A new approach to increase the subsurface flaw detection capability of pulsed eddy current technique

•Variable excitation rise time circuit is designed to detect subsurface flaws.•Modified inductor current equation is fitted to the PEC response signals from GMR.•Signal parameters, time constant and voltage ratio are extracted from the fit.•These signal parameters unambiguously differentiate subsurface and surface flaws. In this paper, the detection of subsurface flaws by pulsed eddy current (PEC) technique has been studied. Experiments have been carried out on an 8.0 mm thick stainless steel plate with subsurface flaws located at different depths below the surface by varying the excitation rise time (Er). The conventional PEC signal parameters like peak amplitude (Vp), time-to-peak (Tp) and rise time (tr) are influenced by noise and could not be effectively used to detect flaws located beyond 4.0 mm below surface. In this context, a new approach based on fitting the PEC response pulse to a modified inductor current equation has been proposed. Based on this approach, two parameters viz. time constant (τ) and voltage parameter (V1/V0) have been derived and used for detection and separation of flaws. The study shows that the two parameters derived from the proposed approach can be used to detect flaws located 6.0 mm below the surface with improved sensitivity. The advantage of the proposed approach is that, it does not require subtraction from reference signal to detect flaws. The derived parameters are also able to differentiate the subsurface and surface flaws.