The complex modal testing method, which was primarily developed for rotor systems, has been proven that the directional frequency response estimates can be effectively used for identification of such system asymmetry as open crack. Unlike an open crack, a breathing crack may repeat opening and closing as the shaft rotates, requiring analysis of a time-varying nonlinear system.
In this work, a two-staged scenario test for effective detection of breathing crack is proposed. In the first stage, an electromagnetic exciter is employed to properly generate a harmonic excitation synchronous to the shaft rotating frequency so that the synchronous excitation allows the breathing crack either completely open or closed during the shaft revolution. In the second stage, the shaft with the completely open or closed crack is modal-tested for estimation of the reverse directional frequency response functions which can indicate the status of crack. For the accurate detection of breathing crack, the initial bow and imbalance of system are identified and balanced by using the magnetic exciter in operation. Experimental results confirm that a breathing crack can be successfully detected by the scenario test, in spite of the presence of other asymmetric and anisotropic properties in the test rotor than a breathing crack.
