Approximation of the second order derivative


I am trying to approximate the second-order derivative using dxx(). But this gives me zero values. P1 and P2 seemed to be not working. I tried using P3 but it gave me an unstable solution. Please help me. Thank you!

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Remember of P1 finite element function the true dxx derivable is zero on each triangle + some jump of
\frac{\partial u}{\partial {n}} on edge. An to get a correct convergence of dxx you need lot of regularity.