Numerical Modeling Study Of Magnetic Flux Leakage Testing Magnetized By Helmholtz Coil

Helmholtz coils consist of two identical coils at a distance the size of a single coil's radius. As a result, the coils generate uniform and linear magnetic fields in the axial direction between them. In this study, the magnetic fields generated by Helmholtz coils were used to magnetize a ferromagnetic rod to investigate magnetic flux leakage (MFL) signals due to defects in the rod. The flux leakage signal is the result of the magnetic flux density components flowing perpendicular to the Hall sensor. These signals were calculated and analyzed through finite element modeling. Results showed that the levels of the overall signals differ depending on the axial position rather than the size of the defect, as defects of the same size were evaluated. Investigation into the cause of this difference provided a correction method, the application of which then resulted in a nearly identical signal pattern. This study, established the necessity of measuring the defect signal with the defect positioned at the center of the Helmholtz coils in order to estimate its exact size. Defect signals from various defect depths and widths were also investigated. It was concluded that although the variation of defect depth was reflected proportionally on the MFL signal, the variation of defect width was not. It was also found that depth change is reflected more sensitively on the signal than changes in width, despite uniform defect volume. The use of Helmholtz coils produced nearly identical signals as those produced by ordinary coils.