Parameter Optimization for Biphasic Unconfined Compression Stress-Relaxation

[maas]

Hi sri,

I suspect that there is a problem with the loadcurve definition. It ramps up to 100, which will result in a total displacement of -25 for the rigid body, even though the whole mesh is only 2.6 tall. So perhaps double check the values here.
You also don't need a biphasic contact. That is only used for contact between two biphasic parts, but here the second part is rigid. So, a sliding-elastic should suffice here. If you find that the contact is not sufficiently constrained, try increasing the penalty factor and see if that helps.

Best,

Steve

[fzmb treuter]

Hi Steve,

I have created an stress relaxation unconfined compression model to determine the mechanical properties of cartilage tissue. However, the cartilage geometry is not defomed by the compression plate. The compression plate moves according to the specified rigid body displacement. Could you please check the model and let me know what is wrong. Thank you very much!
I have attached the feb file.

Best,

Thomas

[maas]

Hi Thomas,

Looks like the penalty factor needs to be increased. Try setting it to a higher value (e.g. 100) and see if that helps.

Best,

Steve

[fzmb treuter]

Hi Steve,

thank you, the model is now running. I still have a question about the material model Tension-Compression Nonlinearity. I would like to simulate the cartilage tissue as biphasic + Neo-Hookean and Tension-Compression Nonlinearity. How can I select this under the material node?

Best,

Thomas

[maas]

Hi Thomas,

Are you talking about the "TC nonlinear orthotropic" material? If so, it looks like that is an uncoupled material, which for certain technical reasons cannot be assigned to the biphasic material in FEBio Studio. However, you can modify the input file manually and assign that material.

To elaborate a bit, uncoupled materials typically are used to represent incompressible materials and have been implemented using a different approach than the regular elastic materials. Since biphasic materials expect elastic materials, by default the uncoupled formulations are not available for biphasic materials in FEBio Studio. But in the input file, you have a bit more freedom.

Best,

Steve

[fzmb treuter]

Hi Steve,

in an older version of FEBIO I simulated cartilage tissue as solid mixture tissue with Neo-Hookean as ground matrix and spherical fiber distribution (alpha = 0 and beta = 2). This is not possible in FEBIO Studio?

Thanks a lot!

Best,

Thomas

[maas]

Hi Thomas,

This is possible in FEBio Studio. I've attached an example file that defines such a material.

To create this material, add a solid mixture. Then set the solid property to 2 (the value of this property defines the number of components). Then, you can add the individual components.

Best,

Steve

[fzmb treuter]

Hi Steve,

thanks a lot. Happy new year!

Best,
Thomas