TY - JOUR
AU - Yoganandan, Narayan
AB - Abstract Blunt impact assessment of the Advanced Combat Helmet (ACH) is currently based on the linear head response. The current study presents a methodology for testing the ACH under complex loading that generates linear and rotational head motion. Experiments were performed on a guided, free-fall drop tower using an instrumented National Operating Committee for Standards on Athletic Equipment (NOCSAE) head attached to a Hybrid III (HIII) or EuroSID-2 (ES-2) dummy neck and carriage. Rear and lateral impacts occurred at 3.0 m/s with peak linear accelerations (PLA) and peak rotational accelerations (PRA) measured at the NOCSAE head center-of-gravity. Experimental data served as inputs for the Simulated Injury Monitor (SIMon) computational model to estimate brain strain. Rear ACH impacts had 22% and 7% higher PLA and PRA when using the HIII neck versus the ES-2 neck. Lateral ACH impacts had 33% and 35% lower PLA and PRA when using HIII neck versus the ES-2 neck. Computational results showed that total estimated brain strain increased by 25% and 76% under rear and lateral ACH impacts when using the ES-2 neck. This methodology was developed to simulate complex ACH impacts involving the rotational head motion associated with diffuse brain injuries, including concussion, in military environments. Advanced Combat Helmet, NOCSAE, Hybrid III, EuroSID-2, CSDM INTRODUCTION Traumatic brain injury (TBI) was considered the “signature injury” of the U.S. military conflicts in Iraq and Afghanistan due to the incidence of TBI in Soldiers who served during Operation Enduring Freedom (OEF), Operation Iraqi Freedom (OIF), and Operation New Dawn (OND).1–4 From 2000 to 2016, the Defense and Veterans Brain Injury Center (DVBIC) documented over 360,000 brain injuries with over 82% classified as mild (i.e., mTBI). Prior to 2006, TBI assessments were not administered unless Soldiers suffered severe injuries and were medically evacuated. However, concerns over the number of undocumented TBIs in OEF and OIF veterans ultimately led to more frequent TBI assessments during pre-deployment and theater.5 Soldiers may suffer mTBI in the form of diffuse brain injuries, including concussion, from various sources of blunt trauma such as falls, collisions, motor vehicles crashes, and parachute jumps.6,7 For most types of head exposures, the Advanced Combat Helmet (ACH) serves as the primary source of head protection for Soldiers.8 The ACH comprises a protective shell, 7-pad fitting system, and 4-point chinstrap/nape strap retention system. The pad fitting system (Zorbium Action Pad, Team Wendy, Cleveland, OH, USA) utilizes pads made from a polyurethane-based foam with rate-dependent properties. The ACH design provides Soldiers with head protection against ballistic and blunt impacts.9–14 The ACH blunt impact performance standard is a modified version of the Federal Motor Vehicles Safety Standard (FMVSS) 218.15 The modified FMVSS 218 reflects specific needs for testing of military helmets, including impact site and subsequent impacts to helmets (CO/PD-05-04).16 The FMVSS 218 regulations for the test drop tower, headforms (DOT size “B,” “C,” or “D”), impact surfaces (4.83 cm hemispherical anvil), and data collection standard (specified by the Society of Automotive Engineers [SAE] Standard J211).17 The ACH must be impacted twice at a velocity of 3.0 m/s at seven different locations including the front, rear, left side, right side, left nape, right nape, and crown of the helmet. For each impact site, the second drop test must occur between 1 and 2 minutes following the first test. Lastly, none of the 14 impacts should exceed the linear acceleration threshold of 150 G.16 Previous studies have used the ACH blunt impact performance standard to evaluate helmet-headform coupling and environmental sensors.18,19 However, recent studies emphasized the need for including a mechanical neck surrogate during testing to induce the angular head motion associated with diffuse brain injuries, including concussion.20–25 The current study describes the development of a new methodology for assessing the blunt impact performance of the ACH. This methodology allows the use of different mechanical neck surrogates (e.g., Hybrid III and EuroSID-2) to enable both linear and angular motion of a helmeted head. Rear and side impact tests were performed at the standard ACH impact velocity with the peak linear acceleration (PLA) and peak rotational acceleration (PRA) measured for each impact location and neck. The experimental data served as inputs for the Simulated Injury Monitor (SIMon) computational model to estimate the brain strain