Eddy Current Testing

Principles. Based on electromagnetic induction, eddy current testing is used to identify or differentiate among a wide variety of physical, structural and metallurgical conditions in electrically conductive ferromagnetic and nonferromagnetic metals and metal parts. The method is based on indirect measurement and on correlation between the instrument reading and the structural characteristics and serviceability of the parts being examined.

With a basic system, the part is placed within or adjacent to an electric coil in which high frequency alternating current is flowing. This excitation current establishes an electromagnetic field around the coil. This primary field causes eddy current to flow in the part because of electromagnetic induction. Inversely, the eddy currents affected by all characteristics (conductivity, permeability, thickness, discontinuities and geometry) of the part create a secondary magnetic field that opposes the primary field. The results of this interaction affect the coil voltage and can be displayed in a variety of methods.

Eddy currents flow in closed loops in the part or air. Their two most important characteristics, amplitude and phase, are influenced by the arrangement and characteristics of the instrumentation and test piece. For example, during the test of a tube the eddy currents flow symmetrically in the tube when discontinuities are not present. However, when a crack is present, then the eddy current flow is impeded and changed in direction, causing significant changes in the associated electromagnetic field.

Applications. An important industrial use of eddy current testing is on heat exchanger tubing. For example, eddy current testing is often specified for thin wall tubing in pressurized water reactors, steam generators, turbine condensers and air conditioning heat exchangers. Eddy current testing is also used often in  aircraft maintenance. The following are some of the typical material  haracteristics that can be evaluated by eddy current testing: cracks, inclusions, dents and holes; grain size and hardness; coating and material thickness; dimensions and geometry; composition, conductivity or permeability; and alloy composition.