Role of helmet in the mechanics of shock wave propagation under blast loading conditions

Experimental investigation of a viscoelastic liner to reduce under helmet overpressures and shock wave reflections

Introduction

Shock wave overpressure exposures can result in blast-induced traumatic brain injury (bTBI) in warfighters. Although combat helmets provide protection against blunt impacts, the protection against blast waves is limited due to the observed high overpressures occurring underneath the helmet. One route to enhance these helmets is by incorporating viscoelastic materials into the helmet designs, reducing pressures imposed on the head. This study aims to further investigate this mitigation technique against under-helmet overpressures by adding a viscoelastic liner to the inside of a combat helmet.

Methods

The liner's effectiveness was evaluated by exposing it to free-field blasts of Composition C-4 at overpressures ranging from 27.5 to 165 kPa (4 - 24 psi) and comparing shock waveform parameters to an unlined helmet. Blasts were conducted using an instrumented manikin equipped with and without a helmet and then with a helmet modified to incorporate a viscoelastic liner. Evaluation of blast exposure results focused on the waveform parameters of peak pressure, impulse and positive phase duration.

Results

The results show that peak overpressure was higher when wearing a helmet compared to not wearing a helmet. However, the helmet with the viscoelastic liner reduced the average peak overpressures compared to the helmet alone. For the lowest overpressure tested, 27.5 kPa, the helmet liner decreased the overpressure on the top of the head by 37.6%, with reduction reaching 26% at the highest overpressure exposure of 165 kPa. Additionally, the inclusion of the viscoelastic material extended the shock waveforms' duration, reducing the rate the shock wave was applied to the head. The results of this study show the role a helmet and helmet design play in the level of blast exposure imposed on a wearer. The testing and evaluation of these materials hold promise for enhancing helmet design to better protect against bTBI.

An End-User Evaluation of Blast Overpressure and Accelerative Impact Body-Worn Sensors

INTRODUCTION

Blast overpressure and accelerative impact can produce concussive-like symptoms in service members serving both garrison and deployed environments. In an effort to measure, document, and improve the response to these overpressure and impact events, the U.S. Army Medical Material Development Activity is evaluating body-worn sensors for use by the Joint Conventional Force. In support, the WRAIR completed a qualitative end-user evaluation with service members from high-risk mission occupational specialties to determine the potential needs, benefits, and challenges associated with adopting body-worn sensors into their job duties.

MATERIALS AND METHODS

WRAIR staff led hour-long semi-structured focus groups with 156 Army, Navy, and Marine Corps participants, primarily representing infantry, combat engineer, explosive ordnance disposal, artillery, mortar, and armor job specialties. Topics included their sensor needs, concepts of operations, and recommended design features for implementing sensors into the force. Dialogue from each focus group was audio recorded and resulting transcripts were coded for thematic qualitative analysis using NVivo software.

RESULTS

Users recommended a single, unobtrusive, rugged, multi-directional sensor that could be securely mounted to the helmet and powered by a battery type (such as rechargeable lithium or disposable alkaline batteries) that was best suited for their garrison and field/deployed environments. The sensors should accurately measure low-level (∼1.0 pounds per square inch) blasts and maintain a record of cumulative exposures for each service member. Discussions supported the need for immediate, actionable feedback from the sensor with the option to view detailed blast or impact data on a computer. There were, however, divergent opinions on security issues regarding wireless versus wired data transfer methods. Participants also expressed a need for the exposure data to integrate with their medical records and were also willing to have their data shared with leadership, although opinions differed on the level of echelon and if the data should be identifiable. Regarding accountability, users did not want to be held fiscally liable for the sensors and recommended having the unit be responsible for maintenance and distribution. Concerns about being held fiscally liable, being overly burdened, and having one's career negatively impacted were listed as factors that could decrease usage. Finally, participants highlighted the importance of understanding the purpose and function of the sensors and supported a corresponding training module.

CONCLUSIONS

Participating service members were generally willing to adopt body-worn sensors into their garrison and deployed activities. To maximize adoption of the devices, they should be convenient to use and should not interfere with service members' job tasks. Providing a clear understanding of the benefits (such as incorporating exposure data into medical records) and the function of sensors will be critical for encouraging buy-in among users and leaders. Incorporating end-user requirements and considering the benefits and challenges highlighted by end users are important for the design and implementation of body-worn sensors to mitigate the risks of blast overpressure and accelerative impact on service members' health.

In Silico Investigation of Biomechanical Response of a Human Brain Subjected to Primary Blast

The brain response to the explosion-induced primary blast waves is actively sought. Over the past decade, reasonable progress has been made in the fundamental understanding of blast traumatic brain injury (bTBI) using head surrogates and animal models. Yet, the current understanding of how blast waves interact with human is in nascent stages, primarily due to the lack of data in human. The biomechanical response in human is critically required to faithfully establish the connection to the aforementioned bTBI models. In this work, the biomechanical cascade of the brain under a primary blast has been elucidated using a detailed, full-body human model. The full-body model allowed us to holistically probe short- (<5 ms) and long-term (200 ms) brain responses. The full-body model has been extensively validated against impact loading in the past. We have further validated the head model against blast loading. We have also incorporated the structural anisotropy of the brain white matter. The blast wave transmission, and linear and rotational motion of the head were dominant pathways for the loading of the brain, and these loading paradigms generated distinct biomechanical fields within the brain. Blast transmission and linear motion of the head governed the volumetric response, whereas the rotational motion of the head governed the deviatoric response. Blast induced head rotation alone produced diffuse injury pattern in white matter fiber tracts. The biomechanical response under blast was comparable to the impact event. These insights will augment laboratory and clinical investigations of bTBI and help devise better blast mitigation strategies.

