×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Poisson's ratio greater than 1
6

Poisson's ratio greater than 1

Poisson's ratio greater than 1

(OP)
Dear all,

I am looking for references, most likely relating to laminated materials, that discuss Poisson's ratios greater than 1.  I'm especially interested in proofs on any type.  I know that we can easily prove that for isotropic incompressible materials, that the poisson's ratio can't be greater than 1, but what about for other non-isotropic materials?

-ldp

RE: Poisson's ratio greater than 1

2
(OP)
please pardon me, the above statement should read "I know that we can easily prove that for isotropic incompressible materials such as rubber, that the poisson's ratio can't be greater than 0.5".  -ldp

RE: Poisson's ratio greater than 1

4
For interesting stuff on unusual poisson's ratios, check out  http://silver.neep.wisc.edu/~lakes/Poisson.html . This is mainly about negative poisson's ratios ('auxetic' materials), and includes instructions for home manufacture of metal and plastic foams which have negative poisson's.

For a 2D "material" like a sheet of honeycomb core, for hexagonal cells the poisson's ratio is 1.0 in both the 12- and 21-directions. If you stretch the core sideways ("over expanding" it), the cells become more rectangular. When the cells are exactly rectangular, the theoretical poisson's ratio is 0 in the 12-direction and infinity in the 21-direction... For a cell shape which deviates from rectangular (90 degree internal angles) by 5 degrees the theoretical 21 poisson's ratio is about 10.5. It's similar at the other extreme: for "under expanded" honeycomb cells, with internal angles 90, 135, 135, 90, 135, 135, the 12-direction theoretical poisson's is 2.41 (actually root(2) + 1). For completely unexpanded core the 12-direction poisson's is infinite while the 21-direction is zero. I have a little Excel 97 spreadsheet illustrating honeycomb core poisson's ratios for different cell shapes that I can e-mail you if you post an e-address.

It is also possible to manufacture 2D "materials" like this with very large negative poisson's ratios. You have to manipulate the cells so that they all become sort of 'X'-shaped.

All these effects rely on the "material" having a microstructure (macrostructure, really, in the case of honeycomb core) which behaves as a mechanism. 3D materials are possible, but get more complicated. Coming up with what we would regard as actual materials with such properties is trickier. I remember an article in Nature in either 1993 or 1994 which featured negative and unusual poisson's rations in single-crystal metallic materials. However, from memory the poisson's were highly anisotropic. More recent stuff can be found from entering  auxetic  into the search box at  http://www.nature.com/nature/ .

There's also some fascinating stuff about materials with a phase having negative stiffness at  http://silver.neep.wisc.edu/~lakes/NegStf.html , though for "with a phase" something like "including micro-mechanisms" might be more appropriate.

Thought for the day: if you mix equal amounts of positive and negative stiffness "materials" together, the flexibilities cancel out and (for just an instant) you have something with theoretically infinite stiffness...

-R.

RE: Poisson's ratio greater than 1

(OP)
RPstress,

Thanks for your help.  I had looked at that (yours?) web site earlier and found it very interesting.  I am looking to advance some theories for "solid" materials.  My interest in high Poisson's ratios comes from my work with fiber-reinforced elastomers.  The only solid references relating to high poisson's ratios I have been able to come up with is "Microstructural Design of Fiber Composites" by Chou, and some geo-tek or soils-related papers.  The web site you suggested has given me some ideas, perhaps I will have to consider Hooke's law using a full anisotropic stiffness matrix.  If you are familiar with composites (classical lamination) theory, you know that we can get v12 = Qbar12/Q22, and plot the poisson's ratio as a function of fiber angle for any orthotropic material.  What I would like to do, is determine theoretical limits for say v12 as a function of oriented orthotropic material properties and fiber angle, much like we can prove that for an incompressible isotropic solid (rubber) v or Poission's ratio will be 1/2.  I keep thinking that I am missing something obvious.

take care,

orangepeel

PS..... I have just read your posting again.... and there may be more correlation between your high poisson's ratios, and what I am seeing, than I originally thought.  This requires some more pondering.  In the mean time, I would like to have you email me the excel file you mentioned, to kfldp00@tamuk.edu.     

