
Re: Why won't SW materials allow a negative Poisson's Ratio material?
Brian Titus Sep 24, 2014 3:02 PM (in response to James Riddell)Materials exhibiting negative Poisson’s ratio (NPR) get fatter when they are stretched or become smaller when compressed, in contrast to conventional materials (like rubber, glass, metals, etc.). I'm pretty sure that SolidWorks Simulation CANNOT handle auxetics due to this behavior.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Mike Pogue Sep 24, 2014 3:34 PM (in response to James Riddell)I don't think you'll find negative Poisson's ratios normal isotropic materials. This is not typically nanoscale behaviors, but more related to micro or macroscopic arrangement of material. In other words, I'd argue that negative Poisson's ratio is not really part of the standard elastic model.
With that said, why can't SW take it and just get an answer? It may be that a negative Poisson's ration causes a negative term in a matrix that needs to be positive definite in order to produce a meaningful solution. I dug through Hughes "The Finite Element Method" trying to find such a case, but it turns out I can no longer understand that book.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
James Riddell Sep 25, 2014 6:15 AM (in response to Mike Pogue)Actually you will find that there are some crystals that exhibit this behavior.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Mike Pogue Sep 25, 2014 10:33 AM (in response to James Riddell)I did not know that.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Jared Conway Sep 29, 2014 8:47 PM (in response to James Riddell)is that the application you're trying to solve?


Re: Why won't SW materials allow a negative Poisson's Ratio material?
Shaun Densberger Sep 30, 2014 2:55 PM (in response to Mike Pogue)"It may be that a negative Poisson's ration causes a negative term in a matrix that needs to be positive definite in order to produce a meaningful solution. "
I don't think that is true (a negative Poisson's ratio causing the stiffness matrix to be something other than positive definite); negative Poisson's ratios are perfectly valid in linearelastic theory.
For example, if you have a relatively short beam and try to compare FEA results against closedform calculations, you'll notice larger displacements on the FEA results. This is because FEA codes typically use Timoshenko Beam Theory whereas most hand calculation are done with EulerBernoulli Beam Theory, where the former accounts for sheardeformation and the latter doesn't. However, you can get FEA beam elements (i.e. Timoshenko beam elements) to act like EulerBernoulli beam elements (at least very close to) by setting the Poisson's ratio for the beam's material to something very close to 1 (but not too close; 0.999 should be good). Doing so drives the shear modulus to a very large number, thereby driving out the effects of sheardeformation (if you look at the limit as G > infinity, Timoshenko Beam Theory becomes EulerBernoulli Beam Theory).


Re: Why won't SW materials allow a negative Poisson's Ratio material?
Jared Conway Sep 24, 2014 8:01 PM (in response to James Riddell)did you check the solidworks Kb? i believe this is answered there.
what is the application? are there other programs that do this?

Re: Why won't SW materials allow a negative Poisson's Ratio material?
James Riddell Sep 30, 2014 7:49 AM (in response to Jared Conway)Checked the KB and there is no info about negative values.
No, I am not currently analyzing anything with this property but you never know. Goretex makes a gasket/seal material that exhibits this behavior, it's really cool to play with.
I was just making up some materials for our shared network material catalog and wanted to put in some 'unobtanium', 'creativium' and some 'imaginarium' materials since the discussion about 'fixing' mass properties to account for things that can't be easily modeled or meshed in FEA.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Jared Conway Sep 30, 2014 8:25 PM (in response to James Riddell)the kb articles i found don't specifically say no negative but i think it is relatively clear that they should be between 0 and 0.5

Re: Why won't SW materials allow a negative Poisson's Ratio material?
James Riddell Oct 1, 2014 6:07 AM (in response to Jared Conway)Jared,
Real world materials can and do vary from 1 to +0.5. There is no good reason why it should not work in FEA software.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Roland Schwarz Oct 1, 2014 9:17 AM (in response to James Riddell)I wonder if this limitation is rooted in the kernel of the FEA engine, not just a SW limitation. Perhaps the elements definitions or solver are such that they are not able to have negative Poisson's ration.
Are there other FEA programs that allow negative Poisson's ratio? If so, what did they need to do to make it happen?

Re: Why won't SW materials allow a negative Poisson's Ratio material?
James Riddell Oct 1, 2014 10:02 AM (in response to Roland Schwarz)Roland,
It's in the material Spec of the modeler really, nothing to do with the FEA except that the 'nu' value is used. The FEA calculation software shouldn't care. I just asked this in here b/c Poisson't ratio really isn't used anywhere else (to my knowledge).

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Roland Schwarz Oct 1, 2014 11:30 AM (in response to James Riddell)I suppose you're right. You seem to know more about the source code than I do.

Re: Why won't SW materials allow a negative Poisson's Ratio material?
Shaun Densberger Oct 1, 2014 3:28 PM (in response to James Riddell)"It's in the material Spec of the modeler really, nothing to do with the FEA except that the 'nu' value is used."
I suspect the is more along the lines of this. I've noticed that SW usually doesn't allow one to enter negative values. For example, if you want to change the direction of a force, enforced displacement, dimension, etc., you usually have to check a box that says "Reverse Direction"; trying to enter a negative values gives an error.
"The FEA calculation software shouldn't care."
It depends; certain Poisson's Ratio values can cause numerical issues. I previously gave an example of using a Poisson's Ratio of 0.999 to causes a Timoshenko beam element to behave like an EulerBernoulli beam element and that 1 would cause numerical issues (specifically, G = E/[2(1+nu)], so nu = 1 will cause division by zero). In linear elastic analysis, the Poisson's Ratio cannot be 0.5 (0.4999 is used instead), but in elastoplastic material models, a Poisson's Ratio of 0.5 for plastic strains to capture the incompressible nature of the material. However, in hyperelastic material models, the Poisson's Ratio is never set to 0.5, but rather 0.4995 so as to prevent dilitational locking, where the element becomes too stiff from a numerical standpoint.
All that being said, there is no theoretical reason preventing a negative Poisson's Ratio from being used. Here's two examples; one with beam elements and one with 3D elements, where one has a Poisson's Ratio of 0.3 and the other has 0.999):
Beam Element with nu = 0.3 vs.nu = 0.999 vs. Euler Beam Calculation
Cube with Enforced Displacement with nu =0.3 vs. 0.999





