- Number of equilibrium iterations used in the last time step (more iterations cause the time step size
to be reduced) - Predictions for nonlinear element status change (time step sizes are decreased when a status change
is imminent) - Size of the plastic strain increment
- Size of the creep strain increment
8.6.2.3.2. Convergence Criteria
The program continues to perform equilibrium iterations until the convergence criteria (CNVTOL) are
satisfied (or until the maximum number of equilibrium equations is reached (NEQIT)). You can define
custom criteria if the default settings are not suitable.
Automatic solution control uses L2-norm of force (and moment) tolerance (TOLER) equal to 0.5%, a
setting that is appropriate for most cases. In most cases, an L2-norm check on displacement with TOLER
equal to 5% is also used in addition to the force norm check.The check that the displacements are
loosely set serves as a double-check on convergence.
By default, the program checks for force (and, when rotational degrees of freedom are active, moment)
convergence by comparing the square root sum of the squares (SRSS) of the force imbalances against
the product of VALUE*TOLER. The default value of VALUE is the SRSS of the applied loads (or, for applied
displacements, of the Newton-Raphson restoring forces), or MINREF (which defaults to 0.01), whichever
is greater. The default value of TOLER is 0.005. If SOLCONTROL,OFF,TOLER defaults to 0.001 and
MINREF defaults to 1.0 for force convergence.
You should almost always use force convergence checking. You can also add displacement (and, when
applicable, rotation) convergence checking. For displacements, the program bases convergence
checking on the change in deflections (∆u) between the current (i) and the previous (i-1) iterations:
∆u=ui-ui-1.
If you explicitly define any custom convergence criteria (CNVTOL), the entire default criteria is overwritten.
Thus, if you define displacement convergence checking, you need to redefine force convergence
checking. (Use multiple CNVTOL commands to define multiple convergence criteria.)
Using tighter convergence criteria improves the accuracy of your results, but at the cost of more equi-
librium iterations. If you want to tighten (or loosen, which is not recommended) your criteria, you should
change TOLER by one or two orders of magnitude. In general, you should continue to use the default
value of VALUE; that is, change the convergence criteria by adjusting TOLER, not VALUE. You should
make certain that the default value of MINREF = 0.001 makes sense in the context of your analysis. If
your analysis uses certain sets of units or has very low load levels, you might want to specify a smaller
value for MINREF.
Also, we do not recommend putting two or more disjointed structures into one model for a nonlinear
analysis because the convergence check tries to relate these disjointed structures, often producing
some unwanted residual force.
Checking Convergence in a Single and Multi-DOF System
To check convergence in a single degree of freedom (DOF) system, you compute the force (and moment)
imbalance for the one DOF, and compare this value against the established convergence criteria
(VALUE*TOLER). (You can also perform a similar check for displacement (and rotation) convergence
Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationPerforming a Nonlinear Static Analysis