Analysis of glass fracture pattern made by .177″ (4.5 mm) Caliber air rifle

At the scene of the occurrence, investigating officer or forensic expert often encounters the shooting at inanimate target such as glass, door, window panes, windows of an automobile, etc. In fact, analysis of fracture pattern is the means and methods for characterization of fractured units or fragments in order to study or identify the mechanism of such fractures. The study of fracture pattern of glass has been of long interest to the forensic community. In literature, the importance of the glass evidence is described by Saferstein, Nabar and O’Hara in their books (Saferstein 2006; Nabar 2008; O’Hara and Osterburg, 1949). Fracture analysis of glass often provides much significant information like cause of fracture, direction of force applied, etc.

Glass can be defined as a semitransparent, transparent, non-crystalline and super cold mixture of metallic silicates. Glass is, therefore, by definition, amorphous or non-crystalline. Glasses are essentially super cooled liquids and they possess a unique combination of properties: transparency with or without colour, durability, electrical and thermal resistance, a range of thermal expansions, with hardness, rigidity and stability density, refractive index (Copley 2001). These properties have been the subject matter of the various investigations (Jauhari et al. 1974). Specially, density and refractive index of a glass that impart the characterization and forensic individualization of the glass evidence (Gogotsi and Mudrik 2010). Silicate glasses are so much part of our everyday life having a forensic importance (Stoney and Thornton 1985).

In contrast to most other materials, the molecular structure of glass does not consist of a regular geometry of crystals, but of an irregular network of silicon and oxygen atoms with alkaline parts in between. In soda lime silica glass, which is generally referred to as glass, the alkaline parts consist of oxides of sodium and calcium. The forces of the inter-atomic bonds describe the theoretical strength of a material. In practice the tensile strength of annealed glass is much lower. Furthermore, the tensile strength is not a material constant, but depends upon many aspects such as the condition of the surface, the size of the glass pane, duration of load and environmental conditions (Overend et al. 2007). The experiment of Griffith on the glass forms the basis for modern fracture mechanics. Griffith stated that fracture always starts from the preexisting flaws that are termed as Griffith’s Flaw (Griffith 1920). Sir Nevill Mott explained the dynamic fracture mechanism that was an extension to the Griffith’s law (Mott 1946).

When a projectile such as a bullet or stone hits a glass surface the impact causes changes, in the form of fractures, to occur within the glass. The glass bends slightly when a projectile hits a glass as shown in Fig. 1. The glass breaks when it reaches limit of tensile stress and the projectile passes through the glass (Saferstein 2006; Nabar 2008).

In accordance to the laws of physics a certain amount of energy from the projectile that is absorbed by the glass will dissipate along the path of least resistance thereby creating cracks. Shock waves of energy originate from the point of impact causing specific types of damage to the glass (Grady 2010).

When a projectile i.e. a bullet or rock hits the glass, it will form two distinct types of fractures: Radial and Concentric. There is another terminology that is known as cone fracture, observed to be caused by a penetration of the high velocity projectile such as a bullet. Projectile leaves a round crater shaped hole which is surrounded by radial and concentric cracks. There is the appearance of the cone as the hole is wider on the exit side and gives the appearance of the cone. This is caused by the high strain and appears at the impact point on the glass. The point of impact and the direction of the impact on the glass can be determined by the cone fractures (Saferstein 2006; Nabar 2008; Mcjijnkins and Thornton 1973).

When a projectile first hits the glass, the glass will be stretched, causing tension on the back side directly behind the projectile. This causes compression around the point of tension. The radial cracks begin on the opposite side of the force at the point where the projectile hits the glass and radiate out from the origin of the impact. They will always end if they encounter an existing fracture line. The concentric cracks begin on the same side as the force, where the tension occurs and formed early in the fracture process. Afterwards the radial fractures are created (Saferstein 2006; Nabar 2008). Figure 2 shows radial and concentric fractures.

The shape of the Mist Zone (chipping) may help to determine the angle of impact. If the bullet strikes at right angle, the chip pattern around the hole is eventually distributed making a symmetrical hole. But if it strikes from the right side, it will make considerable chipping around the glass making an elliptical hole also. As well as when it hit the glass from the left side, these indicators are found to be reversed (Saferstein 2006; Nabar 2008; O’Hara and Osterburg, 1949).

An air gun is special kind of small arm that hurls projectiles by means of mechanically pressurized air or other means which involves no any chemical reaction. All air weapons e.g. rifle and pistol generally propel metallic projectiles. Generally air gun involves three types of action mechanisms that are Spring Piston Mechanism, Pneumatic Air Mechanism and Compressed Gas (CO₂) Mechanism (Vanzi 2005).

It is found to be lethal and may cause grievous injury and even death (Lal and Subrahmanyam 1972) as well as bone micro fracture from the direct impact of the projectile (Kieser et al., 2013).

After test firing, samples were directly analyzed. Following measurements have taken with a Vernier Caliper to analyze the trends in the features:

1. 1.

   Radial Fracture Count

    
2. 2.

   Concentric Fracture count

    
3. 3.

   Diameter of Bullet Hole (Represented by “A” in Fig. 3)

    
4. 4.

   Thickness of Mist Zone (Represented by “B” in Fig. 3)

    
5. 5.

   Diameter of Mist Zone (Represented by “C” in Fig. 3)

    

Measurements were taken and to analyze the consistency in the feature, graphs were drawn. As the radial and concentric fractures are the results of stress travelling and it is not material constant, therefore radial and concentric crack counts have not been taken for graphical representation.

