Triphasic unconfined compression

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  • Farrell1
    Junior Member
    • Oct 2011
    • 25

    Triphasic unconfined compression

    I am modelling an unconfined compression experiment using the triphasic material definition, but seem to be running into some problems. I find that febio crashes when I run the analysis which leads me to believe I have done something very wrong! I have attached my .feb file for comment.

    It is a quarter wedge of height 1.5mm and radius 2.75mm constructed following the guidlines from the example unconfined compression analysis in the users manual.

    I have then input these initial conditions to satisfy a triphasic material:

    1) Effective solute (Na+, Cl-) concentrations are 150mM on the free curved surface.
    2) Effective fluid pressure is -0.3654MPa on the free curved surface (Pe=-R*T*(Bath Osm)).

    It is a 2 step analysis:

    1) Fixed charge density is decreased from 0 to -300mEq/L (t=0 to t=1).
    2) Compressive displacement in the z-direction of 0.075mm (t=1 to t=3.5).

    When I run this analysis but with a biphasic material, it works well and only crashes when I use the triphasic one. I would very much appreciate some feedback as I must be doing something wrong.

    Thanks

    Mark
    Attached Files
  • ateshian
    Developer
    • Dec 2007
    • 1824

    #2
    Hi Mark,

    I checked your file and I found a couple of problems, though I couldn't get it to run even after fixing those:
    1) For some reason, some of the nodal coordinates are huge numbers (E+00150).
    2) You did not apply boundary conditions on the lateral (cylindrical) boundary surface. Though technically not required, I am assuming you intended to prescribe ambient bath conditions?
    3) You chose to have both steps be "steady-state" but I assume you would want the stress-relaxation part to be transient.

    Only (1) above should be cause for error, the other two observations should not cause problems. However, though I modified the coordinates in the input file, I couldn't get it to run either. Since I am away at a conference, I don't have too much time to look more into it, but please note that there is a sample triphasic unconfined compression stress-relaxation problem in the Verify suite, it is tr02.feb (also attached for your convenience). Please try it out and see if that helps you figure out what's not working with your file.

    Best,

    Gerard

    Comment

    • Farrell1
      Junior Member
      • Oct 2011
      • 25

      #3
      Gerard,

      Thank you for taking the time to look at the file. I am, however, confused as to why the nodal coordinates are perhaps causing the problem since I use the same geometry in a biphasic unconfined analysis and it works fine.

      I too have implemented your suggested alterations and it still does not work. The file you attached runs perfectly, and having studied the code I am still unsure as to why mine is not working. Do you have the accomanying preview file for tr02.feb? This would be helpful as I can look at the geometric detailing for this model.

      Regards,

      Mark

      Comment

      • ateshian
        Developer
        • Dec 2007
        • 1824

        #4
        Hi Mark,

        I had a chance to take a closer look at your input file and here are a couple additional observations:

        - Your initial conditions were only applied to the lateral cylindrical boundary surface. They should be applied to the entire volume.
        - Your initial pressure was not calculated correctly: Since the osmolarity of the bath is 150+150 = 300, the pressure should be -0.7308 (you had half of that).

        Once these issues were addressed, I was able to get your file to work.

        Best,

        Gerard

        Comment

        • Farrell1
          Junior Member
          • Oct 2011
          • 25

          #5
          Thanks Gerard, thats it working.

          Mark

          Comment

          • Farrell1
            Junior Member
            • Oct 2011
            • 25

            #6
            I have run into another problem with this model. When I increase the osmolarity of the external solution to a highly hypertonic value, the model crashes at the start of the second step (axial displacement). The model has been working well for the isotonic case and I have some excellent results, but I believe I may have to adapt my boundary conditions to cope with the much greater effective pressures involved with the hypertonic case.

            I have attached my .feb file for comment, and would greatly appreciate some guidence on the matter.

            Regards

            Mark
            Attached Files

            Comment

            • ateshian
              Developer
              • Dec 2007
              • 1824

              #7
              Hi Mark,

              I am not getting a crash with this file. It is running to completion without any trouble. Are you using the latest version of FEBio (1.5.1)?

              Best,

              Gerard

              Comment

              • Farrell1
                Junior Member
                • Oct 2011
                • 25

                #8
                I have tried the file on another computer and it seems to be working fine. The original computer was a high performance machine we use for complex modelling, so not sure why it was not working.

                Thanks,

                Mark

                Comment

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