Computational Modeling For Magnetic-Sensor-Based Three-Dimensional Visualization Of Microcracks
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Computational Modeling For Magnetic-Sensor-Based Three-Dimensional Visualization Of Microcracks Leonid Muratov, David Lederman, and Bernard R. Cooper West Virginia University, Morgantown, WV 26506-6315 ABSTRACT The presence of cracks, phase segregation, or even submicron-sized grain boundaries creates a disruption of the magnetic field response to an externally applied electrical current running through the material. These effects can be detected through the magnetic field leakage in the external near-surface region. Using a computer model of an array of magnetic tunnel junction detectors, magnetic “signatures” of various faults and/or material borders and domains have been calculated using finite element analysis and portrayed by icons. We have considered a number of typical cracks and flaws, of different dimensions and orientations, within the bulk of the component. The database of “signatures” thus generated allows fast recognition of faults and generation of their images in real time. Significant efforts have been made to provide an adequate three-dimensional visualization of the shape and distribution of microcracks, the magnetic field lines, and delineation of the position of the faults in relation to the surface. INTRODUCTION A major problem in aircraft maintenance is the effect of wear and tear on major components. For example, cracks emanating from bolt holes on structural components or turbine blades, located below the surface, appear after years of use. These cracks can cause catastrophic damage if they are not detected sufficiently early. Non-destructive evaluation (NDE) and identification (NDI) methods allow early detection of subsurface cracks and corrosion, so that the damaged components can be replaced or repaired. Current methods used for NDE have difficulties in detecting very small cracks, on the order of a tenth of a millimeter or less, which form below the surface. Our approach consists of fabricating an array of magnetic transistor detectors. The advantage of having an array over a single detector is that the magnetic leakage can be detected simultaneously at different points in space. By modeling the signals from all of the array components simultaneously (a generalized sort of triangulation), it is possible to obtain a three-dimensional image of the cracks and faults. This work focuses on a “proof of concept”, and considers the possibility of detecting a small fringing magnetic field originating from faults and distinguishing among different types of faults. MAGNETIC TUNNEL JUNCTION DETECTOR Magnetic tunneling junction (MTJ) technology is applicable to the present problem because of its great sensitivity in probing the magnetic response of the metallic components to an applied current. If the material is completely homogenous, the resulting magnetic signal is also homogeneous. However, in the presence of phase segregation, cracks, or even submicron-sized grain boundaries, there is a disruption of the normal magnetic flux pattern of the material, causing a leakage field to appear.1 This field
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