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I want to convert the contact of a rigid body and a biphasic material permeable from impermeable during a creep test. What is the best approach for doing this? Thank you.
You can set the fluid pressure to zero on the entire contact surface, while using a sliding-elastic contact interface between the rigid body and the biphasic material (sliding-biphasic can work also, but is not needed if the rigid indenter is porous).
Thank you very much for your reply. I was basically planning to keep the surface impermeable for first half of a creep test and permeable for the rest of the time. So, should I divide the whole test into two steps and set fluid pressure zero in the second step?
Yes, you can do that. Use the "prescribe fluid pressure" (instead of "fix") boundary condition in the second step of your multi-step analysis and set the prescribed value to zero.
Thank you very much for your reply. I have tried according to your direction. I got result which is identical with the case when the surface was always impermeable throughout the whole creep test. I am concerned if I am missing anything. If it is possible, can I send my file to you?
There seems to be a bug in the FEBio2.9.1 code which is not enforcing the prescribed zero fluid pressure correctly in the second step.
Instead, you can try running your file from within FEBioStudio, which uses the beta version of FEBio3. Using FEBio3 from within FEBioStudio, I ran the file you sent me and examined the results, and it does exactly what you want to achieve: During the first step the rigid indenter is impermeable and during the second step it is free-draining. You can verify this result by looking at the effective fluid pressure (which is the nodal degree of freedom on which you prescribe your pressure boundary conditions). Up until time 5 the effective fluid pressure is non-zero inside the contact region. After time 5 it drops to zero exactly, while the effective fluid pressure inside the biphasic layer remains non-zero (as can be verified using a section view).
Please try it out and let me know if that fixes your issue.
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