| With the rapid development of modern industrial technology, the ferromagnetic metalmaterials have been widely used in construction, aerospace, energy, transportation andother fields. In use process, the mechanical strength will be reduced on stress concentrationlocal of metal components, which is easy to cause sudden accident. So stress testing iscritical to the safe operation of pipelines. Compared with the traditional technique ofnondestructive testing, metal magnetic memory testing technique can effectively detectmicroscopic damage of ferromagnetic components, which advantages are fast diagnosis,simple operation, and low price. However, due to the lack of theoretical support andsystematical experimental study, the physical mechanism of magnetic memoryphenomenon is still not settled yet, reliability and authenticity of the magnetic memorysignal is difficult to be convinced. Traditional mechanical and magnetism are difficult toestablish a magnetic relationship to illustrate the mechanism of magnetic memory effect.The microcosmic mechanism of magnetic memory effect is studied based on firstprinciples calculation, and then the magnetic-force effect of ferromagnetic materials, themechanism of spontaneous magnetic flux leakage signals of magnetic memory andrelationship between stress concentration and magnetic memory signal are analysed.The magnetic source of ferromagnetic materials is studied. The lattice constants,electronic structure, electron spin state density, partial wave state density and thequantitative relationship between atomic magnetic moment and the magnetic properties ofmaterials are calculated by using the basic theory of quantum mechanics, Which providetheoretical basis for the power-magnetic coupling of metal magnetic memory effect. Inspin system, the brillouin zone division, exchange connection, plane wave energy andapproximation algorithm are studied. The ground state of magnetic memory effectproperties are researched under the different crystal structures.The influence of the external forces and external magnetic field on magneticproperties of materials, the test model of magnetic memory effect is established. With thechange of the stress concentration state, magnetic moment is closely related to the microstructure change, and magnetic moment of system will change as the most close tothe atomic spacing approximation into linear change, the characteristics of magnetismunder the action of tensile stress and compressive stress is studied.The influence of the lattice distortion and the doping elements on the materialmagnetism is studied. C, Si, Mn and other elements generally are doped in theferromagnetic component in bit or clearance. The magnetic properties of the doping systemare studied. At the same time, the magnetic memory effect is closely related to the changeof lattice structure, the stress concentration may result in isotropy and anisotropy latticestructure distortion distortion, so the flunence of the lattice distortion on the materialsmagnetism is studied.Thesis theoretical research results show that the lattice distortion which is caused bystress concentration is the primary cause of magnetic memory signal, i.e., under tensileeffect, atomic magnetic moment increases, material’s magnetic largens; under compressioneffect, atomic magnetic moment decreases, material’s magnetic abates. And theoreticalcalculation shows that the trace elements which are doped in the steel component will nothave major influence on the material magnetic. A new method is put forward to study thethe physical mechanism of the magnetic memory effect.The X70steel pipe is used for water pressure blasting experiments, and then therelationship between magnetic memory signal and stress is analysed. The main ingredientof X70steel is Fe, and the trace elements such as C, Si, Mn are doped. The trend is thatmagnetic memory signals change with the increase of water pressure, and the experimentphenomenon shows that the application of metal magnetic memory testing technology inon-line detection of pipe stress is feasible. |
