Comparison of Hybrid-III and postmortem human surrogate response to simulated underbody blast loading

Lower Extremity Injury Risk Curve Development for a Human Body Model in the Underbody Blast Environment

Computational human body models (HBMs) provide the ability to explore numerous candidate injury metrics ranging from local strain based criteria to global combined criteria such as the Tibia Index. Despite these efforts, there have been relatively few studies that focus on determining predicted injury risk from HBMs based on observed postmortem human subjects (PMHS) injury data. Additionally, HBMs provide an opportunity to construct risk curves using measures that are difficult or impossible to obtain experimentally. The Global Human Body Models Consortium (GHBMC) M50-O v 6.0 lower extremity was simulated in 181 different loading conditions based on previous PMHS tests in the underbody blast (UBB) environment and 43 different biomechanical metrics were output. The Brier Metric Score were used to determine the most appropriate metric for injury risk curve development. Using survival analysis, three different injury risk curves (IRC) were developed: "any injury," "calcaneus injury," and "tibia injury." For each injury risk curve, the top three metrics selected using the Brier Metric Score were tested for significant covariates including boot use and posture. The best performing metric for the "any injury," "calcaneus injury" and "tibia injury" cases were calcaneus strain, calcaneus force, and lower tibia force, respectively. For the six different injury risk curves where covariates were considered, the presence of the boot was found to be a significant covariate reducing injury risk in five out of six cases. Posture was significant for only one curve. The injury risk curves developed from this study can serve as a baseline for model injury prediction, personal protective equipment (PPE) evaluation, and can aid in larger scale testing and experimental protocols.

Evaluating Pelvis Response During Simulated Underbody Blast Loading

In recent conflicts, blast injury from landmines and improvised explosive devices (IEDs) has been the main mechanism of wounding and death. When a landmine or IED detonates under a vehicle (an under-body blast), the seat acceleration rapidly transmits a high load to the pelvis of the occupants, resulting in torso and pelvic injury. Pelvic fractures have high mortality rates, yet their injury mechanism has been poorly researched. Three (3) fresh-frozen male pelvic specimens were tested under axial impact loading. The pelvis was impacted mounted upside down by dropping a 12 kg mass at target impact velocities ranging from 1 to 8.6 m/s with time to peak velocity ranging from 3.8 to 5.8 ms. Resulting fractures were broadly categorized as involving a bilateral pubis rami fracture, a bilateral ischium fracture, and sacroiliac joint disruption. The study provides insights into the type and severity of pelvic injury that may occur over a range of under-body blast (UBB)-relevant loading profiles.

Computational assessment of leg response to extreme loadings using a detailed finite element model

This study focuses on evaluating the response of the Total Human Model for Safety™ lower extremity finite element model under blast loading. Biofidelity of the lower extremity model was evaluated against experiments with impact loading equivalent to underbody blast. The model response was found to match well with the experimental data for the average impactor speeds of 7 and 9.3 m/s resulting in an overall correlation and analysis rating of 0.86 and 0.82, respectively. The model response was then used to investigate response for antipersonnel mine explosion where the numerical setup consists of a charge mass of 40 g trinitrotoluene placed at a depth of 50 mm below the heel. The explosion was modeled using Multi Material-Arbitrary Lagrangian Eulerian method. The model was subjected to the graded input in terms of variation in standoff distance and mass of explosive to investigate the sensitivity of the model. The model found sensitive to the threat definition and predicted an increase of 110% in peak fluid-structure interaction force with 20% reduction in its time to peak and 29% increase in peak calcaneus axial force with a reduction of 33% in its time to peak when explosive mass varied from 40 g to 100 g. The location of the explosive below the foot was discovered to have significant effect on the injury pattern in near-field explosion. A comparative study suggested that the model predicted similar response and damage pattern compared to experimental data.

The response of Military Lower Extremity and Hybrid III leg using the Hybrid III and EuroSID-2 ATD in vertical loading impacts

OBJECTIVE

Antivehicular landmines (AVLs) and underbelly improvised explosive devices (IEDs) are found to be some of the major threats for military vehicles and their occupants. Anthropomorphic test devices (ATDs) such as the 50th percentile Hybrid III (HIII) and EuroSID2-re (ES2-re) are used to assess injury caused by AVLs or IEDs in order to develop mitigation strategies by analyzing lower leg data in tibia load cells.

