Research On Electromagnetic Ultrasonic Detection Of Bolt Stress In Wind Turbine

With the continuous reduction of global fossil energy,the new energy industry has received widespread attention,and wind energy occupies a significant position in the field of new energy because of its abundant resources and easy access.China is a large country of wind energy resources,and the installed capacity of wind turbine is also in the forefront of the world.Tower in the wind turbine mainly plays a supporting role,wind turbine tower using high-strength bolt connection,the axial stress value of the bolt directly affects the service life of the tower.If the axial stress of the bolt is too large or small may result in the collapse of the wind turbine tower,and even bring about accidental casualties.Detection of bolt axial stress can improve the reliability of the bolt while also ensure the safety of the tower.This thesis used electromagnetic ultrasound technology to detect the axial stress of the bolt,and the main contents were as follows:Firstly,the methods of bolt stress detection were summarized.After analyzing and comparing the application scenarios of different bolt stress detection methods,the thesis selected electromagnetic ultrasonic nondestructive testing technology.And then this thesis studied the mechanism of the electromagnetic ultrasonic transducer and the theory of acoustoelasticity.By analyzing the propagation characteristics of the ultrasonic waveform and the application scenarios and comparing the advantages and disadvantages of the mono-wave method and the bi-wave method,the bi-wave method was proposed as the bolt stress detection method.Due to the theory of acoustoelasticity and the theory of solid acoustics,the thesis gave the calculation formula of the bolt axial stress detection by the bi-wave method.Secondly,based on the electromagnetic ultrasonic transducer mechanism,the thesis established an electromagnetic ultrasonic bolt axial stress detection model in COMSOL by multi-physics field coupling method.In order to improve the transducer efficiency and signal-to-noise ratio,Lorentz force and flux density were selected as optimization indexes to optimize the electromagnetic ultrasonic transducer.By the control variable method,the thesis examined the effect of three parameters,permanent magnet height,width and coil lifting distance,on the peak Lorentz force and flux density and obtained the law of the effect of parameter changes on the peak Lorentz force and flux density.Finally,the optimized transducer parameters were used in COMSOL software to study the working state of the electromagnetic ultrasonic transducer.By applying different sizes of preload force to the bolt and solving,the model was achieved the shear and longitudinal wave echo signals under different preload forces.The ultrasonic transverse time was obtained by the maximum peak over zero method.Based on the bolt stress detection formula,the transverse time was brought into the formula and then the bolt stress was got.Under different bolt types,the thesis carried out multiple simulation experiments.For verifying the accuracy of the simulation model,this thesis used the linear regression method to establish the transition time-stress function relationship in MATLAB and analyzed the results calculated by the bolt stress detection formula and the stress simulation results.