Popping a Balloon

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Popping a Balloon

Postby Himself » Tue Sep 19, 2017 4:37 pm UTC

I've had this question in my head for some years now.
Back when I was in high school a guy in my homeroom brought in a balloon for his birthday. It was clearly overinflated. A friend of his started lightly punching the balloon. After we had settled down and were listening to the day's announcements the balloon, which had been undisturbed for a minute or two, suddenly popped.

Why would the balloon pop then and not while it was being punched?
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Re: Popping a Balloon

Postby SDK » Tue Sep 19, 2017 5:01 pm UTC

Most likely the balloon failed due to creep. Creep is when the molecules or grains of a material are sliding past each other. If enough creep happens, voids will form and failure will occur. It happens slowly, so takes some time, but the only other requirements are that 1) the temperature in degrees K be above half the melting point (clearly the case for the balloon at room temperature) and 2) that some stress is applied (due to the pressure in this case). When a balloon pops for no reason, this is why. If it doesn't pop, and instead slowly deflates, the stress wasn't high enough to cause creep failure before the air escaped (ie: it wasn't overinflated). Make sense?

Compare that to a simple tensile failure, where the material is stressed enough to cause the actual bonds between the molecules to break. When you stretch an elastic band to the breaking point, for example. This is the kind of stress punching the balloon would introduce. Every punch would cause the pressure to increase slightly, increasing the stress on the balloon. If you punched it hard enough, it would break, but any punch weaker than that would not break it. There's a defined line there (known as the yield strength) where any weaker punch won't do anything, and any stretch you've caused will bounce back - it's all elastic so the balloon goes back as if nothing had happened. Any punch stronger than the yield strength will cause permanent stretching, and probably simply failure in this case. Your friend wasn't punching it hard enough, so the balloon didn't fail.

Note that that doesn't necessarily mean that the punches had no effect. The higher the stress is, the higher your creep rate is. Those instantaneous hits where the stress was increased for a split second would have also seen an increased creep rate. That should be a very small effect though. Time is more important, so the balloon likely would have popped all by itself no matter what.
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Re: Popping a Balloon

Postby p1t1o » Wed Sep 20, 2017 9:52 am UTC

I have heard that the term for the physical process of the failure mode of a popping balloon (a bursting polymer) is "catastrophic unzipping".

If accurate, its one of my favorite science terms. Even better than "spaghettification".

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