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.-- MaxwellBuchanan, Nov 29 2017 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_structureWhat 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] Congratulations - you have invented a perpetual motionless machine.-- normzone, Nov 29 2017 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.-- pertinax, Nov 29 2017 It could trickle charge via the day to day bending that doesn't need high voltage correction.
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.
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.-- mylodon, Nov 29 2017 Thankyou for that, [Max]. Very good.
<indicates graciously that [MB] should resume his seat>
<click>
(slide of Münchenstein rail disaster showing engine in river)
<turns to address audience>
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.
(line diagram showing beams of varying aspect ratios)
It is necessary to modify the basic equation when dealing with the class of structural elements referred to as "slender bars".
(diagram showing comparison between ideal bar and slender bar)
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 ...
(colour photograph of unfortunate and crestfallen halfbaker having face pushed into mud by judiciously applied boot on back of neck, surrounded by jeering classmates)
... which you will no doubt recall.
<pauses, glares at victim >
<click >
(set of formulae)
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 ?
Anyone ?
<surveys sea of blank faces, pauses more in hope than expectation >-- 8th of 7, Nov 29 2017 // a new kind of bridge which can fail catastrophically when its batteries go flat//
Certainly. It would be fantastically unsafe in that regard.
And thank you, [8th], for so ably illustrating my point.-- MaxwellBuchanan, Nov 29 2017 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.-- xenzag, Nov 29 2017 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).-- neutrinos_shadow, Nov 29 2017 // I'm no Trekkie //
Do you have anything you wish to say before Summary Execution is carried out ? Your statement will be read out at your subsequent trial.
// 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 //
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.
It's all in the Technical Manual.-- 8th of 7, Nov 30 2017 Mmmmm ... not bad, but maybe a bit more work needed. We'll call you. The way out's over there ... NEXT !!-- 8th of 7, Nov 30 2017 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.
Knowing what a particular strut is taking is still a good idea.-- wjt, Dec 01 2017 // Knowing what a particular strut is taking //
"Bend over, strut, and take it like a ... !" ... no, that's not quite right, somehow ...
Was there something about "Squeal like a pig !", too ?-- 8th of 7, Dec 29 2017 Sp //piezo// pizza-- xenzag, Dec 30 2017 "my trousers contain piezo linear actuators that greatly assist in dancing prowess and the ability to stay upright after a few jars."
Yes, but what are the civilian applications?-- not_morrison_rm, Dec 30 2017 Dispersal of a riotous mob in under ten seconds ?-- 8th of 7, Dec 30 2017 Yes, he went on to found "Thank God It's Faraday's" ...-- 8th of 7, Dec 30 2017 The term you need if you want to look up more on this topic is "active stiffness".
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.-- notexactly, Jun 09 2018 The additional stress imposed by the piezos is negligible.
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.)
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.
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.-- MaxwellBuchanan, Jun 09 2018 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.-- notexactly, Sep 25 2019 random, halfbakery