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Some elastic substances expand sideways as well as in the direction they're being stretched. This can be achieved, for instance, by a kind of crinkly foam which straightens out as it's pulled apart.
One of the ways in which zips fail is when the teeth part laterally due to strain. If the teeth were
made of a material with a positive Poisson ratio, they would mesh more firmly under strain and grip more firmly. However, i'm not sure what would happen when they were fastened or unfastened.
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Hang on - have you got your Poisson ratios the
right way around? A positive ratio means that the
material gets smaller in one (or two) dimensions
as it's stretched in the other. For example, if you
stretch a rubber band it gets narrower - a positive
Poisson ratio. |
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So I think your first paragraph is back to front.
However, the second paragraph is correct - a
positive ratio would clamp the teeth more firmly
together against any zipper-bursting force. |
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Most fabrics have a nicely positive Poisson ratio as
long as the threads run diagonally to the direction
of pull. Perhaps zippers are therefore already like
this; if not, a diagonal warp in the zipper material
would be a good idea. |
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Wouldn't metal zipper teeth solve this? |
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No. The interlocking parts of most zips consist of a
little pyramid-shaped bump on one side of the
tooth, and a corresponding recess on the other side.
Under lateral stress, the force tends to slide the
bump out of the recess, prising he teeth apart. A
zipper fabric with a high positive Poisson ratio would
possibly counteract this. |
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I wouldn't be surprised if it was the wrong way round. Sorry to be vague, but you got what i meant. What's not clear to me, though, is what would happen when the zip pull brought the teeth together because it seems to me that they would widen. I'm thinking of some kind of metal foam, although there may be other similarly anisotropic materials. Or do i mean isotropic? Surely it's normal elasticity which is anisotropic? |
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// what would happen when the zip pull brought the
teeth together because it seems to me that they
would widen.// No, if the material had a positive
Poisson ratio, then the pull (at right-angles to the
length of the zipper) would tend to pull the teeth
together. |
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I see what you mean, [MB]. This is about as hard for
me to visualise as my DNA Zip idea, which i've been
trying to draw on and off for almost a decade. |
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Probably easier if, instead of imagining the zipper
ribbons as long and narrow, you imagine them as
squares, side by side, and with the teeth on their
meeting edges. |
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Stretch the squares directly away from the zipper
(as if trying to burst the zip), and they will want
to become shorter, thereby squashing the teeth
together more firmly. |
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If that visualization doesn't work, imagine instead
two adjacent square fields divided by a hedge,
with rabbits sitting at regular intervals down each
side of the hedge. This won't help understand
how the Poisson ratio works, but it's a nice image. |
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You think too much, [19]. |
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Thanks [MB], that makes a lot more sense now. |
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[UB], i sort of agree but i've thought a lot about whether i think too much or not and have reached the opinion that it's more that i think too much in the wrong way and not enough in the right way. However, i'm not sure which ways those are exactly and am currently cogitating about how i can adumbrate the general details of the differences between these two or more ways of thinking, arriving at a taxonomy for determining how to categorise the right and wrong thoughts, with subcategories for which particular errors of thought might occur in them, as a precursor to exploring them in greater depth at a later date. I will be producing a number of interim reports on the matter which you are free to peruse and am currently drawing up a timetable for the production of those documents, including a number of stages for each draft. I may get back to you on this one. |
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