Fretting cracking behavior of nub of last-stage blade for steam turbine

Nub-and-sleeve of last-stage blade for steam turbine is a part-span damper. The nub is vulnerable to fretting cracking since it is in contact with the sleeve under centrifugal force, and a small-amplitude oscillatory movement occurs at the nub-sleeve contact interface due to the blade vibration during operation of turbine. The present study developed a rig test that mimics fretting behavior at the nub-sleeve contact interface and reproduces fretting cracking of the nub. The rig tests reveal that fretting cracking occurs if the contact interface is in partial slip state and the tangential contact force amplitude is sufficiently large. The minimum tangential contact force amplitude for cracking increases with the normal contact force. Moreover, under the partial slip condition, the tangential contact force amplitude increases with the imposed displacement amplitude and its maximum value increases with the normal contact force.