for each test condition. METHODS The drop test methodology utilized the Medical College of Wisconsin vertical acceleration device (VertAc).26 Modifications to the VertAc test setup allowed a National Operating Committee on Standards for Athletic Equipment (NOCSAE) head to be attached to a mechanical neck surrogate and carriage (Fig. 1). The modified VertAc test setup generated a combined loading response in which the head-neck complex experienced inertial loading followed by a direct impact to the helmet. The test setup differed from the ACH blunt impact test standard in several areas. The test setup used a NOCSAE head due to its anthropomorphic features rather than the Department of Transportation (DOT) headform specified in the ACH standard. The use of the NOCSAE head allowed both the Hybrid III (HII) and EuroSID-2 (ES-2) neck surrogates to be used since each neck is designed and optimized for different loading scenarios. The NOCSAE head connected to the HIII neck through a commercially available adapter (Southern Impact Research Center, Rockford, TN, USA) and to the ES-2 neck through a custom adapter (Medical College of Wisconsin, Milwaukee, WI, USA). The custom-built ES-2 neck adapter consisted of two compartments with the upper compartment attached to the bottom surface of the NOCSAE head and the lower compartment attached to the top surface of the ES-2 neck (Fig. 2).27 FIGURE 1. Open in new tabDownload slide (A) Diagram of vertical acceleration system (VertAc) with modifications for drop tests that induce both linear and angular motion of the National Operating Committee for Standards on Athletic Equipment (NOCSAE) head with the Advanced Combat Helmet (ACH) mounted. (B) Photo showing different features of the ACH drop test methodology and impact sequence. The first impact consisted of the carriage hitting the aluminum honeycomb deceleration medium and the second impact consisted of the ACH hitting the flat steel anvil. FIGURE 1. Open in new tabDownload slide (A) Diagram of vertical acceleration system (VertAc) with modifications for drop tests that induce both linear and angular motion of the National Operating Committee for Standards on Athletic Equipment (NOCSAE) head with the Advanced Combat Helmet (ACH) mounted. (B) Photo showing different features of the ACH drop test methodology and impact sequence. The first impact consisted of the carriage hitting the aluminum honeycomb deceleration medium and the second impact consisted of the ACH hitting the flat steel anvil. FIGURE 2. Open in new tabDownload slide Bottom views of National Operating Committee for Standards on Athletic Equipment head showing the attached (A-1) HIII neck adapter and (B-1) ES-2 neck adapter (upper component shown). (A-2) Side view of the HIII neck showing the nodding blocks and nodding joint. (B-2) Rear view of the ES-2 neck with the lower component of the custom ES-2 neck adapter attached on top. “A” and “B” indicate the type of neck surrogate while “1” and “2” indicate the head versus the neck. FIGURE 2. Open in new tabDownload slide Bottom views of National Operating Committee for Standards on Athletic Equipment head showing the attached (A-1) HIII neck adapter and (B-1) ES-2 neck adapter (upper component shown). (A-2) Side view of the HIII neck showing the nodding blocks and nodding joint. (B-2) Rear view of the ES-2 neck with the lower component of the custom ES-2 neck adapter attached on top. “A” and “B” indicate the type of neck surrogate while “1” and “2” indicate the head versus the neck. Instrumentation for the NOCSAE head consisted of nine linear accelerometers (Endevco 7264B-2000, San Juan Capistrano, CA, USA) mounted on a commercially available mounting block (Southern Impact Research Center, Rockford, TN, USA). This nine-accelerometer package (NAP) was positioned at the head CG and arranged in a 3-2-2-2 configuration with angular accelerations calculated from the corresponding linear accelerations and arm lengths.28 Signals were sampled at 20 kHz and processed under SAE J211 specifications. The NAP utilized a right-handed Cartesian coordinate system with the positive x-axis pointed anteriorly, the positive y-axis pointed laterally toward the right, and the positive z-axis pointed inferiorly. Four ACH drop tests occurred at 3.0 m/s for each impact location and neck (Fig. 3). The methodology included aluminum honeycomb as a deceleration medium. The honeycomb height relative to the floor varied for rear and lateral impact experiments, restricting the degree of motion of the head based on the dimensions of the Hybrid III 50th percentile male dummy. For rear impacts, head motion was restricted to approximately 