIJ 0117.01 standard. Figure 4 presents the corresponding resultant head accelerations (measured at the center of gravity) with respect to time. This graph shows that wearing the helmet alone (orange trace) does not bring any significant reduction in peak head acceleration. On the other hand, adding the face shield (blue trace) dramatically lowers the head acceleration profile, whereby not only is the peak value reduced, but also the rate of increase in head acceleration. These results are in line with those from Nyein et al. and Valverde-Marcos et al., discussed earlier.

Figure 3: Interaction of the blast wave with the Hybrid III mannequin in the 4 protection configurations, for a blast scenario of 2.25kg C4 at a standoff distance of 3m.

Figure 4: Resultant acceleration at the center of gravity of the head for a blast scenario of 2.25kg C4 at a standoff distance of 3m (left: time history, right: peak values).

The addition of the jacket (green trace) further suppresses the initial head acceleration, but without reducing its maximum value by a sizeable amount. On the brighter side, the jacket collar is found to play a key role in reducing the neck shear forces (plotted in Figure 5). The collar also significantly reduces the neck moment (bending in the front and back direction) for the earlier portion of the event (Figure 6, first 2.5 milliseconds). The neck moment then substantially increases due to the effect of the face shield, which significantly enlarges the surface area exposed to the blast. It should be noted however, that although the longer-duration, raw moment values were increased, they still remained well-below established injury thresholds[7].

Figure 5: Neck force (main X-direction) for a blast scenario of 2.25kg C4 at a standoff distance of 3m (left: time history, right: peak values).

Figure 6: Neck moment for a blast scenario of 2.25kg C4 at a standoff distance of 3m (left: time history, right: peak values for the full signal and for the first 2.5ms of the signal). Although the longer-duration long-term moment values were increased with the face shield, they still remained well-below established injury thresholds[7].

Thanks to its ear covers, the EOD helmet, even without the face shield, significantly reduces the pressure at the ear (Figure 7, left). But unsurprisingly, the helmet shell alone (without a face shield) does little to reduce the pressure in front of the eyes (Figure 7, right) which is substantially reduced by incorporating the face shield, as expected. With the face shield, and even more so with the jacket, the peak pressure at the ear is greatly reduced. For this particular small blast case, the addition of the helmet with ear muffs, the visor and then the full EOD PPE reduces the peak pressure at the ear by 53%, 59%, and 73%, respectively.

Figure 7: Pressures at locations of ear and eye in the case of 2.25kg C4 at a standoff distance of 3m.

For small blasts as these, where lethality is generally not at issue when wearing EOD PPE, the part of the body most vulnerable to the overpressure is the eardrum. Generally speaking, there is a 50% chance of perforating the eardrum at an overpressure of 100 kPa with the threshold at a mere 35 kPa[8, 9]. Therefore, the addition of the EOD PPE lowers the chances of eardrum perforation from well over 50% for an unprotected or merely helmeted individual, to pressures just above the threshold for ear damage. Unfortunately, similar injury assessments do not exist for the eyes, however, it should be noted that the peak pressure is reduced by a 87% when donning the full EOD PPE as compared to none at all.

CONCLUSION
While there is no debate in terms of the necessity to wear full EOD PPE including a purpose made EOD helmet with visor when facing large explosive charges, the earlier study from Nyein et al. and especially the recent one from Valverde-Marcos et al. have both demonstrated that even for relatively low blast levels, helmets with face shield protection still play a major role in reducing the potential for blast injuries. Moreover, the numerical simulation data collected from the present study also emphasized the added benefits of wearing a typical EOD jacket equipped with a large collar, in terms of further reducing relevant parameters at the head and neck locations. Wearing a full EOD ensemble (purpose made EOD helmet with faceshield, and EOD jacket) thus plays a key role in minimizing the chances of injury.

To simplify the analysis, the present article only focused on primary (blast overpressure) injuries. With any explosive, even of small size, there is always a risk of fragmentation injuries, when fragments from the explosive device itself or surrounding debris propelled by the blast hit the bomb technician. In such cases, the additional fragmentation protection protected by EOD PPE, both in terms of protection level and amount of coverage, also plays a critical role in reducing the potential for injuries.

