| Horizontal Axis Wind Turbines are the predominant turbine design in use today.The in-service main shaft is not only suffered from the erosion caused by harsh working conditions,but also subjected to complex stress,such as torque,bending moment and axial thrust during its service life.Surface breaking transverse cracks（SBTCs）are easy to arise on the interface of the shaft and the bearing.The cracks can seriously damage the mechanical properties of the main shaft and further threaten the safety of wind turbines.Ultrasonic testing is an effective non-destructive technology,which guarantees the safety of the wind turbine main shaft.In this dissertation,the ultrasonic end-face detection method and crack evaluation technique are proposed to ensure the examination of SBTCs.Wind turbine main shaft is a large rotary part with a complex profile.The end face of the shaft is often used as the detection surface due to the limitation of the service condition.However,the echo signals will be complicated when detected from the end face,which makes it difficult to determine the wave paths.Furthermore,for the detection of shaft examination,not only the identification of cracks are required,but also the accurate quantification of the crack size.This dissertation focuses on the practical detection method on the in-service main shaft.The propagation characteristics of the ultrasonic waves radiated from the end face are analyzed.An end-face detection method based on a sensor array and an integrated system are proposed.Furthermore,a novel ultrasonic testing method based on acoustic diffraction is proposed to quantify the SBTCs,utilizing the central hole of the shaft,which plays an important role in this method.Finally,the extended depths of small cracks are quantified.This research mainly contains four parts:（1）Propagation characteristics of ultrasonic wave radiated from the end face in the main shaft are analyzed.The acoustic modeling method is developed,which is suitable for large-scale and complex structures.The finite element models（FEM）for both ‘pulse-echo’ and ‘pitch-catch’ modes are established for a typical shaft,and the propagation paths of acoustic waves are analyzed in detail.The simulation results clearly illustrate the reflection,mode conversion and side-wall effect in the shaft.The STBC echoes distribution on the end face and the diffracted sound field in the shaft are explored simultaneously.The simulation models are verified through experiments on the shaft samples.（2）Ultrasonic inspection of the SBTCs from the end face of the main shaft is studied.Due to the space limitation,a method for detecting the SBTCs from the end face is proposed.Considering the beam coverage at different depths and utilizing the beam diffusion properties,an optimal sensor array is designed.Moreover,the detection area is completely covered by the sound beam.An ultrasonic on-line detection system is developed,which possesses the functions of ultrasonic excitation,small signal reception,multi-channel multiplexing and wireless transmission.The system not only achieves remote detection and e-scanning on the end face,but also the wireless communication is satisfied for in-service inspection.（3）Evaluation of crack size by ultrasonic diffracted waves is implemented.Taking the end face as the radiating surface and the central hole as the receiving surface,the SBTCs can be quantitatively evaluated.Based on the diffraction characteristics of acoustic waves at the crack tip,the relationships between the position of exciter,receiver and the time of flight（TOF）of the diffraction wave are explored.A crack size quantification model based on double elliptical trajectories is established.Meanwhile,an acoustic FEM of ‘pitch-catch’ mode is developed,in which the quantitative method is well confirmed by the simulation.In addition,the evaluation accuracy of cracks is also studied,and the calibration method for the TOF of diffracted waves is proposed.A shaft sample with a transverse crack was used to implement the experiments,which verifies that the maximum sizing error is less than5 mm.（4）Electromagnetic acoustic transducer（EMAT）,which receives the longitudinal waves in the central hole,is fabricated.The EMAT is developed with a cross shaped permanent magnet array and a spiral coil.The FEM with the reversed Lorentz force mechanism and reversed magnetostriction mechanism is established.The effects of coil turns,wire spacing and coil interval on the received acoustic energy are investigated,and the optimization of coil parameters is carried out.In addition,the influence of different cable types,material types,lift-off distances on sensor impedance is analyzed,and the matching network is designed to enhance the received signals.The receiving characteristics and the directivity of the developed EMAT are measured.Finally,the diffracted wave experiments based on central hole receiving EMATs are carried out.The results show that the crack sizing error is less than 4.5mm. |
