In addition to the option to switch the arc-length control on or off, PLAXIS offers an additional parameter to control the behaviour of the arc-length control. In PLAXIS the use of arc-length control is combined with an automatic failure detection. This automatic failure detection says that if in 5 successive calculation steps the applied load has to be decreased in order for the calculation to converge, failure is assumed and the calculation stops.With a phase parameter called Max unloading steps (under Numerical control parameters in the Phases window) this default value of five (5) can be adjusted in order to overcome e.g. a local failure mechanism.
In order to change this iterative setting parameter for e.g. Phase_5 to a value of 10 instead of 5 (default value), you can also use the set command in PLAXIS 2D and PLAXIS 3D to change its value, and use echo to display the current value. Note that, first, the UseDefaultIterationParams property needs to be deactivated:
0099> _set Phase_5.Deform.UseDefaultIterationParams False
0100> _set Phase_5.Deform.MaxUnloadingSteps 10
0101> echo Phase_2.Deform.MaxUnloadingSteps
When a failure occurs with the default setting, the user may choose to increase the number of decreasing load steps to see if the calculation converges successfully. However, the user should check why the calculation failure occurs in the first place. Use engineering judgement to inspect Output results and judge whether results are:
When the above is considered to be the case the user may choose to increase the number of decreasing load steps. Note that Plaxis does not recommend to change this setting in general.
In PLAXIS 3D 2011, this value was called: Maximum Decreasing Load Steps and its command was:
set Phase_2.MaximumDecreasingLoadSteps 10
Iterative settings: Influence of Arc length control [Tips and Tricks]