I'm running a model that cointais plastic data in the definitionf of the steel mechanical behavior. The plastic region is defined using a hardening modulus (classical plasticity theory), but I obtain several messages of excessive distortion followed by negative eigenvalue messages when the strees reach the yield values. I know that the negative eigenvalues messages are due to the loss of stifness or geometrical instabilities in the structure, but this loss is ruled by the plastic flow of the material and I can´t change this, unless I change the hardening modulus of the material.
I´m using first order, reduced integration hybrid elements to account the volumetric behaviour (incompressibility) in the plastic region.
I try to use the Riks method in order to account the geometrical instabilities in the model, that may occur due to the geometry of my model, but this approach don't work..
Do I have some problems in the definition of the best element formulation for plastic deformations???
Thanks for your help!
P.S. This models runs very well when no plasticity is defined!