The shaft undergoes a full bending reversal from tension to compression on every revolution whether or not the drive torque is fluctuating.

Oscillation in drive torque must be figured in its frequency. You talk about the variation in torque but not its magnitude. You say nothing about the steady-state forces on the shaft. These will factor in any shaft stress limits and fatigue analysis.

Are you trying to apply a fatigue analysis? Be more specific on what you are investigating. ■