Alternating Current Field Measurement
- Alternating Current Field Measurement
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.
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 plant. Applications include in-service inspection of welded items and crack detection in vessels.
The ACFM probe induces a uniform alternating current in the area under test and detects the resulting current flow 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" capability for ongoing monitoring.
*Non-Invasive, inspection without removing any protective coating.
*Works with surface temperatures up to 500 degrees Celsius.
*POD and false call rate is dependent upon application.
Limitations
*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.
*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 and spatter.
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