Comparing options for defining tendon material properties

[cyrusrezvanifar]

Hello all,

I have been comparing different ways to represent tendon material properties in FEBio for simulating shoulder joint motion: 1) defining an uncoupled solid mixture with Mooney-Rivlin bulk material and fiber-exp-pow-uncoupled; 2) defining a trans iso Mooney-Rivlin material with the fiber generator option turned on via "vector" orientation, and 3) default Tendon Material. In option (1), in contrast with (2), I cannot visually check the orientation of the fibers to make sure they are correctly defined, even when I turn on the "Show material fibers" options in Preferences.


1) Does it mean option (1) is not actually defining the fibers? If it does, is it a single fiber or the number/compactness of those fibers needs to be defined by the user?

2) With a trans iso Mooney-Rivlin material, I am wondering how the number/compactness of the fibers affects the overall modulus, and is there a way to modify that?

3) Is there a preference in using either a solid mixture or trans iso Mooney-Rivlin for defining tendon material properties?

4) Are there any advantages/disadvantages in using the default Tendon Material?


I'd really appreciate any thoughts and comments for selecting the most appropriate approach. Thank you!

Cyrus

[maas]

Hi Cyrus,

1) Option 1 is definitely defining the fibers, but currently FEBioStudio can only visualize the fibers or material axes from top-level materials. We are working on better visualization tools for more complex materials.

2) The strain-energy function of a trans-iso Mooney-Rivlin is a simple additive composition of a matrix term and a fiber term. There is no parameter that defines a fiber density or similar. You might be able to correlate the fiber density to the elasticity parameters in some sense, but I'm not sure if that is done commonly.

3) With the solid mixture, you have flexibility since you can choose both the matrix and the fiber model, but if you want to use a Mooney-Rivlin matrix with an fiber-exp-pow material, then it might be more convenient to use the trans-iso MR material. One advantage is that you can see the fibers in FEBioStudio.

4) The Tendon material uses a very different constitutive formulation. You can consult the FEBio User's Manual (section 4.1.2.11 for the Tendon material, and 4.1.2.9 for the related muscle material) and the reference therein, and then decide which formulation fits your needs best.

Cheers,

Steve

[cyrusrezvanifar]

Hi Steve,

Thank you for the clarifying response. I will continue using trans iso MR to be able to verify fiber orientation and will also explore more about the tendon material constitutive formulation.

Best,

Cyrus