Cauchy stress and stretch output data

[NataCalc]

Dear

1) In FEA model I am using BC extracted from 4DCT movement. These BC are simultaneous displacements in x,y,z and rotations in Rx, Ry and Rz. I have experimental uniaxial tensile data Cauchy stress versus Stretch and cyclic tensile data, Force versus Time. I determined my uniaxial experimental data by Force devided by area. I want to consider doing something similar in the computation model (output results). Can I do this in FEBio Studio?


2) My material parameters (Ogden and viscoelastic) were optimised from uniaxial data (Newton versus mm/mm) and cyclic data (N versus time). My model unit is mm. Boundary conditions are in mm. You have mentioned in another post that default Febio unit is SI (Pa=N/m2). I presume that my outcome unit will be N/mm2 (MPa) based on entering parameters?

Regards,
Nataliya

[maas]

Hi Nataliya,

1) You can have FEBio output the rigid body forces, either in the log file or the plot file. To add them to the plot file, add the "rigid force" plot variable to the plotfile section of the input file (Or add it in FEBioStudio, by right-clicking the plotfile item in the model tree, selecting Edit in the popup menu, and then find "rigid force" in the list that shows up.) In either FEBioStudio or PostView you can also calculate the area of a face selection. If you go to the Tools panel (In FEBioStudio this is a tab on the Post panel), you will find the "Measure Area" tool.

2) FEBio does not assume any unit system. It is up to the user to enter values in a consistent set of units. If your dimensions are in mm and force in N, then indeed the stress and stress-like parameters will be in MPa.

Cheers,

Steve

[NataCalc]

Hi Steve,

Thank you for reply.

It seems I am facing the problem of boundary conditions (BC) I produced from 4D CT. Rigid bodies model with produced BC mimics well the kinetic of 4D CT movement. However, these BC prescribe the cauchy stress 30 times higher than in experiment (15 MPa yield strength from cauchy vs stretch curve from uniaxial tesile test along X axis). For comparison, I choose NODE (which shows max value of displacemen and reaction force) in the meshed body. For this NODE I use the total reaction force over time. I calculate the Cauchy stress as Reaction force devided by initial area and multiply by stretch which is the (intial length+displacement) devided by initial length. Would it be possible that different algorithms used to generate BC from 4D CT can significantly affect final simulation results?

Regards,
Nataliya