A man is found dead in an abandoned house. Upon examination the medical examiner finds that the man was killed by a single gunshot to the head. Three local teens where observed near the house two days earlier. They are picked up and questioned by police. When pressured, the teens admit to having stolen one of the boys’ father’s guns. They had taken turns shooting at the windows of an abandoned house.
The teens admit that John shot the gun first, then Jay and last was Fred. At the scene the investigators find three bullet holes in a window. They analyze the angle of each bullet hole in relation to the victim and find that the bullet which passed through the far right side of the window pane is the one which fatally wounded the man.
It is up to the investigators to determine the order of the shots. This will tell them which boy fired the fatal shot and allow the investigators to work out what the charges will be against the shooter. Examination of the fractures in the glass will tell them this.
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. When a projectile smashes into the surface of a glass pane the glass bends slightly before it breaks. Some of the energy required to propel the bullet forward is absorbed by the glass .When the glass reaches a certain point of distortion it breaks and the projectile passes through the glass. In accordance with the laws of physics the energy from the projectile which was absorbed by the glass will dissipate along the path of least resistance. Cracks occur in the glass as energy dissipates through the glass. Shock waves of energy emanate from the point of impact causing specific types of damage to the glass. This is what makes it possible to determine the order of fractures. As well, the various types of fractures in glass can tell an investigator the angle of impact and the side of a glass pane on which the impact occurred.
When the projectile hits a glass surface the glass bends and energy dissipating from the point of impact causes ‘radial’ fractures to occur on the side of the glass opposite the point of impact.
Waves of energy from the projectile reverberate from the point of impact causing the glass to fracture in a cone shape. The result is small entrance hole and a larger exit hole in the glass. Surrounding the hole made by a projectile there will be concentric rings know as ‘concentric fractures’.
Radial fractures will always travel along the path of least resistance. They will always end if they encounter an existing fracture line.
Radial fractures are also useful in determining the direction a pane of glass was struck from. Stress marks on the edges of the radial fractures conform in a specific way which will show the direction of impact. Stress marks are curved. One end of the curve runs parallel to the glass surface. The other end of the curve runs perpendicular to the glass. The side of the glass which has stress marks which are parallel to the surface is the side which sustained the impact (Saferstein, 2004).
Very often the glass in question is shattered and must be pieced back together. An investigator can start with the frame which is labeled as to which side is the inside and which is the outside. Loose pieces of glass can be matched up with pieces still adhering to the frame.
Once the pieces are all in place the point of impact can be determined. The radial fractures can be examined to determine from which side the glass was broken.
Determining the side from which glass was broken is important in burglaries and other crimes involving home invasion. Arson investigators have particular interest in knowing whether a pane of glass was broken from the inside or the outside. Windows broken from the inside at the top of a structure can indicate an attempt to ventilate the building. Ventilation causes the fire to spread upward following the source of oxygen. A fire started at the bottom of a structure can be made to race through a structure more rapidly than normal by ventilating the upper levels.
The concentric fractures form on the same side of the glass surface as the point of impact. When glass breaks most of the glass falls inward. However, the tiny shards produced in the formation the concentric fractures spray backwards towards the person wielding the weapon or tool (Lane, 1992) (Innes, 2002). These tiny bits of glass can get caught in the hair and clothing of a person near the point of impact. Even after washing, tiny fragments of glass can remain caught in the fibers of clothing and footwear. When a gun is fired at a glass surface, tiny particles of glass can spray back up to 18 feet (Innes, 2002)