Johnson Cook model - flow stress match with UTS at ductility?

Hi,

I am trying to use Johnson Cook model for predicting equivalent flow stress. Just to check its validity at extremes, I used ductility as plastic strain, but I don't see it output stress close to ultimate tensile strength. I used plastic strain rate of 1.5% per min as specified for ultimate tensile test in ISO standard. Room temperature considered. All other parameters like A, B, C, n - I have taken from standard available literature. Is it not necessary for equivalent flow stress to match with ultimate tensile strength as plastic strain equals ductility? If so, how can we believe flow stress obtained between yield and ultimate? Please comment.

Thank you.