You can freely switch from the Newton-Raphson iteration method to the arc-length method at the start
of any load step. However, to switch from arc-length to Newton-Raphson iterations, you must terminate
the analysis and restart, deactivating the arc-length method in the first load step of the restart
(ARCLEN,OFF).
An arc-length solution terminates under these conditions:
- When limits defined by the ARCTRM or NCNV commands are reached
- When the solution converges at the applied load
- When you use an abort file (Jobname.ABT)
See the Basic Analysis Guide for information about termination and restart procedures.
Use the load-deflection curve as a guide for evaluating and adjusting your analysis to help you achieve
the desired results. It is usually good practice to graph your load-deflection curve (using POST26 com-
mands) with every analysis.
The total arc-length load factor (SOLU,,ALLF) can be either positive or negative. Similarly,TIME, which
in an arc-length analysis is related to the total arc-length load factor, can also be either positive or
negative. Negative values of ALLF or TIME indicat e that the arc-length feature is applying load in the
reverse direction in order to maintain stability in the structure. Negative ALLF or TIME values are
commonly seen in various snap-through analyses.
8.11.2.1. Checking Arc-Length Results
When reading arc-length results into the database for POST1 postprocessing (SET), always reference
the desired results data set by its load step and substep number (LSTEP and SBSTEP) or by its data
set number (NSET).
Do not reference results by a TIME value, because TIME in an arc-length analysis is not always mono-
tonically increasing. (A single value of TIME might reference more than one solution.) Additionally, the
program cannot correctly interpret negative TIME values (which might be encountered in a snap-
through analysis).
If TIME becomes negative, define an appropriate variable range (/XRANGE or /YRANGE) before creating
any POST26 graphs.
8.11.3. Nonlinear Stabilization vs. the Arc-Length Method
You can use nonlinear stabilization for both local and global instability with few limitations related to
compatibility with other algorithms and materials. However, nonlinear stabilization cannot detect the
negative-slope portion of a load-vs.-displacement curve problem with global instability (if any).
Although the results obtained before the negative slope portion of the problem are always correct, the
results for the substeps after the negative-slope portion are also correct if the materials are not deform-
ation-history-dependent. (Consider the results to be questionable if the materials are deformation-history-
dependent.)
The arc-length method can detect the negative-slope portion of a load-vs.-displacement curve, but it
cannot solve problems with local instability and material softening. Other limitations exist, related mostly
to compatibility with certain algorithms and materials.
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