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Bistable auxetic [message #42302] Fri, 20 January 2012 01:44 UTC Go to previous message
avatar 32Q2  is currently offline 32Q2
Messages: 9
Registered: January 2009
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Junior Member
Hi all,

I've been interested in creating materials with unusual mechanical properties for a while - see this video for some of my older experiments with the sort of thing I'm talking about.

Now I'm looking to develop some of these ideas further through 3d printing and making use of the elastic properties of WSF (inspired also by these 'digi-fabrics')

I'm attempting to create a material which will be both auxetic (having negative Poisson's ratio - so it expands perpendicularly as you stretch it instead of contracting like most materials) and bistable (having two different unstressed states it can snap between, separated by an 'energy hill').
As far as I know this would be the first material to have both these properties.

Here is how my mechanism should (hopefully) work:
http://i.imgur.com/asAT2.gif
(The squares are attached to each other by axles at their corners and to the green spring system by axles through their centres. See how there are 2 different configurations of the squares for which the springs are relaxed)

and here's what I'm actually planning to print:
http://i.imgur.com/t6xxM.png

http://i.imgur.com/UN2M2.png
(dimensions in mm)

Do you think the clearances will be sufficient for the axles to work and not fuse together ?
I've stuck to the 0.6mm guideline, but I'm not sure if this still applies when the gaps go around corners like this.

Also - do you think the springs will be able to bend sufficiently to allow the proposed motion ?
I've printed struts on other models at 0.8mm and they are fairly flexible, though much longer than these.

I want to keep the individual units as small as possible, because I think the interesting behaviour will come from having sheets of many units. (I'm hoping it will be possible to snap different parts of the same sheet into the different stable states, and move the wave between them around).

The clearances can't be too big, otherwise the springs will not be stressed in the in-between state.

This will be printed already assembled, but an alternative option might be to print the expanded squares, and the spring network separately, then snap them together. This could maybe allow tighter fits, but snap-fits would pose their own challenges.

Anyway, I'd be interested in any of your thoughts or ideas on this design and how it might be improved.
Thanks!

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