ACFM is used for detecting and sizing surface breaking flaws. It was initially conceived for use under water to detect flaws in offshore structures. Now, however, ACFM is applied to structures both in and out of the water.
ACFM is an electromagnetic technique. A sensor probe is placed on the surface to be inspected and an alternating current is induced into the surface. When no defects are present, the alternating current produces a uniform magnetic field above the surface. Any defect present will perturb the current forcing it to flow around and underneath the defect; this causes the magnetic field to become non-uniform and sensors in the ACFM probe measure these field variations, providing information concerning the length, depth and position of the defect(s).
Advantages
Advantages of ACFM are that it:
- Works equally well on parent material or welds, ferritic or nonferritic metals
- Can be used on hot surfaces, underwater, or in irradiated environments
- Provides both depth and length information
- Gives accurate sizing of defects up to 25mm in depth
- Can be used after minimal cleaning and can be applied over paint and other coatings up to several millimetres in thickness
TWI Resource
TWI owns the state-of-the-art Amigo ACFM system supplied by TSC.
Selected Clients and Applications
ACFM is particularly suited to the detection and sizing of fatigue cracks at the toes of welds, including all butt, fillet, node and nozzle welds. TWI has applied its AMIGO system in this role for inspection of fillet welds under the orthotropic decks of several highway bridges. These have a paint coating so that ACFM was particularly suitable. In some cases inspections are made to a routine schedule to determine whether crack growth is occurring.
TWI has also used ACFM to inspect fillet welds in Mobile Offshore Drilling Units (MODUs) to detect any possible original fabrication hydrogen cracking present at weld toes.
ACFM is also being used to detect flaws in the links of large mooring chains.
ACFM has also been successfully deployed in the laboratory in a joint industry project to detect stress corrosion cracking in duplex stainless steel pipe welds.
For more infromation, please contact us.