METHODS

This article presents the evaluation of the injury measurement response of the Hybrid III and ES2-re ATDs using both the HIII and Military Lower Extremity (MIL-LX) instrumented lower legs impacted by the Modified Lower Limb Impactor (MLLI). The MIL-LX leg tested with the HIII ATD measures higher forces than when tested with the ES2-re ATD.

RESULTS

In general, the MIL-LX upper tibia load cell measures peak forces that are considerably lower than that measured by the HIII lower leg with both the HIII and ES2-re ATD. The HIII leg fails earlier with both the HIII and ES2-re ATD compared to the MIL-LX leg. The study shows that the HIII and MIL-LX lower leg are not equivalent in their assessment of protective capability of armored vehicles when either attached to the HIII or ES2-re ATD.

CONCLUSIONS

These results show the importance of selecting the correct lower leg surrogate during AVL or IED testing, which can lead to a pass or fail of the armored vehicle. These findings offer insight into the response of each surrogate lower leg with the different ATDs and can be used to develop new mitigation strategies.

Quantifying the Effect of Pelvis Fracture on Lumbar Spine Compression during High-rate Vertical Loading

Fracture to the lumbo-pelvis region is prevalent in warfighters seated in military vehicles exposed to under-body blast (UBB). Previous high-rate vertical loading experimentation using whole body post-mortem human surrogates (PMHS) indicated that pelvis fracture tends to occur earlier in events and under higher magnitude seat input conditions compared to lumbar spine fracture. The current study hypothesizes that fracture of the pelvis under high-rate vertical loading reduces load transfer to the lumbar spine, thus reducing the potential for spine fracture. PMHS lumbo-pelvis components (L4-pelvis) were tested under high-rate vertical loading and force and acceleration metrics were measured both inferior-to and superior-to the specimen. The ratio of inferior-tosuperior responses was significantly reduced by unstable pelvis fracture for all metrics and a trend of reduced ratio was observed with increased pelvis AIS severity. This study has established that pelvis fracture reduces compression forces at the lumbar spine during high-rate vertical loading, thus reducing the potential for fracture to the lumbar spine. Therefore, pelvis injury potential should be considered when implementing lumbar injury criteria specific to UBB.

Verification of High-Rate Vertical Loading Laboratory Skeletal Fractures by Comparison with Theater Injury Patterns

For the current study, an existing theater injury data set was compared to component and whole body experiments meant to replicate the theater high rate vertical loading environment. The theater injury data set was derived from real world events that were within the design range of the Warrior Injury Assessment Manikin. A qualitative and quantitative assessment of the whole body fracture patterns was developed to determine whether the laboratory loading was correctly representing the resulting injuries seen in theater Underbody Blast (UBB) events. Results indicated that most of the experimental test fracture patterns were similar to the theater injuries for Abbreviated Injury Scale body regions of interest (lower extremities, pelvis, and spine); however, some of the body regions had higher similarity scores compared to others. Whole body fracture distribution was less similar than the component tests because of differences in injury distributions. The lower extremity whole body similarity was lower than spine and pelvis similarity. This analysis was able to identify some experimental tests that might not represent theater loading. In conclusion, this analysis confirmed that some laboratory testing produced skeletal injury patterns that are seen in comparable theater UBB events.

Severe Calcaneus Injury Probability Curves Due to Under-Body Blast

The lower extremity is the most frequently injured body region to mounted soldiers during underbody blast (UBB) events. UBB events often produce large deformations of the floor and subsequent acceleration of the lower limb that are not sufficiently mitigated by the combat boot, leaving the calcaneus bone vulnerable to injury. Biomechanical experiments simulating UBB loading scenarios were conducted in a laboratory environment using isolated postmortem human subject (PMHS) leg components. Each leg component was tested twice: one sub-injurious test followed by a injury-targeted test. This enabled the use of interval censoring for each specimen in the survival statistical analysis to generate the human injury probability curves (HIPCs). Foot contact forces were measured in both the hindfoot and forefoot. Strains and acoustic emission signals at the calcaneus and distal tibia were utilized to determine injury timing. The footplate velocities of the injury tests ranged 8-13 m/s with time-to-peak velocity of 1.8-2.5 ms while the velocities of non-injury tests ranged from 4 to 6 m/s with the same time-to-peak. The majority of the injuries were severe calcaneus fractures (Sanders III-IV). Secondary injuries included fractures to the distal tibia, talus, cuboid and cuneiform. These injury outcomes were found to be consistent with those reported in UBB injury literature. The HIPCs for the severe calcaneus fracture were developed using the vertical heel contact force as the injury correlation measure through survival analysis statistical method in the form of lognormal function. This work represents the first set of HIPCs dedicated to the severe calcaneus fracture using the biomechanical force measurement closest to the injury location. This injury probability curve will enable biomechanical response validation of computational models, development of ATD injury assessment reference curve, and injury prediction capability for computational models or ATDs in the UBB environment.

