A parametric analysis of the breathing phenomenon for different shapes of shaft cracks

The propagation of shaft cracks in rotating machines causes changes of the local flexural stiffness of the rotor that, in turn, cause changes of the synchronous and super-synchronous vibrations. In horizontal machine-trains the shaft weight can cause a periodic closure and opening of the transverse crack that significantly affects the machine dynamic behaviour. This paper shows the results of a parametric analysis in which the sensitivity of the breathing phenomenon of shaft cracks to some basic geometrical parameters has been investigated. In particular, two types of transverse cracks having different shapes have been studied. The cracks having a partial-arc circular profile are able to give rise to not negligible supersynchronous changes of the shaft flexural stiffness. Conversely, further investigations have confirmed that full-annular cracks of any depth are able to cause only small amplitudes of the super-synchronous harmonic orders of the area moments of inertia of the cracked section. These results are in good accordance with the experimental evidences found in the dynamic behaviour of a real machine.