Home Contact Sitemap

AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
    • About
    • Contact
    • Contributors
    • PDF Versions
    • Related Links
    • Sections
    • Examples

Section Eddy Current Inspection

The principles of electromagnetic induction are used in eddy current inspections to detect surface and near-surface cracks in electrically-conductive metals.  When an electrically-conductive material is subjected to an alternating magnetic field, small circulating electric currents are generated in the material.  Since these eddy currents are affected by variations in conductivity, magnetic permeability, mass, and material homogeneity, the conditions that affect these characteristics can be sensed by measuring the eddy current response of the material.  In practice, eddy currents are induced in the part to be inspected with a coil carrying an alternating current.  The induced eddy currents generate their own magnetic field, which interacts with the magnetic field of the exciting coil, and changes the impedance of the exciting coil.  By measuring the impedance of the exciting coil, or a separate indicating coil, the inspector can infer the presence of cracks in the material.

An important use of the eddy current NDI method has been in the detection of fatigue or stress corrosion cracks around fastener holes after the cracks have grown beyond the fastener head.  Special bolt hole probes have also been devised for use after the fastener has been removed for locating cracks emanating from the wall of the fastener hole.  This inspection process has been automated to remove operator influence, speed inspections, and produce a permanent inspection record.

Eddy current methods do not require contact with the specimen or clean up, and are generally faster than liquid penetrant and radiographic methods.  Although eddy current methods can detect both surface and subsurface cracks, the depth of inspection below the material surface is limited (approximately 0.25 in.).  Since eddy currents are influenced by many material variables, masked or false indications can easily be caused by sensitivity to part geometry, lift-off, edge effects and permeability variations.  Finally, eddy current methods require well-trained operators to man the test instruments and reference standards are necessary.