For the analysis of a cracked rotor system, a simple rotor model with a breathing crack is proposed assuming that the crack opens, closes or opens/closes depending on the direction of the shaft displacement perpendicular to the crack. By the theoretical analysis of a simple rotor model with a breathing crack, the conditions for the crack to open or close are derived. The nonlinear differential equation for this model is numerically solved by using the 4-th order Runge-Kutta method and the validity of this model is verified by comparing the simulation results with the experiment results. the simulated time responses are found to be fairly accurate in predicting the real responses, in particular the open/close phenomenon.
By comparing the peak to peak values of time responses with varied unbalance vectors, an identification scheme for the orientation and depth of crack is suggested. The crack orientation is identified by searching the unbalance direction such that the time response amplitude becomes minimum and the crack depth is found by comparing the difference between the maximum and minimum time response amplitudes. Simulation results show that the crack can be detected by the method when a crack propagated more than 10-20% of the shaft diameter.
