Development Of A New Metal Magnetic Memory Detector

Metal magnetic memory testing (MMMT) is a new technology of nondestructive testing domain. It can be used to detect the unformed hidden fault in ferromagnetic metallic component so as to accomplish inchoate exploration and diagnose of structural damage and evaluate security of equipment accurately. So MMMT technology is very importance to avoiding the unplanned fatigue failure. Although MMMT has good application prospect, its deficiency of theory research and defect of actual detectors have been become two bottlenecks of restricting its application and development. Present theory study indicates that hidden flaw of structural surface layer can produce the magnetic field which perpendicular to the material surface component (Bn) crossing the zero and parallel to the material surface component (Bx) exhibiting a peak. According to this theory, actual detectors bases on the normal magnetic field component (Bn) whether crossing the zero. This paper's research shows that the magnetic field gradient (G) parameter is an important token of damage of structure. The criterion of normal magnetic gradient has been deeply investigated and the magnetic gradient meter has been developed in this dissertation. The main works of this dissertation summarized as follows.(1) The mechanism of signal of metal magnetic memory has been studied aim at ferromagnetic metallic component. This dissertation's study indicates that magnetic field gradient (G) is better than the parameter of normal magnetic field component whether crossing zero being used to token of damage of structure.(2) The variation of magnetic field and the leakage magnetic field of damage zone has been researched and calculated respectively basing on magnetic dipole model. The big magnetic dipole analysis method is derived. The damage zone has been simulated using big magnetic dipole model and the characteristic of the magnetic field out of the ferromagnetic component and inverse problem has been researched basing on this model.(3)The effect of the ferromagnetic components'shape on MMMT has been investigated and the application scope of MMMT has been analyzed. The emulational results shows that some shape components such as thin and long bars, pipelines and pressure vessels are fit for using MMMT and such as solid spherical and short cylindrical are not fit for using MMMT.(4) A magnetic field gradient detector using MMMT is developed basing on theory study and computation. This detector is different from present magnetic memory detectors .It has higher veracity and better spatial resolution without dolly of measuring displacement.(5)The damage test experiment has been accomplished based on low carbon steel. The result indicates that the distribution of leakage magnetic field gradient of low carbon steel is linear with the maximal tensile stress.