Defect Evaluation in Mild Steel Plate Using Harmonic Ratio Induced by Square Wave Excitation Field and Nonlinear Magnetization

Similar to multi-frequency techniques in eddy current (EC) and magnetic flux leakage (MFL) testings, the harmonic evaluation technique using a square wave excitation field improves the defect detection performance of conductive materials compared to single-frequency techniques. However, in the case of mild steel, its nonlinear frequency-dependent magnetization impacts the intensity and phase of the induced harmonics. In this work, we propose a harmonic ratio technique using a square wave excitation field to enhance defect detection in mild steel. A custom-made probe with a planar differential tunnel magnetoresistance (TMR) sensor and inductor arrays was developed to assess the harmonic responses induced by artificial slits on a 12 mm mild steel. The harmonic dependency on the slit depth, excitation frequency, and intensity was evaluated. The results showed that the ratio of the two highest harmonics exhibited a correlation with slit depth, accompanied by a significant reduction in baseline signal drift as compared to the single-frequency detection mode. It was shown that the proposed harmonic ratio technique, leveraging a square wave excitation field, significantly enhances sensitivity and immunity to background signal drift, thereby improving defect detection capabilities in mild steel.