Convergence criterion

For the incremental solution strategy based on one of the iterative methods to be effective, it is necessary to select suitable criteria (preset tolerances for reaching equilibrium) for the termination of the iteration process.

Note that loose convergence criteria may result in inaccurate results while too tight convergence tolerances may lead to unjustified increase of computational cost spent to arrive at the results of superfluous accuracy.

In the program the convergence is checked against the change of nodal displacement increments, the change of out-of-balanced forces and also the change of internal energy. The last criterion gives a certain idea about how both displacements and forces approach their equilibrium values. The corresponding settings are:

1) Displacement error tolerance – tolerance for the change of displacement increment norm.

2) Out-of-balanced forces tolerance – tolerance for the change of out-of-balance force norm.

3) Energy error tolerance - tolerance of the change of internal energy.

The default setting is 0.01 for all convergence tolerances.