Discussion of the dynamic stability of a multi-degree-of-freedom rotor system affected by a transverse crack

The dynamic behaviour of cracked rotors is one of the most discussed topics in the rotordynamic literature due to the wide range of problems that may arise from this fault. Among them, it is a common notion that cracks in horizontal rotating shafts may cause instability of the system because of the periodic opening and closing of the crack, i.e., the breathing mechanism, determines the stiffness variation and the parametric excitation of the rotor system. Simplified models have been used to study this phenomenon using Jeffcott rotors. For the first time in this paper, a model of a real hyperstatic rotor with several degrees of freedom is used, which also considers the bearings and the foundation of the system, and the stability is discussed by means of the Floquet theory. The sensitivity of the obtained results to the system anisotropy and the crack position is also investigated. The results presented are quite different from those obtained by means of the simple Jeffcott rotor but are consistent with real and documented field experiences.