9.2 Eddy Current Testing

Electromagnetic Induction and the Impedance Plane

Eddy current testing (ET) works by electromagnetic induction. A coil carrying alternating current generates a changing primary magnetic field. When the coil is brought near an electrically conductive part, that field induces circulating eddy currents in the material. Those eddy currents create their own opposing (secondary) magnetic field, which alters the current and voltage in the coil — sensed as a change in the coil's impedance. Anything that changes how easily eddy currents flow — a crack, a change in conductivity, a change in permeability, thickness loss, or the spacing between probe and part — shifts the impedance, and the instrument displays that shift.

Modern instruments plot the response on the impedance plane, where the horizontal axis represents the resistive component and the vertical axis the inductive reactance. Different variables move the operating point in characteristic directions and with characteristic phase angles, which lets a trained operator separate a real crack signal from lift-off or geometry noise. Learning to read those directions — rather than just signal amplitude — is the core skill of ET interpretation.

Test Frequency, Skin Effect, and Depth

Eddy currents are strongest at the surface and decay with depth — the skin effect. The standard depth of penetration is the depth at which eddy current density falls to about 37 percent (1/e) of the surface value. It gets shallower as test frequency, conductivity, or permeability increases. The practical consequences:

• Higher frequency → eddy currents crowd near the surface → shallower penetration but better resolution of small surface cracks.
• Lower frequency → deeper penetration to reach subsurface or far-wall flaws, but with lower resolution and sensitivity to fine defects.

Selecting frequency is therefore a trade-off: a Level III chooses high frequency to find fine surface cracks around fastener holes, and lower frequency to detect deeper wall loss in tubing.

Conductivity, Permeability, and Common Interfering Variables

Conductivity (often expressed in %IACS, the International Annealed Copper Standard, where pure annealed copper = 100% IACS and aluminum is roughly 61% IACS) governs eddy current strength and is the basis for alloy sorting and heat-treat verification. Magnetic permeability is the wild card: in ferromagnetic steel, permeability is high and varies with stress and microstructure, so its signal dominates and masks the flaw signal. That is why conventional eddy current interpretation is far more difficult on ferromagnetic steel than on non-ferrous alloys, and why magnetic saturation (a strong DC bias field) is often applied to drive permeability out of the picture.

• Lift-off is a change in the spacing between the probe and the surface. It causes a large impedance change that can swamp flaw signals, so it is nulled or phase-compensated. Usefully, lift-off is exploited deliberately to measure the thickness of a nonconductive coating or paint over a conductive base.
• Edge effect occurs as a surface probe nears an edge or geometry boundary, distorting the eddy current field and mimicking a flaw; operators keep the probe away from edges or use reference standards with matching geometry.
• Fill factor applies to encircling (feed-through) coils on tubes and bars — the ratio of the part's cross-section to the coil's inside area. A high fill factor (part nearly fills the coil) gives tight coupling and better sensitivity.

Probe and Coil Types

• Surface (pancake) probes for scanning flat or contoured surfaces and fastener holes.
• Encircling coils that surround a tube, bar, or wire for high-speed in-line inspection.
• Bobbin (internal) probes pulled through the bore of installed tubing (heat exchangers, condensers).
• Absolute coils (measure against a fixed reference) versus differential coils (compare two adjacent zones to highlight abrupt changes and suppress slow drift).

Applications and Limitations

ET needs no couplant, works at high speed, is easily automated, and gives an instant electrical readout, making it excellent for nonferromagnetic heat-exchanger tubing, conductivity and alloy sorting, coating-thickness measurement, and surface/near-surface crack screening — including through thin nonconductive coatings that would block penetrant. Its limitations frame every selection decision: it applies only to electrically conductive materials; penetration is shallow (surface and near-surface only, not deep internal flaws); it is highly sensitive to geometry, edges, and lift-off; it requires reference standards with known flaws for calibration; and signal interpretation demands a skilled operator. On ferromagnetic parts, saturation or a different method is usually needed.

Worked Example and a Common Trap

Consider inspecting aluminum tubing carrying a thin, non-removable paint coating for surface cracks near fastener holes. Penetrant is ruled out because the paint blocks the openings, and MT is impossible because aluminum is non-ferromagnetic — leaving eddy current as the best fit: it needs no couplant, screens at high speed, and reads through the thin nonconductive coating, with the paint's thickness simply adding a fixed lift-off that is nulled during standardization. This scenario is a classic Basic-exam item and shows why ET is prized for coated non-ferrous parts.

A frequently missed point is that higher signal amplitude does not always mean a bigger flaw. Because lift-off, edge effect, and conductivity all move the impedance-plane spot, an operator who watches only amplitude can mistake a lift-off wobble for a crack. The correct discipline is to standardize on a reference standard containing known notches or wall-loss zones, note the phase direction a real flaw produces, and reject signals whose phase matches lift-off or geometry instead of a defect. This is why ET is described as a comparative method that always demands calibration standards and a trained interpreter.