13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- The significance of backspatter lies in crime scene reconstruction [2]. Specifically, backspatter travels opposite to the line of fire and can therefore be deposited on the firearm, the shooter and surrounding objects. Hence, factors such as weapon type, range of shot [3], blood stain patterns on the hands of victims proving suicides [5], and homicide information may be derived [8]. A key series of experiments by German pathologist Bernd Karger stands out as the most comprehensive study of backspatter [3, 6, 9]. Nine live New Jersey calves (5-6 months old) destined for slaughter were shot in the right temple 10 cm horizontally below the right eye. Key findings included that backspatter results varied with each shot despite a controlled environment but the pattern was a consistent fine mist with every shot immediately after bullet impact. Synthetic models [2, 4] consisting of blood soaked sponges encased in outer coatings produced more reproducible baskspatter patterns, were inexpensive and avoided ethical issues. There are several case studies [7, 8, 10, 11] that describe backspatter in non-fatal human shootings, suicides, and homicides. These results can provide a specific situation to validate computational models. The three main mechanisms that are thought to contribute to backspatter include; (i) subcutaneous gas effects; (ii) temporary cavitation related to intracranial pressure; and (iii) tail splashing. In general, a combination of all three factors may cause backspatter. Subcutaneous gas effects result from pressurized gases during the muzzle discharge [12]. During close range shots the pressurised muzzle gases enter the wound produced by the bullet and become trapped in the subcutaneous space between the skin and skull. This causes ‘starburst or stellate’ entrance wounds in what is known as a ‘blow-out’ effect where the skin mushrooms and a pocket is created under the skin [2, 9]. The hot, pressurised gases expand within this pocket space and create a backwards streaming of gas escaping ou oft the entrance wound. The accelerating force from the escaping gas drives blood and soft tissue opposite to the direction of fire [2, 4, 9]. Temporary cavitation related intracranial pressure occurs as a bullet passes through a medium creating a temporary cavity in its wake, a feature of all missile wounds [13]. In the case of a bullet wound to the head, a large temporary cavity would be created because of the low retentive forces of brain tissue [13]. The brain is contained within the rigid skull, and therefore as a temporary cavity expands a high pressure is created within the cranium. The high pressure within the cranium and the subsequent collapse of the temporary cavity creates a force to drive tissue and blood back out the entrance wound [7, 9]. Karger proposed that anatomical structures similar to liquid filled cavities provide the best conditions for temporary cavitation [14]. Fackler [15] believed that the collapse of a temporary cavity is the only mechanism that creates backspatter. The phenomenon of ‘tail-splash’ is the backwards streaming of destroyed material or fluid along the lateral surface of a high velocity bullet as it penetrates a dense medium [9, 13, 14]. Karger [14] suggested that ‘tail-splash’ occurs when a bullet penetrates the brain and lateral streaming of brain matter and blood occurs towards the line of fire. This is most closely related to the mechanism evaluated in this study. There have been numerous computational studies of high velocity impacts related to ballistics, but little research has been conducted related to backspatter. Finite element analysis (FEA) of ballistic helmet impacts and the effect on the human cranium [16, 17] stresses within the head and brain were used to evaluate the performance of a helmet. FEA studies have simulated particle projectiles in the opposite direction to the line of fire [18-20], which would appear to be backspatter. These are projectile impacts to human bones, namely the parietal bone of the skull [18] and the mandible (jaw) bone [19, 20]. The use of FEA provided good results when compared to experimental data with the relatively simple material models used. Backspatter or its significance was not the focus of those studies. A further FEA study to recreate the wounding that occurred in an actual death [21]
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