IBBM 2017

Results 1 to 2 of 2

Thread: Tutorial 9: Axisymmetric analysis of biphasic indentation

  1. #1
    Join Date
    May 2017
    Posts
    5

    Default Tutorial 9: Axisymmetric analysis of biphasic indentation

    Hi,

    While working through the tutorial, I have run into a problem with tutorial 9.
    I have even redone the entire model, step by step but I cannot seem to find the source fo the error.

    Right now the errors are:
    Code:
    "Problem is diverging. Stiffness matrix will now be reformed"
    "NAN Detected. "
    ------- failed to converge at time : 0.0166667
    Max. nr of retries reached.
    Thereś a number of things that kind of bother me, hope someone can help clarify them for me:
    1) When I select elements (for mesh editing), clicking "Invert" does not do a lot (in fact, only seems to add a kind of shadow to the elements?)
    2) When welding the two meshes, choosing the given threshold does not really seem to do anything, so to exclude uncertainties I just went with the "Weld" option given during "Attach"-ing the meshes.
    3) What is the difference between absolute and relative moving/rotating? Is it with respect to the original and current frame of reference/ coordinate system, respectively?
    4) When adding the Rigid Constraint>Prescribed force for the indentor, should I leave prescribed force in z-direction as 0, and then only edit the value in the load curve?
    5) In the analysis step, is it correct that the second point is (1 000 , 200) rather then (10 000 , 200)?

    Thanks a lot

    PS: I have attached my files for reference
    Attached Files Attached Files

  2. #2
    Join Date
    Dec 2007
    Posts
    608

    Default

    Hi,

    I have even redone the entire model, step by step but I cannot seem to find the source fo the error.
    You need to add the fixed displacement boundary condition that prevents motion of the bottom surface of the tissue layer. Also, for the indenter load curve, you either specify the value of -0.002 in the load curve itself or you specify that value in the rigid force constraint, but not both places. Whichever has the value -0.002, the other should have the value of 1. FEBio multiplies the prescribed BC's with the load curve value and uses that as the prescribed value.

    1) When I select elements (for mesh editing), clicking "Invert" does not do a lot (in fact, only seems to add a kind of shadow to the elements?)
    This tutorial does not ask to invert elements. However, if you do find the need to invert an element you can check the result of the inversion using the Mesh Inspector tool: Inverted elements will have their Volume or Jacobian switch sign.
    2) When welding the two meshes, choosing the given threshold does not really seem to do anything, so to exclude uncertainties I just went with the "Weld" option given during "Attach"-ing the meshes.
    Tha'ts fine. This option did not exist in PreView when the tutorial was originally written but it performs the same function.
    3) What is the difference between absolute and relative moving/rotating? Is it with respect to the original and current frame of reference/ coordinate system, respectively?
    The absolute position of an object is determined by a rotation matrix about the origin and a vector from the origin to the centroid of the object. Rotations are always performed about the global coordinate system. So specifying absolute rotation/translation means that you directly modify this rotation matrix and translation vector. The "relative" option means that you perform a rotation/translation relative to the current orientation/position of the object. Again, all rotations and translations are performed in the same, global coordinate system.
    4) When adding the Rigid Constraint>Prescribed force for the indentor, should I leave prescribed force in z-direction as 0, and then only edit the value in the load curve?
    Use 1 instead of 0 as explained above.
    5) In the analysis step, is it correct that the second point is (1 000 , 200) rather then (10 000 , 200)?
    In this tutorial this model runs to a final time of 1000, so (1000,200) is the correct value for the second point.

    Best,

    Gerard

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •