Eddy-current testing

Eddy-current testing is a method of nondestructive testing that uses electromagnetic induction to detect irregularities in conductive materials.

In a standard eddy current testing a circular coil carrying current is placed in proximity to the test specimen (electrically conductive). The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current. Variations in the phase and magnitude of these eddy currents can be monitored using a second 'search' coil, or by measuring changes to the current flowing in the primary 'excitation' coil. 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 flow 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.

Uses
Eddy-current testing can detect very small cracks in or near the surface of the material being examined, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements. The testing devices can be made small and portable, can provide immediate feedback, and do not need to contact the item in question.

Limitations
There are several limitations, among them:
 * only conductive materials can be tested,
 * the surface of the material must be accessible,
 * the finish of the material may cause bad readings,
 * the depth of penetration into the material is limited,
 * flaws that lie parallel to the probe may be undetectable.