0697: "Tensile vs. Shear Strength"

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Hale
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0697: "Tensile vs. Shear Strength"

Postby Hale » Wed Feb 03, 2010 5:06 am UTC

Image

title-text: Although really, the damage was done when the party planners took the hole punch to the elevator ribbon to hang up the sign.

I guess everyone stopped laughing, maybe it's just me though. At least until I got the pun.
Last edited by Hale on Wed Feb 03, 2010 2:30 pm UTC, edited 1 time in total.
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Re: "Tensile vs. Shear Strength" Discussion

Postby illxkcdthat » Wed Feb 03, 2010 5:06 am UTC

i dont get it?
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Re: "Tensile vs. Shear Strength" Discussion

Postby SocialSceneRepairman » Wed Feb 03, 2010 5:06 am UTC

And thus they begin again.

Dr Zoidberg
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Re: "Tensile vs. Shear Strength" Discussion

Postby Dr Zoidberg » Wed Feb 03, 2010 5:08 am UTC

I liked the pun.
I mean there were other things but the pun was what tied it all together.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Comic JK » Wed Feb 03, 2010 5:09 am UTC

One would think they would use redundant cables...?

Then again, nothing could stop Mr Hat.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Arancaytar » Wed Feb 03, 2010 5:10 am UTC

After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears

BURMA SHAVE.

So who else thought it?
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lrossouw
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Re: "Tensile vs. Shear Strength" Discussion

Postby lrossouw » Wed Feb 03, 2010 5:12 am UTC


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Re: "Tensile vs. Shear Strength" Discussion

Postby Storm » Wed Feb 03, 2010 5:13 am UTC

Wow 2 for 2 Get out of my head Randall's this week. I was suggesting Space Elevators for a project my girlfriend was doing. :D
Puns are excellent, rhymes too.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Omegaton » Wed Feb 03, 2010 5:13 am UTC

Dr Zoidberg wrote:I liked the pun.
I mean there were other things but the pun was what tied it all together.

I agree, I liked it a lot better when I realized what the title was.

Palpatineli
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Re: "Tensile vs. Shear Strength" Discussion

Postby Palpatineli » Wed Feb 03, 2010 5:14 am UTC

Alt-text fail! the elevator does not hold a lot of force where close to the earth, punching would be just fine.

the comic itself is pretty funny. hat-guy FTW

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Re: "Tensile vs. Shear Strength" Discussion

Postby JustMe » Wed Feb 03, 2010 5:16 am UTC

I almost wish I were teaching structural engineering just so I could use this comic!

Nothing better than an engineering pun + BHG.
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Re: "Tensile vs. Shear Strength" Discussion

Postby Kain » Wed Feb 03, 2010 5:17 am UTC

I'm quite glad I didn't read the title until after reading the comic, or I might have had some idea of where it was going.

So, just out of curiosity, what is the shear stress necessary to cause failure in a one foot wide carbon nanotube ribbon, as a best guess?
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Re: "Tensile vs. Shear Strength" Discussion

Postby fabiocbinbutter » Wed Feb 03, 2010 5:17 am UTC

My theory is that a space elevator is plausible, but the material itself would not need to support the weight of the structure, it would merely be a channel to carry high-energy ions that do all the load bearing, both of the payload and the structure.

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Re: "Tensile vs. Shear Strength" Discussion

Postby sinal » Wed Feb 03, 2010 5:19 am UTC

hmm I didn't even noticed it rhymed till I came here. I need to pay more attention.

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Mr. Burke
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Re: "Tensile vs. Shear Strength" Discussion

Postby Mr. Burke » Wed Feb 03, 2010 5:26 am UTC

Arancaytar wrote:After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears

BURMA SHAVE.

So who else thought it?


Ah yes, it was at the back of my mind. Thanks for getting it out :D

But I think that a real space elevator would have a slightly bigger cable.

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Re: "Tensile vs. Shear Strength" Discussion

Postby wisnij » Wed Feb 03, 2010 5:28 am UTC

fabiocbinbutter wrote:My theory is that a space elevator is plausible, but the material itself would not need to support the weight of the structure, it would merely be a channel to carry high-energy ions that do all the load bearing, both of the payload and the structure.

My theory is that you don't know what any of those words actually mean.
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Kow
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Re: "Tensile vs. Shear Strength" Discussion

Postby Kow » Wed Feb 03, 2010 5:28 am UTC

That guy on the right's like "Oh me yarm!"
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Re: "Tensile vs. Shear Strength" Discussion

Postby musashi1600 » Wed Feb 03, 2010 5:33 am UTC

"In one moment, earth. In the next, heaven."

-Academician Prokhor Zakharov, For I Have Tasted the Fruit

(I figure this quote is required for every space elevator thread.)
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Re: "Tensile vs. Shear Strength" Discussion

Postby fabiocbinbutter » Wed Feb 03, 2010 5:35 am UTC

wisnij wrote:
fabiocbinbutter wrote:My theory is that a space elevator is plausible, but the material itself would not need to support the weight of the structure, it would merely be a channel to carry high-energy ions that do all the load bearing, both of the payload and the structure.

