Advanced Combat Helmet Research Articles

Experimental and computational investigations of the potential improvement in helmet blast-protection through the use of a polyurea-based external coating

Purpose – The design of the Advanced Combat Helmet (ACH) currently in use was optimized by its designers in order to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface/head collisions. Since traumatic brain injury experienced by a significant fraction of the soldiers returning from the recent conflicts is associated with their exposure to blast, the ACH should be redesigned in order to provide the necessary level of protection against blast loads. The paper aims to discuss this issue. Design/methodology/approach – In the present work, an augmentation of the ACH for improved blast protection is considered. This augmentation includes the use of a polyurea (a nano-segregated elastomeric copolymer) based ACH external coating. To demonstrate the efficacy of this approach, blast experiments are carried out on instrumented head-mannequins (without protection, protected using a standard ACH, and protected using an ACH augmented by a polyurea explosive-resistant coating (ERC)). These experimental efforts are complemented with the appropriate combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction finite-element analysis. Findings – The results obtained clearly demonstrated that the use of an ERC on an ACH affects (generally in a beneficial way) head-mannequin dynamic loading and kinematic response as quantified by the intracranial pressure, impulse, acceleration and jolt. Originality/value – To the authors’ knowledge, the present work is the first reported combined experimental/computational study of the blast-protection efficacy and the mild traumatic brain-injury mitigation potential of polyurea when used as an external coating on a helmet.

RETRACTED: Potential improvement in helmet blast-protection via the use of a polyurea external coating: Combined experimental/computational analyses

The Advanced Combat Helmet (ACH) currently in use is designed for maximum protection against ballistic impacts and hard-surface collisions. It has been well-established now that the ACH provides relatively little protection against blast, and that a significant fraction of the soldiers returning from the recent conflicts suffer from traumatic brain injury (TBI). In the present work, an augmentation of the ACH, which involves the use of a polyurea (a nano-segregated elastomeric copolymer)-based external coating for improved blast protection is considered. To test the utility of this approach, blast experiments are carried out on instrumented head-mannequins. Three configurations of the head-mannequins are investigated: (i) unprotected; (ii) protected using a standard ACH; and (iii) protected using an augmented ACH. To provide additional insight into the problem of blast-wave interaction with a head-mannequin, a series of complementary combined Eulerian/Lagrangian transient non-linear dynamics computational...

Behind helmet blunt trauma induced by ballistic impact: A computational model

Behind helmet blunt trauma (BHBT) has emerged as a serious injury type experienced by soldiers in battlefields. BHBT has been found to range from skin lacerations to brain damage and extensive skull fracture. It has been believed that such injuries are caused by forces transmitted from the helmet's back face deformation (BFD), which result in local deformations of the skull and translation or rotation of the head, leading to brain injuries. In this study, head injury risks resulting from the BFD of the Advanced Combat Helmet (ACH) under ballistic impact are evaluated using finite element simulations. The head model developed at KTH in Sweden is adopted, and a helmet shell model (including foam pads) is constructed. The examined mechanical parameters include the maximum von Mises stress in the skull, pressure (mean normal stress) and maximum principal strain in the brain tissue, contact force, and head acceleration. The influences of the foam pad hardness, stand-off distance, helmet shell thickness, and impact direction on head injury risks are studied. It is found that a softer foam pad offers a better protection, but the foam pad cannot be too soft. Also, it is shown that a slightly larger stand-off distance leads to a significant reduction in head injury. In addition, the simulation results reveal that an increase in the helmet thickness reduces the injury risk. It is further observed that a 45-degree oblique frontal impact results in a lower head injury risk than a 90-degree frontal impact. Moreover, for a helmet protected head under ballistic impact, it is seen that a high risk of skull fracture does not necessarily mean an equally high risk of injury to the brain tissue. The predictions from the current model of a helmeted head under ballistic impact agree with experimental findings independently obtained by others. The newly developed model provides a useful tool for studying injury mechanisms of BHBT and evaluating the existing standards for testing and designing combat helmets.

