>
> 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
The test that actually pulls apart test samples until they break measures
the "ultimate strength"
The yield strength (actually yield stress, in pounds per square inch) is
where the material just starts to permanently deform. For a mast, this
would be where it starts to take a permanent bend instead of springing back
straight. You can also test this in a test rig like you're thinking: it
would be the stress where the little test piece (I think they call it a
coupon in that field) takes a little permanent stretch.
You might not think this would happen on masts but it absolutely can without
buckling if the wall thickness and everything else is right. The antenna
windload can be multiplied by a huge factor because of the leverage of the
mast, and if the material starts to stretch on one side and compress on the
other in a permanent way, now your mast is bent. You can also have a
buckling failure where the wall crumples first (then deforms permanently and
breaks) but this all depends on the wall thickness and material and stuff.
Both happen on masts, but I feel like if your wall buckles before the mast
takes a slight permanent bend then you were REALLY using the wrong mast.
If it yields a bit and takes a slight bend, it was just slightly
overloaded. In some sense, that seems like a slighter mistake than if it
folds over because it buckled and that raised all the stresses well beyond
the yield stress near the buckle.
73
Dan
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