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Under what conditions can glass crack spontaneously?

  1. Dec 8, 2018 at 2:51 PM #1
    Are there any conditions under which glass cracks spontaneously?
    Just now, my mother's rear windshield cracked spontaneously.
    The story according to her:
    Slowly approaching a stop sign. No busy traffic. No trees/animals in sight. Few people. Suddenly, the rear windshield cracked with a loud boom with no evidence as to what caused it. A police officer happened to be directing traffic and saw it occur. The officer did nothing about it since there was no indications as to what caused it.

    This is very strange! I don't believe that this could have just happened without a something hitting the window. I suspect it happened so quickly no one saw it. But, apparently, there were no cars, and few people, so they believe that the object impacting the glass scenario is not possible.

    My question now is: is it possible that the glass cracked spontaneously? For reference, the temperature outside was 34 degrees Fahrenheit, the temperature inside about 70 degrees Fahrenheit.

    Thanks for any replies/insight!
    IMG-1203.JPG IMG-1204.JPG
    In this picture above, you can see the indications that some impact occurred.
     
  2. jcsd
  3. Dec 8, 2018 at 3:10 PM #2

    Wrichik Basu

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    Given that the outside temperature was about 1°C and the inside temperature was near 21°C, I would make the trivial guess that it was a drop of water. But can it be spontaneous when temperature difference is 20°C? That's a question. Moreover, a single drop of water will create cracks that originate from a point and spread, but not smash the glass like that. I can surely see some cracks travelling from the impact, but I doubt whether a drop can smash the whole glass.
     
  4. Dec 8, 2018 at 5:17 PM #3

    berkeman

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    That would be my guess as well. Given how many millions of cars are on the road in similar temperature conditions without spontaneous fracture, some outside impact seems likely.

    Search the back of the car -- unless it was a large ice ball from a passing airliner, there may be a rock or branch or other evidence of an impact.

    << I'm moving this thread from the technical forums to the General Discussion forum for now...>>
     
    Last edited: Dec 8, 2018 at 5:27 PM
  5. Dec 8, 2018 at 5:25 PM #4

    DaveC426913

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    You know? It just possibly could have been a meteorite.
    Stranger things have happened.

    Me - I'd take the time to go back to the site and scour the ground.
     
  6. Dec 8, 2018 at 5:28 PM #5

    DaveC426913

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    It's also possible by the way, that there were independent events.
    The first impact may have occurred at any time in the past, creating fractures in the glass that went unnoticed for days, weeks, years.
    Then something (maybe not even a second impact) caused the glass to collapse.
     
  7. Dec 8, 2018 at 7:12 PM #6

    Mark44

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    That would be my guess. A temp difference of only 20 deg (C) wouldn't do it, I don't think.
     
  8. Dec 8, 2018 at 7:19 PM #7

    CWatters

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    That rear window appears to be regular toughened glass not laminated like a front screen is. Toughened glass is prone to sudden failure after minor damage. The chip need not have happened recently, could have happened weeks ago, then the stress of going over a small bump in the road triggered the failure.

    PS: shower screens are also toughened glass and you have to look after the edges during transportation and assembly as a minor nick can cause a later sudden failure.
     
  9. Dec 8, 2018 at 8:28 PM #8
    Thanks for all the replies. They all seem very reasonable and likely. Here's a video on the topic but I didn't seem to come away convinced with a concrete explanation.
     
  10. Dec 8, 2018 at 9:25 PM #9
    Some random bullet fired into the sky from a gun/rifle miles away?
     
  11. Dec 9, 2018 at 3:03 AM #10

    Borek

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  12. Dec 9, 2018 at 6:21 AM #11

    Vanadium 50

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    As some people in another thread have been arguing, it could be aliens! Prove it's not!

    Glass does not spontaneously do this. If it did, we wouldn't use it as we do. By far the likeliest possibility is something like what CWatters said: that it was chipped some moments back and was unnoticed until it failed catastrophically.
     