Effect of blast orientation, multi-point blasts, and repetitive blasts on brain injury

Explosions in the battlefield can result in brain damage. Research on the effects of shock waves on brain tissue mainly focuses on the effects of single-orientation blast waves, while there have been few studies on the dynamic response of the human brain to directional explosions in different planes, multi-point explosions and repetitive explosions. Therefore, the brain tissue response and the intracranial pressure (ICP) caused by different blast loadings were numerically simulated using the CONWEP method. In the study of the blast in different directions, the lateral explosion blast wave was found to cause greater ICP than did blasts from other directions. When multi-point explosions occurred in the sagittal plane simultaneously, the ICP in the temporal lobe increased by 37.8 % and the ICP in the parietal lobe decreased by 17.6 %. When multi-point explosions occurred in the horizontal plane, the ICP in the frontal lobe increased by 61.8 % and the ICP in the temporal lobe increased by 12.2 %. In a study of repetitive explosions, the maximum ICP of the second blast increased by 40.6 % over that of the first blast, and that of the third blast increased by 61.2 % over that of the second blast. The ICP on the brain tissue from repetitive blasts can exceed 200 % of that of a single explosion blast wave.

Twenty Years of Blast-Induced Neurotrauma: Current State of Knowledge

Blast-induced neurotrauma (BINT) is an important injury paradigm of neurotrauma research. This short communication summarizes the current knowledge of BINT. We divide the BINT research into several broad categories-blast wave generation in laboratory, biomechanics, pathology, behavioral outcomes, repetitive blast in animal models, and clinical and neuroimaging investigations in humans. Publications from 2000 to 2023 in each subdomain were considered. The analysis of the literature has brought out salient aspects. Primary blast waves can be simulated reasonably in a laboratory using carefully designed shock tubes. Various biomechanics-based theories of BINT have been proposed; each of these theories may contribute to BINT by generating a unique biomechanical signature. The injury thresholds for BINT are in the nascent stages. Thresholds for rodents are reasonably established, but such thresholds (guided by primary blast data) are unavailable in humans. Single blast exposure animal studies suggest dose-dependent neuronal pathologies predominantly initiated by blood-brain barrier permeability and oxidative stress. The pathologies were typically reversible, with dose-dependent recovery times. Behavioral changes in animals include anxiety, auditory and recognition memory deficits, and fear conditioning. The repetitive blast exposure manifests similar pathologies in animals, however, at lower blast overpressures. White matter irregularities and cortical volume and thickness alterations have been observed in neuroimaging investigations of military personnel exposed to blast. Behavioral changes in human cohorts include sleep disorders, poor motor skills, cognitive dysfunction, depression, and anxiety. Overall, this article provides a concise synopsis of current understanding, consensus, controversies, and potential future directions.

Increased Carbon Dioxide Respiration Prevents the Effects of Acceleration/Deceleration Elicited Mild Traumatic Brain Injury

Acceleration/deceleration forces are a common component of various causes of mild traumatic brain injury (mTBI) and result in strain and shear forces on brain tissue. A small quantifiable volume dubbed the compensatory reserve volume (CRV) permits energy transmission to brain tissue during acceleration/deceleration events. The CRV is principally regulated by cerebral blood flow (CBF) and CBF is primarily determined by the concentration of inspired carbon dioxide (CO₂). We hypothesized that experimental hypercapnia (i.e. increased inspired concentration of CO₂) may act to prevent and mitigate the actions of acceleration/deceleration-induced TBI. To determine these effects C57Bl/6 mice underwent experimental hypercapnia whereby they were exposed to medical-grade atmospheric air or 5% CO₂ immediately prior to an acceleration/deceleration-induced mTBI paradigm. mTBI results in significant increases in righting reflex time (RRT), reductions in core body temperature, and reductions in general locomotor activity-three hours post injury (hpi). Experimental hypercapnia immediately preceding mTBI was found to prevent mTBI-induced increases in RRT and reductions in core body temperature and general locomotor activity. Ribonucleic acid (RNA) sequencing conducted four hpi revealed that CO₂ exposure prevented mTBI-induced transcriptional alterations of several targets related to oxidative stress, immune, and inflammatory signaling. Quantitative real-time PCR analysis confirmed the prevention of mTBI-induced increases in mitogen-activated protein kinase kinase kinase 6 and metallothionein-2. These initial proof of concept studies reveal that increases in inspired CO₂ mitigate the detrimental contributions of acceleration/deceleration events in mTBI and may feasibly be translated in the future to humans using a medical device seeking to prevent mTBI among high-risk groups.

