High Temperature Rotors: Failure Mechanisms and Remnant Life Assessment

This paper presents the common failure mechanisms of high temperature rotors and the engineering approaches to their remnant life prediction. In fatigue crack growth at the rotor bore, cracks from original forging defects or induced during long service life may grow under cyclic loading to its critical size causing fast fracture. In fatigue-creep interaction at the shaft surface, high tensile residual stress relaxation under high operating temperature causes creep crack initiation. The cracks may then grow under the combination of cyclic loading and high operating temperature. Remnant creep life at the center of the rotor is based on the time while accumulated creep strain reaches its threshold level. Creep rupture could occur at other locations such as the outside surface of the shaft at disks∕shaft radii or blade fixings. Finite element analyses were carried out to analyze stresses, temperature transients, creep strain, and reference stress for creep rupture. Fracture mechanics analyses with R5 and R6 approaches were used to estimate the crack initiation and growth rates, the critical crack sizes, and the type of the failure. Appropriate Paris law and Norton creep laws were used for fatigue and creep crack growth. Depending on the failure mechanism, a rotor’s remnant life is defined in terms of allowable starts and operating hours.