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Thread: Output data for a specific set of elements

  1. #1
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    Default Output data for a specific set of elements

    It would be beneficial for me to be able to output data for a specific set of elements. I read in the FEBio manual that it's possible to create element sets for this purpose, but I was unable to find anything later on in the manual about how to actually output data for a specific element set after it has been created. Does anyone know how to use this feature?

    Thanks,

    Stephen

  2. #2
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    Hi Stephen,

    You can do this by adding the "elset" attribute do the element data tag.

    Code:
    <element_data data="sx;sy;sz" elset="set1"/>
    This will only output the data for element set, named "set1", and defined the Geometry section. Note that this was added recently. I think it's available in 2.3, but I'm not sure. If not, it will definitely be available in the upcoming 2.4 release. Give it a try and let me know if that worked for you.

    Cheers,

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

  3. #3
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    Great, thanks! I'll let you know if it works.

    Thanks,

    Stephen

  4. #4
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    Hi,

    The same idea is valid if I wish to track surface elements (type "tri")? Because... in my Matlab script, for instance, I can define a surface set which is seen by PreView with a name, let's say "Surface". However, after I run some simulation, PostView only sees the structure tagged by <elem> for the elements making up the "Part" - unique here.

    By supposing that I need to know quantities in a certain area - in my case, the reaction forces over "Surface" after the domain is deformed -, how do I recover this information from PostView? Is it a right rationale to say that PostView doesn't save the information about "Surface"? (As a matter of fact, I've posted a question
    on this line - awaiting -, since I need to track a surface and know how to recover reaction forces after deformation to compute an apparent Young modulus of the structure.)

    To explain better: my script leaves out Matlab like this

    <Surface name="Surface">
    <tri3 id="1"> 1, 2, 3, 4 </tri3>
    ditto...
    </Surface>
    but this structure is not kept in PostView after I run Febio, so that I can't track it anymore.

    Thank you all!

    Peixoto

  5. #5
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    Hi Peixoto,

    I'm not sure I completely understand the problem, but if you define the surface in the Geometry section of the FEBio input file, then PostView will know it. It will show up in PostView in the Model Editor, under the Mesh item. You should be able to select it and it will be highlighted in PostView. Please give that a try and let me know if that worked for you.

    Cheers,

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

  6. #6
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    Hi Steve,

    To be clearer, I attach two screenshots:

    i) 'preview.png': note that 'Top_surface' is indeed defined (circled in red).

    ii) 'postview.png': note that there is no surface highlighted anymore (circled in black).

    What's wrong?

    preview.jpg postview.jpg
    Thanks!

  7. #7
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    Aha, I see. The Named Selections in PreView don't get stored in the .feb file and that's why they don't show up. That's an interesting suggestion, maybe I can add that.

    The only surfaces that are currently stored in the Geometry section of the input file have to be defined as part of a boundary condition and given a name in the Name field of the Selection box of the boundary condition. I assume that Top_Surface is part of a boundary condition? If so, just name the selection Top_Surface and it should show up in PostView.

    Give that a try and let me know if that worked.

    Cheers,

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

  8. #8
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    Hi Steve,

    'Top_surface' was not a BC, just a named selection like this in .feb file:
    <Surface name="Top_surface">
    <tri3 id="1"> 1, 2, 5</tri3>
    <tri3 id="2"> 1, 3, 4</tri3>
    <tri3 id="3"> 1, 3, 7</tri3>
    (...)


    I had applied displacement over the top nodes. I think I got it.

    Now, I did the following:

    i) applied displacement BC by selecting 'Top_surface'; PostView showed up it, but in the form of node selection;
    ii) applied pressure load BC by selecting 'Top_surface'; PostView showed up it, but in the form of surface selection. <---

    Option ii) is what I want, but brings me questions:

    - To have the full surface selectable (ready to compute area and reaction forces), should I always apply pressure load BCs instead of displacements?
    - Turning back to a question of mine from another post: to achieve a "p%" of strain in the simulations, the scale factor of "p%" set while prescribing
    displacement BCs will produce the same effect as if prescribed for pressure load BCs?

    In a quick run here, ii) didn't converge.

    Thank you!

    Peixoto

  9. #9
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    Hi Peixoto,

    In FEBio, prescribed displacements are applied on the nodes, whereas pressures are prescribed on a surface. Although in PreView you can apply a prescribed displacement to a surface, if you'll look in the .feb input file, you'll see that PreView actually created a node set for the displacement boundary condition.

    So in order to get the surface to show in PostView, you'll have to apply a pressure boundary condition. The applied pressure is not the same as the applied displacement, and in fact, a constant pressure will not result in a constant displacement, so I think for your case, the prescribed displacement is the way to go. Of course, that doesn't give you the surface in PostView, but you can apply this trick: apply a zero pressure boundary condition in addition to the prescribed displacement. The zero pressure will not affect your result and will get your surface into PostView. Give that a try and let me know if that worked.

    Cheers,

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

  10. #10
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    Hi Steve!

    Wow, awesome! It worked!

    Thank you!!!

    P.

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