Hierarchical Validation Prevents Over-Fitting of the Neck Material Model for an Anthropomorphic Test Device Used in Underbody Blast Scenarios

Injury due to underbody loading is increasingly relevant to the safety of the modern warfighter. To accurately evaluate injury risk in this loading modality, a biofidelic anthropomorphic test device (e.g., dummy) is required. Finite element model counterparts to the physical dummies are also useful tools in the evaluation of injury risk, but require validated constitutive material models used in the dummy. However, material model fitting can result in models that are over-fit: they match well with the data they were trained on, but do not extrapolate well to new loading scenarios. In this study, we used a hierarchical approach. Material models created from coupon-level tests were evaluated at the component level, and then verified using blinded component and whole body (WB) tests to establish a material model of the anthropomorphic test device (ATD) neck that was not over-fit. Additionally, a combined metric is introduced that incorporates the well-known correlation analysis (CORA) score with peak characteristics to holistically evaluate the material model performance. A Bergstrom Boyce material model fit to one loop of combined compression and tension experimental data performed the best within the training datasets. Its combined metric scores were 2.51 and 2.18 (max score of 3) in a constrained neck and head neck setup, respectively. In the blinded evaluation including flexed, extended, and WB simulations, similar combined scores were observed with 2.44, 2.26, and 2.60, respectively. The agreement between the combined scores in the training and validation dataset indicated that model was not over-fit and can be extrapolated into untested, but similar loading scenarios.

Characterizing Strain Rate-Dependent Mechanical Properties for Bovine Cortical Bones

Comprehensive knowledge of strain rate-dependent viscoelastic properties of bony materials is necessary to understand the mechanisms of bone fracture under impact loading conditions (e.g., falls, traffic accidents, and military environments). Although the mechanical properties of bones have been studied for several decades, the high strain rate data and corresponding material parameters of the rate-dependent constitutive models are still limited. In this study, split Hopkinson pressure bar technique was used to test bovine cortical bones, to obtain the rate-dependent stress–strain curves in two directions (along and perpendicular to the bone fibers). A constitutive relationship comprising two terms was then applied to identify the material constants with strain rate effect and viscoelastic properties. In this model, the linear elasticity was combined with nonlinear viscoelasticity components to describe the overall nonlinear strain rate dependence. The presented data give strong experimental evidence and basis for further development of numerical biomechanical models to simulate human cortical bone fracture.

Head injury metric response in finite element ATDs and a human body model in multidirectional loading regimes

