Failure analysis of a generator rotor with a deep crack detected during operation: fractographic and fracture mechanics approach

This paper reports the results of an activity of failure analysis performed on a 370 MVA generator rotor that was scrapped after about 133 000 operati ng hours and about 400 start-ups, when the on line monitoring system reveal ed a progressive increase in vibrations amplitude, due to the presence of a deep crack. It was actually detected by automated ultrasonic equipment usi ng advanced signal processing. Failure analysis and laboratory tests demons trated that the crack started from a forging defect located at the root of a longitudinal slot, due to the high stress range induced by oil film insta bility problems. During the following years the crack grew mainly during tr ansients and not during operation. (C) 1999 Elsevier Science S.A. All right s reserved.