Eddy Current Testing (ECT) works on the principle of electromagnetic induction to detect flaws in conductive materials. In a standard ECT a circular coil carrying current is placed in proximity to the test specimen (which must be electrically conductive).The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy currents within the material. Anomalies within the material will cause variations in the phase and magnitude of these eddy currents which are monitored by a second ‘receiver’ coil within the probe.
Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique. Common applications in eddy-current testing include the detection of very small cracks in or near the surface of the material. Other advantages of this method are that the surfaces that are to be inspected need minimal preparation, and complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.