Limin Tao

Nonlinear dynamic behaviors of rotated blades with small breathing cracks based on vibration power flow analysis

Rotated blades are key mechanical components in turbomachinery and high cycle fatigues often induce blade cracks. Accurate detection of small cracks in rotated blades is very significant for safety, reliability, and availability. In nature, a breathing crack model is fit for a small crack in a rotated blade rather than other models. However, traditional vibration displacements-based methods are less sensitive to nonlinear characteristics due to small breathing cracks. In order to solve this problem, vibration power flow analysis (VPFA) is proposed to analyze nonlinear dynamic behaviors of rotated blades with small breathing cracks in this paper. Firstly, local flexibility due to a crack is derived and then time-varying dynamic model of the rotated blade with a small breathing crack is built. Based on it, the corresponding vibration power flow model is presented. Finally, VPFA-based numerical simulations are done to validate nonlinear behaviors of the cracked blade. The results demonstrate that nonlinear behaviors of a crack can be enhanced by power flow analysis and VPFA is more sensitive to a small breathing crack than displacements-based vibration analysis. Bifurcations will occur due to breathing cracks and subharmonic resonance factors can be defined to identify breathing cracks. Thus the proposed method can provide a promising way for detecting and predicting small breathing cracks in rotated blades.

Effects of Crack on Vibration Characteristics of Mistuned Rotated Blades

Rotated blades are key mechanical components in turbine and high cycle fatigues often induce blade cracks. Meanwhile, mistuning is inevitable in rotated blades, which often makes it much difficult to detect cracks. In order to solve this problem, it is important and necessary to study effects of crack on vibration characteristics of mistuned rotated blades (MRBs). Firstly, a lumped-parameter model is established based on coupled multiple blades, where mistuned stiffness with normal distribution is introduced. Next, a breathing crack model is adopted and eigenvalue analysis is used in coupled lumped-parameter model. Then, numerical analysis is done and effects of depths and positions of a crack on natural frequency, vibration amplitude, and vibration localization parameters are studied. The results show that a crack causes natural frequency decease and vibration amplitude increase of cracked blade. Bifurcations will occur due to a breathing crack. Furthermore, based on natural frequencies and vibration amplitudes, variational factors are defined to detect a crack in MRBs, which are validated by numerical simulations. Thus, the proposed method provides theoretical guidance for crack detection in MRBs.

Damage detection in high-speed rotated blades by blade tip-timing method based on compressed sensing

High-speed rotated blades are key mechanical components in turbomachinery. Understanding dynamic characteristics of rotated blades is important for damage detections in turbomachinery. Conventionally contact methods are used for this purpose. However, strain gauges and accelerometers can affect the precision of measurements. So blade tip-timing(BTT) is used to realize non-contact vibration measure and then using accurate measure results to detect damage in rotating blades. However, BTT signals are typically under-sampled. How to extract characteristic features of blade vibrations by under-sampled signals becomes a big challenge. In this paper, a novel of compressed sensing (CS) is proposed to solve the problem. Firstly, a CS mathematical model is built based on BTT method. Then, minimum one normal(L1) algorithm is applied to solve the CS problem. Finally, damage detections are realized based on reconstructed frequency information. Thus the proposed method can provide a promising way for online damage detection in rotated blades for practical engineering.

Effects of crack on modal parameters of mistuned blades

Mistuned blades are inevitable in compressor, and modal parameters of mistuned blades will change significantly compare with tuned blades. In this paper, mistuning stiffness is introduced and a lumped parameter model of coupled multi-blades is established. Mode localization of mistuned blades is analyzed by this model. When a crack appears in mistuned blades, local flexibility method is used to analyze lessened stiffness model of the cracked blade. A local parameter is introduced. It is used to quantitatively analyze that positions and depths of a crack affect modal parameter of blades. When depth of a crack is shallow or a crack locate near tip of a blade in mistuned blades system, these cannot change local parameter of modal amplitude in mistuned blades with a crack. In other words, this kind of the crack cannot change mistuned degree of blades. However, with depth of a crack increasing or a crack being close to root of a blade, local parameter of modal amplitude can increase sharply. The analytical results establish a based model for diagnosis of a crack in mistuned blades.