Eddy Current Testing: Near surface flaw detection

Eddy Current Testing Near surface flaw detection By Jim Cox September 2000 As far back as the late 1700's, scientific investigation into electromagnetism was conducted by a number of creative people. They didn't have very sophisticated...

Signal Analysis
The system should now be ready to look at our test samples that may contain flaws. Scanning of the critical regions where damage might typically occur needs to be performed carefully to assure complete coverage of those critical regions. If the probe is moved on the surface and the dot doesn't move (very much) that tells us that nothing is really changing in the test environment. If the dot does move, we have to try to decide why.
GraphicWe 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 material conductivity as a variable. Cracks should 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.
Discontinuities 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 2 this would mean that any flaw would be represented by a "positive" or upward response on the screen from the balance point (NULL). Any response on the horizontal would be related to lift-off (geometry).

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.

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