R. Delille

Experimental study of the expansion dynamic of 9 mm Parabellum hollow point projectiles in ballistic gelatin

We study in this paper the expanding behaviour of hollow point 9 mm Parabellum projectiles (Hornady XTP(®) and Speer Gold Dot(®)). We defined a deformation rate that takes into account both the diameter increase and the length reduction. We plotted the behaviour of this parameter versus impact velocity (we refer to this curve as the expanding law). This expanding law has been plotted for different gelatin weight ratios and different gelatin block lengths. We completed our experiments with a set of high speed movies in order to correlate the deceleration to the state of expansion and size of the temporary cavity. Our results pointed out that full expansion is reached shortly after the projectile fully penetrates the gelatin. This result shows that the key point to accurately simulate human body interaction with a hollow point projectile is to accurately simulate the interface (skin, skull, clothes thoracic walls). Simulating accurately organs is only an issue if a quantitative comparison between penetration depths is required, but not if we only focus on the state of expansion of the projectile. By varying the gelatin parameters, we discovered that the expanding law exhibits a velocity threshold below which no expansion occurs, followed by a rather linear curve. The parameters of that expanding law (velocity threshold and line slope) vary with the gelatin parameters, but our quantitative results demonstrate that these parameters are not extremely critical. Finally, our experiments demonstrate that the knowledge of the expansion law can be a useful tool to investigate a gunshot in a human body with a semi-jacketed projectile, giving an estimation of the impact velocity and thus the shooting distance.

Skull wounds linked with blunt trauma (hammer example). A report of two depressed skull fractures – Elements of biomechanical explanation

The lesions of the skull following perforating traumas can create complex fractures. The blunt traumas can, according to the swiftness and the shape of the object used, create a depressed fracture. The authors describe through two clinical cases the lesional characteristic of the blunt traumas, perforating the skull using a hammer. In both cases the cranial lesions were very typical: they were geometrical, square shaped, of the same size than the tool (head and tip of the hammer). On the outer table of the skull, the edges of the wounds were sharp and regular. On the inner table, the edges of the wounds were beveled and irregular. The bony penetration in the depressed fracture results from a rupture of the outer table of the bone under tension, in periphery, by the bend of the bone to the impact (outbending) and then, from the inner table with comminuted bony fragmentation. Breeding on the fractures of the size and the shape of the blunt objects used is inconstant and differs, that it is the objects of flat surface or wide in opposition to those of small surface area. Fractures morphologies depend on one hand on these extrinsic factors and on the other hand, of intrinsic factors (structure of the bone). To identify them, we had previously conducted experimental work on cranial bone samples. The bone was submitted to a device for three-point bending. This work had shown properties of thickness and stiffness of the various areas of the vault. Our cases are consistent with these results and illustrate the variability of bone lesions according to region and mode of use of blunt weapons. Many studies have identified criteria for identification of the weapons and the assistance of digital and biomechanical models will be an invaluable contribution with this aim in the future.

Geometrical and material parameters to assess the macroscopic mechanical behaviour of fresh cranial bone samples

The present study aims at providing quantitative data for the personalisation of geometrical and mechanical characteristics of the adult cranial bone to be applied to head FE models. A set of 351 cranial bone samples, harvested from 21 human skulls, were submitted to three-point bending tests at 10 mm/min. For each of them, an apparent elastic modulus was calculated using the beams theory and a density-dependant beam inertia. Thicknesses, apparent densities and percentage of ash weight were also measured. Distributions of characteristics among the different skull bones show their symmetry and their significant differences between skull areas. A data analysis was performed to analyse potential relationship between thicknesses, densities and the apparent elastic modulus. A specific regression was pointed out to estimate apparent elastic modulus from the product of thickness by apparent density. These results offer quantitative tools in view of personalising head FE models and thus improve definition of local injury criteria for this body part.

