Validation of a computational biomechanical mouse brain model for rotational head acceleration.

Recent mouse brain injury experiments examine diffuse axonal injury resulting from accelerative head rotations. Evaluating brain deformation during these events would provide valuable information on tissue level thresholds for brain injury, but there are many challenges to imaging the brain's mechanical response during dynamic loading events, such as a blunt head impact. To address this shortcoming, we present an experimentally validated computational biomechanics model of the mouse brain that predicts tissue deformation, given the motion of the mouse head during laboratory experiments. First, we developed a finite element model of the mouse brain that computes tissue strains, given the same head rotations as previously conducted in situ hemicephalic mouse brain experiments. Second, we calibrated the model using a single brain segment, and then validated the model based on the spatial and temporal strain responses of other regions. The result is a computational tool that will provide researchers with the ability to predict brain tissue strains that occur during mouse laboratory experiments, and to link the experiments to the resulting neuropathology, such as diffuse axonal injury.

Dynamic strain fields of the mouse brain during rotation.

Mouse models are used to better understand brain injury mechanisms in humans, yet there is a limited understanding of biomechanical relevance, beginning with how the murine brain deforms when the head undergoes rapid rotation from blunt impact. This problem makes it difficult to translate some aspects of diffuse axonal injury from mouse to human. To address this gap, we present the two-dimensional strain field of the mouse brain undergoing dynamic rotation in the sagittal plane. Using a high-speed camera with digital image correlation measurements of the exposed mid-sagittal brain surface, we found that pure rotations (no direct impact to the skull) of 100-200 rad/s are capable of producing complex strain fields that evolve over time with respect to rotational acceleration and deceleration. At the highest rotational velocity tested, the largest tensile strains (≥ 21% elongation) in selected regions of the mouse brain approach strain thresholds previously associated with axonal injury in prior work. These findings provide a benchmark to validate the mechanical response in biomechanical computational models predicting diffuse axonal injury, but much work remains in correlating tissue deformation patterns from computational models with underlying neuropathology.

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.

An Improved Method for Developing Injury Risk Curves Using the Brier Metric Score.

Many injury metrics are routinely proposed from measured or derived quantities from biomechanical experiments using post mortem human subjects (PMHS). The existing literature did not provide guidance on deciding between parameters collected in an experiment that would be best to use for the development of human injury probability curves (HIPC). The objective of this study was to use the Brier Metric Score (BMS) to identify the most appropriate metric from an experiment that predicts injury outcomes. The Brier Metric Score assesses how well a metric predicts the outcome for a censored data point (a lower BMS is better). Survival analysis was then conducted with the selected metric and the best distribution was selected using Akaike information criterion (AIC). Confidence intervals (CIs) and the normalized confidence interval width (NCIS) were calculated for the injury probability curve. The testing and validation of the methods described were performed using biomechanics data in the open literature. The methods for the HIPC development procedure detailed herein have been rigorously tested and used in the generation of WIAMan HIPCs and Injury Assessment Reference Curves (IARCs) for the WIAMan ATD, but can also be used in other ATD or PMHS injury risk curve development.

Combat Helmet Suspension System Stiffness Influences Linear Head Acceleration and White Matter Tissue Strains: Implications for Future Helmet Design.

Combat helmets are expected to protect the warfighter from a variety of blunt, blast, and ballistic threats. Their blunt impact performance is evaluated by measuring linear headform acceleration in drop tower tests, which may be indicative of skull fracture, but not necessarily brain injury. The current study leverages a blunt impact biomechanics model consisting of a head, neck, and helmet with a suspension system to predict how pad stiffness affects both (1) linear acceleration alone and (2) brain tissue response induced by both linear and rotational acceleration. The approach leverages diffusion tensor imaging information to estimate how pad stiffness influences white matter tissue strains, which may be representative of diffuse axonal injury. Simulation results demonstrate that a softer pad material reduces linear head accelerations for mild and moderate impact velocities, but a stiffer pad design minimizes linear head accelerations at high velocities. Conversely, white matter tract-oriented strains were found to be smallest with the softer pads at the severe impact velocity. The results demonstrate that the current helmet blunt impact testing standards' standalone measurement of linear acceleration does not always convey how the brain tissue responds to changes in helmet design. Consequently, future helmet testing should consider the brain's mechanical response when evaluating new designs.

