Vumat -Elastic Plastic Problem

Greetings,

Im running a model with using the vumat subroutine and the elastic part compatible with the experiment results, but once pass yielding point,

the von mises become constant (supposely the vonmises will increase with the updating trial stress)

and the updated stress and force become almost constant..

The simple VUMAT subroutine im defined is


if ( stepTime .eq. zero ) then   
        do k = 1, nblock
       
        stressNew(k,1) = stressOld(k,1) .........
       
      end do