Revealing the Effect of Skull Deformation on Intracranial Pressure Variation During the Direct Interaction Between Blast Wave and Surrogate Head

Intracranial pressure (ICP) during the interaction between blast wave and the head is a crucial evaluation criterion for blast-induced traumatic brain injury (bTBI). ICP variation is mainly induced by the blast wave transmission and skull deformation. However, how the skull deformation influences the ICP remains unclear, which is meaningful for mitigating bTBI. In this study, both experimental and numerical models are developed to elucidate the effect of skull deformation on ICP variation. Firstly, we performed the shock tube experiment of the high-fidelity surrogate head to measure the ICP, the blast overpressure, and the skull surface strain of specific positions. The results show that the ICP profiles of all measured points show oscillations with positive and negative change, and the variation is consistent with the skull surface strain. Further numerical analysis reveals that when the blast wave reaches the measured point, the peak overpressure transmits directly through the skull to the brain, forming the local positive ICP peak, and the impulse induces the local inward deformation of the skull. As the peak overpressure passes through, the blast impulse impacts the nearby skull supported by the soft and incompressible brain tissue and extrudes the skull outward in the initial position. The inward and outward skull deformation leads to the oscillation of ICP. These numerical analyses agree with experimental results, which explain the appearance of negative and positive ICP peaks and the synchronization of negative ICP with surface strain. The study has implications for medical injury diagnosis and protective equipment design.

Evaluation of Blast Simulation Methods for Modeling Blast Wave Interaction with Human Head

Blast induced traumatic brain injury (bTBI) research is crucial in asymmetric warfare. The finite element analysis is an attractive option to simulate the blast wave interaction with the head. The popular blast simulation methods are ConWep based pure Lagrangian, Arbitrary-Lagrangian-Eulerian, and Coupling method. This study examines the accuracy and efficiency of ConWep and Coupling methods in predicting the biomechanical response of the head. The simplified cylindrical, spherical surrogates and biofidelic human head models are subjected to field-relevant blast loads using these methods. The reflected overpressures at the surface and pressures inside the brain from the head models are qualitatively and quantitatively evaluated against the available experiments. Both methods capture the overall trends of experiments. Our results suggest that the accuracy of the ConWep method is mainly governed by the radius of curvature of the surrogate head. For the relatively smaller radius of curvature, such as cylindrical or spherical head surrogate, ConWep does not accurately capture decay of reflected blast overpressures and brain pressures. For the larger radius of curvature, such as the biofidelic human head, the predictions from ConWep match reasonably well with the experiment. For all the head surrogates considered, the reflected overpressure-time histories predicted by the Coupling method match reasonably well with the experiment. Coupling method uniquely captures the shadowing and union of shock waves governed by the geometry driven flow dynamics around the head. Overall, these findings will assist the bTBI modeling community to judiciously select an objective-driven modeling methodology.

Biomechanical Analysis of Head Subjected to Blast Waves and the Role of Combat Protective Headgear Under Blast Loading: A Review

Blast-induced traumatic brain injury (bTBI) is a rising health concern of soldiers deployed in modern-day military conflicts. For bTBI, blast wave loading is a cause, and damage incurred to brain tissue is the effect. There are several proposed mechanisms for the bTBI, such as direct cranial entry, skull flexure, thoracic compression, blast-induced acceleration, and cavitation that are not mutually exclusive. So the cause-effect relationship is not straightforward. The efficiency of protective headgears against blast waves is relatively unknown as compared with other threats. Proper knowledge about standard problem space, underlying mechanisms, blast reconstruction techniques, and biomechanical models are essential for protective headgear design and evaluation. Various researchers from cross disciplines analyze bTBI from different perspectives. From the biomedical perspective, the physiological response, neuropathology, injury scales, and even the molecular level and cellular level changes incurred during injury are essential. From a combat protective gear designer perspective, the spatial and temporal variation of mechanical correlates of brain injury such as surface overpressure, acceleration, tissue-level stresses, and strains are essential. This paper outlines the key inferences from bTBI studies that are essential in the protective headgear design context.

Epidemiology of Blast Neurotrauma: A Meta-analysis of Blast Injury Patterns in the Military and Civilian Populations

BACKGROUND

Mass casualty incidents (MCIs) due to bombing-related terrorism remain an omnipresent threat to our global society. The aim of this study was to elucidate differences in blast injury patterns between military and civilian victims affected by terrorist bombings.

METHODS

An analysis of the Global Terrorism Database (GTD) and a PubMed literature search of casualty reports of bombing attacks from 2010-2020 was performed (main key words: blast injuries/therapy, terrorism, military personnel) with key epidemiological and injury pattern data extracted and statistically analyzed.

RESULTS

Demographic analysis of casualties revealed that military casualties tend to be younger and predominantly male (P < 0.05) compared with civilians. Military casualties also reported higher amounts of head/neck injury (P < 0.01) compared with civilians. The proportion of instantaneous fatalities along with injuries affecting the thoracoabdominal and extremity regions remained approximately equal across both groups.

