Insight Vol 51 No 1 January 2009 21 PEC A 3D visualisation of transient magnetic ield mapping via inite element simulation for the characterisation of complex defects is presented in this paper. In this work, we analyse the Pulsed Eddy Current (PEC) testing application on detection and characterisation of complex geometry defects, for example angular slots, through the mapping of magnetic ield distribution. The investigation is implemented via time- stepping 3D Finite Element Analysis (FEA) with features from the mapping extracted for angular slots characterisation. The results indicate the potential static or dynamic features for complex geometry defect characterisation based on the visualisation and mapping of the magnetic ield distribution. It is expected that the simulation and magnetic ield distribution will help in sensor (array) design and inverse models for complex geometry characterisation. Keywords: Transient magnetic ield mapping, Pulsed eddy current, Complex geometry defects, Finite element analysis 1. Introduction Eddy current testing (ECT) is one of the Non-Destructive Testing (NDT) methods which work on electromagnetic principles and is widely used on electrically conductive samples. It allows the inspection of surface-breaking as well as subsurface discontinuities, due to the eddy current distribution in the samples. The excitation frequencies directly affect the probability of detection of subsurface discontinuities due to the skin effect [1] . In ECT, the excitation coil driven by an alternating current generates an alternating magnetic ield which induces eddy currents in conductive materials. In contrast to most eddy current (EC) techniques which use continuous sinusoidal waveforms, Pulsed Eddy Current (PEC) uses transient waveforms for their coil excitation. The wideband pulse consists of a series of frequency components leading to richness of information gathered about the defect [2-4] . For the purpose of unveiling the electromagnetic phenomena underlying the PEC systems, theoretical study has been conducted for several years and began with analytical modelling for time- harmonic on layered structures [5] , followed by numerical simulations using finite element analysis (FEA) [6, 7] . After the time-stepping solver was introduced in FEA, 2D and 3D FE simulations became prevalent in solving PEC problems. Li et al provided analytical and FEA models to predict the eddy current response and variation of magnetic field to flawed specimens such as conductors with corrosions or inclusions when the inspection probe moves over the samples [6] . To the knowledge of the authors, the analytical modellings for EC and PEC are mostly focused on specimens with simple geometries such as stratified conductors [8] or samples with slots [9] . One significant advantage of FEA over analytical modelling [10, 11] lies in the fact that it is intricate for analytical approaches to predict magnetic field interaction with complex geometrical defects which precludes closed-form analytical approaches [12] . In this paper, to analyse the PEC testing application towards complex geometrical defects, ie angular slots, a time-stepping method using Finite Element Analysis (FEA) was applied to a 3D transient problem. In this time-stepping method, accurate time- domain results can be obtained by using adequate short time steps for the pulse waveform excitation. We have taken the forward approach of characterising the complex geometry defects, based on the visualisation and extracted features of the resultant magnetic field mapping from the interaction between the eddy current and the defects in the sample. In particular, the potential of 3D visualisation of magnetic field distribution for complex geometry defects characterisation is explored. This paper is organised as follows: Section 2 presents the model set-up for the PEC simulation; Section 3 presents the 3D visualisation of the results and feature extraction; Section 4 gives the conclusion of this work. 2. Simulation model and set-up In the Electromagnetic Non-Destructive Evaluation (ENDE) ield, it is essential to build the relationship between magnetic ield distribution and different defects including 3D shape, size and location, which facilitates not only forward problems but also the inverse process involving sensor array coniguration, pattern recognition, defect quantiication and reconstruction of 3D defects. Consequently, a series of numerical modelling functions with regard to temporal magnetic ield distribution under complex defects are conducted. In the simulation, the coil used has the dimensions of 14 mm inner diameter, 16 mm outer diameter, 1 mm height and 1000 turns of wire, and is current driven with a rectangular waveform of 1 A amplitude to generate a varying magnetic field. Figure 1 illustrates the excitation current input waveform used to drive the coil. Aluminium samples with long angular slots of depth 5 mm DOI: 10.1784/insi.2009.51.1.21 3D transient magnetic ield mapping for angular slots in aluminium I Mukriz, G Y Tian and Yong Li Ilham Mukriz, Gui Yun Tian and Yong Li are with the School of Electrical, Electronic and Computer Engineering, Merz Court, Newcastle University, Newcastle Upon Tyne NE1 7RU, UK. Tel: +44 (0)191 222 5639; Email: i.m.zainal-abidin@ncl.ac.uk / g.y.tian@ncl.ac.uk / yong.li@ncl.ac.uk Figure 1. Coil excitation current input waveform Submitted 15 August 2008 Accepted 5 December 2008