Frontal-Crash Occupant Protection in the Rear Seat: Submarining and Abdomen/Pelvis Response in Midsized Male Surrogates.

Frontal-crash sled tests were conducted to assess submarining protection and abdominal injury risk for midsized male occupants in the rear seat of modern vehicles. Twelve sled tests were conducted in four rear-seat vehicle-bucks with twelve post-mortem human surrogates (PMHS). Select kinematic responses and submarining incidence were compared to previously observed performance of the Hybrid III 50th-percentile male and THOR-50M ATDs (Anthropomorphic Test Devices) in matched sled tests conducted as part of a previous study. Abdominal pressure was measured in the PMHS near each ASIS (Anterior Superior Iliac Spine), in the inferior vena cava, and in the abdominal aorta. Damage to the abdomen, pelvis, and lumbar spine of the PMHS was also identified. In total, five PMHS underwent submarining. Four PMHS, none of which submarined, sustained pelvis fractures and represented the heaviest of the PMHS tested. Submarining of the PMHS occurred in two out of four vehicles. In the matched tests, the Hybrid III never underwent submarining while the THOR-50M submarined in three out of four vehicles. Submarining occurred in vehicles having both conventional and advanced (pretensioner and load limiter) restraints. The dominant factors associated with submarining were related to seat pan geometry. While the THOR-50M was not always an accurate tool for predicting submarining in the PMHS, the Hybrid III could not predict submarining at all. The results of this study identify substantive gaps in frontal-crash occupant protection in the rear seat for midsized males and elucidates the need for additional research for rear-seat occupant protection for all occupants.

Comparison of conjunctival pedicle flap to corneal fixation strength achieved by Tisseel® fibrin glue, ethyl cyanoacrylate adhesive, ReSure® hydrogel sealant, and conventional suturing with 8-0 VICRYL® ophthalmic suture.

OBJECTIVE: To determine and compare the fixation strength of conjunctival pedicle flaps to cornea achieved via conventional ophthalmic suture and three different adhesive compounds. ANIMALS STUDIED: Ex vivo porcine globes. PROCEDURES: Following a 6 mm wide 500-micron-restricted depth lamellar keratectomy, conjunctival pedicle flaps were secured to the keratectomy site with either 8-0 VICRYL® suture or one of three adhesive products, including Tisseel® bioadhesive, ReSure® synthetic adhesive, or ethyl cyanoacrylate adhesive (n = 10 per surgical group). Adhesive application protocol varied by product based upon adhesive biocompatibility. Corneoconjunctival tissues were then harvested, clamped in a tensile testing device, and loaded at a rate of 1 mm/s under video surveillance until the point of failure. Peak load was determined for each test and used to compare fixation strength between samples. RESULTS: Forty conjunctival flaps were performed, with 6 omitted from evaluation due to dehiscence prior to tensile testing. Of the 34 flaps analyzed, 10 were secured with suture, 10 with cyanoacrylate, 8 with ReSure®, and 6 with Tisseel®. Flaps secured with suture withstood significantly higher applied tensile force compared with cyanoacrylate (p = .02474), ReSure® (p = .00000), and Tisseel® (p = .00002). Flaps secured with cyanoacrylate withstood significantly greater force than those secured with ReSure® and Tisseel® (p = .01194 and 0.01798, respectively). There was no significant difference in fixation strength between ReSure® and Tisseel® glue (p = .95675). CONCLUSIONS: Conjunctival pedicle flap fixation using 8-0 VICRYL® suture fixation was able to withstand significantly greater maximum tensile force compared to ReSure®, Tisseel®, or cyanoacrylate adhesives. Fixation strength achieved with cyanoacrylate adhesive was significantly greater than that achieved with ReSure® or Tisseel®.

Effects of loading rate, age, and morphology on the material properties of human rib trabecular bone.

