Eddy current testing is an electromagnetic technique used on conductive materials to detect a number of conditions, including sub-surface defects, variations in material structure, thickness and corrosion. When an energized coil is brought near to the surface of a metal component , eddy currents are induced into the specimen. These currents create a magnetic field that tends to oppose the original magnetic field. Sub-surface defects will distort the eddy currents and alter the opposing magnetic field. By measuring how the impedance of the coil in close proximity to the specimen is affected by variations in the magnetic field, a skilled operator can precisely locate and classify sub-surface defects.
Eddy currents are subject to a "skin effect", meaning they flow in a thin skin beneath the surface adjacent to the coil. The thickness of this skin is determined by the operating frequency of the coil (usually 10Hz to 10Mhz) plus the conductivity and permeability of the material under inspection. Eddy current inspection is effective when the skin is as thin as 5μm or as thick as 10μm or more. By altering the operating frequency of the coil it possible to evaluate a specimen at various depths.
Eddy current inspection is a non-contact procedure that is unaffected by the presence of oils, paints and coatings on the test specimen. This greatly reduces preparation time prior to inspection.
Frequently used for on-site inspection for corrosion in heat exchangers tubes, eddy current testing is very fast — making it ideal for high volume sorting of small parts for flaws, size variations and material anomalies.
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