Abstract Since World War I, helmets have been used to protect the head in warfare, designed primarily for protection against artillery shrapnel. More recently, helmet requirements have included ballistic and blunt trauma protection, but neurotrauma from primary blast has never been a key concern in helmet design. Only in recent years has the threat of direct blast wave impingement on the head–separate from penetrating trauma–been appreciated. This study compares the blast protective effect of historical (World War I) and current combat helmets, against each other and ‘no helmet’ or bare head, for realistic shock wave impingement on the helmet crown. Helmets included World War I variants from the United Kingdom/United States (Brodie), France (Adrian), Germany (Stahlhelm), and a current United States combat variant (Advanced Combat Helmet). Helmets were mounted on a dummy head and neck and aligned along the crown of the head with a cylindrical shock tube to simulate an overhead blast. Primary blast waves of different magnitudes were generated based on estimated blast conditions from historical shells. Peak reflected overpressure at the open end of the blast tube was compared to peak overpressure measured at several head locations. All helmets provided significant pressure attenuation compared to the no helmet case. The modern variant did not provide more pressure attenuation than the historical helmets, and some historical helmets performed better at certain measurement locations. The study demonstrates that both historical and current helmets have some primary blast protective capabilities, and that simple design features may improve these capabilities for future helmet systems.

Citation: Op ‘t Eynde J, Yu AW, Eckersley CP, Bass CR (2020) Primary blast wave protection in combat helmet design: A historical comparison between present day and World War I. PLoS ONE 15(2): e0228802. https://doi.org/10.1371/journal.pone.0228802 Editor: David Zonies, Oregon Health and Science University, UNITED STATES Received: August 20, 2019; Accepted: January 23, 2020; Published: February 13, 2020 Copyright: © 2020 Op ‘t Eynde et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All the data underlying the analysis can be found in a public repository hosted by the Duke Data Repository. All 46 files are avalible at https://doi.org/10.7924/r4r49m981. Funding: We would like to gratefully acknowledge funding and support from the Josiah Charles Trent Memorial Foundation Endowment Fund at Duke University for directly funding this research. There is no particular grant associated with the work and funding was not provided to a particular member of the research group. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction ‘That men do not learn very much from the lessons of history is the most important of all the lessons that history has to teach.’—Aldous Huxley. At the start of World War I (WWI) in July 1914, helmets were not part of the standard military equipment for any of the allied or central powers [1]. Most headwear consisted of cloth (e.g. French Kepi [2]) or leather (e.g. German Pickelhaube [3]) and did not offer the wearer any protection from blasts, shrapnel, or ballistic impacts. Multiple reports at the time estimated that at the start of the war, over fifty per cent of fatalities occurred due to shrapnel or artillery shell fragments, often striking the head, for which steel helmets could be effective [1, 4]. In 1915, France was the first nation in WWI to equip soldiers with steel helmets, utilizing the M15 Adrian helmet, named after the design by General Adrian [5]. Inventor John L. Brodie addressed the British need for head protection in late 1915 with a helmet design aimed at shrapnel protection while focusing on ease of manufacturing [6]. Other nations also used the Brodie helmet, including the United States when they joined the war in late 1917 [7]. After extensive testing of Allied helmets, the Stahlhelm (translation: steel helmet) was rolled out to German soldiers at the start of 1916 [8]. These helmets were effective in their design to protect against artillery shell shrapnel [1]. Besides propelling shrapnel, exploding artillery shells also create a shock wave. The shock wave is referred to as primary blast, while the projectiles launched during an explosion are considered secondary blast. In WWI, the effects of these blast waves were experienced on a large scale for the first time in the combat theatre. Soldiers who experienced explosions in close vicinity were delivered to field hospitals despite having little to no signs of external trauma. British physician Charles Myers used the term ‘Shell Shock’ in 1915 to describe an array of symptoms experienced by soldiers after shell explosions [9], today believed to be potentially caused by a combination of traumatic brain injury (TBI) and psychological trauma [10]. Since the early 1900s, exploding artillery shells have been the largest cause of combat casualties in major conflicts [11]. In US wars since WWI, there has been an increasing trend towards greater number of casualties being caused by explosions, with one study reporting 78% of all injuries in the 2001–5 period of the conflict in Iraq being caused by explosions [12]. During the conflicts in Iraq and Afghanistan, over 65% of reports of TBI were associated with an explosion [13]. A 2008 study of US Army Infantry soldiers returning from deployment in Iraq found more than 15% of them suffered some form of mild traumatic brain injury (mTBI) [14]. In the past decade, there has been an increase in the awareness of long-term debilitating effects of primary blast mTBI, such as axonal injuries [15, 16]. Blast exposures causing minimal acute injuries might cause functional brain changes over time or with repeated exposures. Since most blast TBI is classified as ‘mild,’ there is an increasing demand for combat helmets to protect against these exposures. Risk assessment of human blast injury to the pulmonary system was developed in the 1960s [17] and recently given a stronger experimental basis [18, 19]. The bulk of historical blast work implies that pulmonary tolerance is much lower than neurotrauma tolerance for blast [20–22]. This