CONCLUSIONS

Though the increased number of head/neck injuries was unexpected, we also found that the number of nonlethal head injuries also increased, predicating that more military blast neurotrauma patients survived their injuries. These data can be used to increase blast MCI preparation and education throughout the international neurosurgical community.

Numerical Analysis of EOD Helmet under Blast Load Events Using Human Head Model

Brain injury resulting from improved explosives devices (IEDs) is identified as a challenge for force securities to improve protection equipment. This paper focuses on the mechanical response of explosive ordnance disposal (EOD) helmet under different blast loadings. Limited published studies on this type of helmet are available in the scientific literature. The results obtained show the blast performance of the EOD helmet because a decrease in the maximum values in the measured damage parameters is found. Therefore, an EOD helmet minimizes the risks of the severity of injuries on the user showing a low probability of injury.

Simulation of the Strain Amplification in Sulci Due to Blunt Impact to the Head

Traumatic brain injury (TBI) has become a concern in sports, automobile accidents and combat operations. A better understanding of the mechanics leading to a TBI is required to cope with both the short-term life-threatening effects and long-term effects of TBIs, such as the development chronic traumatic encephalopathy (CTE). Kornguth et al. (1) proposed that an inflammatory and autoimmune process initiated by a water hammer effect at the bases of the sulci of the brain is a mechanism of TBI leading to CTE. A major objective of this study is to investigate whether the water hammer effect is present due to blunt impacts through the use of computational models. Frontal blunt impacts were simulated with 2D finite element models developed to capture the biofidelic geometry of a human head. The models utilized the Arbitrary Lagrangian Eulerian (ALE) method to model a layer of cerebrospinal fluid (CSF) as a deforming fluid allowing for CSF to move in and out of sulci. During the simulated impacts, CSF was not observed to be driven into the sulci during the transient response. However, elevated shear strain levels near the base of the sulci were exhibited. Further, increased shear strain was present when differentiation between white and gray matter was taken into account. Both of the results support clinical observations of (1).

Assessment of the shock adsorption properties of bike helmets: a numerical/experimental approach

In this paper, a numerical and experimental study of the shock absorption properties of bike helmets is presented. Laboratory compression and tensile tests were carried out on samples of expanded polystyrene (EPS) and polycarbonate (PC), respectively constituting the internal shock absorption layer and the external hard shell of composite helmets. The measured responses of the two materials were then exploited to calibrate the relevant elasto-plastic constitutive models, adopted in full-scale finite element analyses of a helmet subject to standardized impacts. The simulations allowed assessing the time evolution of the acceleration measured inside the headform (according e.g., to EN 1078) and the failure mechanisms of the helmet, if any, as induced by the localization of plastic deformations.

Primary blast wave protection in combat helmet design: A historical comparison between present day and World War I

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.

Development of Phantom Material that Resembles Compression Properties of Human Brain Tissue for Training Models

There is a need to quantify and reproduce the mechanical behavior of brain tissue for a variety of applications from designing proper training models for surgeons to enabling research on the effectiveness of personal protective gear, such as football helmets. The mechanical response of several candidate phantom materials, including hydrogels and emulsions, was characterized and compared to porcine brain tissue under similar strains and strain rates. Some candidate materials were selected since their compositions were similar to brain tissue, such as emulsions that mimic the high content of lipids. Others, like silicone, were included since these are currently used as phantom materials. The mechanical response of the emulsion was closer to that of the native porcine brain tissue than the other candidates. The emulsions, created by addition of oil to a hydrogel, were able to withstand compressive strain greater than 40%. The addition of lipids in the emulsions also prevented the syneresis typically seen with hydrogel materials. This allowed the emulsion material to undergo freeze-thaw cycles with no significant change in their mechanical properties.

A Comprehensive Review of Experimental Rodent Models of Repeated Blast TBI

We reviewed the relevant literature delineating advances in the development of the experimental models of repeated blast TBI (rbTBI). It appears this subject is a relatively unexplored area considering the first work published in 2007 and the bulk of peer-reviewed papers was published post-2011. There are merely 34 papers published to date utilizing rodent rbTBI models. We performed an analysis and extracted basic parameters to capture the characteristics of the exposure conditions (the blast intensity, inter-exposure interval and the number of exposures), the age and weight of the animal models most commonly used in the studies, and their endpoints. Our analysis revealed three strains of rodents are predominantly used: Sprague Dawley and Long Evans rats and wild type (C57BL/6J) mice, and young adult animals 8 to 12-week-old are a preferred choice. Typical exposure conditions are the following: (1) peak overpressure in the 27–145 kPa (4–21 psi) range, (2) number of exposures: 2 (13.9%), 3 (63.9%), 5 (16.7%), or 12 (5.6%) with a single exposure used for a baseline comparison in 41.24% of the studies. Two inter-exposure interval durations were used: (1) short (1–30 min.) and (2) extended (24 h) between consecutive shock wave exposures. The experiments included characterization of repeated blast exposure effects on auditory, ocular and neurological function, with a focus on brain etiology in most of the published work. We present an overview of major histopathological findings, which are supplemented by studies implementing MRI (DTI) and behavioral changes after rbTBI in the acute (1–7 days post-injury), subacute (7–14 days), and chronic (>14 days) phases post-injury.

