Confusion about lam_max parameter in von-Mises fiber model

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  • LEO
    Junior Member
    • Feb 2020
    • 8

    Confusion about lam_max parameter in von-Mises fiber model

    Hello
    I have a question about "Mooney-Rivlin von Mises Fibers" model. There is a parameter in the model called "lam_max". I was not able to figure out what exactly this parameter does. I read the FEBio help section 4.1.2.13 when it says:

    "lam_max (λm) is the stretch at which the fibers are straightened"
    . This definition is not clear to me.

    The question is: How can I know what value I should use for this parameter? For example if I use <lam_max>1</lam_max> the results is very different compared to when <lam_max>10</lam_max>. For example for a hemisphere under internal pressure:

    fig.png

    The feb file is attached. Thanks in advance for your help.
    Attached Files
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi,

    This material uses a fiber formulation that is defined by an exp-linear stress behavior. This means that at low stretch, the fiber will have an exponential stress response, but at large stretch, the stress will be linear. The lam_max parameter is the point of transition between these two regions. The motivation for this, and the explanation for the lam_max parameter, is that at low stretch, most fibers are in a crimped state and don't contribute to the stress. As the stretch increases, fibers begin to uncrimp and contribute to the stress response. This rapid growth in the number of fibers that begin to resist the stretch results in an exponential growth in the stress. At some stretch value (i.e. lam_max) all fibers are considered uncrimped, and from thereon the response becomes linear.

    Thus, when you set the value of lam_max to 10, you are essentially only using the exponential part of this formulation. If you set it to 1, then you are only using the linear part. So, you indeed would get a very different response as you observed.

    I hope this helps. Let us know if you have any further questions.

    Cheers,

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

    Comment

    • LEO
      Junior Member
      • Feb 2020
      • 8

      #3
      Thank you for your response. It is now clear what this parameter is. I appreciate your help. By the way, I was just hoping if you could take a look at the other question of mine here:



      Once thanks again for your help.

      Comment

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