This actually could go in the modeling side as well but the question stands. Since Poisson's Ratio can vary from -1.0 to 0.5 (-1.0<nu<0.5) why can't a negative number be entered in the material properties?

This actually could go in the modeling side as well but the question stands. Since Poisson's Ratio can vary from -1.0 to 0.5 (-1.0<nu<0.5) why can't a negative number be entered in the material properties?

I don't think you'll find negative Poisson's ratios normal isotropic materials. This is not typically nano-scale 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.

"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 linear-elastic theory.

For example, if you have a relatively short beam and try to compare FEA results against closed-form 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 Euler-Bernoulli Beam Theory, where the former accounts for shear-deformation and the latter doesn't. However, you can get FEA beam elements (i.e. Timoshenko beam elements) to act like Euler-Bernoulli 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 shear-deformation (if you look at the limit as G -> infinity, Timoshenko Beam Theory becomes Euler-Bernoulli Beam Theory).

did you check the solidworks Kb? i believe this is answered there.

what is the application? are there other programs that do this?

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.

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

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.

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?

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).

"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 Euler-Bernoulli 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

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.