Understanding Cranial Injury – Virtual Forensics
Creators: name: Chattrairat, Akanae
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401602
subject: Composite and hybrid materials
classificationCode: 401605
subject: Functional materials

description: In the essence of firearm-related crime scene investigation and its resolution in court, one of the critical aspects is to distinguish the cause of mortality between homicide and suicide. This context can be determined by investigating the nature and conditions leading to the injury and accessing viable biomechanical scenarios. In cases of cranial injuries, a comprehensive understanding of principal biomechanics, biomaterial dispersion, and wounding mechanisms can promote the development of valid methods to distinguish the cause of fatality. 
The virtual crime scene investigation employing numerical models holds significant potential to assist in forensic investigations of firearm-related fatalities, particularly in cases where ethical concerns and resource constraints limit physical experiments. Furthermore, traditional experiments involving animal models and human cadavers have limitations in investigating various factors influencing different cranial ballistic injuries, which require repetitive experimental conditions, unlike biomaterials. However, research on ballistic impacts on accurate numerical-based human cranial models using three primary anatomical components (skin-skull-brain) is still scarce. This includes the usage of appropriate biomaterials that replicate the properties of the human scalp and their properties at dynamic strain rates, which are essential for ballistic applications, thus hindering the reliability of numerical models. 
Therefore, there is a need for appropriate models to investigate cranial ballistic responses. The present research aims to i) develop suitable human scalp biomaterials across a wide range of dynamic loadings and ii) enhance understanding of cranial ballistic responses through numerical and experimental approaches involving various ballistic factors. The human cranial numerical model developed in this study integrates three primary layers: skin, skull, and brain, with dynamic properties of biomaterials, addressing previous limitations in incorporating high strain rate skin biomaterial properties. The present study also develops and characterises silicone-based composite skin biomaterials with short polyethylene fibre and bioactive glass reinforcements across a range of strain rates through the micromechanics design of composites. The experiment and numerical analysis reveal their suitability for both quasi-static conditions and high-speed dynamic events. Notably, the study identifies a 3% reinforcement composite as the optimal skin simulant compared to pure silicone, and it also demonstrates a large variation in material stiffness with strain rate. The skin biomaterial subjected to a strain rate of 4000 s-1 demonstrates a 9-time greater stiffness than the same biomaterial subjected to a quasi-static strain rate of 0.48 s-1. This variation underscores the necessity of incorporating high-strain rate properties to yield accurate cranial injury simulations in ballistic impact scenarios. 
The importance of considering the biomaterial strain rate becomes evident when employing different skin material properties in the numerical human cranial head model impacted by a 9mm projectile, where the skin with quasi-static properties exhibits excessive cranial injuries compared to its physical counterpart. In contrast, using high-strain rate properties yields results similar to those of the existing experiments. Additionally, the necessity of incorporating blood vessels and pressure was determined to be insignificant through the micromechanics finite element analysis, and a simplified human cranial model without a circulatory system was deemed sufficient. 
The simulation results of ballistic impact on the human cranium demonstrate a correlation with ballistic experiments conducted on physical counterparts and animal models, establishing the reliability of the numerical model. The study establishes relationships between post-ballistic impact cranial injuries, such as wound diameter, wound shape, and temporary cavity, and various ballistic factors, namely impact location, projectile velocity, and angle of impact. This analysis facilitates the elimination of specific crime scene situations from reassessment consideration.
The research offers comprehensive numerical and experimental approaches to study cranial biomechanics and injury mechanisms that generate primary evidence in crime scenes. It integrates detailed anatomical human cranial geometry, including skin, skull, and brain components, and employs biomaterials closely resembling real human skin properties. This study constitutes the first analysis of various ballistic factors using an anatomical human cranial simulation, providing valuable insights into the interplay between ballistic factors and resultant cranial injuries. Moreover, it stands as the inaugural study characterising a specific composite skin biomaterial that integrates both mechanical and bio-integrative properties. The comprehensive exploration of strain rates highlights the importance of adopting high-strain rate material properties in dynamic simulations.
Overall, this research advances the understanding of cranial biomechanics and injury mechanisms, with implications for composite biomaterials design, impact dynamics, and forensic investigation.