This thesis reports a comprehensive literature survey on Electromagnetic Non-destructive
Evaluation (ENDE) and the investigation of forward problems and inverse problems of
Sweep-frequency and Pulsed Eddy Current NDE using numerical and analytical
modelling in collaboration with an experimental approach.

Firstly, Finite Element simulations have been conducted to (1) evaluate and assess the
commercial Finite Element simulation packages, COMSOL and ANSOFT MAXWELL
EM in terms of feasibility for simulation of ENDE forward problems including Eddy
Current and Magnetic Flux Leakage; (2) apply Finite Element modelling as a
comparative approach to verify the proposed analytical methods for forward models and
inverse schemes.

In parallel with the numerical simulations, much more effort is put into implementation of
efficient magnetic-field-based Eddy Current modelling, particularly for Sweep-frequency
Eddy Current and Pulsed Eddy Current inspections on multilayered structures. The
Extended Truncated Region Eigenfunction Expansion (ETREE) method has been
proposed and verified. The formulation of closed-form expressions, which are in series of
expansions of eigenfunctions, is useful to rapidly predict signals from solid-state
magnetic field sensors rather than the traditional impedance signals measured using
induction coils, as the dimension of sensors is taken into account in the model.
Based on ETREE models, the analytical expression of Lift-Off Intersection (LOI)
occurring in eddy current inspection is formulated. This is adopted to investigate the
characteristics of LOI and set up the database for inverse schemes. Following that, the
inverse schemes for the estimation of lift-off of probes and conductivity of samples have
been implemented by finding the measured LOI in the established database which
comprises the relations of variable lift-off as well as conductivity with the corresponding
LOI. The results from Finite Element simulations and experiments have proved the
validity of the analytical modelling results as well as the proposed inverse schemes.

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