Alternating Current Field Measurement (ACFM) is an electromagnetic technique for detection and sizing of surface breaking cracks. It works on all metals and, not requiring direct, electrical contact, works through coatings.

Use

The system was originally developed in the early 1990 for the sub-sea and topside inspections of offshore structures without the need to remove the item protective coating. Since then it has been successfully applied to onshore process plants. Applications include in-service inspection of welded items and crack detection in vessels.

Method

The ACFM probe induces

Electromagnetic induction

Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

 a uniform alternating current in the area under test and detects the resulting current near to 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.

Capabilities

• Detects and sizes cracks both length and depth.
• Can inspect any electrically conductive material.
• Data recorded electronically for off-line evaluation if necessary. Permanent record of indications.
• "Fingerprint
  Fingerprint
  A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
  
  " capability for ongoing monitoring.
• Non-Invasive, inspection without removing any protective coating.
• Works with surface temperatures up to 500 degrees Celsius
  Celsius
  Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
  
  .
• POD and false call rate is dependent upon application.

Limitations

• Not recommended for short sections or small items.
• Locations of weld
  Weld
  Weld most commonly refers to a joint formed by welding.Weld may also refer to:-People:* Weld family, an extended family of New England** Theodore Dwight Weld** Tuesday Weld* Weld-Blundell family* Cecil Weld-Forester, 1st Baron Forester...
  
   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.
• POD and false call rate is generally good but application dependent.

Preparation

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

Mill scale

Mill scale, often shortened to just scale, is the flaky surface of hot rolled steel, iron oxides consisting of Iron oxide, hematite, and magnetite....

and spatter.