13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Understanding Backspatter due to Skull Fracture from a Ballistic Projectile Raj Das1,*, Justin Fernandez2, Alistair Collins1 , Anurag Verma 1, Michael Taylor3 1Department of Mechanical Engineering, University of Auckland, Auckland 1010, New Zealand 2Auckland Bioengineering Institute, University of Auckland, Auckland 1010, New Zealand 3Institute of Environmental Science and Research (ESR), Christchurch 8041, New Zealand * Corresponding author: r.das@auckland.ac.nz Abstract In forensics, a challenge arises from relating observed evidence to the actual events. Specifically, in cranial wounds resulting from a gunshot, the study of backspatter patterns (material propagated opposite to the direction of the projectile) can provide information about the cause by linking material to the firearm, shooter or surrounding objects. Firstly, this study investigates the physics during backspatter from a high speed projectile impact by evaluating two skull simulant materials. Secondly, we evaluate the suitability of a mesh-free method called Smoothed Particle Hydrodynamics (SPH) to model the fracturing and splashing mechanism during backspatter. The study has shown that projectile impact causes fragmentation of material at the impact site, whilst transferring momentum to fragmented particles. The particles travel along the path of least resistance, leading to partial material movement in the reverse direction of the projectile causing backspatter. The amount of backspatter depends on the strain limit of each material and how rapidly the bullet hole closes. The path of resistance is dependent on the constitutive properties of the materials. MDF was found to be a better simulant for a human skull than polycarbonate as demonstrated by the backspatter pattern. SPH was a suitable numerical method for modeling the high speed impact fracture, fragmentation during backspatter. The simulation predictions agreed well to the experimental data of medium density fiberboard (MDF). Keywords Backspatter, cranial injury, skull simulant, impact, SPH. 1. Introduction Wound ballistics is the study of phenomena that arises when a projectile strikes and penetrates a human or animal [1]. Due to variability in biological wounds from projectiles, computational predictions are increasingly playing a role in drawing conclusions. One important feature of wound ballistics is ‘backspatter’, a term used to describe any tissue ejected from a gunshot entrance in the opposite direction to the line of fire [2, 3]. However, while well documented, backspatter mechanics is not fully understood and involves multiple factors including transfer of kinetic energy, rapid expansion of gas, and high deformation of biological material. Following a firearm discharge, “high velocity” blood spatter [4] is created and often characterised by a finely spattered pattern [2]. The spatter pattern is usually circular when the projectile is at right angles to the surface and a narrow elongated pattern forms when the projectile is at narrower angles [5]. These larger elongated patterns may be analysed to determine the angle of impact and origin [5, 6]. The distance travelled by backspatter is reported as highly variable in the literature. For example, close gunshots to the head of live calves produced backspatter between 0-50 cm with a maximum distance of 119 cm [6]. A case study of an atypical gunshot wound by Verhoff and Karger [7] involved a suicide where extensive backspatter was observed to travel up to 4.6 m. Physical experiments from shots to a bloodied sponge covered in a rigid material resulted in backspatter travelling 30-60 cm [2, 4]. The biological contents of backspatter include brain tissue, bone fragments, skin tissue, adipose tissues and blood. Factors affecting the pattern include muzzle to target distance, calibre of firearm [2] and anatomical location with most studies focused on the cranium.
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