Research On Dynamic Characteristics Of Cracked Rotors And Crack Diagnosis Methoeds

Rotating machinery has been widely used not only in key industries of national economy such as transportation,power,mining and manufacturing,but also in key fields of national defense such as aviation,aerospace and navigation.It is the key important kind of equipment for national basic industries.Due to the demanding operational environment and the long-lasting alternating and even random axial,bending and torsional loads,cracks and other damage may easily arise.Rotors are one of the most important components of rotating machines and cracks in rotors are a very dangerous kind of fault that may lead to a sudden and catastrophic failure of equipment.In order to ensure the safe service of a rotating machine,it is of vital significance to monitor and detect a crack in its early state and provide the crack location and severity information.A basic two-disc rotor-bearing system is used as the research object.Currently in the field of cracked rotor research,dynamic characteristics of cracked rotors need to be improved.In addition,there are not reliable methods for crack diagnosis in rotors.Therefore,research in this paper will be carried out focusing on the shortage and ready-to-solve problem in cracked rotors mentioned above.The rotor-bearing system without cracks is established by Timoshenko beam elements based on finite element method.The strain energy release rate approach in linear fracture mechanics is adopted to derive stiffness matrix of a crack element.By assembling the elements with and without cracks,the cracked rotor-bearing system is established.Crack breathing phenomenon is simulated based on zero stress intensity factors approach.The modelling method for a rotor-bearing system is validated by comparing the results in this paper with results from published literature,and with the results from Abaqus,as well as with the results from experiments.Therefore,the modelling method and established model can be used for the forward and inverse problem of cracked rotor dynamics.Coupled vibration characteristics from external excitations are investigated based on the established rotor with a transverse and slant breathing crack,which involve the bending-torsion couple induced by eccentricity and the coupled phenomena under external excitations,in addition to the influence of steady torsion on the coupled response characteristics.Characteristics of unbalanced response are studied when a cracked rotor rotates at subcritical speed,pass-critical speed and supercritical speed and their mechanism are revealed.Besides,characteristics of' transverse and torsional responses of a run-up cracked rotor are studied.Results show that there is bending-torsion couple in an un-cracked rotor with eccentricity.A rotor with a transverse crack is only coupled in longitudinal and bending directions and uncoupled in bending and torsional directions as well as longitudinal and torsional directions.While for a rotor with a slant crack,vibrations in all directions are coupled.With reasonably applied external excitations,the crack detection can be realized by monitoring the combinational resonance components.The steady torsion transmitted has no effect on dynamic responses of rotors with transverse cracks,but it will affect the breathing phenomenon of slant cracks,and then will have influences on dynamics of rotors with slant cracks.Super-harmonics components and subcritical resonances are the obvious indictors of a cracked rotor.The breathing phenomena show different rules when rotors rotate at subcritical,pass-critical and supercritical speeds and those rules will be influenced by relative eccentric phase angle between two discs.For a run-up rotor with a breathing crack,higher order components are easier to appear in the torsional vibration,which will lead to subcritical torsional resonances at a low speed.In real rotors,misalignments will not be eliminated and will affect crack detection in rotors.Focusing on this problem,methods for crack detection under the interference of misalignments are studied.Features of rotors with faults of misalignment,crack and misalignment-crack are investigated based on full-spectrum and short time full-spectrum methods respectively.Afterwards,qualitative method for crack detection in rotors with misalignments is proposed.Further,by investigating full-spectrum characteristics of the cracked rotor with various crack parameters,a crack sensitive feature named ratio between backward and forward whirls is proposed for quantitative crack detection in rotors with misaliguments.Crack localization especially multi-crack localization in operating rotors is still a challenge.Focusing on this challenge,from the aspect of time and space information fusion,how to realize multi-crack localization in a rotor without prior information about the health rotor model is studied.The proper orthogonal decomposition(POD)is introduced into rotors,and a multi-crack localization method based on the characteristic proper orthogonal mode(CPOM)with gapped smoothing method(GSM)and fractal dimension(FD)is proposed.Further,considering the steps in real rotors which will reduce the local stiffness of the rotor and may confuse the crack localization results,the super-harmonic characteristic deflection shape(SCDS)which is sensitive to cracks is extracted based on singular value decomposition in frequency domain,and a multi-crack localization method based on the SCDS is proposed which can exclude the interference of steps.Based on surrogate model techniques,crack identification methods for operating rotors are studied from the points of pattern recognition and target optimization.Firstly,crack parameters sensitivity is analyzed,and the super-harmonic feature vector sensitive to crack parameters is extracted.After that,two crack identification methods based on super-harmonic feature vector and BP networks as well as super-harmonic feature vector combining with Kriging surrogate mode are propose respectively.Identification probability of crack location and depth reaches 95%by the method based on super-harmonic feature vector and BP networks.For the crack identification method based on super-harmonic feature vector and Kriging surrogate model,the samples demanded are reduced and both the crack location and depth can be identified accurately,besides,the method is robust to noise.Multivariate empirical mode decomposition(MEMD)is introduced for multi-channel signals de-noising in rotors,and the reconstruction components are automatically chosen by correlation coefficient method to realize multi-channel signals de-noising simultaneously.Experiment rigs for crack detection and location are constructed.And the qualitative method based on full-spectrum and short time full-spectrum analysis for crack detection in rotors with misalignments is validated by experiment.Ratio between backward and forward whirls based quantitative crack detection method in rotors with misalignments is validated.Afterwards,two crack localization methods based on proper orthogonal decomposition and singular value decomposition in frequency domain are validated by the established experiment rig.To sum up,focusing on dynamic characteristics of cracked rotor and crack diagnosis in rotors,research has been carried from the aspects of cracked rotor-bearing system modelling,dynamic characteristics of cracked rotors,crack detection methods for rotors,cracks localization methods for rotors,crack identification methods for rotors and experiment validating.From the research above,dynamic characteristics of cracked rotors are supplemented,and reliable crack detection?crack localization and crack identification methods are put forward.Research in this paper can provide methods for the development of condition monitoring system for rotating machines,and is meaningful in theory and engineering.