Cortical thinning in military blast compared to non-blast persistent mild traumatic brain injuries

In the military, explosive blasts are a significant cause of mild traumatic brain injuries (mTBIs). The symptoms associated with blast mTBIs causes significant economic burdens and a diminished quality of life for many service members. At present, the distinction of the injury mechanism (blast versus non-blast) may not influence TBI diagnosis. However, using noninvasive imaging, this study reveals significant distinctions between the blast and non-blast TBI mechanisms. A cortical whole-brain thickness analysis was performed using structural high-resolution T1-weighted MRI to identify the effects of blasts in persistent mTBI (pmTBI) subjects. A total of 41 blast pmTBI subjects were individually age- and gender-matched to 41 non-blast pmTBI subjects. Using FreeSurfer, cortical thickness was quantified for the blast group, relative to the non-blast group. Cortical thinning was identified within the blast mTBI group, in two clusters bilaterally. In the left hemisphere, the cluster overlapped with the lateral orbitofrontal, rostral middle frontal, medial orbitofrontal, superior frontal, rostral anterior cingulate and frontal pole cortices (p < 0.02, two-tailed, size = 1680 mm²). In the right hemisphere, the cluster overlapped with the lateral orbitofrontal, rostral middle frontal, medial orbitofrontal, pars orbitalis, pars triangularis and insula cortices (p < 0.002, two-tailed, cluster size = 2453 mm²). Self-report assessments suggest significant differences in the Post-Traumatic Stress Disorder Checklist-Civilian Version (p < 0.05, Bonferroni-corrected) and the Neurobehavioral Symptom Inventory (p < 0.01, uncorrected) between the blast and non-blast mTBI groups. These results suggest that blast may cause a unique injury pattern related to a reduction in cortical thickness within specific brain regions which could affect symptoms. No other study has found cortical thickness difference between blast and non-blast mTBI groups and further replication is needed to confirm these initial observations.

Effect of human head morphological variability on the mechanical response of blast overpressure loading

A methodology is introduced to investigate the effect of intersubject head morphological variability on the mechanical response of the brain when subjected to blast overpressure loading. Nonrigid image registration techniques are leveraged to warp a manually segmented template model to an arbitrary number of subjects following a procedure to coarsely segment the subjects in batch. Finite element meshes are autogenerated, and blast analysis is conducted. The template model is initially constructed to enable the full automated implementation and application of the proposed methodology. The application of the proposed approach for an anterior-oriented blast has been demonstrated, and the results reveal that the pressure response in the brain does exhibit some dependence on head morphological variability. While the magnitude of the peak pressure response can vary by more than 30%, its location within the brain is unaffected by head morphological variability. A linear least squares analysis was conducted to demonstrate that the peak magnitude of pressure is uncorrelated with head volume while it is correlated with aspect ratio relating to the amount of exposed surface area to the blast. These features of the pressure response are likely due to the peak pressure occurring during the early stages of stress wave transmission and reflection. As a result, the pressure response due to blast overpressure loading is predominantly loading dependent while morphological variability has a secondary effect.

Human Skin-Like Composite Materials for Blast Induced Injury Mitigation

Armors and military grade personal protection equipment (PPE) materials to date are bulky and are not designed to effectively mitigate blast impacts. In the current work, a human skin-like castable simulant material was developed and its blast mitigation characteristics (in terms of induced stress reduction at the bone and muscles) were characterized in the presence of composite reinforcements. The reinforcement employed was Kevlar 129 (commonly used in advanced combat helmets), which was embedded within the novel skin simulant material as the matrix and used to cover a representative extremity based human skin, muscle and bone section finite element (FE) model. The composite variations tested were continuous and short-fiber types, lay-ups (0/0, 90/0, and 45/45 orientations) and different fiber volume fractions. From the analyses, the 0/0 continuous fiber lay-up with a fiber volume fraction close to 0.1 (or 10%) was found to reduce the blast-induced dynamic stresses at the bone and muscle sections by 78% and 70% respectively. These findings indicate that this novel skin simulant material with Kevlar 129 reinforcement, with further experimental testing, may present future opportunities in blast resistant armor padding designing.

Assessment of the Effectiveness of Combat Eyewear Protection Against Blast Overpressure

It is unclear whether combat eyewear used by U. S. Service members is protective against blast overpressures (BOPs) caused by explosive devices. Here, we investigated the mechanisms by which BOP bypasses eyewear and increases eye surface pressure. We performed experiments and developed three-dimensional (3D) finite element (FE) models of a head form (HF) equipped with an advanced combat helmet (ACH) and with no eyewear, spectacles, or goggles in a shock tube at three BOPs and five head orientations relative to the blast wave. Overall, we observed good agreement between experimental and computational results, with average discrepancies in impulse and peak-pressure values of less than 15% over 90 comparisons. In the absence of eyewear and depending on the head orientation, we identified three mechanisms that contributed to pressure loading on the eyes. Eyewear was most effective at 0 deg orientation, with pressure attenuation ranging from 50 (spectacles) to 80% (goggles) of the peak pressures observed in the no-eyewear configuration. Spectacles and goggles were considerably less effective when we rotated the HF in the counter-clockwise direction around the superior-inferior axis of the head. Surprisingly, at certain orientations, spectacles yielded higher maximum pressures (80%) and goggles yielded larger impulses (150%) than those observed without eyewear. The findings from this study will aid in the design of eyewear that provides better protection against BOP.

