The symptoms of the propagation of shaft cracks in rotating machines, which can be brought out of the vibration data collected by condition monitoring systems, are often not very evident, especially when the crack depth is not very high. In fact, shaft cracks commonly propagate rather far from the cross-sections where the vibration probes have been mounted. At these measurement planes, the sensitivity of the machine dynamic behavior to the effects caused by a shaft crack can be fairly low. Therefore, it is very important to improve the capabilities of diagnostic techniques aimed at the early detection of shaft cracks.
In this paper, the dynamic behavior of the cracked shaft of a HP-IP steam turbine is shown. The results of investigations on the influence of the periodic and non-periodic breathing of the crack on the rotor response, measured in different machine thermal states and at unusual cross-sections of the shaft, are shown and discussed.