110 mm, which corresponded to half the chest depth of the Hybrid III dummy. For lateral impacts, head motion was restricted to approximately 214 mm, which corresponded to half the shoulder-to-shoulder width of the Hybrid III dummy (Fig. 3). PLA and PRA were measured for each test condition and then compared statistically using unpaired t-tests (α = 0.05). FIGURE 3. Open in new tabDownload slide Representative images of the VertAc system for rear impacts with the (A) HIII and (B) ES-2 necks as well as lateral impacts with the (C) HIII and (D) ES-2 necks. FIGURE 3. Open in new tabDownload slide Representative images of the VertAc system for rear impacts with the (A) HIII and (B) ES-2 necks as well as lateral impacts with the (C) HIII and (D) ES-2 necks. Linear and angular velocities were calculated via integration of the filtered acceleration signals and then used as inputs for the Simulated Injury Monitor (SIMon) model. This computational model originated from CT scans of a 50th percentile male head and was validated using data from previous animal and post-mortem human subject (PMHS) experiments.29 Susceptibility to brain injury was determined from the Cumulative Strain Damage Measure (CSDM), an injury metric that indicates the percentage of the brain model that exceeds a predefined strain threshold. For all test conditions, the strain threshold was set to 10%. CSDM(10) values are reported as the percentage of the brain (or segmented portion of the brain) exceeding the strain threshold. CSDM(10) values were calculated using an approach that decomposed the total CSDM value for the entire brain into separate CSDM values for individual brain structures (Fig. 4).30 FIGURE 4. Open in new tabDownload slide Representative images that show the (A) coronal and (B) mid-sagittal views of the SIMon model including the cerebrum, cerebellum, brainstem, and ventricles. The SIMon model consists of additional structures including the skull, falx, tentorium, parasagittal blood vessels, and combined layer of cerebrospinal fluid and the pia-arachnoid complex, which are not shown. FIGURE 4. Open in new tabDownload slide Representative images that show the (A) coronal and (B) mid-sagittal views of the SIMon model including the cerebrum, cerebellum, brainstem, and ventricles. The SIMon model consists of additional structures including the skull, falx, tentorium, parasagittal blood vessels, and combined layer of cerebrospinal fluid and the pia-arachnoid complex, which are not shown. RESULTS Linear head acceleration profiles exhibited a shorter time-to-peak (TTP) during rear impacts compared to lateral impacts (Fig. 5). Rear impacts exhibited linear accelerations (Ax) with a TTP of 10–18 ms while lateral impacts exhibited linear accelerations (Ay) with a TTP of 54–58 ms. Angular acceleration profiles displayed similar response times with rear impacts (αy) producing a TTP of 10–18 ms and lateral impacts producing a TTP of 54–58 ms (Fig. 6). FIGURE 5. Open in new tabDownload slide Representative linear accelerations from (A) rear impacts with the HIII neck, (B) rear impacts with the ES-2 neck, (C) lateral impacts with the HIII neck, and (D) lateral impacts with the ES-2 neck. FIGURE 5. Open in new tabDownload slide Representative linear accelerations from (A) rear impacts with the HIII neck, (B) rear impacts with the ES-2 neck, (C) lateral impacts with the HIII neck, and (D) lateral impacts with the ES-2 neck. FIGURE 6. Open in new tabDownload slide Representative angular accelerations from (A) rear impacts with the HIII neck, (B) rear impacts with the ES-2 neck, (C) lateral impacts with the HIII neck, and (D) lateral impacts with the ES-2 neck. FIGURE 6. Open in new tabDownload slide Representative angular accelerations from (A) rear impacts with the HIII neck, (B) rear impacts with the ES-2 neck, (C) lateral impacts with the HIII neck, and (D) lateral impacts with the ES-2 neck. Variations in the ACH impact location and neck surrogate led to differences in the PLA and PRA of the NOCSAE head (Fig. 7). Rear impacts produced a higher PLA with the HIII neck (86.9 G) compared with the ES-2 neck (67.2 G) and a higher PRA with the HIII neck (4,585.0 rad/s2) compared to the ES-2 neck (4,242.2 rad/s2). Unpaired t-tests showed that relative to the HIII neck, the ES-2 neck underestimated the NOCSAE head PLA by 22.6% (p = 0.001) and PRA by 7.4% (p = 0.43). For lateral impacts, the NOCSAE head experienced a higher PLA with the ES-2 neck (37.3 G) compared to the HIII neck (24.9 G) and a higher PRA with the ES-2 neck (2,294.1 rad/s2) compared to the HIII neck (1,469.4 rad/s2). Relative to the ES-2 neck, the HIII neck underestimated PLA by 33.7% (p = 0.0005) and PRA by 35.9% (p = 0.0094). FIGURE 7. Open in new tabDownload slide Comparison of (A) PLAs and (B) PRAs based on Advanced Combat Helmet impact location and neck selection. Values are represented as mean ± SD. FIGURE 7. Open in new tabDownload slide Comparison of (A) PLAs and (B) PRAs based on Advanced Combat Helmet impact location and neck selection. Values are represented as mean ± SD. Variations in impact conditions also led to differences in CSDM(10). Rear impacts to the ACH produced a higher CSDM(10) with the ES-2 neck (25.5%) compared to the HIII neck (0.6%). Additionally, lateral impacts to the ACH generated a higher CSDM(10) with the ES-2 neck (49.3%) versus the HIII neck (30.8%). A comparison of the individual CSDM(10) values for different brain regions, which included the cerebrum, cerebellum, and brainstem, revealed that the cerebrum contributed the most to the total CSDM regardless of ACH impact location or the neck surrogate chosen for testing (Fig. 8). FIGURE 8. Open in new tabDownload slide CSDM comparison according to Advanced Combat Helmet impact location (rear versus lateral) and neck selection (HIII versus ES-2). The cerebrum contributed the most to the total CSDM value while the cerebellum and brainstem made smaller contributions. For rear impacts with the HIII neck, less than 1% of the SIMon model exceeded the 10% strain threshold. FIGURE 8. Open in new tabDownload slide CSDM comparison according to Advanced Combat Helmet impact location (rear versus lateral) and neck selection (HIII versus ES-2). The cerebrum contributed the most to the total CSDM value while the cerebellum and brainstem made smaller contributions. For rear impacts with the HIII neck, less than 1% of the SIMon model exceeded the 10% strain threshold. DISCUSSION The ACH drop test methodology replaced the standard DOT headform with a NOCSAE head due to concerns of helmet fit and potential helmet/head decoupling during testing. Because the DOT headform does not extend significantly past the Frankfort plane, foam padding must be used as a surrogate chin for the ACH to be mounted properly prior to testing.6 However, the NOCSAE head possesses both a chin and a nape that allow the ACH to be fastened more securely using the helmet retention system. A previous study compared the shapes of the NOCSAE and Hybrid III heads due to their frequent use in testing helmets. Although the study included only one sample of each head type and neglected the effects of instrumentation, neck, and inertial properties, the authors concluded that substantial morphological differences exist near the chin and nape thus affecting helmet fit.31 Unlike the DOT headform, the NOCSAE head possesses another advantage in its capacity to be instrumented with a NAP for obtaining measurements of both linear and angular head acceleration. The drop test methodology incorporated a mechanical neck surrogate to induce angular head motion, which cannot be achieved using the current ACH test standard because the DOT headform attaches directly to the carriage. The choice of neck was investigated because the HIII neck was optimized for flexion/extension while the ES-2 neck was optimized for lateral bending.25,32,33 In addition to interchangeable dummy necks, the methodology allows for variations in the range of head motion based on the type of impact selected. Head motion for rear impacts was limited to 110 mm, approximately half the chest depth of the Hybrid III, while head motion for lateral impacts was limited to 214 mm, approximately half the shoulder width of the Hybrid III (Fig. 3). A comparison between these dimensions and average measurements from a 2012 anthropomorphic study of U.S. Army personnel revealed a 13% difference in the chest depth and a 2% difference in the shoulder-to-shoulder width.34 By accounting for anthropomorphic measurements, the drop test methodology can be adapted to simulate a variety of military impact scenarios. The NOCSAE head attained higher PLA and PRA with the HIII neck during rear impacts but with the ES-2 neck during lateral impacts. These differences in peak head kinematics can be attributed to the unique design of each neck surrogate. The design and nodding joint of the HIII neck contribute to its flexion/extension response but generate a stiffer lateral response whereas the design and neck buffers of the ES-2 neck facilitate a more biofidelic lateral response. The combined loading sequence and adjustable neck movement in the drop testing procedure also affected peak kinematic measures. The increased range of neck motion during the lateral impact test condition allowed the inertial loading phase to dominate