Experimental study of the strain rate dependence of a synthetic gel for ballistic blunt trauma assessment

The mechanical characterization of a polymer gel used as reference backing material for blunt ballistic impact interpretation is performed at room temperature from quasi-static (0.002s-1) up to high strain rates (1500s-1). As very high strain tensile tests (350%) are conducted, an appropriate gripping device and particular strain measurement techniques are used, as well as high strain compressive tests (80%) based on retro lighting imaging. One major challenge is to carry out reliable compressive tests at high strain rates with polymeric split Hopkinson pressure bars using high-speed imaging and specific signal processing software. These mechanical tests provide a primary response to the strain rate dependence of the hyperelastic material behavior. Indeed, the material exhibits a higher stress response when the strain rate increases. Moreover, dynamic compression tests highlight a larger radial strain propagating along specimen axis with higher strain rates. This preliminary study on the characterization of the gels mechanical behavior, constitutes an interesting step for an evaluation of human surrogate material. The extensive constitutive law can therefore be implemented for numerical simulations, with an aim of impact biomechanics analysis and body armor assessment.

A three-dimensional geometric quantification of human cortical canals using an innovative method with micro-computed tomographic data

The complex architecture of bone has been investigated for several decades. Some pioneer works proved an existing link between microstructure and external mechanical loading applied on bone. Due to sinuous network of canals and limitations of experimental acquisition technique, there has been little quantitative analysis of three-dimensional description of cortical network. The aim of this study is to provide an algorithmic process, using Python 3.5, in order to identify 3D geometrical characteristics of voids considered as canals. This script is based on micro-computed tomographic slices of two bone samples harvested from the humerus and femur of male cadaveric subject. Slice images are obtained from 2.94 μm isotropic resolution. This study provides a generic method of image processing which considers beam hardening artefact so as to avoid heuristic choice of global threshold value. The novelty of this work is the quantification of numerous three-dimensional canals features, such as orientation or canal length, but also connectivity features, such as opening angle, and the accurate definition of canals as voids which ranges from connectivity to possibly another intersection. The script was applied to one humeral and one femoral samples in order to analyse the difference in architecture between bearing and non-bearing cortical bones. This preliminary study reveals that the femoral specimen is more porous than the humeral one whereas the canal network is denser and more connected.

THORAX INJURY CRITERIA ASSESSMENT THROUGH NON-LETHAL IMPACT USING AN ENHANCED BIOMECHANICAL MODEL

Ballistic injury refers to the interaction of a projectile and the human body, resulting in penetration or blunt trauma. In order to consider both consequences, a hydrodynamic elastoplastic constitutive law was implemented in a numerical FE model of the human torso to simulate soft tissues behavior and to evaluate their injury risk. This law, derived from 20% ballistic gelatin, was proven to be very efficient and biofidelic for penetrating ballistic simulation in soft tissues at very high velocity. In this study, the ability of the hydrodynamic law to simulate blunt ballistic trauma is evaluated by the replication of Bir et al.’s (2004) experiments, which is a reference test of the literature for nonpenetrating ballistic impact. Lung injury criteria were also investigated through the Bir et al.’s experiments numerical replication. Human responses were evaluated in terms of mechanical parameters, which can be global (acceleration of the body, viscous criteria and impact force) or local (stress, pressure and displacement). Output results were found to be in experimental corridors developed by Bir et al., and the maximum pressure combined with the duration of the peak of pressure in the lungs seems to be a good predictor for lung injury.

Identification protocol of skull human bone using a mono-layer law

The purpose of this work is to finalize an experimental protocol in order to identify a law of behaviour monolayer of the skull (Tables and diploë are represented in the model only by a single homogeneous layer). Further applications are pedestrian shock simulations. Many authors work on this identification (Delille, et al. 2002). They use various stress types (traction, compression and bending) and work with various means of conservation (Winckler, Formol and frozen). Crandall showed that the means of conservation deteriorates the mechanical properties. Moreover, different parameters as sex, age, cutting quality of the samples and geometry bring great dispersions in the results. These do not facilitate the comparison between tests. The LAMIH and the CEESAR began an experimental test on 8 skulls. We defined our protocol with unembalmed male subjects who were lower than 80 years. All the skulls were scanned to obtain a real geometry.

