Demand of higher speeds in rotor systems has elevated the possibility of numerous instabilities. Internal damping is one of the key parameters for instability in such machinery. Its studies are generally conducted for a possible range of chosen internal damping values. Hence, experimental estimation of internal damping along with external damping is very vital for an accurate prediction of rotor stability, which is scarcely addressed in the literature. To fill this gap, current paper presents an experimental identification methodology for estimation of the internal and external damping in a cracked rotor system. Other crucial unknown fault parameters of rotor system, like additive crack stiffness and unbalance, have also been identified. In present work, internal damping due to rub between transverse fatigue crack faces has been envisioned for the first time. Hence, internal damping has been considered due to combined effect of material hysteretic, i.e. the rub between transverse fatigue crack faces, and the rub between disc and shaft during shaft rotation. Measurements much below critical speeds ensures the main contribution of crack in internal damping due to weight dominance effect as compared to shaft material damping. In experimental setup, a hairline fatigue crack was artificially developed by three-point bending procedure in a shaft at a notch location. The opening and closure behavior of crack faces during rotation of the shaft leads to the forward and backward whirls of the rotor at multiple frequencies, which are apparent in the full spectrum. Mathematical modeling of the rotor system for development of identification algorithm is based on these behaviors of cracked rotor system. Experimentally measured responses have been converted into the full spectrum (both amplitude and phase) based on regression analysis and fast Fourier transform (FFT). Earlier, amplitudes of full spectrum have been used for qualitative indication of crack. But in the present work, corresponding phase has also been considered in quantitative estimation of crack parameter along with other system fault parameters. With the help of a multi-frequency reference signal, phase anomalies have been removed during processing of full spectrum. The bow effect of shaft has been removed from full spectrum responses with the help of a slow roll measurement. Estimated parameters are consistent for different sets of rotor speeds and for different frequency ranges of excitation forces due to fatigue crack. Validations of identified parameters have been done by comparing experimental responses with numerically generated system responses. The latter was generated using experimentally identified parameters in the mathematical model of cracked rotor system.

中文翻译：

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全谱疲劳裂纹转子系统内外阻尼实验识别

转子系统对更高速度的需求增加了许多不稳定性的可能性。内部阻尼是此类机械不稳定的关键参数之一。它的研究通常针对选定的内部阻尼值的可能范围进行。因此，内部阻尼和外部阻尼的实验估计对于转子稳定性的准确预测非常重要，这在文献中很少涉及。为了填补这一空白，本文提出了一种用于估计有裂纹转子系统中的内部和外部阻尼的实验识别方法。还确定了转子系统的其他关键未知故障参数，如附加裂纹刚度和不平衡。在目前的工作中，由于横向疲劳裂纹面之间的摩擦而产生的内部阻尼是首次设想。因此，由于材料滞后的综合影响，即横向疲劳裂纹面之间的摩擦，以及轴旋转过程中圆盘和轴之间的摩擦，内部阻尼已被考虑在内。与轴材料阻尼相比，由于重量优势效应，远低于临界速度的测量确保裂纹在内部阻尼中的主要贡献。在实验装置中，在轴的缺口位置通过三点弯曲程序人为地产生了细线疲劳裂纹。轴旋转期间裂纹面的打开和关闭行为导致转子在多个频率下向前和向后旋转，这在整个频谱中都很明显。用于开发识别算法的转子系统数学建模基于裂纹转子系统的这些行为。基于回归分析和快速傅立叶变换 (FFT)，实验测量的响应已转换为全谱（幅度和相位）。早先，全谱振幅已被用于裂纹的定性指示。但在目前的工作中，在定量估计裂纹参数和其他系统故障参数时也考虑了相应的相位。在多频参考信号的帮助下，在全频谱处理过程中消除了相位异常。在慢滚测量的帮助下，轴的弓形效应已从全谱响应中消除。对于不同的转子速度组和由于疲劳裂纹引起的不同频率范围的激振力，估计参数是一致的。已通过将实验响应与数值生成的系统响应进行比较来验证已识别参数。后者是在裂纹转子系统的数学模型中使用实验确定的参数生成的。