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.

Evaluation of the Whole Body Spine Response to Sub-Injurious Vertical Loading.

It is critical to understand the relationship between under-body blast (UBB) loading and occupant response to provide optimal protection to the warfighter from serious injuries, many of which affect the spine. Previous studies have examined component and whole body response to accelerative based UBB loading. While these studies both informed injury prediction efforts and examined the shortcomings of traditional anthropomorphic test devices in the evaluation of human injury, few studies provide response data against which future models could be compared and evaluated. The current study examines four different loading conditions on a seated occupant that demonstrate the effects of changes in the floor, seat, personal protective equipment (PPE), and reclined posture on whole body post-mortem human surrogate (PMHS) spinal response in a sub-injurious loading range. Twelve PMHS were tested across floor velocities and time-to-peak (TTP) that ranged from 4.0 to 8.0 m/s and 2 to 5 ms, respectively. To focus on sub-injurious response, seat velocities were kept at 4.0 m/s and TTP ranged from 5 to 35 ms. Results demonstrated that spine response is sensitive to changes in TTP and the presence of PPE. However, spine response is largely insensitive to changes in floor loading. Data from these experiments have also served to develop response corridors that can be used to assess the performance and predictive capability of new test models used as human surrogates in high-rate vertical loading experiments.

Kinematic and Biomechanical Response of Post-Mortem Human Subjects Under Various Pre-Impact Postures to High-Rate Vertical Loading Conditions.

Limited data exist on the injury tolerance and biomechanical response of humans to high-rate, under-body blast (UBB) loading conditions that are commonly seen in current military operations, and there are no data examining the influence of occupant posture on response. Additionally, no anthropomorphic test device (ATD) currently exists that can properly assess the response of humans to high-rate UBB loading. Therefore, the purpose of this research was to examine the response of post-mortem human surrogates (PMHS) in various seated postures to high-rate, vertical loading representative of those conditions seen in theater. In total, six PMHS tests were conducted using loading pulses applied directly to the pelvis and feet of the PMHS: three in an acute posture (foot, knee, and pelvis angles of 75°, 75°, and 36°, respectively), and three in an obtuse posture (15° reclined torso, and foot, knee, and pelvis angles of 105°, 105°, and 49.5°, respectively). Tests were conducted with a seat velocity pulse that peaked at ~4 m/s with a 30-40 ms time to peak velocity (TTP) and a floor velocity that peaked at 6.9-8.0 m/s (2-2.75 ms TTP). Posture condition had no influence on skeletal injuries sustained, but did result in altered leg kinematics, with leg entrapment under the seat occurring in the acute posture, and significant forward leg rotations occurring in the obtuse posture. These data will be used to validate a prototype ATD meant for use in high-rate UBB loading scenarios.

Similitude assessment method for comparing PMHS response data from impact loading across multiple test devices.

Biological tissue testing is inherently susceptible to the wide range of variability specimen to specimen. A primary resource for encapsulating this range of variability is the biofidelity response corridor or BRC. In the field of injury biomechanics, BRCs are often used for development and validation of both physical, such as anthropomorphic test devices, and computational models. For the purpose of generating corridors, post-mortem human surrogates were tested across a range of loading conditions relevant to under-body blast events. To sufficiently cover the wide range of input conditions, a relatively small number of tests were performed across a large spread of conditions. The high volume of required testing called for leveraging the capabilities of multiple impact test facilities, all with slight variations in test devices. A method for assessing similitude of responses between test devices was created as a metric for inclusion of a response in the resulting BRC. The goal of this method was to supply a statistically sound, objective method to assess the similitude of an individual response against a set of responses to ensure that the BRC created from the set was affected primarily by biological variability, not anomalies or differences stemming from test devices.

Evaluation of WIAMan Technology Demonstrator Biofidelity Relative to Sub-Injurious PMHS Response in Simulated Under-body Blast Events.