Injury Risk Curves for the Human Cervical Spine from Inferior-to-Superior Loading.

Cervical spine injuries can occur in military scenarios from events such as underbody blast events. Such scenarios impart inferior-to-superior loads to the spine. The objective of this study is to develop human injury risk curves (IRCs) under this loading mode using Post Mortem Human Surrogates (PMHS). Twenty-five PMHS head-neck complexes were obtained, screened for pre-existing trauma, bone densities were determined, pre-tests radiological images were taken, fixed in polymethylmethacrylate at the T2-T3 level, a load cell was attached to the distal end of the preparation, positioned end on custom vertical accelerator device based on the military-seating posture, donned with a combat helmet, and impacted at the base. Posttest images were obtained, and gross dissection was done to confirm injuries to all specimens. Axial and resultant forces at the cervico-thoracic joint was used to develop the IRCs using survival analysis. Data were censored into left, interval, and uncensored observations. The Brier score metric was used to rank the variables. The optimal metric describing the underlying response to injury was associated with the axial force, ranking slightly greater than the resultant force, both with BMD covariates. The results from the survival analysis indicated all IRCs are in the "fair" to "good" category, at all risk levels. The BMD was found to be a significant covariate that best describes the response of the helmeted head-neck specimens to injury. The present experimental protocol and IRCs can be used to conduct additional tests, matched-pair tests with the WIAMan and/or other devices to obtain injury assessment risk curves (IARCs) and injury assessment risk values (IARVs) to predict injury in crash environments, and these data can also be used for validating component-based head-neck and human body computational models.

Role of age and injury mechanism on cervical spine injury tolerance from head contact loading.

OBJECTIVE: The objective of this study was to determine the influence of age and injury mechanism on cervical spine tolerance to injury from head contact loading using survival analysis. METHODS: This study analyzed data from previously conducted experiments using post mortem human subjects (PMHS). Group A tests used the upright intact head-cervical column experimental model. The inferior end of the specimen was fixed, the head was balanced by a mechanical system, and natural lordosis was removed. Specimens were placed on a testing device via a load cell. The piston applied loading at the vertex region. Spinal injuries were identified using medical images. Group B tests used the inverted head-cervical column experimental model. In one study, head-T1 specimens were fixed distally, and C7-T1 joints were oriented anteriorly, preserving lordosis. Torso mass of 16 kg was added to the specimen. In another inverted head-cervical column study, occiput-T2 columns were obtained, an artificial head was attached, T1-T2 was fixed, C4-C5 disc was maintained horizontal in the lordosis posture, and C7-T1 was unconstrained. The specimens were attached to the drop test carriage carrying a torso mass of 15 kg. A load cell at the inferior end measured neck loads in both studies. Axial neck force and age were used as the primary response variable and covariate to derive injury probability curves using survival analysis. RESULTS: Group A tests showed that age is a significant (P < .05) and negative covariate; that is, increasing age resulted in decreasing force for the same risk. Injuries were mainly vertebral body fractures and concentrated at one level, mid-to-lower cervical spine, and were attributed to compression-related mechanisms. However, age was not a significant covariate for the combined data from group B tests. Both group B tests produced many soft tissue injuries, at all levels, from C1 to T1. The injury mechanism was attributed to mainly extension. Multiple and noncontiguous injuries occurred. Injury probability curves, ±95% confidence intervals, and normalized confidence interval sizes representing the quality of the mean curve are given for different data sets. CONCLUSIONS: For compression-related injuries, specimen age should be used as a covariate or individual specimen data may be prescaled to derive risk curves. For distraction- or extension-related injuries, however, specimen age need not be used as a covariate in the statistical analysis. The findings from these tests and survival analysis indicate that the age factor modulates human cervical spine tolerance to impact injury.

Foot-ankle complex injury risk curves using calcaneus bone mineral density data.