Furthermore, increasing evidence from the literature that suggests that repetitive low-level blasts, even at levels that do not cause injury for a single exposure, can lead to long term brain issues sometimes revealing themselves long after actual blast exposure[10]. As such, even when dealing with relatively low explosive charges, it is always recommended and much more prudent for bomb technicians to properly dress up in their full-fledged EOD PPE. ■

REFERENCES

  1. U.S. National Institute of Justice (NIJ), Public Safety Bomb Suit Standard, NIJ-0117.01, 2016
  2. IG Bowen, ER Fletcher, DR Richmond, Estimate of Man’s Tolerance to the Direct Effects of Air Blast, Lovelace Foundation for Medical Education and Research, Albuquerque, New Mexico, DASA 2113, DA-49-146-XZ- 372, 1968
  3. Michelle K. Nyein, Amanda M. Jason, Li Yu, Claudio M. Pita, John D. Joannopoulos, David F. Moore, Raul A. Radovitzky. In silico investigation of intracranial blast mitigation with relevance to military traumatic brain injury. Proceedings of the National Academy of Sciences Nov 2010, 107 (48) 20703-20708; DOI: 10.1073/ pnas.1014786107
  4. Valverde- Marcos, B., Rubio, I., Antona-Makoshi, J., Chawla, A., Loya, J.A., Rodríguez-Millán, M., Numerical Analysis of EOD Helmet under Blast Load Events using Human Head Model, Applied Sciences 2020, 10, 8227; doi:10.3390/app10228227
  5. Antona-Makoshi, J. Traumatic Brain Injuries: Animal Experiments and Numerical Simulations to Support the Development of a Brain Injury Criterion; Chalmers University of Technology: Gothenburg, Sweden, 2016; ISBN 978-91-628-9848-9.
  6. Giudice, J.S., Park, G., Kong, K., Bailey, A., Kent, R., Panzer, M., Development of Open-Source Dummy and EOD HELMET PROTECTION IN SMALL BLAST SCENARIOS counteriedreport.com 7 Impactor Models for the Assessment of American Football Helmet Finite Element Models, Annals of Biomedical Engineering, Vol. 47, No. 2, February 2019
  7. R Eppinger, E Sun, F Bandak, M Haffner, N Khaewpong, M Maltese, S Kuppa, T Nguyen, E Talhounts, R Tannous, A Zhang & R Saul, Development of Improved Injury Criteria for the Assessment of Advanced Automotive Restraint Systems – II, National Highway Traffic Safety Administration (NHTSA), November 1999
  8. Jensen JH, Bonding P. Experimental pressure induced rupture of the tympanic membrane in man. Acta Otolaryngol 1993;113:62–7. doi:10.3109/00016489309135768.
  9. Stapczynski JS. Blast injuries. Ann Emerg Med 1982;11:687 – 94.
  10. Carr, W., Polejaeva, E., Grome, A., Crandall, B., LaValle, C., Eonta, S.E., Young, L.A., Relation of Repeated Low- Level Blast Exposure With Symptomology Similar to Concussion, Journal of Head Trauma Rehabilitation, June 2014, DOI: 10.1097/HTR.0000000000000064

ABOUT THE AUTHORS

Dr. Aris Makris is VP of Research, Development & Engineering and Chief Technology Officer at Med-Eng, which is a brand of The Safariland Group. He holds Masters and Ph.D. degrees in Mechanical Engineering, specializing in explosions and protection against blast effects, with over 30 years of related experience. He has led numerous programs involving the design and development of advanced personal protective systems to protect against IEDs, landmines, and explosive threats. Dr. Makris and his team have also conceived and developed a number of person borne blast sensors to assist in quantifying individual blast exposure under diverse operational circumstances and guide improvements in design, medical understanding and optimization of Techniques, Tactics and Procedures. Dr. Makris has been an active member of several equipment performance standards, including the NIJ Bomb Suit standard, NATO, UN working groups and a member of the IABTI Advisory Board.

Dr. Jean-Philippe Dionne holds a Ph.D. in Mechanical Engineering from McGill University with over 20 years expertise in the fields of numerical simulations of detonations, blast waves and combustion. Since joining Med-Eng in 2000, he has been involved in numerous projects including explosive tests on demining and bomb disposal personal protective equipment, performance testing of personal cooling systems, blast dosimeters and blast mitigation seats. Dr. Dionne manages a group of R&D engineers and technologists dedicated to research related to all Med-Eng products. He has significantly contributed to the development of the National Institute of Justice NIJ 0117.01 standard for Public Safety Bomb Suits. During the 2004 Personal Armour Systems Symposium, taking place in The Hague, Netherlands, Dr. Dionne received the Young Talent Award, recognizing his contribution in personal protection against blast. He is also the author of the book “Presentation Skills for Scientists and Engineers” published by Springer Nature (2021).

Ming Cheng, Ph.D., has worked on blast effects on the human body for more than 15 years through computational modeling and simulations as well as physical experiments. He has published many journal and conference papers, and was awarded with the prestigious Louis & Edith Zernow Award for fundamental progress in ballistics at the International Symposium on Ballistics in 2011.

 

Jeffrey Levine has held the position of Research Engineer at Med-Eng for more than fifteen years. His research has generally focused on shock physics, human injury and survivability, and blast exposure monitoring. These concentrations have been specifically applied to the fields of Explosive Ordnance Disposal (EOD) and Explosive Forced Entry (EFE).


 

Download PDF: 53-60 MED-ENG article – C-IED Report SS2021