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Mythbuster Video for reference

In the imploding tank car story, hot steam gas at atmospheric pressure was put inside a steel tank car, and the valve closed overnight. Gradually the steam condensed and cooled down, severely lowering the pressure inside the tank, and then atmospheric pressure squeezed the tank car.

But my question is, why does the implosion take place so suddenly and rapidly? Why doesn't the tank car implode slowly and gradually as the pressure continuously drops inside (ie. the dent slowly grows)?

Compare this with, say, an empty water bottle. When you suck the air out, the bottle continuously deforms and flattens out, it doesn't retain its shape and then suddenly collapse. How are the two containers designed differently?

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  • $\begingroup$ The materials and mechanical design are designed to withstand some pressure difference and deform elastically. Once the materials or mechanical limits are exceeded, plastic deformation starts and there is nothing holding it back (is a simplified sense - work hardening can hold it back a bit longer, but...). So, the tank car hits the maximum stress and that is that. $\endgroup$ – Jon Custer Apr 5 '17 at 14:59
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    $\begingroup$ the dent disrupts what was initially an even, distributed pressure force about the tank walls. Small imperfections concentrate stress, force and this escaltes further stress, pressure that cascades to implosive results. $\endgroup$ – docscience Apr 5 '17 at 15:24
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Tanker cars that carry fluids at normal temperatures and pressures are generally designed to withstand the outward pressure that the fluid exerts on the walls. The construction is usually cylindrical or ellipsoidal in shape. These shape help to distribute the hydrostatic forces evenly and eliminating sharp corners reduces the likelihood that pressure stresses will concentrate over small areas.

But if you empty a tank of the same construction, seal it and provide any means to evacuate or reduce the pressure within, there are two factors that can lead to sudden collapse. (1) the vacuum pressure force is in the opposite direction that the normal hydrostatic forces impose on the tank and (2) the vacuum forces could conceivably reach greater pressure than what the hydrostatic forces reach (up to 14.7 psig). If the tank uses external gussets that reinforce the strength of the tank, the gussets no longer assist in supporting the tank wall under vacuum since they act to support in the opposite direction.

But to answer your primary question, all it takes is one minor deflection, fold in the tank wall, the even distribution of pressure is lost and all of the sudden stress is concentrated at that one point. This leads to rapid escalation of the collapse.

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    $\begingroup$ I'll leave it to your judgement, but it might be nice to mention buckling, since that is likely what happens to it. You have a thin walled structure under compression, at a certain pressure it will start to buckle, and once it starts the failure would cascade quickly. $\endgroup$ – JMac Apr 5 '17 at 16:14
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    $\begingroup$ Thinking about it a little more, it may even be some sort of multi-modal buckling since there would be multiple slender dimensions being compressed. I realized that's essentially the same as what you said about pressure, the small deflection would be like buckling starting. $\endgroup$ – JMac Apr 5 '17 at 16:22
  • $\begingroup$ Got any links on this buckling phenomenon? Also, what is it about a plastic water bottle that makes it compress slowly whereas the iron tank car does so quickly? what material property is different? $\endgroup$ – Faraz Masroor Apr 7 '17 at 19:19
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    $\begingroup$ @FarazMasroor I think it's simply that the plastic water bottle is much weaker, so that first buckling appears when you've only built up a small pressure difference, and the resulting collapse to equalise the pressure is only small. The further slow collapse of the water bottle is simply it keeping pace with the change in pressure as you suck out more air, which is a relatively slow process. Whereas the tank doesn't have its first failure until there's already a large pressure difference, which it can no longer withstand once it buckles, so it collapses rapidly at that point. $\endgroup$ – Ben Mar 24 '18 at 0:43

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