Investigation of lift-off-intersection characteristics and lift-off derivation in eddy current non-destructive evaluation using magnetic field camera

摘要

Lift-off intersection (LOI) has gained much attention in the field of eddy current evaluation due to its intrinsic characteristics of independence of lift off variation, and thus been regarded as a prominent feature in eddy current inspection on the integrity of conductive specimens under inspection. This paper dilates the ongoing research of LOI to investigate its dependence of eddy current configurations, and the potential utilisation in eddy current inverse process. The generalised analytical equations of LOI are formulated for eddy current evaluation of a stratified conductor and a conductive half-space. From the expression LOI has been found highly dependent on the configuration of eddy current probe apart from the lift-off. The conductivity of the sample and the lift-off can be inversely obtained by adopting LOIs found from magnetic field signals after introducing a predefined lift-off increment. Based on this, an inverse scheme for acquisition of the conductivity of a half-space and the lift-off has been proposed. The experimental study using magnetic field camera is subsequently conducted to corroborate the inverse scheme. It has been found that by using the inverse scheme, the unknowns, namely the conductivity of a sample being inspected and the lift-off of the eddy current probe can be estimated with acceptable discrepancy.