Fracture Mechanics Evaluation of Generator Rotors
Although a very few generator rotors have failed in comparison to the total number of turbine generator units presently in service, the amount of stored energy in a rotor when at rated speed is high enough to cause a catastrophic event if brittle failure does occur. With this in mind, the designer of generator rotors must use all the tools available, to design against the possibility of brittle failure.
By using the theory involved in fracture mechanics, the designer may input the design stresses to calculate their effects on fatigue crack growth and brittle fracture resistance due to the presence of internal discontinuities in rotor components. Due to the number of sonic indications found in many of the older generator rotors, a computerized approach is used to calculate the interaction effects of sonic indications within close proximity to neighboring indications and also within close proximity to the free surface of the prepanned center bore.
The computer program simulates the total number of cyclic start-stops that a generator rotor would normally see in its lifetime. The various cyclic stress states are applied to the sonic indications to estimate the crack growth of each indication. Once the crack growth is calculated, the fracture resistance capability of the generator rotor is found by comparing the maximum stress intensity of the internal discontinuities to the critical stress intensity found by measuring the fracture toughness of each rotor.
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