Analysis of fretting cracking condition for nub of steam turbine blade

Abstract

Nub-and-sleeve of last-stage blade for steam turbine is a part-span damper. The nub is susceptible to fretting cracking since the nub is in contact with the sleeve under centrifugal force and vibration of blade causes a small-amplitude oscillatory movement at the nub-sleeve contact interface. Previous study of the authors achieved the experimental result that the minimum tangential contact force amplitude for cracking increases almost proportionally to the normal contact force when the nub-sleeve contact interface is in partial slip state. Present study aims at establishing the fretting cracking condition for nub that can be applicable to design of nub-and-sleeve. For this purpose, the experimental result was analyzed with fatigue damage parameters: Smith-Watson-Topper, Fatemi-Socie and Crossland. Finite element analyses were carried out to obtain stress and strain data for estimation of fatigue damage. It is demonstrated that the fretting cracking of nub can be predicted with a single fatigue damage parameter value in a range of the normal contact force if plasticity is considered. However, if plasticity is ignored, the minimum fatigue damage parameter value for cracking increases with the normal contact force.