Faults or other disturbances in an electrical distribution system can potentially feed back to the turbine-generator and produce large-amplitude torsional oscillations in the turbine-generator shaft. These transient torques, in combination with gravity-induced and misalignment-induced bending stresses in the shaft, can give rise to significant combined-mode fatigue crack growth phenomena. Prediction of the fatigue damage or of the growth rate of existing flaws under these complex loading conditions has been the subject of numerous investigations in the past. The work described here focuses upon the development of a correlation of the experimentally measured combined-mode fatigue crack growth rate (FCGR) with a suitable fracture mechanics characterizing parameter.
Since the torsional stress levels involved in transient oscillations can be comparable to the yield strength, linear elastic fracture mechanics (LEFM) parameters (for example, K, the stress-intensity factor range) are not appropriate for correlating the crack growth rate. Consequently, a set of elastic-plastic crack driving force solutions was developed for part-circumferential through cracks in a thin-walled tube. Another similar set of elastic-plastic crack driving force solutions for circumferential cracks in a solid cylinder was also developed. Elastic-plastic finite-element analysis techniques, including the virtual crack extension method, were used to develop these solutions, in terms of the energy release rate, J. The geometry and the constitutive behavior of the models mimicked that of the corresponding specimens tested in the associated experimental portion of this program.
Pure Mode II fatigue crack growth was obtained in thin-walled tubular specimens through pure torsional loading, while combined Mode I plus Mode II crack growth was obtained by superimposing in-phase tensile cycling. Similarly, Mode III fatigue crack growth was achieved via torsional loading of the solid cylindrical specimens.
The analytical and experimental results discussed here demonstrate that the measured FCGR can be correlated with the range of the energy release rate, J, computed by elastic-plastic means, or with the corresponding range of the elastic-plastic effective stress-intensity factor range, Kj(EJ). In addition, for the shaft material studied here, the FCGR relation for all combined-mode cases was found to be essentially identical to that for pure Mode I fatigue crack growth.