AC fields around short cracks in metals induced by rectangular coils

In the alternating current field measurement method, information about the magnetic field distribution above the metal surface is used to detect and size surface-breaking cracks in metals. In this measurement, the incident field is produced by a set of wires carrying a high-frequency current. This paper presents a mathematical technique for the theoretical prediction of the distribution of the magnetic field around a short crack due to a current-carrying rectangular coil located horizontally above the metal surface. The crack is assumed to have a profile that can be approximated by a rectangular boundary. The mathematical technique is developed and its experimental verification is discussed. The technique, an extension of one recently reported by the authors, is accurate and computationally efficient. The effects of crack length and depth on the crack signal, obtained theoretically, are illustrated and discussed