Large Von Mises Stress Value in a Multilinear Plastic Model

Question - When using multi linear plasticity model, is it possible for von Mises or max principal stress to go beyond ultimate stress?

Answer - Equivalent stress must follow the stress-strain curve. For metal plasticity by definition we use the von Mises equation (a.k.a. equivalent stress) to relate the stress tensor to a scalar quantity that we match to the input multilinear stress-strain curve. Maximum principal stress will not follow the stress-strain curve. It is definitely possible that maximum principal stress will be above ultimate stress if the hydrostatic component of stress is large.