Issue with Muscle Model Stresses for Stretches < lofl

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  • dougblake
    Member
    • Oct 2017
    • 48

    Issue with Muscle Model Stresses for Stretches < lofl

    I am trying to match a model with the muscle material model to some stress-stretch data I have. Since the fiber stresses are zero below the lofl, I wanted to to start there. I am using a rectangular bar of material, applying uniaxial stretching to it. Whenever the stretching is less than the lofl, the stresses essentially go to zero and vary substantially across the length, instead of a constant value (away from the ends) that was a function of g1 and g2. I further ran a test case with an isotropic incompressible model (since I was using g1=g2). I downloaded the source code and noticed that there was a g3 that was added to "address the zero-stress problem". Is there something I should be doing differently to get a non-zero response below lofl? I have attached my .feb file.

    Thanks,
    Doug Blake
    Attached Files
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    We've been communicating about this via email, but just wanted to add something here for the record.

    The original formulation of the muscle and tendon materials had some shortcomings. A long time ago we had come up with a solution to these problems. Although the implementation in FEBio was updated these fixes were not documented, mostly since they were never fully validated. I will try to find some time to revisit this at some point in the near future, but in the mean time, anybody who wants to know more about how we attempted to fix the formulation, feel free to reach out to me.

    Cheers,

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

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