Brake output shaft fractured at the shoulder

It’s a disaster, everyone.

The output shaft of a magnetic particle brake suddenly snapped during normal operation. The fracture occurred at the fillet radius of the shaft shoulder; the fracture surface shows distinct "beach marks" (striations), indicating a classic torsional fatigue failure.

The equipment had been in service for about a year and a half; loads were generally light, and there were no shock loads. Upon inspection, the fillet radius at the shoulder was found to be too small, leading to stress concentration. The material is standard 45 steel with no surface hardening treatment.

Does this failure indicate an insufficient design safety margin? Or was there some overlooked alternating torque during operation? Would replacing the shaft solve the problem, or does the entire brake unit need to be swapped for a different model?

Fortunately, no one was nearby at the time; otherwise, the consequences could have been unthinkable. I would appreciate an analysis of the cause and suggestions for future improvements. 
Thanks.