Fatigue Stresses Can Initiate Cracking
Problem: Many generator components are susceptible to fatigue stresses that can initiate cracking. With rotors, cyclic operation can cause low-cycle fatigue cracking in rotor-forging tooth tops, snap ring grooves and fluted areas, windings, slot wedges and retaining rings. The failure of a forged component can initiate circumstances that completely destroy a generator and cause loss of life. Photo at top left shows copper turn fatigue cracking attributed to low-cycle fatigue stresses. This can lead to arcing and burning, causing shorts and rotor unbalance. Rotating fan blades (photo at center left) are more susceptible to high-cycle fatigue cracking. Here, the failure occurred in the blade base area, not in a weld joint.
Prevention: Regular nondestructive examination (NDE) helps you find cracks early, before a failure occurs and the component can be repaired or replaced. The right inspection method depends on material, accessibility of parts, and other criteria. Select from among ultrasonic, wet fluorescent magnetic particle (photo at bottom left), eddy current, and simple dye penetrant testing.