1. Junior Member
Join Date
Apr 2014
Posts
1

## 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!  Reply With Quote

2. ## 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  Reply With Quote

3. Junior Member
Join Date
Jul 2019
Posts
14

## 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  Reply With Quote

4. ## 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  Reply With Quote

5. Junior Member
Join Date
Jul 2019
Posts
14

## 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  Reply With Quote

6. ## 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  Reply With Quote

7. Junior Member
Join Date
Jul 2019
Posts
14

## 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?  Reply With Quote

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

Cheers,

Steve  Reply With Quote

9. tff Junior Member
Join Date
Mar 2019
Posts
16

## Hi all,

I have a question about the difference between applying a nodal force and a pressure load to a rigid body. In first case, I apply 1N nodal force, and in the second case I calculate the corresponding pressure for 1N force, based on the area under the load, and then apply the pressure load to the rigid body. I want to know if these two cases resulting in an approximately similar result.  Reply With Quote

10. ## Assuming that the nodal force is applied in the same direction as the average surface normal and the surface area isn't too big, then, yes, I think those two cases should produce similar loads in the initial configuration. However, if the object rotates, the constant nodal force will not rotate with the body, whereas the pressure load will (since it's always applied normal to the surface), so over time, the net applied load might become significantly different.

Cheers,

Steve  Reply With Quote

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