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

Abstract

<jats:p>&lt;p&gt;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.&lt;/p&gt;</jats:p>

Lift-Off Effect Evaluation by Using Eddy Current Testing Technique on Copper (C101)

“…Lift-off is well-known as the limitation of eddy current signal because its strong signals can mask the defect. However, lift-off can also be beneficial since it can be used to determine the thickness of nonconductive materials, conductivity, and permeability of the test piece [25] [26]. Eddy Current Testing (ECT) is highly affected by the impedance change that occurs when the distance between the excitation coil and the material surface changes.…”

“…In addition, the samples' thickness and the test materials' signal frequency also influence the lift-off variation [26], besides the conductivity invariance lift-off exists for the non-magnetic conductive and ferromagnetic materials. The test material for this research experiment was chosen based on its application used in the industry and the physical properties, e.g., conductivities and permeabilities, based on their material data sheet.…”