GLASS

Explain Glass types and glass fractures with Collection and preservation of Glass evidence:

Glass Types:

Safety Glass:

Tempered glass- is made strong by rapid heating- and –cooling process that introduces stress to the glass

When the tempered glass breaks it breaks into tiny cubes with no sharp edges.

Tempered glass is used in the side and back windows of the car.

Laminated glass- Safety glass very strong because it is made of 2 layers of glass sandwiching a layer of plastic, Auto window shields are this type.

Other types are- Wire glass, Colored glass, Light sensitive glass.

Glass bends in response to any force that is exerted on anyone of its sur­faces; when the limit of its elasticity is reached, the glass fractures. Fre­quently, fractured window glass will reveal information that can be related to the force and direction of an impact; such knowledge may be useful for reconstructing events at a crime-scene investigation.

The penetration of ordinary window glass by a projectile, whether it is a bullet or a stone, produces a familiar fracture pattern in which cracks both radiate outward and encircle the hole, as shown in Figure 4-16. The radiating lines are appropriately known as radial fractures, and the cir­cular lines are termed concentric fractures.

Radial Fracture - A crack in a gloss that extends outward like the spoke of a wheel from the point at which the glass was struck.

Concentric Fracture - A crack in a glass that forms a rough circle.

Often it is difficult to determine just from the size and shape of a hole in glass whether it was made by a bullet or by some other projectile. For instance, a small stone thrown at a comparatively high speed against a pane of glass will often produce a hole very similar to that produced by a bullet. On the other hand, a large stone can completely shatter a pane of glass in a manner closely resembling the result of a close-range shot. How­ever, in the latter instance, the presence of gunpowder deposits on the shattered glass fragments does point to damage caused by a firearm.

When it penetrates glass, a high-velocity projectile such as a bullet often leaves a round, crater-shaped hole that is surrounded by a nearly symmetrical pattern of radial and concentric cracks. The hole is inevitably wider at the exit side and hence its examination is an important factor in determining the direction of impact. However, as the velocity of the penetrating projectile decreases, the irregularity of the shape of the hole and of its surrounding cracks increases, so that at some point the hole shape will provide no assistance for determining the direc­tion of impact. At this time, information derived from an examination of the radial and concentric fracture lines may prove a useful alternative for determining the direction of impact.

When a force pushes on one side of a pane of glass, the elasticity of the glass permits it to bend in the direction of the force applied. Once the elas­tic limit is exceeded, the glass begins to crack. As the first fractures form on the surface opposite that of the penetrating force, and these fractures develop into radial lines. The continued motion of the force places tension on the front surface of the glass, resulting in the' formation of concentric cracks. An examination of the edges of the radial and concentric cracks frequently reveals stress markings whose shape can be related to the side on which the window first cracked.

Stress marks are shaped like arches that are perpendicular to one glass surface and curved nearly parallel to the opposite surface. The importance of stress marks stems from the observation _ that the perpendicular edge always faces the surface on which the crack originated. Thus, in examining the stress marks on the edge of a radial _ crack near the point of impact, the perpendicular end is always found to be located opposite the side from which the force of impact was applied. For a concentric fracture, the perpendicular end always faces the surface on which the force originated. A convenient way for remembering these observations is the 3R Rule-Radial cracks form a Right angle on the Reverse side of the force. These facts will enable the examiner to deter­ mine readily the side on which a window was broken. Unfortunately, the absence of radial or concentric fracture lines prevents the above observations from being applied to broken tempered glass

COLLECTION AND PRESERVATION OF GLASS EVIDENCE

The gathering of glass evidence at the crime scene and from the suspect must be thorough if the examiner is to have any chance to individualize, the fragments to a common source. If even the remotest possibility exists that fragments may be pieced together, every effort must be made to col­lect all the glass found. For example, collection of evidence at hit-and-run scenes must include all the broken parts of the headlight and reflector lenses. This evidence may ultimately prove to be an invaluable means of placing a suspect vehicle at the accident scene by actually matching the fragments with glass remaining in the headlight or reflector shell of the suspect vehicle. In addition, an examination of the headlight's filaments may resolve any questions regarding whether or not an automobile's head­ lights were on or off before the impact.

When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene. This standard/reference glass should always be taken from any remaining glass in the window or door fral1l.es, as close as possible to the point of breakage. About one square inch of sample is usually adequate for this purpose. The glass fragments should be packaged in solid containers to avoid further breakage. If the suspect's shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually;' wrapped in paper and transmitted to the laboratory. It is best that the field investigator avoid removing such evidence from garments unless it is thought absolutely necessary for its preservation.

When a determination of the direction of impact is desired, all broken glass must be recovered and submitted for analysis. Wherever possible, the exterior and interior surfaces of the glass must be indicated. In. cases in which this is not immediately apparent, the presence of dirt, paint, grease, or putty may provide an indication as to the exterior surface of the glass.

