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