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Most materials are at least as strong in compression as they are in tension. Many are far stronger. However, long thin struts fail easily when taking a compressive load.
This failure has nothing to do with the compressive strength of the material. Instead, it happens because the strut has only
finite stiffness, and can easily bow to one side or the other. Once this begins, it becomes a runaway process and the strut either bends or breaks (the latter being a _tension_ break starting on the outside of the bend). For this reason, struts under compression are best made out of a stiff - rather than a strong - material, because stiffness resists the initial bowing.
MaxCo. has looked into this problem, and has developed its Piezo-stabilized strut. The strut is made of a fairly nondescript material, but has three lengthwise stripes of piezoelectric material bonded to its surface.
As soon as the strut begins to bow under a compressive load, the piezo strips generate a voltage which is detected by what our marketing team propose to call "the detector". This signal is fed to a control box (we are thinking of calling it "the control box"), which then applies a high voltage to the appropriate piezo strips. This, in turn, provides a modest corrective force, opposing the bowing before it has become at all significant.
In this way, bowing failure from a purely compressive load is almost eliminated - so that the long, thin strut can take as much load as a very short one of the same diameter. Failure will ultimately happen only when the forces are large enough to crush the strut - many times larger than the forces it could otherwise support.
Dynamically-stabilised vacuum balloon
by [MaxwellBuchanan]. Mentioned in my anno. Links to this idea. [notexactly, Sep 25 2019]
Wikipedia: Active structure
https://en.wikipedi...ki/Active_structure
What this is an example of. Wikipedia conflates them, but any of the type that needs power to maintain its shape is liable to //fail catastrophically when its batteries go flat//. [notexactly, Sep 26 2019]
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Congratulations - you have invented a perpetual motionless machine. |
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You've got a "voltage" coming from the strip and a "high voltage"
going to it. This difference suggests a new kind of bridge which
can fail catastrophically when its batteries go flat. |
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It could trickle charge via the day to day bending that
doesn't need high voltage correction. |
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But then it would fail if things were still for too long, so
you'd need some mechanism to introduce bend to ensure
voltage output remained constant. |
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If you suspended pairs of ropes from the bottom that
went down about 100 meters and joined in small wooden
platforms, and then had people climb down the ropes, sit
on the platforms, and swing their legs back and forth, this
could do the trick. |
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Thankyou for that, [Max]. Very good. |
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<indicates graciously that [MB] should resume his seat> |
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(slide of Münchenstein rail disaster showing engine in river) |
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<turns to address audience> |
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This introduces us nicely to an extension of Euler-Bernoulli beam
theory and its limitation in real-world situations, as opposed to
assumed perfectly uniform beams to which infinitesimal uniform
strain is applied. |
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(line diagram showing beams of varying aspect ratios) |
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It is necessary to modify the basic equation when dealing with
the class of structural elements referred to as "slender bars". |
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(diagram showing comparison between ideal bar and slender bar) |
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You may recall that we briefly touched upon this divergence from
theory when conducting a mercilessly efficient and unnecessarily
ruthless but academically rigorous demolition of an unrelated
halfbakery idea ... |
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(colour photograph of unfortunate and crestfallen halfbaker
having face pushed into mud by judiciously applied boot on back
of neck, surrounded by jeering classmates) |
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... which you will no doubt recall. |
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<pauses, glares at victim > |
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Now, restricting the analysis to two-dimensional form for the
purposes of demonstration, who would like to explain to the
class the small but critical flaw in [Max]'s proposal ? |
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<surveys sea of blank faces, pauses more in hope than
expectation > |
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// a new kind of bridge which can fail catastrophically when its batteries go flat// |
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Certainly. It would be fantastically unsafe in that regard. |
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And thank you, [8th], for so ably illustrating my point. |
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I think it's cool as an idea to create the exact opposite of a
stable bridge, that being a dancing bridge. Make it so and
earn some crumbs. |
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Maybe this is what Star Trek does when they "apply power to structural integrity" (or something...) on the Enterprise?
Perhaps His Borginess could provide deeper insight (I'm no Trekkie). |
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Do you have anything you wish to say before Summary Execution
is carried out ? Your statement will be read out at your
subsequent trial. |
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// Maybe this is what Star Trek does when they "apply power to
structural integrity" (or something...) on the Enterprise? Perhaps
His Borginess could provide deeper insight // |
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As well as its external deflectors, the Federation's Constitution
and Galaxy class starships have an internal network of
waveguides which distribute forcefield power through the
infrastructure to support it under extreme stress. The Structural
Integrity Field works in conjuction with the Inertal Damping Field. |
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It's all in the Technical Manual. |
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Mmmmm ... not bad, but maybe a bit more work needed. We'll
call you. The way out's over there ... NEXT !! |
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Most struts would be designed to their specific use. [Max] is designing a strut that would never take the loading he suggests or it wouldn't be that particular strut. |
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Knowing what a particular strut is taking is still a good idea. |
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// Knowing what a particular strut is taking // |
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"Bend over, strut, and take it like a ... !" ... no, that's not quite
right, somehow ... |
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Was there something about "Squeal like a pig !", too ? |
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"my trousers contain piezo linear actuators that greatly assist in dancing prowess and the ability to stay upright after a few jars." |
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Yes, but what are the civilian applications? |
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Dispersal of a riotous mob in under ten seconds ? |
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Yes, he went on to found "Thank God It's Faraday's" ... |
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The term you need if you want to look up more on this topic
is "active stiffness". |
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The problem I see is that the piezo strips will apply
additional stress to the substrate. I'm not sure exactly how
the superimposed stresses will turn out, but they'll probably
result in failure somewhere, because while they'll cancel out
in some places, they'll add in others. |
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The additional stress imposed by the piezos is negligible. |
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If you want to visualize this, take a long, thin strip of wood
and stand it vertically on the ground. Push down on the top
end, and it will bow very easily. (You can also do this with
a 12" plastic ruler, if it's one of the very flexible ones.) |
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Now get someone else to push down on the top, but this
time use your fingers in the middle of the strip to stop it
bowing. Even a tiny constraining force from your fingers in
the middle will stop the strip bowing; you only need to
apply a large corrective force if the strip is already very
bowed. |
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Incidentally, this is basically how knees work when you're
standing. As long as the leg is more or less straight, only
very small (and unconscious) exertion of the muscles is
needed to keep it straight. These small forces stop any
bend before it develops. But if you bend at the knees (a
lot of "bow"), your muscles have to work much harder to
straighten the leg again. |
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I just happened to see the tagline "The leaning tower of
Piezo", and guessed it came from this idea (which I had been
reminded of by the link on the idea "Dynamically-stabilised
vacuum balloon" [link]), but I guess not. |
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