A Twice Subtraction Method for Obtaining LOI in Pulsed Eddy Current Signals of Ferromagnetic Samples

Abstract

Lift-off point of intersection (LOI), as a desirable signal feature independently of the lift-off effect, is expected to be applied in pulsed eddy current (PEC) testing of ferromagnetic samples. The time-domain LOI in PEC signals is easily recorded and easier to be extracted for evaluating ferromagnetic samples, compared with the frequency domain LOI in spectral PEC signals. However, the time-domain LOI cannot be obtained in the PEC signals of ferromagnetic samples. In this article, a twice subtraction method was proposed to obtain an LOI feature in PEC signals of ferromagnetic samples for evaluating the sample thickness, which is verified through simulations and experiments. In the twice subtraction method, the first subtraction is performed between PEC signals from the sample and air to obtain a difference in the PEC signal. The secondary subtraction is implemented between the difference in the PEC signal and its standard deviation. As a result, an LOI can be extracted from the twice subtraction signals when only lift-off distances are introduced. The results demonstrate that the twice subtraction method is a feasible program to obtain an LOI in twice subtraction signals of ferromagnetic samples, and the obtained LOI has the potential to be used to measure the thickness of ferromagnetic samples without lift-off effect.