Dynamic Analysis and Crack Detection in Stationary and Rotating Shafts

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Abstract

The sustainability, smooth operation and operational life of rotating machinery significantly rely on the techniques that detect the symptoms of incipient faults. Among the faults in rotating systems, the presence of a crack is one of the most dangerous faults that dramatically decreases the safety and operational life of the rotating systems, thereby leading to catastrophic failure and potential injury to personnel if it is undetected. Although many valuable techniques and models have been developed to identify a crack (or cracks) in stationary and rotating systems, finding an efficient technique (or model) that can identify a unique vibration signature of the cracked rotor is still a great challenge in this field. This is because of the unceasing necessity to develop high performance rotating machines and driving towards significant reduction of the time and cost of maintenance. Most of the crack identification techniques and models in the available literature are based on vibration-based methods. The main idea of the vibration-based method is that the presence of a crack in a rotor induces a change in the mass, damping, and stiffness of the rotor, and consequently detectable changes appear in the modal properties (natural frequencies, modal damping, and mode shapes). Among all these modal properties, the choice of the modal natural frequency change is more attractive as a tool for crack identification. The changes in natural frequencies due to a crack can be conveniently measured from just a few accessible points on the cracked rotor. Furthermore, measuring the natural frequencies does not require expensive measuring instruments, and the natural frequency data is normally less contaminated by experimental noise. However, the change that a crack induces in the natural frequencies is usually very small and can be buried in the ambient noise. Moreover, the natural frequencies are not affected if the crack is located at the nodes of modes or far from the location of inertia force and out-of-unbalance force that the disc generates in the shaft.To overcome these problems (or limitations), therefore, this study is conducted using the idea of the roving mass (roving disc in rotor case). The modal natural frequencies are used for the identification and location of cracks of various severities at different locations in both stationary and rotating shafts. The fundamental idea of the roving disc is that an extra inertia force is traversed along the cracked rotor to significantly excite the dynamics of the rotor near the crack locations. In other words, the location of a crack can be anywhere on the shaft which is contrary to the developed techniques in the available literature in which the location of a crack should be close to the disc. Along with the roving disc idea, three crack identification techniques are developed in this study using the natural frequencies of the cracked and intact shafts. Each of these techniques has its merits and limitations for crack identification. These techniques are implemented using data that are numerically generated by the finite element method based on the Bernoulli-Euler shaft elements and experimentally validated in the laboratory environment.The numerical and experimental results clearly demonstrate the capability of the suggested approach for the identification and location of cracks in stationary and rotating shafts.

Keyword(s)

rotor dynamics crack identification or detection cracked rotor roving disc or mass natural or modal frequency non-destructive testing vibration analysis finite element modelling open crack

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