What Is ACFM?
Alternating Current Field Measurement (ACFM) is an electromagnetic NDT technique for
detecting and sizing surface-breaking cracks in electrically conductive materials. An alternating
current is induced into the material surface, producing a uniform surface current. Where a
surface-breaking crack is present, it disturbs the current flow — creating characteristic
perturbations in the associated magnetic field above the surface that ACFM probes detect and
measure.
The key feature that distinguishes ACFM from other electromagnetic surface inspection
methods is its ability to provide quantitative crack sizing — estimating both the surface length
and the through-wall depth of detected cracks — from the shape of the detected magnetic field
perturbation. This sizing capability is based on a mathematical model of the crack-field
interaction, validated through extensive laboratory and field testing.
ACFM key characteristics:
- Detects and sizes surface-breaking cracks — not just detects them
- Effective through non-conductive coatings up to approximately 5mm — no coating
removal required
- No couplant required — completely dry, near-contact inspection
- Applicable to welds and base material in carbon steel, stainless steel, aluminium, and
titanium
- Semi-quantitative crack depth sizing — sufficient for fitness-for-service assessment inputs
- Effective in difficult access environments — underwater, elevated temperature, offshore