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Studying how a drinking straw deforms

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choppergirl

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Joined
Jan 30, 2015
Messages
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Location
air-war.org
A lot of aircraft seem to be built out of tubes, so I've been playing around with plastic drinking straws, of the Burger King / McDonald's variety.

When a sideways load is placed against them (or a compression load, that almost always turns into a sideways load), they always fail in the same predictable way.... they crease on the inside, and flatten on the outside, at the point of failure. Try it with any drinking straw you have lying about.

If you could stop that crease from beginning (with a structural member inside the tube), you increase the strength of the tube.

I first considered placing a square tube like structure around a tube, making a structure that had 4 flat surfaces to attach to, but would not twist like a plain square would, and you could bolt through the center of 4 sides... like for a motor mount or fuselage boom (on a fuselage boom type ultralight0. A square tube that gave you four flat surfaces to bolt to, enhanced to resist twist, so to speak. Each corner acts like an L channel, but reinforced by being attached to the others, and the round tube inside resists twist.

Then I considered a tube with an equilateral triangle inside of a tube, for something such as a general structural member or strut. I think an equilateral triangle holds maybe the best promise, as a way to strengthen any tube, particularly, if any point of the triangle is placed in the direction a sideways load is most likely to develop. I played around with triangle paper tubes slid inside straws and tested them.

If you were to fill the voids inside the tube further with something lightweight that is strong under compression, that would also strengthen the tube. If the edges of your interior structure were glued/attached to the tube, it would further increase the strength. A lot of things are very strong under compression, but not otherwise... I'm wondering if there is some material I don't know of that is very strong under compression, but also very lightweight, that one could further fill tubes with.

Pondering.... I-beams, corrugated cardboard, bridge trusses, and the like.... I wonder if structural engineers have solved tubes bending already... and how an equilateral triangle inside a tube reinforced against side loads, holds up under tests under 120 degrees of side load testing.
 

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