Convergence failure during automatic time-stepping in ANSYS Mechanical implicit solver

Termination at the Minimum Time Step

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.

Numerical Failure vs. Physical Instability

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:

• Numerical Failure: Residuals decrease steadily but fail to reach the tolerance before the iteration limit is hit, or they fluctuate slightly before failing. This suggests the time step is still too large or the solver needs more iterations.
• Physical Instability: Residuals diverge rapidly or oscillate wildly regardless of how small the time step becomes. This typically indicates a lack of structural stiffness, unstable contact definitions, or an unconstrained model.

Recommended Resolution Steps

1. Adjust Time Step Aggressiveness: Reduce the Maximum Time Step to prevent the solver from jumping too far ahead after a successful step, which often prevents the subsequent failure.
2. Increase Iteration Limits: Increase the maximum number of iterations allowed per sub-step to give the Newton-Raphson method more opportunity to converge.
3. Enable Line Search: In the Analysis Settings, enable Line Search. This helps the solver find a stable path to convergence in highly non-linear problems.
4. Audit Contact and Constraints: Verify that all parts are properly constrained and that contact formulations (e.g., Augmented Lagrange vs. Pure Penalty) are appropriate for the expected stiffness.

Verification

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.