Three laboratory simulated sub-injurious under-body blast (UBB) test conditions were conducted with whole-body Post Mortem Human Surrogates (PMHS) and the Warrior Assessment Injury Manikin (WIAMan) Technology Demonstrator (TD) to establish and assess UBB biofidelity of the WIAMan TD. Test conditions included a rigid floor and rigid seat with independently varied pulses. On the floor, peak velocities of 4 m/s and 6 m/s were applied with a 5 ms time to peak (TTP). The seat peak velocity was 4 m/s with varied TTP of 5 and 10 ms. Tests were conducted with and without personal protective equipment (PPE). PMHS response data was compiled into preliminary biofidelity response corridors (BRCs), which served as evaluation metrics for the WIAMan TD. Each WIAMan TD response was evaluated against the PMHS preliminary BRC for the loading and unloading phase of the signal time history using Correlation Analysis (CORA) software to assign a numerical score between 0 and 1. A weighted average of all responses was calculated to determine body region and whole body biofidelity scores for each test condition. The WIAMan TD received UBB biofidelity scores of 0.62 in Condition A, 0.59 in Condition B, and 0.63 in Condition C, putting it in the fair category (0.44-0.65). Body region responses with scores below a rating of good (0.65-0.84) indicate potential focus areas for the next generation of the WIAMan design.

Biomechanical Response of Military Booted and Unbooted Foot-Ankle-Tibia from Vertical Loading.

A new anthropomorphic test device (ATD) is being developed by the US Army to be responsive to vertical loading during a vehicle underbody blast event. To obtain design parameters for the new ATD, a series of non-injurious tests were conducted to derive biofidelity response corridors for the foot-ankle complex under vertical loading. Isolated post mortem human surrogate (PMHS) lower leg specimens were tested with and without military boot and in different initial foot-ankle positions. Instrumentation included a six-axis load cell at the proximal end, three-axis accelerometers at proximal and distal tibia, and calcaneus, and strain gages. Average proximal tibia axial forces for a neutral-positioned foot were about 2 kN for a 4 m/s test, 4 kN for 6 m/s test and 6 kN for an 8 m/s test. The force time-to-peak values were from 3 to 5 msec and calcaneus acceleration rise times were 2 to 8 msec. Compared to the neutral posture, the "off-axis" measures (e.g. shear and bending moment) were much greater in magnitude in plantar- or dorsi-flexed posture. The results as a function of velocity demonstrated uniform increases with increasing test velocities. The response corridors supplied from the present investigation will serve as initial design parameters for the ATD lower leg, and can also be used for validation for a human computational model.

Foot-Ankle Fractures and Injury Probability Curves from Post-mortem Human Surrogate Tests.

This purpose of this study was to replicate foot-ankle injuries seen in the military and derive human injury probability curves using the human cadaver model. Lower legs were isolated below knee from seventeen unembalmed human cadavers and they were aligned in a 90-90 posture (plantar surface orthogonal to leg). The specimens were loaded along the tibia axis by applying short-time duration pulses, using a repeated testing protocol. Injuries were documented using pre- and post-test X-rays, computed tomography scans, and dissection. Peak force-based risk curves were derived using survival analysis and accounted for data censoring. Fractures were grouped into all foot-ankle (A), any calcaneus (B), and any tibia injuries (C), respectively. Calcaneus and/or distal tibia/pilon fractures occurred in fourteen tests. Axial forces were the greatest and least for groups C and B, respectively. Times attainments of forces for all groups were within ten milliseconds. The Weibull function was the optimal probability distribution for all groups. Age was significant (p < 0.05) for groups A and C. Survival analysis-based probability curves were derived for all groups. Data are given in the body of paper. Age-based, risk-specific, and continuous distribution probability curves/responses guide in the creation of an injury assessment capability for military blast environments.

Cervical spine injuries, mechanisms, stability and AIS scores from vertical loading applied to military environments.

PURPOSE: The purpose of this study was to determine injuries to osteo-ligamentous structures of cervical column, mechanisms, forces, severities and AIS scores from vertical accelerative loading. METHODS: Seven human cadaver head-neck complexes (56.9 ± 9.5 years) were aligned based on seated the posture of military soldiers. Army combat helmets were used. Specimens were attached to a vertical accelerator to apply caudo-cephalad g-forces. They were accelerated with increasing insults. Intermittent palpation and radiography were done. A roof structure mimicking military vehicle interior was introduced after a series of tests and experiments were conducted following similar protocols. Upon injury detection, CT and dissection were done. Temporal force responses were extracted, peak forces and times of occurrence were obtained, injury severities were graded, and spine stability was determined. RESULTS: Injuries occurred in tests only when the roof structure was included. Responses were tri-phasic: initial thrust, secondary tensile, tertiary roof contact phases. Peak forces: 1364-4382 N, initial thrust, 165-169 N, secondary tensile, 868-3368 N tertiary helmet-head roof contact phases. Times of attainments: 5.3-9.6, 31.7-42.6, 55.0-70.8 ms. Injuries included fractures and joint disruptions. Multiple injuries occurred in all but one specimen. A majority of injury severities were AIS = 2. Spines were considered unstable in a majority of cases. CONCLUSIONS: Spine response was tri-phasic. Injuries occurred in roof contact tests with the helmeted head-neck specimen. Multiplicity and unstable nature of AIS = 2 level injuries, albeit at lower severities, might predispose the spine to long-term accelerated degenerative changes. Clinical protocols should include a careful evaluation of sub-catastrophic injuries in military patients.