Effective testing of personal protective equipment in blast loading conditions in shock tube: Comparison of three different testing locations

We exposed a headform instrumented with 10 pressure sensors mounted flush with the surface to a shock wave with three nominal intensities: 70, 140 and 210 kPa. The headform was mounted on a Hybrid III neck, in a rigid configuration to eliminate motion and associated pressure variations. We evaluated the effect of the test location by placing the headform inside, at the end and outside of the shock tube. The shock wave intensity gradually decreases the further it travels in the shock tube and the end effect degrades shock wave characteristics, which makes comparison of the results obtained at three locations a difficult task. To resolve these issues, we developed a simple strategy of data reduction: the respective pressure parameters recorded by headform sensors were divided by their equivalents associated with the incident shock wave. As a result, we obtained a comprehensive set of non-dimensional parameters. These non-dimensional parameters (or amplification factors) allow for direct comparison of pressure waveform characteristic parameters generated by a range of incident shock waves differing in intensity and for the headform located in different locations. Using this approach, we found a correlation function which allows prediction of the peak pressure on the headform that depends only on the peak pressure of the incident shock wave (for specific sensor location on the headform), and itis independent on the headform location. We also found a similar relationship for the rise time. However, for the duration and impulse, comparable correlation functions do not exist. These findings using a headform with simplified geometry are baseline values and address a need for the development of standardized parameters for the evaluation of personal protective equipment (PPE) under shock wave loading.

Computational modeling of blast induced whole-body injury: a review

Blast injuries affect millions of lives across the globe due to its traumatic after effects on the brain and the whole body. To date, military grade armour materials are designed to mitigate ballistic and shrapnel attacks but are less effective in resisting blast impacts. In order to improve blast absorption characteristics of armours, the first key step is thoroughly understands the effects of blasts on the human body itself. In the last decade, a plethora of experimental and computational work has been carried out to investigate the mechanics and pathophysiology of Traumatic Brain Injury (TBI). However, very few attempts have been made so far to study the effect of blasts on the various other parts of the body such as the sensory organs (eyes and ears), nervous system, thorax, extremities, internal organs (such as the lungs) and the skeletal system. While an experimental evaluation of blast effects on such physiological systems is difficult, developing finite element (FE) models could allow the recreation of realistic blast scenarios on full scale human models and simulate the effects. The current article reviews the state-of-the-art in computational research in blast induced whole-body injury modelling, which would not only help in identifying the areas in which further research is required, but would also be indispensable for understanding body location specific armour design criteria for improved blast injury mitigation.

Effect of cerebrospinal fluid modeling on spherically convergent shear waves during blunt head trauma

The MRI-based computational model, previously validated by tagged MRI and harmonic phase imaging analysis technique on in vivo human brain deformation, is used to study transient wave dynamics during blunt head trauma. Three different constitutive models are used for the cerebrospinal fluid: incompressible solid elastic, viscoelastic, and fluid-like elastic using an equation of state model. Three impact cases are simulated, which indicate that the blunt impacts give rise not only to a fast pressure wave but also to a slow, and potentially much more damaging, shear (distortional) wave that converges spherically towards the brain center. The wave amplification due to spherical geometry is balanced by damping due to tissues' viscoelasticity and the heterogeneous brain structure, suggesting a stochastic competition of these 2 opposite effects. It is observed that this convergent shear wave is dependent on the constitutive property of the cerebrospinal fluid, whereas the peak pressure is not as significantly affected.

Face shield design against blast-induced head injuries

Blast-induced traumatic brain injury has been on the rise in recent years because of the increasing use of improvised explosive devices in conflict zones. Our study investigates the response of a helmeted human head subjected to a blast of 1 atm peak overpressure, for cases with and without a standard polycarbonate (PC) face shield and for face shields comprising of composite PC and aerogel materials and with lateral edge extension. The novel introduction of aerogel into the laminate face shield is explored and its wave-structure interaction mechanics and performance in blast mitigation is analysed. Our numerical results show that the face shield prevented direct exposure of the blast wave to the face and help delays the transmission of the blast to reduce the intracranial pressures (ICPs) at the parietal lobe. However, the blast wave can diffract and enter the midface region at the bottom and side edges of the face shield, resulting in traumatic brain injury. This suggests that the bottom and sides of the face shield are important regions to focus on to reduce wave ingress. The laminated PC/aerogel/PC face shield yielded higher peak positive and negative ICPs at the frontal lobe, than the original PC one. For the occipital and temporal brain regions, the laminated face shield performed better than the original. The composite face shield with extended edges reduced ICP at the temporal lobe but increases ICP significantly at the parietal lobe, which suggests that a greater coverage may not lead to better mitigating effects.

