Biphasic Question

[Curt Voss]

Hey guys, a quick question:

Is the stress tensor returned from febio biphasic runs the mixture stress or the elastic stress? I was assuming the latter, but in the attached run I apply 0.05 MPa pressure on the top surface, and the y-axis stress comes out exactly 0.05MPa, which makes me think its the mixture stress (solid matrix + fluid pressure).

Thanks!

[maas]

Hi Curt,

Yes, the stress stored in the plot file is the mixture stress, not the elastic stress.

Cheers,

Steve.

[Curt Voss]

Thanks, Steve. Next time you're editing documentation, that might be worth specifying, as I think that's the opposite of Abaqus.

[michalacadova]

Hi,
I have a question concerning stress using the biphasic material, concretely where can I find the results for the stress in the solid matrix and where for the fluid pressure? Above you say that in the „effective stress“ field is stored the mixture stress (solid matrix + fluid pressure) and (in another thread) that the fluid pressure is stored in the „temperature“ field.

But then how can I get the fluid pressure value bigger than the effective stress ? In my case for one particular node the value for the effective stress is app. 0,5 (the 1.principal stress -0,57) and in the temperature field the value for the same node is 0,9 (value in the pressure field equals to 0,88)

Thanks for the explanation

Michala

[ateshian]

Hi Michala,

The mixture (or total) stress is

T = -p*I + Te

where p is the fluid pressure, I is the identity tensor, and Te is the elastic (or effective) stress in the solid matrix.

PostView displays T under the various Stress entries, and p in the Temperature field. (The Pressure field does not contain fluid pressure information.) In most problems involving compressive loading, the fluid pressure p is a positive quantity (there are exceptions of course). However, the Cartesian and principal components of T can be positive or negative.

For example, it is possible to have the "1 Principal Stress" be T1=0.1 while the "Temperature" is p=0.2. In this case, T1 = -p + Te1 or 0.1 = -0.2 + Te1, which means that Te1 = 0.3 (the first principal stress of the elastic stress tensor is tensile).

Alternatively, you can have the "3 Principal Stress" be T3=-0.25 and "Temperature" be p=0.2, or -0.25 = -0.2 + Te3, implying that Te3 = -0.05 (the third principal stress of the elastic tensor is compressive).

I hope this clarifies mixture stress versus elastic stress.

Gerard

[michalacadova]

Yes, thank you, now it makes sense.
Michala