The Mechanism Revealing and Law Explorating for the Nonlinear Response of Blade Disk Rotor System Under the Coupling Effects of Crack and Aerodynamic Force

Blade disk rotor system is a typical structure of industrial equipments such as aeroengines and gas turbines. The research on the response characteristics and mechanism of the system under the coupling effects of aerodynamic force and blade crack is of great significance to the interpretation of vibration phenomena and diagnosis of faults. From the numerical solution based response characteristic analysis to the kinematics and dynamics based essential response mechanism revealing, from the model based special case study to the Number Theory based general law establishing, in this paper, the response mechanism of blade disk rotor system under the coupling effects of crack and aerodynamic force is studied comprehensively and deeply. Firstly, a simplified dynamic model of typical blade disk rotor system is constructed by using the classical continuous parameter modeling method. Based on the dynamic model, for two structural forms of moving and stationary blades, the typical characteristics of vibration response under the actions of aerodynamic force and blade crack are analyzed by means of numerical solution. Then, from the perspective of kinematics and dynamics, the internal mechanism between the vibration responses and the excitations are revealed. Finally, based on Number Theory, the response characteristics and mechanism of typical structures are summarized, and the general laws of responses with general structural forms are established.

An improved analytical dynamic model for rotating blade crack: With application to crack detection indicator analysis

Rotating blade is one of the most important components for turbomachinery. Blade crack is one of the most common and dangerous failure modes for rotating blade. Therefore, the fault mechanism and feature extraction of blade crack are vital for the safety assurance of turbomachinery. This study is aimed at the nonlinear dynamic model of rotating blade with transverse crack and the prior feature extraction of blade crack faults based on the vibration responses. First and foremost, a high-fidelity breathing crack model (HFBCM) for rotating blade is proposed on the basis of criterion for stress states at crack section. Since HFBCM is physically deduced from the perspective of energy dissipation and the coupling between centrifugal stress and bending stress is considered, the physical interpretability and the accuracy of the crack model are enhanced comparing with conventional models. The validity of the proposed HFBCM is verified through the comparison study among HFBCM, conventional crack models, and finite element-based contact crack model (FECCM). It is suggested that HFBCM behaves best among the analytical models and matches well with FECCM. With the proposed HFBCM, the nonlinear vibration responses are investigated, and four types of blade crack detection indicators for rotating blade and their quantification method are presented. The numerical study manifests that all these indicators can well characterize the occurrence and severity of crack faults for rotating blade. It is indicated that these indicators can serve as the crack-monitoring indexes.

Rotor imbalance detection and quantification in wind turbines via vibration analysis

Rotor imbalance is a common fault in wind turbines, which may enhance radial loads that induce faults on the main bearing and gearbox. It usually results from the asymmetry of the air dynamics caused by blade crack, icing, etc. A simple and effective method on rotor imbalance detection and quantification is presented using the vibration signal collected from the accelerometer monitoring the wind turbine drive train. A vibration model describing the rotor imbalance under blade crack is proposed. The complex morlet wavelet transform is applied to the detection of the rotational frequency of rotor hub which represents the rotor hub. A health indicator that can quantify the degree of the rotor imbalance is designed. The proposed methods have successfully detected and quantified the rotor imbalance caused by blade crack in an on-site wind turbine.

Identification and influence factors analysis of blade crack mistuning in hard-coated blisk based on modified component mode mistuning reduced-order model

Blade crack will cause severe mistuning of hard-coated blisks, which will lead to vibration localization. To identify crack mistuning and analyze influence factors, in this study, a mistuning identification method of blade cracks in hard-coated blisks is presented based on modified component mode mistuning reduced-order model, in which the hard-coated blisk with blade crack is decomposed into a substructure of tuned hard-coated blisk and a substructure of coated blade with cracks. Crack mistuning of each coated blade can be obtained by a single identification calculation. After verifying the rationality of this identification method, the influence factors of blade crack mistuning are analyzed. The influence factors include the crack location on the coated blade (cracks occurring only in coating or only in blade substrate or both in blade substrate and coating), crack length, crack position in the radial direction of the blisk, and modal data type of coated blisk used for mistuning identification calculation. The research results show that, with the increase of crack length, the mistuning of crack occurring only in the coating does not increase continuously but decreases firstly and then increases. For the first bending modes, the closer the blade crack is to the blade root, the larger the mistuning is. For the second bending modes, the blade crack located at the position of maximum modal displacement will produce large mistuning. For hard-coated blisk with blade crack, these crack mistuning variation rules are of great significance to the dynamic analysis and the determination of the crack location.

Detection of Cracks in Turbomachinery Blades by Online Monitoring

The presence of a crack in a blade can change the natural frequencies of that blade. It has long been a goal to detect blade cracks by assessing the change in a measured vibration frequency of the blade over time. It has been found that prior frequency assessment methods can be less accurate than is desirable to reliably detect the relatively small frequency changes that are typically associated with blade crack sizes of practical interest. This paper describes a method in which potential temporal changes in the frequencies of individual blades are assessed by periodically analyzing complete rows of blades using mistuning analysis techniques that treat the blade rows as coupled systems, in contrast to other techniques that consider each blade individually in turn. This method, while computationally complicated and challenging, has been found to be capable of detecting blade root cracks that are much smaller than those that can be detected using other techniques. Moreover, this method has been demonstrated to detect cracks that are much smaller than the critical size for mechanical separation of the blade from the rotor. This improved frequency assessment technique has been used to identify more than 30 blades with frequency changes that were considered to be potential indicators of blade cracks. Subsequent inspections verified indications in all of those blades. In addition to providing operational guidance, the frequency change data were used to infer the time periods during which crack growth had occurred.