Modeling of Advanced Combat Helmet Under Ballistic Impact

The use of combat helmets has greatly reduced penetrating injuries and saved lives of many soldiers. However, behind helmet blunt trauma (BHBT) has emerged as a serious injury type experienced by soldiers in battlefields. BHBT results from nonpenetrating ballistic impacts and is often associated with helmet back face deformation (BFD). In the current study, a finite element-based computational model is developed for simulating the ballistic performance of the Advanced Combat Helmet (ACH), which is validated against the experimental data obtained at the Army Research Laboratory. Both the maximum value and time history of the BFD are considered, unlike existing studies focusing on the maximum BFD only. The simulation results show that the maximum BFD, the time history of the BFD, and the shape and size of the effective area of the helmet shell agree fairly well with the experimental findings. In addition, it is found that ballistic impacts on the helmet at different locations and in different directions result in different BFD values. The largest BFD value is obtained for a frontal impact, which is followed by that for a crown impact and then by that for a lateral impact. Also, the BFD value is seen to decrease as the oblique impact angle decreases. Furthermore, helmets of four different sizes—extra large, large, medium, and small—are simulated and compared. It is shown that at the same bullet impact velocity the small-size helmet has the largest BFD, which is followed by the medium-size helmet, then by the large-size helmet, and finally by the extra large-size helmet. Moreover, ballistic impact simulations are performed for an ACH placed on a ballistic dummy head form embedded with clay as specified in the current ACH testing standard by using the validated helmet model. It is observed that the BFD values as recorded by the clay in the head form are in good agreement with the experimental data.

Auditory localization performance in the azimuth for tactical communication and protection systems

An important consideration in assessing auditory abilities is the effect personal protective equipment (PPE) can have on performance. Tactical Communication and Protection (TCAP) systems are a type of PPE that is becoming more common in military applications. These types of systems provide radio communications, while also protecting the user from hazardous noise through passive and active attenuation. These systems have an active acoustic pass-through mechanism that utilizes an external microphone and internal earphone to restore environmental hearing. Past research has shown that this type of pass-through mechanism can negatively affect localization ability. In the present study, six participants localization ability was tested for two different signals (AK-47 impulse, 500 ms white noise) across an array of eight loudspeakers arranged in a ring around the test participant. The systems tested consisted of three types of in-ear TCAPs, one over-the-ear TCAPs, and one passive nonlinear hearing protection device. All systems were tested with the Advanced Combat Helmet (ACH) donned. In general, localization performance was worst for the AK-47 impulse signal. Across signals, the over-the-ear TCAPs had significantly more front/back confusions and greater localization error than the in-ear systems.

Examination of the protective roles of helmet/faceshield and directionality for human head under blast waves

A parametric study was conducted to delineate the efficacy of personal protective equipment (PPE), such as ballistic faceshields and advanced combat helmets, in the case of a blast. The propagations of blast waves and their interactions with an unprotected head, a helmeted one, and a fully protected finite element head model (FEHM) were modeled. The biomechanical parameters of the brain were recorded when the FEHM was exposed to shockwaves from the front, back, top, and bottom. The directional dependent tissue response of the brain and the variable efficiency of PPE with respect to the blast orientation were two major results of this study.

Sound localisation ability of soldiers wearing infantry ACH and PASGT helmets

Helmets provide soldiers with ballistic and fragmentation protection but impair auditory spatial processing. Missed auditory information can be fatal for a soldier; therefore, helmet design requires compromise between protection and optimal acoustics. Twelve soldiers localised two sound signals presented from six azimuth angles and three levels of elevation presented at two intensity levels and with three background noises. Each participant completed the task while wearing no helmet and with two U.S. Army infantry helmets – the Personnel Armor System for Ground Troops (PASGT) helmet and the Advanced Combat Helmet (ACH). Results showed a significant effect of helmet type on the size of both azimuth and elevation error. The effects of level, background noise, azimuth and elevation were found to be significant. There was no effect of sound signal type. As hypothesised, localisation accuracy was greatest when soldiers did not wear helmet, followed by the ACH. Performance was worst with the PASGT helmet.

