Penetration Thresholds of Porcine Limbs for Low Sectional Density Projectiles in High-Rate Impact.

INTRODUCTION: With similar prevalence to injuries from fires, stings, and natural disasters, soft tissue injuries may occur from fireworks, industrial accidents, or other explosives. Surgeons are less familiar with treating high-velocity penetration from small debris, which may increase the chance of infection and subsequent fatality. Penetration risk curves have been developed to predict V50, the velocity with 50% probability of penetration, for various sized projectiles. However, there has been limited research using nonmetallic materials to achieve lower density projectiles less than 1 g cm-2, such as sand or rocks. MATERIAL AND METHODS: To emulate the size and density of these energized particles, 14 ball bearings of stainless steel, silicon nitride, or Delrin acetal plastic ranging from 1.59 mm (1/16") to 9.53 mm (3/8") with sectional densities between 0.3 g cm-2 and 5 g cm-2 were launched toward porcine legs at a range of velocities to determine the penetration thresholds. High-speed videography was captured laterally at 40 kHz and impact velocity was captured using a physics-based tracking software. A generalized linear model with repeated measures and a logit link function was used to predict probability of penetration for each projectile. A total of 600 impacts were conducted to achieve at least 15 penetrating impacts for each projectile over a range of velocities. RESULTS: Higher impact velocities were required to penetrate the skin as sectional density of the projectile decreased, and the relationship between velocity and sectional density exhibited an exponential relationship (V50, $ = 184.6*S{D^{ - 0.385}}$, R2 = 0.95) with substantial change for nonlinearity in sectional densities ranging from 0.3 g cm-2 to 1 g cm-2. Compared to previous studies, the empirical relationship was consistent in the linear region (2-5 g cm-2), and novel experimentation filled in the gaps for sectional densities less than 1 g cm-2, which expressed more nonlinearity than previously estimated. For low-density projectiles with diameters of 1.59 (1/16") or 3.18 (1/8"), 32 impacts were lodged into the epidermis but did not penetrate through the dermis; however, penetration was defined as displacement into or through the dermis. CONCLUSIONS: These experimental results may be used to develop and validate finite element simulations of low-density projectile impacts to address complex, multivariate loading conditions for the development of protective clothing to reduce wounding and subsequent infection rates.

In vitro skin puncture methodology for material characterization.

Quantifying the mechanical behavior of skin has been foundational in applications of cosmetics, surgical techniques, forensic science, and protective clothing development. However, previous puncture studies have lacked consistent and physiological boundary conditions of skin. To determine natural skin tension, excision of in situ porcine skin resulted in significantly different diameter reduction (shrinkage) in leg (19.5 %) and abdominal skin (38.4 %) compared to flank skin (28.5 %) (p = 0.047). To examine effects of initial tension and pre-conditioning, five conditions of initial tension (as percentage of diameter increase) and pre-conditioning were tested in quasistatic puncture with a 5 mm spherical impactor using an electrohydraulic load frame and custom clamping apparatus. Samples with less than 5 % initial tension resulted in significantly greater (p = 0.011) force at failure (279.2 N) compared to samples with greater than 25 % initial tension (195.1 N). Eight pre-conditioning cycles of 15 mm displacement reduced hysteresis by 45 %. The coefficient of variance was substantially reduced for force, force normalized by cutis thickness, displacement, stiffness, and strain energy up to 46 %. Pre-conditioned samples at physiological initial tension (14-25 %) resulted in significantly greater (p = 0.03) normalized forces at failure (278.3 N/mm) compared to non-conditioned samples of the same initial tension (234.4 N/mm). Pre-conditioned samples with 14-25 % initial tension, representing physiological boundary conditions, resulted in the most appropriate failure thresholds with the least variation. For in vitro puncture studies, the magnitude of applied initial tension should be defined based on anatomical location, through a shrinkage experimentation, to match natural tension of skin. Characterizing the biological behavior and tolerances of skin may be utilized in finite element models to aid in protective clothing development and forensic science analyses.

Surface wave analysis of the skin for penetrating and non-penetrating projectile impact in porcine legs.