The material and morphometric properties of trabecular bone have been studied extensively in bones bearing significant weight, such as the appendicular long bones and spine. Less attention has been devoted to the ribs, where quantification of material properties is vital to understanding thoracic injury. The objective of this study was to quantify the compressive material properties of human rib trabecular bone and assess the effects of loading rate, age, and morphology on the material properties. Material properties were quantified via uniaxial compression tests performed on trabecular bone samples at two loading rates: 0.005 s-1 and 0.5 s-1. Morphometric parameters of each sample were quantified before testing using micro-computed tomography. Rib trabecular bone material properties were lower on average compared to trabecular bone from other anatomical locations. Morphometric parameters indicated an anisotropic structure with low connectivity and a sparser density of trabeculae in the rib compared to other locations. No significant differences in material properties were observed between the tested loading rates. Material properties were only significantly correlated with age at the 0.005 s-1 loading rate, and no morphometric parameter was significantly correlated with age. Trabecular separation and thickness were most strongly correlated with the material properties, indicating the sparser trabecular matrix likely contributed to the lower material property values compared to other sites. The novel trabecular bone material properties reported in this study can be used to improve the thoracic response and injury prediction of computational models.

Comparative biofidelity of the Hybrid III and THOR 50th male ATDs under three restraint conditions during frontal sled tests.

OBJECTIVE: The purpose of this study was to provide a whole-body biofidelity assessment of the Hybrid III (HIII) and THOR 50th percentile male anthropomorphic test devices (ATDs) during frontal sled tests, incorporating data from kinematics, chest deflection, and test buck reaction load cells. Additionally, the accuracy of the injury risk prediction capabilities for each ATD was evaluated against injuries observed in matched postmortem human surrogate (PMHS) tests. METHODS: Sled tests, designed to simulate a United States New Car Assessment Program (US-NCAP) frontal test, were conducted using the HIII, THOR, and 8 approximately 50th percentile male PMHS under 3 restraint conditions. The test buck was instrumented with load cells on the steering column, knee bolster supports, and foot supports. ATD and PMHS reaction force-time histories were quantitatively compared using the ISO/TS-18571 objective rating metric. Previously published biofidelity analyses of kinematic and chest deflection data from the same tests were combined with the reaction force analyses to perform an overall assessment of the comparative biofidelity of each ATD. Injury risk predictions from existing HIII and proposed THOR injury risk curves for the US-NCAP were compared to observed injuries. RESULTS: For the reaction forces, the HIII and THOR had similar levels of biofidelity on average, except for 2 locations. The HIII produced more biofidelic knee bolster support forces, and the THOR lap belt forces were more biofidelic. The comparative biofidelity of the ATDs also varied by body region. The THOR head response was more biofidelic, whereas the HIII thorax and lower extremity responses had higher biofidelity. When all body regions were pooled, the HIII was more biofidelic, but differences between ATDs were generally small. Both ATDs were able to predict the observed injuries, except for the HIII chest, HIII neck, and THOR neck, all of which underpredicted PMHS injury outcomes. CONCLUSIONS: This study revealed that biofidelity assessed through response time histories and accuracy of injury risk predictions do not always align. Specifically, the HIII had marginally better time history biofidelity, whereas the THOR had better injury prediction. However, not all THOR responses could be fully assessed, so more work is needed to assess the THOR in complex loading environments.

A comparison of rib cortical bone compressive and tensile material properties: Trends with age, sex, and loading rate.

The objectives of this study were to develop novel methods for quantifying human rib cortical bone material properties in compression and to compare the compressive material property data to existing tensile data for matched subjects. Cylindrical coupons were obtained from the rib cortical bone of 30 subjects (M = 19, F = 11) ranging from 18 to 95 years of age (Avg. = 48.5 ± 24.3). Two coupons were obtained from each subject. One coupon was tested in compression at 0.005 strain/s, while the other coupon was tested in compression at 0.5 strain/s. Load and displacement data were recorded so that the elastic modulus, yield stress, yield strain, ultimate stress, ultimate strain, elastic strain energy density (SED), plastic SED, and total SED could be calculated. All compressive material properties were significantly different between the two loading rates. An ANOVA revealed that sex alone had no significant effect on the compressive material properties. The interaction between sex and age was significant for some material properties, but this may have been a consequence of the lack of older females in the subject pool. None of the compressive material properties were significantly correlated with age, but were more correlated with sample density. This finding differed for the tensile material properties, which showed stronger correlations with age. When comparing between tension and compression, significant differences were observed for all material properties except for the total SED, once the effects of loading rate and age had been accounted for. This was the first study to quantify the material properties of human rib cortical bone in compression. The results of this study demonstrated that rib and thorax finite element models should consider the effects of loading rate, loading mode, and age when incorporating material properties published in the literature.

Subject-specific rib finite element models with material data derived from coupon tests under bending loading.

