I have read questions from users as to how to model plastic behavior in Prepomax and thought that I can contribute a little bit. The analysis in this post comes from one of my Professors at the University of Michigan Joseph Datsko. Quite interesting approach which is based only on the Yield Strength and the Tensile strength of the material being modeled. I have used with Prepomax myself.

I have programed an Excel file that describes how to calculate the strain hardening from the Yield strength and the Tensile strength, but I could not upload the excel file.

Thanks for the tip! Here are the two files: The description of the science behind the approach, and the excel file that describes the calculation to obtain the Stress factor, the strain hardening exponent, and the uniaxial plastic behavior.

First, thank you very much for sharing your notes and Excel file. There are different approaches to fit the experimental Strass-Strain data and this documents are always welcome.

May I ask you something?. It can be a good opportunity known we can also have Mr. Datsko support.

1-You are using test data (Su and Sy) . Those are engineering Stresses not directly usable in ccx. Ccx nonlinear material cards are defined in terms of logarithmic (true) plastic strain vs Cauchy Stresses.

Does your S0 and m parameters fit/define the Engineering or the True Stress/Strain curve?

2-Ccx has Ramberg-Osgood material law which is also an potential relationship. Is there any way to relate your s0 and m with ccx parameters?

I ‘have been looking at your notes in detail. Mr.Datsko is imposing that the Ultimate Strength or instability point is a maximum on the Engineering Stress Strain curve (the derivative is =0 at that point).
I did not saw it at first as the original proposed function is expressed as True values and it is monotonically increasing. That condition plus imposing a true strain of 0.002 at Yield point is enough to define the curve.

That’s very cleaver . Once again thanks for sharing .
Still working on the last two points.

The procedure from my professor Datsko allows you to Calculate So and the strain hardening exponent, and you can use the formulas in the expression above to calculate the Ramberg-Osgoord equation.
Enjoy!

C3D8 is fine but C3D8R and C3D8I doesn’t work with Ramberg. Just in case someone wants to try.
I need to do more testing but it is not a Prepomax issue.