Fatigue based design and life estimation of viscoelastic rotors

Abstract

PurposeHigh-speed rotor subjected to cyclic stress during operation may cause fatigue failure. Thus, the design of a rotor-shaft should involve the fatigue analysis for predicting safe life.Design/methodology/approachA damped rotor shaft with a centrally mounted disc, which is simulated as steam turbine rotor, is considered for fatigue analysis. The shaft is subjected to thermal load and axial torque. It is supported by two orthotropic flexible bearings at its two extreme ends. The bearings are modelled with two-element Voigt model along each orthogonal direction to consider the elastic damped behaviour. Finite element modelling is done through Rayleigh beam theory, where each element is also considered as a Voigt model. The mathematical model involves effect of external axial torque, softening and compressive action of the shaft due to thermal load by the high temperature steam.FindingsThis paper attempts to find dynamic stresses in a viscoelastic rotor-shaft subject to combined bending and torsional loading and is exposed to thermal environment during operation. The dynamic stress is then used to determine fatigue and also the life of rotors.Research limitations/implicationsInternal damping plays an important role in deciding dynamic behaviour of rotor shaft systems. Because of this, the rotor shaft becomes unstable after a certain spin speed. Thus, the design of the rotor based on fatigue analysis is limited to the stable zone.Practical implicationsFor this purpose, equations of whirl motion of a viscoelastic rotor-shaft are first obtained after discretizing the continuum with finite beam elements and then the time domain solution of rotor displacement is used to find the bending stress and shear stress at various locations of the rotor. Location for the maximum stress decides the failure point. Safe rotor dimensions have been predicted by comparing dynamic stresses with the Soderberg diagram.Originality/valueDesign of rotor for safe life operation and prediction of stability could serve a good reference for designers.