Research On Magnetic Memory Signal Characteristic Of Defect In Ferromagnetic Material

The Metal Magnetic Memory Testing?MMMT?technique has been used in many fields as it is more superior in fast identifying defect and internal stress concentration,non-work-polluting and efficient detecting than that of traditional Non-destructive testing techniques.However,the existing theoretical and experimental researches of MMMT technique are still in progressing.For example,the magnetic field model of buried defect has not been established,and the quantitative relation between MMFP?Metal Magnetic Field Parameter?of welding crack and the size of crack has not obtained satisfactory results.Also,the current standards cannot support for its further popularizing.Based on the above insufficients,the presented dissertation conducted a study for the variation laws of MMFP of typical defects in ferromagnetic material and investigated the method for assessing defect,which has a significant theoretical and practical value for improving MMMT.The mechanic model of defect in typical shell was established,gaining the relation between stress and damage variable.Based on above model,the magnetic field models near hole and crack were established,obtaining the relation between magnetic memory field and the stress;After that,the magnetic field intensity model in elastic zone and plastic zone of?type crack tip and the critical magnetic field intensity were established respectively.The results show that two magnetic poles of the defect are opposite,and the magnetic field intensity decreases rapidly away from the defect.Furthermore,the magnetic field intensity increases with increasing stress at one fixed position.The magnetic field model for buried defects was establieshed by equivalent magnetic core method,obtaining the relation between the magnetic field intensity and the size of defect.The buried depth and the width of defect were inversed by the new model,and the inversion results were consistent with experimental results.The impacts of placement orientation of specimen and lift-off value of sensor on MMFP were ressearched,which obtained the impact laws of above factors on MMFP.And the quantitative relation between MMFP and the size of defects were studied.The results show that the magnetic field parameters,including K_ave,K_max,S?K?,m_max,W_P-P-P and W_J-J,increase with increasing defect size.Furthermore,the parameters W_P-P-P and W_J-J,used for identifying defect size,were defined.The experiments were performed to study the mutual relations among W_P-P,W_J-J and the defect size.And the results show that the parameters,such as K_max,W_P-P-P and W_J-J,have a positive and linear relation with defect size.The MMFP of specimen with cracks and other welding defects were analyzed,based on which the method of separating welding defects from welding residual stress was proposed.Then the concepts of?magnetic defect‘and?non-magnetic defect‘were proposed according to the theory of magnetic induction refraction,giving the identification method for magnetic and nonmagnetic defects.The variation laws of the MMFP for typical defects such as groove defects,welding crack under different tensile loads were studied.Especially,the variation laws of MMFP for groove defects in five stages comprising loading into early,middle,late,before the critical fracture and after fracture were studied,presenting that the magnetic field intensity and gradient of defects decrease first and then increase.The testing results of MMFP of defects indicate that the magnetic field intensity and gradient of defects are weakened obviously with increasing buried depth.The MMFP of buried defects of line cutting production versus welding crack were studied,and the results show that the MMFP variation laws of the two type defects are very different.Under no or low external loads,the MMFP tested from the back of the groove defect is difficult to identify buried defects,concluding that both sides of the metal members should be inspected to prevent leak detection of defect in unmeasured surface.As to welding cracks,their locations and buried depth could be detected accurately by MMFP under no or low loads,and the MMFP show obvious change with increasing loads,which proves that the MMMT is more suitable for detecting the ferromagnetic members under working loads.Finally,the impacts of heat treatment and stress concentration on parameters of leakage magnetic field of defects were studied.The testing results indicate that the heat treatment has a significant impact on magnetic memory field.Before heat treatment,the MMFP could not only locate defects,but also identify the size of defects.Besides,the peak values of magnetic field intensity and gradient of defects increase first and then decrease with increasing loads.However,after heat treatment,the MMFP could no longer identify the location of hole defect,and there is an obvious weaken for MMFP of cracks.The magnetic field intensity and gradient peak values of the defect show a continual increase with the increasing loads until the specimen is fractured as well.Therefore,the impact of stress-relief heat treatment on magnetic memory testing results should be taken into account when the MMMT is adapted.In identifying defect features,the impact of stress concentration and size of the defect on MMFP should be taken into account separately.The results also indicate that the MMMT can be used to test the effect of stress relieving heat treatment.