Soda Can Crunch - Writ Large

Those paper thin soda cans can hold soda at up to 50 psi at times, as a more familiar example, of course. Actually, attaching a strain gage to a full can and opening to check the internal pressure by reading the delta strain is a popular undergrad exercise, I understand.

Brian Whatcott

Interesting. A naive analyst might wonder how 15 psi of external pressure can crush a pressure vessel rated to 100 or 500 psi.

It's a pity he didn't discuss that.

Cheers

Greg

50 psi surprises me. WAG on thickness =0.1mm. 3*10^5*60/2/.1/10^6 = 90 N/mm^2 OK, about right for aluminium

Cheers

Greg

I am naive, then why ?

A thin structure can stand tension quite nicely - think of the balloon, for example. But a thin structure is in trouble with compression - it buckles easily

Brian Whatcott Altus OK

jobijoba thought carefully and wrote on 11/21/2004 6:16 AM:

I don't know if you're naive, but there are two entirely different ways to design pressure vessels (like railroad tank cars) depending on if they are subjected to internal or external pressure.

The primary consideration for internal pressure design is to prevent stresses from exceeding the strength of the material. An exploding tank car is an example of stresses exceeding material strength.

The primary consideration for external pressure design is to prevent buckling. I don't know too much about buckling, but I know it's more complicated design process and difficult to predict. Your steam-cleaned tank car is an example of buckling.

Here's a site that shows some typical equations for internal and external pressure vessel design. Depending on which code you based your design, the equations may be slightly different:

In addition to these equations, we also consider secondary effects like welding, corrosion, material weight, local stresses created by penetrations, dynamic forces, etc.

BTW, someone on your site mentioned that the tank car would probably shrink perfectly in zero gravity. That's not true, the car would buckle in much the same way.

Lance

*****

Soda can thickness is typically .007"

Years ago at an unnamed nuclear power station, a large filament wound water storage tank was hydro tested by filling it with water from a fabric fire hose through a top-mounted connection and by then pressurizing it. At the completion of the test, the tank was emptied through the bottom drain. The crew had failed to disconnect the fill hose or otherwise vent the tank, the hose sucked flat, and the tank imploded in impressive fashion.

Just because something isn't in the procedure doesn't mean you don't have to do it.

Fred Klingener