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By counting the cracks in pierced glass, forensic scientists can now tell how fast an object was going when it pierced through it.

Whenever an object, like a stone or a bullet, pierces through brittle materials like glass or Plexiglass, the energy expended by the impact results in a characteristic cracking pattern that extends radially outward from the entry point, producing a kind of star-shaped pattern. Like a snowflake, these patterns are unique — but up to a point. It turns out that the number of cracks created by a projectile holds some important clues about the nature of the impact.

This is the conclusion reached by Nicolas Vandenberghe and colleagues at Aix-Marseille University after shooting BB-sized metal balls into plexiglass plates of various thickness and strengths. As a high-speed camera chronicled each impact, the speed of the bullets were increased until reaching 268 mph (432 km/h). The team shot over 100 plates, and then analyzed and counted the cracks that extended outward.

Then, after considering the material’s strength and thickness, Vandenberghe’s team devised a global scaling law for the number of radial cracks produced. It turns out that the number of cracks double for every fourfold increase in an object’s speed.

###### In the example above, the four cracks (left) indicate a speed of 70 km/h, whereas the eight cracks (right) indicate a bullet moving four-times as fast, 280 km/h. Credit: N. Vandenberghe/Aix-Marseille University.

The simple equation, which was based on Griffith’s theory of fracture, could be used by forensic scientists (e.g. to determine location of a shooter, or hypothesize about type of gun used; or to determine the speed at which a car was traveling when an object pierced through the window, etc.). The rule could also be used in archaeology or astronomy (e.g. to analyze impact events on terrestrial surfaces).

Read the entire study in Physical Review Letters: “Star-Shaped Crack Pattern of Broken Windows.”

Image: Shutterstock/EGD.