Research On The Principles And Methods Of Testing Stress In Ferromagnetic Components Under The Excitation Of Constant Magnetic Field

It is of great sigificance to testing the working stress in the load-bearing components for their safe operation.There are shortages in the existing electromagnetic method as time-varying magnetic field is usually adopted as the excitation magnetic field.In this case,the induced eddy current and the coil heating can influence the testing results,also there are difficulties in winding coils on the tested component.Thus,this dissertation researches the method of testing stress in ferromagnetic components under the excitation of constant magnetic field.A spatially-varying constant magnetic field is excited on ferromagnetic components to test the stress in the component.The aim of this method is to overcome the shortage of the existing electromagnetic method.Firstly,this dissertation puts forward the theory of testing stress under the excitation of constant magnetic field based on the magnetoelastic effect to overcome the influence of eddy current on the detection results.Also the method for calculating the initial magnetization curve of the ferromagnetic members with uniform stress is given.In this method,the spatially-varying constant magnetic field is used as the excitation field,the magnetization curve are deduced from the superficial axial and normal magnetic flux densities in the region where the excitation field that satisfies the uniform distribution condition and the amplitude distribution condition.A simulation experiment is conducted and the results show that the proposed method is correct.These researchs provide theoretical supports for obtaining magnetic parameters to characterize the stress under the excitation of constant magnetic field.Secondly,combined with the actual requirement of testing the stress in ferromagnetic components,a method for detecting the initail magnetization curve is proposed in this dissertation.Permanent magnets instead of DC coils are adopted to excite the spatially-varying constant magnetic field which satisfies the uniform distribution condition and the amplitude distribution condition.Also,the axial and normal magnetic flux densities at different lift-offs from the surface of the ferromagnetic member are extracted.The superficial axial and normal magnetic flux densities are deduced by the polynomial extrapolation method and the Gaussian theorem extrapolation method,respectively.An experimental platform is set up to conduct experments to verify the feasibility of this method.These reseaches afford a new way to obtain the magnetic parameters to characterize the stress.Thirdly,the mehods for characterizing stresses by differential permeability and by magnetic flux densities from the surface of the ferromagnetic member are clarified.The principle of methods are analyzed.Also,the applicability of the two methods for stresses in ferromagnetic members with different materials is discussed.The two methods are compared according to the linearity and sensitivity of characteristic parameters changing with the stress.The differential permeability[?d](H=10000)and the reciprocal amplitude of initial differential susceptibility RAIDS[1/?d]H=0 are adopted to characterize the stress in steel wires made of high carbon steel and steel bars made of medium carbon steel,respectively.Finally,a set of principle prototype for testing stress under the excitation of constant magnetic field is developed.The prototype consists of the testing sensor,the data acquisition unit and t the testing software.Results of the testing experiments show that the prototype can test the stress in ferromagnetic members with the maximum error less than 7%,also detect the intial magnetization curve with the maximum error less than 10%.The dissertation provides a new method for testing the stress in ferromagnetic components.It may be developed to be a convenient and efficient method for testing the working stress in ferromagnetic components.At the same time,the dissertation provides a new idea for obtaining the initial magnetization curve of ferromagnetic members with the circular section.