Research On Theory And Method Of Micromagnetic Testing For The Stress Distribution Of The Ferromagnetic Components Along The Depth Direction

The residual stress widely exists in various bearing structures and various pressure vessels;the surface heat treatment and the machining of steels all produce residual stress.The presence of residual stress will have a significant impact on mechanical parts or engineering structures,in some cases,it is beneficial to artificially implant residual compressive stress on the work pieces by shot peening,which can increase the surface fatigue strength and abrasion resistance of the work pieces;however in most cases,the presence of residual stress is harmful,such as the existence of residual tensile stress will reduce the bearing capacity of the engineering structure,reduce the surface fatigue strength of the work pieces,etc.,and even bring catastrophic failure effects on the bearing equipment or key links.In some cases,the residual stress level of the material surface of the load-bearing structure is identical,but a series of mechanical properties such as the load-bearing capacity of the structure are very different,and also the failure form is different.This is due to the difference in the distribution of residual stress along the depth direction of the material.Therefore,in the fields of high-end precision machinery manufacturing and precision instrument design and manufacturing,it is not enough to only detect the surface residual stress level of the material.It is extremely necessary to detect the distribution of residual stress along the depth direction.Based on the above engineering practical application requirements,combined with the existing experimental conditions and research foundation of the laboratory,this thesis is devoted to the study of using micro-magnetic non-destructive testing technology to realize the quantitative detection of(residual)stress distribution along the depth direction.The research contents of the thesis include:(1)Based on the detection standardization implementation requirements,from the system design perspective,a computer dynamic simulation experiment was conducted through the MATLAB^?/Simulink^?software to study the stable control strategy of excitation magnetic field strength and the distortion correction method of excitation magnetic field waveform.(2)Based on multi-frequency eddy current detection technology,the layered coupling model between permeability and stress is established based on without considering the propagation attenuation characteristics of the electromagnetic field and considering the propagation attenuation characteristics of the electromagnetic field respectively,in addition the stress gradient inversion algorithm is given,combined with the four-point bending experiment(the specimen material is 45#steel),the feasibility of the layered coupling model and the inversion algorithm for detecting the distribution of stress along the depth direction is verified.(3)Based on the multi-frequency incremental magnetic permeability method,a layered analytical model between incremental magnetic permeability and stress was established based on the physical mechanism of incremental magnetic permeability generation,combined with a four-point bending experiment(the testing material is Q235steel),verifying the feasibility of this layered analytical model for detecting stress distribution along the depth.(4)Based on the distributions along the depth direction of the magnetic Barkhausen noise energy density of different sub-bands are different,a theoretical model between the magnetic Barkhausen noise energy density and the stress was established,combining with the four-point bending experiment(the sample material is Q235 steel),it was verified that the theoretical model can valid characterize the stress gradient under different loading deflections.This thesis starts from the needs of the engineering practical application,under the current working conditions of the laboratory,based on different stress gradients under different loading deflections generated by the four-point bending experimental device,combined with two test materials(45#steel and Q235 steel),it was verified that the correctness and feasibility of the established theoretical model and the proposed inversion algorithm which can be used to detect(residual)stress distribution along the depth direction.Based on the requirements of the instrument detection standardization,through the computer dynamic simulation experiments,the stability control strategy of excitation magnetic field strength and the distortion correction method of excitation magnetic field waveform were studied,the results showed that the system design method was feasible.