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Thread: pinching of an elastic ring

  1. #1
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    Default pinching of an elastic ring

    DESCRIPTION:
    An elastic ring of inner radius 6 and outer radius 8 is pressed by means of prescribed vertical displacements on the outer edge of two stiff elastic plates. Plane strain conditions are enforced. All materials are assumed neo-Hookean and three sliding interfaces are defined: 1) contact of top plate with ring, 2) contact of bottom plate with ring, 3) self contact of ring.

    REQUIRES:
    FEBio 1.1 (or up)

    EST. RUNNNG TIME:
    Intel Core2 @ 2.4 GHz : 12s
    Last edited by maas; 08-05-2010 at 11:32 AM.
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

  2. #2
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    Default question about syntax

    Hi, I'm using FEBio to simulate compression of tissue between stiff plates. I have a similar problem to this example and I'm not clear on the syntax used in defining surfaces. A simplified version of my problem would be this example with an additional material within the ring that also undergoes compression. I am writing out my .feb file using Matlab and when I run it, I come across a "negative jacobian" error, which leads me to believe that I've defined my contact surfaces or elements incorrectly. How are the contact surfaces defined? What should the node order be for both surface definitions and element definitions? From looking at a few examples, it seems like the elements and surface node ordering changes depending on different materials, but I can't find how the order should be in either of the manuals. In this example the first 9 surfaces for the master surface is the same in the first two contact definitions, why is this?
    Sorry for the confused rambling, I'm very new to FEM. I'd appreciate any help you can provide.

    Thanks!

  3. #3
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    Default

    Regarding the node numbering, the element connectivity used in FEBio is described in the FEBio Theory Manual (which should be part of the installation) in chapter 4 (Element Library). For the contact surfaces, the important thing to remember is that the node numbering defines the surface normal, and this normal must be pointed outwards. If not, the surfaces will actually attract each other instead of repel and cause all sorts of havok.
    Regarding your observation that the first 9 faces are the same, that's most likely a mistake. There is no reason why this should be and I'll see if I can fix this. Thanks for catching this. Let me know if you have any other questions.

    Cheers,

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

  4. #4
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    Default

    Thanks for your reply. I have seen the example you mention in the theory manual, unfortunately that did not match with what I saw in one of the examples so I did not know if that numbering was subjective depending on the model being run.

    The example I am referring to is co22.feb, again with compression. I looked at the last 3 elements defined and then cross-referenced their associated nodes and to me they all appear to be numbered in a different order. Since this does not refer to the above example I won't go into specifics.

    As far as the surface node numbering, this also confuses me. My model is pretty complicated with a random distribution of "fiber" like tissues and it isn't clear which direction or where exactly they would make contact with the plates or with one another. If I do not know which surfaces are going to make contact, how do I define the surfaces?

    Thanks!
    Christina

  5. #5
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    Default

    Hi Christina,

    Can you upload your model or an image of it? If I can take a look at it, I might be able to offer some suggestions on how to deal with the contact definitions.

    Thanks,

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

  6. #6
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    Default

    it only took me 12 seconds to run it.
    ________
    milf Cam
    Last edited by kangli2; 04-07-2011 at 12:09 PM.

  7. #7
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    Default

    I think this problem was created before we had fast linear solvers in FEBio. On my machine it now also takes only 12 seconds to run. I've updated the timings.

    Cheers,

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

  8. #8
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    Jul 2014
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    Default

    Hi Steve,

    I am solving a similar problem that involves collapse of an elastic ring with an asymmetric shape. I would like to know whether it would be possible to a) Import the point data defining the shape of the ring (and its outer thickness) and model the solid domain in Preview, since I am unable to import the .iges CAD file of the same b) Replace the prescribed vertical displacements by a prescribed pressure on the top and bottom plates.

    Thanks & Regards
    Dhananjay

  9. #9
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    Default

    Hi Dhananjay,

    a) PreView can only import FE meshes, not CAD, so you will have to figure out a way to convert the CAD file to a mesh. Perhaps the software that you used to create the CAD drawing might be able to do this?

    b) Yes, you should be able to do this in PreView. If you run into any problems, let us know.

    Cheers,

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

  10. #10
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    Feb 2019
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    Default

    Im trying to make a simillar case to tutorial9. First problem is with meshing. The menus and gui is totally different with the preView tutorials here:

    https://www.youtube.com/watch?v=ytBc...QYOSbx&index=3

    Is that another version?

    The second problem is that the tissue deformation does not match the indenter movement.
    Attached Files Attached Files

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