Solving Geometrically Nonlinear Problems
Depending on the geometrically nonlinear effects that appear in your model, you may have to use different solution strategies. Some problems in this class are strongly nonlinear, while others show only a weak deviation from linearity. Some guidelines are:
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If the problem has a path dependent solution, then it must be solved in an incremental way in order to give a correct solution. Problems including for example plasticity or friction belong to this class. If you do the analysis in time domain, then the solution is inherently incremental. If the analysis is stationary, invoke the parametric continuation solver by adding an Auxiliary Sweep, and ramp up some loading parameter. In either case, make sure that the step size is not too large.
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Problem that have a unique solution, like an elastic model subjected to large rotations or strains can be solved in a single static load step without loss of accuracy. It is however possible that such an approach will not converge, in which case a parametric continuation solver must be used.
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In problems involving large rotations, the default settings of the nonlinear solver will sometimes give a solution strategy that is too conservative. You can often decrease the solution time significantly by modifying the settings under Method and Termination in the settings for the Fully Coupled or Segregated Step node in the solver sequence. Set Nonlinear method to Constant (Newton) and use a rather high Damping factor. In most cases the value 1 will work.
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If you model a situation which to a large extent is a rigid rotation, it is often necessary to use tighter tolerances than usual in order to avoid spurious stresses. Since the strains are computed from the differences of the displacements in an element, even a small relative error in the displacements (which are large) can cause significant strains. This will be visible in a case where the actual stresses are small.