First, a little history. The alternating current field measurement (ACFM) nondestructive testing technique was developed in the ‘80s to detect and estimate penetration depth of fatigue cracks in underwater welded tubular intersections of offshore oil platforms. Prior to the advent of ACFM, these defects were generally detected and their length measured with magnetic particle inspection.
The severity of fatigue cracks (directly related to the remaining life of structures) is commensurate with how deep they are. At the time, conventional eddy current testing systems were not well suited for use underwater or for welds in ferritic steel. They were unable to accurately measure defects deeper than 5 mm (0.2 in). Alternating current potential drop (ACPD) was the technique used to measure depth, but it was slow and very difficult to use underwater because of the need to maintain very good electrical contact between the voltage probe and the steel surface.