Secondary blast injuries may result from high-velocity projectile fragments which ultimately increase medical costs, reduce active work time, and decrease quality of life. The role of skin penetration requires more investigation in energy absorption and surface mechanics for implementation in computational ballistic models. High-speed ballistic penetration studies have not considered penetrating and non-penetrating biomechanical properties of the skin, including radial wave displacement, resultant surface wave speed, or projectile material influence. A helium-pressurized launcher was used to accelerate 3/8″ (9.525 mm) diameter spherical projectiles toward seventeen whole porcine legs from seven pigs (39.53 ± 7.28 kg) at projectile velocities below and above V50. Projectiles included a mix of materials: stainless steel (n = 26), Si3N4 (n = 24), and acetal plastic (n = 24). Tracker video analysis software was used to determine projectile velocity at impact from the perpendicular view and motion of the tissue displacement wave from the in-line view. Average radial wave displacement and surface wave speed were calculated for each projectile material and categorized by penetrating or non-penetrating impacts. Two-sample t-tests determined that non-penetrating projectiles resulted in significantly faster surface wave speeds in porcine skin for stainless steel (p = 0.002), plastic (p = 0.004), and Si3N4 ball bearings (p = 0.014), while ANOVA determined significant differences in radial wave displacement and surface wave speed between projectile materials. Surface wave speed was used to quantify mechanical properties of the skin including elastic modulus, shear modulus, and bulk modulus during ballistic impact, which may be implemented to simulate accurate deformation behavior in computational impact models.

Repeated measures analysis of projectile penetration in porcine legs as a function of storage condition.

Buried blast explosions create small projectiles which can become lodged in the tissue of personnel as far away as hundreds of meters. Without appropriate treatment, these lodged projectiles can become a source of infection and prolonged injury to soldiers in modern combat. Human cadavers can be used as surrogates for living humans for ballistic penetration testing, but human cadavers are frozen during transport and storage. The process of freezing and thawing the tissue before testing may change the biomechanical properties of the tissue. The goal of the current study was to understand penetration threshold differences between fresh, refrigerated, and frozen tissue and investigate factors that may contribute to these differences. A custom-built pneumatic launcher was used to accelerate 3/16″ stainless steel ball bearings toward porcine legs that were either tested fresh, following refrigerated storage, or following frozen storage. A generalized linear mixed model, accounting for within-animal dependence, owing to repeated observations, was found to be the most appropriate for these data and was used for analysis. The "generalized" model accommodated non-continuous observations, provided a straight-forward way to implement the repeated measures, and provided a risk estimate for projectile penetration. Both storage condition (p = 0.48) and leg (p = 0.07) were shown to be not significant and the confidence intervals for those variables were overlapping. As all covariates were found to be non-significant, a single model containing all impacts was used to develop a V50, or velocity at which 50% of impacts are expected to penetrate. From this model, 50% probability of penetration occurs at 137.3 m/s with 95% confidence intervals at 132.0 and 144.0 m/s. In this study, the fresh legs and previously frozen legs allowed penetration at similar velocities indicating that previously frozen legs were acceptable surrogates for fresh legs. This study only compared the penetration threshold in tissues that had been stored in differing conditions. To truly study penetration, more conditions will need to be studied including the effects of projectile mass and material, the effects of projectile shape, and the effects of clothing or protective layers on penetration threshold.

Analysis of Force Mitigation by Boots in Axial Impacts using a Lower Leg Finite Element Model.

Lower extremity injuries caused by floor plate impacts through the axis of the lower leg are a major source of injury and disability for civilian and military vehicle occupants. A collection of PMHS pendulum impacts was revisited to obtain data for paired booted/unbooted test on the same leg. Five sets of paired pendulum impacts (10 experiments in total) were found using four lower legs from two PMHS. The PMHS size and age was representative of an average young adult male. In these tests, a PMHS leg was impacted by a 3.4 or 5.8 kg pendulum with an initial velocity of 5, 7, or 10 m/s (42-288 J). A matching LS-DYNA finite element model was developed to replicate the experiments and provide additional energy, strain, and stress data. Simulation results matched the PMHS data using peak values and CORA curve correlations. Experimental forces ranged between 1.9 and 12.1 kN experimentally and 2.0 and 11.7 kN in simulation. Combat boot usage reduced the peak force by 36% experimentally (32% in simulation) by compressing the sole and insole with similar mitigations for calcaneus strain. The simulated Von Mises stress contours showed the boot both mitigating and shifting stress concentrations from the calcaneus in unbooted impacts to the talus-tibia joint in the booted impacts, which may explain why some previous studies have observed shifts to tibia injuries with boot or padding usage.

A novel bridge wire model of blast traumatic brain injury - biomed 2013.

