Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

# Methods of Controlling Lift-off in Conductivity Invariance Phenomenon for Eddy Current Testing

Version 1 : Received: 30 May 2020 / Approved: 31 May 2020 / Online: 31 May 2020 (18:37:15 CEST)

How to cite: Jin, Z.; Meng, Y.; Yu, R.; Huang, R.; Lu, M.; Xu, H.; Meng, X.; Zhao, Q.; Zhang, Z.; Peyton, A.; Yin, W. Methods of Controlling Lift-off in Conductivity Invariance Phenomenon for Eddy Current Testing. Preprints 2020, 2020050491 (doi: 10.20944/preprints202005.0491.v1). Jin, Z.; Meng, Y.; Yu, R.; Huang, R.; Lu, M.; Xu, H.; Meng, X.; Zhao, Q.; Zhang, Z.; Peyton, A.; Yin, W. Methods of Controlling Lift-off in Conductivity Invariance Phenomenon for Eddy Current Testing. Preprints 2020, 2020050491 (doi: 10.20944/preprints202005.0491.v1).

## Abstract

Previously, a conductivity invariance phenomena (CIP) has been discovered – at a certain lift-off, the inductance change of the sensor due to a test sample is immune to conductivity variations, i.e. the inductance – lift-off curve passes through a common point at a certain lift-off, termed as conductivity invariance lift-off. However, this conductivity invariance lift-off is fixed for a particular sensor setup, which is not convenient for various sample conditions. In this paper, we propose using two parameters in the coil design – the horizontal and vertical distances between the transmitter and the receiver to control the conductivity invariance lift-off. The relationship between these two parameters and the conductivity invariance lift-off is investigated by simulation and experiments and it has been found that there is an approximate linear relationship between these two parameters and the conductivity invariance lift-off. This is useful for applications where the measurements have restrictions on lift-off, e.g. uneven coating thickness which limits the range of the lift-off of probe during the measurements. Therefore, based on this relationship, it can be easier to adjust the configuration of the probe for a better inspection of the test samples.

## Subject Areas

Conductivity Invariance Phenomenon; Conductivity invariance lift-off; Sensor design; Eddy current testing; Electrical conductivity; Non-destructive testing

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