Advanced Concepts for Eddy Current NDT Applications

As well as being able to be used across a multitude of standard Eddy Current (ECT) NDT Applications, there are a selection of non destructive testing applications that Eddy Current NDT can be used for including:

Remote Field

Remote field is a branch of eddy current testing that has evolved over the last decade or so. By using specially designed equipment and ID probes it is possible to obtain indications of wall thickness changes on magnetic material. 

Simultaneous Use of Multiple Frequencies

Choice of frequency determines how well surface and subsurface defects may be detected. By using more than one frequency it is possible to achieve both good detection of surface defects and sub-surface defects. Further, generally the more information that is available from a test, the easier it is to categorise difficult-to- interpret defects (for example ferrous inclusions in non-ferrous material).

Mixing of signals from a test at two frequencies allows unwanted signals to be suppresses, for example support plate signals in heat exchanger inspection. 

With a single frequency ID Probe, support plate and wobble signals (shown in black) mask a small flaw signals, especially near the support plate.

With two test frequencies the support plate signal can be removed by mixing.

Illustrations explained: A mix exploits the changes in phase separation and amplitude that occur when testing at different frequencies on an unwanted signal. By suitable manipulation of the phase and x/y gain it is possible to minimise the signal from the unwanted signal, after subtraction of the two frequencies whilst maintaining sensitivity to wanted signals. 

Simutaneous Use of Absolute and Differential Tests

Differential testing is excellent for finding small defects but can be poor at detecting very large defects.

By testing simultaneously in absolute and differential then it is possible to preserve good sensitivity to small defects (for example cracks, small pits) in the differential channel and large defects (for example corrosion, material property changes) in the absolute channel.

The risk in both simultaneous testing modes is that the data becomes more complex to analyse whilst not greatly improving the reliability of defect detection. 

Spatial Considerations in Selecting Probes

Probe geometry has an influence on the efficiency of an eddy current test.

Smaller probe elements will give better signals from smaller defects. Shielded probes will further improve this.

Larger probes will allow the eddy current signal to penetrate more deeply (probe diameter should be typically two times higher or more than the depth of the material to be penetrated. Note this means that choice of frequency is not the only consideration in determining depth of signal penetration.

Noise Sources and How To Minimise Them

There are numerous noise sources in eddy current testing but they may be summarised as follows: 

  • Intrinsic electronic noise from the instrument electronics
  • External electronic noise also known as electromagnetic interference
  • Noise from the material, in that noise can be defined as any signal that may obscure the signal that it is required to detect.

A probe that is well matched to the instrument being used will produce a better signal-to-noise ratio (NOTE: signal-to-noise ratio is the ratio of the wanted to unwanted component, usually expressed in dB). Further larger probes will be better at averaging out noise caused by surface roughness and other small variations.

By ensuring that the probe drive signal is as high as possible,  then both forms of electronic noise may be minimised. The probe type and whether the instrument electronics become saturated will limit this.

External electronic noise is also influenced by the quality of the cables used and earthing of both the test-piece and the instrument.

Similarly, the intrinsic electronic noise may be further minimised by using as high an input (or pre-amplifier gain).

Filters may be used to ensure the signal is detected using the smallest possible bandwidth (NOTE: bandwidth is the numeric difference between the low and high-pass filters).

The setting of the lift-off signal in the horizontal is one simple form of optimising the removal of unwanted material signals.

Using two frequencies or more to minimise unwanted material signals by mixing can further improve the situation but the electronic noise will generally decrease by 6 dB for each mix. 

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