Applying prestrain to biphasic material

[TLaue]

Hello!

I am trying to apply a prestrain to a biphasic material using febio version 2.0. The error FATAL ERROR: unrecognized tag "elastic" is returned.
Do I have to use a different tag? Upgrade to the newest version? Or is this not possible?

Here is an example file using an isotropic elastic and a biphasic (with Mooney-Rivlin) material. I tried to apply a tensile prestrain, which works for two elastic materials.
BiphasicPrestrain.feb

Thank in advance!
Greetings,
Torben

[TLaue]

Hi!

I am still interested in applying prestrain to biphasic materials. I would be very pleased to receive an answer.

Cheers!
Torben

[maas]

Hi Torben,

You cannot put a biphasic material inside a "prestrain elastic" material. You can, however, set the "prestrain elastic" as the elastic component of a biphasic material. In other words, switch the order of the biphasic and prestrain elastic material. You will also need to set the "Module" to "biphasic". Give that a try and let us know if that worked for you.

Cheers,

Steve

[TLaue]

Hi Steve,

Thank you for the reply! The material definition is recognized now.
Unfortunately, a negative Jacobian appears in the second iteration. I was not able to solve the problem.

The current .feb-file is attached here:
BiphasicPrestrainTest.feb

In this example, a specimen is fixed at its ends. It consists of two parts: One elastic prestrain material, which a tensile prestrain is applied to, and a biphasic prestrain material.
The stiffness of the elastic prestrain material is ramped.
This example works, if two elastic prestrain materials are used.
It still doesn't work using the biphasic prestrain material, even if no prestrain is applied.

Cheers,
Torben

[maas]

Hi Torben,

In the attached file you did not set the module to biphasic. If you change the Module tag neat the top of the file to:

<Module type="biphasic"/>

The file runs for me.

Cheers,

Steve

[TLaue]

Hi Steve!

That was a silly fault. The file runs now for me, too.

Unfortunately, I get deformations in the result, although no deformations should appear due to the prestrain update.
I checked the .log-file and the update doesn't seem to be done. The solver converges after one iteration in each load step.

The following lines are missing in the .log-file:

........................ augmentation # n
max norm = x.xxxxxxxx (0.001)
max norm = x.xxxxxxxx (0.001)

Cheers,
Torben

[maas]

Hi Torben,

When I run the file I do get the augmentation lines. I've attached my log file for comparison.

Cheers,

Steve

[TLaue]

Hi Steve,

It works now.
Thank you for your help!

Cheers,
Torben

[TLaue]

Hi again,

I updated my code to the version 2.5 using FEBiO 2.8.5. Here are two example files, which should produce the same results.

The first file is in version 2.0 from a previous post in this thread. This one works fine.
BiphasicPrestrainTest.feb

The second file is in version 2.5.
BiphasicPrestrainTest2.8.feb

Although the prestrain tensor is defined in the ElementData Section, no prestrain is applied and the update algorithm converges after one augmentation.
Any suggestions?

Cheers,
Torben

[TLaue]

Hi Steve,

The prestrain plugin seems not to work for me using FEBio 2.5. I still haven't solved this problem. Can you have a look at my last post?

I also checked with the information in these two threads from MiguelA:

https://forums.febio.org/showthread.php?1714-Prestrain-in-FEBio-2-5

https://forums.febio.org/showthread.php?1720-PreStrain-in-Membrane-element

Even in the posted files in these two threads, I can't observe a prestrain to be present in results.

I would be very pleased, if you could help.

Cheers,
Torben

[maas]

Hi Torben,

It appears that in the 2.8 file the prescribed displacement boundary condition is missing. I only see the fixed BCs. You also have a rather large value for the min_residual. If that was not intentional you may want to put it back to its default value of 1e-20. This is for both files, by the way. I hope that solves the issue.

Cheers,

Steve

[TLaue]

Hi Steve,

Thank you for the quick reply!
In this example model I only use fixed BCs on purpose. I introduce a prestrain in y-direction in material 2. Due to the fixed BCs a stress should remain after solving. (In each loadstep the Young's modulus of material 1 is increased to ramp the prestrain.)
When you look at the results, you will observe a stress in y-direction in the first model (2.0), but no stress in y-direction in the second model (2.8). I think, the F0 assignment does not work in this case.

Cheers,
Torben

[maas]

But what I was pointing out was that in one of the files you have a prescribed BC, and in the other one you don't. If I remove the prescribed BC from BiphasicPrestrainTest, both files are giving me the exact same answer, including a non-zero stress. Can you try that on your end and see what happens for you? If this still doesn't work, I can give you my executable and prestrain plugin. It is possible that I changed something in the mean time that fixes the issue you are running into. Also, if you are not using the latest version of FEBio and the prestrain plugin, please make to sure to update your version first and see if that helps. Let me know what you find.

Cheers,

Steve

[TLaue]

Hi Steve,

Sorry! I forgot one boundary in the 2.8-version file. Although this should have fixed the problem, it still remained.
By updating from version 2.8.5 to 2.8.6 the problem disappeared. It was either the version or my environment variable was not set correctly (pointing to an older version).

Thank you!

Cheers,
Torben