Thanks for your responde Peter,
maybe I haven't been precise on the problem. I am currently doing static structural simulations in Workbench on a real-world object (a sheave) with its loads and constraints. Because there is a lot of compression in the model, and the material may yield locally, I wanted to include in the model data the behaviour from stress-strain data, and I created a command APDL as you saw before. I took the stress-strain curve from literature for the tensile part, and from a datasheet for the compressive part. All I have is a graph in excel with a bunch of points. Maybe you were thinking that I am doing uniaxial test simulation on Ansys but unfortunately I am doing something more "practical", as to say.
The data in my excel, which are the points that are in the APDL code, have been transformed in True Stresses and True Strains so from that point of view it is correct.
I don't know if with this information you can help me with my doubts. I am doing some tests in the lab in some weeks so I will have the stress-strain curves with precise data. About your last question, I really don't know, I am only a student and I have never been in the lab doing tests :')
Eventually, thank you for your time, I really appreciate it.