ACFM (Alternating Current Field Measurement)

Alternating Current Field Measurement (ACFM) is an electromagnetic technique for detection and sizing of surface breaking cracks.


  • Works on all metals
  • Does not require direct, electrical contact
  • Works through coatings

Applications include in-service inspection of welded items and crack detection in vessels.

PrinciplesThe ACFM probe induces a uniform alternating current in the area under test and detects the resulting current flow 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 measures these disturbances in the field and uses mathematical modeling to estimate crack size.
How and where is it used?An electromagnetic technique that is a faster alternative to magnetic particle inspection with the added benefit of permitting crack sizing
Works through coatings and does not require direct, electrical contact
Detection and sizing of surface breaking fatigue cracks.
In service inspection of welded items and crack detection in vessels.
Inspection of equipment on-line at temperatures up to 500C
Form of report or deliverable?Computerized reporting software provides immediate, detailed images of the test.
AdvantagesIdentifies defects and size of cracks both length and depth
Can inspect electronically conductive material
Non invasive
Permits inspection without removing insulation or coatings
LimitationsDifficult for short sections/small items
Weld repairs and areas of local grinding can cause spurious indications
Cracks need to be longer than 5mm
Equipment is more bulky than MT and indications are more difficult to interpret
Certifications of technicians performing the work?
Rope access applicable?