has been recently confirmed in direct comparative experimentation in rabbits [23]. In concert with these studies, researchers have recently developed risk functions for potential neurotrauma from blast [23, 24] that provide assessment tools for primary blast effects on the head. Initially perplexing, the incidence of blast pulmonary trauma following blast exposure in current military conflicts is quite low compared with blast neurotrauma [25], despite the difference in tolerance. This apparent contradiction is resolved by the almost universal use of body armor which dramatically increases the pulmonary tolerance of blast relative to that of the brain [26]. The battlefield conditions of WW1 on the Western Front provide similar blast conditions to those seen today. Battles fought from trenches in short spurts of unit advancement largely result in the helmeted head being exposed to the blast, while the torso is more distant or covered, decreasing the potential for blast pulmonary trauma. No current fielded helmet system has been specifically designed for blast protection, though careful studies suggest that modern helmets have a degree of blast protective effects [27–29]. This study compares the blast protective capabilities of principal military helmets from WWI combatants with a modern composite helmet. For the three historical helmets discussed in this study, no record of primary blast evaluation was found in the scientific literature. The current study is, to our knowledge, the first to assess the protective capabilities of these historical combat helmets against blast. Brain injury due to primary blast was first recognized around the same time these helmets were being developed [30], and primary blast is now a generally recognized mechanism of injury to the brain. This study is an investigation into whether improvements have been made in combat helmet primary blast protection or if there is a lesson to be learned from these 100-year-old designs.

Discussion Blast exposure to the bare head was more severe than any helmeted test for every blast intensity and at every measurement location. The bare head experienced three to five times higher peak pressures (Fig 8) at the crown of the head (at similar positive phase durations), which corresponds to higher risk of meningeal bleeding and other potential brain injuries [24]. Helmets provided more shock wave attenuation at lower pressure levels than at higher pressure levels (Fig 8), suggesting that helmets might play an especially important role in protection against mild primary blast induced brain trauma. The effect of wearing a helmet, especially for short positive phase durations (0.5–5 ms), is a significant reduction in risk of blast brain injury at the crown of the head for overhead blast scenarios. In other orientations, blast wave measurements are complicated by the difference between reflective (measured with pressure gauges oriented parallel to the direction of the blast) and incident (measured with pressure gauges oriented perpendicular to the direction of the blast) pressures, leading to conflicted reports of helmets possibly increasing the risk of primary blast injury [36–42]. This risk has to be carefully evaluated because reflected pressure measurements can be two to eight times greater than incident pressure measurements for the same blast scenario [32]. An interesting result from these experiments is the blast protective effect provided by the French Adrian helmet, which had a lower crown pressure than all other helmets, despite being manufactured using similar materials as the Stahlhelm and Brodie Helmet, with a thinner helmet wall (Table 1). This result might stem from the deflector crest along the midline of the helmet (Fig 1a). Specifically added with overhead shrapnel in mind [43], this feature of the helmet could deflect the shock wave off to the side of the head, rather than allow shockwave impingement onto a more planar surface seen in the other helmets. The crest also provides an added first layer for shock wave reflection before reflecting a second time off the helmet itself. The crown pressure sensor used in the measurements was located under the deflector crest and may have experienced a decreased peak pressure because of this. Further studies are needed to see if surface geometry manipulation or helmet attachments may augment the protective capabilities of helmets against blast exposure. Peak pressures measured in locations other than the crown of the head were much lower because of measurement at an orientation incident to the blast wave and being partly or completely covered by the helmets. In these locations, the Adrian helmet did not provide the same protective advantage seen at the crown. Pressure attenuation was seemingly determined by the width of the brim and/or coverage of the helmet (Fig 2). At the ear, the small brim and limited coverage of the Adrian helmet resulted in higher pressures than the other helmets (Fig 11d), with a corresponding increased risk in eardrum damage (Fig 12). The ACH, without a brim as seen in the historical helmets, had increased pressures at the eye (Fig 11c) but provided similar protection at the other measurement locations. While ballistic protection provided by helmets has increased significantly since WWI and saved many lives [39], the results found here suggest that the ACH did not perform quantitatively or qualitatively better than the historical helmets, and performed worse than the Adrian helmet for overhead primary blast at the crown of the head. On the other hand, while ballistic protection has been an active focus in combat helmets design, protection from primary blast has not been an important design element [39], and the level of protection from primary blast from all of the helmets tested is large compared with the bare head. One of the reasons for this is that the mechanism for blast protection was poorly understood for the first sixty years following WWI. While the exact injury mechanism for primary blast is still unknown, the scientific community (cf. Cooper, 1991) [44] identified acoustic impedance as one important protection mechanism against blast waves. The acoustic impedance protection