A method of magnetic scanning imaging for detecting defects in ferromagnetic materials

The paper presents a novel magnetic scanning imaging method for detecting defects in ferromagnetic material, which uses a small permanent magnet to magnetize the ferromagnetic material and measures the single vertical components of magnetic flux densities by bi-reverse-connection Hall sensors. The measured signal is the magnitude of the vertical component of the magnetic flux density rather than the time derivative of total magnetic fluxes. The detection mechanism is researched by finite element simulation for a three-dimensional transient electromagnetic field. The magnetic flux densities produced by the eddy currents generated by a permanent magnet moving over the ferromagnetic material at a speed of less than 1 m s−1 are also simulated, and the results show that the magnetic flux densities produced by eddy currents are very small and can be ignored. The permanent magnet probe and the magnetic scanning imaging system for detecting defects in ferromagnetic material are designed and developed, in which wavelet algorithms are used to enhance the image's quality. Small crack imaging and forged number imaging of a vehicle's engine are realized. Experiments show that this method is simple, suitable for detecting small defects and able to provide shape information on the defects because of its high sensitivity and spatial resolution.