Deformation Gradient/Piola–Kirchhoff Stress Output

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  • ntjacobs
    Member
    • Nov 2009
    • 79

    Deformation Gradient/Piola–Kirchhoff Stress Output

    I am currently trying to compare FE predictions to experimental results for a biaxial tension experiment using fiber reinforced tissue. Experimentally, it is common to report stress using 1st Piola-Kirchhoff stress, whereas FE results are based on Cauchy Stress. It is relatively easy to convert between the two knowing the Jacobian and the defomation gradient tensor. It seems PostView outputs the Jacobian under the Volume Ratio data field, but I am not aware of a way to access the deformation gradient tensor either directly through an output to the logfile from FEBio or through PostView. Is this currently possible in any way? If not, I will post a feature request.

    I know that nodal displacements and position coordinates can be output to a logfile from FEBio and this can be used to calculate the deformation gradient through a script in Matlab, but I think that since FEBio specializes in biomechanics and many laboratories corroborate experimental and FE results, it would be great complement to the existing system to be able to directly access these measures in PostView or through a logfile.

    Thanks!

    Nathan
  • ateshian
    Developer
    • Dec 2007
    • 1824

    #2
    Hi Nathan,

    I updated FEBio to allow saving the deformation gradient to the logfile. A sample code is shown below:

    Code:
    <logfile>
    	<element_data file="defgrad.dat" data="Fx;Fxy;Fxz;Fyx;Fy;Fyz;Fzx;Fzy;Fz;J" name="Fx,Fxy,Fxz,Fyx,Fy,Fyz,Fzx,Fzy,Fz,J">1:1:1</element_data>
    </logfile>
    This updated code should be available for download soon.

    Best,

    Gerard

    Comment

    • ntjacobs
      Member
      • Nov 2009
      • 79

      #3
      Gerard,

      Thanks for the quick reply. I'll keep a lookout for the next release version to be available for download.

      Thanks!

      Nathan

      Comment

      • dsrawlins
        Developer
        • Dec 2008
        • 366

        #4
        Nathan,

        The updated executables were uploaded yesterday, so give it a try.

        Cheers,

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

        Comment

        • ntjacobs
          Member
          • Nov 2009
          • 79

          #5
          Thanks Dave. I checked this morning, but I must have had a cached version of the webpage saved. When I hit refresh it showed up.

          Comment

          • ntjacobs
            Member
            • Nov 2009
            • 79

            #6
            I just downloaded FEBio version 1.8 and noticed that I am no longer able to export the deformation gradient using the code above, which previously worked for me, and I don't see anything about writing the deformation gradient to the logfile in the Chapter 3 of the user manual documentation. Is there still a way to export this variable? I am running a biphasic simulation if that matters.

            Thanks

            Nathan

            Comment

            • ntjacobs
              Member
              • Nov 2009
              • 79

              #7
              Sorry, I found the problem. The "Fx" variable has been replaced with "Fxx", and I did end up finding it in the user manual, not sure how I missed it the first time. Thanks

              Comment

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