My theory is that you don't know what any of those words actually mean.

I'll play the part of the philosopher and say that all meaning is semantic anyway. Words mean what we want them to mean and yet mean nothing at the same time. But seriously, there aren't any mysterious words in my post.

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Alsadius
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Re: "Tensile vs. Shear Strength" Discussion

Postby Alsadius » Wed Feb 03, 2010 5:35 am UTC

If the elevator is just a single ribbon like that, wouldn't the fact that the elevator car has an off-centre centre of gravity be enough to do damage? That would seem to be at least somewhat a test of shear strength, imparting enough force to zero out that torque.

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Nintendon't
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Re: "Tensile vs. Shear Strength" Discussion

Postby Nintendon't » Wed Feb 03, 2010 5:37 am UTC

PLEEEASE get out of my head, Randall.

I always wondered why it was so weird, until he came into my head.

Also, pun.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Krenn » Wed Feb 03, 2010 5:39 am UTC

depending on the specific design of the elevator, wouldn't the ribbon just hover above the earth's surface by a few inches? the orbital counterweight should keep it in position, yes?

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Re: "Tensile vs. Shear Strength" Discussion

Postby Pierrot » Wed Feb 03, 2010 5:43 am UTC

For some reason I am not shocked that the elevator was cut. What I'm worried about is that who is going to clean up the 80km of cable that is connected to the counterweight moving at rotational speed. Which we all know it's going to inevitably cause tremendous damage.

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Re: "Tensile vs. Shear Strength" Discussion

Postby sibwarra26 » Wed Feb 03, 2010 5:43 am UTC

ahh you beat me to the alpha centauri reference, musashi1600

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Re: "Tensile vs. Shear Strength" Discussion

Postby nqdp » Wed Feb 03, 2010 5:44 am UTC

Wait, shouldn't the elevator be in geostationary orbit? In which case, it wouldn't really matter if BHG cut the ribbon, since the anchor only stops the elevator from blowing around a bit in the wind, and doesn't actually hold it up or anything like that. In fact, I'm pretty sure that there's no need for the elevator to touch the ground at all.

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Re: "Tensile vs. Shear Strength" Discussion

Postby nqdp » Wed Feb 03, 2010 5:45 am UTC

Krenn wrote:depending on the specific design of the elevator, wouldn't the ribbon just hover above the earth's surface by a few inches? the orbital counterweight should keep it in position, yes?

As far as I know, yes.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Flewellyn » Wed Feb 03, 2010 5:47 am UTC

Arancaytar wrote:After countless
Engineers
Spend trillions over
Fifty years
A modern Babel
Disappears
Because some fuck brought
Pruning shears

BURMA SHAVE.

So who else thought it?


I was going to post exactly that.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Caffeine » Wed Feb 03, 2010 5:52 am UTC

Black Hat Guy is Francis de Groot!!

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Re: "Tensile vs. Shear Strength" Discussion

Postby fabiocbinbutter » Wed Feb 03, 2010 6:00 am UTC

Counterweight-based elevator models still seem too far-fetched to me. The material strengths required are too great. What I'm suggesting is not nevel and has actually been analyzed in some detail:
http://en.wikipedia.org/wiki/Space_fountain

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Re: "Tensile vs. Shear Strength" Discussion

Postby Ghandi 2 » Wed Feb 03, 2010 6:12 am UTC

I just realized reading the forum this whole thing is a pun for shear. UGH.

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Re: "Tensile vs. Shear Strength" Discussion

Postby Kow » Wed Feb 03, 2010 6:26 am UTC

fabiocbinbutter wrote:Counterweight-based elevator models still seem too far-fetched to me. The material strengths required are too great. What I'm suggesting is not nevel and has actually been analyzed in some detail:
http://en.wikipedia.org/wiki/Space_fountain


http://en.wikipedia.org/wiki/Carbon_nanotube

In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). (This, for illustration, translates into the ability to endure tension of a weight equivalent to 6300 kg on a cable with cross-section of 1 mm^2.)


Now imagine even a foot-wide ribbon.
Image

firestar27
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Re: "Tensile vs. Shear Strength" Discussion

Postby firestar27 » Wed Feb 03, 2010 6:35 am UTC

I don't get the pun...

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Re: "Tensile vs. Shear Strength" Discussion

Postby WontonSoup » Wed Feb 03, 2010 6:37 am UTC

As an engineer (in the making), I find this hilarious. :P

rocketrat
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Re: "Tensile vs. Shear Strength" Discussion

Postby rocketrat » Wed Feb 03, 2010 6:39 am UTC

I think BHG is losing it. Surely he could do something more interesting and classhole-ish than cutting the cable.

Also, what's the point? It won't do much if he cuts the cable.

