Suppose a victim calls the police and reports that someone committed a shooting from a vehicle in their home. Crime scene investigators (CSI) then show up to investigate the crime. One of the things they look at is the broken glass that the bullet or bullets pass through. Such evidence can tell a story to the technician with a keen eye for details found in broken glass.
The way glass is broken many times presents clues to how it was broken. For example, glass that was smashed by a high-speed baseball will have a different fracture pattern than glass that was smashed by a high-speed bullet.
The particular characteristics of the break allow a forensic scientist to determine the direction from which the impact came. They include a special type of stress fracture lines called conchoidal lines. The conchoidal lines are lines that emanate from the place of impact. When looking at the glass through its thickness, these lines curve outward and away from the point of impact in a curve that resembles the curve of a seashell.
If the investigator takes a closer look at the conchoidal lines, he can see tiny lines that radiate almost 90 degrees from the conchoidal lines. These fracture lines are known as hackle marks.
Cracks in windows and other flat sheets of glass, such as windshields, tend to radiate and concentrate. When looking at the glass perpendicularly from the surface, you can see radial cracks radiating outward from the point of impact. Imagine this as the spokes radiating from the center of a wheel.
Again, when viewing the glass perpendicularly from the surface, you would also find cracks that are concentrated around the point of impact and are called concentric cracks. Concentric cracks are cracks that form progressively larger circles around the point of impact. Imagine this as a circle within a circle within a circle … all around the point of impact.
If a high velocity bullet hits a window and penetrates it but does not completely rupture it, the destruction may leave a hole accompanied or not by the aforementioned fracture lines.
On one side of the impact (the inlet side), the hole will be small and clean. On the other side of the impact (the exit side), a small conical piece of glass would have been removed. CSIs would normally view the glass from both sides to determine the trajectory of a bullet; that is, if the bullet was fired from outside the house as in the case of a shooting from a vehicle or vice versa.
Next time you look at your favorite CSI gold Forensic files TV show, you will better understand how broken glass is studied in a crime lab to bring the perpetrator to justice.