GLASS FRACTURES

Glass bends in response to any force that is exerted on anyone of its sur­faces; when the limit of its elasticity is reached, the glass fractures. Fre­quently, fractured window glass will reveal information that can be related to the force and direction of an impact; such knowledge may be useful for reconstructing events at a crime-scene investigation.

The penetration of ordinary window glass by a projectile, whether it is a bullet or a stone, produces a familiar fracture pattern in which cracks both radiate outward and encircle the hole, as shown in Figure 4-16. The radiating lines are appropriately known as radial fractures, and the cir­cular lines are termed concentric fractures.

Radial Fracture - A crack in a gloss that extends outward like the spoke of a wheel from the point at which the glass was struck.

Concentric Fracture - A crack in a glass that forms a rough circle.

Often it is difficult to determine just from the size and shape of a hole in glass whether it was made by a bullet or by some other projectile. For instance, a small stone thrown at a comparatively high speed against a pane of glass will often produce a hole very similar to that produced by a bullet. On the other hand, a large stone can completely shatter a pane of glass in a manner closely resembling the result of a close-range shot. How­ever, in the latter instance, the presence of gunpowder deposits on the shattered glass fragments does point to damage caused by a firearm.

When it penetrates glass, a high-velocity projectile such as a bullet often leaves a round, crater-shaped hole that is surrounded by a nearly symmetrical pattern of radial and concentric cracks. The hole is inevitably

  wider at the exit side (see Figure 4-17), and hence its examination is an important factor in determining the direction of impact. However, as the velocity of the penetrating projectile decreases, the irregularity of the shape of the hole and of its surrounding cracks increases, so that at some point the hole shape will provide no assistance for determining the direc­tion of impact. At this time, information derived from an examination of the radial and concentric fracture lines may prove a useful alternative for determining the direction of impact.

When a force pushes on one side of a pane of glass, the elasticity of the glass permits it to bend in the direction of the force applied. Once the elas­tic limit is exceeded, the glass begins to crack. As shown in Figure 4-18, the first fractures form on the surface opposite that of the penetrating force, and these fractures develop into radial lines. The continued motion of the force places tension on the front surface of the glass, resulting in the' formation of concentric cracks. An examination of the edges of the radial and concentric cracks frequently reveals stress markings whose shape can be related to the side on which the window first cracked.

Stress marks, shown in Figure 4-19, are shaped like arches that are perpendicular to one glass surface and curved nearly parallel to the opposite surface. The importance of stress marks stems from the observation _ that the perpendicular edge always faces the surface on which the crack originated. Thus, in examining the stress marks on the edge of a radial _ crack near the point of impact, the perpendicular end is always found to be located opposite the side from which the force of impact was applied. For a concentric fracture, the perpendicular end always faces the surface on which the force originated. A convenient way for remembering these observations is the 3R Rule-Radial cracks form a Right angle on the Reverse side of the force. These facts will enable the examiner to deter­ mine readily the side on which a window was broken. Unfortunately, the absence of radial or concentric fracture lines prevents the above observations from being applied to broken tempered glass

When there have been successive penetrations of glass, it is fre­quently possible to determine the sequence of impact by observing the existing fracture lines and their points of termination. A fracture always terminates at an existing line of fracture. In Figure 4-20, the fracture on the left preceded that on the right; we know this because the latter's radial fracture lines terminate at the cracks of the former.

COLLECTION AND PRESERVATION OF GLASS EVIDENCE

The gathering of glass evidence at the crime scene and from the suspect must be thorough if the examiner is to have any chance to individualize, the fragments to a common source. If even the remotest possibility exists that fragments may be pieced together, every effort must be made to col­lect all the glass found. For example, collection of evidence at hit-and-run scenes must include all the broken parts of the headlight and reflector lenses. This evidence may ultimately prove to be an invaluable means of placing a suspect vehicle at the accident scene by actually matching the fragments with glass remaining in the headlight or reflector shell of the suspect vehicle. In addition, an examination of the headlight's filaments may resolve any questions regarding whether or not an automobile's head­ lights were on or off before the impact (see Figure 4-21). .

When an individual fit is thought improbable, the evidence collector must submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene. This standard/reference glass should always be taken from any remaining glass in the window or door fral1l.es, as close as possible to the point of breakage. About one square inch of sample is usually adequate for this purpose. The glass fragments should be packaged in solid containers to avoid further breakage. If the suspect's shoes and/or clothing are to be examined for the presence of glass fragments, they should be individually;' wrapped in paper and transmitted to the laboratory. It is best that the field investigator avoid removing such evidence from garments unless it is thought absolutely necessary for its preservation.

When a determination of the direction of impact is desired, all broken glass must be recovered and submitted for analysis. Wherever possible, the exterior and interior surfaces of the glass must be indicated. In. cases in which this is not immediately apparent, the presence of dirt, paint, grease, or putty may provide an indication as to the exterior surface of the glass.