I get that once the material exceeds the yield stress it then moves to the strain hardening part of the stress-strain curve, but is there an ultimate stress? I used the ASTM A-36 material from the material library. The reason I ask is that one of my models had stresses around 120ksi, where as ASTM A-36 ultimate is around 50ksi.
The analysis does not use the ultimate stress in the manner that you asked. The elements would need to break apart to show the effect, and the software does not have that capability.
So, the analysis treats the stress-strain curve as extending to infinity. (I think that it follows the slope of the last segment, but I do not remember that with 100% certainly. Perhaps the wiki help (documentation) indicates what is used beyond the "last" data point.)
The software can use the ultimate stress to estimate a strain hardening modulus as shown by the equation in the wiki Help (documentation).
16 years experience with Simulation Mechanical