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Cracked Windshields Reveal Impact Physics

Many commuters can relate to the common plight of cracked windshields. The ride may be going smoothly until a pop signals a small crack in the corner of the windshield — a small crack that will soon radiate into a spider-like obstruction.

Recently, researchers from Aix-Marseille University in Marseille, France published research on this topic, and they revealed a relatively simple relationship between the velocity of an impacting object and the number of radial cracks in the glass. Nicolas Vandenberghe and his colleagues found that the number of cracks is proportional to the square root of the impact speed for small steel projectiles hitting samples of plexiglass.

For example, quadrupling the speed of a small rock would double the number of triangular cracks emanating from the impact site. While this may provide little solace for an angry motorist, the research may prove useful in ballistics testing, forensics, and even protecting spacecraft from the dangers of the cosmos.

Videos of high speed steel projectiles breaking through plexiglass targets.
Top: A projectile traveling at about 50 MPH impacts a 1 mm-thick target.
Bottom: A 126 MPH projectile hits a plexiglass target.
Video Credit: N. Vandenberghe/Aix-Marseille Univ.

The research team used high-speed cameras to record the plexiglass impacts, and you can see two of the impacts above. The researchers shot at sheets of plexiglass with a variety of thickness, and they varied the speed of the projectiles as well. The fastest experimental bullets traveled at over 260 MPH.

When I was an undergraduate doing physics research, we studied similar sorts of impacts on a much smaller scale. My research focused on determining how small space dust particle impacts translated into electrical signals on the Student Dust Counter aboard the New Horizons spacecraft.

Although the instrument used a different type of material, the basic impact mechanics were quite similar. When I worked on the project, I was initially surprised by how destructive even microscopic projectiles can be, given a high enough velocity.

A fair amount of this research has been devoted toward finding better ways to protect satellites and spacecraft from extremely fast-moving micrometeoroid. Although they're tiny, some micrometeorites can reach impact speeds of tens of kilometers per second.

Just a few months ago, one such micrometeoroid likely left a "bullet hole" in the International Space Station's solar array. Thankfully, it didn't affect any of the crew.

I imagine this research (or further research in this area) could prove especially useful for forensic scientists looking to trace bullet velocities at crime scenes. To read the full story behind this research, check out this APS Physics summary article or the research paper itself.


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