Methodology for ballistic blunt trauma assessment

Over the past decade, numerous studies have been undertaken to understand the human body response during blunt ballistic impact. Indeed, non-penetrating ballistic trauma can occur during the use of Less-Lethal Kinetic Energy projectiles (LLKE). A study about rigid LLKE projectile impacts on Post Mortem Human Subjects (PMHS) has been carried out (Bir et al. 2004). It leads to the development of biomechanical response corridors and the use of the injury criterion VCmax to establish the probability of skeletal injury (Bir and Viano 2004). Therefore, the French Ministry of the Interior and the authors develop a methodology with a coupled experimental-numerical based approach to assess blunt trauma. Among various backing materials employed as human body substitutes, the polymer gel SEBS (styrene-ethylene-butylene-styrene) has been identified as an appropriate ballistic gelatin susbtitute (Mauzac et al. 2010). Although gel transparency allows direct impact analysis using high-speed camera and provides information such as the dynamical wall displacement, the volume of deformation, these data are not sufficient to establish a direct link towards blunt ballistic trauma. Hence, the authors focus their research on the use of numerical tools as finite element (FE) model. Extend information can be obtained such as, strain, strain rate and pressure. In addition, the human torso FE model called HUByx is used in this study and has shown its biofidelic ability to replicate high-speed loading accidents (Roth et al. 2013). The robustness of numerical tools may lead to the correlation of predicted results with the experimental work and the investigation of case reports. The aim of this work is to introduce a reliable method and first results based on experimental and numerical study for ballistic trauma assessment. 2. Methods

Development of a mechanical model of the human skull bone by morphological study

With the aim to improve the safety of people, it is essential to know the phenomena of deformation, damage, and fracture of the various parts of the human body. This kind of study necessitates the creation of numerical models allowing making biologically relevant simulations of accidents. Several models of skull are proposed in the literature, the homogeneous macroscopic models of the bone of the human skull brands, the macroscopic models making the difference between cortical bone and spongy bone (Raul et al. 2006), or the microscopic models of Halgrin 2011. For the macroscopic models, the modeling is not refined enough to represent the bone of the skull in a correct way. With regards to the microscopic models, the setups allow good modeling but ask for a significant calculation time in order to be usable in simulations of crash of vehicle. There are also several studies which tried to identify the mechanical parameters of the human bone according to the morphological parameters, these equations do not allow to obtain a good precision in the calculation of the modulus. We can quote Van Eijden et al. (2006), Van Lenthe et al. (2006), and Chatelin et al. (2011) who identified the Eapp modulus according to morphological parameters in particular BV/TV (percentage of bone in a volume). The main criticism that can be made for this kind of model is the globalized vision of the morphology of the bone; while the geometry of the skull bone makes that there are differences inside the same sample having an impact on the mechanical behavior. This article suggests an intermediate model, precise enough to model with finite elements the human skull bone by taking into account the morphology of the head and with a reasonable calculation time. The consideration of the variability of the bone will be represented as layers of elements of different mechanical properties. We limit ourselves for this study to the linear modeling of a quasi-static compression test.

Experimental protocol to characterise damage of appendicular bone

Appendicular bone looks like composite material with long fibres and a matrix; the fibres are replaced by porous Haversian canals in the longitudinal direction and Volkmann canals in the transverse direction (Figure 1 (a)). Due to its architecture, orthotropic behaviour model with transverse isotropy can be used (Bry et al. 2012). The porosity introduced by the Haversian andVolkmann canals leads to the identification of apparent behaviour and consequently non-accurate mechanical properties (stiffness and resistance) when performing classical mechanical tests. To identify the true mechanical properties of appendicular bone, the porosity and its evolution during loading have to be modelled and measured (Xiang-Dong et al. 1994). The continuum damagemodel is a good candidate to compute the effective stress and the damage evolution. The damage is defined by a second-order tensor in the orthotropic frame and canbe identified easily by using tensile tests (Lemaitre 1992). To identify the mechanical behaviour of this damaged material, an experimental protocol has been developed. A ‘normalised’ shape closer as possible as a parallelepiped form is done to the samples harvested from appendicular bone in longitudinal and transverse directions. The initial porosity is measured in longitudinal and transverse directions by using X-ray microtomography (Figure 1(a)). To avoid intra-variability, tensile and compressive cyclic tests are done at first to identify the stiffness (elasticity modulus and Poisson’s ratio) and followed by loading/unloading tests, without taking down the specimen, carriedout until rupture to compute thedamage evolution by measuring the elasticity modulus degradation. This experimental protocol is used to characterise the behaviour until the rupture of male PMHS humeri.