Primary blast waves induced brain dynamics influenced by head orientations

There is controversy regarding the directional dependence of head responses subjected to blast loading. The goal of this work is to characterize the role of head orientation in the mechanics of blast wave-head interactions as well as the load transmitting to the brain. A three-dimensional human head model with anatomical details was reconstructed from computed tomography images. Three different head orientations with respect to the oncoming blast wave, i.e., front-on with head facing blast, back-on with head facing away from blast, and side-on with right side exposed to blast, were considered. The reflected pressure at the blast wave-head interface positively correlated with the skull curvature. It is evidenced by the maximum reflected pressure occurring at the eye socket with the largest curvature on the skull. The reflected pressure pattern along with the local skull areas could further influence the intracranial pressure distributions within the brain. We did find out that the maximum coup pressure of 1.031 MPa in the side-on case as well as the maximum contrecoup pressure of -0.124 MPa in the back-on case. Moreover, the maximum principal strain (MPS) was also monitored due to its indication to diffuse brain injury. It was observed that the peak MPS located in the frontal cortex region regardless of the head orientation. However, the local peak MPS within each individual function region of the brain depended on the head orientation. The detailed interactions between blast wave and head orientations provided insights for evaluating the brain dynamics, as well as biomechanical factors leading to traumatic brain injury.

Experimental Investigation of Cavitation as a Possible Damage Mechanism in Blast-Induced Traumatic Brain Injury in Post-Mortem Human Subject Heads

The potential of blast-induced traumatic brain injury from the mechanism of localized cavitation of the cerebrospinal fluid (CSF) is investigated. While the mechanism and criteria for non-impact blast-induced traumatic brain injury is still unknown, this study demonstrates that local cavitation in the CSF layer of the cranial volume could contribute to these injuries. The cranial contents of three post-mortem human subject (PMHS) heads were replaced with both a normal saline solution and a ballistic gel mixture with a simulated CSF layer. Each were instrumented with multiple pressure transducers and placed inside identical shock tubes at two different research facilities. Sensor data indicates that cavitation may have occurred in the PMHS models at pressure levels below those for a 50% risk of blast lung injury. This study points to skull flexion, the result of the shock wave on the front of the skull leading to a negative pressure in the contrecoup, as a possible mechanism that contributes to the onset of cavitation. Based on observation of intracranial pressure transducer data from the PMHS model, cavitation onset is thought to occur from approximately a 140 kPa head-on incident blast.

Evaluation of brain tissue responses because of the underwash overpressure of helmet and faceshield under blast loading

Head protective tools such as helmets and faceshields can induce a localized high pressure region on the skull because of the underwash of the blast waves. Whether this underwash overpressure can affect the brain tissue response is still unknown. Accordingly, a computational approach was taken to confirm the incidence of underwash with regards to blast direction, as well as examine the influence of this effect on the mechanical responses of the brain. The variation of intracranial pressure (ICP) as one of the major injury predictors, as well as the maximum shear stress were mainly addressed in this study. Using a nonlinear finite element (FE) approach, generation and interaction of blast waves with the unprotected, helmeted, and fully protected (helmet and faceshield protected) FE head models were modeled using a multi-material arbitrary Lagrangian-Eulerian (ALE) method and a fluid-structure interaction (FSI) coupling algorithm. The underwash incidence overpressure was found to greatly change with the blast direction. Moreover, while underwash induced ICP (U-ICP) did not exceed the peak ICP of the unprotected head, it was comparable and even more than the peak ICP imposed on the protected heads by the primary shockwaves (Coup-ICP). It was concluded that while both helmet and faceshield protected the head against blast waves, the underwash overpressure affected the brain tissue response and altered the dynamic load experienced by the brain as it led to increased ICP levels at the countercoup site, imparted elevated skull flexure, and induced high negative pressure regions. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

The Complexity of Biomechanics Causing Primary Blast-Induced Traumatic Brain Injury: A Review of Potential Mechanisms

Primary blast-induced traumatic brain injury (bTBI) is a prevalent battlefield injury in recent conflicts, yet biomechanical mechanisms of bTBI remain unclear. Elucidating specific biomechanical mechanisms is essential to developing animal models for testing candidate therapies and for improving protective equipment. Three hypothetical mechanisms of primary bTBI have received the most attention. Because translational and rotational head accelerations are primary contributors to TBI from non-penetrating blunt force head trauma, the acceleration hypothesis suggests that blast-induced head accelerations may cause bTBI. The hypothesis of direct cranial transmission suggests that a pressure transient traverses the skull into the brain and directly injures brain tissue. The thoracic hypothesis of bTBI suggests that some combination of a pressure transient reaching the brain via the thorax and a vagally mediated reflex result in bTBI. These three mechanisms may not be mutually exclusive, and quantifying exposure thresholds (for blasts of a given duration) is essential for determining which mechanisms may be contributing for a level of blast exposure. Progress has been hindered by experimental designs, which do not effectively expose animal models to a single mechanism and by over-reliance on poorly validated computational models. The path forward should be predictive validation of computational models by quantitative confirmation with blast experiments in animal models, human cadavers, and biofidelic human surrogates over a range of relevant blast magnitudes and durations coupled with experimental designs, which isolate a single injury mechanism.

