Surface & Near-Surface NDT · ACFM
Alternating Current Field Measurement
Crack detection and sizing — straight through the coating.
Alternating current field measurement (ACFM) is an electromagnetic technique that detects and sizes surface-breaking cracks through paint and coatings. A probe induces a uniform alternating current in the component surface; a crack diverts that current around its ends and depths, and the resulting magnetic field disturbances reveal both the length and depth of the defect — without any coating removal.
The explainer
Watch how it works.
Schematic · loops automatically
1Induce a uniform current
The probe couples a locally uniform AC sheet into the component surface — through the coating, no contact with bare metal needed.
2The crack diverts the flow
Current sweeps around the crack ends and dives beneath its deepest point, distorting the field above the surface.
3Read Bx for depth
The along-crack field dips where current descends — the deeper the crack, the deeper the trough.
4Read Bz for length
The vertical field peaks at one crack end and troughs at the other, bracketing the defect length.
5Size without stripping
The instrument converts both signals into crack length and depth — recorded, positioned and repeatable, with the paint still on.
The physics
ACFM starts where eddy current testing stops: instead of a small circulating field, the probe induces a locally uniform sheet of alternating current flowing in one direction across the inspection surface. Above an unflawed surface, the associated magnetic field is smooth and predictable.
A crack interrupts the sheet. Current cannot cross the gap, so it dives beneath the crack and sweeps around its ends. Two field components tell the story: Bx (along the crack) dips in proportion to how deep the current had to dive — measuring depth — while Bz (vertical) forms a peak and trough at the crack ends — measuring length. Software converts the pair directly into defect dimensions.
Because the input field is broad and theoretical models link signal to size, ACFM tolerates lift-off from coatings that would cripple conventional eddy current work — several millimetres of paint on structural steel is routine. That is its defining economics: crack inspection of coated structures without blasting, on offshore steelwork, cranes, bridges and plant, with quantified depth rather than a simple hit/no-hit.
Where it earns its keep
Typical applications
- In-service weld inspection on coated structural steel
- Offshore and marine structure surveys
- Crane, lifting and fatigue-loaded equipment examination
- Follow-up sizing of indications found by MPI or visual inspection
Standards & schemes
| Designation | Covers |
|---|---|
| Qualified written procedures | ACFM is applied to project-specific procedures with documented probe qualification and calibration |
| ISO 9712 (ACFM sector schemes) | Personnel certification for electromagnetic techniques |
ACFM inspection is delivered by certified electromagnetic-technique specialists working to qualified procedures, with defect records integrated into structural integrity assessments.
What does ACFM offer that MPI does not?
Two things: it works through intact coatings — no blasting, no reinstatement costs — and it sizes what it finds, returning crack depth as well as length. MPI gives a sensitive but binary indication on bare metal; ACFM gives engineering numbers on painted structures.
How accurate is ACFM depth sizing?
For simple surface-breaking planar cracks it typically sizes depth to within fractions of a millimetre to around a millimetre, degrading with branching, clustering or awkward geometry. That is normally sufficient to feed a fitness-for-service assessment rather than a repair-everything default.
Do we still need MPI if we use ACFM?
Often both: MPI remains the fastest, most sensitive screen on bare or lightly coated steel, while ACFM earns its keep on coated structures and wherever a found crack must be sized without stripping. The methods are complementary, and we advise per structure.
Delivered through
Testing · Inspection · Certification
Need ACFM on your asset?
Describe the component, the material and the question — a specialist confirms the technique, the standard and the turnaround.