Detection of cracks in rotor based on the 2× and 3× super-harmonic frequency components and the crack–unbalance interactions

The purpose of this paper is to investigate the use of the 2× and 3× super-harmonic frequency components for detecting the presence of a single transverse breathing crack in a non-linear rotor system. This procedure is based on the detection of the super-harmonic frequency components of the non-linear dynamical behaviour at the associated sub-critical resonant peaks.The non-linear behaviour of the rotor system with a breathing crack is briefly analysed numerically: it will be illustrated that the effects of the crack size and location induce the variation of non-linear responses and the emerging of new resonance – antiresonance peaks of the cracked rotor at second, third and fourth harmonic frequency components. Then, the influence of the crack–unbalance interactions and more particularly the relative orientation between the front crack and the unbalance are also undertaken with considerations of various crack depths, and unbalance magnitudes. It is demonstrated that for a given crack depth, the unbalance does not only affect the vibration amplitude of the 1× amplitudes, but also the 12 and 13 sub-critical resonant peaks. Finally, it is illustrated that the emerging of super-harmonic frequency components provides useful information on the presence of cracks and may be used on an on-line crack monitoring rotor system. Using this methodology, the detection of small levels of damage may be easily undertaken.