Significance of Poisson's Ratio in FEA modeling 1

[hummad]

I have a material model i created in abaqus representing a polypropylene soft knit surgical mesh
the poissons ratio of polypropylene is v=0.45 (I do not know if it being soft knit has any impact in altering the v value)
However when i create my model and use 0.45 (Hyperelastic model ogeden n=1) I get results that are much lower than my experimental data (uniaxial tests)
when i select use volumetric test data (even though i have no values plugged in) i get results that almost perfectly matches my experimental data
It seems when i select the volumetric test data option it uses a poissons ratio of v=0.5 (i have run the model using other Poisson ratios to confirm this)
Polyproplene should not be incompressible however this set up in abaqus modles my experimental results
Any idea why this would occur? Any suggestions on how i could justify a poissons ratio of 0.5 for this model when the ratio for the material is already known?




Note:
I have run into this problem with another material model as well, were i tried to model soft tissue (abdominal wall)
When using any poissons ratio less than .48 the model gave results much lower than the experimental data
luckily for me there is no specific poissons ratio for soft tissue in literature and most literature state to model soft tissue as a nearly incompressible material
is it a coincidence that both material types only seem to match my results when v=0.5, or am i doing something in particular that is causing this?

 

[burningDiesel]

In general, the Poisson's ratio matters more and more if you are performing constrained compression. I.e. in Hooke's law, you have the constrained modulus E ((1 - n) / (1 + n)(1 - 2n)), which shoots up when going to n=0.5
Incompressibility is nothing but a mathematical simplification. It simplifies the constitutive model formulation. Numerically, it's a difficult thing, in Abaqus, a hydrostatic penalty term (the bulk modulus) is introduced. For perfect incompressibility, special elements (C3D8H) are used, treating pressure as an independent solution variable.

For a woven structure, poisson's can be anything, depending on the weave, material etc. going from 0.2 to close to 0.5
For the abdominal aorta soft tissue, it's a bit weird that your outcome differs much, as the poisson ratio should only slightly influence strains:
for a cylinder:
eps_theta = 1/E * (sig_theta - n * sigma_r)
And on the outside surface, sigma_r = 0, while on the inside, it's always an order of magnitude lower than sig_theta, so for a cylindrical inflation, poisson should not matter much.

1. Maybe you are getting volumetric locking
2. Check if your material model is stable for the range of stress/strain you have
3. Give more info, what is "much lower", how are you loading the model etc.

if you like reading and formula's, you can check out

http://www.maths.nuigalway.ie/~destrade/publications/destrade_83.pdf

 

[IceBreakerSours]

Thanks for the interesting reference, sdebock.

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