Objective: The objective was to quantify head injury metric sensitivity of the 50th percentile male Hybrid III, THOR, and Global Human Body Models Consortium simplified occupant (GHBMC M50-OS) to changes in loading conditions in loading regimes that may be experienced by occupants of spaceflight vehicles or highly autonomous vehicles (HAVs) with nontraditional seating configurations.Methods: A Latin hypercube (LHD) design of experiments (DOE) was employed to develop boundary conditions for 455 unique acceleration profiles. Three previously validated finite element (FE) models of the Hybrid III anthropomorphic test device (ATD), THOR ATD, and GHBMC M50-OS were positioned in an upright 90°-90°-90° seat and with a 5-point belt. Acceleration pulses were applied to each of the three occupants in the ± X, +Y, and ± Z directions, with peak resultant acceleration magnitudes ranging from 5 to 20 G and times to peak ranging from 32.5 to 120.8 ms with duration 250 ms, resulting in 1,248 simulations. Head injury metrics included peak linear head acceleration, peak rotational head acceleration, head injury criteria (HIC15), and brain injury criteria (BrIC). Injury metrics were regressed against boundary condition parameters using 2nd order multiple polynomial regression, and compared between occupants using matched pairs Wilcoxon signed rank analysis.Results: Across the 416 matched-simulations that reached normal termination with all three models, HIC15 values ranged from 1.0-396.5 (Hybrid III), 1.2-327.9 (THOR), and 0.6-585.6 (GHBMC). BrIC ranged from 0.03-0.95 (Hybrid III), 0.03-1.21 (THOR), and 0.04-0.84 (GHBMC). Wilcoxon signed rank analysis demonstrated significant pairwise differences between each of the three occupant models for head injury metrics. For HIC15, the largest divergence between GHBMC and the ATDs was observed in simulations with components of combined underbody and rear impact loading. The three models performed most similarly with respect to BrIC output when loaded in a frontal direction. Both the GHBMC and the Hybrid III produced lower values of BrIC than the THOR on average, with the differences most pronounced in rear impact loading.Conclusion: In conclusion, observed differences between the occupant models' head injury metric output were quantified. Loading direction had a large effect on metric outcome and metric comparability across models, with frontal and rear impacts with low vertical acceleration tending to be the most similar. One explanation for these differences could be the differences in neck stiffness between the models that allowed more rotation in the GHBMC and THOR. Care should be taken when using ATDs as human volunteer surrogates in these low energy events.

Lower Limb Posture Affects the Mechanism of Injury in Under-Body Blast

Over 80% of wounded Service Members sustain at least one extremity injury. The 'deck-slap' foot, a product of the vehicle's floor rising rapidly when attacked by a mine to injure the limb, has been a signature injury in recent conflicts. Given the frequency and severity of these combat-related extremity injuries, they require the greatest utilisation of resources for treatment, and have caused the greatest number of disabled soldiers during recent conflicts. Most research efforts focus on occupants seated with both tibia-to-femur and tibia-to-foot angles set at 90°; it is unknown whether results obtained from these tests are applicable when alternative seated postures are adopted. To investigate this, lower limbs from anthropometric testing devices (ATDs) and post mortem human subjects (PMHSs) were loaded in three different seated postures using an under-body blast injury simulator. Using metrics that are commonly used for assessing injury, such as the axial force and the revised tibia index, the lower limb of ATDs were found to be insensitive to posture variations while the injuries sustained by the PMHS lower limbs differed in type and severity between postures. This suggests that the mechanism of injury depends on the posture and that this cannot be captured by the current injury criteria. Therefore, great care should be taken when interpreting and extrapolating results, especially in vehicle qualification tests, when postures other than the 90°-90° are of interest.

Analysis of the Frequency and Mechanism of Injury to Warfighters in the Under-body Blast Environment

During Operation Iraqi Freedom and Operation Enduring Freedom, improvised explosive devices were used strategically and with increasing frequency. To effectively design countermeasures for this environment, the Department of Defense identified the need for an under-body blast-specific Warrior Injury Assessment Manikin (WIAMan). To help with this design, information on Warfighter injuries in mounted under-body blast attacks was obtained from the Joint Trauma Analysis and Prevention of Injury in Combat program through their Request for Information interface. The events selected were evaluated by Department of the Army personnel to confirm they were representative of the loading environment expected for the WIAMan. A military case review was conducted for all AIS 2+ fractures with supporting radiology. In Warfighters whose injuries were reviewed, 79% had a foot, ankle or leg AIS 2+ fracture. Distal tibia, distal fibula, and calcaneus fractures were the most prevalent. The most common injury mechanisms were bending with probable vehicle contact (leg) and compression (foot). The most severe injuries sustained by Warfighters were to the pelvis, lumbar spine, and thoracic spine. These injuries were attributed to a compressive load from the seat pan that directly loaded the pelvis or created flexion in the lumbar spine. Rare types of injuries included severe abdominal organ injury, severe brain injury, and cervical spine injury. These typically occurred in conjunction with other fractures. Mitigating the frequently observed skeletal injuries using the WIAMan would have substantial long-term benefits for Warfighters.

