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Name: Selden
Status: educator
Grade: N/A
Location: CA 
Country: N/A
Date: 1/9/2006


Question:
I am an English teacher but like to talk science sometimes. Some years ago, I had an air-pump wine bottle opener that with a needle into the cork pumped air into the bottle and popped the cork out. I liked using it because it was innovative and very cool. Once, I used it on a triangular-shaped bottle of sangria from Spain and the bottle exploded, which in retrospect seemed logical, and I learned a lesson: "science is everywhere," my scientist colleague used to say.

Recently, I got in a rather in-depth argument with one of my students. We both agreed that the non-circular bottle shape caused the problem, and would every time. I said the bottle exploded because the triangular shape caused there to be different pressure at different points on the bottle surface area. He said that pressure was distributed equally to all points by the liquid, but the bottle exploded because the glass at different points was weaker than at others. Who was right?


Replies:
In a liquid or any fluid at rest, the pressure is the same in all directions. The failure of the bottle is due to stresses that remain in the glass when the bottle is cast. These stresses tend to be greater in places where the curvature of the bottle is greatest. If there were any differences in the pressure in different regions of a fluid, gas or low viscosity liquid, the fluid would move to equalize the pressure difference.

Vince Calder


Your student was partially correct. What most likely cause you explosion was an increase in stress at the corners of the triangular bottle. A fluid contained in a bottle will exert the same amount of force perpendicular to the walls of the container at every point of the bottle. What caused the problem was the fact that the angular shape of the container creates what is know as stress intensities. The change in shape, i.e. the sharp corner of the triangle, causes an increase in stress. This is a common problem in mechanical behavior of materials in many contexts. For instance, a simple hole in a plate will increase the stress around the hole by sometimes up to a factor of 3. Once you increased the pressure of the bottle contents, you increased the stress on the bottle. At the corners you probably increased the stress by some multiplication factor that exceeded the yield strength of the bottle. Also, with glass, slight flaws such as micro-cracks around a corner (or anywhere for that matter) can be exacerbated by an increase in stress. Throw in the multiplication factor of stress intensities, and you get the explosion. That is the brittle nature of ceramics and glasses. This is one of the reasons I still use a cork screw to release my bottle of merlot.

Christopher Murphy, P.E.
Mechanical Engineer
Air Force Research Laboratory



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