8.12.1.4. Apply Loading Gradually
For nonconservative, path-dependent systems, you need to apply the load in small enough increments
to ensure that your analysis closely follows the structure's load-response curve.
You can sometimes improve the convergence behavior of conservative systems by applying the load
gradually, so as to minimize the number of Newton-Raphson equilibrium iterations required.
8.12.2. Overcoming Convergence Problems
This section provides information to help you fix convergence problems in a nonlinear analysis. The
following topics are available:
8.12.2.1. Overview of Convergence Problems
8.12.2.2. Performing Nonlinear Diagnostics
8.12.2.3. Tracking Convergence Graphically
8.12.2.4. Automatic Time Stepping
8.12.2.5. Line Search
8.12.2.6. Nonlinear Stabilization
8.12.2.7. Arc-Length Method
8.12.2.8. Artificially Inhibit Divergence in Your Model's Response
8.12.2.9. Use the Rezoning Feature
8.12.2.10. Dispense with Extra Element Shapes
8.12.2.11. Using Element Birth and Death Wisely
8.12.2.12. Read Your Output
8.12.2.13. Graph the Load and Response History8.12.2.1. Overview of Convergence Problems
When performing a nonlinear analysis you may encounter convergence difficulties due to a number of
reasons. Some examples may be initially open contact surfaces causing rigid body motion, large load
increments causing nonconvergence, material instabilities, or large deformations causing mesh distortion
that result in element shape errors. Solution control (SOLCONTROL) automatically adjusts solution
parameters and attempts to obtain a robust, accurat e solution. In addition, several tools are available
to help you identify potential problems before, during, and after a solution.
CHECK,MCHECK, and CNCHECK commands help you verify if there are any obvious problems with
the model before you start the solution.The CHECK command does an overall verification of the
model, including missing elastic properties, unconstrained model, and element shape checks. The
MCHECK command can help you identify defects in the mesh such as holes or cracks, especially when
the mesh is imported from a third party software. The CNCHECK command provides the initial contact
status of contact pairs, identifying whether the contacts are initially open or closed. If, for example, a
part in your model is constrained only through contact with other parts and if the contact surfaces are
open, the CNCHECK command can help you identify this potential error condition.
When you analyze models with large deformations, some portions of the initial mesh can become highly
distorted. Highly distorted elements can take on unacceptable shapes, providing inaccurat e results. This
can cause your nonlinear solution to stop. When this happens, use the ESCHECK command to perform
shape checking of deformed elements in the postprocessor (based on the current set of results in
database). This deformed-shape checker helps you to identify the portions of your model that require
different meshing, thereby allowing them to retain acceptable shapes. Using ESCHECK at different time
points helps you to identify the load conditions that cause mesh deterioration.
Release 15.0 - © SAS IP, Inc. All rights reserved. - Contains proprietary and confidential informationGuidelines for Nonlinear Analysis