Last modified: 2015-05-04
Abstract
Investigation of crime requires rigorous testing and sound scientific understanding of the evidence to reconstruct the criminal event. From the multitude of forensic specializations, blood spatter evidence from cranial gunshot wounding is of particular interest because of the high mortality rate resulting from head wounding compared to other body parts. Traditionally, animal models and physical models of the human anatomy have been used to study the mechanism and extent of ballistic spatters, including backspatter. Backspatter is a retrograde spattering of the target material from the entry wound. The reverse directionality of the backspatter has specific evidential value, as it may establish a link between the victim and the shooter via transfer of biological matter. Backspatter evidence has also been used in courts to distinguish between a homicide and a suicide. Despite the importance of backspatter, the understanding of its mechanism has remained inadequate due to ethical issues, difference in anatomical geometry associated with various animal samples, or material property difference among biological and synthetic materials used in physical experiments. Hence there is a need to develop simulation tools that will use numerical models of cranium geometry and configurations relatively similar to those of humans. Such numerical models can act as alternatives to the animal or physical models for investigation of backspatter in a variety of situations.
In this study, a mesh-free method called Smoothed Particle Hydrodynamics (SPH) is used to develop a numerical model to simulate high speed ballistic impacts. The complex geometry of the human cranium was reduced to a simplified box model equivalent to human anatomical internal volumes. The inhomogeneous and anisotropic behaviors of the biological materials in a cranium (skin, skull and brain) were simplified to homogeneous and isotropic materials for each component. A physical equivalent model has been manufactured and tested under the same ballistic conditions for numerical model validation. The numerical model matched well with its physical equivalent experimentation in both material deformation characteristics and the timing of key events, demonstrating the potential of the simulation models as a better alternative to animal and physical models. The simulation captures the temporary cavity development in the brain simulant well, as well as showing realistic fragmentation, including backspatter. The temporary deformation of the skin entry wound was also a good match to the physical experimentation. The simulation also identified the best suitable material models to simulate ballistic impact of the brain simulant. Also, a parametric study into the effect of material properties on the dynamic response of different simulants has been conducted. This work provides basis for a more complicated, anatomically accurate geometric cranium model to further develop reliable and robust simulation of cranial ballistic impact.