Anatomically Realistic Physical and Computational Models of Cranial Ballistic Impact and Associated Backspatter

Abstract

Cranial ballistic injury and associated blood spatter evidence is of particular forensic interest because of the high mortality rate of a head wound compared to other body parts. The magnitude and speed of a ballistic impact means the ballistic blood spatter are often crucial in crime scene reconstruction and determination of guilt. Appropriate models are required to investigate how a complex structure (cranium) fails under ballistic impact. This research aims to enhance the understanding of the cranial ballistic impact by two approaches: i) To use animal models to investigate factors influencing ballistic wounding quantitatively and ii) To develop novel physical and computational model pairs to study cranial ballistic impact. Conventional use of animal models lack control over sample variation or specific experimental conditions. The influence of the ballistic, biological, and experimental factors on the ballistic outcome of porcine and ovine models were determined by statistical analysis to show the bullet calibre and animal species had a significant influence on the retrograde ballistic responses. In addition, sex, mortality state, and time since death were also factors that influenced the ballistic response. Physical models of anatomically realistic and simple geometries were developed, with skin-skull-brain tri-layer construction for study of ballistic cranial wounding. The realistic MRI-based model successfully replicated the ballistic failure characteristics of a human cranium in both quantitative and qualitative measures, while being capable of simulating ballistic blood spattering as well. The influence of model geometry was demonstrated by the four geometries employed. The physical model is free from ethical complications and has a higher degree of control over its geometry and composition compared to animal models. The computational model had matching geometry, structure, and materials to the physical models. The use of the SPH method allowed simulation of ballistic spatter and produced a computational model that was comparable to its physical equivalent, successfully replicating quantitative trends between different geometries and qualitative ballistic response characteristics such as temporary cavitation and bevelling. The unique analysis and visualisation ability of the computational model provided insights to cranial wounding mechanisms such as the exit wound enlargement or distant skull crack formation.