jimlux wrote:
> Steve, W3AHL wrote:
>
>> In my opinion, there is no way to do any stress analysis of pipe
>> without a material testing lab, and then the results only apply to
>> the specific batch of pipe sampled, at best. Pipe is not a
>> structural element.
We shouldn't lump all pipe together. There is structural steel tube,
which is really pipe and comes in both schedule 40 and 80. That is what
I have been using It has far more "spring" and strength than what we
think of as regular pipe. It will bend much farther than regular pipe
and just spring back while the regular pipe will take a set.
>> Even if you know the yield strength of the
>> steel, the manufacturing process does not result in a predictable
>> structural component. Ask a P.E. to certify a mast using "pipe"....
>> Hundreds of hams use pipe successfully for years. But it isn't a
>> sound practice that I would ever recommend to others as "safe enough,
>> most of the time".
>>
>>
The "structural steel" is used in buildings which require a predictable,
structural component. It's just this is pipe you don't purchase at the
local hardware store.
>
> Actually, one doesn't need a full-up lab, if you know what you want to
> measure and have some ingenuity. For instance, you could buy your piece
> of pipe, and test just that piece of pipe as a single instance in a
> purpose built jig.
>
> For instance, you could set up a piece of tower or some blocks of wood
> with holes in the appropriately places horizontally with sand
> bags/concrete blocks to hold it down, shove in your would-be mast, and
> hang weights on it to simulate the antenna loads and measure the deflection.
>
> One can even "proof test" by overloading it.
>
And if you are willing to sacrifice a piece of the stuff you keep
loading it to the point of a permanent deflection. That gives you not
only where it essentially fails, but the inbetween where it springs
back. This will even give you some concrete figures as to how much
safety margin you have or do not have.
> This is pretty common when you don't have enough information about
> material properties, or where the properties vary a lot (e.g. hand layup
> of composites).
>
> High performance experimental sailboat masts get tested this way (attach
> them to the wall, hang weights), as do composite airplane wings and
> such. There's an interesting video on youtube (I think) of a full size
> wing being tested under load
>
The Boeing 777 wing was tested to failure. It had a huge bend before it
*broke* abruptly. The breaks in the individual winds were almost
identical at the same locations, pressure, and deflections.
Another step is vibrational analysis, which predicts flutter or
"resonances" where feedback becomes positive.
> However, designing the test to meaningful also requires some cleverness
> and knowledge.
>
Maybe a bit of mechanical ingenuity but it's not all that difficult or
elaborate.
73
Roger (K8RI)
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