The clear relation between shaft cracks in turborotors and vibration effects measured in bearings can be established by model-based methods very well. Here a new concept has been presented, based on the theory of disturbance rejection control, extended for nonlinear systems and applied on a turborotor. Simulations have been done, showing the theoretical success of this method, especially for reconstructing disturbance forces as inner forces caused by the crack. Calculating the relative crack compliance as the ratio of additional compliance caused by the crack and undamaged compliance a clear statement about the opening and closing, and therefore for the existence of the crack, and about the crack depth is possible. Theoretically it has been shown that it is possible to detect a crack with very small stiffness changes which corresponds to a crack depth of 5 percent of the radius of the rotor.

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