Brain Response to Primary Blast Wave Using Validated Finite Element Models of Human Head and Advanced Combat Helmet

Blast-induced traumatic brain injury has emerged as a “signature injury” in combat casualty care. Present combat helmets are designed primarily to protect against ballistic and blunt impacts, but the current issue with helmets is protection concerning blasts. In order to delineate the blast wave attenuating capability of the Advanced Combat Helmet (ACH), a finite element (FE) study was undertaken to evaluate the head response against blast loadings with and without helmet using a partially validated FE model of the human head and ACH. Four levels of overpressures (0.27–0.66 MPa) from the Bowen’s lung iso-damage threshold curves were used to simulate blast insults. Effectiveness of the helmet with respect to head orientation was also investigated. The resulting biomechanical responses of the brain to blast threats were compared for human head with and without the helmet. For all Bowen’s cases, the peak intracranial pressures (ICP) in the head ranged from 0.68 to 1.8 MPa in the coup cortical region. ACH was found to mitigate ICP in the head by 10–35%. Helmeted head resulted in 30% lower average peak brain strains and product of strain and strain rate. Among three blast loading directions with ACH, highest reduction in peak ICP (44%) was due to backward blasts whereas the lowest reduction in peak ICP and brain strains was due to forward blast (27%). The biomechanical responses of a human head to primary blast insult exhibited directional sensitivity owing to the different geometry contours and coverage of the helmet construction and asymmetric anatomy of the head. Thus, direction-specific tolerances are needed in helmet design in order to offer omni-directional protection for the human head. The blasts of varying peak overpressures and durations that are believed to produce the same level of lung injury produce different levels of mechanical responses in the brain, and hence “iso-damage” curves for brain injury are likely different than the Bowen curves for lung injury.

Making Army Helmets Tougher and Safer with Realistic Simulation

Abstract Researchers at MIT have evaluated a wide range of helmet liner designs and materials for potential use in the U.S. Army's advanced combat helmet (ACH). To measure the effectiveness of candidate materials, the investigators rely on finite-element simulations that are validated by shock tube testing at Purdue University. Of the various filler materials examined, glass beads, glycerin, and water show the most promise for blast attenuation.

Performance of an advanced combat helmet with different interior cushioning systems in ballistic impact: Experiments and finite element simulations

This paper presents results from both experiments and numerical simulations of frontal and lateral ballistic impacts on a Hybrid III headform equipped with Advanced Combat Helmets (ACH) of two different interior cushioning designs, namely the strap-netting system and the Oregon Aero (OA) foam padding. It aims to study the differences between two different impact orientations for these two different interior cushioning systems. The experiments involve frontal and lateral ballistic impacts of a 11.9g spherical steel projectile traveling at speeds of approximately 200m/s, striking on the helmet-cushion-headform assemblies. The dynamic interaction between various components of the helmet-cushion-headform assemblies are investigated with the use of high-speed photography while post-test evaluation of the damaged helmets are performed using visual observation, optical microscopy and computed tomography (CT) scan. A series of ballistic impact simulations with finite element (FE) models of the two assemblies reconstructed from CT images, are performed to correlate with experimental results. The commercial software, Abaqus™, is used for the FE analyses. In general, there is reasonable correlation between numerical and experimental observations and on quantitative parameters, such as head accelerations, helmet damage and deflections. It is also found that softer foams with low stiffness are more effective as shock absorbing cushion against ballistic impacts under certain condition. Additionally, results of the two interior cushioning systems are compared with various injury criteria to assess their acceleration levels. It is found that, for frontal impact, the helmet with strap-netting system fails both the Wayne State Tolerance Curve (WSTC) and Federal Motor Vehicles Safety Standards (FMVSS) 218 criteria while the one with OA foam-padding passes both.