Manganese-Enhanced Magnetic Resonance Imaging as a Diagnostic and Dispositional Tool after Mild-Moderate Blast Traumatic Brain Injury.

Traumatic brain injury (TBI) caused by explosive munitions, known as blast TBI, is the signature injury in recent military conflicts in Iraq and Afghanistan. Diagnostic evaluation of TBI, including blast TBI, is based on clinical history, symptoms, and neuropsychological testing, all of which can result in misdiagnosis or underdiagnosis of this condition, particularly in the case of TBI of mild-to-moderate severity. Prognosis is currently determined by TBI severity, recurrence, and type of pathology, and also may be influenced by promptness of clinical intervention when more effective treatments become available. An important task is prevention of repetitive TBI, particularly when the patient is still symptomatic. For these reasons, the establishment of quantitative biological markers can serve to improve diagnosis and preventative or therapeutic management. In this study, we used a shock-tube model of blast TBI to determine whether manganese-enhanced magnetic resonance imaging (MEMRI) can serve as a tool to accurately and quantitatively diagnose mild-to-moderate blast TBI. Mice were subjected to a 30 psig blast and administered a single dose of MnCl2 intraperitoneally. Longitudinal T1-magnetic resonance imaging (MRI) performed at 6, 24, 48, and 72 h and at 14 and 28 days revealed a marked signal enhancement in the brain of mice exposed to blast, compared with sham controls, at nearly all time-points. Interestingly, when mice were protected with a polycarbonate body shield during blast exposure, the marked increase in contrast was prevented. We conclude that manganese uptake can serve as a quantitative biomarker for TBI and that MEMRI is a minimally-invasive quantitative approach that can aid in the accurate diagnosis and management of blast TBI. In addition, the prevention of the increased uptake of manganese by body protection strongly suggests that the exposure of an individual to blast risk could benefit from the design of improved body armor.

Vertical accelerator device to apply loads simulating blast environments in the military to human surrogates.

The objective of the study was to develop a simple device, Vertical accelerator (Vertac), to apply vertical impact loads to Post Mortem Human Subject (PMHS) or dummy surrogates because injuries sustained in military conflicts are associated with this vector; example, under-body blasts from explosive devices/events. The two-part mechanically controlled device consisted of load-application and load-receiving sections connected by a lever arm. The former section incorporated a falling weight to impact one end of the lever arm inducing a reaction at the other/load-receiving end. The "launch-plate" on this end of the arm applied the vertical impact load/acceleration pulse under different initial conditions to biological/physical surrogates, attached to second section. It is possible to induce different acceleration pulses by using varying energy absorbing materials and controlling drop height and weight. The second section of Vertac had the flexibility to accommodate different body regions for vertical loading experiments. The device is simple and inexpensive. It has the ability to control pulses and flexibility to accommodate different sub-systems/components of human surrogates. It has the capability to incorporate preloads and military personal protective equipment (e.g., combat helmet). It can simulate vehicle roofs. The device allows for intermittent specimen evaluations (x-ray and palpation, without changing specimen alignment). The two free but interconnected sections can be used to advance safety to military personnel. Examples demonstrating feasibilities of the Vertac device to apply vertical impact accelerations using PMHS head-neck preparations with helmet and booted Hybrid III dummy lower leg preparations under in-contact and launch-type impact experiments are presented.

Hybrid III Lower Leg Injury Assessment Reference Curves Under Axial Impacts Using Matched-Pair Tests.

