Negative jacobian in 2nd step of analysis

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  • konris87
    Member
    • Nov 2019
    • 36

    Negative jacobian in 2nd step of analysis

    Greetings,

    I am a postgraduate student and i would like to study the biomechanics of the knee. I am new to FEBio. I have set a multistep analysis where I want to rotate the femur in the 1st step and then, in the 2nd step to apply an anterior force on tibia. For this, i create cylindrical joints using dummy rigid bodies. In the 1st step i am using prescribed constraints for the tibia rigid body with zero value loadcurves in order to keep it stable. The same goes for the femur and i only keep free the Rx, Ry rotation bc. I apply a loadcurve that descibes the angles that the cylindrical joint consisting of the femur material and dummy material will rotate. Anything until now works as planned.

    In the 2nd step , i am using prescribed constraints for the femur rigid body with zero value loadcurves in order to keep it stable and free the tibial x,y translations. Now here come the Negative Jacobians. Whatever i have tried i cant run the simulation. Either i have used a cylindrical joint or i have applied a prescribed force to the y bc of tibia i am always getting a Jacobian error.

    I would appreciate any help or ideas.
    Thanks in advance,
    Kostas
    Konstantinos Risvas
    Dipl.-Ing. MSc Electrical and Computer Engineer
    Ph.D. Candidate, Research Associate, VVR group
    Department of Electrical and Computer Engineering
    University of Patras
    26500, Rio-Patras, Greece
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi Kostas,

    Are you able to share your model? That would allow us to reproduce the issue on our end and hopefully figure out a solution. If it's too big to upload, feel free to send it to steve dot maas at Utah dot edu (or you can use a file sharing service of course).

    Cheers,

    Steve
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

    Comment

    • konris87
      Member
      • Nov 2019
      • 36

      #3
      Hi Mr Maas,

      I apologize for the late answer. I sent you the necessary feb files via email.

      Thanks in advance,
      Kostas
      Konstantinos Risvas
      Dipl.-Ing. MSc Electrical and Computer Engineer
      Ph.D. Candidate, Research Associate, VVR group
      Department of Electrical and Computer Engineering
      University of Patras
      26500, Rio-Patras, Greece

      Comment

      • konris87
        Member
        • Nov 2019
        • 36

        #4
        An additional issue i have, is that i would like to apply a pretension in the ligament graft that i have between the femur and the tibia. The graft material is a trans Mooney Rivlin. The graft is fixed with rigid connectors to the femur and tibia surfaces.

        Can this pretensioning be achived using the active-contaction property? Or the prestrain plugin?

        Any other idea on this matter?

        Thanks in advance,
        Kostas
        Konstantinos Risvas
        Dipl.-Ing. MSc Electrical and Computer Engineer
        Ph.D. Candidate, Research Associate, VVR group
        Department of Electrical and Computer Engineering
        University of Patras
        26500, Rio-Patras, Greece

        Comment

        • maas
          Lead Code Developer
          • Nov 2007
          • 3400

          #5
          Hi Kostas,

          In principle, you could use either. The active contraction adds a contractile stress, whereas the prestrain feature applies an initial strain. So, if you know the amount of stress you would like to achieve, the active contraction might be a better option, but if you want to apply an initial strain, the prestrain plugin is probably the way to go.

          Cheers,

          Steve
          Department of Bioengineering, University of Utah
          Scientific Computing and Imaging institute, University of Utah

          Comment

          • konris87
            Member
            • Nov 2019
            • 36

            #6
            Ok i will try both! Thanks for your help Steve!

            Best regards,
            Kostas.
            Konstantinos Risvas
            Dipl.-Ing. MSc Electrical and Computer Engineer
            Ph.D. Candidate, Research Associate, VVR group
            Department of Electrical and Computer Engineering
            University of Patras
            26500, Rio-Patras, Greece

            Comment

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