Potential Improvements in Shock-Mitigation Efficacy of a Polyurea-Augmented Advanced Combat Helmet

The design of the currently used Advanced Combat Helmet (ACH) has been optimized to attain maximum protection against ballistic impacts (fragments, shrapnel, etc.) and hard-surface collisions. However, the ability of the ACH to protect soldiers against blast loading appears not to be as effective. Polyurea, a micro-segregated elastomeric copolymer has shown superior shock-mitigation capabilities. In the present work, a combined Eulerian/Lagrangian transient non-linear dynamics computational fluid/solid interaction analysis is used to investigate potential shock-mitigation benefits which may result from different polyurea-based design augmentations of the ACH. Specific augmentations include replacement of the currently used suspension-pad material with polyurea and the introduction of a thin polyurea internal lining/external coating to the ACH shell. Effectiveness of different ACH designs was quantified by: (a) establishing the main forms of mild traumatic brain injury (mTBI); (b) identifying the key mechanical causes for these injuries; and (c) quantifying the extents of reductions in the magnitude of these mechanical causes. The results obtained show that while the ACH with a 2-mm-thick polyurea internal lining displays the best blast mitigation performance, it does not provide sufficient protection against mTBI.

Reply to Moss et al.: Military and medically relevant models of blast-induced traumatic brain injury vs. ellipsoidal heads and helmets

Moss et al. (1) acknowledge the second main conclusion of Nyein et al. (2): that a face shield may significantly mitigate blast-induced traumatic brain injury (TBI). However, they obviate the first and most important finding: that the advanced combat helmet (ACH) does not amplify the overpressure experienced by the head, as suggested by Moss et al. in the letter in ref. 3; therefore, it is safe for blast exposure. As has been shown repeatedly in theater, the ACH provides significant protection against shrapnel and ballistic threats.

In silico investigation of intracranial blast mitigation with relevance to military traumatic brain injury

Blast-induced traumatic brain injury is the most prevalent military injury in Iraq and Afghanistan, yet little is known about the mechanical effects of blasts on the human head, and still less is known about how personal protective equipment affects the brain's response to blasts. In this study we investigated the effect of the Advanced Combat Helmet (ACH) and a conceptual face shield on the propagation of stress waves within the brain tissue following blast events. We used a sophisticated computational framework for simulating coupled fluid-solid dynamic interactions and a three-dimensional biofidelic finite element model of the human head and intracranial contents combined with a detailed model of the ACH and a conceptual face shield. Simulations were conducted in which the unhelmeted head, head with helmet, and head with helmet and face shield were exposed to a frontal blast wave with incident overpressure of 10atm. Direct transmission of stress waves into the intracranial cavity was observed in the unprotected head and head with helmet simulations. Compared to the unhelmeted head, the head with helmet experienced slight mitigation of intracranial stresses. This suggests that the existing ACH does not significantly contribute to mitigating blast effects, but does not worsen them either. By contrast, the helmet and face shield combination impeded direct transmission of stress waves to the face, resulting in a delay in the transmission of stresses to the intracranial cavity and lower intracranial stresses. This suggests a possible strategy for mitigating blast waves often associated with military concussion.

Fluid/Structure Interaction Computational Investigation of Blast-Wave Mitigation Efficacy of the Advanced Combat Helmet

