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Question about pressure explosions

  1. Jun 1, 2005 #1
    Hi, now, i've observed liquid nitrogen in a closed plastic bottle, will eventually explode pretty violently once the pressure gets high enough (that's what i'm assuming happens, the nitrogen gets warmer which increases the pressure, plus it goes from liquid to gas form). But my question is this:

    Why does it end in a explosion but not a small tear/rift in the bottle? I'd expect that it does have the energy to cause a rift/tear/break in the material since it does have enough energy to make it explode.
     
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  3. Jun 1, 2005 #2

    FredGarvin

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    Most likely it is due to the rate at which it is being heated. Since liquid nitrogen is at such a low temperature compared to average room temperature, LN2 in an uninsulated container is being heated at a pretty substantial rate.
     
  4. Jun 1, 2005 #3

    russ_watters

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    I would say its probably more due to the material properties. Depending on the plastic, it can get very brittle at low temperatures. Also, explosions sometimes do start as a small tear, but the tear itself weakens the surrounding material, causing a sort of cascading failure.
     
  5. Jun 1, 2005 #4

    brewnog

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    The pressures a Coke bottle can withstand are surprisingly high. Once failure initiates, the bottle is weakened significantly (almost instantly), so catastrophic failure occurs with a bit of a bang. As russ suggested, the low temperatures are likely to induce embrittlement of the polymer structure, which acts to reduce the failure stress, and causes propogation of a defect to happen much faster.
     
  6. Jun 1, 2005 #5

    PerennialII

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    Polymers typically have "non-existent" leak-before break characteristics, i.e. fracture toughness, when we're considering behavior near/below crystalline temperatures etc.
     
  7. Jun 1, 2005 #6
    I'm afraid i don't know much about explosions in general, but in this case is it basicly just the effect of a highly compressed gas escaping and thus expanding very quickly and hurling the parts of the bottle around? And also if so what is it that causes the noise that follows the explosion?
     
  8. Jun 1, 2005 #7

    brewnog

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    Yes, pretty much. The bottle is too weak to withstand such high pressures, and once a critical pressure is reached, a crack will appear in the bottle. This greatly reduces the bottle's strength, and so a tear will form very rapidly.

    The 'bang' is likely to be a combination of the plastic being torn apart rapidly, and of the sudden expansion of gas escaping from the bottle.
     
  9. Jun 2, 2005 #8

    PerennialII

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    .... you can try to further visualize the failure part by considering behavior of 'brittle' materials ... after reaching a critical state (which would be best evaluated using fracture mechanics) the applied loading exceeds the material resistance for crack propagation by a healthy margin, and next thing you know you've a crack propagating with speed comparable to the speed of sound. And when the material is brittle, there is nothing to arrest the crack other than complete structural failure (like hitting a window with a sledge hammer).
     
  10. Jun 4, 2005 #9

    Clausius2

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    I wouldn't dare to call it an "explosion"....

    the term "explosion" is more accurated for sudden combustion explosive reactions...
     
  11. Jun 4, 2005 #10

    Danger

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    Not like I'm qualified to or anything, but I agree with that. 'Explosion' presumes the production of heat. This would more accurately be described as an 'overpressure', similar to a balloon bursting. Incidentally, if you have a chance to you should check out high-speed photography of things like this happening. You'll see that the initial activity is very similar to that which would happen in a low-pressure rupture of the same material, but it happens a lot faster. I have some nice pictures of bullets going through balloons and oranges and such. It's fascinating. (Don't ask me to post them; they're in a gun magazine and I don't know how to work my scanner.)
     
  12. Jun 5, 2005 #11

    Clausius2

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    Exactly. I agree.
     
  13. Jun 5, 2005 #12

    Danger

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    Wait a minute! You can't agree with me; I was agreeing with you!
    Maybe it's just because I'm getting into my 11th beer, but I just noticed that the word 'agreeing' looks a bit like a weenie dog. I think that I should head back down to GD now...
     
  14. Jun 5, 2005 #13

    Clausius2

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    I don't know. You should try with the 12th. Maybe you see clearer the stuff. :rofl:
     
  15. Jun 5, 2005 #14

    Danger

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    Aw nuts; they've worn off. Go to sleep for a couple of hours, and look what happens. Anyhow, I'm sort of getting used to some of this internet stuff now that I've had it for a couple of months, so maybe I'll see if I can find some of those pictures on line.
     
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