A convergence failure can also indicat e a physical instability in the structure, or it can merely be the
result of some numerical problem in the finite element model.
The following sections detail some of the techniques that you can use to attempt to improve the con-
vergence performance of your analysis.
8.12.2.2. Performing Nonlinear Diagnostics
The nonlinear diagnostics tool NLDIAG can help you find problems in your model when an analysis
does not converge.
Identify Regions of High Residual Forces Issue the NLDIAG,NRRE command to write the Newton-
Raphson residuals from equilibrium iterations to a file (Jobname.nrxxx). You can then contour plot
the residual forces via the PLNSOL,NRRE command, which helps to identify regions of high residual
forces.
Such a capability can be useful when you experience convergence difficulties in the middle of a load
step, where the model has a large number of contact surfaces and other nonlinearities. You can restart
the analysis and issue an NLDIAG,NRRE command to write out the residuals. By tracking the way the
residuals change over several equilibrium iterations you can identify a portion of your model where
large residuals persist. You can then focus on the nonlinearities in that area (for example, a contact
pair's properties) instead of having to deal with the entire model.
Identify Problem Elements Typically, nonlinear analyses fail to converge for the following reasons:
- Too large a distortion
- Elements contain nodes that have near zero pivots (nonlinear analyses)
- Too large a plastic or creep strain increment
- Elements where mixed u-P constraints are not satisfied (mixed u-P option of current-technology solid
elements only)
The program has default limits which, when exceeded, determine when convergence criteria have been
violated. Some limits are user-controlled; for example, the CUTCONTROL command sets the maximum
plastic/creep strain increments allowed in an iteration. Other limits are fixed.
The NLDIAG,EFLG command identifies elements that violate the above criteria and records them in a
file (Jobname.ndxxx).
Convergence problems may occur when material algorithms fail (for example, local element level
Newton-Raphson convergence failure, or extreme element distortion). The error message identifies the
corresponding element number and/or the material ID for these cases. Be sure to read any error messages
generated during solution.
Process the Tracked Results Issue the NLDPOST command to process the .ndxxx nonlinear dia-
gnostics files. The command creates components of elements that violate a certain criterion for a par-
ticular equilibrium iteration (or iterations).
Identify contact pairs causing convergence difficulties Issue the NLDIAG,CONT command to write
various contact information for all defined contact pairs to a single Jobname.cnd text file. The file is
written during solution at a user-specified frequency (each iteration, substep, or load step).
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