Mechanical APDL Structural Analysis Guide

8.11.1.1.3. Using the Options for the First Substep

When stabilization is active, you can activate artificial dashpot elements (STABILIZE) for the first substep
of a load step.

In most analyses, stabilization is unnecessary because the structure is initially stable, so the first substep
should converge if the substep size is reasonable. When SubStpOpt = NO, the program calculates all
necessary data for stabilization more accurat ely and achieves convergence more easily; therefore, ANSYS,
Inc. recommends using this option whenever possible.

Convergence Problems at the First Substep

There are some situations where convergence is an issue at the first substep. For such cases, you can
specify substep option (STABILIZE,,,,SubStpOpt) MINTIME or ANYTIME.

The MINTIME option activates stabilization only when the time increment reaches the

minimum time increment and the analysis still has not converged. Use this option for

the first load step only.

The ANYTIME option activates stabilization for any time increment tried for the first

substep. Use this option for any load step other than the first load step where constant

stabilization is active (STABILIZE,CONSTANT ).

The program uses the damping factor calculated at the previous load step to calculate the stabilization
forces for the first substep. If no such value is available, the program assumes a deformation mode for
the first substep and calculate a damping factor for the first substep. In either case, the program recal-
culates the damping factor after a successful convergence based on the solution of the first substep
and uses the new value for all subsequent substeps.

Use caution with either substep option and check the final result to verify that the stabilization forces
and energies are not excessive.

Example

Specify SubStpOpt = ANYTIME for the current load step after you have applied a con-

stant stabilization force (STABILIZE,CONSTANT) in the previous load step and the first

substep did not converge, yet the current load step also requires stabilization. This option

is especially useful if you do not want to rerun the previous load step using the reduced

method (STABILIZE,REDUCE).

8.11.1.1.4. Setting the Limit Coefficient for Checking Stabilization Forces

When the L2-norm of the stabilization force exceeds the product of the L2-norm of the internal force
and the stabilization force coefficient, the program issues a message displaying both the stabilization
force norm and the internal force norm. The message indicat es that the stabilization force may be too
large. In such cases, verify the results carefully, and consider adjusting the stabilization force by updating
either the energy-dissipation ratio (STABILIZE,,ENERGY) or the damping factor (STABILIZE,,DAMPING).

If you want to change the stabilization force limit coefficient (by default 0.2, or 20 percent), issue a
STABILIZE,,,,,FORCELIMIT command. ( To omit a stabilization force check, specify a value of 0.)

The program checks the norms (and reports them if necessary) only after a substep has converged. The
stabilization force check has no effect on convergence.

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Unstable Structures