ACFM – Alternating Current Field Measurement

Alternating Current Field Measurement (ACFM)

ACFM is ideally suited to in-service defect detection and is able to size a crack for length and depth without the need for further evaluation by techniques such as ultrasonics.

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).

ACFM provides the following main advantages over more traditional inspection techniques:

  • Able to size for length and depth at the same time;
  • Requires minimal surface preparation and can be applied over paint and other coatings, therefore detect and size defects through coatings up to 10mm thick;
  • Accurate sizing of defects up to 25mm in depth
  • Suitable for inspecting most electrically conducting materials;
  • Able to operate at elevated temperatures nominally in the range of 400-500°C; also underwater or in irradiated environments
  • Provides a permanent record of results for improved auditability and off-line analysis;
  • Allows for a high degree of automation making the system very user-independent;
  • Provides for real-time reporting and defect evaluation enabling engineering personnel to make site decisions which can affect plant operations;
  • 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 bridgesInspection 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 & gas and power generation industries
  • Inspection of drilling tools
  • Inspection of various components in automotive industries