TechCorr Non-Destructive Testing Alternating Current Field Measurement

Electromagnetic Crack Detection

Alternating Current Field Measurement (ACFM) is an electromagnetic technique that identifies and sizes surface breaking cracks in welds and metallic components.

The ACFM probe induces a uniform alternating current in the test area and detects the resulting magnetic field near the surface. If there is a crack, the current redirects around the crack ends and face. The ACFM equipment measures these disturbances and analytical software estimates the crack size.

This method replaces conventional dye penetrants, magnetic particle testing, and ultrasonic testing.

No Contact Required


  • can inspect any electrically conductive material
  • does not need to contact the material surface
  • has the ability to work through insulation and non-conductive coatings, such as paint, scale, or rust.

Because there is no need for surface contact, there also is no need to clean or otherwise prepare the material surface for inspection, which can save time and money.

However, the testing is only accurate if insulation and coatings are strictly adhered to the surface. Non-adherent or loose insulation must be removed.

Used in Many Environments

ACFM can be used:

  • in high-temperature applications up to 900°F (480°C)
  • underwater
  • in irradiated environments.

Broad Applications

ACFM is used to detect cracks in:

  • structural welds on platforms and drilling rigs
  • pipeline girth welds and supports
  • pressure vessel system welds
  • cooling tower welds
  • compressor fin surfaces and threads
  • drill collar thread and other drilling tools
  • fillet welds in highway bridges
  • rail components
  • gear box, gear teeth, crank shafts, cylinder heads, turbines, and other rotating equipment.

Complementary Techniques

ACFM is intended for use with other testing methods.

  • It is not recommended for short sections of pipes or for small components.
  • Locations of weld repairs and localized grinding can cause spurious indications.
  • Crack length needs to be longer than 5 mm. Multiple defects reduce the ability to estimate defect depth.