RE: Poisson's ratio greater than 1

Hi
Can u point out a site for application of negative poisson's ratio materials for textiles.With regard to manufacture,properties and uses etcetc

RE: Poisson's ratio greater than 1

hi
and i also had a basic question .
in a positive honey comb strcuture when extended the area would remain the same i suppose since the overall perimeter remains same.For a negative poissons auxetic material ,under stretch the material expands in all directions,how does this influence the overall area and volume of air contained in it.

RE: Poisson's ratio greater than 1

(OP)
Simisastry,

I am not aware of any such web sites for textiles, on the honeycomb question, you might have better luck posting it as a separate topic.  I can't claim any significant expertise on deformed honeycomb structures.... Perhaps RPstress might be able to help.

Good luck!

RE: Poisson's ratio greater than 1

I have no info on textiles, either. Sorry. While a bias cut fabric can have a relatively high Poisson's (see Professor J.E. Gordon's books on materials and structures), I can't personally think of a way to get a negative poisson's out of a woven or even a knitted material. Fabrics tend not to have "proper" compressive properties unless embedded in a matrix. For a negative poisson's, I think one direction or the other has to be in compression (or just perhaps straining negatively from an initial tensile pre-load). You might do it by making a fabric out of stiff fibers that looked like a honeycomb core with the squashed cells, and then either embedding it in a soft thermoplastic or rubber matrix, or by making the "diagonal" fibers in "A" (see below) rigid (little tubes? Resin impregnation?). See label in B.

Attempt at graphic in fixed pitch:

A.                  B.
                     ^^^^^^^^^^^^^^^^^^^^^^^^ PULL!
                          ||          ||
                          ||          ||
   ||      ||         ____||____  ____||____
|\ || /||\ || /|     |          ||          |
| \||/ || \||/ |     |          ||          |
|      ||      |     |          ||          |
| /||\ || /||\ |     |          ||          |
|/ || \||/ || \|     |____  ____||____  ____|
   ||      ||             ||          ||  \     somehow make
                          ||          ||   \___ these bits rigid.
                          ||          ||        
                     vvvvvvvvvvvvvvvvvvvvvvvv PULL!


As can be seen from the above "diagram", the area and perimiter both increase. For a flat "material" such as a honeycomb, the area changes for any cell shape other than hexagonal (all internal angles 120 deg). For a 2D material, no change in area means a poisson's of 1.0.

[Also, note that for these sorts of pure mechanisms, stiffnesses and stresses are nominally zero and "poisson's" is the ratio of strains. For actual solid materials, poisson's relates the stress in one direction to the strain in others.]

Bias-cut cloth is renowned for its "clinginess". As opposed to negative poisson's, what you might achieve with an ultra-high positive poisson's material I don't know...is this how fishnet stockings work?

RE: Poisson's ratio greater than 1

RPstess, FYI

I know some work was done in this area in the development of nonwoven materials (nylon). There was considerable effort to make a material/composite that could be used as a reinforcing material for tires.
I do remember that they made thick multilayer products with both random/random, random/bias, combinations of others materials in the layers.  There were always discussions of the directional properties in the terms you discussed.  They only thing that I’ve seen come close to these  materials in appearance is the 3M pads.

I was involved in the mechanical end and didn’t stay around for these discussions, whish I had now.
 
I don’t think anything was published.

There is some interesting reading around concerning the use of honeycomb material as shock absorbers.

RE: Poisson's ratio greater than 1

I am looking for its applications in textiles other than in composites or foams
I read professor lakes site on Auxetics
(http://silver.neep.wisc.edu/~lakes/Poiss...) and application of Auxetics in textiles in http://research.dh.umu.se/dynamic/artikl.....
In the first,there was a picture of open structured foam of small thickness which showed the change in size and shape of a auxetic material with stretch.This gave me the idea that perhaps auxetics could be used in functional textiles where vaiable coverfactor is required for functions such a smart moisture or thermal management.As Rpstress pointed out it might be difficult  to implement due to the compressive properties of textiles.But perhaps Auxetic textiles would be realised in the future.

RE: Poisson's ratio greater than 1

(OP)
Dear all,

Back to high Poisson's ratios.......RPstress mentioned "fishnet" stockings, or perhaps something like an onion sack.  That may be similar to what I am trying right now.  Anyone else tried a scissoring net like that for structural purposes?  What kind of structural response did you get?

thanks,

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close