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

Non-linear Transient Analysis

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.

Time-Step Oscillation

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?