Radial Cracks Definition
Theodora Andy
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).
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.
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).
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.
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.
Now a days glass pane are coated with a SCF to maintain privacy in house, office and automobile as well as for fancy purpose. To analyze the fracture pattern on the glass coated with SCF, the sample were test fired as same as that of previous. The test firing was conducted on the glass panes of 3 mm, 4 mm privacy panes and 5 mm coated with a thin SCF. Samples were measured and analyzed as that of previous. These measurements have been tabulated in Table 3.
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.
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.
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.
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The complicated subsurface indentation crack system has been studied by observing the cross-section using optical microscopy8,11,12 and scanning electron microscopy (SEM)13,14. Focused ion beam (FIB) tomography can be used as a serial sectioning technique15,16. However, these sectioning methods affect the stress field around the process zone, so that it may alter the original morphology of crack system. X-ray computed tomography (CT) is a powerful technique to observe the internal cracks non-destructively17. Lacondemine et al.18 performed in-situ Vickers indentation experiment by means of X-ray tomography, and assessed the displacement field using a Digital Volume Correlation routine (DVC). Okuma19 clearly detected crack-like defects formed during powder processing and sintering of alumina by using a multiscale X-ray computed tomography, which was developed by Takeuchi and co-workers in SPring-820,21.
Figure 1 shows Vickers indentation crack patterns for various loads in translucent CAS-GC observed by optical microscopy. The indentation cracks are symmetric at high loads, 196 N and 98 N, as schematically illustrated in Fig. 1a. The subsurface lateral crack is seen as a brilliant circular area (Fig. 1b,c), in contrast to smooth lateral cracks observed in glasses (Supplementary Fig. S1). Four radial cracks on the surface emanate from corners of the indent. The microcrack zone is defined as a circular opaque white area at the center (dashed line in Fig. 1c). The lateral crack system is composed of four circular sectors, the shape and size of which are irregular at loads less than or equal to 29.4 N (Fig. 1d,e). The radii of radial crack c, lateral crack r, microcrack zone R, and indent size a increase with indentation load P according to power law relationship
where L represents the radii and n is an exponent (Supplementary Fig. S2). The exponents for radial crack and lateral crack were 0.57 and 0.65, respectively. They were approximately equal to the theoretical value of 2/3 for radial crack2 and 5/8 for lateral crack3. The exponent of 0.73 for microcrack zone was closer to the value of 0.57 for radial and 0.66 for lateral crack systems than the value of 0.5 for indent size.
The subsurface crack system was divided into three layers in Fig. 3; (a) upper layer, (b) middle layer, and (c) lower layer. The shallow lateral cracks located in the upper layer near the surface (Fig. 3a). The topography of the surface of shallow lateral crack was rough and wavy, as shown in Supplementary Fig. S3 in detail. The rough surface is the origin of the brilliant appearance of lateral cracks of CAS-GC observed by optical microscopy. The rough crack surface has been attributed to crack deflection by plate-like CAS crystals, which form a house-of-cards structure36. The evidence of crack deflection was seen not only for lateral cracks, but also for median crack from the crack profiles presented in Fig. 2c. The middle layer (Fig. 3b) indicates the circular microcrack zone at the center and radial cracks outside the zone. In the lower layer below the microcrack zone (Fig. 3c), there was only one median crack, which was connected with radial crack R4. The connection between the median crack and radial cracks R2 and SR2 was not detected unambiguously. There was no median crack, which connected radial cracks R1 and R3. As Cook and Pharr5 pointed out, cracks emanating from the indentation corners are not always originated from the median cracks. Figure 4a illustrates the schematic representation of crack system by Vickers indentation around the indent and the semi spherical process zone.
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