Mechanical APDL Structural Analysis Guide

for your single DOF.) However, in a multi-DOF system, you might want to use a different method of
comparison.

The program provides three different vector norms to use for convergence checking:

• The infinite norm repeats the single-DOF check at each DOF in your model.


• The L1 norm compares the convergence criterion against the sum of the absolute values of force (and
  moment) imbalance for all DOFs.


• The L2 norm performs the convergence check using the square root sum of the squares of the force
  (and moment) imbalances for all DOFs. (Of course, additional L1 or L2 checking can be performed
  for a displacement convergence check.)

Example

For the following example, the substep is considered to be converged if the out-of-bal-

ance force (checked at each DOF separat ely) is less than or equal to 5000*0.0005 (that

is, 2.5),and if the change in displacements (checked as the square root sum of the

squares) is less than or equal to 10*0.001 (that is, 0.01).

CNVTOL,F,5000,0.0005,0

CNVTOL,U,10,0.001,2

8.6.2.3.3. Maximum Number of Equilibrium Iterations

Automatic solution control sets the value of NEQIT to between 15 and 26 iterations, depending upon
the physics of the problem. The idea is to employ a small time step with fewer quadratically converging
iterations.

This option limits the maximum number of equilibrium iterations to be performed at each substep
(default = 25 if solution control is off). If the convergence criteria have not been satisfied within this
number of equilibrium iterations, and if auto time stepping is on (AUTOTS), the program attempts to
bisect. If bisection is not possible, then the analysis either terminates or moves on to the next load step
(according to NCNV command settings).

8.6.2.3.4. Predictor-Corrector Option.

Automatic solution control activates a predicator (PRED,ON) if no SOLID65 elements are present. If the
time step size is reduced greatly in the current substep,PRED is deactivated. The predictor is also de-
activated for transient analyses.

You can activate a predictor on the DOF solution for the first equilibrium iteration of each substep. This
feature accelerates convergence and is particularly useful if nonlinear response is relatively smooth, as
in the case of ramped loads.

8.6.2.3.5.VT Accelerator

This option selects an advanced predictor-corrector algorithm based on Variational Technology to reduce
the overall number of iterations (STAOPT,VT for static analyses,TRNOPT,VT for transient). It is applicable
to analyses that include large deflection (NLGEOM), hyperelasticity, viscoelasticity, and creep nonlinear-
ities. Rate-independent plasticity and nonlinear contact analyses may not show any improvement in
convergence rates; however, you may choose this option with these nonlinearities if you wish to rerun
the analysis with changes to the input parameters later.

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Nonlinear Structural Analysis