Some advances in numerical analysis techniques for quantitative electromagnetic nondestructive evaluation

In this paper, some progresses in numerical techniques mainly made in our research group for the forward and inverse simulation of electromagnetic nondestructive evaluation (ENDE) signals are introduced. For the first part, efficient forward analysis schemes for the simulation of eddy current testing (ECT), remote field ECT (RFECT) and magnetic flux leakage testing (MFLT) signals are described respectively, in addition to some numerical examples. Fast and accurate ECT signal simulation is realised by introducing a database type strategy using precalculated unflawed potential field data. To meet the high accuracy requirement of the simulation of RFECT signals, a hybrid scheme using 2D and 3D geometry and a new formula for pickup signal are proposed. To improve the efficiency of MFLT signal simulation, a fast scheme is developed based on a FEM-BEM hybrid code of polarisation method. In addition, a phenomenological method is also described in the first part, which is developed for the qualitative estimation of eddy current distribution and pickup signals. The second part of this review paper is on the reconstruction of defect from the detected ENDE signals (mainly ECT signals). Reconstruction schemes based on conjugate gradient (CG) method of deterministic category and NN method, metaheuristic methods of stochastic category are developed and sizing of both artificial and natural cracks are performed by using measured signals. It is clarified through applications that a deterministic optimisation method is more efficient for treating simple cracks, while a stochastic way is prefer for defects of complicated geometry such as a stress corrosion crack and multiple cracks. In the crack modelling and parameterisation, an element of discontinuous material property is introduced to treat crack of arbitrary shape based on a given regular mesh. Several numerical models are proposed for natural cracks, which makes the reconstruction of some natural cracks become possible.