Apply moment to a single point or several desired points in the model

[lyra]

Hi,

I was wondering if we can apply a moment to a single point or several points of the model? if so, would you please let me know which force type in the FEBio I should use to model a moment applied to the desired point(s) of a rigid body or non-rigid body object?

Best

[apaz]

Hi Lyra,

As far as I know, moments can only be applied to rigid bodies. So, depending on the problem you are interested in, you can define a rigid material and locate the center of mass of the body at some important point for you. Then, through rigid constraints or joints definitions, you can apply those conditions you need. You can also use rigid contacts to transfer those rigid-body conditions to deformable bodies. Another approach would be to use a pair of nodal forces in opposite directions.

Regards,
Alex

[lyra]

Thanks for your reply.

I have another question, in the rigid body constraints/rigid forces, there are torque constraints. Can I use them for applying a moment at the center of the mass of the rigid body? What is the corresponding unit for these torque values? Are they have a N.m in the case that I am using SI units (N for force and meters for length)?

For applying a specific torque in a specific direction, should I first specify the decomposed directions in the x,y, and z, and then for the magnitude of it should I change the corresponding load curves for the torque components to the desired value? Would be any differences if I multiply that desired value with the decomposed values of x,y, and z and use them as the values of the torque in x, y, and z while keeping the load curve as the default value which starts from (0,0) to and ends at (1,1)?

[maas]

Hi Lyra,

In order to apply a torque to a rigid body's center of mass, you need to specify three rigid constraints, one for each of the torque vector components. (Actually, you don't need to specify the components that are zero.) The actual torque values used in FEBio are the product of the load curve (if one was assigned) and the value you specified in FEBio Studio (or in the input file). What I recommend is setting the target value in the edit fields in FEBio Studio and use the load curves to ramp the effective value from zero to its target. In other words, leave the load curve from (0,0) to (1,1). The units are indeed N.m in SI.

Cheers,

Steve

[lyra]

Dear Steve,

Thank you so much for your useful and complete answer.

Best

[Sara_kr]

Hello,
I am currently engaged in the simulation of the knee joint and am in need of applying joint reaction forces and moments at specific points within the model. My endeavor involves applying moments in various directions. However, according to my research, moments can only be applied to rigid bodies. As my model does not incorporate any rigid materials, I am seeking alternative methods for application.

Could you kindly advise on how I may proceed in this matter?
Thank you very much

[maas]

Hi Sara_kr,

As you mentioned, applying reaction forces and moments, requires rigid bodies. If you don't have rigid bodies, you can either add them by adding a "rigid body" material, and then use a "rigid" boundary condition to connect parts of the deformable mesh to the rigid body. (The rigid body does not require any geometry if you are only using it to apply loads.) The alternative would be to emulate the effect of an applied force or moment through a distributed load (e.g. surface pressure), but that can be very complicated.
If you are still unsure how to proceed, please share a bit more detail about your model and the loads you wish to apply, and then we might be able to provide some additional suggestions.

Best,

Steve