Review of High Sensitivity AC Field Measurement — Recent Advances and Future Work

The high sensitivity ac field measurement (ACFM) is a powerful alternative to the conventional eddy current technique. This technique does not require an electronic bridge or a differential probe. It uses a current-carrying rectangular coil or a rhombic loop for producing the eddy current in the work-piece, and a linear probe attached to the inducer in a particular location to sample the field tangent to the metal surface. In this paper, initially a review of the ACFM technique in crack detection and sizing is given. Then the principles behind the high sensitivity ACFM are addressed. Recent advances in this technique, including the development of flat inducers, accurate modelling of field-flaw interaction, and techniques for crack signal inversion, are reported. The application of the technique in detection of cracks using high lift-off probes, shallow cracks, cracks in welded areas and heat affected zones, is highlighted. Also, a linear array system based on the high sensitivity ACFM for detection of defects in large metal surfaces, is discussed. The final part of the paper concerns with future work on the high sensitivity ACFM which is of particular interest in practical NDE