Relevance of Blood Vessel Networks in Blast-Induced Traumatic Brain Injury

Cerebral vasculature is a complex network that circulates blood through the brain. However, the role of this networking effect in brain dynamics has seldom been inspected. This work is to study the effects of blood vessel networks on dynamic responses of the brain under blast loading. Voronoi tessellations were implemented to represent the network of blood vessels in the brain. The brain dynamics in terms of maximum principal strain (MPS), shear strain (SS), and intracranial pressure (ICP) were monitored and compared. Results show that blood vessel networks significantly affected brain responses. The increased MPS and SS were observed within the brain embedded with vessel networks, which did not exist in the case without blood vessel networks. It is interesting to observe that the alternation of the ICP response was minimal. Moreover, the vessel diameter and density also affected brain dynamics in both MPS and SS measures. This work sheds light on the role of cerebral vasculature in blast-induced traumatic brain injury.

Impact of complex blast waves on the human head: a computational study

Head injuries due to complex blasts are not well examined because of limited published articles on the subject. Previous studies have analyzed head injuries due to impact from a single planar blast wave. Complex or concomitant blasts refer to impacts usually caused by more than a single blast source, whereby the blast waves may impact the head simultaneously or consecutively, depending on the locations and distances of the blast sources from the subject, their blast intensities, the sequence of detonations, as well as the effect of blast wave reflections from rigid walls. It is expected that such scenarios will result in more serious head injuries as compared to impact from a single blast wave due to the larger effective duration of the blast. In this paper, the utilization of a head-helmet model for blast impact analyses in Abaqus(TM) (Dassault Systemes, Singapore) is demonstrated. The model is validated against studies published in the literature. Results show that the skull is capable of transmitting the blast impact to cause high intracranial pressures (ICPs). In addition, the pressure wave from a frontal blast may enter through the sides of the helmet and wrap around the head to result in a second impact at the rear. This study recommended better protection at the sides and rear of the helmet through the use of foam pads so as to reduce wave entry into the helmet. The consecutive frontal blasts scenario resulted in higher ICPs compared with impact from a single frontal blast. This implied that blast impingement from an immediate subsequent pressure wave would increase severity of brain injury. For the unhelmeted head case, a peak ICP of 330 kPa is registered at the parietal lobe which exceeds the 235 kPa threshold for serious head injuries. The concurrent front and side blasts scenario yielded lower ICPs and skull stresses than the consecutive frontal blasts case. It is also revealed that the additional side blast would only significantly affect ICPs at the temporal and parietal lobes when compared with results from the single frontal blast case. By analyzing the pressure wave flow surrounding the head and correlating them with the consequential evolution of ICP and skull stress, the paper provides insights into the interaction mechanics between the concomitant blast waves and the biological head model.

The mechanics of traumatic brain injury: a review of what we know and what we need to know for reducing its societal burden

Traumatic brain injury (TBI) is a significant public health problem, on pace to become the third leading cause of death worldwide by 2020. Moreover, emerging evidence linking repeated mild traumatic brain injury to long-term neurodegenerative disorders points out that TBI can be both an acute disorder and a chronic disease. We are at an important transition point in our understanding of TBI, as past work has generated significant advances in better protecting us against some forms of moderate and severe TBI. However, we still lack a clear understanding of how to study milder forms of injury, such as concussion, or new forms of TBI that can occur from primary blast loading. In this review, we highlight the major advances made in understanding the biomechanical basis of TBI. We point out opportunities to generate significant new advances in our understanding of TBI biomechanics, especially as it appears across the molecular, cellular, and whole organ scale.

Mechanics of blast loading on the head models in the study of traumatic brain injury using experimental and computational approaches

Blast waves generated by improvised explosive devices can cause mild, moderate to severe traumatic brain injury in soldiers and civilians. To understand the interactions of blast waves on the head and brain and to identify the mechanisms of injury, compression-driven air shock tubes are extensively used in laboratory settings to simulate the field conditions. The overall goal of this effort is to understand the mechanics of blast wave-head interactions as the blast wave traverses the head/brain continuum. Toward this goal, surrogate head model is subjected to well-controlled blast wave profile in the shock tube environment, and the results are analyzed using combined experimental and numerical approaches. The validated numerical models are then used to investigate the spatiotemporal distribution of stresses and pressure in the human skull and brain. By detailing the results from a series of careful experiments and numerical simulations, this paper demonstrates that: (1) Geometry of the head governs the flow dynamics around the head which in turn determines the net mechanical load on the head. (2) Biomechanical loading of the brain is governed by direct wave transmission, structural deformations, and wave reflections from tissue-material interfaces. (3) Deformation and stress analysis of the skull and brain show that skull flexure and tissue cavitation are possible mechanisms of blast-induced traumatic brain injury.