Hello,
1. it is not clear if you wish to
a) define a material through prescribing a stress-strain law
or
b) create stress-strain plots for some points in the analysis
?
2. no, the material law are only defined by mulitilinear approximations in case of the User functions (NLCem2User material).
If the explanation in sections 2.1.2.4 Compression before Peak Stress and 2.1.2.5 Compression after Peak Stress of the Theory Manual is not enough, I have to ask my colleagues who have developed the SBETA material model - please be as specific as possible.
3. I still do not understand what you are doing/wishing to do.
If you have a direct tensile test, you are typically measuring the average strain over some relatively large distance (the shorter the distance, the larger strain you are going to get after a single crack localizes).
Do you wish to determine the critical displacement w_d from a uniaxial experiment? Why do you think the default value is not good enough for your case?
Or do you only wish to compare measured experimental results to your analysis? - then, displaying the plastic/fracturing/principal strains instead of crack openings may be a way.
4. confinement is already considered in the material model - please see section 2.1.5.1 Compressive Failure for SBETA material, or the corresponding sections for the Cementitious2 and Cementitious3 materials. The Cementitous3 material is based on the model developed by Papanikolaou and Kappos. If you need that confinement model, I suggest to use the NLCem3 material instead of trying to modify SBETA.