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

Multi-frequency Eddy Current (EC) techniques such as Pulse Eddy Current (PEC) and rectangular EC techniques have gained widespread adoption for non-destructive tests of conductive materials. However, in the case of mild steel, the harmonics induced by a square wave excitation field can be affected by the nonlinear frequency-dependant magnetization. In this work, we study the harmonic magnetic responses induced by artificial slits on a mild steel plate upon a square wave excitation field. The result reveals that the harmonics' percentage increment is increased slightly, suggesting the effect of the mild steel's B-H characteristic. The harmonic ratio of the real and imaginary components can be used to evaluate the slit depth in the mild steel plate with improved sensitivity and immunity to the lift-off effect.