A Methodology for Predicting Torsional Fatigue Crack Initiation in Large Turbine-Generator Shafts

This document presents results of a three-year program in which an analytical method was developed to estimate the cumulative fatigue damage sustained by a turbine-generator shaft system during a torsional transient. The work was conduted at the Steam Turbine-Generator Engineering and Manufacturing Department of the General Electric Company undr the sponsorship of the Electric power Research Institute (Proect RP1531-1). The product of this program was a methodology that used the results of analysis and torsional fatigue tests on 25.4 mm diameter laboratory specimens to predict the fatigue life of large-diameter shafts subjected to torsional transients. These predicted results were verified with 127 mm diameter specimens subected to simulated torsional transient load history tests. To account for complicated deformation cycles, range pair cycle counting and linear damage summation techniques were used. Notch root deformation response was characterized with a form of Neuber´s rule. The results of tests of 25.4 mm and 127 mm diameter specimens showed that the proposed methodology characterized the torsional cumulative fatigue damage within a factor of two of the measured damage under laboratory conditions.