8.12.2.4. Automatic Time Stepping
Place an upper limit on the time step size (DELTIM or NSUBST), especially for complicated models.
Doing so ensures that all of the modes and behaviors of interest are accurat ely included. This can be
important in the following situations:
- Problems that have only localized dynamic behavior (for example, turbine blade and hub assemblies)
in which the low-frequency energy content of the system could dominate the high-frequency areas. - Problems with short ramp times on some of their loads. If the time step size is allowed to become
too large, ramped portions of the load history may be inaccurat ely characterized. - Problems that include structures that are continuously excited over a range of frequencies (for example,
seismic problems).
Exercise caution when modeling kinematic structures (systems with rigid-body motions).These following
guidelines can usually help you to obtain a good solution:
- Incorporate significant numerical damping (0.05 < γ < 0.1 on the TINTP command) into the solution
to filter out the high frequency noise, especially if a coarse time step is used. Do not use α-damping
(mass matrix multiplier, ALPHAD command) in a dynamic kinematic analysis, as it dampens the rigid
body motion (zero frequency mode) of the system. - Avoid imposed displacement history specifications, because imposed displacement input has (theor-
etically) infinite jumps in acceleration, which causes stability problems for the Newmark time-integration
algorithm.
8.12.2.5. Line Search
Line search (LNSRCH) can be useful for enhancing convergence, but it can be expensive (especially
with plasticity). You might consider setting line search on in the following cases:
- When your structure is force-loaded (as opposed to displacement-controlled).
- If you are analyzing a "flimsy" structure which exhibits increasing stiffness (such as a fishing pole).
- If you notice (from the program output messages) oscillatory convergence patterns.
8.12.2.6. Nonlinear Stabilization
You can use the nonlinear stabilization method to solve both locally and globally unstable problems,
and to overcome convergence for general problems. For more information, see Using Nonlinear Stabil-
ization (p. 258).
8.12.2.7. Arc-Length Method
You can use the arc-length method (ARCLEN and ARCTRM) to obtain numerically stable solutions for
many physically unstable structures. For more information, see Using the Arc-Length Method (p. 263).
8.12.2.8. Artificially Inhibit Divergence in Your Model's Response
If you do not want to use both nonlinear stabilization and the arc-length method to analyze a force-
loaded structure that starts at, or passes through, a singular (zero stiffness) configuration, you can
sometimes use other alternatives to artificially inhibit divergence in your model's response:
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