Dear Hamed, >From the code you've provided you don't seem to impose a dirichlet boundary condition on any "y" degree of freedom. You need to fix at least one degree of freedom in each coordinate direction in order for your problem to have a unique solution. In some cases the CG method can solve such ill-posed problems <http://doi.org/10.1109/20.497322> (i.e. it might have hidden the problem from you until now), but in the context of elasticity the solution would still not be physically satisfactory. I bet that if you examine your displaced solution from different runs then you'd find that your solution "floats" in the y-direction, i.e. its solved up to a constant value that remains undetermined.
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