Multisine excitation technique for rotor crack detection

Rotordynamic systems have had many applications for decades. Well known examples include gas turbines and compressors used intensively in many important industry fields. Driven by rotating shafts transmitting large amounts of kinetic energy and utilized with continuous heavy loads, these machines very often suffer from sudden unpredicted failures. Such failure usually results in abrupt release of the transmitted energy, machine damage and a catastrophic accident. Propagating fatigue cracks are dangerous malfunctions, which had been recognized as the most frequent causes of similar accidents in the past. The fatigue shaft crack initiates from a small surface imperfection or a micro-crack due to a combination of corrosive, thermal, loading and other processes. Advancing slowly yet continuously the crack may achieve the critical depth at which an immediate shaft fracture occurs. Therefore the fatigue shaft crack, and especially the methods for its early detection and warning have a great deal of attention. It is widely known that a developing shaft crack manifests itself in the appearance of nonlinear effects resulting in higher harmonics in a vibration spectrum. However, such symptoms are characteristic not only for developing shaft cracks but also for other malfunctions such as a shaft bow, a coupling misalignment, etc. That is why novel shaft crack detection methods introduce a specially designed diagnostic force applied to the shaft in order to amplify the particular symptoms of the crack. Most often a simple harmonic force is used for such purposes, yet the results may not be very reliable. The present paper analyzes the possible application of the multisine excitation technique for an early shaft crack detection problem. This technique has been previously used for many problems where the influence of nonlinear distortions on the linear system behavior is to be evaluated and measured. By using well chosen periodic excitations, called random multisines, it is poss- ble not only to detect but also to quantify these distortions. In the present paper the multisine technique is adopted for the rotating cracked shaft. The approach is illustrated with the numerical results of the flexible rotating shaft modeled with the rigid finite element method. In order to asses the effectiveness of the proposed method the numerical results obtained for the uncracked and cracked rotors are compared. The robustness of the method to additional disturbances is checked by introducing some noise to the system. The results demonstrate a potential of the presented method. Possible problems of its practical implementation are briefly discussed.