Can elasto-plastic stiffness matrix be unsymmetric?

[ananan88]

I think elasto-plastic stiffness matrix will be unsymmetric when yielding function and plastic potential function are not the same one. Am I right?
If it is true, how to solve the equations in nonlinear finite element anasysis?

 

[bukac1]

Generaly yes. Elastoplastic problems are generaly non-linear, especially whwn hysteresis is considered.