Star-like cracks can reveal the speed of a bullet

[Nipuna]

PIECING together details of a car accident or crime scene could become a bit easier thanks to star-like cracks in glassy substances. These characteristic patterns can reveal the speed of the objects that made them.

It has long been known that different materials require different amounts of energy to crack. But until now few studies have examined the patterns of cracks left behind to trace back details about the impact.

Nicolas Vandenberghe and colleagues at Aix-Marseille University in France used an air gun to fire small metal cylinders at glass plates at increasingly higher speeds, reaching 432 kilometres per hour. A high-speed camera filmed each shot, and the team counted the radial cracks formed by the impact. "As surprising as it might be, that had not been done before," says team member Emmanuel Villermaux.

The team found a unique relationship between the number of cracks and the projectile's speed. They were able to use this to develop a simple equation that can tell how fast an object was travelling, based on the type of material it hit (Physical Review Letters, doi.org/mdd).

Robert Ritchie, a materials scientist at Lawrence Berkeley National Laboratory in California, says the work is a useful analysis. "For forensics, you could use this to find the energy of a bullet." The work could also reveal how fast a car was moving just before an accident, by looking for cracks in the windshield created by stones kicked up from the road.

This article appeared in print under the headline "Starburst fractures can help forensic experts crack a case"