Question
Convergence failure during automatic time-stepping in ANSYS Mechanical implicit solver
Nia Atlas
0 reputation · 08 Dec 2020, 13:34 UTC
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0 reputation · 08 Dec 2020, 13:34 UTC
In ANSYS Mechanical, the implicit solver manages non-linear transient analyses using an automatic time-stepping algorithm. This process relies on the Newton-Raphson method to monitor residuals against defined tolerance criteria, employing bisection to reduce the time step when convergence is not achieved within the maximum iteration limit.
A specific behavior occurs when the solver reaches the defined 'Minimum Time Step' setting but continues to fail convergence. In certain configurations, the solver may oscillate between the minimum step size and convergence failure without triggering a standard exit or providing a clear diagnostic for the instability.
Given the constraints of the 'Initial', 'Minimum', and 'Maximum' time step settings in the Analysis Settings, how does the solver determine the final termination point when the minimum step is reached but residuals remain above the tolerance? Is there a mechanism to distinguish between a physical instability and a numerical failure at the minimum time step limit?
32775 reputation · 08 Dec 2020, 22:03 UTC
When the ANSYS Mechanical implicit solver reaches the defined Minimum Time Step and still cannot reduce the residuals below the convergence tolerance within the maximum allowed iterations, the solver terminates the analysis with a convergence failure error. It does not oscillate indefinitely; once the bisection algorithm hits the hard floor of the Minimum Time Step and fails to converge, the simulation stops.
The solver does not inherently distinguish between a numerical failure (e.g., a time step too large for a sharp nonlinearity) and a physical instability (e.g., buckling or material failure). However, the behavior of the residuals provides the necessary clue:
To verify the fix, monitor the Solution Information. Check the convergence plot (Residual Norm vs. Iteration); a healthy simulation should show a clear downward trend in residuals for every sub-step. Ensure that the final results are not skewed by excessively high damping or artificial stabilization.
Diagnostic Request: To determine if this is a numerical or physical issue, please provide the iteration count and the residual norm value at the exact moment the solver failed at the minimum time step.
Use comments to ask for clarification. Post a solution as an answer.
32,775 reputation · 08 Dec 2020, 21:47 UTC
ANSYS writes a line such as Minimum Time Step Reached – Convergence Aborted to the *.log file. Search for this entry to confirm that the solver actually hit the hard floor before aborting.
If the failure occurs at the start of a dynamic step, enable the Load/Displacement Increment option in the analysis settings. This lets the solver split the initial load or displacement into smaller sub‑increments, often restoring convergence without changing the minimum time step.
In the results tree, expand Transient Results → Step Size History and Residual History. A flat step‑size line at the minimum value with a steep residual rise signals a numerical failure; a persistent residual spike regardless of step size hints at a physical instability.
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