Alternating Current Field Measurement (ACFM) is an electromagnetic technique for non-destructive testing detection and sizing of surface breaking cracks. It was 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 ACFM probe induces a uniform alternating current in the area under test and detects magnetic field of the resulting current near the surface. This current is undisturbed if the area is defect free. 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 analysed; 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. It can inspect any electrically conductive material. Data is recorded electronically for off-line evaluation if necessary and provides a permanent record of indications. Tests can be repeated and compared over time for on-going monitoring.


The method is non-invasive and can carry out inspection without removing any protective paint coating. With suitable probes, the method can be used on hot surfaces.

• Not recommended for short sections or small items.
• Locations of weld repairs and localised grinding can cause spurious indications.
• Crack length needs to be longer than 5 mm. Multiple defects reduce the ability to estimate defect depth.
• Equipment more bulky than for MT and indications may be more difficult to interpret.
The probability of detection and false detection rate is generally good, but it is application dependent.

Non-adherent protection such as insulation must be removed. The system can operate through non-conductive adherent coatings, but there may be a need to remove heavy or loose scale and spatter.