Alternating Current Field Measurement
ACFM is an electromagnetic technique for non-destructive testing detection and sizing of surface breaking cracks.
It is derived from the methods used in eddy-current testing and works on all metals, ferrous or non-ferrous. Since it doesn’t require direct electrical contact with the surface it can work through coatings such as paint or rust.
The system was originally developed in the early 1990’s for the sub-sea and topside inspections of offshore structures without the need to remove the item’s protective coating.
The ACFM probe induces a uniform alternating current in the area under inspection and detects magnetic field of the resulting current near the surface.
This current is undisturbed if the area is defect free but a crack redirects the current around the ends and faces of the crack, the ACFM instrument measures these disturbances in the field and uses mathematical modelling to estimate crack size. The lateral and vertical components of the magnetic field are analyzed; disturbances indicate a crack is present, and the size and depth of the crack can be calculated.
The method both detects cracks and estimates their size and length providing the crack length is greater than 5 mm. It can inspect any electrically conductive material.
The method is non-invasive and can carry out inspection without removing any protective paint coating. The probability of detection and false detection rate is generally good.