Alternating Current Field Measurement ACFM

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ACFM is a non-destructive technique predominantly utilized for inspecting for surface breaking cracks through coated surfaces. ACFM introduces an alternating current into the surface of a component to detect surface-breaking cracks. Not having to remove coatings for inspection and immediate defect sizing and recording are the main benefits of this method compared to more traditional NDT techniques.

ACFM is used for in-service inspection as an alternative to MT/PT. ACFM is ideal for detecting cracks in welds, through coatings and with less extensive cleaning. In addition, it can be used to estimate crack depths.

The system is both remote and robust so can be utilized in all environments effectively by our qualified Technicians with little or no surface preparation required.

IRISNDT applies ACFM for:

  • Inspections through coatings including paint, epoxy coatings, oxide layers, fire protection layers, and marine growth
  • Inspecting piping, pressure vessels, pipelines, drill pipe, and structural supports in petrochemical facilities
  • Inspecting pipe for ovality deviations
  • Detecting size growing fatigue cracks in in-service pressure vessels
  • Inspecting threads
  • Inspections on equipment operating at temperatures as high as 500 ⁰C (932 ⁰F)
  • Inspections during welding without having to allow each bead to cool down and without subsequent reheating
  • Inspections of wind turbines


  • As a non-contact inspection method, ACFM can be used to inspect through coatings of various thickness (including dirt), saving on coating removal and reapplication costs
  • Equipment is highly portable, easy to deploy through rope access
  • High temperature applications allow for in-service inspections, avoiding the cost of equipment shutdown
  • Highly reproducible electronic data for reference/audits and integration into plant software
  • In-depth defect morphology allows for prioritization of resources


  • ACFM sizing models currently only sizes accurately crack depths in steel
  • The length assessment is not greatly affected in any material
  • When sizing cracks in other materials, a calibration chart is needed based on defects of known depth; the chart can then be used to convert the ACFM indicated depths into true depths
  • ACFM sizing models be readily applied to cracks with a simple morphology (fatigue); they are less suitable for networks of branched cracks (stress corrosion cracking)