How to Visualize Fiber Stretch for Multiple Solid Mixture Materials?

[mper162]

My model simulates muscle contraction with the distal and proximal aponeurosis attached, and fascia surrounding the muscle. I've modeled the aponeuroses and fascia as uncoupled solid mixture materials. The simulation runs without errors, but now I want to examine the fiber stretch in each tissue. With a single solid mixture, I can use fiber stretch['solid[0]'] = fiber stretch, but how do I modify the output to visualize the fiber stretch for each of the three tissues?

[maas]

Hi,

You can indeed use the "fiber stretch['solid[1]']=fiber stretch 1" plot variable. That will give you the fiber stretch for AA and PA and the first component for Fascia. You can get the second component for Fascia, similarly by using solid[2]. So you'll have several fiber stretch variables, but you can use the alias (the string after the = sign) to distinguish between the different fields.
Please give that a try and let me know if that worked for you.

Best,

Steve

[mper162]

Hi Steve,

Thank you so much for your response. It worked!

By the way, I’m now running another simulation using the same geometry, but this time without the fascia around the muscle. I’ve removed just the fascia and re-added the fiber data for the muscle. Please take a look at the attached file. However, the simulation isn't running and is giving an error related to negative Jacobians.

I’m wondering if this issue might be caused by the mesh—did removing the fascia leave the mesh open-ended, and could that be causing the problem? If so, what would be the best way to resolve this?

Looking forward to your insights.

Thank you and best regards,
Randika

[maas]

Hi Randika,

I suspect something happened to the "muscle1" mesh data since many of the values are 0,0,0, which I suspect is not the intention. What exactly do you mean when you say that you "re-added the fiber data for the muscle"? Did you regenerate using the fiber generator or something else?

Thanks,

Steve

[mper162]

Hi Steve,

Thank you for prompt reply. That's odd. I can clearly see the fibers in my simulation, and I haven't found any zeros in the mesh data either. I assign fiber directions to each element by modifying the FEB file using DTI data, rather than using the fiber generator tool. But that's fine, even if you can't visualize the fibers. Please assign a vector to the muscle and try running the simulation. Even with the simplest fiber types, I couldn't get the model to converge. (I assume you opened the correct file, named 'model3-febio,' which was attached in the previous reply.)

Looking forward to hearing from you soon.


Thanks
Randika.

image.png

[mper162]

Hi Steve,

Let me share the FEB file with you just in case. It should include the fiber data. https://drive.google.com/file/d/1Vyi...ew?usp=sharing

Best Regards,
Randika.

[maas]

Hi Randika,

In the file you sent me, the 0 values start at line 24806 and you can see the 0 values in FEBio Studio starting at row 3636.

Screenshot 2024-11-11 064936.png
I suspect these zero values are the source of the problem.

Best,

Steve

[mper162]

Hi Steve,

Thank you so much for your response. I’ve switched to a new model, and I’d appreciate it if you could take a look at the attached file. I’ve replaced the tet4 elements (muscle) with tet10, and I’ve modeled the fascia and aponeurosis as shell elements (TRI6). The rest of the model remains the same as before. However, it’s not running now, and I’m getting a negative Jacobian error immediately after starting the simulation. Do you have any insights into what might be causing this? I’d really appreciate your help.

Model2_tet10_improved.fs2

[maas]

Hi Randika,

One problem with our shell formulation is that when placed on top of solid elements, it pushes the solid nodes inward until it achieves the desired shell thickness. Depending on the shell thickness and the underlying solid mesh resolution, this can invert the solid elements. The easiest way to get around this, is to invert the shell elements so that their normals point inwards towards the solid. Perhaps give that a try and see if that helps.

Best,

Steve

[mper162]

Hi Steve,

Thanks for your valuable input. I believe that's likely the issue. When I ran the model without any shell elements, the error didn’t occur, so I suspect it's related to the shells. I tried inverting the normals, but I keep getting the same direction regardless of what I do. I used the 'Invert Face and Elements' option in the mesh panel, but it didn’t seem to work. Could you take a look and see why this might be happening? Would you recommend using an external meshing software to invert the normals? I’d appreciate any suggestions you have.

Best regards,
Randika.

[maas]

Hi Randika,

I think I have that issue fixed in FEBio Studio. Please update to the latest development version and then give it another try. Let me know what you find.

Best,

Steve

[mper162]

Hi Steve,

The latest version worked perfectly—thank you very much for that!

Now, I’m trying to create a simple muscle model without the fascia using the same setup. However, I’m encountering some unusual muscle activation behavior and was wondering if you could take a look and provide some feedback. It would be greatly appreciated!

Thank you and best regards,
Randika No_fascia_sctrach_3_0_activation.fs2

[maas]

Hi Randika,

What do you mean by unusual muscle activation? The model seems to run fine, so I'm not sure what you find unusual about it. I did notice that the fibers don't all seem to align in the same direction and there are pockets with fibers running in a different direction. Could that perhaps be the reason?

Best,

Steve