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.

Definition

Eddy Current Testing (ET) uses an alternating-current coil to induce small circulating currents — eddy currents — in an electrically conductive test piece. Discontinuities, conductivity variations, and geometry changes alter the eddy current flow and therefore the coil's impedance. The resulting impedance shift is plotted on the impedance plane for interpretation.

Technical Context

Probe selection depends on application: encircling coils for tubes, bobbin coils for heat-exchanger tubes, surface coils for plates and welds, and rotating probes for bolt holes. Performance is affected by lift-off, edge effect, and fill factor.

When It Is Used

Heat-exchanger and condenser tube inspection
Aircraft skin and fastener-hole crack detection
Coating thickness and conductivity sorting

Related Standards

ASME Section V Article 8, ASTM E309, ASTM E571, ISO 15549.

Synonyms

Also called ECT. Advanced variants include multi-frequency ET, ACFM, and Eddy Current Array (ECA).

How it works

An alternating current in a coil induces circulating currents in a conductive part. Anything that disturbs those currents — a crack, a change in conductivity, a change in the gap between coil and surface — reflects back as a change in coil impedance, which is displayed as a movement in the impedance plane.

What it finds

Surface and near-surface cracking in conductive materials, including through thin non-conductive coatings, and it does so without couplant. In tubing, bobbin and array probes find wall loss, pitting and cracking at production speed. Conductivity and coating-thickness measurement use the same physics.

What it will not find

Anything deep. Penetration falls off exponentially with depth, and faster as frequency, conductivity or permeability rise, so eddy current is a surface and near-surface method whatever the probe. Ferromagnetic materials complicate it further because permeability variation swamps the flaw signal unless the material is saturated.

How it is actually done

Frequency is chosen for the depth of interest, the instrument is nulled on sound material, and the phase rotation is set on a reference standard with known artificial defects so that lift-off moves along a known direction and flaw signals separate from it. Reading the impedance plane is the skill; the equipment only presents it.

Governing codes and standards

ASME Section V Article 8 covers eddy current examination; ISO 15548 addresses equipment characterisation; ASME Section V Article 26 and the various tubing appendices cover heat-exchanger tube examination in service.

Where it goes wrong

Mistaking lift-off for a defect, or worse, phasing lift-off out so aggressively that a genuine shallow flaw goes with it. The reference standard exists so the two are separated deliberately rather than by eye.

Frequently asked questions

Can eddy current testing be used on steel?

Yes, but with care. Carbon steel is ferromagnetic, and permeability variation produces signals far larger than most flaws. Magnetic saturation probes or alternatives such as magnetic flux leakage are usually specified instead, depending on what is being looked for.

Does paint have to be removed first?

Usually not, which is one of the method's real advantages. The coating simply adds lift-off, and provided it is reasonably uniform and accounted for during setup, examination proceeds through it. Thick or irregular coatings erode sensitivity, and at some point removal becomes the honest option.

Where Eddy Current Testing fits in an inspection programme

A term is only useful when it connects to a decision. Eddy Current Testing 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

Impedance Plane — The impedance plane is a 2D display in eddy current testing showing the probe coil's resistive (X-axis) and inductive reactive (Y-axis) components, on which signals from lift-off, defects, and conductivity changes trace characteristic vectors.
Lift-Off — Lift-off is the change in eddy current signal caused by a variation in the distance between the probe coil and the test surface, used both as a calibration tool and as a nuisance variable to be minimized.
Bobbin Coil — A bobbin coil is a differential or absolute eddy current probe that travels axially inside a tube, inspecting the entire tube circumference simultaneously for wall loss, pitting, and through-wall defects.
Surface Coil — A surface coil is a small handheld eddy current probe used to scan accessible surfaces of components — typically a pencil-style absolute or reflection probe — for surface-breaking crack detection.
Multi-Frequency Eddy Current — Multi-frequency eddy current testing uses two or more simultaneous frequencies whose responses are mixed in software to suppress unwanted variables (support-plate signals, tube-end effects) and isolate the defect signal of interest.
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.

More method terms

Ultrasonic Testing · Radiographic Testing · Magnetic Particle Testing · Penetrant Testing · Visual Testing · Acoustic Emission Testing · Leak Testing · Thermography / Infrared Testing · Microwave Testing · Phased Array Ultrasonic Testing

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.