Eddy current testing is an electromagnetic technique and can only be used on conductive materials. Its applications range from crack detection, to the rapid sorting of small components for either flaws, size variations, or material variation. Commonly it is used in the aerospace, automotive, marine and manufacturing industries.
When an energized coil is brought near to the surface of a metal component, eddy currents are induced into the specimen. These currents set-up magnetic field that tend to oppose the original magnetic field. The impedance of coil in close proximity to the specimen is affected by the presence of the induced eddy currents in the specimen.
When the eddy currents in the specimen are distorted by the presence of the flaws or material variations, the impedance in the coil is altered. This change is measured and displayed in a manner that indicates the type of flaw or material condition.