Quantitative evaluation of deep-shallow compound defects using frequency-band-selecting pulsed eddy current testing

Deep-shallow compound defects (DSCD) may occur in both single-layer and multi-layer structures and commit great loss of structural mechanical strength. Their quantification, especially in depth, is hence imperatively required for guaranteeing the integrity and safety of engineering structures. In this paper, the defect parameters of DSCD are identified by frequency-band-selecting pulsed eddy current testing (FSPECT), and a strategy of component separation is proposed. The high-frequency component is separated from the FSPECT responses for the quantification of shallow defects so that the parameters of deep defects in DSCD can be reconstructed, which is the ultimate objective of FSPECT method. Finite element analysis (FEA) was conducted on the single-layer structure to preview the feasibility of the proposed method and to highlight the signal feature of better sensitivity. Subsequently, validation experiments were implemented on the double-layer structure and identified the features suitable for the shallow and deep defects. Besides, error propagation of the proposed method was also discussed. Results from FEA simulations and experiments show that (a) the strategy of component separation enables the quantitative evaluation of DSCD, (b) peak value (PV) should be chosen for the quantification of shallow defects while for the identification of deep defects time to zero-cross (TZC) is suggested, and (c) the deep defect can still be characterized despite the little error in the estimation of shallow defects.