eddy-current testing

Eddy-current testing finds surface and near-surface flaws in metals without any contact, using nothing but a changing magnetic field. Bring a small AC-driven coil near a conductive part and it induces little swirling loops of current — eddy currents — in the metal, like whirlpools spinning just under the surface. A crack, a change in thickness, or a change in the alloy disturbs those whirlpools, and the coil feels the disturbance as a change in its own electrical behaviour.

Alternating current in the probe coil makes an alternating magnetic field; that field induces eddy currents in the metal, and those eddy currents make their own opposing field that pushes back on the coil, changing its impedance (its effective resistance to AC). A crack forces the eddy currents to detour, weakening them, and the coil registers the shift. Because the induced currents crowd near the surface (the skin effect), and shallower the higher the frequency, eddy-current testing is most sensitive to surface and slightly sub-surface defects, and the choice of frequency trades depth against sensitivity.

Eddy current is fast, needs no couplant, and works without touching the part, so it is ideal for high-speed automated scanning: sorting alloys, inspecting tubes in heat exchangers, checking aircraft fastener holes for fatigue cracks, and measuring coating thickness or conductivity. Honest limits: it works only on electrical conductors (useless on plastics and most ceramics), it reaches only shallow depths, its signals are affected by many variables at once (lift-off, geometry, conductivity, permeability) so they need careful calibration, and it is easily confused near edges.