  13. Dec 9, 2018 at 7:18 AM #12

    Borek

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    We are talking about glass that is mounted in a car, so it is under many different stresses all the time - like temperature gradients and car frame shaking, bending and twisting. Plenty of forces that could start non-spontaneous failure.

    That doesn't rule out earlier damage, could be a combination of both.
     
  14. Dec 9, 2018 at 7:18 AM #13
    It looks like the cause was as the others have said. Edge damage (circled) and particularly the impact mark (arrow) very likely initiated stress cracking which went unnoticed until the final, catastrophic failure.

    Chipped rear window(annotated).jpg

    Once compromised, normal stressors such as thermal cycling of the defroster wire grid will contribute to final failure.

    Unless your mom enjoys loud music this doesn't apply, but I once rented an SUV for a work-related road trip, and while driving east listening to Aes Dana's Leylines noticed caustics from late afternoon sunlight dancing on the dashboard. Looked back, and rear window was visibly pulsing in time with the kick drum.
     
  15. Dec 9, 2018 at 10:37 AM #14

    russ_watters

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    If that were true, it would have to be a design or manufacturing flaw. Engineers don't design things to fail under normal operating conditions.
     
  16. Dec 9, 2018 at 11:10 AM #15

    Vanadium 50

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    That's right. Car glass gets impacts and abrasions all the time. Even if it looks "spontaneous", it's often not.
     
  17. Dec 9, 2018 at 11:11 AM #16

    Borek

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    1. Which both happen all the time despite best efforts of the engineers designing the car itself and the manufacturing process.

    2. Yes, in most cases such things don't happen, which is why there was probably some additional element. We know nothing about how old the car is and what its history was.

    My window (story linked earlier in the thread) wasn't designed to crack, yet it did for no apparent reason.
     
  18. Dec 10, 2018 at 1:19 PM #17

    collinsmark

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    Yes, it could have been a manufacturing flaw. Tempered glass contains a lot of internal stresses by its very nature. That's what makes it tempered glass. (One could reduce the internal stresses by annealing it, or simply not tempering it, but then it wouldn't be tempered glass.) The tricky bit is make sure the internal stresses are distributed roughly evenly throughout the glass, and such that the internal stresses are within acceptable thresholds at every location. I'm guessing that's easier said than done.

    On the other hand, stresses might also come from outside the manufacturing process. If the car was ever in a fender-bender -- even a mild one -- the auto shop will likely pound out or replace the dented fender, but would never replace the rear window if there were no visible signs of damage in the rear window. The new, extra, uneven stresses [edit: might*] remain in the window though. The OP might know more about the car's history, of course, unless the OP's mother obtained the car second-hand; in which case it's anybody's guess.

    *[depends on the nature of the fender-bender and the rear window's mounting, etc.]
     
    Last edited: Dec 10, 2018 at 1:45 PM
  19. Dec 10, 2018 at 1:51 PM #18

    DaveC426913

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    If it were a flaw, could it fail in a way that looks like the closeup in post #13?

    I mean, that really looks like an impact.
     
  20. Dec 10, 2018 at 2:18 PM #19

    Klystron

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    Regrettably must agree with earlier posts that impact from gunfire remains probable.
     
  21. Dec 10, 2018 at 2:31 PM #20

    collinsmark

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    I agree. It does visually appear like an impact. The pattern seems to radiate from a specific point.

    I guess what I'm saying is that, particularly given the testimony by the OP's mother,
    • We don't really know how long before shattering that the impact spot was there,
    • If some sort of impact did coincide with the shattering (maybe a small, hard pebble or piece of porcelain thrown out from the street by a nearby truck tire), the impact might not have been the only stress factor involved, and
    • maybe there wasn't an impact, and that spot just happened to be the point on the glass that was most brittle. Even if the shattering happens spontaneously, it has to start from somewhere.
    Or maybe it was from an small caliber firearm or air rifle (i.e., BB gun). It's kind of difficult to tell from the picture. It seems kind of odd though that somebody would be brazen enough to shoot at traffic with a police officer right there directing traffic. So let's not jump to conclusions.
     
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