A new differential eddy current testing sensor used for detecting crack extension direction Peng Xu a,n , Songling Huang b , Wei Zhao b a Jiangsu Key Laboratory of New Energy Generation and Power Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China b State Key Lab of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing, PR China article info Article history: Received 3 August 2010 Received in revised form 15 January 2011 Accepted 26 January 2011 Available online 3 February 2011 Keywords: Eddy current testing Crack extension direction Circumferential gradient winding spiral coil Differential sensor abstract In this paper a new differential eddy current testing sensor is proposed which is composed of two planar circumferential gradient winding spiral coils. The eddy current testing sensor can detect out the existence of crack in conductive material surface and furthermore the crack extension direction because of the circumferential gradient winding spiral structure of coil. We study the detection principle of crack extension direction and carry out experimental test. The test results show that the impedance of circumferential gradient winding coil mainly varies with crack extension direction and width. Based on impedance plane analysis we extract the features which can indicate the crack characteristics and study on the inverse quantitative evaluation of crack extension direction and crack width. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Eddy current testing (ECT) as a non-destructive testing (NDT) method has been rapidly developed in past several decades. Based on the principle of electromagnetism eddy current tests can be made on all electrically conducting materials [1]. An important application of ECT is to detect surface and sub-surface cracks [2–5]. Usually the ECT sensor implements scan in 2 dimen- sions for test area, which is operated by an X–Y positioning system [6]. Then the measurement results including various crack information are obtained. Based on the inverse non-destructive evaluation (NDE) methods like time-frequency analysis and impedance analysis, the crack characteristics like position, size, shape, depth and extension direction can be quantitatively estimated [5,7–9]. Various planar coils, such as round coil, rectangle coil, mean- der coil and mesh coil, have been studied in past studies [10–14] and these coils can be called even winding coil. We have proposed the gradient winding and studied on the straight gradient wind- ing coils in our past researches. The studies show that the differential ECT sensor composed of double straight gradient windings can detect out the location of a surface crack in 2D area by implementing line-scan. Comparing with the point-scan detec- tion, the line-scan detection can reduce measurement and advance test efficiency [15,16]. In this paper firstly based on the novel structure of gradient winding a new planar circumferential gradient winding coil is proposed. Then a new differential ECT sensor with two planar circumferential gradient windings is proposed. We study the detection principle of crack extension direction using the new planar ECT sensor which is different from the detection principle of crack extension direction using the ECT sensor with two cross- coils vertical to test surface [17]. Finally we study on the inverse quantitative NDE method of crack extension direction based on the impedance plane analysis [18,19]. The crack extension direc- tion as a key NDE characterisation can be applied to predict the structural stress in material and monitor the status of material. 2. Eddy current testing principle The equivalent circuit of eddy current testing is shown in Fig. 1. Based on Kirchhoff’s voltage laws, two Eqs. (1) and (2) are obtained. R 0 _ I þ j 2p fL 0 _ I j 2p fM _ I e ¼ _ U , ð1Þ R e _ I e þ j 2p fL e _ I e j 2p fM _ I ¼ 0 ð2Þ where f is the excitation frequency of coil; R 0 , and L 0 are the resistance and inductance of coil, respectively; R e , and L e are the resistance and inductance of the induced eddy current loop, respectively; and M is the mutual inductance between the two loops. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2011.01.010 n Corresponding author. Tel.: +86 15905180715. E-mail addresses: xupeng@nuaa.edu.cn, noonmoonwing@yahoo.com.cn (P. Xu). NDT&E International 44 (2011) 339–343