how can boundary conditions affect prescribed material response?

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how can boundary conditions affect prescribed material response?

Nikolay T
Hello,
I'm simulating a Representative Volume Element (RVE) to estimate it's
homogenised properties under pure compression in X direction.
After some attempts I decided to try just an isotropic material model with
one-element simulation, but with the similar boundary conditions, which
emulate Periodic Boundary Conditions (PBC) in my loading case: symmetry on
X on one side, symmetry on Y on both sides and symmetry on Z on both sides
as well. One side is subjected to uniform displacement in X direction. I
tested the Extended Drucker-Prager model with perfect plasticity after 120
MPa

What I found out in results is that after yield had begun stresses (mises,
pressure, principles and S11) kept on rising up to the end of simulation in
the same "straight" manner they had benn rising before yield. It doesn't
even seem to look like plastic behaviour.

If I change my boundary conditions to simple support on one side
perpendicular to X and retain the displacement on the opposite side, the
result picture of stresses begins to look more "plastic-like".

Can anyone explain me the performance of material model in first case
(PBC)? Thanks in advance

I'm attaching the plots of the both cases. Units for stress are
TerraPascals (i.e. my yield strength is 120e-6TPa), horizontal axis is the
applied strain in X direction (recalculated from displacement and X size of
the element: strain=displacement/x-size)

Thanks in advance
С уважением,

Н.Т.


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