[TowerTalk] terminology

[jimlux]

Dayle Johnson wrote:
> 
> 
> 
>  I have seen a lot of mention about "yeild strength" of this and that.....is this the right term?
>  
> In order to measure this material function, test samples are pulled apart.
> A tower mast to me would not get this type of force?
>  
> Buckling would be a better description and measure of the force on a mast wouldn't it?
>  
> 

They're all related.  You're looking at the basic properties of a chunk 
of the metal.  When the stress on it is greater than the yield, it 
deforms permanently. You can put the stress on in a variety of ways 
(bending, for instance, stretches one side and squishes the other).

Buckling is a structural thing (as opposed to a material property)..

Think of trying to crush a steel hockey puck 2" in diameter (a short, 
large diameter column).   It fails when the metal just can't take the 
pressure, and it squishes down.  The force is in a straight line, and is 
pretty uniform over the whole puck.  The failure load for the 
*structure* (the column) is basically the cross sectional area * yield 
strength of the material.

Now take a 20 foot long rod, 2" in diameter.  Start applying the load. 
At first, the stress on the rod is the same everywhere in the cross 
section.  However, what will happen is that the rod will start to bend 
pretty early.  As soon as it starts to bend, the force per unit area on 
the inside of the bend is bigger than the force per unit area on the 
outside of the bend. (just as if you held the rod horizontally by one 
end, and let it droop..  the top stretches, the bottom shrinks.)

THe more pressure you apply, the more the rod bends, and the more the 
force is bigger on the inside than on the outside (because the average 
force has to be the same...nothing else is pushing on it).  As soon as 
the load gets bigger than the yield strength, it fails, and generally 
catastrophically, because once it starts to go, the bend gets bigger 
real fast.

So you can see that the "critical" part of the rod is the surface of the 
rod on the inside of the bend.  In fact, that's why tube works almost as 
well as solid rod, for long skinny columns.