On the suitability of induction coils for crack detection and sizing in metals by the surface magnetic field measurement technique

The surface magnetic field measurement (SMFM) technique has proved to be an accurate means for crack detection and sizing cracks in ferrous metals. Th e technique involves the use of two U-shaped current-carrying wires of suff iciently high frequency while measuring the discontinuity in the resultant magnetic field at the crack edge with an appropriate magnetic field sensor. In this work, we describe a mathematical algorithm to obtain the crack sig nal from the output of an induction coil used in a SMFM probe. We also disc uss the measurement errors due to the coil size and shape. To reduce the me asurement errors, we present an algorithm in which the crack signal is reco vered by appropriate deconvolution of the coil output signal and its spatia l transfer function. The algorithm is then used to recover crack signals fo r various coil shapes and sizes. The study of the results demonstrates the effectiveness of the algorithm in the case of large coils. (C) 2001 Elsevie r Science Ltd. All rights reserved.