Detecting internal defects of a steel plate by using low-frequency magnetic flux leakage method

Steel is an important material in modern society. It is widely used in our lives, such as in buildings, bridges, and airplanes. We previously reported an analysis method for examining the internal defects of metal structures with eddy current testing (ECT) over a wide range of frequencies. However, internal defects in steel plates are difficult to detect using ECT. In this paper, we present a low-frequency magnetic flux leakage (MFL) method for the nondestructive evaluation of internal defects in steel structures, and an analysis and imaging method that uses the phase of a magnetic field to examine internal defects. By analyzing the phase change in the detected signal, the internal defects can be detected with no magnetic fluctuation. The phase map of the measured magnetic field exhibits good correlation with the simulation results. The two-dimensional magnetic phase map can be used to visualize the internal defects. This new method has been validated through both experiment and simulation.