First off, let me thank you guys for all the hard works. Next, I get to the project I started working on recently.
In Slope-Pipeline interaction analysis, there is a need for nonlinear shell elements. Unfortunately provided shell element in the FLAC3D only accept linearly elastic material. Therefore, I decided to write my own FISH function to go through all the shell/liner elements calculate the principal strain based on local displacement and from there modify each shell’s elastic modulus based on the strain-stress curve provided by the user.
This FISH function works fine as shown in the figure below, but problem is that this method is only valid for the strain hardening curves. For the case with strain softening strain-stress curves, either negative young modulus should be used or somehow the internal force (i.e. stress) in the shell element should be manipulated similar to the approach used in UDM-C++. Negative modulus is not an option, so the best method is to modify the internal stress (or force) at each shell element.
Now problem is that all the shell stress FISH functions (i.e. struct.shell.stress or struct.shell.resultant and etc.) are getters and there is no way to modify the internal local stress. So, my questions are:
1- Is there any way to modify the internal force/stress for each shell elements?
2- Is there any work in progress in the Itasca for implementing nonlinear shell element? this definitely helps me to save times and also avoid some headaches 
The best,
Roozbeh