Rib fractures are common thoracic injuries in motor vehicle crashes. Several human finite element (FE) human models have been created to numerically assess thoracic injury risks. However, the accurate prediction of rib biomechanical response has shown to be challenging due to human variation and modeling approaches. The main objective of this study was to better understand the role of modeling approaches on the biomechanical response of human ribs in anterior-posterior bending. Since the development of subject specific rib models is a time-consuming process, the second objective of this study was to develop an accurate morphing approach to quickly generate high quality subject specific rib meshes. The exterior geometries and cortical-trabecular boundaries of five human 6th-level ribs were extracted from CT-images. One rib mesh was developed in a parametric fashion and the other four ribs were developed with an in-house morphing algorithm. The morphing algorithm automatically defined landmarks on both the periosteal and endosteal boundaries of the cortical layer, which were used to morph the template nodes to target geometries. Three different cortical bone material models were defined based on the stress-strain data obtained from subject-specific tensile coupon tests for each rib. Full rib anterior-posterior bending tests were simulated based on data recorded in testing. The results showed similar trends to test data with some sensitivity relative to the material modeling approach. Additionally, the FE models were substantially more resistant to failure, highlighting the need for better techniques to model rib fracture. Overall, the results of this work can be used to improve the biofidelity of human rib finite element models.

Stiffness of a type II external skeletal fixator and locking compression plate in a fracture gap model.

OBJECTIVE: To compare the stiffness of constructs fixed with a type II external skeletal fixator (ESF) or a 3.5-mm locking compression plate (LCP) in axial compression and bending with a fracture gap model. STUDY DESIGN: Quasi-static four-point bending and axial compression tests. SAMPLE POPULATION: Ten LCP and 10 ESF immobilizing epoxy cylinders with a 40-mm fracture gap. METHODS: Five constructs of each type were tested in nondestructive mediolateral (ML) four-point bending and then rotated and tested in nondestructive craniocaudal (CC) four-point bending. Five additional constructs of each type were tested in nondestructive axial compression. Stiffness was compared between loading modes by construct type and between construct types by loading mode. RESULTS: Type II ESF were stiffer than LCP in ML bending (difference, 1474 N/mm, P < .0001) and in axial compression (difference, 458 N/mm, P = .008) but not in CC bending (P = .1673). Type II ESF were stiffer in ML bending than in CC bending (difference, 999 N/m, P < .0001), while LCP were stiffer in CC bending than in ML bending (difference, 634 N/mm, P < .0001). CONCLUSION: Type II ESF generated stiffer constructs compared with LCP in ML bending and in axial compression without a difference in CC bending. External skeletal fixator and LCP bending stiffness varied by loading direction. CLINICAL SIGNIFICANCE: A type II ESF should be considered in a comminuted fracture requiring increased stability in ML and axial directions.

Effects of sex, age, and two loading rates on the tensile material properties of human rib cortical bone.

The objective of this study was to evaluate the effects of sex, loading rate, and age on the tensile material properties of human rib cortical bone over a wide range of subject demographics. Sixty-one (n = 61) subjects (M = 32, F = 29) ranging in age from 17 to 99 years of age (Avg. = 56.4 ±â€¯26.2 yrs) were used in this study. Two rectangular coupons of cutaneous rib cortical bone were obtained from each subject and milled into dog-bone coupons for testing. For each subject, one coupon was tested to failure in tension on a material testing system at a targeted strain rate of 0.005 strain/s, while the other coupon was tested at 0.5 strain/s. A reaction load cell was used to measure axial load, and an extensometer was used to measure displacement within the gage length of the coupon. Data were obtained from fifty-eight (n = 58) subjects at 0.005 strain/s and fifty-eight (n = 58) subjects at 0.5 strain/s, with fifty-five (n = 55) matched pairs. The elastic modulus, yield stress, yield strain, failure stress, failure strain, ultimate stress, elastic strain energy density (SED), plastic SED, and total SED were then calculated for each test. There were no significant differences in material properties between sexes and no significant interactions between age and sex. In regard to the differences in material properties with respect to loading rate, yield stress, yield strain, failure stress, ultimate stress, elastic SED, plastic SED, and total SED were significantly lower at 0.005 strain/s compared to 0.5 strain/s. Spearman correlation analyses showed that all material properties had significant negative correlations with age at 0.005 strain/s except modulus. At 0.5 strain/s, all material properties except yield strain had significant negative correlations with age. Although the results revealed that the material properties of human rib cortical bone varied significantly with respect to chronological age, the R2 values only ranged from 0.15 to 0.62, indicating that there may be other underlying variables that better account for the variance within a given population. This is the first study to analyze the effects of sex, loading rate, and age on tensile material properties of human rib cortical bone using a reasonably large sample size. Overall, the results of this study provide data that will allow FEMs to better model and assess differences in the material response of the rib cage for nearly all vehicle occupants of driving age.

