Excitation with phase shifted fields-enhancing evaluation of deep cracks in eddy-current testing Ladislav Janousek * , Zhenmao Chen, Noritaka Yusa, Kenzo Miya International Institute of Universality, Imon Ikenohata Bldg 7F, 2-7-17 Ikenohata, Taito-ku, Tokyo 110-0008, Japan Received 10 July 2004; revised 5 January 2005; accepted 24 January 2005 Available online 17 March 2005 Abstract This study enhances the applicability of eddy current testing to the evaluation of deep surface-breaking cracks in the front face of thick structures, while keeping the advantages of higher frequency inspection. The key idea is to suppress eddy currents on the surface of a test- piece, and thus realize the deeper penetration of eddy currents. Based upon this idea, a mutual induction eddy current testing probe, consisting of four coaxial rectangular tangential exciter coils and a pancake pick-up coil, has been designed. The exciters are driven by AC currents with different phases and amplitudes, which contrive to emulate the desired behavior. Numerical simulations and consecutive experimental verifications are presented in the paper. It is revealed that when the new probe is used for inspections under a frequency of 10 kHz there is a 20% difference between signals due to an electro-discharge machined (EDM) notch of 15 mm depth and those due to one of 20 mm depth. On the other hand, a conventional probe driven with the same frequency gives only 1% difference between the signals of the two EDM notches. q 2005 Elsevier Ltd. All rights reserved. PACS: 06.60.Mr; 07.07.Df; 07.05.Tp; 41.20.-q Keywords: Eddy current probe; Excitation with phase shifted fields; Deeper penetration of eddy currents; Evaluation of deep defects 1. Introduction Eddy current testing (ECT) is a non-destructive inspec- tion method applied to conductive materials. Numerous advantages such as high sensitivity, rapid scanning, contact- less inspection, and versatility contribute to its wide utilization. With the evolution of computer systems and numerical methods, the interest in ECT has grown as problems that had been difficult to solve practically can now be dealt with by numerical means [1]. It is presently possible to very precisely and rapidly perform forward numerical simulations [2–4] concerning different applications of the ECT. Defect characterization and evaluation, even for stress corrosion cracks [5] has become feasible by solving the inverse problem using computational physics [6–9]. This ability has made ECT a sophisticated method able to satisfy the particular demands connected with inspections. In spite of its very useful advantages, ECT has a particular drawback that indeed originates from its under- lying principles. A nearby applied alternating electromag- netic field creates eddy currents in a conductive test-piece. A field produced by the eddy currents is opposite to the exciting field and thus attenuates it. Therefore, eddy currents are quite dense at the surface and decay exponentially with depth in the material being tested. This is the well known skin-effect [10]. The defect signal gets saturated from the increasing depth of a defect due to the skin-effect. There- fore, evaluation of a deep defect is quite difficult when the defect is deeper than a certain value. In general, a lower frequency excitation provides deeper penetration of eddy currents; however, very low frequency driving brings additional problems such as a decreased signal to noise ratio, a smaller phase separation of the crack signals, and the necessity to utilize magnetic sensors, not inductive coils. Since a true penetration of induced eddy currents is affected not only by the frequency, conductivity, and permeability of a test-piece but also by the probe dimensions [10,11], some restricted improvement might also be obtained by optimiz- ing the exciting coils [12]. For instance, tangential coils induce eddy currents that penetrate deeper than pancake NDT&E International 38 (2005) 508–515 www.elsevier.com/locate/ndteint 0963-8695/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ndteint.2005.01.012 * Corresponding author. Tel.: C81 3 5814 5350; fax: C81 3 3827 0682. E-mail address: janousek@iiu.co.jp (L. Janousek).