Extraction of mass matrix and internal forces

[tdetroux]

Hello,

I would also be interested in evaluating nonlinear frequency responses for simple mechanical systems (material+fixation) by computing periodic time responses using FEBio and MATLAB, and would have the following questions:

1) I assume that I can retrieve the internal forces for x-y-z at each node from "Rx-Ry-Rz" in the logfile. Are these the internal forces represented by "T" in Eq. 7.0.1 of the theory manual? Should I also consider internal forces related to rotations (xy, xz, yz)? If so, how can I retrieve them?

2) How could I easily compute the internal forces associated to a given displacement field (in a static way) for my problem?

3) How can I extract the mass matrix for my problem?

4) Is there a way to start a dynamic analysis in FEBio by setting an initial displacement field (together with the initial velocity field), i.e. imposing U(t = 0) = U0 and V(t = 0) = V0 where U0 and V0 are defined by the user.

Thank you very much for your help!

[ateshian]

Hi,

I assume that I can retrieve the internal forces for x-y-z at each node from "Rx-Ry-Rz" in the logfile.

The only non-zero internal forces you can retrieve are the reaction forces at nodes where displacements have been prescribed. At all other nodes the output will be zero.

Are these the internal forces represented by "T" in Eq. 7.0.1 of the theory manual?

No, the internal forces represented by "T" in that equation represent the global force vector. The contents of this vector cannot be viewed in the log file.

Should I also consider internal forces related to rotations (xy, xz, yz)? If so, how can I retrieve them?

Only rigid bodies have rotational degrees of freedom. For all other (deformable) solid materials there are only nodal displacement DOFs, no rotational DOFs.

How could I easily compute the internal forces associated to a given displacement field (in a static way) for my problem?

See equations 7.1.30 and 7.1.31 in the Theory Manual.

How can I extract the mass matrix for my problem?

The mass matrix is calculated as shown in equation 7.1.33 (and the equation before it).

Is there a way to start a dynamic analysis in FEBio by setting an initial displacement field (together with the initial velocity field), i.e. imposing U(t = 0) = U0 and V(t = 0) = V0 where U0 and V0 are defined by the user.

The initial displacement field is always assumed to be zero everywhere in a FEBio analysis. The initial velocity can be prescribed using Physics->Add Initial Condition...->Velocity in PreView/FEBioStudio.

Best,

Gerard