Practical Testing using Eddy Current NDT

For any Eddy Current Non Destructive Testing Application you will need the following equipment and probes:

  • A suitable probe.
  • An instrument with the necessary capabilities.
  • A good idea of size, location and type of the flaws it is desired to find.
  • A knowledge of the material conductivity and whether it is magnetic or not.
  • A suitable test standard to set up the equipment and verify correct operation.
  • A procedure or accept/reject criteria based on the above.
  • The necessary operator expertise to understand and interpret the results. 

Operating Frequency

Selection of operating frequency is the primary eddy current test parameter under operator control. Frequency selection affects both the relative strength of response from different flaws and the phase relationship. Thus, selection of operating frequency is very important in obtaining good resolution of flaw signals in the presence of other variables which may affect the test. 

Instrument Set Up

While the precise details of setting up an instrument will vary

depending on the type and application, the general procedure is usually the same. Once the application has been tested the required values for many test parameters will be known, at least approximately.

  1. Connect up the appropriate probe and set any instrument configuration parameters (mode of operation, display type etc.).
  2. Set the frequency as required for the test.
  3. Set gain to an intermediate value, for example 40 dB.
  4. Move the probe on/over the calibration test-piece and set phase rotation as desired (for example lift-off or wobble horizontal on a phase plane display). It may help the stability of the readings to attenuate the horizontal (x axis) gain by 12 dB (1⁄4 of the vertical gain).
  5. Move over the defects and adjust gain (and horizontal/vertical gain ratio if fitted) to obtain the desired trace size/meter indication. It may be necessary to re-balance after changing gain.
  6. Further optimise phase rotation as required by setting the dominant source of noise whilst scanning the probe in the horizontal axis.
  7. Use filters etc. to further optimise signal-to-noise ratio (see below).
  8. Set alarms etc. as required.
  9. Run over the calibration test-piece again and verify that all flaws are clearly detected.
  10. Perform the test, verifying correct operation at regular intervals using the calibration test piece. 

Use of Filtering

Searching for defects in an eddy current test conventionally implies probe movement. So when indications are detected then, due to the probe size, these will vary with time in a way which is fairly consistent (assuming that the probe movement speed is reasonably constant). As a result of this speed and the probe size, defects have a characteristic frequency of response (probe width divided by probe speed).

For example, if an absolute probe with diameter 2 mm moves over a narrow crack at a speed of 1 m/s the resulting indication will last for approximately 2 ms. If the material composition, thickness or probe lift-off is also varying gradually, the indication from this will change much more slowly. Therefore, a high-pass filter set to a frequency around 100 Hz or so will pass the rapidly changing signal from the defect but not the slowly varying changes.

Further rapidly varying signals such as electronic noise or noise caused by surface roughness may be reduced by low-pass filtering. It is good practice to ensure that the low-pass filter is set sufficiently low to ensure the test signal displays the lowest amount of high-frequency noise but high enough to ensure that the smallest target defect is not attenuated by the filter. 

If you have any questions regarding Eddy Current NDT testing, please do not hesitate to contact the team at ETher NDE at sales@ethernde.com and we will be happy to answer your questions.

You can see what Eddy Current equipment from ETher NDE suits which application by taking a look at our equipment and probes versus application table.

 

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