Assessing expired fatigue life in large turbine shafts

Rotating equipment, such as turbo-generator rotors used for automotive electrical power generator and transmission shafts, are often subjected to transient high-amplitude torsional oscillations that may severely limit the useful life of the structure through subcritical crack growth of undetected flaws. High strain amplitudes, approaching full-scale yielding of the shaft, can arise following electrical transients from particular line switching events in electric power generation and transmission systems. At such high torques, fatigue growth occurs in antiplane shear along transverse and/or longitudinal shear planes. The objective of this report is to provide an experimental and theoretical basis for characterizing antiplane shear fatigue crack growth under both small-scale yielding and elastic-plastic conditions in a low-strength, low-alloy steel. The approach combines continuum fracture mechanics and preliminary mechanistic modeling to serve as a framework for the development of defect-tolerant life estimation procedures for components loaded to high torques.