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Research on Metal Magnetic Memory Testing and Fatigue Evaluation

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Tutor: RenShangKun
School: Nanchang University of Aeronautics and
Course: Measuring Technology and Instruments
Keywords: magnetic memory test,damage model,fatigue life,stress concentration
CLC: TG115.284
Type: Master's thesis
Year:  2011
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As a new nondestructive testing method, metal magnetic memory testing attractedmany interests due to its potential in damage evaluation of ferromagnetic materials. Inthis study£¬metal magnetic memory testing has been lead into fatigue damage andresidual life evaluation, relevant numerical simulation was made as a foundation forfatigue damage, mass research made on the basis of multi-subjects such as physics ofmagnetism magnetic materials, ferromagnetism, material fatigue, fracture mechanics.In paper, main research is characterization rule of magnetic memory signals evolutionmechanism through fatigue damage, parameter model of residual life evaluation wasset up for fatigue damage, which were on the ground of stress intensity factor ¦¤Kand magnetic memory signals. Meanwhile, with electromagnetism basic theoriesguided, the ANSYS finite elements software was used to numerical simulation for themetal magnetic memory phenomenon of ferromagnetic specimen loaded.By compact tensile fatigue of30CrMnSiNi2A steel, with high frequency,magnetic memory signals changing along with fatigue process was studied. Throughanalyzing the relations of magnetic memory signals and fatigue residual life, It¡¯s foundthat magnetic memory signals B(x) maximizing at the crack tip parts; the mean valueof magnetic memory signals B(x)avgwere divided into two stages, in the first stagebetween fifty thousand cycles and two hundred thousand cycles, the leakage magneticfield B(x) arised rapidly, and£¬in the second stage, after the leakage magnetic field B(x)reached maximum, along with crack deepening, stress released constantly, B(x)decreased gradually. Parameter model of fatigue damage was set up based on stressintensity factor¦¤K and magnetic memory signals, which evaluate residual life offerromagnetic specimen well. It was found that the specimen was brittle at first, andlater, ductile fracture happened by fracture morphology observation. The earlyflattening fracture formation was brittle fracture, and cleavage fracture appeared ininterim, dimple turned up indicated it¡¯s ductile fracture in later term.Because the gap exists, the largest tensile stress took place around the crack tip,and the biggest compressive stress present in the left area around the crack tip,extension of Macroscopic crack played a leading role in the process of instability and failure for the component, which influenced the stress intensity factor and integralvalue significantly.The phenomenon of surface leakage magnetic field distortioncaused by stress for ferromagnetic specimen was simulated, mathematical model wasbuilt of stress concentration and magnetic memory parameter, on account of ANSYSfinite element software, quantitative evaluation research of stress-magnetic effect wasdone on stress concentration of ferromagnetic specimen, It¡¯s analysed that the loadingcondition of tested specimen could be reflected by stress concentration area and theintersection angle variation between stress concentration lines.Finally, Through compression experimental, the magnetic memory effectproduced by compressive stress was unobvious, and it affected magnetic intensity ofmaterials was weak, which was far less than the magnetic intensity produced by tensilestress, so the effect on the strength of tensile testing and compression testing, themagnetic field distortion of compressive stress direction was small.
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