Research On The Magnetic Memory Signals Characterization Of Buried Ⅴ-type Defect

Metal Magnetic Memory Testing(MMMT)is a new nondestructive testing technique developed in recent years.Compared with the traditional non-destructive testing methods(NDT),this method provides early warning of materials or components in use.According to the basic principles of ferromagnetism,stress concentration will lead to changes in magnetic properties,in the form of leakage magnetic field,called magnetomechanical effect.By testing the leakage magnetic field on the surface of the specimen and combining with the actual operating conditions and the structural characteristics of the specimen,the stress and deformation state of the specimen are comprehensively evaluated.Test the potential danger before damaging and achieve the purpose of early diagnosis.Aiming at the present situation of damage detection and technical application of the buried defects of the ferromagnetic specimens,a mathematic model of the buried V-type defects’ leakage magnetic field is established.And the theoretical expression of the designated detection point’s leakage magnetic field is obtained.Using MATLAB program the formula for analysis,the result shows that under the condition of geomagnetic field,the depth and width of buried V-type defects increase,the leakage magnetic field strength increases.And the increase of defect width has great influence on the leakage magnetic field strength.The magnetic charge density is the source of the change of magnetic field caused by external stress.The effect of defect size and tensile load on the magnetic memory signals was experimentally studied,and the variation of magnetic memory signals at buried V-type defects was obtained.The results show that the magnetic field strength and magnetic field gradient can locate buried defects.Because the gradient of the magnetic field can eliminate the influence of the initial magnetic field,it is more accurate to locate the buried defect by the magnetic field gradient.The magnetic field gradient is into a non-linear positive correlation with the defect size.Due to the magnetic shielding effect of the ferromagnetic specimen,the defect depth plays a major role in the change of the magnetic memory signal.In the experiment,the curve of the magnetic memory signal during the tensile process of the ferromagnetic specimen was obtained.On the basis of the experimental measurement results,Lissajous figure of normal component and tangential component differential synthesis was introduced into the analysis.The normalized area of closed area of Lissajous figure is proposed and adopted as the physical quantity for judging stress concentration.The results show that the normalized area value increases slowly with the increase of the load.The normalized area value decreases when the specimen enters the yielding stage.Then,as the load increases,the normalized area value increases again and the rate of increase increases.Therefore,the normalized area value of Lissajous figure can be used to quantitatively evaluate the degree of stress concentration.Magnetomechanical effect simulation of ferromagnetic specimen was carried out by COMSOL.It can be seen that in the plastic deformation phase,the change of physical quantities of the magnetic memory signal is more significant than that of the elastic phase as the stress increases.Changes in magnetic memory signals are more sensitive to plastic deformation.Therefore,metal magnetic memory testing can determine whether the stress concentration area of the specimen enters the plastic deformation stage through nonlinear changes of magnetic field physical quantities.