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Thread: Applying a Force in a different angle

  1. #1
    Join Date
    May 2020
    Posts
    6

    Default Applying a Force in a different angle

    Hi everyone,
    is it possible to apply a force in a different direction then x-/y- or z-direction? I would like to apply a force on a screw in a certain angle but I could not find how I can do it.
    Hope somoene can help me and thanks a lot in advance.
    Best regards
    Jeff

  2. #2
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
    Posts
    2,329

    Default

    Hi Jeff,

    I assume the screw is a rigid body? You can apply a force in a particular direction by decomposing the force in its vector components and then applying three forces, one for each direction. For instance, in 2D, since you know the angle, you can a force F in direction a, by applying two forces: Fx = F*cos(a), Fy = F*sin(a).

    Cheers,

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

  3. #3
    Join Date
    May 2020
    Posts
    6

    Default

    Hi Steve,
    thanks a lot for your help. I understand the decomposing of the force. But how do I enter it in FEBio. If I add a load, I can only enter a number for the load and not a cos(a) or sin(a).
    Best regards,
    Jeff

  4. #4
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
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    2,329

    Default

    Hi Jeff,

    We have some support for defining equations, but not for all features yet, and I don't think you can use equations in rigid body forces (if that's what you are using?). I can definitely look into adding that. In the meantime however, you'll have to do all your calculations beforehand, and enter the final numbers in the value fields for the three components of the force. Or is there something more to what you want to do that I am missing?

    Cheers,

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

  5. #5
    Join Date
    May 2020
    Posts
    6

    Default

    Hi Steve,
    thank you for your support!
    I have entered all the values but now I cannot run the simulation. Am I missing something?
    I just want a simple simulation of one vertebra and a force in a certain angle on a pedicle screw (see the link).
    https://www.dropbox.com/sh/ksb3i9uyk...TiV9MiQ_a?dl=0

  6. #6
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
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    2,329

    Default

    Hi Jeff,

    The main problem is that the two rigid parts (the vertebra and the screws) are not attached to each other. When you apply the force to one of the screws it just flies off to infinity. You need to attach the screws to the vertebra. Given that both are rigid bodies, I think the easiest way to do this is to make them one rigid body. (Assign the same rigid body material to both the vertebra and the screws). Also, constraints on rigid bodies have to be applied via "rigid constraints". The "fixed displacement" that is applied to the vertebra will not have any effect.

    By the way, I had some problems with this model. It seems that some parts had the same ID, which made it impossible to select the screws. Do you have this problem as well? I was able to fix that issue so if you want the fixed model, please reach out to me via email (steve dot maas at utah dot edu).

    Cheers,

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

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