Effects of Lumbar Spine Assemblies and Body-Borne Equipment Mass on Anthropomorphic Test Device Responses During Drop Tests

When simulating or conducting land mine blast tests on armored vehicles to assess potential occupant injury, the preference is to use the Hybrid III anthropomorphic test device (ATD). In land blast events, neither the effect of body-borne equipment (BBE) on the ATD response nor the dynamic response index (DRI) is well understood. An experimental study was carried out using a drop tower test rig, with a rigid seat mounted on a carriage table undergoing average accelerations of 161 g and 232 g over 3 ms. A key aspect of the work looked at the various lumbar spine assemblies available for a Hybrid III ATD. These can result in different load cell orientations for the ATD which in turn can affect the load measurement in the vertical and horizontal planes. Thirty-two tests were carried out using two BBE mass conditions and three variations of ATDs. The latter were the Hybrid III with the curved (conventional) spine, the Hybrid III with the pedestrian (straight) spine, and the Federal Aviation Administration (FAA) Hybrid III which also has a straight spine. The results showed that the straight lumbar spine assemblies produced similar ATD responses in drop tower tests using a rigid seat. In contrast, the curved lumbar spine assembly generated a lower pelvis acceleration and a higher lumbar load than the straight lumbar spine assemblies. The maximum relative displacement of the lumbar spine occurred after the peak loading event, suggesting that the DRI is not suitable for assessing injury when the impact duration is short and an ATD is seated on a rigid seat on a drop tower. The peak vertical lumbar loads did not change with increasing BBE mass because the equipment mass effects did not become a factor during the peak loading event.

Mediastinal injury is the strongest predictor of mortality in mounted blast amongst UK deployed forces

BACKGROUND

Blast injury has been the most common cause of morbidity and mortality encountered by UK forces during recent conflicts. Injuries sustained by blast are categorised by the injuring component of the explosion and depend upon physical surroundings. Previous work has established that head injuries and intra cavity haemorrhage are the major causes of death following exposure to under body (mounted) blast but has yet to explore the precise nature of these torso injuries nor the effect of particular injuries upon survival. This study examines the patterns of torso injury within the mounted blast environment in order to understand the effect of these injuries upon survivability.

METHODS

This retrospective study examined the UK Joint Theatre Trauma Registry to determine precise injury patterns of mounted blast casualties within a 13year period of UK military deployments. Survival rates of individual injuries were compared and a multivariable logistic regression model was developed in order to assess the effect that each injury had upon likelihood of death.

RESULTS

426 mounted casualties were reviewed of whom 129 did not survive. Median NISS and ISS for non-survivors was found to be 75. Torso injuries were significantly more common amongst non-survivors than survivors and high case fatality rates were associated with all haemorrhagic torso injuries. Multivariable analysis shows that mediastinal injuries have the largest odds ratio for mortality (20.4) followed by lung laceration and head injury.

CONCLUSIONS

Non-compressible torso haemorrhage is associated with mortality amongst mounted blast. Of this group, mediastinal injury is the strongest predictor of death and could be considered as a surrogate marker of lethality. Future work to link blast loading characteristics with specific injury patterns will inform the design of mitigating strategies in order to improve survivability of underbody blast.

Mechanical characterization and finite element implementation of the soft materials used in a novel anthropometric test device for simulating underbody blast loading

Soft materials (e.g. polymers) are widely used in biomechanical devices to represent the nonlinear viscoelastic properties inherent in biological soft tissues. Knowledge of their mechanical properties is used to inform design choices and develop accurate finite element (FE) models of human surrogates. The goal of this study was to characterize the behavior of eight polymeric materials used in the design of a novel anthropomorphic test device (ATD) and implement these materials in an FE model of the ATD. Tensile and compressive tests at strain rates ranging from 0.01s^-1 to 1000s^-1 were conducted on specimens from each material. Stress-strain relationships at discrete strain rates were used to define strain rate-dependent hyper-elastic material models in a commercial finite element solver. Then, the material models were implemented into an FE model of the ATD. The performance of the material models in the FE model was evaluated by simulating experiments that were conducted on the ATD lower limb. The material characterization tests revealed viscoelastic strain rate-dependent properties in the flesh and compliant elements of the ATD. Higher modulus polymers exhibited rate-dependent, strain-hardening properties. A strong agreement was seen between the material model simulations and corresponding experiments. In component simulations, the materials performed well and the model reasonably predicted the forces observed in experiments.