2D finite element using a plane strain formulation

[Luciano Jacinto]

Dear NextFEM Support Team:
As far as I can understand, the 2D finite element implemented by NextFEM Designer (triangle or quadrangle) is good to analyze a membrane problem using a plane stress formulation. What about a membrane problem using a plane strain formulation instead? Is that possible in NextFEM? This kind of formulation is very common in geotechnics problems.
Thanks in advance,
Luciano Jacinto

[NextFEM Admin]

Dear Luciano,
we support plane strain as well. To convert quad or tria elements to plane strain, select the elements, the use the command Edit / Change elem. type / select the option Is -> PlaneStrain and finally press Apply.

[Luciano Jacinto]

Thank you very much for your support. Now I know how to solve a plane strain problem in NextFEM Designer.

I made some search in the NextFEMpy module (in GitHub) to see if there is a Python method equivalent to the “Change elem. type” command in the GUI, but I didn’t find any. Perhaps this command is not yet implemented in the API. Am I correct?

Thank you for the free version of NextFEM. It’s a nice tool.
Luciano

[NextFEM Admin]

You’re right and we expected your enquiry – that’s why we’re adding the command to APIs in the next patch.

[Luciano Jacinto]

Thank you very much!

[Luciano Jacinto]

It seems that solving a plane strain model, NextFEM gives only nodal displacements, not stresses. I’m attaching a planar model (defined in the XY plane, as stated in the user manual) in which, as I said, I got no stresses, only displacements.
Can you verify this, please? Thank you.
Luciano

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[NextFEM Admin]

Thanks for the sample, if you have one also for planestress we’ll test in deep the correction we made, available on the next patch.

[Luciano Jacinto]

The same sample can also be used in plain stress analysis. The model is planar and the loads are defined in the plane of the model. However, running the model selecting the plain stress formulation gives the following error:

ERROR:Likely lability detected at node 25 – dof z
Solving …
ERROR:Likely lability detected at node 25 – dof z
No success in solving system.
ZZNodalRecoveryModel :: unknown size of InternalStateType IST_ShellMomentTensor

This is an unexpected error, because the model runs without problem selecting Shell analysis. Because this is a planar model (both in elements and in loads), the shell analysis and the plane stress analysis should give the same results.

Thanks for your time spent on this.
Luciano

[NextFEM Admin]

In plane stress, planar elements acts as membrane, hence the same model cannot be used. Z direction should be fixed for all free nodes, because in our case the analysis is still performed in 3D (other solver limitates the number of dofs in such cases).

We’ll look to the other error, which involves output only – it seems however a solver limitation, hence we reloaded the patch (reinstall the program from the full installer).

[Luciano Jacinto]

Thanks for implementing the method setPlaneStrainElement().

I express my gratitude sending a Python sample (I used to test the method) and a doc describing the model and some of its results.

Warm regards
Luciano

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[NextFEM Admin]

Dear Luciano,
thanks for this, we’ll publish it soon in out GitHub repo!

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