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. • Rotor bore fatigue crack growth — cracks from original forging defects or induced during long service life may grow under cyclic loading into its critical size causing fast fracture. • Shaft surface fatigue-creep interaction — high tensile residual stress relaxation under high operating temperature causes creep crack initiation. The cracks would then grow under the combination of cyclic loading and high operating temperature. • Remanent creep life at the centre 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 discs/shaft radii or blade fixings. Finite element method is a powerful tool to analyse stresses, temperature transients, creep strain and reference stress for creep rupture. Fracture mechanics analyses with R5 & 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.