Suppose a victim called police and notified that somebody committed a drive-by shooting on his house. Crime scene investigators (CSI’s) then show up to investigate the crime. One of the things they look at is the broken glass through which the bullet or bullets pass. Such evidence can tell a story to the technician with a keen eye for the details found in broken glass.
The manner in which glass breaks many times presents clues with regard to how it was broken. For instance, glass that was broken by a speeding baseball will have a different pattern of fracture than glass that was broken by a speeding bullet.
Particular break characteristics allow a forensic scientist to ascertain the direction from which the impact came. They include a special kind of stress-fracture lines called conchoidal lines. Conchoidal lines are lines that emanate from the site of impact. When viewing glass through its thickness, these lines curve out and away from the point of impact in a curve resembling a seashell curve.
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 plates of glass such as windshields tend to radiate and concentrate. When viewing the glass perpendicularly from the surface, one can see radial cracks that radiate outward from the impact point. Picture this like the spokes that radiate from the center of a wheel.
Again, when viewing the glass perpendicularly from the surface, one would also find cracks that concentrate around the impact point and are called concentric cracks. Concentric cracks are cracks that form progressively larger circles around the impact point. Picture this as a circle within a circle within a circle….all surrounding the impact point.
If a speeding bullet strikes a window and penetrates it but does not completely break it, the destruction may leave behind a hole accompanied or not accompanied by the aforementioned fracture lines.
On one side of the impact (the entrance side), the hole will be small and clean. On the other side of impact (the exit side), a small conical piece of glass would have been removed. CSI’s would normally view the glass from both sides to determine a bullet’s trajectory; i.e. whether the bullet was fired from the outside of the house in as in the case of a drive-by shooting or vice versa.
The next time you watch your favorite CSI or Forensic Files TV show, you will have a better understanding of how broken glass is studied in a crime lab in order to bring a perpetrator to justice.