TWI Knowledge Summary

Eddy Current Testing

How does it work?

In standard eddy current testing, a circular coil carrying an AC current is placed in close proximity to an electrically conductive specimen. The alternating current in the coil generates a changing magnetic field, which interacts with the test object and induces eddy currents. Variations in the phase and magnitude of these eddy currents can be monitored using a second 'search' coil, or by measuring changes to the current flowing in the primary 'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.

The penetration depth of eddy currents is indicated by a parameter known as the 'skin depth'. This is dependent upon operating frequency (lower frequencies give deeper penetration) and specimen conductivity and permeability, and is typically between 5 µm (0.2 thou) and 1 mm (0.04") or more. For most inspection applications, eddy current probe frequencies in the range 1kHz to 3MHz are used.

To help simplify the often complex eddy current response, changes in amplitude and phase are displayed on an impedance plane diagram (a plot of system inductance against resistance). In this way, changes in operator variability, such as the distance between the probe and the test piece (lift-off) will cause a horizontal shift in the spot forming the trace, while the presence of any flaws causes the spot to shift vertically.

What will it find?

Standard eddy current testing is essentially a near-surface technique. It is useful for detecting surface-breaking or near-surface cracking and variations in material composition. It can also be used to measure the thickness of non-electrically conductive coatings on electrically conductive substrates. In most steels, eddy current testing is limited to surface examination due to the relatively high permeability of these materials. The inspection of welds in ferritic steels can be problematic as the response is dominated by changes in the magnetic permeability across the weld. However, special types of probes have been developed (differential coils) to lessen the effects to material changes and permit the detection of small flaws.

Where is it used?

In general, standard eddy current methods are only used in plant inspection for non-ferritic materials (where eddy current penetration is deeper) or for special applications, such as in the inspection of heat exchanger tubing for cracking or corrosion thinning.

Further information

TWI offers training courses on eddy current testing.

You can use the Weldasearch literature database to supplement what you find in JoinIT.