Evaluation of Hybrid III and THOR-M neck kinetics and injury risk under various restraint conditions during full-scale frontal sled tests.

OBJECTIVE: The objective of this research was to compare the kinetics and predicted injury risks of the Hybrid III (HIII) and Test device for Human Occupant Restraint (THOR)-M necks during full-scale frontal sled tests under 3 safety restraint conditions: knee bolster (KB), knee bolster and steering wheel airbag (KB/SWAB), and knee bolster airbag and steering wheel airbag (KBAB/SWAB). METHODS: Twelve sled tests were performed for the HIII and THOR-M, and 8 matched sled tests were performed using postmortem human surrogates (PMHSs). The tests were designed to match the 2012 Toyota Camry New Car Assessment Program (NCAP) full-scale crash test. Upper and lower neck forces and moments were collected from the HIII and THOR-M load cells. Inverse dynamics was used to calculate PMHS upper neck forces and moments from acceleration data until the time of head contact. The PMHSs experienced head contact with the SWAB before appreciable neck loading occurred. Therefore, PMHS neck forces and moments were only compared to the HIII and THOR-M for the KB condition. Neck injury risks were calculated for the HIII and THOR-M and were compared to the injuries observed for the PMHSs. RESULTS: The HIII had greater upper and lower neck shear forces than the THOR-M, whereas both surrogates had similar upper and lower neck axial forces. The HIII also experienced greater peak upper neck bending moments than the THOR-M, which experienced negligible upper neck bending moments. Before head contact, the PMHSs experienced upper neck flexion, and the HIII experienced extension. The HIII and THOR-M injury risk curves predicted less than a 50% risk of an Abbreviated Injury Scale (AIS) 3+ injury. No AIS 3+ neck injuries were observed for the PMHS tests, but at least one AIS 2 injury was observed per condition. CONCLUSIONS: The results of this study showed that the HIII and THOR-M had different neck kinetics for these restraint conditions. In particular, the THOR-M experienced lower upper neck shear forces and bending moments. These differences are likely due to the very different neck designs of the anthropomorphic test dummies (ATDs), particularly the increased compliance of the THOR-M neck. Despite these differences, both ATDs still predicted a similar risk of AIS 3+ neck injury.

Effects of postmortem time and storage fluid on the material properties of bovine liver parenchyma in tension.

In motor vehicle collisions (MVCs), liver injuries are one of the most frequently reported types of abdominal organ trauma. Although finite element models are utilized to evaluate the risk of sustaining an abdominal organ injury in MVCs, these models must be validated based on biomechanical data in order to accurately assess injury risk. Given that previous studies that have quantified the tensile failure properties of human liver parenchyma have been limited to testing at 48 h postmortem, it is currently unknown how the material properties change between time of death and 48 h postmortem. Therefore, the objective of this study was to quantify the effects of postmortem degradation on the tensile material properties of bovine liver parenchyma with increasing postmortem time when stored in DMEM or saline. A total of 148 uniaxial tension tests were successfully conducted on parenchyma samples of fourteen bovine livers acquired immediately after death. Liver tissue was submerged in DMEM or saline and kept cool during sample preparation and storage. Twelve livers were stored as large blocks of tissue, while two livers were stored as small blocks and slices. Tension tests were performed on multiple dog-bone samples from each liver at three time points: ~6 h, ~24 h, and ~48 h postmortem. The data were then analyzed using a Linear Mixed Effect Model to determine if there were significant changes in the failure stress, failure strain, and modulus with respect to postmortem time. The results of the current study showed that the failure strain of bovine liver parenchyma decreased significantly between 6 h and 48 h after death when stored as large blocks in saline and refrigerated. Conversely, neither the failure stress nor failure strain changed significantly with respect to postmortem time when stored as large blocks in DMEM. The modulus did not change significantly with respect to postmortem time for tissue stored as large blocks in either saline or DMEM. Cellular disruption increased with postmortem time for tissue stored as large blocks, with tissue stored in saline showing the greatest increase at each time point. In addition, preliminary results indicated that reducing the tissue storage size had a negative effect on the material properties and cellular architecture. Overall, this study illustrated that the effects of postmortem liver degradation varied with respect to the preservation fluid, storage time, and storage block size.

