Thursday, May 2, 2013

Counting cracks in glass gives speed of projectile

Simple relationship between velocity and number of spokes in star-shaped fracture

A shattered windshield has a story to tell. The key to hearing it is counting the cracks.

The number of cracks that emerge in a plate of glass or Plexiglas relates to the speed of the object that broke it, researchers demonstrate April 26 in Physical Review Letters. This simple relationship could prove useful for forensic scientists, archeologists and even astronomers.

Over the past century, most research into cracks has focused on parameters that determine whether a material remains intact when struck. 

Nicolas Vandenberghe and his colleagues at Aix-Marseille University in France decided to try something different: They wanted to push glass and other materials past their breaking points and study the resulting fractures. They wondered if they could connect the patterns of cracks to the properties of the impact that created them, something no one had done before, Vandenberghe says.

So he and his team set up a shooting gallery. The targets were small squares of glass and Plexiglas between 0.5 millimeters and 3 millimeters thick. The researchers’ weapon was a gun filled with pressurized air that fired 4-millimeter-wide steel balls, about the size of BB’s, at speeds ranging up to 432 kilometers per hour.

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