Research into the mechanics of blast-induced traumatic brain injury requires a device capable of reproducing pressures of the same magnitude and time scale as a blast wave. A blast simulator based on the exploding bridge wire mechanism was created with these capabilities. Peak blast pressures in the range of 5 – 29 psi were generated with a positive phase duration less than 20 µs. A series of experiments using 0.008 inch diameter wires (10-20 psi) were used to demonstrate the ability of the blast simulator to injure in vitro primary brain cell cultures at 1, 24, and 48 hours following blast. Blast exposure caused a rapid loss of cells which was significant over controls. Propidium iodide uptake indicated limited injury to cellular membranes but the cytoskeletal structure showed signs of degeneration 1 hour following blast. These results indicate that the bridge wire blast simulator can serve as a suitable in vitro model of blast injury.

Vibrational frequency response to impact loading of skull models.

More than 73% of soldiers returning from duty are injured by explosive devices. The shock waves generated are believed to cause injury via intracranial pressure and skull flexure. Prior modal analyses of spherical shells as skull substitutes using analytical solutions to the wave equation indicate the impact point and opposite side as areas of intense bending. In this study, finite element models extend modal analyses and applied impulse scenarios for a variety of altered spherical geometries. Holes of differing sizes, the direction of impact, and the presence of water inside were considered. The finite element model matched the analytical modal frequencies within 4%. The discrete modal frequencies are lost as the geometry deviates from the ideal sphere. The frequency response to impact was complex with many participating modal frequencies. The deformation near holes increased as the hole increased in size. Impacts in line with holes increased the minimum to maximum spread by 30% whereas angled impacts caused more pronounced motion near holes. Filling the sphere interior with liquid diverted some load from the shell and decreased the maximum deflections by 80%. Avenues of further research focused on more accurate geometries are discussed.

Evaluation of the Accuracy of NASS/CDS Delta-V Estimates from the Enhanced WinSmash Algorithm.

The National Automotive Sampling System / Crashworthiness Data System (NASS/CDS) uses the WinSmash program to reconstruct changes in vehicle velocity for real world crashes. Vehicle change in velocity, or delta-V, is a measure of crash severity and a predictor of injury risk. Earlier studies have demonstrated that WinSmash 2.42 underestimated the delta-V by 23% on average with the use of categorical stiffness values for vehicles identified as a source of error. An enhanced version of WinSmash, WinSmash 2008, was developed to employ vehicle specific stiffness values whenever possible. A total of 478 General Motors vehicles equipped with event data recorders (EDRs) and involved in real-world crashes were collected from years 2000 - 2008 of the NASS/CDS database and the delta-V was computed using the enhanced WinSmash. All vehicles were involved in frontal impacts. The enhanced reconstruction algorithm reduced the underestimation of delta-V from 23% to 13% on average for all vehicles. Delta-V estimates for cars only were greatly improved but still understated by 16% on average. Less than 5% error in delta-V was observed for pickup trucks and utility vehicles. The amount of structural overlap for the vehicle and investigator confidence in the reconstruction continued to have an effect on accuracy. No difference in average delta-V was observed when using either updated categorical stiffness values or vehicle specific stiffness values. The changes in WinSmash delta-Vs have important policy implications for NHTSA as the NASS/CDS delta-Vs are the basis for traffic and safety regulations as well as the speeds for vehicular crash testing and costs/benefits analyses.

Validation of an improved injury device for in vitro study of neural cell deformation - biomed 2009.

Traumatic Brain Injury is hypothesized to occur as a function of the strain and strain rate experienced by neural tissues during a traumatic event. In vitro studies of TBI at the cellular level have used a variety of methods to subject neural cell cultures to potentially injurious strains and strain rates. The Advanced Cell Deformation System (ACDS) has been developed which has the ability to independently control strain and strain rate and can strain cell cultures grown on a stretchable membrane from 0.1 to 0.60 at rates up to 25 s-1. The ability to control strain and strain rate independently or to simulate quick repetitive loading was not available in previous devices. Here we present the experiments testing the ability of the ACDS to replicate the results of in vitro experiments of neural cell deformation conducted by earlier researchers. This is a first step toward future experiments which will use the more advanced capabilities of the ACDS.

Influence of the missing vehicle and cdc only delta-v reconstruction algorithms for prediction of occupant injury risk - biomed 2009.

Delta-V, the change in velocity of a vehicle, is a widely used predictor of occupant injury in vehicle collisions. In real worldv crashes, delta-V is commonly estimated from measurements of vehicle deformation using absorbed energy based methods. The accuracy of these estimates is highly dependent on the availability of deformation measurements for both vehicles involved in a crash. Specialized algorithms have been developed for those cases in which complete information is not available from a crash (e.g. the missing vehicle algorithm) or has been estimated (e.g. the collision deformation classification, or CDC only algorithm). The objectives of this study are to evaluate (1) the accuracy of the missing vehicle and CDC only algorithms and (2) the influence of these algorithms upon estimates of occupant injury risk. The approach is to develop and critically evaluate occupant injury risk curves using the standard, missing, and CDC only reconstruction algorithms for 1899 real vehicles extracted from the National Automotive Sampling System / Crash Data System for 2006.

