A Novel Eddy Current Testing Scheme by Transient Oscillation and Nonlinear Impedance Evaluation

Eddy current testing technique is widely used in nondestructive testing area. A novel eddy current testing scheme by transient oscillation and nonlinear impedance evaluation is proposed in this paper. A sensing coil with a ferrite core is used as the eddy current probe. The probe is connected in series with a capacitor and a simulated negative resistor to establish a second-order circuit. A square-wave voltage signal is utilized as the excitation source. By adjusting the value of the negative resistor within an appropriate range, the response signal will exhibit free damped oscillations within the transient durations. The transient oscillations are sampled as the response signal and first processed by singular spectrum analysis to remove noises. Then, Hilbert transform is further used to calculate the instantaneous frequency and the instantaneous damping coefficient of the transient oscillation. After that the nonlinear impedance carrying the defect information of the specimen under test can be finally obtained. Subsurface defects of 304 stainless steel specimens are experimentally detected. The results indicate that the proposed scheme is competent to detect subsurface defects. An interesting phenomenon is that the impedance nonlinearity of the probe-specimen system can be extracted from the transient oscillations. The nonlinear impedance causes critical damping in theory. Therefore, an online defect detection can be implemented by simply measuring the envelope of the transient oscillations.