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Impact on glass and consequences of said impact

  1. Sep 18, 2014 #1
    Hello everyone!

    I happen to read this forum a lot, but I've never posted/became a member since I didn't have a specific question that wasn't already asked before or because I did not feel I had anything relatively original to contribute. So, I have a question to which I cannot seem to find an answer to and hope that someone with the knowledge can answer it.

    So my situation is as follows:

    I am driving down a road at about 40 mph when suddenly a small crack on my windshield appeared. The crack then spread to about 6-8 inches within minutes. Someone was cutting the grass using an edge trimmer on the sidewalk where I drove by.

    I would like to know if it is possible for a small, smooth pebble/rock to be propelled from the ground with the weed-eater at a fast enough speed to make a small, clean hole a few mm in length without having a star-like-crack effect on the glass, and only perforating the first glass layer of the windshield? Conversely. if the same rock was flying at much lower speed, would that then cause a messier/star-like crack instead of a clean little hole? Additionally, only the first outer layer of the windshield is damaged.

    My take on it, is that it is definitely possible for this to happen, because if you look close enough, the rock seems to have made a bit of an "oscillation/ripple" effect as it was leaving the area of impact in the direction it was heading. I believe the rock came from the right side (from the driver's perspective), and headed off toward the left side (again from the driver's perspective).

    What's your take on this situation?
  2. jcsd
  3. Sep 19, 2014 #2

    Andy Resnick

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    Automotive (safety) glass is a composite material- layers of tempered glass separated by a plastic film. What you observe us likely due to the tempered glass- the residual stress energy means cracks form very easily.
  4. Sep 19, 2014 #3


    Staff: Mentor

    Definitely, although I would probably call anything that didn't perforate through all the layers a "chip" rather than a "hole".

    Regarding whether the initial damage is clean or messy, I am not sure what causes that. I would think that lots of details are involved including the mass and composition of the projectile.
  5. Sep 19, 2014 #4
    I am no expert either, but love to speculate...

    ...I was going to say that the clean-cut vs star-like scratch probably has to do with the angle of impact or in other words, the component of the velocity perpendicular to the plane of the windshield...if you are driving along the side walk and a pebble is launched by the lawn mower, chances are the impact is going to come from a rather small angle with the pebble almost going by sideways...as opposed to be driving on the highway and hitting a pebble that has been lifted into the air right in front you, this is more of a frontal impact.
  6. Sep 22, 2014 #5
    As expected--good responses guys, gave me a bit to think about. Thank you!
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