Shell model

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  • SimonH
    Junior Member
    • Feb 2008
    • 9

    Shell model

    Hi,

    I am having trouble running a shell model imported from ABAQUS.

    Please see attached. I just applied a simple pressure load and fixed the top of the model. No iterations are converging and I'm not sure why.

    Orginally there was no thickness associated with the elements, but I think I have fixed that. I'm not sure what else is required to obtain a solution?

    Cheers!
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi Simon,

    In the attached file you specify a zero-thickness for the shells, which is not allowed in the current implementation. You will have to specify a non-zero thickness. However, I'll add a warning messsage to FEBio so it will notify you of this.

    Cheers,

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

    Comment

    • SimonH
      Junior Member
      • Feb 2008
      • 9

      #3
      Steve,

      Sorry, sent the wrong file (pre thickness change)

      Correct is attached. I get the same output with a thickness included.

      Have I missed something else?

      Simon

      Comment

      • maas
        Lead Code Developer
        • Nov 2007
        • 3400

        #4
        Hi Simon,

        I think there are still two problems with your mesh. Run the problem in debug mode (add -g2 at the end of the command line). This will create some timesteps and then open the file in PostView. Turn on the normals and you'll see the first problem. At a certain section in your mesh you'll see that the normals flip their direction abruptly. This causes the elements at the edge of this section to invert. However, this is not your problem yet. Your problem is caused by the sharp edge at the top of the femur. Currently, the shell formulation can only deal with "soft" edges. To correctly deal with hard edges such as in your model I'd need to define two sets of rotational degrees of freedom at the edge nodes, which the code can't do right now. Note that the sharpness of an edge is also determined by the thickness (you want the half-thickness to be less than the curvature radius to be precise, otherwise your shell will invert). So, what I did was remove all the shells at the top of the femur to remove the sharp edge. If you do so, then you'll run into the problem I mentioned before. My suggestion would be to go back to your mesh generator and see if you can fix the normal issue first. Let me know if you have any further questions.

        Cheers,

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

        Comment

        • SimonH
          Junior Member
          • Feb 2008
          • 9

          #5
          Thanks Steve,

          I suspected it was a shell-normals type problem, but was unsure how to check them.

          Simon

          Comment

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