Dear Wolfgang Problem solved, thank you! For other people's references, here is what I did: 1. when setting the nonzero_constraints, always query the current value first and then substract it from the target value so that the solution delta is constrained correctly. 2. Apply nonzero_constraints at the first Newton iteration in every time step. 3. By not eliminating constrained dofs from the sparsity pattern, no need to recreate it when the constraints change, thus no need to reinitialize the matrices and vectors.
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