General Mechanical

General Mechanical

Hyperelastic – strain energy density plot

    • Lovisa Svanborg
      Subscriber

      Hi!

      I wonder about strain energy density plot for hyperelastic. How can a plot it to see material instabilities? Can I plot before simulation, because I want to see before if the material is instable?

      I have done curve fit with mooney Rivlin 3 and 9. My simulation works for MR 3 and not for 9. But is MR 9 instable?I don't think it looks like that in the picture.

      Plot MR 3

      Plot MR 9

      Reards Lovisa

    • Mike Rife
      Ansys Employee

      Hi Lovisa

      Why not create a 1 element cube model and run the three typical stress conditions (uniaxial tension, shear etc) for a range of stretch ratios and post process?  The MAPDL solver does this behind the scenes when a hyperelastic material is present but will only show the results if the material is unstable (shows as a list of conditions & ranges that were unstable in the output file).

      Mike

    • Lovisa Svanborg
      Subscriber

      Ok, thanks. Can you please explain a litter more how I post process to plot strain energy density.

      Regards Lovisa

    • Mike Rife
      Ansys Employee

      Strain energy is a typical result available.  If you want strain energy density then in the Details of the Analysis Settings; Output Controls make sure that Nonlinear Data is set to Yes.  Solve.  Then use a user defined result with the expression NLELWK (Non Linear ELastic WorK).

       

    • Lovisa Svanborg
      Subscriber

      Thanks, I will try.

    • Lovisa Svanborg
      Subscriber

      It didn't work. I have turn on nonlinear data. It says I have error and also an invalid scopping method. I have choosen the body for hyperelastic material.

    • Lovisa Svanborg
      Subscriber

       

      It works now to show strain energy density, but I would like to plot the contour plot of strain energy density. To se if the material is instable or not. How can I make contour plot of strain energy density?

       

    • Mike Rife
      Ansys Employee

      Hi Lovisa

      So if the plot worked, and it's a contour plot, then I don't get the question.

    • Lovisa Svanborg
      Subscriber

      Hi!

      I have read that to see if a material model is stable/unstable, you can plot the contour plot of strain energy density function. And it should looks like this in the rapport

      If it is one local minimum it is stable (picture to the left) with more unstable (picture to the right).

      The result I get in Ansys with that command you gave me

      So is there another way to get the contour plot? Have I undetstand wrong?

       

    • Mike Rife
      Ansys Employee

      Hi Lovisa

      In traditional FEA parlance a contour plot is a color-coded plot of some field result plotting on the FEM.  Which is what that strain energy density plot you posted is showing.  If I understand the isoline plot you show, then the strain energy density can be calculated from the materials specific strain energy density function for differing values of lambda (principal stretch ratios).  Stretch ratios are engineering strain plus 1.  

      Mike

       

    • Lovisa Svanborg
      Subscriber

      Thank you. Do you know how to plot the isoline plot?

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