Hi Pouya,
Just to add, mesh size dependency after plastic localization is an open question in solid mechanics. It is known that localized plastic response can be significantly affected by element size (generally, it is not surprising to see that plastic deformation in localized regions keeps increasing by reducing the element size with no apparent tendency for convergence to a solution).
There are some numerical techniques proposed in the literature such as regularization methods that may be of help to you. You can look up online for "mesh regularization for localized plastic deformation" and you should be able to find some articles in this area.
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