Measuring Poisson Ratio

[CycloWizard]

I was wondering if anyone could point me in the direction of how I might go about measuring the Poisson Ratio (or any sort of compressibility) of a very soft material. I can find it for concrete and steel, but the methods suggested don't work for odd geometries that I am constrained by. The shape is a lens (as in the one in your eye), so it has very irregular geometry. Any ideas or links would be much appreciated. :beer:

 

[Gibsons]

this won't get a ratio like that, but just for simple compressibility, you could put it in a liquid and then compress the liquid. Liquids have very low delta V usually, so you can probably get a reasonable pressure/volume graph.

 

[white]

you would have to get a uniform sample, measure the axial strain and the transverse strain. that's kind of hard to do for a lens. but for soft biological materials you can approximate it as 0.5.

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[CycloWizard]

Originally posted by: Gibsons
this won't get a ratio like that, but just for simple compressibility, you could put it in a liquid and then compress the liquid. Liquids have very low delta V usually, so you can probably get a reasonable pressure/volume graph.

This is sort of what I was thinking.

Originally posted by: white
you would have to get a uniform sample, measure the axial strain and the transverse strain. that's kind of hard to do for a lens. but for soft biological materials you can approximate it as 0.5.

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Yeah, I've read that too. The problem is that a lot of people are trying to show just using math that it's 0.42, 0.47, or 0.49. Believe it or not, these small changes completely change the method in which the eye would actually focus.



Anyway, I'll be down in Florida this week for ARVO, so maybe I can glean some insight from people who have been doing this longer than myself. Thanks for the help guys. :beer::beer:

 

[onix]

Here's an idea taken from Archimedes. Put your sample in water, or some liquid where you can measure its combined volume, apply a force, notice the change in volume of the water and with the help of some FEM tool for your arbitrary geometry, you should be able to extract some effective poisson's ratio. Materials that are "incompressible" have a poisson's ratio of 0.5 -- most solids are between 0.2-0.35, and they do change in volume with a diliatation.

Follow-up

Just noticed the poster reply to himself. I guess he agrees. I don't see why measuing volume change should be so difficult. Use a capillary tube if you expect a small change and do some math to calculate any change fluid volume after measured change in fluid level.

 

[CycloWizard]

Originally posted by: onix
Just noticed the poster reply to himself. I guess he agrees. I don't see why measuing volume change should be so difficult. Use a capillary tube if you expect a small change and do some math to calculate any change fluid volume after measured change in fluid level.

Well, the lens is about 8-10 mm in diameter and 3-5 mm thick. Getting it inside a container with a capillary tube apparatus is therefore very difficult. Further, for me to measure the force applied (which is what my device does) requies lots more space. 😛 Thanks for the help though! :beer:

After learning way too much about the lens' architecture in the last week, I no longer have any expectation of a uniform Poisson ratio for the lens. The orientation of fibers is much too complex and my current theory on accommodation (how the eye focuses) changes the lens in very specific ways that would cause variable Poisson ratio with position within the lens. This is going to be lots of fun modeling in FEM... 😛