Eddy-current testing

Principle and inspection objective

Eddy-current testing uses a coil to induce currents in a conductive component. Both material variation and discontinuities affect the measured amplitude and phase. Evident: introduction to eddy-current testing

Evident defines standard penetration depth as the depth where current density has decreased to approximately 37% of its surface value. Frequency, electrical conductivity, magnetic permeability, and coil geometry affect sensitivity and penetration. Higher frequency generally favors near-surface response while reducing penetration. Evident: penetration and probe considerations

Define the purpose before selecting equipment: surface-crack detection, tube degradation, conductivity comparison, sorting, or coating/thickness measurement. These require different probes, references, processing, and acceptance criteria. Conventional eddy-current inspection requires an electrically conductive test material. A nonconductive coating may be inspected indirectly through its effect on separation from a conductive substrate; that is different from detecting flaws in the coating itself.

ISO 15549:2019, published April 2019, defines general principles for repeatable eddy-current examination and preparation of application-specific documents. Its general scope is not a universal product acceptance specification. Establish the applicable product standard, written procedure, personnel requirements, and acceptance limits for the particular inspection. ISO: ISO 15549 scope

Frequency, material, and penetration

Standard penetration depth is not a guaranteed flaw-detection depth. A discontinuity's orientation, dimensions, location, surrounding geometry, and signal-to-noise ratio determine whether the selected system can detect it. Avoid selecting a frequency solely from a skin-depth calculation or treating the lack of an indication as proof that the material is free of discontinuities.

Ferromagnetic materials need particular care because permeability variation can produce strong background signals and alter penetration. TWI identifies this as a difficulty rather than a blanket prohibition on testing steel. Demonstrate capability for the actual material condition, including relevant weld or heat-treatment variation. TWI: eddy-current limitations

Probe, geometry, and coverage

FHWA's bridge-oriented eddy-current-array guidance warns that irregular surfaces, probe lift-off, and tilt can produce signals resembling defects. It also identifies weld magnetic properties, probe orientation, and access restrictions as limitations. Its discussion of inspection through bridge paint concerns suitable applications and equipment, not unlimited coating tolerance. FHWA: eddy-current array testing

Define scan paths, overlap, start and end regions, inaccessible areas, and any orientation changes in the procedure. A wider probe does not by itself demonstrate complete coverage. Reference responses should remain detectable under the actual motion and permitted handling variation, not only with the probe held stationary in an ideal position.

Reference standards and system checks

The IAEA Training Course Series 48, printed February 2011, describes eddy-current signal analysis as largely comparative. Its reference-standard section uses known responses to establish sensitivity and phase rotation and examine influences such as material resistivity, curvature, thickness, flaw dimensions, and lift-off. These training principles remain useful, while current product procedures must supply the actual inspection requirements. IAEA: Eddy Current Testing at Level 2, reference-standard section

TWI emphasizes reference blocks representative of material, heat-treatment condition, shape, and size. Artificial discontinuities provide controlled comparison responses, while thickness steps may support corrosion applications. Identify reference geometry and condition rather than referring only to a “calibration block.” TWI: standards and calibration

Keep instrument performance verification separate from application standardization and detection qualification. Record probe/cable identity, frequency, gain, phase, filters, drive, balancing conditions, references, and scan method. Repeat the checks required by the procedure after changes and at its specified intervals. If a closing check fails, assess the affected inspection interval rather than assuming earlier results remain valid.

Illustrative limitation: a system adjusted to show a particular machined notch at a selected screen height has demonstrated that response under those conditions. It has not thereby demonstrated that every fatigue crack of the same depth produces the same amplitude. Geometry, orientation, opening, and material response need separate consideration before sizing or acceptance decisions.

A bounded instrument example

Evident's undated NORTEC 600 product page lists an instrument frequency range of 10 Hz–12 MHz and separate N600, N600C, N600S, and N600D variants. The feature table assigns conductivity measurement, rotary-scanner support, and dual-frequency/mixing capabilities according to model. The frequency range is an equipment capability, not a recommended inspection range or a statement that one probe works throughout it. Evident: NORTEC 600 specifications and model table

Confirm the supplied variant, compatible probe, cables, software functions, and procedure. An automatic preset or probe-recognition feature can assist setup but does not establish suitability for an unqualified component or acceptance criterion.

Arrays and other electromagnetic methods

TWI describes eddy-current arrays as assemblies of coils covering a larger region, with multiplexing used to manage interaction between nearby coils. Array inspection can improve acquisition coverage and recording, but the selected channels, scan arrangement, and interpretation still require validation. Do not transfer a conventional pencil-probe qualification to an array solely because both use eddy currents. TWI: eddy-current array principles

FHWA describes spatially referenced array data using position encoders and stored data for later processing. Such maps help locate and review responses, but a color-coded region remains a processed signal interpretation. Retain settings, calibration relationships, and relevant position information with the recorded data. FHWA: acquisition and interpretation

Remote-field, pulsed eddy-current, and other electromagnetic techniques require their own system and application qualification. Their capabilities should not be assumed from conventional surface-probe behavior or substituted without checking the inspection objective.

Evaluation and reporting

Characterize indications using the approved procedure, including location, amplitude/phase behavior, repeatability, relation to geometry, and any qualified sizing method. Separate detection, characterization, and acceptance. Use complementary inspection when the required distinction cannot be demonstrated with the selected eddy-current technique.

Report component/material condition, inspected region and exclusions, procedure edition, equipment and probe, settings, reference artifacts and check results, scan coverage, relevant indications, interpretation, and acceptance criterion. State detection or sizing limitations explicitly. “No reportable indications under this procedure in the inspected area” is a more bounded result than claiming the entire component is defect-free.

Related reference

Inspection and quality documents