Occupant kinematics of the Hybrid III, THOR-M, and postmortem human surrogates under various restraint conditions in full-scale frontal sled tests.

OBJECTIVE: The objective of this research was to compare the occupant kinematics of the Hybrid III (HIII), THOR-M, and postmortem human surrogates (PMHS) during full-scale frontal sled tests under 3 safety restraint conditions: knee bolster (KB), knee bolster and steering wheel airbag (KB/SWAB), and knee bolster airbag and steering wheel airbag (KBAB/SWAB). METHODS: A total of 20 frontal sled tests were performed with at least 2 tests performed per restraint condition per surrogate. The tests were designed to match the 2012 Toyota Camry New Car Assessment Program (NCAP) full-scale crash test. Rigid polyurethane foam surrogates with compressive strength ratings of 65 and 19 psi were used to simulate the KB and KBAB, respectively. The excursions of the head, shoulders, hips, knees, and ankles were collected using motion capture. Linear acceleration and angular velocity data were also collected from the head, thorax, and pelvis of each surrogate. Time histories were compared between surrogates and restraint conditions using ISO/TS 18571. RESULTS: All surrogates showed some degree of sensitivity to changes in restraint condition. For example, the use of a KBAB decreased the pelvis accelerations and the forward excursions of the knees and hips for all surrogates. However, these trends were not observed for the thorax, shoulders, and head, which showed more sensitivity to the presence of a SWAB. The average scores computed using ISO/TS 18571 for the HIII/PMHS and THOR-M/PMHS comparisons were 0.527 and 0.518, respectively. The HIII had slightly higher scores than the THOR-M for the excursions (HIII average = 0.574; THOR average = 0.520). However, the THOR-M had slightly higher scores for the accelerations and angular rates (HIII average = 0.471; THOR average = 0.516). CONCLUSIONS: The data from the current study showed that both KBABs and SWABs affected the kinematics of all surrogates during frontal sled tests. The results of the objective rating analysis indicated that the HIII and THOR-M had comparable overall biofidelity scores. The THOR-M slightly outperformed the HIII for the acceleration and angular velocity data. However, the HIII scored slightly better than the THOR-M for the excursion data. The most notable difference in biofidelity was for the knee excursions, where the HIII had a much higher average ISO score. Only the biofidelity of the HIII and THOR-M with regard to occupant kinematics was evaluated in this study; therefore, future work will evaluate the biofidelity of the ATDs in terms of lower extremity loading, thoracic response, and neck loading.

Assessment of Thoracic Response and Injury Risk Using the Hybrid III, THOR-M, and Post-Mortem Human Surrogates under Various Restraint Conditions in Full-Scale Frontal Sled Tests.

A total of 20 full-scale frontal sled tests were conducted using the Hybrid III (HIII), THOR-M and post-mortem human surrogates (PMHSs) to evaluate the thoracic biofidelity of the HIII and THOR-M under various belted restraint conditions. Each surrogate was tested under three belted restraint conditions: knee bolster, knee bolster and steering wheel airbag, and knee bolster airbag and steering wheel airbag. In order to assess the relative biofidelity of each ATD, external thoracic deflections were quantitatively compared between the ATDs and PMHSs using an objective rating metric. The HIII had slightly higher biofidelity than the THOR-M for the external thoracic deflections. Specifically, the THOR-M lower chest was more compliant compared to the other surrogates. However, the THOR-M exhibited expansion of the lower chest opposite belt loading, which was also observed to some degree in the PMHSs. The efficacy of the current injury risk prediction instrumentation and criteria were also evaluated for each surrogate. The THOR-M and its proposed injury risk criteria predicted the injuries observed in the PMHS tests better than the HIII. The PMHS injury criteria over-predicted the amount of chest deflection necessary to produce a severe injury and, consequently, under-predicted injury risk. The results of this study indicate that further testing should be performed to evaluate the biofidelity of the THOR-M thorax under more conditions. Furthermore, current thoracic injury risk criteria, which were developed using censored data, may not be effective at predicting injuries for all restraints and experimental conditions.

