Eddy Current Testing
Near surface flaw detection
By Jim Cox
Material and Test Parameters
One of the historical arguments against using eddy current testing is that it is too sensitive. The signal changes that we might detect during our inspection could be created by some combination of material changes. The possible variables include the material's electrical conductivity, or its magnetic permeability, and/or the geometrical factors that are encountered while performing the examination. With that many possible responses, how do we figure out what really changed?
Conductivity (s) is an electrical property. It determines how well electrons will move through a material. Metals are normally classified as conductors. Theoretically, we could do an eddy current test on any piece of metal.
Resistivity (r) is another term that we can use to describe the electrical properties of a material.
Permeability (m) is a magnetic property.
Not all metals have significant levels of permeability. The permeability value of a metal determines how it will alter a magnetic field moving through it. In metals with high permeability values (carbon steels) the eddy current test process is only capable of detecting material changes that occur right on the surface immediately in front of the probe. Fortunately, the largest percentage of aircraft structures and surfaces are made from non-ferromagnetic materials (i.e., aluminum). This means that the magnetic field created by the coil moves through aluminum easier than if we were trying to inspect a piece of steel with the same test conditions. In aluminum we can detect changes that are much deeper in the material. The normal limit for this test process is about .2 to .3 inches into the nonferromagnetic materials.
The third major test variable is geometry. In surface scanning operations we typically define our geometry response as Lift-Off. This will occur any time that the coil detects a "change in the spacing" between the coil and the material. This type of signal would be created by the operator if the probe were rocked from side to side by just a few degrees. Geometry responses are considered to be a limiting factor in our ability to detect true material changes. Anything that we can do to limit these geometry responses during the test will improve the quality of the test.
1. If the system allows for multiple modes of coil drive operation (absolute, differential, driver/pick-up) then determine which is correct for this particular surface probe.
2. Select a frequency that is within the specified range of the coil.
3. Set the gain control to a setting that is in the low to medium range of its scale.
4. Place the probe on a good (non-flawed area) of the "cal" block.
5. Perform the electrical "balance" or "Null" function provided on the system.
At this time you should see a dot on the screen.
6. Rock the probe slightly from side to side to create a "lift-off" response. You do not have to completely remove the probe from contact with the surface. Note the direction of the lift-off response.
7. Rotate the "phase" adjustment on the system so that the lift-off response leaves the screen directly to the left. This would be the nine o'clock position on a clock. In ECT, we call that screen position "zero degrees" or we say that lift-off is now "on the horizon." Ninety degrees is at the 12 o'clock position and we continue through the range until we reach 360 degrees back at the 9 o'clock point.
8. Keeping the probe in a vertical position, move it slowly over the range of discontinuities on the standard. These will probably be EDM notches that represent cracks. The screen response should be a rapid movement of the dot into the upper left-hand quadrant of the screen.
Once you can verify that you have flaw detection capability then the only issues that remain are how to optimize the signal variation between flaw and non-flaw conditions. You may need to adjust gain to increase signal amplitude from the smaller EDM notches. You might want to alter the V/H ratio to improve the displayed signal-to-noise ratio. Keep in mind that filters can be used in some applications to improve flaw sensitivity but they should never be used with any manually controlled scanning process.
ECT requires trained personnel to understand it and apply the techniques properly. This generic discussion has been limited to detection of near-surface cracking in non-ferromagnetic alloys. However, there are many applications of ECT. Weld inspection, near and far surface crack detection, sub-surface corrosion detection and sizing, material properties measurements (conductivity), hardness measurement, thickness measurements, etc., can all be accomplished when the correct style of probes and techniques are applied.
There are a lot of misconceptions about the limitations of eddy current testing. With the advances in ECT equipment and probe technology that have been achieved in the last few years, it is now possible to apply ECT to many test scenarios that previously could not be handled. Inspection procedures and codes that have been in place for a number of years probably do not address these newer options. The eddy current method, when combined with modern tools and techniques, may actually be faster, cheaper, and even more sensitive to the defined flaw conditions than those other NDE testing processes that are specified or authorized. Even if ECT is not addressed in your inspection criterion, you might want to give it a try.