A magnetic flux leakage (MFL) method was applied to detect damage for inspecting the steel bar. A multi-channel MFL sensor head was fabricated using Hall sensors and permanent magnets. The MFL sensor head scanned the damaged specimen that have 5 levels of depth to measure the magnetic flux density. A signal processing process including the enveloping process based on Hilbert transform was performed to clarify the flux leakage signal. The enveloped signals were then analyzed for objective damage detection by comparing with the threshold value. For improvement of quantitative analysis, new damage indexes that utilize the relationship between the enveloped MFL signal and the threshold value were additionally proposed. By using the proposed damage indexes and the general damage index for the MFL method, the detected MFL signals were quantified and analyzed according to the size of damage increase.
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