Studies On The Magnetomechanical Theory And Experiment Of Ferromagnetic Materials Under Weak Magnetic Field

Metal magnetic memory (MMM) method is a novel nondestructive testing(NDT) technology for ferromagnetic material. This method evaluates the stress (orstrain) state of component by testing its distribution of surface magnetic field. Thestress concentration induces the abnormality of the surface magnetic field.Compared to routine magnetic testing methods, the MMM method can not onlylocate the stress concentration accurately, but also this method has followingadvantages: unwanted expensive magnetization devices, unnecessary to clean testedsurface, easy in operation, portable equipment and etc. This method provides a newdirection for the non-destructive evaluation techniques in estimating the stress stateof the component, and so it has promising future. The shortage in the theoreticalresearch of MMM method has become the bottleneck hindering its application.In essence, the MMM can be defined as the change in magnetic parameters of amagnetic material resulting from a change in applied stress (or strain) undergeomagnetic field. The nature of MMM is magnetomechanical effect under weakmagnetic field. The research of magnetomechanical effect can be traced back to over100years ago, and the study on it has formed a relatively mature theory system. Themagnetomechanical effect is the change of magnetization of a magnetic materialresulting from a change in applied stress. In this dissertation, aiming at ambiguousmechanism of the MMM phenomenon, combined with the experimental results ofthe MMM test and based on partial results of the magnetomechanical effect, themagnetization models for different stress states are established, respectively. To bemore specific, the main works are as follows:(1)To incorporate the asymmetry in magnetic property behavior under tensileand compressive stress in the J-A model and make up defect of basic J-A model indescribing asymmetry, the asymmetry properties caused by stress demagnetizationterm, a variable pinning coefficient, stress-dependence domain coupling coefficientand stress-dependence saturation magnetostriction are incorporated in the basic J-Amodel. It is found that the modified model can not only present the phenomena offield reverse and magnetization approach, but also provides a much betterdescription of asymmetrical magnetic property under tension and compression.Moreover, the accuracy of calculations is improved considerably, particularly in thecompressive stress situation;(2) The change of magnetization caused by plastic deformation can beconcluded as following three mechanisms: first, the plastic deformation induces mass multiplication of dislocation; second, the plastic deformation causesmagnetoplastic energy; third, the plastic deformation gives rise to the residual stress.In order to describe the change of magnetization with the plastic deformation, theabove three mechanisms are equivalent to the responses of the effective field andmodel parameters to the plastic deformation, where the effective field incorporatesthe contributions caused by residual stress and magnetoplastic energy, and the modelparameters prime consider the magnetoplastic effect on the pinning coefficient, theinterdomain coupling coefficient and the scaling constant. The computedmagnetization exhibits sharp change in the preliminary stage of plastic deformation,and then decreases slowly with the increase of plastic strain, in agreement withexperimental results;(3) Magnetization simulation caused by cyclic stress not only should considercourse of magnetization change with stress, but also take into account the influenceof fatigue damage on magnetization. The magnetization mechanisms are differentfor different fatigue stages: In the stage I of fatigue, cyclic softening or hardeningoccur in material. In the process of simulating the magnetization accommodation,the effects of softening or hardening on the pinning coefficient and the interdomaincoupling coefficient are considered; in the stage II of fatigue, it is a stable stage offatigue and the parameter of MMM testing is also stable. From the views ofdamage-to-heat and defects-to-iron atoms per cycle, the immeasurability isdiscussed; In the stage III of fatigue, macroscopic cracks appear in material and theparameter of MMM testing changes greatly. The leakage magnetic field caused bymacroscopic cracks is considered;(4) Peak-to-peak saltation in magnetization occurs at the instant of fracture, andthe magnitude of saltation is without parallel. This saltation is simulated with thehelp of theory of magnetic dipole. Assume that the distribution ofhardness-dislocation-magnetic charge is consistent, based on the test result ofhardness, the models of dislocation-magnetic dipole for tensile fracture andtensile-tensile fatigue fracture are established, respectively: The distribution ofmagnetic charge for tensile fracture is linear increase from distant to the fracturezone, while the distribution of magnetic charge for tensile-tensile fatigue fracture isconverging only at the fracture zone. It is found that the established model can notonly present the saltation of peak-to-peak in magnetization for tensile fracture andtensile-tensile fatigue fracture, but also provides an explanation to the experimentalphenomena that the peak value of saltation for tensile-tensile fatigue fracture ishigher that that for tensile fracture.