The objective of the present study was to derive injury probability curves applicable to the Hybrid III dummy (also termed the Anthropomorphic Test Device, ATD) lower leg under axial impacts for military applications. A matched-pair approach was used. Axial impacts were delivered to below knee foot-ankle complex preparations of the lower leg of the ATD using pendulum and custom vertical accelerator devices. Military boot was used in some tests. Post mortem human surrogate (PMHS) preparations were used as matched-pair tests for injury outcomes. The alignment was such that the foot-ankle complex was orthogonal to the leg (below knee tibia-fibula complex), termed as the normal 90-90 posture. Injury outcomes from the biological surrogate focused on calcaneus and or distal tibia fractures with or without the involvement of articular surfaces. Peak lower tibia load cell forces were obtained from matched-pair dummy tests. Injury and force data were paired, censoring was assigned based on injury outcomes and survival analysis was done using the Weibull distribution to derive dummy-based probability curves. Mean peak forces were extracted at 5, 10, 20 and 50% probability levels. Normalized confidence interval sizes (NCIS) at ± 95% level were computed to determine the tightness-of-fit of the confidence bands. The NCIS data ranged from 0.34 to 0.78 and a peak force of 8.2 kN was associated at the ten percent injury probability level. Other data and curves are given in the body of the paper. The present Injury Assessment Reference Curves and Values (IARC and IARV) may be used in future tests for advancing safety in military environments. These survival analysis processes and IARC and IARV data may also be used in other applications.

Development and validation of a statistical shape modeling-based finite element model of the cervical spine under low-level multiple direction loading conditions.

Cervical spinal injuries are a significant concern in all trauma injuries. Recent military conflicts have demonstrated the substantial risk of spinal injury for the modern warfighter. Finite element models used to investigate injury mechanisms often fail to examine the effects of variation in geometry or material properties on mechanical behavior. The goals of this study were to model geometric variation for a set of cervical spines, to extend this model to a parametric finite element model, and, as a first step, to validate the parametric model against experimental data for low-loading conditions. Individual finite element models were created using cervical spine (C3-T1) computed tomography data for five male cadavers. Statistical shape modeling (SSM) was used to generate a parametric finite element model incorporating variability of spine geometry, and soft-tissue material property variation was also included. The probabilistic loading response of the parametric model was determined under flexion-extension, axial rotation, and lateral bending and validated by comparison to experimental data. Based on qualitative and quantitative comparison of the experimental loading response and model simulations, we suggest that the model performs adequately under relatively low-level loading conditions in multiple loading directions. In conclusion, SSM methods coupled with finite element analyses within a probabilistic framework, along with the ability to statistically validate the overall model performance, provide innovative and important steps toward describing the differences in vertebral morphology, spinal curvature, and variation in material properties. We suggest that these methods, with additional investigation and validation under injurious loading conditions, will lead to understanding and mitigating the risks of injury in the spine and other musculoskeletal structures.

Development of a Human Cranial Bone Surrogate for Impact Studies.

In order to replicate the fracture behavior of the intact human skull under impact it becomes necessary to develop a material having the mechanical properties of cranial bone. The most important properties to replicate in a surrogate human skull were found to be the fracture toughness and tensile strength of the cranial tables as well as the bending strength of the three-layer (inner table-diplöe-outer table) architecture of the human skull. The materials selected to represent the surrogate cranial tables consisted of two different epoxy resins systems with random milled glass fiber to enhance the strength and stiffness and the materials to represent the surrogate diplöe consisted of three low density foams. Forty-one three-point bending fracture toughness tests were performed on nine material combinations. The materials that best represented the fracture toughness of cranial tables were then selected and formed into tensile samples and tested. These materials were then used with the two surrogate diplöe foam materials to create the three-layer surrogate cranial bone samples for three-point bending tests. Drop tower tests were performed on flat samples created from these materials and the fracture patterns were very similar to the linear fractures in pendulum impacts of intact human skulls, previously reported in the literature. The surrogate cranial tables had the quasi-static fracture toughness and tensile strength of 2.5 MPa√ m and 53 ± 4.9 MPa, respectively, while the same properties of human compact bone were 3.1 ± 1.8 MPa√ m and 68 ± 18 MPa, respectively. The cranial surrogate had a quasi-static bending strength of 68 ± 5.7 MPa, while that of cranial bone was 82 ± 26 MPa. This material/design is currently being used to construct spherical shell samples for drop tower and ballistic tests.

Response of individual thoracolumbar spine ligaments under high-rate deformation.

