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Alternating Current Field Measurement - ACFM - Electromagnetic NDT

Alternating current field measurement
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Alternating current field measurement (ACFM) is an electromagnetic technique used for the detection and sizing of surface breaking cracks in metallic components and welds. It combines the advantages of the alternating current potential drop (ACPD) technique and Eddy Current Testing (ECT) in terms of defect sizing without calibration and ability to work without electrical contact respectively.

The ACFM probe introduces an electric current locally into the part and measures the associated electromagnetic fields close to the surface. The presence of a defect disturbs the associated fields and the information is graphically presented to the system operator. The ends of a defect are easily identified to provide information on defect location and length. Through wall extent of the flaw plays an important role in determining structural integrity and the same is calculated using mathematical computation thus allowing an immediate evaluation of the implication of the indication. ACFM inspection can be performed through paint and coatings, hence it is considered to be a faster and economic technique than others (E.g. magnetic particle inspection (MPI).

TWI Resources

TWI owns the state-of-the-art Amigo ACFM system supplied by Technical Software Consultants (TSC) accompanied by special probes. The induction coil typically operates at 5 kHz with standard probes and 50 KHz with special high-frequency probes. The 50 kHz high-frequency probes are typically used to obtain higher test sensitivity for nonmagnetic conductive materials.


  • Applicable for base material or welds, ferritic or non-ferritic conductive metals

  • Can be used on hot surfaces, underwater, or in irradiated environments

  • Provides both depth and length information

  • Accurate sizing of defects up to 25mm in depth

  • Requires minimal surface preparation and can be applied over paint and other coatings

  • Applicable for under water inspection as well as normal inspection

  • Inspection data can be stored and analysed offline

  • Inspection can be encoded


  • Detection and sizing of fatigue cracks and hydrogen cracking

  • Inspection of fillet welds in mobile offshore drilling Units

  • Inspection of fillet welds in highway bridges

  • Inspection of rail components

  • Inspection of gear box, gear teeth, crank shafts, cylinder heads, turbines etc. in aerospace industries

  • Detection of cracks and corrosion in vessels, storage tanks and piping in oil and gas and power generation industries

  • Inspection of drilling tools

  • Inspection of various components in automotive industries

For more information about TWI's NDT services, please email

For more information please email: