[ABAQUS] Hoffman's plasticity UMAT help

Hi everybody,

This is my first post here and I'm looking for some help with UMAT subroutine I am currently working on. I am writing UMAT subroutine to account for Hoffman plasticity in field of paper mechanics. By design it is supposed to deal with Hoffman plasticity and all of its simplier forms (Hill, Mises). I use Newton-Raphson scheme with plastic multiplier and stresses as unknowns (though I am aware that problem can be simpliefied to one-variable only). I've started testing routin by comparing results to Abaqus-implemented models. I have 1 Plain Strain Element, with 1 face fixed and 1 node of fixed displacement. So far I found that routin is dealing well with elasticity (Isotropic, Orthotropic), but I've got some problems with plasticity. Until some certain threshold displacement of node, results between my routin and Abaqus doesn't differ (in Mises plasticity), but then they diverge. I figured that mean that basicly routin algorithm must be working well, but there is some bug. Unfortunately when I tested Anisotropic Elasticity + Hill criterion results diverge from the very beginning. I've figured that there might be some problem with finite strains, or rigid body rotation as far as material properties are concerned - is it possible? I based my work on "Computational Methods for Plasticity. Theory and Aplications" by Souza Neto, Peric and Owen. I am attaching plots of Mises Stress against Controlled Displacement of certain nodes (though I suppose Mises Stress isn't appropriate measure to plot for Hill criterion) and UMAT file. Thank you for any response.