Constraints
Make sure that the bodies are sufficiently constrained. If the bodies are not in contact in the initial configuration, and there are no constraints on them, there will be possible rigid body displacements. This causes the solver to fail and must be avoided.
For a more detailed discussion about sufficient constraints, see Constraints under Stationary Analysis.
Sometimes, as when simulating mounting processes, the structure is fully constrained only once the contact is fully established. There are some strategies for how you can deal with this problem.
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Create the geometry or set initial values for the displacement variables so that there is a small penetration in the initial configuration.
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Use boundary conditions giving a prescribed displacement rather than a prescribed force. When possible, this is usually the best solution to the stability problem. Note that you can always obtain the force actually applied from the reaction forces.
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Add a temporary weak spring during the beginning of the simulation. Assuming that a parameter p, ranging from 0 to 1, is used for applying the external load, you can introduce a stabilizing spring with stiffness kx in the x direction as
and similarly in any other direction that needs to be restrained. It is not important whether the spring is applied to domains, boundaries, or edges, but it should not create local forces. The value for the stiffness k should be chosen so that the force generated by the spring balances the external load at a sufficiently small displacement. A too weak spring will give a too large initial overclosure of the contact boundaries. A too stiff spring might influence the solution too much.