12 September 2018
- Last edited 12 September 2018
I learned something new from Sandeep above.
Below is a plot that shows you three bisections followed by an almost converged set of iterations, then the solver stopped.
You can see that the initial number of substeps was 100 (Time of 1e-2). Three bisections is 2x2x2 or a factor of 8 and that was not enough. I would use a factor of 10 and set the Initial Substeps to 1000 on the next try.
One case when a Bisection occurs is when it has used up the maximum number of iterations, which by default is set to 26. This is generally a good number, but when you see an almost converged condition, like the last part of the graph above, you want the solver to keep iterating, since it's almost there! There is a command snippet that can override the default number of 26, and that is
where you can type in larger numbers than even 50, like 100. I use this a lot. The only time it is wasteful is when it's never going to converge, it keeps the solver running on a futile task.
I sometimes watch the N-R Residual graph plot new points as it solves and if I see it diverging for many points instead of converging, I will interrupt the solution and perhaps change to a smaller time step (larger number for Initial Substeps), and then restart the solution. That way I don't have to wait for 26 iterations for a bisection to occur. That is helpful on large models that might take tens of minutes per iteration.
In the plot above, step 1 needed the substeps to converge, but step 2 has a Minimum Substeps that is larger than required. I often set Minimum Substeps to a large number to get a smooth curve of the history from step 1 to step 2, but if all you were interested in was the result at step 2, the plot shows you to try a larger Initial and larger Minimum number of substeps in step 2 to get to the final result.
You must set the number of Newton-Raphson Residuals to a number larger than the default of 0, so that when the solver fails to converge, you can see which elements were having difficult converging and take some corrective action, like using smaller elements or softening the contact stiffness. Here is an example.
I also have a post with a video example.