When running problems using Implicit solution sheme in LS-DYNA, the default iterative non-linear solver used is the BFGS method that employs a ‘Quasi-Newton’ method in which the global stiffness matrix is reformed only every ILIMIT steps and in between these a relatively inexpensive update to the stiffness matrix is performed. This default stiffness matrix update scheme works very well and relative fast when there is no significant contact is involved and the non-linearily of the problem is moderate. The default method often fails to converge when the non-linearity of the problem grows or when significant amount of contact is involved. For such problems, a more expensive Full-Newton method is recommended by setting ILIMIT=1 which forces LS-DYNA to reform the stiffness matrix at every iterative step to yield a more accurate stiffness estimation. When full-newton method is employed, it is often necessary to set a large maximum allowable reformations using the MAXREF parameter to allow sufficient number of stiffness reformations before terminating or reduction of the solution timestep (when AUTO timestep control is activated). Recommended value of MAXREF depends on the problem but typical values could be around 200 which should be sufficient for any large non-linear problem to converge on a solution.
Related keywords include *CONTROL_IMPLICIT_SOLUTION, *CONTROL_IMPLICIT_SOLVER