Extraction of LOI Features From Spectral Pulsed Eddy Current Signals for Evaluation of Ferromagnetic Samples

Different applications of pulsed eddy current (PEC) testing have been reported in safety detection due to its characteristics of high sensitivity, non-contact, and rich information. In the determination of metal thickness by PEC testing, the liftoff effect commonly gives rise to inaccurate measurement. As a desirable signal feature immune to liftoff effect, liftoff point of intersection (LOI) is a common point where PEC signals intersect when only the liftoff distance changes. However, for ferromagnetic samples, a LOI point could not be obtained directly from the PEC signals when the liftoff distance changes. This paper investigated spectral PEC signals of ferromagnetic metal plate by numerical simulations and experiments. Excitingly, a frequency LOI was first observed in spectral PEC signals for ferromagnetic samples, and the behavior of LOI was further analyzed when sample conductivity, permeability, and thickness change. This paper concludes that the LOI in spectral PEC signals is affected by sample conductivity, permeability, and thickness. In addition, the frequency LOI allows for accurate evaluation of ferromagnetic samples with the variation of liftoff distance.