Eddy Current crack detection units are normally limited to surface defects but are also sensitive to changes in shape, heat treatment, coating thickness and the edges of a part. The effective depth of penetration of eddy current is usually less than 2mm. Eddy currents are particularly useful for detecting surface cracking.
Whereas all the previously mentioned methods are suitable for detecting defects over the entire surface of a part, eddy current techniques must be established to identify a particular size discontinuity, in a specific location on a part. This requires the use of reference standards, usually with a known defect similar to that for which the inspection is being set up. The instrument must be calibrated prior to inspection using the reference standards. The reference standards will tell the operator what type of indication to expect from an actual defect thus increasing operator confidence and the reliability of the inspection.
Eddy current of a weld scan
Once calibrated, eddy current inspection is both quick and clean as readings are instantaneous and no oils or contaminating liquids are required. Thin, sound coatings need not be removed except perhaps to smooth any surface roughness. Eddy current instruments are available to gauge coating thickness and metal conductivity as well as for crack detection.