Basic Research On The Identification Of Cracks Under The Condition Of Spontaneous And Artificial Magnetization

Ferromagnetic materials having a magneto-mechanical effect,and the magnetic domain structure having spontaneous magnetization characteristics,which most commonly used in mechanical engineering fields.How to evaluate the ferromagnetic material injury situation timely and effectively has been a important technical problems to be solved.As a novel passive weak magnetic NDT method,Metal Magnetic Memory Testing?MMMT?method has attracted many researchers attention due to its potential to be used in damage detection and assessment of ferromagnetic material.In this paper,three-dimensional magnetic sensor was used in this study,and the variation and distribution characteristics of three-dimensional self-emission leakage magnetic field of different ferromagnetic materials were investigated under static and fatigue tensile load.The characteristic method by magnetic memory technique and magnetic parameters to the characterization of stretching damage and crack propagation of ferromagnetic materials were explored.Moreover,the effect of external weak magnetic field excitation to MMM crack detection was also discussed.Main results of this research are summarized as follows:?1?Based on the three-dimensional magnetic sensor,the variation regularity of self-emitting magnetic signals under different load levels were studied and correlations between magnetic parameters and applied stress were explored.The results indicated that applied stress induced the spontaneous magnetization of ferromagnets,and the spontaneous magnetization increase with the increase of axial stress.The relationship between applied stress and parameters B_xaveave and K_Bz was approximately linear.The elastic and plastic deformation stage can be distinguished according to the relationship between applied stress and parameters K_Bzz andB_yave.?2?Three-dimensional magnetic signals?B_x?B_y?B_z?have different sensitivities to the crack direction.The defect parallel to the detection direction exhibits almost no abnormal magnetic signals,however the defect vertical to the detection direction exhibits the maximum abnormal magnetic signals.Compared with the components B_x and B_z,the component B_y is more sensitive to the defects non-parallel to the scanning direction.Three-dimensional magnetic signals has showed distinctive variation features,which can be used to indicate the defect position accurately.Parameters?B_x and K_G increase with the applied stress,which are possible to be used as the stress-related magnetic parameters to evaluate the mechanical damage of the ferromagnetic materials.?3?The variation regularity of both tangential and normal components of magnetic signals during fatigue process were investigated.The results showed that the initial abnormal signals which appeared at the notch were reversed after cyclic loading.The abnormal magnetic signals became more significant with the increase of fatigue cycles and reversed again after failure.The characteristic parameters B_tpp and K_gmax showed different variation characteristics during the fatigue process.An approximate linear relationship was found between the characteristic parameters and fatigue crack length 2a.What's more,the variation and distribution of the magnetic signals were also analyzed based on the theory of magnetic charge.?4?Based on Helmholtz coil excitation device,the changes in distribution of magnetic memory signals of defects under steady weakening incentive condition were discussed.The magnetic memory signals of cracks under Geomagnetic and weakening incentives condition were analyzed comparatively.The results showed that magnetic memory signals under steady weak magnetic field excitation were more pure,the magnetic memory signals strengthened and became more prominent.What's more,the interference has been effectively suppressed and significantly improved signal noise ratio,which make the magnetic characteristic parameter extraction more conducive.