Glass is a supercooled mixture of the metallic silicates. Generally, glass is referred to as transparent, semitransparent, non-crystalline, and amorphous solid having number of physical properties, such as durability, rigidity, refractive index, and density (Copley 2001). These physical properties, especially refractive index, provide means of investigation for the forensic purpose (Jauhari et al. 1974; Stoney and Thornton 1985; Gogotsi and Mudrik 2010). Glass or glass fragments serve as physical evidence of a great value for the investigation of many offenses such as the burglary, arson, hit and run cases, shooting, and assault. A large variety of physical evidences is included under the term “Trace Evidence”. Locard’s Principle of Exchange is the fundamental concept in the production of physical and trace evidence (Caddy 2001; Robertson and Roux 2010; Mistek et al. 2018).
The analysis of glass fracture has been a subject matter of interest to the forensic community for a long time as it enables the investigator to establish lots of considerations such as cause of fracture, direction of impact, and angle of impact. The term “Fractography” represents the study of surface fracture to identify the mechanism of such fracture (Caddy 2001; Harshey et al. 2017).
Many eminent researchers gave their efforts to explain the mechanism of the fracture. Griffith echoed that fracture starts from the previous flaws known as Griffith flaw’s (Griffith 1920). Law of Griffith was further extended as dynamic fracture mechanism by Mott (1964). Theoretically, the strength of any material is defined in terms of inter-atomic bond. Tensile stress depends upon various factors such as condition and size of surface, duration of load, and environmental conditions (Overend et al. 2007). As the projectile hits the glass pane, cracks are created due to the transfer of projectile’s energy. Cracks propagates through the path of least resistance. The energy shock waves cause specific damage to the glass, originated from the point of impact (Grady 2010). In the context of forensic glass fracture analysis, the aspect of energy loss was experimentally demonstrated by Waghmare et al. (2003).
When any projectile, e.g., bullet or stone, hits the glass surface, stretching and compression occur. As the limit of tensile stress is crossed, it results in the breaking of the glass. The impact of the bullet or stone causes two special types of fractures, namely termed as radial and concentric (also known as spiral) fractures as demonstrated in Fig. 1. As the high velocity projectile penetrates the glass pane, there is a formation of a crater-shaped hole that is termed as cone fracture. The appearance of the cone may be observed at wide exit side of the hole (Mcjijnkins and Thornton 1973). In this context, the shape of the chip pattern (also called mist zone) around the hole is indicative of the direction of impact (Harshey et al. 2017). Hackle marks, stress marks, mirror zone, etc., are some other phenomena that may be observed in the fractured glass pane (Caddy 2001). These characters often provide a significant help to the investigator in the investigation of variety of offenses.
In this study, the soda-lime glass was taken. Soda-lime glass majorly contributes in the manufacture of windowpanes (flat glass), containers, etc. Sand (SiO2, 63–74%), soda ash (Na2CO3, 12–16%), and limestone (CaO, 7–14%) contributes as raw material in the manufacturing of soda lime glass (Curran et al. 2000).
No chemical reaction is used in the air gun and the mechanism of air guns involves the mechanically pressurized air or other means. When the trigger is released, compressed air is let off into the barrel, pushing along a specially made slug or pellet in front of it. On the basis of mechanism, there are three types of air gun, namely spring piston air gun, pneumatic air gun, and compressed gas (CO2) air gun (Vanzi 2005; Abhyankar et al. 2018). They have been utilized as a part of chasing, sporting, and fighting. These are easily available in an open market with fewer rules and regulations and also considered relatively safe. Although these have been found to have the potential to cause fatal and grievous injuries such as bone micro-fracture (Kieser et al. 2013), and ocular injuries (Schein et al. 1994) and even may cause death (Lal and Subrahmanyam 1972). It is evident from the literature that air guns (both rifle and pistol) are being used in illicit activities for a long period and in current time also (Burnt and Packey 1979; Harshey et al. 2017; Morgan et al. 2019). Various researchers studied the different aspects of glass and air gun that may be of forensic interest such as wounding potential and behavior of air gun pellet in ballistic gel (DiMaio et al. 1982; Wightman et al. 2010; Hallikeri et al. 2012; Stankov et al. 2013; Ansari and Chakrabarti 2017; Vedrtnam and Pawar 2017; Hsiao and Meng 2018). In firing incidences, glass may also serve as an intermediate object. Therefore, in the light of above facts, this study was performed with the aim to analyze the multiple fractures on a glass pane made by air rifle.