| As one of the most important components of wind turbine,the force of high strength bolts at the blade root increases with the increase of the power of wind turbine.Fracture of high strength bolts at blade root has become one of the key issues in fan safety research.In this paper,the fracture of high strength bolts at the blade root of a 1.5MW wind turbine in a wind farm is studied,and the following research work is completed:(1)The breaking bolt of blade root in wind farm is studied experimentally,including fracture analysis,spectrum analysis,hardness test,mechanical property test and metallographic structure analysis.The results show that the fracture of blade root high strength bolt belongs to fatigue fracture,which lays a foundation for the feasibility of the following mechanical analysis.(2)Mechanical analysis and finite element analysis models are applied to the blade root and root high strength bolts respectively.Firstly,the blade coordinate system and the blade root coordinate system are established;in the blade coordinate system,the force analysis of the blade is carried out based on the relevant theories of aerodynamics and statics,and it is concluded that the moving blade mainly bears three kinds of external loads(aerodynamics,gravity and centrifugal force),and the general formula of blade load is summarized;the general formula is carried out with the help of MATLAB software.Secondly,the resultant force is transformed to the root of the blade in the root coordinate system to determine the stress state and the maximum area of the blade root under the resultant force,and then the stress state of the bolt and the position of the dangerous section in the maximum area of the blade root stress are analyzed.Then,the ANSYS finite element software is used to further analyze the blade root and the blade.The stress state of the root bolt is analyzed,and the stress nephogram of the root bolt and the stress nephogram of the root bolt group are obtained,thus the position of the bolt with the greatest stress is determined.The two analysis methods show that the position of the bolt with the greatest stress on the blade root is the same as that of the dangerous section of the bolt with the greatest stress,and the relative error of the maximum stress value obtained by the two methods is only 6.8%.The theoretical analysis and finite element analysis are proved to be mutually valid,which provides the analysis basis for the acoustic emission test.(3)Acoustic Emission(AE)test design of bolts with maximum strength.The design of excitation loading system and acoustic emission signal testing system are completed.The hexagonal bolts made of the same material as the double studs in the field are selected as the test objects,and the method of bending moment is adopted to excite the test objects.In addition,the test model is built by means of the acoustic emission test signal acquisition system(signal acquisition software,sensor and amplifier type selection,etc.),and the acoustic emission test signal data acquisition is completed.(4)Acoustic emission test data of high strength bolts at the maximum stress are analyzed.Taking the amplitude as the object of acoustic emission signal extraction,and then analyzing it in time domain and frequency domain(FFT),the general signal characteristics before the breaking of high strength bolts are obtained.The test shows that the detection method of acoustic emission technology is a feasible method to diagnose the breaking of bolts. |
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