Torsion response of a cracked stainless steel shaft

A B S T R A C T 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. The growth of fatigue cracks in rotating components in the power generation industry, especially pump shafts, is a significant economic and safety concern. 1 Fatigue cracks typically initiate at stress concentrators due to some combination of thermal, bending and torsional loads. In many cases the equipment is required to operate continuously for extended periods of time, highlighting the importance of on-line structural health monitoring. The measurement of vibrational response features can be used to monitor structural damage, such as that due to a fatigue crack because crack growth decreases structural stiffness, which changes the vibrational characteristics. The work presented herein was undertaken in response to the occurrence of fatigue cracks in reactor coolant pump shafts for nuclear power plants. In this case, the crack apparently initiates at a pinhole due to thermal loading and then propagates due to bending stresses. These cracks have been observed to propagate as either a semi-elliptical surface crack or a circumferential crack. Shaft deflection-based vibration monitoring systems were ineffective in providing an early warning to the plant operators. Recent work has shown the potential of torsional vibration monitoring to detect structural damage. Laboratory tests have shown the ability to detect tor-sional natural frequency changes as …