Surface potential distributions due to eddy currents around long cracks in metals, induced by U-shaped current-carrying wires

Sizing of surface-breaking cracks in metals by the AC field measurement (ACFM) technique by producing the surface field by inducing current into the specimen under test is considered. The surface potential distribution produced by an inducing mechanism around a long crack in a ferrous metal is formulated, and the crack signal, which is the variation of the potential difference around the crack, is investigated by varying the crack depth, the dimensions of the inducer, and the inducer´s lift-off distance from the metal surface. The inducer consists of two parallel U-shaped wires symmetrically attached to the two sides of an ACFM probe. It is shown that the direct application of the one-dimensional formula based on the uniform field assumption for the interpretation of the crack signal gives rise to an underestimation of the crack depth. However, it is shown that the error in depth measurement by this formula can be negligible if the horizontal sections of the inducer´s wires are sufficiently long