PSI - Issue 39

Nabam Teyi et al. / Procedia Structural Integrity 39 (2022) 333–346 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Rotor defects due to crack(s): Here the defect in rotor is due to presence of crack or cracks either in shaft or in the disc or in both. A crack is a small opening like a slit cut on the rotor’s surface which may occur due to poor manufacturing of the rotor material, or which occurs during the operating period of the component (Fig. 4). Such cracks are so small to the tune of microns that it is impossible to detect its presence visually even just after the rotor is freshly manufactured. Cracks are definitely impossible to be detected visually in a rotating rotor. Presence of cracks in a rotor system is very harmful for its health.

Fig. 4. Rotor defect due to crack. Defect due to cracks is the most complex amongst the four. Cracks are just not visible, neither in the static condition nor the dynamic condition of the rotor. Cracks can neither be detected using displacement gauges nor can these be corrected using balancing machineries. Therefore a rotor with cracks is the riskiest in comparison with rotors with any other defect. Even a freshly manufactured shaft which lets say, is free from any defect, still would have some deflection about its centre due to its own weight. And the longitudinal fibers of the shaft material experience, both tension for certain time, compression at other times, in every single rotation. This is similar to R. R. Moore rotating beam fatigue model. Similar model has been used to study fatigue life of micro arc oxidation coated 6061-T6 Al alloy by Ramakrishna et al. (2017) and Madhavi et al. (2019), and to study high cycle fatigue behaviour of hard turned 300M ultra-high strength steel by Ajaja et al (2019). With continuous rotor spin, the shaft motion may induce a crack. The basic reason of emphasizing cracks in rotors is to avoid unwarranted and uneconomical shutdown of the entire machinery for the crack analysis. Therefore, the required work has to be done in situ. However, difficulties are inherent to analyse any dynamic system. Also the roundness of the shaft and disk in rotor further complicates the study, as any physical reference point in the system is impossible to fix. Therefore, cracks are identified by signal based methods, by parametric methods, by model based methods and by modal based methods. Signal based methods use some sensors and instrumentations for data acquisition and signal processing to obtain solutions. Parametric methods represent the shaft rotor system as a function of mass, damper and spring. Here the mass resists acceleration as in Newtonian force, the damper resists velocity as in viscous force and the spring resists deflection as in spring force, and together, they make up for the dynamic force in the system as a function of time. In model based methods, there is no physical involvement and every effect is represented by a mathematical equation. In modal based methods, the inherent dynamic characteristics of the shaft rotor system in forms of their natural frequencies and mode shapes are used to formulate a process for its behaviour. During the last four decades, significant study has been conducted into the development of various problem solving strategies for the efficient management of rotor cracks, which has resulted in a large number of technical papers on the subject being published. As a result, a succinct periodic review of such a corpus of work is desperately needed. Sabnvavis et al. (2004) presented a review article of the published papers on crack shaft detection and diagnostics from 1990 to 2003, which was published in the The Shock and Vibration Digest. Also, Kushwaha and Patel (2020) conducted an exhaustive study of methodologies and modelling approaches in crack analysis the previous year. Additionally, this research conducts a review of cracked rotor literature from the recent decade, specifically from the years 2010 to 2021, in chronological order.