Half a croissant, on a plate, with a sign in front of it saying '50c'
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Negative Poisson ratio zip teeth

Gets stronger as it's pulled apart
  (+2, -1)
(+2, -1)
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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.

nineteenthly, Feb 05 2013


       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.   

       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.   

       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.
MaxwellBuchanan, Feb 05 2013

       Wouldn't metal zipper teeth solve this?
whlanteigne, Feb 05 2013

       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.
MaxwellBuchanan, Feb 05 2013

       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?
nineteenthly, Feb 06 2013

       // 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.
MaxwellBuchanan, Feb 06 2013

       chinese finger trap ?
FlyingToaster, Feb 06 2013

       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.
nineteenthly, Feb 06 2013

       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.   

       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.   

       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.
MaxwellBuchanan, Feb 06 2013

       You think too much, [19].
UnaBubba, Feb 07 2013

       Thanks [MB], that makes a lot more sense now.   

       [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.
nineteenthly, Feb 07 2013

       Take your time.
UnaBubba, Feb 07 2013


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