Fatigue cracking of the generator rotor damper bars, Jocassee Pumped Storage Station

Two Generator/Motor rotors were removed for a generator stator rewind outage. Visual inspection of the rotor poles revealed two poles with a horizontal crack on a damper bar. Further non-destructive testing on all four units, identified 17 out of 240 poles with the same type damper bar cracking. Subsequent non-destructive testing, mapping, destructive metallurgical failure analysis, and finite element analysis has attributed low cycle fatigue to be the primary cause of cracking. The crack initiates at a point of constraint that coincides with an indeterminate notch resulting from the design. A lack of post-weld heat treatment of the hardenable steel also may have contributed to the crack initiation. The cracks then propagated due to the normal operational cycling of the units.