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Pasta breakage

  1. Apr 20, 2006 #1

    Danger

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    Hi, all;
    An interesting question was posed this morning by the science correspondent on Canada AM. Apparently, it's also in New Scientist magazine. He claims that it's unsolved, and I figured there's no better place than PF to go when searching for engineering solutions. I'm putting it in GE because I really don't know whether it's structural or materials or what.
    The question is 'why can't you break a piece of dry spaghetti into only 2 pieces by holding the ends and bending it?' It always ends up in 3 or more segments.
    Any thoughts?
     
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  3. Apr 20, 2006 #2

    Bystander

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  4. Apr 20, 2006 #3

    FredGarvin

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    Oooh. That's an old one. I forgot about that discussion.
     
  5. Apr 20, 2006 #4

    Danger

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    Bloody hell! That was a lot of reading to get to the one reference to Feynman's spaghetti conjecture. It doesn't seem related to the majority of that thread, since there's no impact involved and we're dealing with a much more flexible substance. Is there some place where it's treated in more detail? Crosson... it was you that brought it up. Where are you?
     
  6. Apr 20, 2006 #5

    Bystander

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    "Holding by both ends" might be interpreted a number of ways: just did the experiment with one interpretation, axial compression of a slender column between fingers, and got two pieces 2/3 time. Haven't done the other interpretation, pinch each end between thumb and finger and rotate; haven't figured out how to control tension/compression forces applied between two hands. Other loading arrangements would be as a free beam supported at two ends, uniform load (or point loads), and centrifuge the setup for high enough "g" loading to get breakage.
     
  7. Apr 20, 2006 #6

    Danger

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    The specific instance involved pinching the ends and attempting to 'fold' the noodle in half, similar to the force applied when stringing a bow. At least one sub-section always snaps out of the middle.
     
  8. Apr 24, 2006 #7

    Mech_Engineer

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    If I had to guess, I would say that as the spaghetti bends, it stores a force like a beam in bending, and once the bend is enough to break the spaghetti, it breaks very quickly in a brittle fashion.

    Once this fracture occurs, the bend in the spaghetti is released and the spaghetti "springs" back, which is enough to break it again at some weakest point in the smaller piece.

    Alternatively, perhaps the shock travelling through the spaghetti piece directly after the break adds to the spring-back effect, causing a second or possibly third break, but eventually the breaks absorb enough energy to prevent it from simply shattering into a million pieces.

    Additionally, perhaps the spaghetti is pre-stressed from the process of drying from the outside inward, in which case it would be similar to a pre-stressed piece of safety glass, where once one break is initiated, the entire plate of glass shatters.

    Those are the factors I might try to look into if I were a "spaghetti scientist." Most telling might be to see if the breaks happen at exactly the same time, or one occurs and then another (using an ultra-high speed camera). Also, magnifying the break surface would give some insight into what may have caused the break (pre-stress, etc.)
     
  9. Apr 24, 2006 #8

    Danger

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    Good analysis, Mech. I suppose that some sort of testing could be done with small strain gauges or something. It seems to me that the breakages were not simultaneous, but it was a bit hard to be sure because it was so fast.
     
  10. Apr 24, 2006 #9

    PerennialII

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    ... interesting stuff, the spring back would be tolerably easy to assess fracture mechanically, first consider the geometrically nonlinear shape change & stress redistribution up to first failure, and assess whether the 'spring back' would be sufficient to cause another unstable fracture event. If spaghetti is "truly" brittle, has limited rate sensitivity etc. (???) would be quite doable by minutely 'adjusting' the analysis to the experimental chain of events, has anyone come across any fracture properties of spaghetti (remember seeing somewhere, was it Pf?), like fracture toughness? I'd say the breakages being not simultaneous makes 'sorta sense' :rolleyes: .
     
  11. Apr 24, 2006 #10

    enigma

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    I think it would have to be done with photography. I think that strain gages, even if they could get small enough to fit on the spaghetti, would change the dynamics of the problem.
     
  12. Apr 25, 2006 #11

    PerennialII

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    .....something like 'laser holography'/speckle interferometry would be nice.
     
  13. Apr 25, 2006 #12

    Danger

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    Yeah, there's that. I'm glad that Perennial mentioned interferometry. I was wondering if that could be applied, but don't know enough about it. Good high-speed photography can for sure show what's happening, but it might not provide enough data to know why it's happening.
    The spring-back effect never crossed my mind, simply because it's hard from a macroscopic viewpoint to think of something like a noodle having enough momentum to cause it. Makes sense, though. I'm not quite sure that I understand it exactly, but I'm assuming that it means that the massive compression force on one side of the strand and the (tensile?) force on the other are both instantly relieved at the point of first breakage and snap back to beyond their initial states to reverse the stress application. Is that a proper interpretation?
     
  14. Apr 25, 2006 #13
    I initially misread this post. Is there a link to the New Scientist Magazine article about this? I am having trouble finding it on their website.
     
  15. Apr 26, 2006 #14

    Danger

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    Sorry, but I'm not much of a web person. All I know is that the CTV science guy is involved with the magazine and said that there's a free-forum Q&A section near the back where this was posed. I got the impression that it's the current issue. It's also involved in a book that I believe was put out by the same magazine entitled "Does Anything Eat Wasps?"
     
  16. Apr 27, 2006 #15

    PerennialII

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    .... we've one interferometer for non-contact stress/strain measurement in our lab and it has a measurement area of something like X cms * X cms (think order of 10 cm2), and it extracts some thousands (I think the max was something like 5k) datapoints (="strain gages") out of that area (in 3D). Use it to observe behavior of defects, it's a really cool piece of equipment, with that resolution the results are just :!!) . My take on the spring back is pretty much the same, failure relaxes the stress state and off you go.
     
  17. Apr 27, 2006 #16
    Danger,

    If you wanna know everything on this, check out THIS website

    Enjoy

    regards
    marlon
     
  18. Apr 27, 2006 #17

    Danger

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    Wow! That's really cool. Thanks, Marlon. I e-mailed that link to the Canada AM science address. I'm sure that they'll be pleased.:smile:
     
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