Corrosion Fatigue Life Evaluation Method for the Bladed Disk Attachments of Low-Pressure Steam Turbine

The mechanism of disk cracking was investigated and an evaluation method for its failure occurrence rate was developed. It was found that the disk cracking was caused by the corrosion pit growth, the superposition of the multiple vibration modes, and the increase in the scatter of the natural frequency due to the interface condition change after long-term operation. The effects of several uncertainties on the failure occurrence rate were examined and the values of the uncertainties were obtained by solving the inverse problem according to the failure analysis; the examined uncertainties were the standard deviation of the natural frequency and the stimulus ratio. It is recommended to perform the replica inspection by removing the blades and to take the proper maintenance actions based on the remaining life evaluations because relatively small corrosion pits can cause crack initiation. It was found that a continuous cover blade is superior to the conventional tenon-shroud-type grouped blade because the former reduces resonance points in the interference diagram and eliminates any tangential modes which are main contributors for the disk cracking.