Evaluation of the accuracy of side impact crash test reconstructions - biomed 2009.

Each year in the U.S., vehicle side crashes result in over 6,000 fatalities. Delta-V, the vehicle change in velocity, is a widely used measure of crash injury risk in real world crashes. However, delta-V is difficult to estimate accurately for side crashes using reconstruction codes such as CRASH3. Such codes are the source of a large portion of the delta-V values in crash databases, so their accuracy has a direct impact on injury risk prediction data. In this study, delta-V was first reconstructed for a series of 42 staged side impact crash tests using CRASH3. This reconstructed delta-V was then compared to the delta-V recorded by the crash test instrumentation to determine the accuracy of the reconstructed value.

NASS/CDS delta-V estimates: the influence of enhancements to the WinSmash crash reconstruction code.

The change in velocity (delta-V) crash severity metric in the NASS/CDS (National Automotive Sampling System / Crashworthiness Data System) is computed using the WinSmash crash reconstruction code. Beginning in 2008, NASS/CDS investigators have started to use an enhanced version of WinSmash, WinSmash 2008, which features a comprehensive vehicle specific library for over 5000 vehicle make-model-year combinations and updated categorical stiffness values. The use of WinSmash 2008 is expected to greatly improve delta-V estimates. However, there is concern that this may result in a step change in the NASS/CDS delta-V estimates, making it difficult to compare NASS/CDS 2008 with earlier years. A total of 1,808 collisions were recomputed using data from NASS/CDS 2007. The new version of WinSmash shows improved accuracy, but still underpredicts delta-V. The use of WinSmash 2008 increased the delta-V by 7.9% or 1.9 kph on average. The changes in delta-V were not evenly distributed. Delta-V increases were larger for side impacts (8.3%) than for back impacts (5.3%). The calculation type had little effect on the delta-V changes. For vehicles, pickup trucks showed a small increase (3.3%) and utility vehicles increased the most (9.6%). This jump in delta-V may prevent the data from NASS/CDS 2008 and later from being readily aggregated with previous years.

Mechanical properties of polytetraflouroethylene elastomer membrane for dynamic cell culture testing.

A wide body of existing research on cellular injury has been conducted using cell cultures grown on flexible elastomer membranes deformed by a transient pressure pulse. However, there has been little published information on the material properties of these elastic membranes. In order to facilitate the development of a finite element model of cellular injury, the material properties of the underlying membrane must first be known. A series of static tests of an elastomer yielded a set of pressure-deflection data for applied pressures of 2.5, 5.0, 7.5, 10, 12 and 14 PSIG. Using an optimization technique, the material properties for an elastic finite element model were iteratively changed and compared to these experimental results in order to minimize the difference between experiment and simulation. The final material properties were found to be quite different from the initial guess, with a final modulus of 950,000 Pa, a Poisson's ratio of 0.499, and a density of 5.5*10-4 g/mm3. The comparison between the experimental and finite element models was conducted using a sum of squares difference for each of the six pressures, yielding an average sum of squares difference of 0.271 mm. The average percent error of the deflection measurements was 3.57%, with errors measured at each pressure ranging between 0% and 12%. Parameter sensitivity was examined and the most influential property was the modulus of elasticity. The least influential parameter was the density, having almost no effect on the maximum deflection.

Numerical simulation of a cellular-level experiment to induce traumatic injury to neurons.

Previous research has developed a pneumatically driven device for delivering a controlled mechanical insult to cultured neurons. The neuronal cell culture was injured by applying a transient air pulse to a culture well fitted with a highly elastic Silastic culture well bottom. In response to the pressure pulse, he Silastic culture well bottom deformed, stretched the attached cell culture, and resulted in observable cell injuries and death. The goal of this paper was to computationally model the spatial distribution of membrane strain, stress, and strain rate to which these cultures were subjected. The simulation results, using a finite element model of the culture well membrane, compared well with the results from the original experiments. When peak air pressure was varied from 69 kPa to 345 kPa (10 to 50 psig), numerical simulations showed that the corresponding membrane strains varied from 20 to 95% and the stress response varied from 0.5 to 1.2 MPa.