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Thread: Interface elements

  1. #1
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    Default Interface elements

    Hi Steve,

    Does FeBio offer interface elements (for instance similar to INTER205 in Ansys)? I couldn't find anything about that in the manual. And if not, would you consider adding that capability?

    Thanks a lot!

    Bahar

  2. #2
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    Default

    Hi Bahar,

    I'm not familiar with interface elements. Could you briefly explain what they are and what they are used for?

    Thanks,

    Steve.
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

  3. #3
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    Default

    Hi Steve,

    Interface elements are used to model the cohesive zone between two surfaces. The material model for these elements would define the normal and shear components of traction at the interface as functions of normal and tangential separations (normal and tangential displacement between initially coincident nodes on the two surfaces). These elements can be used to model a weaker (more complaint) or a stronger (stiffer) cohesive zone. Also the stiffness of the cohesive zone can be defined differently in normal and shear directions. They can also be used to model separation between the two surfaces. In my case I need to use them to model cell-matrix cohesion.

    Iím not at all an expert on interface elements. I just know they are what I need to use. So I hope this helps.

    Thanks,

    Bahar

  4. #4
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    Default

    Hi Bahar,

    I must admit this is all very new to me. I'll need to do some more reading to fully understand these interface elements. In any case, I don't think this is something I can add quickly. But perhaps, if you can share some more details about what you would like to do, maybe the other interface types in FEBio might be able to help you.

    Cheers,

    Steve.
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

  5. #5
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    Default

    Hi Steve,

    What I need to do is to be able to model the interface between cell and their extra cellular matrix. There are a bunch of proteins that form this interface. So my first instinct was to use discrete elastic elements (springs) between the cell mesh and the ECM mesh to represent the proteins. I want to look at the effect of the strength of the cohesive zone on strain distributions in and on the surface of the cell under various loading conditions. For example I want to know what happens if some proteins are missing. I do this by modifying the stiffness of the discrete spring elements. However there are a couple of problems with this approach:

    1- There are as many spring elements as there are nodes so the stiffness of the cohesive zone becomes mesh dependent and mesh density is not constant everywhere in the model.

    2- I canít have different normal and shear stiffnesses for the cohesive zone

    3- The length scale at which the adhesive proteins exist is smaller than my element size so I really donít have a physical basis to claim these springs directly represent the proteins.

    All of this has so far been Nike3d, not in FEBio. I am looking to switch to FEBio if it gives me more suitable options. I look forward to hearing your suggestions.

    Thanks a lot!

    Bahar

  6. #6
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    Default

    Hi Bahar,

    I've given some thought to how you might be able to do this with the current modeling capabilities in FEBio. It will depend on what exactly you expect the interface to do. Would it have to be possible that the two surfaces can slide over each other? If not (which I think is the case?) I suggest the following options:
    - model the interface with a thin layer of elements and assign an elastic material (e.g. neo-Hookean) to these elements.
    - If the interface is much smaller than your element size, perhaps we can build the interface with shell elements and tie the shells to the surfaces with a tied interface.

    Let me know what you think. I would be more than happy to help you set up these models.

    One last questoin, when you say that all of this work has been done in Nike3D, am I correct in assuming that you were referring to your spring models? We work with Nike3D as well here, but as far as I know it doesn't have any interface elements like ansys.

    Thanks,

    Steve.
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

  7. #7
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    Default

    Hi Steve,

    The interface is much thinner than the size of my elements. The two surfaces can separate or slide but not freely. There will be a surface traction (normal or tangent to the surfaces) in response to relative displacement. Sort of like a thin row of solid elements, but since it has no thickness the stresses are functions of displacements (in the material model) as opposed to being functions of strain. Also, the only tractions are normal and tangential to the surfaces.

    Shell elements wonít work because, as I understand, they canít have shear or normal stresses in the through-thickness direction.

    Yeah, Nike3d doesnít have interface elements either. I meant I did the springs in Nike. Problem is, Ansys doesnít have the material model I need to use (transversely isotropic and active) and adding that as a user defined material to Ansys is going to be Ö very painful. So, do you think you could add an interface element into FEBio sometime in the future? If you do, I think it will be very valuable for a lot of people Ö.

    Thanks a lot for your help and your time.

    Bahar

  8. #8
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    Steve - I wonder if the contact formulation could be modified to allow for resistance normal and tangent to the sliding surface? Sounds like a force and stiffness term for each direction would handle this. Much easier to implement than friction but still a good amount of work...

    Jeff
    Jeffrey A. Weiss
    Professor, Department of Biomedical Engineering, University of Utah
    Director, Musculoskeletal Research Laboratories
    jeff.weiss@utah.edu

  9. #9
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    Hello,

    I've been digging deep into the threads. Has there been any updates to enable an adhesive feature like this? I'd like to use FEBio for a similar application.

    Thanks!

    Andre

  10. #10
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    Hi Andre,

    Several contact formulations now support friction. I recommend that you take a look at the sliding-elastic contact formulation and let us know if you have any questions.

    Best,

    Steve
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

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