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Me321
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Re: "Tensile vs. Shear Strength" Discussion

Postby Me321 » Wed Feb 03, 2010 6:49 am UTC

for those of you that did not get the pun,

Spoiler:
http://en.wikipedia.org/wiki/BurmaShave

macrocephalic
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Re: "Tensile vs. Shear Strength" Discussion

Postby macrocephalic » Wed Feb 03, 2010 6:54 am UTC

Unless I'm fundamentally wrong in my understanding of the space elevator, there has to be tension in the cable at all times to counteract the weight of the item that's being transported.

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Re: "Tensile vs. Shear Strength" Discussion

Postby StClair » Wed Feb 03, 2010 7:01 am UTC

And Black Hat Man is either killed by the energy released when the cable parts and starts whipping around, or executed on the spot by some equatorial-country soldier with little regard for due process.
And there was much rejoicing, because he ****ing deserved it. (Though of course, his fellow sociopaths and internet admirers will staunchly disagree.)

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Re: "Tensile vs. Shear Strength" Discussion

Postby hollowclown » Wed Feb 03, 2010 7:14 am UTC

m
macrocephalic wrote:Unless I'm fundamentally wrong in my understanding of the space elevator, there has to be tension in the cable at all times to counteract the weight of the item that's being transported.


The thing to remember, though, is that the cable doesn't actually need to be connected to the ground -- this is actually a good thing, since it prevents worries about things like earthquakes. Instead of the originally-proposed design, with the cable ending in a large counterweight sitting in geosynchronous orbit, it's possible to use a much longer cable with its center in geosynchronous orbit. You'll then have gravity pulling down on the bottom part of the cable, and centrifugal force keeping the top part of the cable taut. The trick is to get the cable just long enough so that the gravity and centrifugal force balance one another out, leaving the cable's center of mass in geostationary orbit. The Earth-end of the cable would essentially hover above the ground, with no need for connection whatsoever.

As an added bonus to using a longer cable, you can use the space-end of the elevator to cheaply launch stuff all over the solar system. Basically, you just take your payload to the end of the cable, wait for the right moment, and let it go; the centrifugal force will chuck it out of Earth's gravity well.

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Re: "Tensile vs. Shear Strength" Discussion

Postby tacvek » Wed Feb 03, 2010 7:16 am UTC

Kow wrote:
fabiocbinbutter wrote:Counterweight-based elevator models still seem too far-fetched to me. The material strengths required are too great. What I'm suggesting is not nevel and has actually been analyzed in some detail:
http://en.wikipedia.org/wiki/Space_fountain


http://en.wikipedia.org/wiki/Carbon_nanotube

In 2000, a multi-walled carbon nanotube was tested to have a tensile strength of 63 gigapascals (GPa). (This, for illustration, translates into the ability to endure tension of a weight equivalent to 6300 kg on a cable with cross-section of 1 mm^2.)


Now imagine even a foot-wide ribbon.


If tensile strength scales the way I would assume it does, (but I am no materials scientist, so perhaps it does not scale like this) then we have:
"6300kg/(1mm^2)*(pi*(6 in)^2) in pounds" then we have approximately 1.02 Billion pounds of tensile strength given a 1 foot diameter cable. One would almost certainly use a cable rather than a ribbon. Of course trying to cut a cable with a 1 foot diameter would require some pretty significant pruning shears.

As the the idea of a counterweighted ribbon staying in place, not going to happen. The counterweight must be more than perfect equilibrium with Earth. So not only must it have enough mass to counteract the earthward force of gravity on the cable, but it must also be able to counteract the weight of a fully loaded elevator platform, plus the additional force caused by the elevator platform climbing.

Even if we assume exactly no safety margin, consider what happens when the elevator platform is at the cable's center of gravity. At that time the earthward forces of the elevator platform climbing and the elevator platform weights are no longer pulling the cable earthward, (remember the center of gravity for the cable is geostationary orbit). Thus the counterweight is proving an uncanceled spaceward force of slightly more than the platform's fully loaded weight, so the anchor must be able to counteract said weight.

Plus the fact that in reality there would be a safety margin, so even with the platform loaded and just a foot off the ground, the anchor is still providing some level of earthward force to overcome the safety margin which tends to pull the cable into space. So when the cable is cut, the part below the cut may fall to earth (although a foot wide cable would actually still stand straight up like a pole), and the rest would float into space, probably settling at some higher orbit.

(Firefox's spellchecker knows "earthward", but not "immersive", or "plasmatic" !?)

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Re: "Tensile vs. Shear Strength" Discussion

Postby wagner » Wed Feb 03, 2010 7:29 am UTC

hollowclown wrote:The thing to remember, though, is that the cable doesn't actually need to be connected to the ground -- this is actually a good thing, since it prevents worries about things like earthquakes.


It actually does need to be anchored. If the elevator were a completely balanced system, any external stimulus would be catastrophic. Adding mass to the shuttle on the ground would cause the elevator to start drifting eastward, sink, start dragging ribbon, and eventually get pulled out of orbit. Being anchored and slightly fast for its orbital altitude allow it to handle addition of mass to, and acceleration of, the shuttle. Current designs generally have it anchored to a floating platform out at sea.


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