FEBio Units

[Plat_Sar]

Hello I am trying to setup a simulation and I am getting as Outputs rather low values.I want to make sure that the Loads, Material parameters and Boundary Conditions have consistent Units.
Browsing through the FEBio Manual I found the following instructions :
image.png
In my case I have can only define 4 quantities for the simulation.
1) The lengths of the geometry (nodal coordinates) which are currently given in units of [ m ] .
2) The time units of the simulation, currently in [ sec ] .
3)The Material Properties in [ Pa ] .
4)The applied Pressure Load in [ Pa ] .

In the manual (pic above) it is stated that pressure units must not be used as primary units as they are derivable from the combination of two primary units.
I don't have parameters such as Mass or Force to input.
How can I make sure that the units are applied correctly?

Thanks,
Plato.

[maas]

Hi Plato,

Assuming you are doing a quasi-static analysis with only elastic materials, then your units are applied correctly. In this case, the only units in your model are length and pressure, and you are free to choose whatever units you want for these quantities. The only thing you need to make sure of is that the values you enter for the material are in the same unit that you choose for the applied pressure. (So, for instance, if your Young's modulus is expressed in MPa, then your load cannot be in Pa. It must also be in MPa.)

Also, to clarify, in the highlighted section above, you cannot choose [P] as a primary unit, since [F] and [L] were chosen as the primary units and [P] can be expressed in those. However, you could choose [P] as one of the primary units and then express [F] in terms of [P]: [F]=[P]*[L]^2.

Best,

Steve

[Plat_Sar]

Thank you very much Steve, you completely cleared my confusion.
You assumed correctly that I am running a quasi static simulation with elastic materials.
I want to run it with SI units, but I am having a problem with the convergence because with non SI units the first time steps were converging more easily for a ramp pressure loadcurve from 0 to 0.015 MPa but with SI units this is very different from 0 to 1.5e4 Pa. How can I override this difficulty in converging?

Thanks in advance,
Plato.

[maas]

Hi Plato,

Could you please attach your model so I can give it a try on my end?

Thanks,

Steve

[Plat_Sar]

Hello Steve,
I am attaching my feb file link so you can give it a try.

https://drive.google.com/drive/folders/1JmrbnUsyOtgn24a2L_D-LglFUDS2e-3z?usp=sharing

Thanks,
Plato.

[maas]

Hi Plato,

I downloaded the file, but this model doesn't run at all. (Fails with negative jacobians.) I understood that the model runs, but the convergence is worse than with different material parameters. Did I misunderstand?
Also, if the model should run with different material parameters/load, can you please let me know which values should work?

Thanks,

Steve

[Plat_Sar]

Hi Steve,
the file above that doesn't run is of a simulation that converges but with changed units from MPa,mm,sec to SI (Pa,m,sec).
The reason I changed the units is because I was getting very low stress values (way lower than expected according to bibliographic studies) and I wanted to test if it is an issue of inconsistent unit definition.
I can attach the proper feb file so you can give it a try.

https://drive.google.com/drive/folders/1JmrbnUsyOtgn24a2L_D-LglFUDS2e-3z?usp=sharing

Thanks,
Plato.

[maas]

Hi Plato,

It looks like this last file you attached is the exact same file is the previous one. I don't see any difference between the material parameters, loads, or nodal coordinates. Can you please double check that this is the correct file? I'd like to see the file that runs, so I can see exactly what changes were made.

Thanks,

Steve