Pulsed eddy current imaging and frequency spectrum analysis for hidden defect nondestructive testing and evaluation Yunze He a,b,n , Mengchun Pan a , Feilu Luo a , Guiyun Tian b a College of Mechatronics and Automation, National University of Defense Technology, Changsha 410073, PR China b School of Electrical, Electronic and Computer Engineering, Merz Court, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, UK article info Article history: Received 25 September 2010 Received in revised form 10 January 2011 Accepted 13 January 2011 Available online 2 February 2011 Keywords: Hidden defect Pulsed eddy current Imaging technique Frequency spectrum analysis Defect classification Defect quantification. abstract Hidden defect characterisation in some complex structures is difficult. Pulsed Eddy Current (PEC) imaging based on rectangular excitation coil is investigated in this paper and hidden defect nondestructive testing and evaluation (detection, classification, and quantification) is carried out based on the various C-scan images. Experimental results have illustrated that hidden defects can be identified effectively by particular character in C-scan imaging results and sub-surface defects can be discriminated to correct class by selecting the rising time from response in time domain. The quantification information of hidden defects is preliminarily obtained based on the contour and 3D images. In addition, PEC imaging and frequency spectrum analysis are effective to detect, classify, and evaluate the sub-surface defects under the influence of edge effect of specimen. To sum up, PEC imaging is an effective approach to characterise hidden defects and sub-surface defects. Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved. 1. Introduction It is difficult to detect and evaluate hidden defects and sub-surface defects in many manufacturing processes and nondestructive testing and evaluation (NDT&E) applications. Imaging technique can get the shape of a hidden defect and is more intuitive than traditional non- destructive testing methods. In recent years, some eddy current (EC) imaging methods are investigated to detect the defects in metal structures, such as Magneto-optical imaging [1,2], multi-frequency eddy current imaging [3], transient eddy current imaging [4], low-frequency eddy current imaging [5], and pulsed eddy current imaging [6]. In all these methods, pulsed eddy current (PEC) testing can be used not only to measure the parameters of metal, such as thickness and conductivity [7,8], but also in defect nondestructive testing and evaluation [9,10], such as crack measurements, crack depth estimation, and crack reconstruction [11–13]. In defect char- acterisation, PEC testing has shown many advantages over conven- tional eddy current testing, including more extended detection depth, richer information in frequency domain, higher robustness of anti- interference, and lower power consumption [14–16]. In PEC testing, the probe usually comprises pick-up units and driving units. The pick-up units detecting the magnetic field disturbed by presence of a defect [17] are various in detection principle and structure, such as a differential coil [16,18], a three-dimensional coil [19], a magneto-resistive sensor [20], a giant magneto-resistive (GMR) sensor [21–23], a Hall-effect sensor [24–27], or a super- conducting quantum interference device (SQUID) magnetometer [28]. The driving unit inducing eddy current is usually a cylindrical coil. In contrast to cylindrical coil, rectangular excitation coil can induce uniform eddy current in specimen [29,30] and has been widely used in alternating current field measurement (ACFM) technique [31–33]. In 2006, rectangular excitation coil has been investigated and used in PEC testing and the experimental results have indicated that it is capable of detecting and evaluating defects [34–36]. Therefore, the main objective of this paper is to investigate PEC imaging based on rectangular excitation coil for the purpose of hidden defect detection and evaluation. The rest of the paper is arranged as follows. First, experimental set-up is shown in Section 2. Next, the PEC imaging based on rectangular excitation coil is introduced in Section 3. Then, the nondestructive testing and evaluation of hidden defects based on C-scan imaging is carried out in Section 4 and the proposed PEC imaging and frequency spectrum analysis are used for defect characterisation under the edge effect of specimen in Section 5. Finally, conclusions are outlined in Section 6. 2. Experimental set-up The PEC experimental set-up in this work is a PC-based system, with the key elements contained in an industrial control Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter Crown Copyright & 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2011.01.009 n Corresponding author at: College of Mechatronics and Automation, National University of Defense Technology, Changsha 410073, PR China. Tel. + 86 13467698133. E-mail addresses: hejicker@163.com, hejicker@gmail.com, y.he2@newcastle.ac.uk (Y. He), pmc_nudt@vip.163.com (M. Pan), flluo@nudt.edu.cn (F. Luo), g.y.tian@ncl.ac.uk (G. Tian). NDT&E International 44 (2011) 344–352