To analyze the fracture pattern, the samples were test fired. Some measurements were taken to analyze the glass fracture pattern and afterwards the graphs were drawn to find the regularity in features and the trends of the characteristics. First of all, the glass panes of 3 mm thickness without Sun Control Film (SCF) were test fired at fixed distance. Similarly 4 mm and 5 mm window panes were test fired. Measurements are tabulated and graphically represented. Table 2 summaries the measurements for the glass panes without SCF and Figs. 4, 5, 6 and 7 shows graphical representation of 3 mm, 4 mm Privacy, 4 mm Transparent and 5 mm thickness glass respectively.

Sr No.	Sample No.	No. of radial fractures	No. of concentric Fractures	Hole diameter	Thickness of mist zone	Diameter of Mist zone
3 mm
1	1	11	9	6.35 mm	11.21 mm	28.76 mm
2	2	8	8	5.96 mm	10 mm	25.96 mm
3	3	11	8	5.86 mm	12.33 mm	30.52 mm
4	4	11	7	5.6 mm	12.8 mm	25.6 mm
4 mm Privacy Glass
5	1	8	5	6.10 mm	13.52 mm	33.14 mm
6	2	10	6	6.46 mm	13.35 mm	33.16 mm
7	3	9	8	5.63 mm	12.62 mm	30.86 mm
8	4	5	2	5.52 mm	13.17 mm	31.86 mm
4 mm Transparent Glass
9	1	6	1	7.5 mm	15.45 mm	38.4 mm
10	2	7	3	6.2 mm	13.37 mm	32.94 mm
11	3	8	0	6.4 mm	13.7 mm	33.8 mm
12	4	12	13	5.1 mm	11.27 mm	27.64 mm
13	5	10	3	5.57 mm	13.22 mm	32.00 mm
5 mm
14	1	11	6	5.37 mm	12.37 mm	30.10 mm
15	2	10	7	6.07 mm	12.9 mm	31.87 mm
16	3	11	0	6.70 mm	16.0 mm	38.70 mm
17	4	7	4	5.45 mm	15.44 mm	36.30 mm

These all glass panes show somewhat similarities in the features that it reveals on the general visual examination. Figures 8, 9 and 10 show that fractured glass pane of 3 mm, 4 mm Privacy 4 mm Transparent, 5 mm thickness glass respectively.

To analyze the trends of the hole diameter, linear graphs have been drawn. It also provides a comparison between the glass panes coated with sun control film and those without sun control film of same thickness. Figures 11, 12, 13 and 14 shows the linear trends for the 3 mm, 4 mm Privacy, 4 mm Transparent and 5 mm thickness glass panes respectively. Further, to establish the consistency in hole diameter i.e. whether all the values are consistent (equals or near to the mean value), test of goodness of fit has applied. The findings of chi-square test have been summarized in Table 4.

Chai Square Observations
Null Hypothesis: All the values are consistent i.e. equals to the mean value.
For 3 mm thickness
Total no of observations	8
Calculated value of chi square	0.470328
Tabulated value For 7 degree of freedom at 1% and 5% level of significance respectively	1.239, 2.167
Conclusion	Calculated value of χ2 < tabulated value; null hypothesis accepted
4 mm Privacy
Total no of observations	8
Calculated value of chi square	.13867
Tabulated value For 7 degree of freedom at 1% and 5% level of significance respectively	1.239, 2.167
Conclusion	Calculated value of χ2 < tabulated value; null hypothesis accepted
4 mm Transparent
Total no of observations	5
Calculated value of chi square	0.540878
Tabulated value For 4 degree of freedom at 1% and 5% level of significance respectively	.297, .711
Conclusion	Calculated value of χ2 < tabulated value; null hypothesis accepted
5 mm
Total no of observations	8
Calculated value of chi square	0.406755
Tabulated value For 7 degree of freedom at 1% and 5% level of significance respectively	1.239, 2.167
Conclusion	Calculated value of χ2 < tabulated value; null hypothesis accepted

On the basis of observations of all the glass panes, it is observed that diameter of hole shows a consistency that can be observed in the above graphs and has been proved in the Chi Square Observations.

They found that the diameter of bullet hole is somewhat double to the caliber of the weapon in case of .315″, .303, A.K. 47 rifle. While 9 mm pistol produces hole of diameter ranging from 11.04 to 14.63 mm. As well as a large variation was observed in the hole diameter, made by Improvised Pistol, ranging from 11.35 mm to 19.43 mm.

Thus the findings of this work may help to opine whether the fracture is made by standard/ regular arm or by air gun by examining the hole diameter. As well as shape of mist zone also have some indication like in 3 mm glass without SCF, mist zone is somewhat spherical in shape, in 4 mm privacy glass panes mist zone is slightly triangular with rounded edges and 5 mm glass panes have irregular shape of mist zone. 5 mm glass pane shows some markings in the mist zone that are easily visible.

The aim of this work is to study the fracture pattern of the glass made by air rifle of .177″/4.5 mm caliber. The graphical trends shows the consistency in hole diameter, thickness of mist zone and the diameter of the mist zone. The hole diameter is ranging from 4.77 to 7.5 mm in case of air rifle. It can lead to distinguish the weapon by which fracture was made, whether it is standard or air weapon. No significant difference has been observed in pattern on the pane coated with sun control film and the pane without sun control film as well as no any significant differences observed in the fracture pattern of the glass panes when force is applied to the same side of coating of sun control film and when direction of the impact is opposite to the coating. The finding of this work may lead provide extreme help to a forensic expert in criminal investigation.