new implementation of a non-associated flow rule in rate-independent plasticity

one of the new research fields in plasticity is related to choosing a proper non-associated flow rule (nafr), instead of the associated one (afr), to predict the experimental results more accurately. the idea of the current research is derived from combining von mises and tresca criteria in the places of yield and plastic potential surfaces in rate-independent plasticity.  this idea is implemented using backward euler method in non-linear finite element simulation. the results are compared with the experimental data for an internally pressurized thick-walled cylinder and it is demonstrates that, using the proposed nafr in rate-independent plasticity, the experimental results could be predicted more accurately. finally, it can be said that the current research confirms the results of the previous works on rate-dependent plasticity (viscoplasticity) in steady state conditions.