10.1 Electromagnetic / Eddy Current Testing

10.1 Electromagnetic / Eddy Current Testing

Quick Answer: Eddy current testing (ET) is an electromagnetic method for electrically conductive materials. An alternating current in a coil induces eddy currents in the part; cracks, thinning, lift-off, and conductivity/permeability changes disturb those currents and shift coil impedance. Frequency sets skin depth; ET is excellent for surface cracks and tubing but weak for deep volumetric flaws in thick sections.

ET sits on the ASNT NDT Level III Basic method list next to MT, PT, UT, and RT. You are not certifying as an ET Level III here, but you must select ET correctly, explain its physical limits, and know when related techniques such as remote-field testing (RFT) or alternating current field measurement (ACFM) enter the picture.

Conductive Materials Only

Eddy currents require a closed electrical path in the test object. Practical ET materials include:

• Aluminum, copper, brass, bronze, and titanium alloys
• Austenitic stainless steels (nonmagnetic or weakly magnetic grades used in tubing and vessels)
• Carbon and low-alloy steels (with stronger permeability effects and more complex signals)
• Conductive coatings and clad layers when the procedure is designed for them

Nonconductors (most plastics, ceramics, glass, composites without conductive paths) do not support conventional eddy-current flaw detection. Conductive fibers or foils in composites can produce specialized responses, but that is not the Basic default model.

Exam trap: “Metal” is not enough. ET needs electrical conductivity. Magnetic permeability in ferromagnetic steels multiplies complexity: permeability variations can dominate impedance and mask small cracks unless frequency, magnetization, and coils are chosen carefully.

Physics: Eddy Currents and Impedance

A coil driven with alternating current produces a changing magnetic field. In a nearby conductor, Faraday’s law drives circulating eddy currents. Those currents create their own magnetic field that opposes the coil field (Lenz’s law) and loads the coil electrically. Instruments display this as impedance—a combination of resistance and inductive reactance—often plotted on an impedance plane (horizontal and vertical channels after phase rotation).

Anything that changes eddy-current paths changes impedance:

• Surface-breaking or near-surface cracks interrupt or lengthen current paths
• Wall thinning or corrosion reduces the amount of metal carrying current
• Conductivity differences (alloy, heat treat, cold work) change current density for a given field
• Permeability changes (ferritic content, residual magnetism, stress) strongly affect ferromagnetic responses
• Lift-off and fill factor change coupling between coil and metal

Level III selection language: ET is a surface / near-surface electromagnetic method with property-measurement capability—not a deep volumetric replacement for UT or RT in thick forgings.

Frequency and Skin Depth

Eddy current density is highest at the surface and decays with depth. Standard depth of penetration (skin depth δ) is the depth at which current density falls to about 1/e of the surface value. Conceptually:

• Higher frequency → smaller skin depth → surface emphasis (fine cracks, shallow layers)
• Lower frequency → larger skin depth → more subsurface / wall information (within practical limits)

Skin depth also depends on conductivity and permeability. High-conductivity copper shows shallower penetration at a given frequency than lower-conductivity alloys. High-permeability carbon steel has very small skin depth at common ET frequencies, which is one reason surface crack ET on steel often uses specialized setups and why ferromagnetic tubing inspection often moves toward RFT or other techniques for mid-wall and OD defects.

Practical multi-frequency work: Many procedures use more than one frequency to separate lift-off, conductivity, and flaw signals on the impedance plane. Basic exam items rarely demand calculation of δ, but they do expect the qualitative trade-off: raise frequency for surface sensitivity; lower frequency for deeper sensing—until noise, coil design, or material physics stop you.

Lift-Off, Fill Factor, and Geometry

Lift-off is the gap between coil and surface (or the effective gap created by paint, scale, or probe tilt). Lift-off almost always produces a strong impedance shift. Uncontrolled lift-off is a leading cause of false calls and missed cracks. Controls include:

• Rigid fixtures, spring-loaded probes, and wobble-resistant designs
• Phase rotation and mix channels that suppress lift-off loci
• Calibration on the same coating/paint condition as production parts when coatings remain

For internal tubing probes, fill factor (how fully the coil fills the tube ID) plays a similar role: undersize probes reduce sensitivity and alter phase relationships.

Geometry limits ET everywhere edges, holes, thickness changes, and welds create baseline impedance shifts. Scanning strategies and filters must distinguish geometric signals from flaw signals—the ET analog of nonrelevant MT leakage at section changes.

Coils and Inspection Modes (Overview)

Arrays and multi-coil probes improve coverage and reduce scan time on large surfaces; Basic-level recognition is enough—detailed array processing is method-exam territory.

Signature Applications

1. Surface crack detection — Aircraft skins, fastener holes (with rotating probes), welds in conductive alloys, and critical machined surfaces. ET can find tight surface cracks that might be missed if PT cleaning is imperfect, and it often needs less consumable mess than PT. Orientation still matters: coil current paths and scan direction must interact with expected crack orientation.

2. Tubing inspection — Power-plant and process heat-exchanger tubes are classic ET territory. Bobbin probes detect pits, cracks, and wall loss from the ID. Codes and owner procedures define calibration standards (ASME-style tube standards with artificial flaws), reporting thresholds, and expansion criteria. Ferromagnetic tubing may require RFT, partial saturation ET, or other electromagnetic variants for effective wall assessment.

3. Conductivity sorting — Portable conductivity meters (often %IACS) sort aluminum alloys, verify heat-treat condition trends, and screen mixed stock. Conductivity is not a complete material certification by itself, but it is a fast NDT property check tied to composition and temper.

4. Coating/thickness and other property work — With proper calibration, ET estimates nonconductive coating thickness over metal (lift-off calibration) and can support other specialized measurements.

Related Methods at Overview Level: RFT and ACFM

• Remote-field testing (RFT): A low-frequency electromagnetic tubing technique especially useful for ferromagnetic tubes. A separated exciter and detector sense wall loss and some discontinuities via the remote-field zone through the tube wall. Think “ferromagnetic tubing wall assessment” when standard high-frequency ID ET is limited by permeability and skin effect.
• ACFM: An electromagnetic surface-crack technique that models magnetic field disturbances above cracks; often marketed for inspection through coatings with less surface preparation than wet MT/PT. Still an electromagnetic surface method—not a volumetric deep-flaw tool.

Strengths and Limitations (Exam Table)

Level III Takeaway for Method Selection

Choose ET when the part is conductive, the concern is surface or near-surface (or thin-wall tubing), and you need speed, automation, or conductivity data. Do not choose ET as a substitute for UT/RT on thick volumetric integrity problems. For ferromagnetic heat-exchanger tubes, plan for RFT or specialized ET, not naive high-frequency bobbin practice. Document frequency strategy, calibration standards, acceptance thresholds, and how lift-off and geometry are controlled—those are the procedure hooks Basic exam scenarios revolve around.