To combat the problem of traumatic brain injury (TBI), a signature injury of the current military conflicts, there is an urgent need to design head protection systems with superior blast/ballistic impact mitigation capabilities. Toward that end, the blast impact mitigation performance of an advanced combat helmet (ACH) head protection system equipped with polyurea suspension pads and subjected to two different blast peak pressure loadings has been investigated computationally. A fairly detailed (Lagrangian) finite-element model of a helmet/skull/brain assembly is first constructed and placed into an Eulerian air domain through which a single planar blast wave propagates. A combined Eulerian/Lagrangian transient nonlinear dynamics computational fluid/solid interaction analysis is next conducted in order to assess the extent of reduction in intra-cranial shock-wave ingress (responsible for TBI). This was done by comparing temporal evolutions of intra-cranial normal and shear stresses for the cases of an unprotected head and the helmet-protected head and by correlating these quantities with the three most common types of mild traumatic brain injury (mTBI), i.e., axonal damage, contusion, and subdural hemorrhage. The results obtained show that the ACH provides some level of protection against all investigated types of mTBI and that the level of protection increases somewhat with an increase in blast peak pressure. In order to rationalize the aforementioned findings, a shockwave propagation/reflection analysis is carried out for the unprotected head and helmet-protected head cases. The analysis qualitatively corroborated the results pertaining to the blast-mitigation efficacy of an ACH, but also suggested that there are additional shockwave energy dissipation phenomena which play an important role in the mechanical response of the unprotected/protected head to blast impact.

Traumatic Brain Injury Risk While Parachuting: Comparison of the Personnel Armor System for Ground Troops Helmet and the Advanced Combat Helmet

Military paratroopers are inherently at risk for a variety of injuries when they jump, including traumatic brain injuries (TBIs). U.S. Army paratroopers rely on their ballistic helmets for protection against TBIs when jumping. Currently, two different helmets are available to Army paratroopers, that is, the personnel armor system for ground troops helmet and the advanced combat helmet. This study compared the incidence of self-reported, jump-related TBIs in a small sample of paratroopers (N = 585) using each type of helmet. Data were obtained from surveys of soldiers at Fort Bragg, North Carolina. The overall relative risk of sustaining a TBI while jumping was 2.3 times (95% confidence interval, 1.3-4.3) higher for personnel armor system for ground troops helmet users. Most of the increase in risk was accounted for by the most-minor TBIs (American Academy of Neurology grade 1 or 2 concussion).

How Satisfied Are Soldiers with Their Ballistic Helmets? A Comparison of Soldiers' Opinions about the Advanced Combat Helmet and the Personal Armor System for Ground Troops Helmet

Many factors are considered during ballistic helmet design, including comfort, weight, fit, and maintainability. These factors affect soldiers' decisions about helmet use; therefore, rigorous research about soldiers' real-life experiences with helmets is critical to assessing a helmet's overall protective efficacy. This study compared soldiers' satisfaction and problem experience with the advanced combat helmet (ACH) and the personal armor system for ground troops (PASGT) helmet. Data were obtained from surveys of soldiers at Fort Bragg, North Carolina. Ninety percent of ACH users were satisfied overall with their helmet, but only 9.5% of PASGT users were satisfied (p < 0.001). The most frequently reported problems for the ACH involved malfunctioning helmet parts. The most frequently reported problems for the PASGT involved discomfort. This analysis indicated that there was a strong soldier preference for the ACH over the PASGT, which could enhance its already superior protective qualities. It also demonstrated the usefulness of soldiers' assessments of protective equipment.

The impact of helmet design on sound detection and localization

The shape and absorptiveness of a ballistic helmet changes the sound signal reaching the soldier’s ear. Although the attenuation of current helmets is minimal, soldiers have reported removing their Personnel Armor System for Ground Troops (PASGT) helmet when it was necessary to determine sound source direction. The Advanced Combat Helmet (ACH) has reduced ear coverage and absorptive internal padding that decreases localization errors. In an effort to provide more integrated headgear, the U.S. Army is testing a new, Scorpion (R4) helmet that has rings that allow the insertion of earmuffs and a modified version without the rings (R4-R). Two studies were conducted to measure the effect of these four designs on sound detection and localization. None of the helmets differed significantly in the degree of attenuation nor did attenuation correlate with localization. However, localization performance varied greatly. The current ACH and the R4-R had the least impact on localization. The older, unpadded PASGT, caused high error rates despite having the lowest attenuation. Despite being otherwise similar to the R4-R, the R4 impaired localization just as much as the PASGT and its rings increased the attenuation by more than a decibel.