HomeBackNuclearEngineering ServicesEnergy and Utilities

Specialised Tube Inspection | Eddy Systems | Centest 91 | Internal Rotary Inspection | Helium Inspection | Hydrotesting | Floor Testing | Magnetic Particle Inspection | Ultrasonic Inspection | Corrosion Mapping | Time of Flight Diffraction | Computerised Analysis | Saferad | Radiography | Welding Department | Mechanical Testing | Furnace Facilities | Heat Treatment

Surface Crack Detection by A.C.F.M


ACFM (Alternating Current Field Measurement) is an electromagnetic technique for the rapid detection and sizing of cracks in conductive materials. It is non-contacting and the inspection can be carried out through up to 5mm of non-conductive coating such as paint and scale.

Advantages & Cost Savings

  • Through coatings
  • Minimum preparation/cleaning therefore no recoating
  • Accurate sizing
  • High temperature operation
  • Remote operation.
  • Hard copy via PC


The ACFM technique was developed from the AC potential drop (ACPD) technique to combine the ability of ACPD to size without calibration with the ability of eddy current techniques to work without electrical contact. This is achieved by maintaining the uniform input field (induced rather than injected) but measuring the magnetic fields above the specimen surface instead of the surface voltages. The presence of a defect in the specimen will influence the magnetic fields in a predictable way.

Unlike eddy current techniques calibration defects are not required, sizing is based on theoretical models. This removes a major source of error, since artificial calibration defects are rarely representative of service induced defects.

Site Implementation

The ACFM system operated by Torch Integrated Inspection Services consists of a probe, a TSC U9 Crack Microgauge and a notebook computer for the display, analysis and storage of data. The system can be either battery or mains operated and a probe umbilical is available for operation remote from the equipment in areas with difficult access. The inspection is usually carried out using a two man team, one trained and approved to view and interpret the results, the other to manipulate the probe.

The ACFM probes are designed primarily to run along the line expected to be followed by a crack, such as a weld toe. A standard ACFM probe can inspect a band about l0 mm either side of the probe centre so that complete welds up to 20 mm wide can be inspected in this way. For wider welds, additional scans along the weld cap are required. For inspection of an area where no preferred crack orientation exists, two scans are made at right angles to each other. Typical scanning speeds are in the range of 50 mm – 100 mm per second.

As the probe is scanned, the operator views the trace on the computer screen. The presence of a defect is readily apparent. When a defect is detected, measurements are made on the computer trace and the software calculates the predicted crack dimensions based on these measurements. Surface breaking cracks as small as l0 mm long x 1 mm deep, are readily detectable in steel welds through the coating. Full details of the inspection, including defect traces, can be stored and re-run on the computer for future reference, and as such, the system is ideal as a ‘fingerprinting’ tool.


ACFM can be applied to the detection of cracks and other defects in most conductive materials. Its main applications to date have been for the inspection of welds without the requirements to remove paint and scale, particularly off-shore, both sub-sea and topside. It has also been used to inspect operating process plant at up to 500oC. Other uses have been in the nuclear, steel and railway industries.