Alternating current field measurement
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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.
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[edit] 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.
[edit] Method
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.[1] 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.
[edit] Capabilities
The method both detects cracks and estimates their size and length. It can isnpect 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 ongoing 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.
[edit] 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.
The probability of detection and false detection rate is generally good, but it is application dependent.
[edit] 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.
[edit] References
- ^ Paul E. Mix,Introduction to nondestructive testing: a training guide, John Wiley and Sons, 2005, ISBN 0471420298 pages 124-126
