Discussion Forum

Hi

fisrt to be sure: are you in 2D-axi mode (using a true 2D-axisymmetric analysis with a rectangular section, or are you in 3D with a axi-symmetric cylinder as domain ?

because in 2D-axi you are restraining your model such that the full stiffness tensor has specific symmetric behaviour. THis is no problem per say just to better understand.

Because if you use E, nu, rho you are also defining a specific symmetry of your stiffness tensor.

Anyhow what strikes me are your values:
the compressive stiffness of a cylindrical slab (fixed on a lower section is

k[N/m] = E[Pa] * Area[m^2] / Length[m] or about 120 kN/m,

loading that with some 1E5[Pa] or about 1[Atm] should give almost a 1m displacement/compression, which is not really compatible with the 1 [cm] thickness, or have I missed something ? These are not "small" displacements

Then if you are fixing the rim of your cylinder and its a membrane, you should get out something like the shape of a well filled balloon.

Anyhow for such large displacement you must use more advanced material properties as the linear theory

Could it be that E= 6 [GPa] and not 6 [kPa] ? that would change a lot, and still be more compatible with something like a plastic material ?

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Good luck
Ivar

Hi Ivar,
thank you for very fast response.
I'm in 2D-axi mode.
I know that the Young's Modulus is very small, but the material is a polyurethane and it's very soft.
In the Analysis I expect that when the pressure is too high the compression stop, but it isn't.
I know that for this E the load on material must be low, in particular under the 8[kPa] ,but in real application the load can reach values of more than 8 [kPa]. In the Realty if the pressure grown over a limit the compression show a saturation and the displacement of upper boundary stop to about 90% of the thickness of 1 [cm]. I'd like this behavior.
I'd like see the compression of material until it become a thin material of 10% of initial 1[cm] thick. Than the displacement must stop his growth.
Can't I limit the displacement?
There is the possibility of constrain the upper boundary to stay over a particular Z-coordinate, for example 0.001 [m]?

Hi

well that means that you have a non-linear material, and you need to add that to the material physics/parameters. It's probybly more a hyperelastic material than truely a "linear" one. So check with the other material definition possibilities and enter rather the correct physics for your material, than searching for "tricks" or workarounds.

For me that seems to make more sens

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Good luck
Ivar

Hi,
I don't interested about the non linearity of material, I'd like make analysis for linear material, but I'd like take a constrain on the maximum displacement for upper boundary. Isn't it possible?
Thank You

Hi

you can adapt at will so long you respect physics that is add in some equations that governs correctly your displacemen. If you apply with a large force on a soft material it will compress, but linear theory is based on "small displacements up to a few permille or a percent, you are at 10'000%, the hypithesis of "small displacement is not really being respected.

You can say you add asmall metallic spacerthat touches (i.e. contact problem) or you add a non-linear spring that sets the system in equilibrium, before you have sweezed tzhe shim to "0" thickness. But you must respect the physics nd the underlaying hypothesis, or have I missed something in you explanations ?

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Good luck
Ivar

Great, these can be the solutions I search. In particular I've tried to put a small metallic spacer under the elastic material, but with the same error: the elastic material penetrates in metallic material.
I don't try the idea of spring, but I'm not a mechanics so I search the easiest.
I think we are on the right way, you understand my problem.
Thank You

Hi

just be aware that contact problems are also rather tricky to get solved, and are based on non linear springs

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Good luck
Ivar