Neck forces and moments of human volunteers and post mortem human surrogates in low-speed frontal sled tests.

OBJECTIVE: The objective of this study was to quantify the effects of active muscles (e.g. conscious bracing, resting tone, and reflex response) and acceleration severity on the neck forces and moments generated during low-speed frontal sled tests with adult male human volunteers and post mortem human surrogates (PMHSs). METHODS: A total of 24 frontal sled tests were analyzed including male volunteers of approximately 50th percentile height and weight (n = 5) and PMHSs (n = 2). The tests were performed at two acceleration severities: low (∼2.5 g, Δv ≈ 5 kph) and medium (∼5.0 g, Δv ≈ 10 kph). Each volunteer was exposed to two impulses at each severity, one relaxed and one braced, while each PMHS was exposed to one impulse at each severity. Linear acceleration and angular velocity of the head were measured at a sampling rate of 20kHz, then filtered using SAE Channel Frequency Class 180 and 60, respectively, and transformed to the head center of gravity (CG). The location of the head CG, external auditory meatus, and occipital condyle (OC) were approximated using pretest photos and literature values. Neck forces (Fx and Fz) and sagittal plane moments (My) were calculated at the OC by applying the equations of dynamic equilibrium to the head. RESULTS: Peak Fx, Fz, and My increased significantly with increasing acceleration severity (p < 0.1). Minimal differences were observed between the magnitudes of the peak forces and moments for each subject type. Qualitatively, differences in the timing of peak neck forces and moments and the overall shape of the time histories were evident. Maximum Fx, Fz, and My occurred earliest in the event for the braced volunteers and latest for the PMHSs. However, these differences were not supported statistically for the volunteers (p > 0.05). The timing of neck loading was visibly augmented by the increased stiffness of the volunteer necks as a result of muscle activation. Although differences were observed between the volunteer muscle conditions, the volunteer subsets were more similar to each other than the PMHSs. CONCLUSIONS: This study examined the effects of active muscles, in the form of conscious and reflexive muscle activity, on the biomechanical response of occupants in low-speed frontal sled tests. Although active bracing did not result in significantly different peak neck loads or moments, the timing of these peak values were affected by muscle condition. The findings of this study provide insight to the kinetics experienced during low-speed sled tests and are important to consider when refining and validating computational models and ATDs used to assess injury risk in automotive collisions.

Non-censored rib fracture data during frontal PMHS sled tests.

OBJECTIVE: The purpose of this study was to obtain non-censored rib fracture data due to three-point belt loading during dynamic frontal post-mortem human surrogate (PMHS) sled tests. The PMHS responses were then compared to matched tests performed using the Hybrid-III 50(th) percentile male ATD. METHODS: Matched dynamic frontal sled tests were performed on two male PMHSs, which were approximately 50(th) percentile height and weight, and the Hybrid-III 50(th) percentile male ATD. The sled pulse was designed to match the vehicle acceleration of a standard sedan during a FMVSS-208 40 kph test. Each subject was restrained with a 4 kN load limiting, driver-side, three-point seatbelt. A 59-channel chestband, aligned at the nipple line, was used to quantify the chest contour, anterior-posterior sternum deflection, and maximum anterior-posterior chest deflection for all test subjects. The internal sternum deflection of the ATD was quantified with the sternum potentiometer. For the PMHS tests, a total of 23 single-axis strain gages were attached to the bony structures of the thorax, including the ribs, sternum, and clavicle. In order to create a non-censored data set, the time history of each strain gage was analyzed to determine the timing of each rib fracture and corresponding timing of each AIS level (AIS = 1, 2, 3, etc.) with respect to chest deflection. RESULTS: Peak sternum deflection for PMHS 1 and PMHS 2 were 48.7 mm (19.0%) and 36.7 mm (12.2%), respectively. The peak sternum deflection for the ATD was 20.8 mm when measured by the chest potentiometer and 34.4 mm (12.0%) when measured by the chestband. Although the measured ATD sternum deflections were found to be well below the current thoracic injury criterion (63 mm) specified for the ATD in FMVSS-208, both PMHSs sustained AIS 3+ thoracic injuries. For all subjects, the maximum chest deflection measured by the chestband occurred to the right of the sternum and was found to be 83.0 mm (36.0%) for PMHS 1, 60.6 mm (23.9%) for PMHS 2, and 56.3 mm (20.0%) for the ATD. The non-censored rib fracture data in the current study (n = 2 PMHS) in conjunction with the non-censored rib fracture data from two previous table-top studies (n = 4 PMHS) show that AIS 3+ injury timing occurs prior to peak sternum compression, prior to peak maximum chest compression, and at lower compressions than might be suggested by current PMHS thoracic injury criteria developed using censored rib fracture data. In addition, the maximum chest deflection results showed a more reasonable correlation between deflection, rib fracture timing, and injury severity than sternum deflection. CONCLUSIONS: Overall, these data provide compelling empirical evidence that suggests a more conservative thoracic injury criterion could potentially be developed based on non-censored rib fracture data with additional testing performed over a wider range of subjects and loading conditions.