Under-Body Blast (UBB) has emerged as the predominant threat to ground vehicles and Warfighter survivability. The force transference from the vehicle structure to the human body has resulted in serious injuries, with the thoracolumbar spine frequently damaged. Computational models of the human body are being generated to model human response and develop injury mitigation strategies. To effectively model the spine mechanics, the thoracolumbar ligaments, which serve varying roles in contributing to spine stability, must be characterized at relevant strains and strain rates. Adaptation of cervical spine testing methods has allowed for testing of isolated spinal ligaments including the Anterior Longitudinal Ligament (ALL), Posterior Longitudinal Ligament (PLL), and Ligamentum Flavum (LF). A high-rate servo-hydraulic test machine was used to execute a tensile test protocol for 24 complexes with loading rates ranging from 240 - 2800 mm/s and displacements of 25%, 50%, 75%, 100%, and 300% of the measured ligament length. Non-contact strain field measurements were recorded to produce a three dimensional strain field of the ligament surface. In order to provide the ligament data in a form which can be incorporated in the human computational models, analytical methods for modeling the ligament response are being investigated. Ultimately, this model will be optimized to be utilized in computational models of the lumbar spine.

Method for obtaining simple shear material properties of the intervertebral disc under high strain rates.

Predicting spinal injury under high rates of vertical loading is of interest, but the success of computational models in modeling this type of loading scenario is highly dependent on the material models employed. Understanding the response of these biological materials at high strain rates is critical to accurately model mechanical response of tissue and predict injury. While data exists at lower strain rates, there is a lack of the high strain rate material data that are needed to develop constitutive models. The Split Hopkinson Pressure Bar (SHPB) has been used for many years to obtain properties of various materials at high strain rates. However, this apparatus has mainly been used for characterizing metals and ceramics and is difficult to apply to softer materials such as biological tissue. Recently, studies have shown that modifications to the traditional SHPB setup allow for the successful characterization of mechanical properties of biological materials at strain rates and peak strain values that exceed alternate soft tissue testing techniques. In this paper, the previously-reported modified SHPB technique is applied to characterize human intervertebral disc material under simple shear. The strain rates achieved range from 5 to 250 strain s-1. The results demonstrate the sensitivity to the disc composition and structure, with the nucleus pulposus and annulus fibrosus exhibiting different behavior under shear loading. Shear tangent moduli are approximated at varying strain levels from 5 to 20% strain. This data and technique facilitates determination of mechanical properties of intervertebral disc materials under shear loading, for eventual use in constitutive models.

Thoracic response to high-rate blunt impacts using an advanced testing platform.

ehind Armor Blunt Trauma (BABT) is a persistent concern for both the military and civil law enforcement. Although personal protective equipment (PPE), including soft and hard body armor, mitigates penetrating injuries from ballistic threats, the impact generates a backface deformation which creates a high-rate blunt impact to the body and potential internal injury (i.e., BABT). A critical need exists to understand the mechanics of the human response and subsequently evaluate the efficacy of current and proposed PPE in mitigating BABT injury risk. Current human surrogate test platforms lack anatomical fidelity or instrumentation for capturing the dynamic transfer of energy during the event. Therefore, we have developed and tested a Human Surrogate Torso Model (HSTM) composed of biosimulants representing soft tissues and skeleton of the human torso. A matrix of pressure transducers were embedded in the soft tissue and a custom displacement sensor was mounted to the skeletal structure to measure sternum displacement. A series of non-penetrating, high energy ballistic tests were performed with the HSTM. Results indicate that both sternum displacement and internal localized pressure are sensitive to impact energy and location. These data provide a spatial and temporal comparison to the current standard (static clay measurements) and a method for evaluating the applicability of thoracic injury metrics, including the Viscous Criterion, for BABT. The HSTM provides an advanced, biomechanically relevant test platform for determining the thoracic response to dynamic loading events due to non-penetrating ballistic impacts.

The pathobiology of blast injuries and blast-induced neurotrauma as identified using a new experimental model of injury in mice.

