Learning Forum

[Lovisa Svanborg]

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]

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]

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

Regards Lovisa

[Mike Rife]

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]

Thanks, I will try.

[Lovisa Svanborg]

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]

 

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]

Hi Lovisa

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

[Lovisa Svanborg]

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]

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]

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