Research on Sensing Shot Peening Residues Using Different Responsivity Between Magnet and Nonmagnet in Super Low-Frequency Eddy Current Testing

During the machining of castings, shot peening is often used for surface strengthening. However, a portion of metal shots tend to remain in the internal cavity of casting and disable the workpiece. In order to detect the shot peening residues in conductive castings, we researched the eddy current testing (ECT) theory and derived out that: in the inspection of a double-coil probe, at a super low-frequency (SLF) excitation, the conductivity of the detected does not affect the imaginary part of system’s trans-impedance, while the imaginary part is very sensitive to its permeability, namely, there is a distinct responsivity difference between magnet and nonmagnet. Subsequently, the research verified this derivation by experiments and simulations, and we believe that the conclusion is valid when the frequency is less than 1 kHz. Therefore, this article proposed a flexible sensor based on this different low-frequency responsivity, and it could efficiently and consistently find out the steel shot residues from aluminum alloy tubes, which is not influenced by the changing distance from the probe to the wall of the tube. Also, a reliability analysis of the sensing system was conducted, as well this article proved the necessity and rationality of our design with controlled experiments. Last but not least, the parameter solution of double-coil ECT at SLF is derived out for the first time, which can be further applied to the wilder field.