Research On Early Damage Diagnosis Method Of Ferromagnetic Materials Based On Magnetic Memory Testing Technique

Non-destructive testing techniques have very important influence onguaranteeing reliable equipment operation and life safety. With the increasingdevelopment of modern equipment towards high load, high speed, high temperatureand high pressure, it is vital significant to early discover various micro-defects andlocal stress concentration zones causing mechanical structure and equipment failureto avoid sudden accident, but these hidden damage occurring before macro-cracksor defects can not be detected by traditional non-destructive testing methods. Anewly arisen technique named metal magnetic memory is deemed to have potentialsin detecting early damage, whereas it is lack of systematic research on magneticmemory testing mechanism and method in early damage of ferromagneticcomponents.A method suitable to detect initial plastic deformation and early fatiguedamage of ferromagnetic materials is proposed via research on the metal magneticmemory testing technique, in order to discover component damage early and avoidfailure in advance. The feasibility of the magnetic memory technique applied todetect stress state and fatigue damage is first explored, and the early damage testingon ferromagnetic materials is kept as the central task. The main problems includingstatic and dynamic tensile tests, the magneto-mechanical effect modeling in theplastic region, and the magnetic field distortion modeling on the early fatiguedamage have been systematically investigated. To be specific, the main contents areexplored as follows.Based on the effective field theory and approaching principle, themagneto-mechanical effect model of ferromagnetic materials under the action ofunaxial stress state is derived in detail, and the variation of magnetization withstress in the earth’s magnetic field is numerically simulated, showing that the initialmagnetic state tends continuously towards the anhysteretic magnetization on theapplication of stress. The variation of the magnetic signal in one certain point atdifferent locations of the specimen rotated a period is recorded via the rotarybending fatigue experiment, which is consistent with its actual stress-strain state. Inthe meantime, the variation characteristics of the magnetic memory signal withcycle number are investigated, indicating that there is correlation between the magnetic curve and the fatigue damage.On the basis of the research on the feasibility of the magnetic memorytechnique applied to detect early damage, the evolution law of the magneticmemory signal on loading is studied via the static tensile experiments withundemagnetized and demagnetized plate specimens, and the magnetic signalcharacteristics to recognize elastic and plastic deformation stages are given.Combined with the approaching principle, the influence and reason of differentinitial residual magnetization on the magnetic field variation induced by stress areanalyzed, which helps to guide engineering application. The variation regularities ofthe magnetic memory signal with cycle number are investigated throughtensile-tensile fatigue experiments, and the magnetic field gradient in the stressconcentration zone as the key parameter to characterize fatigue damage is putforward. The result calculated by the established damage variable model coincideswith the evolution law of different damage degree in the four stages during thedynamic fatigue process.The modified magneto-mechanical effect model applicable to plasticdeformation stage is developed, and the variation relationship betweenmagnetization and strain is obtained. Considering the limit that the presentmagneto-mechanical effect models are only valid in the elastic range, the moreuniversal stress-magnetism coupling model correlating the magnetization and theeffective field is derived based on the law of conservation of energy. The piningcoefficient and the effective field scaling parameter are both regarded as thefunctions of dislocation density in the plastic range in the model, and the effectivefield components induced by elastic and plastic deformation are differentiateddefinitely in the effective field expression on loading, and the effective field afterunloading contains the additional magnetic field induced by residual stress. Thenumerical simulation results show that the magnetization by the rather small plasticdeformation decreased sharply, and the magnetic memory signal after unloadingonline also changed suddenly on yielding in the static tensile experiment withdemagnetized smooth plate specimens. The theoretical results have a goodagreement with the experimental phenomena, indicating that the proposed modelcan be used to detect the initial plastic deformation of ferromagnetic materials.The microscopic fatigue experimental system including XH-500metallograhpic microscope, DIG300special digital camera and magnetic memorytesting device, combined with the magnetic memory microscopic fatigue observation method is adopted, and the tensile-tensile fatigue experiment withspecimens made of steel45are conducted. The evolution process of fatigueshort-crack initiation and growth is observed dynamically from the microscopicscale, and the variation regularity of microscopic Vickers hardness during thefatigue procedure is also given. At the same time, the variation of the magneticmemory signal on the surface of the V-shaped notch with cycle number is analyzed,and the magnetic signal curve distribution regularity corresponding to differentcrack length is investigated. On this basis, assuming that the dislocation density inthe plastic zone is linear distribution, the leakage magnetic field distribution in thestress concentration zone is calculated in terms of the magnetic dipole model, andthe leakage magnetic field model on the surface of the plastic zone in the crack tipis set up. The tangential and normal components of the leakage magnetic field areboth simulated, and the peak Hxpof the tangential component and the width betweenabnormal wave crest and trough△xHyp-pof the normal component as two keyparameters to represent damage degree and range are proposed respectively,achieving early diagnosis on fatigue damage.