Alternating Current Field Measurement (ACFM)
ACFM is an electromagnetic NDT technique that induces a uniform AC field in a conductive component and measures the field disturbance over surface-breaking cracks to size their length and depth without surface cleaning.
Definition
ACFM induces a uniform alternating-current magnetic field parallel to the inspected surface. A surface-breaking crack disturbs the resulting current flow; an array of sensors above the surface measures the orthogonal magnetic field components Bx and Bz, from which crack length and depth are calculated.
Technical Context
Because ACFM is calibrated mathematically rather than against a reference flaw, it works through paint and coatings and on hot surfaces. It is widely used for in-service inspection of structural welds in offshore platforms, ships, and onshore plant.
When It Is Used
- Offshore structural weld inspection (through coating)
- Topside piping and bridge weld inspection
- High-temperature crack monitoring
How it works
An electromagnetic method that induces a uniform AC field and reads the magnetic-field perturbations a surface-breaking crack causes — yielding detection AND length/depth sizing without cleaning to bare metal.
What it finds
Fatigue cracks at welds through paint and thin coatings — its home ground is offshore structural inspection, where removing coatings for MT costs more than the examination.
What it will not find
Subsurface flaws; complex geometry complicates the field model behind the sizing; very short or shallow cracks approach the noise floor.
How it is actually done
Probe passes along the weld toe with software matching signal to a crack model. Operator skill concentrates in recognising geometry and seam signals versus genuine indications — the classic ACFM error is a false call at a geometry change.
Governing codes and standards
Recognised in offshore inspection practice; personnel typically certified under ISO 9712 or employer schemes with method-specific training.
Where it goes wrong
Trusting depth sizing at the extremes. The inversion model is calibrated for a crack-shape family; unusually long-shallow or short-deep cracks size worst exactly when the number matters most.
Where Alternating Current Field Measurement fits in an inspection programme
A term is only useful when it connects to a decision. Alternating Current Field Measurement appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Eddy Current Testing (ET) — Eddy Current Testing (ET) is an electromagnetic NDT method that induces circulating currents in conductive materials using an AC coil; changes in coil impedance reveal surface and near-surface defects, conductivity variations, and coating thickness.
- Magnetic Particle Testing (MT) — Magnetic Particle Testing (MT) is a surface and near-surface NDT method for ferromagnetic materials that detects discontinuities by applying a magnetic field and observing the leakage flux attract magnetic particles to the flaw indication.
More method terms
Ultrasonic Testing · Radiographic Testing · Penetrant Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing · Phased Array Ultrasonic Testing · Time-of-Flight Diffraction
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.