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Research on the Magnetic Effects of Ferromagnetic Material in the Process of Fatigue

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Tutor: XuMinQiang
School: Harbin Institute of Technology
Course: General and Fundamental Mechanics
Keywords: Metal magnetic Memory,Fatigue,Magnetomechanical,Dislocation,Leakage magnetic fie
CLC: TM271
Type: PhD thesis
Year:  2012
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Fatigue failure is a common failure pattern of material in engineering practice,and fatigue damage evaluation of material has important significance. The metalmagnetic memory £¨MMM£© technique has great potential in the fatigue damageevaluation of ferromagnetic material. However, the quantitative evaluation isdifficult for lack of enough theoretical bases, which makes the application of thistechnique hampered. Therefore, mechanism study of this method in the process offatigue is the key task for propelling this method in application.The MMM phenomenon is the embodiment of material magnetization, on thesurface of material, and the magnetization is combined action of geomagnetic field,stress and material¡¯s microstructure. Magnetomechanical effect, dislocationmagnetization effect and magnetic leakage effect are mainly three effects connectedwith MMM phenomena. In this dissertation, a series of fatigue experiments aredesigned to research the three magnetic effects, and the MMM phenomenon in theprocess of fatigue is researched using theoretical and experimental method.A magnetomechanical model that is suitable for MMM phenomenon isestablished considering the combined action of constant field and cyclic stress. First,the magnetization characteristic caused by cyclic stress is analyzed, and themagnetization stable state M0is constructed; and then, the J-A-F model suitable forMMM phenomena is obtained based on the J-A theory. The results show that, themagnetization of material reaches stable state M0gradually under geomagnetic fieldand cyclic stress, which is accord with experimental results; M0changes regularlywith stress, external field and pinning parameter k1; the J-A-F model can describemagnetization features in tension-release processes better and explain the signchange of dB/d that has been observed in experiments more reasonably. And theJ-A-F model can describe the magnetization change in the process of fatigue by thevariation of pinning parameter k1.The relationship between M0and dislocations is established to describe themagnetization caused by dislocations, considering dislocations¡¯ influence to domainwall motion in the process of fatigue. First, the change characters and correlationsof plastic deformation, dislocations of material and the MMM signal in the processof fatigue are analyzed synchronously in experiments. And then, the dislocationmagnetization effect is formulated using M0, considering the influence ofdislocation density, dislocation structure and plastic strain. The results show that,rapid plastic deformation, stable plastic deformation and fast fracture composethree stages of high stress fatigue; at the first stage, dislocation density and dislocation structure both change, which makes the MMM signal change quickly; atthe second stage, only dislocation structure changes, which makes the MMM signalchange slowly. Dislocation density ¦Ñ and the average displacement of dislocationaffect stress range and pinning parameter k1. increases as fatigue damagedevelops, which makes the MMM signal increase. Because that dislocation changemanifests as plastic deformation at the macro level, dislocations¡¯ influence tomagnetization can be expressed using plastic strain.A magnetic flux leakage model is constructed to describe the variation ofMMM signal, since the MMM phenomenon is the external reflection ofmagnetization caused by material damage. First, the generation and compose ofMMM signal is analyzed from the view of magnetic charge; after that, the magneticflux leakage model is constructed to formulate the leakage field considering thatmaterial damage happens in a certain area and cracks generate and develop in thisarea; and then, connect the leakage field with M0via the expression of magneticcharge density. The results show that, the MMM signal variation is caused byleakage magnetic field from damage area; the tangential component distribution ofthe leakage magnetic field has peak value, and the normal component distributionof the leakage magnetic field is slope shape; the leakage magnetic field increases asfatigue damage develops, and its distribution fluctuates at the position of crack. Themagnetic charge density of damage area has the same variation trend with M0, soM0can be used to represent MMM signal variation before cracks emerge. Themagnetic charge density of crack is much bigger than that of damage area, so theleakage message reflects in the surface leakage field, and makes the MMM signalabnormal at the late stage of fatigue. Contrast the variation laws of plastic strain,hardness and magnetic signal, the conclusion is obtained that MMM signal is moresensitive to fatigue damage than plastic strain and hardness, and the MMM signal ismore suitable for fatigue damage detection.Combine the three magnetic effects, the relation between MMM signal, stress,plastic strain and crack impact factor is obtained; the relation between MMM signalHp, plastic strainpand crack impact factorgfor a certain point in unloadcondition is obtained. The results show that, the MMM signal is influenced bystress, damage status, material, load condition and environment field etc., andchanges with plastic strain and cracks size; the formula between Hp,pandgcandescribe the Hpvariation in the process of fatigue, and explain why the MMMsignal has different variation trends at late fatigue stage.
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