Torsion response of a cracked stainless steel shaft

Pump shafts used for power generation are susceptible to fatigue cracking while often in near-continuous operation. Technology based on torsional vibration is under development for condition-based assessment of shaft health. The focus of this paper is on the relationship between a crack, which propagated due to bending loads, and the torsional stiffness of the shaft. An analytical method to determine the compliance associated with a crack has been implemented for a semi-elliptical surface crack. A 3-D finite element model of a shaft section with a crack has also been used to predict the effect of a crack on stiffness. Fatigue cracks were seeded in shafts on a three-point bend apparatus. A benchtop test rig was constructed to evaluate the torsional natural frequencies of a cracked shaft system. Quasistatic torsional stiffness tests indicated that crack closure has an effect on the results. A torsional finite element model of the benchtop test rig indicates that the first torsional natural frequency is reduced by the propagation of a crack. The reduction in torsional stiffness of the shaft inferred from the natural frequency results is in reasonable agreement with the quasistatic results and the model predictions.