OBJECTIVE: Biomechanical data from post mortem human subject (PMHS) experiments are used to derive human injury probability curves and develop injury criteria. This process has been used in previous and current automotive crashworthiness studies, Federal safety standards, and dummy design and development. Human bone strength decreases as the individuals reach their elderly age. Injury risk curves using the primary predictor variable (e.g., force) should therefore account for such strength reduction when the test data are collected from PMHS specimens of different ages (age at the time of death). This demographic variable is meant to be a surrogate for fracture, often representing bone strength as other parameters have not been routinely gathered in previous experiments. However, bone mineral densities (BMD) can be gathered from tested specimens (presented in this manuscript). The objective of this study is to investigate different approaches of accounting for BMD in the development of human injury risk curves. METHODS: Using simulated underbody blast (UBB) loading experiments conducted with the PMHS lower leg-foot-ankle complexes, a comparison is made between the two methods: treating BMD as a covariate and pre-scaling test data based on BMD. Twelve PMHS lower leg-foot-ankle specimens were subjected to UBB loads. Calcaneus BMD was obtained from quantitative computed tomography (QCT) images. Fracture forces were recorded using a load cell. They were treated as uncensored data in the survival analysis model which used the Weibull distribution in both methods. The width of the normalized confidence interval (NCIS) was obtained using the mean and ± 95% confidence limit curves. PRINCIPAL RESULTS: The mean peak forces of 3.9kN and 8.6kN were associated with the 5% and 50% probability of injury for the covariate method of deriving the risk curve for the reference age of 45 years. The mean forces of 5.4 kN and 9.2kN were associated with the 5% and 50% probability of injury for the pre-scaled method. The NCIS magnitudes were greater in the covariate-based risk curves (0.52-1.00) than in the risk curves based on the pre-scaled method (0.24-0.66). The pre-scaling method resulted in a generally greater injury force and a tighter injury risk curve confidence interval. Although not directly applicable to the foot-ankle fractures, when compared with the use of spine BMD from QCT scans to pre-scale the force, the calcaneus BMD scaled data produced greater force at the same risk level in general. CONCLUSIONS: Pre-scaling the force data using BMD is an alternate, and likely a more accurate, method instead of using covariate to account for the age-related bone strength change in deriving risk curves from biomechanical experiments using PMHS. Because of the proximity of the calcaneus bone to the impacting load, it is suggested to use and determine the BMD of the foot-ankle bone in future UBB and other loading conditions to derive human injury probability curves for the foot-ankle complex.

Male and Female Cervical Spine Biomechanics and Anatomy: Implication for Scaling Injury Criteria.

There is an increased need to develop female-specific injury criteria and anthropomorphic test devices (dummies) for military and automotive environments, especially as women take occupational roles traditionally reserved for men. Although some exhaustive reviews on the biomechanics and injuries of the human spine have appeared in clinical and bioengineering literatures, focus has been largely ignored on the difference between male and female cervical spine responses and characteristics. Current neck injury criteria for automotive dummies for assessing crashworthiness and occupant safety are obtained from animal and human cadaver experiments, computational modeling, and human volunteer studies. They are also used in the military. Since the average human female spines are smaller than average male spines, metrics specific to the female population may be derived using simple geometric scaling, based on the assumption that male and female spines are geometrically scalable. However, as described in this technical brief, studies have shown that the biomechanical responses between males and females do not obey strict geometric similitude. Anatomical differences in terms of the structural component geometry are also different between the two cervical spines. Postural, physiological, and motion responses under automotive scenarios are also different. This technical brief, focused on such nonuniform differences, underscores the need to conduct female spine-specific evaluations/experiments to derive injury criteria for this important group of the population.

Human Foot-Ankle Injuries and Associated Risk Curves from Under Body Blast Loading Conditions.

Under body blast (UBB) loading to military transport vehicles is known to cause foot-ankle fractures to occupants due to energy transfer from the vehicle floor to the feet of the soldier. The soldier posture, the proximity of the event with respect to the soldier, the personal protective equipment (PPE) and age/sex of the soldier are some variables that can influence injury severity and injury patterns. Recently conducted experiments to simulate the loading environment to the human foot/ankle in UBB events (~5ms rise time) with variables such as posture, age and PPE were used for the current study. The objective of this study was to determine statistically if these variables affected the primary injury predictors, and develop injury risk curves. Fifty belowknee post mortem human surrogate (PMHS) legs were used for statistical analysis. Injuries to specimens involved isolated and multiple fractures of varying severity. The Sanders classification was used to grade calcaneus severity and the AO/OTA classification for distal tibia fracture. Injury risk curves were developed using survival regression analysis and covariates were included whenever statistically significant (p<0.05). With peak force as the injury predictor and age and boot as covariates, the model was statistically significant. However, boot use changed the pattern of injury from predominately calcaneus to predominantly tibia. Also, a severity based risk curve showed tolerance differences between calcaneus (minor/major) and tibia (severity-I/ severity- II) injuries. The tibia demonstrated higher tolerance as compared to either minor or major calcaneus injury. These findings can play a vital role in development of safety systems to mitigate injuries to the occupant.

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.

Deriving injury risk curves using survival analysis from biomechanical experiments.

Injury risk curves from biomechanical experimental data analysis are used in automotive studies to improve crashworthiness and advance occupant safety. Metrics such as acceleration and deflection coupled with outcomes such as fractures and anatomical disruptions from impact tests are used in simple binary regression models. As an improvement, the International Standards Organization suggested a different approach. It was based on survival analysis. While probability curves for side-impact-induced thorax and abdominal injuries and frontal impact-induced foot-ankle-leg injuries are developed using this approach, deficiencies are apparent. The objective of this study is to present an improved, robust and generalizable methodology in an attempt to resolve these issues. It includes: (a) statistical identification of the most appropriate independent variable (metric) from a pool of candidate metrics, measured and or derived during experimentation and analysis processes, based on the highest area under the receiver operator curve, (b) quantitative determination of the most optimal probability distribution based on the lowest Akaike information criterion, (c) supplementing the qualitative/visual inspection method for comparing the selected distribution with a non-parametric distribution with objective measures, (d) identification of overly influential observations using different methods, and (e) estimation of confidence intervals using techniques more appropriate to the underlying survival statistical model. These clear and quantified details can be easily implemented with commercial/open source packages. They can be used in retrospective analysis and prospective design of experiments, and in applications to different loading scenarios such as underbody blast events. The feasibility of the methodology is demonstrated using post mortem human subject experiments and 24 metrics associated with thoracic/abdominal injuries in side-impacts.

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.

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.

Full-field bulge test for planar anisotropic tissues: part I--experimental methods applied to human skin tissue.

The nonlinear anisotropic properties of human skin tissue were investigated using bulge testing. Full-field displacement data were obtained during testing of human skin tissues procured from the lower back of post-mortem human subjects using 3-D digital image correlation. To measure anisotropy, the dominant fiber direction of the tissue was determined from the deformed geometry of the specimen. Local strains and stress resultants were calculated along both the dominant fiber direction and the perpendicular direction. Variation in anisotropy and stiffness was observed between specimens. The use of stress resultants rather than the membrane stress approximation accounted for bending effects, which are significant for a thick nonlinear tissue. Of the six specimens tested, it was observed that specimens from older donors exhibited a stiffer and more isotropic response than those from younger donors. It was seen that the mechanical response of the tissue was negligibly impacted by preconditioning or the ambient humidity. The methods presented in this work for skin tissue are sufficiently general to be applied to other planar tissues, such as pericardium, gastrointestinal tissue, and fetal membranes. The stress resultant-stretch relations will be used in a companion paper to obtain material parameters for a nonlinear anisotropic hyperelastic model.

Full-field bulge test for planar anisotropic tissues: part II--a thin shell method for determining material parameters and comparison of two distributed fiber modeling approaches.

A thin shell method is presented to analyze the results of the bulge test presented in Part I of this paper. The method accounts for the effects of bending, which can be significant for thick tissues inflated from a planar state. We fit two commonly used hyperelastic distributed fiber constitutive models to the stretch-stress resultant data for human skin tissue calculated in Part I from the measured inflation pressure and deformed geometry of the tissue. To validate the method, the resulting parameters were implemented in a specimen-specific finite-element analysis. The method was capable of reproducing the experimentally measured pressure-stretch response of the tissue for a fully integrated distributed fiber model, but not for the pre-integrated distributed fiber models. The parameters obtained for the pre-integrated models significantly underestimated the anisotropic properties of the tissue. The thin shell method presented in this work has been applied to human skin tissues but is sufficiently general to be applied to analyze the inflation response of other planar tissues.

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.