Flat Metal Surface Defect Perturbation Of The Magnetic Field Distribution

The Alternating Current Field Measurement (ACFM) technique is an electromagnetic non-destructive testing (NDT) technique used to detect and size surface cracks in electrically conducting, metallic components. In this paper, the status and development home and abroad of ACFM is illustrated, And then based on its characteristic and principle, the mathematical modeling of the electromagnetic field in the neighbourhood of a surface crack is precisely built up. Considering the motion of exciting coil, the finite element numerical simulation analysis of the distribution of the perturbed magnetic is accomplished and far-field Element is adopted to dispose a three-dimensional unbounded field problem. Compared to the analysis of coil impedance introduced by conventional eddy current testing ( ECT), the distributing characteristic of the magnetic field component is capable to identify cracks more accurately. The simulationresults show that the length of crack is estimated by amplitude peculiarity of B,, the component of magnetic field perpendicular to metal surface. The relation between the depth of crack and Bx ,the tangential field component perpendicular to induced current inside the conductor, iscorresponding, it is described by function curve of Bx based on the flaw depth, this conclusionis available to measure the depth quantitatively. Meanwhile, in order to improve the result of numerical calculation, the method of sliding mesh is applied to mesh the important field invariably. And implement it in ANSYS software. Finally, According to idea of realizing quantitative measurement by means of magnetic field, the magnetic field sensors which can accomplish the accurate "point" measurement of magnetic field is designed. The results of experimental measurement verify all the conclusions from FEM numerical simulation analysis. These can be used as the theoretic base to achieve quantitative measurement of surface cracks.