Eddy Current Testing (ECT) is a form of the Electromagnetic Testing (ET) method usually used for the detection of surface-breaking or near-surface defects in materials. It is typically used to perform the inspection of non-ferromagnetic metals, although the material under test can be a ferromagnetic metal. ECT is also less commonly used to inspect composite materials such as carbon fiber reinforced polymer (CFRP).
An electrical conductor (usually a copper coil) is placed in a circuit where an AC current passes through it, resulting in a magnetic field being generated around the coil based on the principle of self-inductance. The right-hand rule determines the direction of the magnetic field. The magnetic field strength depends on the magnitude of the coil’s excitation current, as well as a frequency equal to the AC frequency of the excitation current.
When the coil is placed in the proximity of a metal sample, eddy currents are induced in the material. Eddy currents in materials are analogous to eddy currents in fluids and thus how they received their name. In electromagnetic induction, eddy currents flow in a direction opposite to the coil current and thus also producing a counter force, or “secondary” field. The circuitry in an ECT system is normalized (a.k.a., “balanced”) to this opposing secondary field.
If the coil is moved to a location on the sample with a defect, the flow of eddy currents is disrupted, causing their paths and density to change. A subsequent change in the secondary magnetic field strength causes the balance of the system to change, which is then registered as a change in the impedance in the coil. A typical ECT system represents that change as a voltage measurement. Modern permutations of ECT technology include Eddy Current Array (ECA), Remote Field Testing (RFT), Magnetic Flux Leakage (MFL), and Pulsed Eddy Current (PEC). While the Electro Magnetic Acoustic Transducer (EMAT) technique relies on ultrasonic waves rather than eddy currents, it shares the same fundamental theories as Eddy Current Testing.