the combined loading response. In a previous study, inertial loading of a NOCSAE head resulted in a 36% higher angular velocity and a 4 ms longer rise time under lateral bending with the ES-2 neck attached versus the HIII neck.27 Therefore, the helmeted NOCSAE head likely experienced higher PLA and PRA due to the presence of the ES-2 neck under combined loading conditions (Fig. 5). However, rear impacts to the ACH did not yield similar results. This is partly due to the restricted range of neck motion, which altered the combined loading response by reducing the effect of inertial loading while increasing the effect of the direct impact. The differences in PLA and PRA under rear impacts also occurred due to the stiffer response of the HIII neck compared to the ES-2 neck under extension. These results highlight the importance of the neck in helmet evaluations due to its influence on head kinematics. The ACH impact data reported in this study compared reasonably with similar work by previous investigators. In 2010, Hopping et al conducted ACH blunt impact experiments using a standard test rig, standard loading conditions, and multiple headforms (e.g., DOT, ISO, and NOCSAE).6 Rear impacts by Hopping et al with the ACH mounted on a NOCSAE head resulted in PLAs of 70–75 G, which was lower than the average PLA generated in the current study with the Hybrid III neck (86 G) but higher than the average PLA generated with the ES-2 neck (67 G). Although the two studies utilized different test rigs, similar results were attained due to the limited range of neck motion during rear impacts. Despite the difference in stiffness between the HIII and ES-2 necks, the restricted range of neck motion caused the head-neck system to behave similarly to the head-only system. For side impacts, however, Hopping et al produced PLAs of approximately 55–70 G while the current study generated lower PLAs in the range of 24–37 G. This difference in side impact PLAs can be attributed to the inclusion of a neck surrogate and the increased range of allowable neck motion, which not only generated rotational head motion but also reduced the head’s linear response upon impact.6 Compared to Hopping et al., Rooks et al generated average PLAs from rear and side impact tests that were similar despite the use of a traditional DOT headform.18 These findings suggest that fall-related head impacts may require different ACH locations to have different peak acceleration thresholds to properly account for the corresponding neck motion in complex loading environments that generate linear and angular head motion. The SIMon computational model was included because the brain’s response to blunt impacts cannot be captured using peak head kinematics alone. Linear and angular measures collected at the head CG were subsequently used as inputs for the SIMon model. The SIMon model was chosen due to its minimal use of computational resources and its experimental validation with animal and PMHS experiments.29,35,36 The SIMon model has been used in conjunction with helmet testing to determine the strains in the brain under various impact conditions.30,37 Previous investigators have chosen different CSDM strain thresholds ranging from 10% to 25%. Takhounts et al reported that a CSDM(25) equal to 49% corresponded to a 50% probability of sustaining diffuse axonal injury (DAI).36 Knowles and Dennison selected a strain threshold of 15% because CSDM(15) had been correlated to injury using the SIMon model.37 Given the severity of DAI, however, this study applied a CSDM strain threshold of 10% since Kimpara and Iwamoto showed that a CSDM(10) equal to 18% corresponded to a 50% probability of mTBI.38 One notable finding from the SIMon model was that it generated higher CSDM(10) values in rear impact simulations with the ES-2 neck even though rear impact tests with the HIII neck produced higher peak linear and angular accelerations. This unexpected result could be attributed to the transient response of the brain model and its contribution to the strain observed. It is worth noting that more detailed computational models, including the Worcester Head Injury Model (WHIM), have characterized the localized strain in white matter brain structures associated with DAI (e.g., corpus callosum).39 Furthermore, investigators have examined not only the susceptibility of white matter brain regions to injury but also the effects of alternative measures such as strain rate.40–42 The current study includes some limitations. First, mass differences between the two necks and associated adapters were not considered. However, there was a 25% difference in the total mass of the ES-2 neck and