Basic Research About Mechanism Of Magnetic Memory Method

The thesis is aimed at the magnetic memory method that is a new non-destructive inspection method and characterized by prior inspection. Based on the magneto-physics theory, and by experiments and finite element method, the influences of stress on magnetic flux leakage (MFL) and magnetic domain structure of ferromagnetic materials were studied, the physical mechanism of the Magnetic Memory Method were also discussed.With different loading, tensile velocity and temperature, the tensile loading experiments of 0.20%C and 0.45%C (all mass fraction) and 12CrMoV steel were conducted in the experiments. The MFL distributions of specimens before applied uniaxial tension and after removal of applied uniaxial tension at different loadings were tested by the weak magnetic measurement system. At the same time, simulations of stress of loaded specimens were carried out by finite element method. Then a contrast was taken between the results of magnetic measurement and stress simulation. It shows that at the stress concentration zone, the MFL curve had a maximum and a minimum. The inflexion of the MFL curve was near the most stress point. When the loading less than the yield strength of the specimens, changes of MFL were relatively weak with slight deformation of the specimens. When the loading larger than the yield strength of the specimens, with the deforming to plastic and further the changing of MFL exhibits more distinct, and when necking emerging a very intensive change takes place. With the increase of loading, the difference of the max and min point on the MFL curve was increased. Under the same loading, with the increasing of tensile velocity or temperature, the difference of the maximum and minimum on the MFL curve was increased.And then, this thesis regards silicon steel and steel 20 as the research object, carries on magnetic domain observing experiments by the Bitter method. The result shows that changes of domains were induced by stress concentration. Instead of lamellar domains, maze domains were found at the stress concentration zone of above two materials. With analyzing the first derivative MFL signal of the stress concentration zone, K was defined as the max of the first derivative MFL signal of the stress concentration zone, when the loading near the yield strength of the specimens. The K estimated the zone of stress concentration. It is indicated that this method would be useful on the Metal Magnetic Memory Testing.The conclusions by the research will contribute to really revealing the mechanism of the magnetic memory and present basic data for early establishing the prior evaluation standards about the failure.