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Thread: Load Choice

  1. #1
    Join Date
    Apr 2014
    Posts
    1

    Default Load Choice

    Dear all,

    I would like to apply a force acting on a surface and am unsure which loading option to use. I have a resultant vector from a uniform force distribution and I would like to assign a load to an arbitrary surface to yield that resultant. I tried using a traction load but I am unsure of the input format for the traction vector. Is the traction vector input for an overall resultant force or is it somehow in terms of force/area? It seems like it is relative to area because I made two models of different sizes and got the same stresses in both models. I was also considering just applying nodal forces over the surface but that would involve counting the nodes in order to determine what value I assign to reach my resultant. Unless there is a way to count the nodes from a surface without individually selecting them, I would rather not use nodal forces. Any advice would be greatly appreciated, thank you!

  2. #2
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
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    2,064

    Default

    Hi,

    The traction load is indeed a force per unit area so if you want to apply a net force it may not be the most suitable option. Have you considered attaching the surface to a rigid body and then applying the force to the rigid body? That way you won't have to worry about area or counting nodes. Let me know if that would work for you.

    Best,

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

  3. #3
    Join Date
    Jul 2019
    Posts
    5

    Default

    Hi,

    I have a question about the type of loads in FEBio. Let's say I want to apply 1 N force to push an object, which type of force do you suggest? As I understood the traction and pressure load, both are not net forces and the entered values should be calculated based on N/m^2. Therefore, no matter if the object is a rigid body or any material, we need to calculate the area under the load and derived the pressure based on the underlying area. However, I got confused with the answer to the previous post in the thread. Would you please explain it in more details? Also, I want to know whether a nodal load can be used as Newton. Would you please explain nodal force? can it be used as the net force applied to the object? I realised that for the nodal force we can select either nodes or faces of the mesh. Can it affect the results of the simulation?

    Thanks

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

    Hi,

    Even if you know the area of the surface, applying a pressure or traction force may not give you a 1N force. If the surface is curved, you would need to evaluate the surface integral of the traction, taking the surface normal into account, to get to the net force. Even more, as a result of deformation, the surface and its area and normal can change over time, which would change the net force as well.

    The nodal force option allows you to apply a force (in Newtons) directly to a node. However, applying a force to a single node creates a singularity in the solution. This means that the displacement of that node will get bigger as you refine the mesh around that node, making the solution very dependent on the mesh size. When you apply a "nodal force" to a surface, FEBio will apply this force to every node on that surface. In this case the net force will depend on the number of nodes and thus would change if you change the mesh.

    The only way to truly apply a 1 N force, is by applying the force to a rigid body. You can then connect the rigid body to a deformable body to transfer the load.

    Cheers,

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

  5. #5
    Join Date
    Jul 2019
    Posts
    5

    Default

    Dear Steve,

    Thank you for your detailed reply. Please correct me if I am wrong, so it means that I should apply a nodal force with a magnitude of 1N to a rigid body and use rigid-contact to connect that rigid body to the deformable model that I have?

    Best

  6. #6
    Join Date
    Nov 2007
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    Default

    You could apply a nodal force to a node of the rigid body, but that may also create a torque on the rigid body unless the nodal force's line of action goes through the rigid body's center of mass. If you just want to apply a force, i.e. apply the force to the center of mass, you should use a prescribed rigid force (Physics -> Rigid Constraints -> Prescribed rigid force).

    Cheers,

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

  7. #7
    Join Date
    Jul 2019
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    5

    Default

    Is there any way to apply the prescribed rigid force in any desired directions? or I need to decompose a force vector let's say (1,1,0) into an x-force and a y-force?

  8. #8
    Join Date
    Nov 2007
    Location
    Salt Lake City, UTAH
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    Default

    Indeed, you need to decompose the vector and apply separate rigid force constraints for each component.

    Cheers,

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

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