its adapter versus the total mass of the HIII neck and its adapter. Second, this study analyzed only rear and lateral impacts to the ACH. Although the ACH blunt impact performance standard includes several other impact locations, the rear and lateral impact sites were prioritized due to the design of the HIII and ES-2 necks. The HIII and ES-2 necks were originally designed and optimized for flexion/extension and lateral bending, respectively. Therefore, loading within the sagittal and coronal planes were approximated as reasonable starting points for developing the drop test methodology before proceeding to other impact locations on the ACH. Another limitation was that testing occurred only at the ACH blunt impact standard velocity of 3.0 m/s even though Soldiers may sustain head impacts that exceed the standard level of protection.14 Finally, tests were conducted using a single NOCSAE headform despite the potential for variability among individual NOCSAE headforms as well as the availability of other headforms (e.g., ISO, ISOC, Hybrid III, FOCUS). Future testing should include multiple NOCSAE headforms and alternative headforms. CONCLUSIONS The current method for evaluating the blunt impact protection of the ACH focuses on mitigating the peak linear response of the head but diffuse brain injuries, including concussions, can also involve rotational head kinematics.43 Therefore, a methodology was developed to test the ACH under complex head impacts that induce angular head motion. This methodology features an anthropomorphic head that allows for a more realistic fit inside the ACH and the ability to attach two different mechanical neck surrogates depending on the loading configuration. Results from experiments and SIMon model simulations indicate that incorporating the ES-2 neck into this ACH drop test methodology ultimately produces higher estimations of brain strain than with the stiffer HIII neck. The hybrid experimental/computational approach presented in this work could be used to assess the blunt impact performance of the ACH under complex impact conditions and serve as a supplement to the existing ACH blunt impact standard. Future studies will analyze the influence of various loading conditions on ACH blunt impact attenuation using the proposed drop test methodology. The ACH will be evaluated at velocities above the blunt impact standard to correlate different helmet impact energies with head kinematics. Variations in standard ACH test locations (e.g., oblique impacts at nape), non-standard test locations (e.g., ear cup), and the allowable range of head motion will also be examined. Scenarios involving neck preflexion and increased helmet mass from mounted accessories, such as night vision goggles, could also be explored with this technique. Although the helmet testing protocol is currently used for the ACH, the methodology could be adapted for blunt impact evaluations of other military helmets. Presentation Presented as an oral talk at the 2017 Military Health System Research Symposium, August 2017, Kissimmee, FL; abstract # MHSRS-17–1730 Funding This work was supported by the U.S. Army Medical Research and Materiel Command under Contract Number W81XWH-16-1-0010. This supplement was sponsored by the Office of the Secretary of Defense for Health Affairs. Acknowledgments This material is the result of work supported with resources and use of facilities at the Zablocki VA Medical Center (ZVAMC) in Milwaukee, Wisconsin, the Department of Neurosurgery at the Medical College of Wisconsin (MCW), the U.S. Army Medical Research and Materiel Command in Fort Detrick, Maryland, W81XWH-16-1-0010, and the U.S. Army Aeromedical Research Laboratory. The authors NY and FAP are part-time employees of the ZVAMC. 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The views, opinions, and/or findings contained in this manuscript are those of the authors and should not be construed as an official Department of the Army position, policy, or decision, unless so designated by other official documentation. Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2019. This work is written by (a) US Government employee(s) and is in the public domain in the US. Published by Oxford University Press on behalf of the Association of Military Surgeons of the United States 2019.
TI - Development of a Methodology for Simulating Complex Head Impacts With the Advanced Combat Helmet
JO - Military Medicine
DO - 10.1093/milmed/usy282
DA - 2019-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/development-of-a-methodology-for-simulating-complex-head-impacts-with-XKBejXPDcU
SP - 237
EP - 244
VL - 184
IS - Supplement_1
DP - DeepDyve
ER -