GOTOH Y ET AL: 'Study on problems in detecting plural cracks by alternating flux leakage testing using 3-D nonlinear Eddy current analysis', IEEE TRANSACTIONS ON MAGNETICS, IEEE SERVICE CENTER, NEW YORK, NY, US, vol. 39, no. 3, May 2003 (2003-05-01), pages 1527 - 1530, XP011096478, ISSN: 0018-9464