Use the solution transition method to achieve convergence in the following situations:
Rigid body motion causing lack of convergence in a static solution. This can be due to a lack of constraints or due to a loss of contact during an iteration. Both the mass matrix and algorithmic damping of the quasi-static solution help achieve convergence for this case.
Convergence difficulty due to contact chattering. The algorithmic damping of the transient solution helps to achieve convergence.
Collapse or buckling analysis. The algorithmic damping helps to achieve convergence.
Follow these recommendations when using the solution transition method:
Since the procedure relies on the mass matrix and algorithmic damping to achieve convergence when the static solution fails to converge, it is important to define density for all materials when using this method.
Important: Because SOLOPTION can be issued in a restart after the base static analysis has failed to converge, you must define the densities during the static solution. The restart framework does not permit material density to be defined in the restart analysis.
During the transient solution phase, the default backward Euler time integration (SOLOPTION,STOT, ,QUASI) is recommended since the algorithmic damping in this method helps to keep the transient solution close to the static solution.
Monitor the kinetic energy to potential energy ratio during the transient solution to make sure it is close to a static solution. You can reduce the densities and/or add damping (ALPHAD command) to lower the kinetic energy. When adding damping, also monitor the damping energy. The kinetic and damping energies are representative of the solution approximations made to achieve convergence and should be a very small percentage (<1%) of the potential energy to achieve an accurate solution.
If the static solution fails to converge due to rigid body motion or due to loss of contact, it might be more efficient to use
Type= RBM or CONT on the SOLOPTION command in order to trigger the transition to a transient solution at the first occurrence of rigid body motion (RBM) or loss of contact of an entire pair (CONT). This will save on the number of iterations performed in the static solution since it will transition before the point of convergence failure.For most problems, it can be advantageous to use the SOLOPTION command during the restart. Run the static simulation as usual, but add material densities. Then issue SOLOPTION,STOT,FORC in the restart to force a transition to the quasi-static solution. This approach can help to achieve convergence.