In order to do that we have to go through a "calibration" process when we start this test. We need to be able to recognize certain types of signals and organize them on our read-out mechanism. This will create a logical, repeatable pattern of responses so we can train our visual and mental process to recognize the differences between signals that are "flaw-like" and those that are normal or expected non-flaw information for an acceptable specimen.
We are always going to have some geometry response as we move the probe across the test specimen. We always try to limit our visual sensitivity to geometry by putting that response at a known position on the screen, normally horizontal. Permeability changes are not a major factor in most aircraft examinations. That only leaves conductivity as a variable. Cracks and corrosion exhibit a change along what is referred to as the conductivity line. Once we understand this relationship, it is a fairly straightforward process to decide what is a true flaw and what isn't.
Any discontinuity that appreciably changes the normal eddy current flow can be detected. Discontinuities (cracks or corrosion) generally cause the effective conductivity of the test object to be reduced. If we look at the arrangement of signals on the screen in Figure 3, this would mean that any flaw would be represented by a positive or upward response on the screen. Any response that was only side-to-side (horizontal) would be related to a liftoff or geometry-related event. Discontinuities open to the near-surface would be much more easily detected than subsurface or far-surface discontinuities.
The displayed signal change due to discontinuities will vary based on the size (volume) and location (phase) of the signal source. Phase is normally equated to depth or location within the specimen. Voltage is more dependent upon total volume (length, depth, width) of the discontinuity.
In the test results shown in Figure 4 we would need to look at our procedure. We may not be meeting the required flaw sensitivity. The tester has been "calibrated" properly, but it is likely that the frequency choice was not correct. If we need to be able to detect cracks throughout this plate, then the fact that we cannot get a reasonable response from crack C is a major problem. Notice that at whatever frequency we were operating, we had a depth of penetration (1d) of about one-fourth of the way through the block. When we scan over crack C we have very little energy left (3d= 5 percent). We used 95 percent of our energy just to reach this point in the block. What if we were to decrease our operating frequency? Wouldn't that give us more depth of penetration into the sample? This should put more energy deeper in the plate and we should get a larger amplitude response from crack C.
That would not be the only signal change. Remember that since we are working at a new depth of penetration, the phase angle of all of the responses would be different. The advantage would be that the new phase angle response of crack C should be more like the original response that we saw from the B indication at the original frequency. By running at a lower frequency we have dramatically improved our test for crack indications throughout this plate.
Let's wrap it up
What can we really say about eddy current testing?
• Eddy current testing is not a black-box art form. It has many different applications. It can provide material sorting capability, it can measure material properties (i.e., conductivity in percent IACS), or it can be used to detect different types of service-related damage in a wide range of structures.
• ECT can be used on many different metallic surfaces. Within reason, you don't need to remove paint or other protective coatings to perform the test.
• ECT works best for near-surface indications but can also be used to detect subsurface cracking and corrosion. We don't normally concern ourselves with sizing front surface flaws. Once you detect a surface crack, the acceptance criterion becomes one of location and probability of crack length extension. It is possible to set up sizing criteria for subsurface corrosion issues. This is becoming more and more common in aging aircraft inspection procedures.
• ECT requires trained personnel to understand it and apply the techniques properly. This is especially true if you don't have a dedicated procedure defining exactly how to set up a given test system for a given application. A good manufacturer's applications lab can assist you in making the right choices or at least let you know if the test is feasible. Some provide this service free of charge.
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