Current experimental models of blast injuries used to study blast-induced neurotrauma (BINT) vary widely, which makes the comparison of the experimental results extremely challenging. Most of the blast injury models replicate the ideal Friedländer type of blast wave, without the capability to generate blast signatures with multiple shock fronts and refraction waves as seen in real-life conditions; this significantly reduces their clinical and military relevance. Here, we describe the pathophysiological consequences of graded blast injuries and BINT generated by a newly developed, highly controlled, and reproducible model using a modular, multi-chamber shock tube capable of tailoring pressure wave signatures and reproducing complex shock wave signatures seen in theater. While functional deficits due to blast exposure represent the principal health problem for today's warfighters, the majority of available blast models induces tissue destruction rather than mimic functional deficits. Thus, the main goal of our model is to reliably reproduce long-term neurological impairments caused by blast. Physiological parameters, functional (motor, cognitive, and behavioral) outcomes, and underlying molecular mechanisms involved in inflammation measured in the brain over the 30 day post-blast period showed this model is capable of reproducing major neurological changes of clinical BINT.

Assessing behind armor blunt trauma (BABT) under NIJ standard-0101.04 conditions using human torso models.

BACKGROUND: Although soft armor vests serve to prevent penetrating wounds and dissipate impact energy, the potential of nonpenetrating injury to the thorax, termed behind armor blunt trauma, does exist. Currently, the ballistic resistance of personal body armor is determined by impacting a soft armor vest over a clay backing and measuring the resulting clay deformation as specified in National Institute of Justice (NIJ) Standard-0101.04. This research effort evaluated the efficacy of a physical Human Surrogate Torso Model (HSTM) as a device for determining thoracic response when exposed to impact conditions specified in the NIJ Standard. METHODS: The HSTM was subjected to a series of ballistic impacts over the sternum and stomach. The pressure waves propagating through the torso were measured with sensors installed in the organs. A previously developed Human Torso Finite Element Model (HTFEM) was used to analyze the amount of tissue displacement during impact and compared with the amount of clay deformation predicted by a validated finite element model. All experiments and simulations were conducted at NIJ Standard test conditions. RESULTS: When normalized by the response at the lowest threat level (Level I), the clay deformations for the higher levels are relatively constant and range from 2.3 to 2.7 times that of the base threat level. However, the pressures in the HSTM increase with each test level and range from three to seven times greater than Level I depending on the organ. CONCLUSIONS: The results demonstrate the abilities of the HSTM to discriminate between threat levels, impact conditions, and impact locations. The HTFEM and HSTM are capable of realizing pressure and displacement differences because of the level of protection, surrounding tissue, and proximity to the impact point. The results of this research provide insight into the transfer of energy and pressure wave propagation during ballistic impacts using a physical surrogate and computational model of the human torso.

Assessing behind armor blunt trauma in accordance with the National Institute of Justice Standard for Personal Body Armor Protection using finite element modeling.

BACKGROUND: To assess the possibility of injury as a result of behind armor blunt trauma (BABT), a study was undertaken to determine the conditions necessary to produce the 44-mm clay deformation as set forth in the National Institute of Justice (NIJ) Standard 0101.04. These energy levels were then applied to a three-dimensional Human Torso Finite Element Model (HTFEM) with soft armor vest. An examination will be made of tissue stresses to determine the effects of the increased kinetic energy levels on the probability of injury. METHODS: A clay finite element model (CFEM) was created with a material model that required nonlinear properties for clay. To determine these properties empirically, the results from the CFEM were matched with experimental drop tests. A soft armor vest was modeled over the clay to create a vest over clay block finite element model (VCFEM) and empirical methods were again used to obtain material properties for the vest from experimental ballistic testing. Once the properties for the vest and clay had been obtained, the kinetic energy required to produce a 44-mm deformation in the VCFEM was determined through ballistic testing. The resulting kinetic energy was then used in the HTFEM to evaluate the probability of BABT. RESULTS: The VCFEM, with determined clay and vest material properties, was exercised with the equivalent of a 9-mm (8-gm) projectile at various impact velocities. The 44-mm clay deformation was produced with a velocity of 785 m/s. This impact condition (9-mm projectile at 785 m/s) was used in ballistic exercises of the HTFEM, which was modeled with high-strain rate human tissue properties for the organs. The impact zones were over the sternum anterior to T6 and over the liver. The principal stresses in both soft and hard tissue at both locations exceeded the tissue tensile strength. CONCLUSIONS: This study indicates that although NIJ standard 0101.04 may be a good guide to soft armor failure, it may not be as good a guide in BABT, especially at large projectile kinetic energies. Further studies, both numerical and experimental, are needed to assist in predicting injury using the NIJ standard.