Unexplored Physical Phenomena in Pulsed or Transient Eddy Current NDE

Abstract

Electromagnetic eddy current testing is a well recognised and widely used non-destructive evaluation (NDE) technique for detecting subsurface defects, thickness and conductivity measurements, corrosion studies etc., in conducting structures over several decades. The eddy current NDE techniques operated in frequency domain or in time domain are based on the eddy current induction in the target which is under investigation and acquiring its response. In both cases, the eddy current induced in the target is due to the physical phenomenon of Faraday’s Law of electromagnetic induction and the mode of measurement is different. Since from the beginning of the discovery of eddy current, there are only two magnetic fields such as primary and secondary magnetic fields have been playing a role in eddy current NDE. Here, it is proposed that the contribution of an additional magnetic field in the process of eddy current induction is the tertiary magnetic field and it has been explicitly observed through transient eddy current NDE measurements. The proposed novel hypothesis for the generation of tertiary magnetic field has been experimentally verified through specially designed pulsed eddy current NDE experiments using suitable targets. The proposed hypothesis explains the existence of self induced tertiary magnetic field in the absence of the target which is altered by not only with the target and also with lift-off. In this work, a set of NDE measurements have been carried out with targets of different thicknesses. The response of the tertiary magnetic field has also been observed by repeating the experiments with targets located at different lift-offs. The experimental data and subsequent analysis confirmed the proposed hypothesis that the contribution of an additional tertiary magnetic field in transient or pulsed eddy NDE measurements is responsible for the observation of peculiar PEC signal for the lift-off variations and it must be taken into account while analysing the data. This observation pave the way to develop novel methods on sub-surface defect detection, thickness measurements, corrosion studies, etc. in conducting structures, particularly on the most commonly used structural materials by various industries.