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What are the advantages and disadvantages of eddy current testing?

Frequently Asked Questions

The advantages of eddy current testing are:

Sensitivity to surface defects. Able to detect defects of 0.5mm in length under favourable conditions.

Can detect through several layers. The ability to detect defects in multi-layer structures (up to about 14 layers), without interference from the planar interfaces.

Can detect through surface coatings. Able to detect defects through non-conductive surface coatings in excess of 5mm thickness.

Accurate conductivity measurements. Dedicated conductivity measurement instruments operate using eddy currents.

Can be automated. Relatively uniform parts can be inspected quickly and reliably using automated or semi-automated equipment, e.g. wheels, boiler tubes and aero-engine disks.

Little pre-cleaning required. Only major soils and loose or uneven surface coatings need to be removed, reducing preparation time.

Portability. Portable test equipment is very small and light, some of the latest equipment being as small as a video cassette box and weighing less than 2kg.

The disadvantages of eddy current testing are:

Very susceptible to magnetic permeability changes. Small changes in permeability have a pronounced effect on the eddy currents, especially in ferromagnetic materials. This makes testing of welds and other ferromagnetic materials difficult but, with modern digital flaw detectors and probe design, not impossible.

Only effective on conductive materials. The material must be able to support a flow of electrical current. This makes testing of fibre reinforced plastics unfeasible.

Will not detect defects parallel to surface. The flow of eddy currents is always parallel to the surface. If a planar defect does not cross or interfere with the current then the defect will not be detected.

Not suitable for large areas and/or complex geometries. Large area scanning can be accomplished, but needs the aid of some type of area scanning device, usually supported by a computer, both of which are not inexpensive. The more complex the geometry becomes, the more difficult it is to differentiate defect signals from geometry effect signals.

Signal interpretation required. Due to the many factors which affect eddy currents, careful interpretation of signals is needed to distinguish between relevant and non-relevant indications.

No permanent record (unless automated). Normally the only permanent record will be a paper print out or computer file when using automated systems.

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