Identification of a Crack in a Rotor System using a Model-based Wavelet Approach

The dynamics and diagnostics of a cracked rotor have been gaining importance in recent years. Vibration monitoring during start-up or shut-down is as important as during steady-state operations to detect cracks especially for machines such as aircraft engines which start and stop quite frequently and run at high speeds. In the present study, a model-based method is proposed for the on-line identification of cracks in a rotor while it is passing through its flexural critical speed. The fault-induced change of the rotor system is taken into account by equivalent loads in the mathematical model. The equivalent loads are virtual forces and moments acting on the linear undamaged system to generate a dynamic behaviour identical to the measured one of the damaged system. The rotor has been modelled using FEM, while the crack is considered through local flexibility change. The crack has been identified for its depth and location on the shaft for different rotor accelerations. The nature and symptoms of the fault – crack – are further ascertained using the continuous wavelet transform.