Speaking of DEFLECTION, how about a way to deflect this subject? How about
BIRDS, owls, ospreys, wenches, winches, or mother-in-laws, in no particular
order. Thanks,.... - M
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Steve, W3AHL
Sent: Saturday, March 20, 2010 3:23 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] material testing Re: 2" OD pipe.. again
If all you want to do is determine deflection under a static load, yep,
that's all you need to do -- hang a weight on it. But that is the least of
the issues. What is the composition of the steel? How brittle is it, will
it fatigue with cyclic stresses?
But the major issue in pipe is the weld joint. In electrical resistance
welding, there is no high tensile strength filler used. The joint is made
of what the pipe is made of. If the machine speed is too fast (to cut
costs) and the current too high (because the speed was increased) then you
have poor penetration, a brittle weld because of the impurities of the base
metal and the improper heating / cooling gradients, porosity due to cheap
flux and martensite formations if it wasn't annealed properly. After a few
tens of thousands of cycle swaying back and forth the weld joint develops
microfractures. One strong gust splits the seam someday, ruining a couple
of thousand dollars worth of antennas,coax and installation.
If the submittal form for the material doesn't say it was tested, it
probably wasn't, because it wasn't intended for an application that required
it. Hydrostatic testing of pipe at 2X its working pressure isn't worth much
peace of mind in a structural application.
It just doesn't make sense to use pipe instead of real mast tubing to save a
$100 or so. Buy tubing from a reputable supplier, manufactured and TESTED
to the correct spec for the application.
Steve, W3AHL
Date: Sat, 20 Mar 2010 07:48:22 -0700
From: jimlux <jimlux@earthlink.net>
Subject: [TowerTalk] material testing Re: 2" OD pipe.. again
To: "Tower and HF antenna construction topics."
<towertalk@contesting.com>
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. 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".
>
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.
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.
However, designing the test to meaningful also requires some cleverness
and knowledge.
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