mechanism against blast trauma is different than against ballistic trauma. An ideal protection against ballistic impacts can locally absorb high energy impacts without failure or excessive deformation by distributing the energy through the material [45]. Desirable materials have high strength, high modulus, and a high local speed of sound. Protection from primary blast waves can be obtained by attenuating the blast wave using an acoustic impedance mismatch at an interface the wave is travelling through. An increased difference in acoustic impedance causes a higher proportion of the blast wave to be reflected, rather than penetrate into the body where it causes local stresses and tissue damage [44]. The reflection coefficient R can be calculated from Eq 2. (2) In Eq 2, Z helmet is the acoustic impedance of the helmet and Z air is the acoustic impedance of the air. Acoustic impedance Z is calculated as the product of speed of sound in the material and density of the material. Ideal materials have a high local speed of sound and a high density. Steel has a greater acoustic impedance (~38⋅106 Pa⋅s/m3 for hardened manganese steel [46], used in WWI helmets) than composite fibers (~12⋅106 Pa⋅s/m3 for Kevlar® 129, used in ACH [47]), but since both impedances are orders of magnitude higher than air (~440 Pa⋅s/m3), reflection will be relatively similar (R = 0.999977 for steel and R = 0.999927 for Kevlar® 129). This explains the similar results for the ACH, Brodie helmet, and Stahlhelm. Many helmet and body armor materials have properties that are desirable for both ballistic and blast trauma. Because a shock wave reflection occurs at every interface where there is an acoustic impedance mismatch, primary blast protection can be improved by using multi-layered configurations of high and low acoustic impedance, with each layer reflecting a proportion of the penetrating wave. Not every layer of material will be beneficial to blast wave protection, and if a material has an acoustic impedance in between two neighboring materials, it will enhance blast wave penetration. The layered structure of the ACH might contribute to its blast protection, but future studies are needed to evaluate the effect of a layered structure. Helmet wall thickness improves ballistic protection by providing higher strength and energy absorption, but it doesn’t affect blast protection much since reflection only occurs at interfaces. While the Adrian helmet provided superior blast protection at the crown of the head for overhead blast in this study, Dean [1] noted that the ballistic protection it provided was less than both the Brodie helmet and Stahlhelm. One of the limitations of this study is that only an overhead blast scenario was examined. While this would be an accurate approximation of blasts in trench warfare as in WWI or air bombings of soldiers in the field during major unit action, it would not be as applicable to other cases such as improvised explosive devices (IEDs) used as roadside bombs, a significant cause of injury and death in conflicts in Iraq and Afghanistan [48]. The current study evaluated primary blast protection without considering reflective surfaces. In combat scenarios, reflection of a blast wave off of a surface can change outcomes considerably [49], such as when a soldier lies on the ground with the crown of the head towards the blast, or is confined within a trench. Another limitation is that the historical helmets tested are over one hundred years old, and their material properties might not be the same as they were originally. While properties of steel are relatively stable, the helmet linings may have degraded. However, there is no guarantee that replicas would be identical copies of the original either, so this study stays as true to the original helmets as possible. Finally, this study did not include the potential from blunt neurotrauma from impacts of the helmet on the head following acceleration from the transiting shock overpressure. This effect may be large with blasts that had larger positive phase duration and larger impulse than for the shells considered in this study.

Conclusions Interestingly, though primary blast protection was not a design objective, both historical and modern combat helmets provide primary blast protection. Tested modern helmets provide similar protection to historical ones in an overhead blast scenario. All tested helmets provided significant protection against primary blast brain injury compared to a bare head scenario. This protection substantially decreased the potential for primary blast neurotrauma from typical World War I artillery threat equivalents based on available injury criteria. While the helmets also provided protection against eardrum rupture based on current eardrum injury risk assessments, the resulting pressures were still injurious even with the helmets with extended brims. Major improvements made in helmet technology to increase ballistic protection do not provide the same increase in blast protection. At certain measurement locations, some historical helmets provided more blast attenuation than the modern helmet even though the modern helmets based on modern fiber composites are far more protective from typical ballistic threats. Specifically, the French 1915 Adrian helmet produced a lower peak pressure at the crown of the head compared to the Advanced Combat Helmet and the other historical helmets. These results show that there is considerable overlap in materials that have good qualities for ballistic and blast protection, but the protection mechanisms are different. Protection against primary blast focuses largely on impedance mismatches that reduce the amplitude of the transmitted waves to the head. The introduction of steel helmets during World War I reduced the toll of both blast and ballistics injuries at the front. In the future, helmet protection against primary blast might be improved by material choice, multiple material layers with different acoustic impedance, or the geometry of the helmet.

Acknowledgments The authors gratefully acknowledge the Department of Biomedical Engineering of the Pratt School of Engineering at Duke University for their support in this study.