12.2.2 Turbine rotors
Rotor design is influenced by the often conflicting requirements of steady and cyclic operation. One example of this is the rotor bore size. A large rotor bore, often with a diameter of half the external diameter, gives improved performance for transient or cyclic operation, as the temperature gradients and thus thermal fatigue is reduced.
However, large diameter bores cause an increase in the hoop stress which will accelerate creep at steady load. In large modern machines, the requirement to reduce the hoop stress dictates that the bore diameter should be kept to a practical minimum. The main requirement for rotors is to reduce thermal fatigue by ensuring that all stress-concentrating features are controlled so as to minimise the stress levels.
A fracture mechanics approach is adopted to assess the growth of any initial inherent forging defects. The repeated application of stress during cycling could cause the defect to propagate to the stage where fast fracture of the forging would occur. With knowledge of the operational requirements of the turbine and assuming that the largest undetectable defect is initially present, the criticality of that defect can be assessed.