combining bodyforces

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  • theilenb
    Junior Member
    • Sep 2010
    • 14

    combining bodyforces

    Hi!

    We tried to combine the non-constant bodyforce with a constant bodyforce to add gravity to our model.
    We did this by applying a fixed constraint to one face of a cylinder and then ramping up a "normal" bodyforce, thus getting a a hanging cylinder. We then ramped up the non-constant bodyforce while leaving the gravity constant. This new force is directed against the gravity, ergo pushing the cylinder back up.

    Calculating the same model without gravity leads to a certain displacement. After adding the gravity we expected that the displacement relative to the state with full ramped up gravity but no non-constant bodyforce should be more or less the same as in the model without gravity. However, we found that the displacement increases significantly (factor ~2.5).

    Since we have no explanation for this behavior we did some tests. The effect stays exactly the same with different elementsizes. So we tried different material models and did the exact same thing with an isotropic elastic material. Surprisingly we found that in this case the displacement decreases (up to a factor of ~0.5). Since the displacements caused through the gravity are not that big we believe neo-hookean and isotropic elastic material models should behave almost the same.

    All these calculations were static calculations with a nearly incompressible (v=0.499) neo-hookean material with a Young's-Modulus of 10kPa and a density of 1 g/cm³.

    We also did some simplified tests. We did the same thing with a cube with one face constraint in all three spatial dimensions. This time we used E =2000kPa and v=0.3 to get rid of the problems with nearly incompressible materials. We then did the same thing as before, but this time we used a more simple non-constant bodyforce (e.g. we set it to a constant). Although the effect this time was really small it is still there.

    Do you have any suggestion where this behavior might come from? We are out of ideas at the moment.

    Thanks for any input,
    Sebastian


    P.S.: This is the way we configured the forces:

    <Globals>
    <body_force type="non-const">
    <z lc="1" data="exp(-12.5*(x^2+y^2))*0.007355*exp(-0.345*z)">-1000.0</z>
    </body_force>
    <body_force>
    <z lc="2">981</z>
    </body_force>
    </Globals>

    <LoadData>
    <loadcurve id="1" type="linear" extend="constant">
    <loadpoint>0.010,0.0</loadpoint>
    <loadpoint>0.020,1.0</loadpoint>
    </loadcurve>
    <loadcurve id="2" type="linear" extend="constant">
    <loadpoint>0.000,0.0</loadpoint>
    <loadpoint>0.010,1.0</loadpoint>
    </loadcurve>
    </LoadData>
    Last edited by theilenb; 07-20-2011, 02:58 AM. Reason: added density
  • ateshian
    Developer
    • Dec 2007
    • 1824

    #2
    Can you please post some sample files and provide more details about what you think is going wrong? I am unable to reproduce your problem otherwise.

    I have attached a file where the effects of a standard body force are perfectly countered by a "non-const" body force that has been prescribed to be uniform. There seems to be no discrepancy.

    Best,
    Gerard

    Comment

    • theilenb
      Junior Member
      • Sep 2010
      • 14

      #3
      My files are rather big (several 10 MB) because we have a very dense mesh.

      Consider a linear-elastic material modell. We apply a bodyforce which stretches a geometry a certain amount. Now we additionally apply a second (but smaller) bodyforce in opposite direction to the first one. The geometry is now compressed a little bit, but still stretched compared to the state without any forces.

      Now we are interested in the displacement caused by the second bodyforce compared to the state where only the first bodyforce is applied. Since the material model is linear this displacement should not be dependent on how far the geometry was stretched due to the first bodyforce (or so we believe). But we see a strong dependency: the bigger the first bodyforce is (and therefore the bigger the geometry is strechted), the smaller the displacement caused by the second bodyforce is.

      Comment

      • maas
        Lead Code Developer
        • Nov 2007
        • 3400

        #4
        Hi Sebastian,

        Have you made any progress on this issue? Perhaps the problem is related to the fact that FEBio does not have a proper linear elastic material model. All material response is therefore nonlinear for sufficiently large deformations. Just a thought.

        Cheers,

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

        Comment

        • theilenb
          Junior Member
          • Sep 2010
          • 14

          #5
          Hi Steve,

          we haven't made any real progress on this issue yet. I wasn't aware that one has to distinguish isotropc elastic from linear elastic, thanks for pointing that out. Though it seems the deformations we see behave linearly.

          I will take a closer look in the next days and let you know wether this could leed to a solution.

          Cheers,
          Sebastian

          Comment

          • theilenb
            Junior Member
            • Sep 2010
            • 14

            #6
            Just to finish this Thread:

            Steve, you were right, it has to do with the non-linearity of the isotropic material. Thanks a lot again for pointing this out!

            Cheers,
            Sebastian

            Comment

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