Comparison of ATD to PMHS Response in the Under-Body Blast Environment.

A blast buck (Accelerative Loading Fixture, or ALF) was developed for studying underbody blast events in a laboratory-like setting. It was designed to provide a high-magnitude, high-rate, vertical loading environment for cadaver and dummy testing. It consists of a platform with a reinforcing cage that supports adjustable-height rigid seats for two crew positions. The platform has a heavy frame with a deformable floor insert. Fourteen tests were conducted using fourteen PMHS (post mortem human surrogates) and the Hybrid III ATD (Anthropomorphic Test Device). Tests were conducted at two charge levels: enhanced and mild. The surrogates were tested with and without PPE (Personal Protective Equipment), and in two different postures: nominal (knee angle of 90°) and obtuse (knee angle of 120°). The ALF reproduces damage in the PMHS commensurate with injuries experienced in theater, with the most common damage being to the pelvis and ankle. Load is transmitted through the surrogates in a caudal-to-cranial sequential fashion. Damage to the PMHS lower extremities begins within 2 ms after the initiation of foot/floor motion. The Hybrid III cannot assume the posture of the PMHS in rigid seats and exhibits a stiffer overall response compared to the PMHS. The ATD does not mimic the kinematic response of the PMHS lower extremities. Further, the Hybrid III does not have the capability to predict the potential for injury in the high-rate, vertical loading environment. A new ATD dedicated to under-body blast is needed to assist in the effort to mitigate injuries sustained by the mounted soldier.

Finite element comparison of human and Hybrid III responses in a frontal impact.

The improvement of finite element (FE) Human Body Models (HBMs) has made them valuable tools for investigating restraint interactions compared to anthropomorphic test devices (ATDs). The objective of this study was to evaluate the effect of various combinations of safety restraint systems on the sensitivity of thoracic injury criteria using matched ATD and Human Body Model (HBM) simulations at two crash severities. A total of seven (7) variables were investigated: 3-point belt with two (2) load limits, frontal airbag, knee bolster airbag, a buckle pretensioner, and two (2) delta-v's - 40kph and 50kph. Twenty four (24) simulations were conducted for the Hybrid III ATD FE model and repeated with a validated HBM for 48 total simulations. Metrics tested in these conditions included sternum deflection, chest acceleration, chest excursion, Viscous Criteria (V*C) criteria, pelvis acceleration, pelvis excursion, and femur forces. Additionally, chest band deflection and rib strain distribution were measured in the HBM for additional restraint condition discrimination. The addition of a frontal airbag had the largest effect on the occupant chest metrics with an increase in chest compression and acceleration but a decrease in excursion. While the THUMS and Hybrid III occupants demonstrated the same trend in the chest compression measurements, there were conflicting results in the V*C, acceleration, and displacement metrics. Similarly, the knee bolster airbag had the largest effect on the pelvis with a decrease in acceleration and excursion. With a knee bolster airbag the simulated occupants gave conflicting results, the THUMS had a decrease in femur force and the ATD had an increase. Preferential use of dummies or HBM's is not debated; however, this study highlights the ability of HBM metrics to capture additional chest response metrics.

Effects of Filtering on Experimental Blast Overpressure Measurements.

When access to live-fire test facilities is limited, experimental studies of blast-related injuries necessitate the use of a shock tube or Advanced Blast Simulator (ABS) to mimic free-field blast overpressure. However, modeling blast overpressure in a laboratory setting potentially introduces experimental artifacts in measured responses. Due to the high sampling rates required to capture a blast overpressure event, proximity to alternating current (AC-powered electronics) and poorly strain-relieved or unshielded wires can result in artifacts in the recorded overpressure trace. Data in this study were collected for tests conducted on an empty ABS (“Empty Tube”) using high frequency pressure sensors specifically designed for blast loading rates (n=5). Additionally, intraocular overpressure data (“IOP”) were collected for porcine eyes potted inside synthetic orbits located inside the ABS using an unshielded miniature pressure sensor (n=3). All tests were conducted at a 30 psi static overpressure level. A 4th order phaseless low pass Butterworth software filter was applied to the data. Various cutoff frequencies were examined to determine if the raw shock wave parameters values could be preserved while eliminating noise and artifacts. A Fast Fourier Transform (FFT) was applied to each test to examine the frequency spectra of the raw and filtered signals. Shock wave parameters (time of arrival, peak overpressure, positive duration, and positive impulse) were quantified using a custom MATLAB® script. Lower cutoff frequencies attenuated the raw signal, effectively decreasing the peak overpressure and increasing the positive duration. Rise time was not preserved the filtered data. A CFC 6000 filter preserved the remaining shock wave parameters within ±2.5% of the average raw values for the Empty Tube test data. A CFC 7000 filter removed experimental high-frequency artifacts and preserved the remaining shock wave parameters within ±2.5% of the average raw values for test IOP test data. Though the region of interest of the signals examined in the current study did not contain extremely high frequency content, it is possible that live-fire testing may produce shock waves with higher frequency content. While post-processing filtering can remove experimental artifacts, special care should be taken to minimize or eliminate the possibility of recording these artifacts in the first place.

Modeling the biomechanical and injury response of human liver parenchyma under tensile loading.

The rapid advancement in computational power has made human finite element (FE) models one of the most efficient tools for assessing the risk of abdominal injuries in a crash event. In this study, specimen-specific FE models were employed to quantify material and failure properties of human liver parenchyma using a FE optimization approach. Uniaxial tensile tests were performed on 34 parenchyma coupon specimens prepared from two fresh human livers. Each specimen was tested to failure at one of four loading rates (0.01s(-1), 0.1s(-1), 1s(-1), and 10s(-1)) to investigate the effects of rate dependency on the biomechanical and failure response of liver parenchyma. Each test was simulated by prescribing the end displacements of specimen-specific FE models based on the corresponding test data. The parameters of a first-order Ogden material model were identified for each specimen by a FE optimization approach while simulating the pre-tear loading region. The mean material model parameters were then determined for each loading rate from the characteristic averages of the stress-strain curves, and a stochastic optimization approach was utilized to determine the standard deviations of the material model parameters. A hyperelastic material model using a tabulated formulation for rate effects showed good predictions in terms of tensile material properties of human liver parenchyma. Furthermore, the tissue tearing was numerically simulated using a cohesive zone modeling (CZM) approach. A layer of cohesive elements was added at the failure location, and the CZM parameters were identified by fitting the post-tear force-time history recorded in each test. The results show that the proposed approach is able to capture both the biomechanical and failure response, and accurately model the overall force-deflection response of liver parenchyma over a large range of tensile loadings rates.

Membrane characteristics for biological blast overpressure testing using blast simulators.

Blast simulators often use passive-rupture membranes to generate shock waves similar to free-field blasts. The purpose of this study was to compare rupture patterns and pressure traces of three distinct membrane materials for biological and biomechanical blast studies. An Advanced Blast Simulator (ABS) located at the Center for Injury Biomechanics at Virginia Tech was used to test membrane characteristics. Acetate, Mylar, and aluminum sheets with different thicknesses were used to obtain pressures between 70–210 kPa. Static pressure was measured inside the tube at the test section using piezoelectric pressure sensors. Peak overpressure, positive duration, and positive impulse were calculated for each test. Rupture patterns and characteristic pressure traces were unique to each membrane type and thickness. Shock wave speed ranged between 1.2-1.8 Mach for static overpressures of 70–210 kPa. Acetate membranes fragmented sending pieces down the tube, but produced ideal (Friedlander) pressure traces. Mylar membranes bulged without fragmenting, but produced less-than-ideal pressure traces. Aluminum membranes did not fragment and produced ideal pressure traces. However, the cost of manufacturing and characterizing aluminum membranes should be considered during membrane selection. This study illustrates the advantages and disadvantages of using Mylar, acetate, and aluminum for passive rupture membranes for blast simulators.