Why is there a difference between the stress plots for PIPE18 (elastic) and PIPE60 (plastic) elements?
The way the stresses are computed for the PIPE18s and the PIPE60s are very different. They are pretty much the same for the straight pipes, which is why you don`t see a significant difference.
For the PIPE18s, the stress is computed directly using the constitutive equation, while for the 60s it is computed in 3 different phases:
1. Computing the modified total strains
2. Using the modified total strains, computing the change in plastic strains and then the current elastic strains
3. Computing the current stresses based on the current elastic strains
So it uses the basic constitutive stress equation, but with the current values of the elastic strains.