Eddy Current NDE

Applications

Eddy current testing plays a critical role in ensuring the safety of energy supply systems and transportation, especially in the inspection of power plant and aircraft.   For example, it is used to detect cracks in steam generator tubes and welds. In aircraft structures, tests are carried out around fasteners, in fastener holes and on aircraft engines parts.

Principles

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Figure 1: Eddy current probe coil over a cracked plate.

In eddy current testing an inductive probe carry an alternating current has impedance that changes when it is placed on a conductor and if there is a crack in the
conductor it has a small but measurable effect on the probe impedance. This means that a crack in a metal can be detected by monitoring small impedance changes of an eddy current probe.

The probe senses the crack because it generates a magnetic field that drives an induced current near the surface of the metal. The current in the conductor also produces and magnetic field which has an inductive effect of the probe impedance.  Hence we have a two-way process of mutual induction between the probe and the conductor. The inductive process refers to the fact that a changing magnetic field produces and electric field to drive a current in the conductor and to modify the probe impedance.
The penetration of the magnetic field and the induce current into the conductor is limited by the skin effect. This effect means that the field decays rapidly with depth. A crack at, or near the surface, disrupts the current flow which is sensed by the probe through the effect on of this perturbation on the magnetic field.

Although deep subsurface flaws are difficult or impossible to detect due to the skin effect, eddy current NDE is sensitive to surface flaws and can be non-contacting.

It is helpful to be able to predict the signals derived from changes in eddy current probe impedance. The calculations enable us to get quantitative results on the effect of flaw geometry, material parameters and probe characteristics on the signals that are detected. Some models that calculate the field around the probe, in the conductor including the region of a flaw, take a long time to generate useful results and it may require specialist knowledge to set up the calculations. For this reason we prefer to develop semi-analytical methods which are fast, accurate and are easy to run.