Eddy current testing

f = 4(5) (.2)2 = 20 (.04) = 500 Hertz

If we plug that frequency into our depth of penetration calculation, you can see that all we have done is put the position of d at the bottom of this plate. That means that we would still have 37 percent of our original surface energy when the primary magnetic field creates eddy current at the back surface. That should give us plenty of detection capability.

Let's assume that our test procedure is being created for a possible corrosion problem. In a multilayered segment we typically let (t) equal just the upper layer thickness. When you recalculate the test frequency for this type of application you see that we have intentionally put (d) at the interface between the two layers. This optimizes the detection potential at that point without sacrificing detection below that point.

Coils Options
Test coils can be categorized into three main mechanical groups. These are probe coils, bobbin coils, and encircling coils. Each of these three basic designs might be driven or operated in one of three modes: absolute, differential, or hybrid. The coil configuration and mode of operation would tell us what type of screen response to expect.

Most of the coils that we use in aircraft component inspection would be addressed as "probe" coils. We are normally scanning across the surface of a material. The mode of operation for surface crack detection would typically be either absolute or differential, based on how the coil or coils were connected to the tester. A differential coil pair is normally used to reduce the effect on non-flaw related responses (such as edge effect).

For most subsurface flaw detection we find that a hybrid or driver-pickup coil tends to provide a better response. The hybrid coils are usually designed to provide a higher S/N ratio in a specific application. If we were trying to get better detection or characterization from second- or third-layer corrosion, a hybrid coil designed specifically for that type of test would probably perform better than selecting a normal surface probe and trying to do the same test.

Weld inspection with normal surface probes would be extremely difficult. Responses from the weld heat-affected zone (HAZ), localized conductivity changes due to recrystallization, weld metal content vs. base material composition (filler wire), variations in permeability, and variations in geometry (weld splatter, surface condition of crown, etc.) would all be occurring simultaneously. Probes have been designed to compensate for these variables that still maintain their sensitivity to cracks.

Eddy current coils are usually optimized for certain frequency ranges. This is due, in part, to certain physical design requirements of probes. In order to obtain an adequate flux density, for example, a low frequency probe requires a larger amount of wire compared to a high frequency probe. Very often, the designing of a probe becomes a compromise between several different factors, i.e., operating frequency, size limitations, flaw sensitivity, and durability.

You have already won half the battle if you can calculate a reasonable test frequency and choose a probe style that is going to provide you a reliable flaw response in the area of interest.

Eddy Current Testing

Coming of age

By Jim Cox

March 1998

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Signal Displays/ Calibration
Electromagnetic variations are detected by the eddy current system and displayed on some type of read-out mechanism. Meter-based testers are the simplest to use but have definite application-specific limitations. Cracking and corrosion issues for aging aircraft sometimes require the use of impedance plane (X-Y) displays.

When we are performing an eddy current test on an X-Y system we watch a dot on a screen. As long as that dot doesn't move that tells us that nothing is changing in the test environment. If the dot does move, we now have to try to understand how much the dot moved (signal amplitude) and in what direction the dot moved (its phase). If we can do that, then we can probably estimate "why" the dot moved. Hopefully, we can quantify that electromagnetic change to a specific change in the material properties. With that information, we can make our decision about whether or not this part is acceptable or rejectable.

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