An Efficient Modeling Technique for Analysis of AC Field Measurement Probe Output Signals to Improve Crack Detection and Sizing in Cylindrical Metallic Structures

The paper proposes an efficient modeling technique for predicting the output signal of an AC field measurement (ACFM) probe when encountering a fatigue crack in a cylindrical metallic structure. The proposed technique is used to determine the electromagnetic fields around an arbitrary-shape surface crack in a cylindrical metal, induced by a high frequency alternating-current-carrying coil of arbitrary shape. The modeling technique involves two steps. First, the finite-difference method is used to solve the governing Laplace equation inside the crack region. Then, the normal component of magnetic field at the crack opening on the metal is obtained to derive a closed-form solution in the Fourier domain for the regions outside the crack. Experimental and theoretical results associated with prototype ACFM probes confirm the validity of the proposed technique while demonstrating its remarkable computation efficiency. The latter is an important feature of the proposed modeling technique as a fast “forward” solver that results in a remarkable improvement in solving the “inverse” problem required for the sizing of fatigue cracks in cylindrical metallic structures, particularly in real-time applications.