Ever had to inspect a surface with eddy currents only to realize that your results are not quite right? One of the most difficult things about using eddy current arrays (ECA) in surface applications is selecting a probe that adequately matches conditions. But it doesn’t have to be. Here are 8 things that you should know to make your life easier.
It’s not enough to know that it’s made out of stainless steel, for example. As you may know, there are various types of stainless steels and they all have their own very specific electromagnetic properties (e.g., 300-series stainless steel is non-ferromagnetic, while 400-series is, and both series have exceptions), which can influence the configuration of the selected probe and affect inspection results.
2. What’s the shape of the surface under test?
Is it completely flat? Is it concave or convex? If so what’s the radius? Does it have a unique or distinctive shape? Does it have an uneven geometry? Having CAD or clear pictures of the surface to be tested can make short work of determining the level of flexibility necessary of your probe.
3. What are the precise types of flaws you’re looking for?
Looking for corrosion isn’t like trying to characterize cracking. Knowing whether you’ll be inspecting for pitting, stress-corrosion cracking, general corrosion, porosities, or something else helps choose the right inspection frequencies and topology — that is, how the coils are organized and activated inside the probe.
4. Typically, how large are the flaws you expect to find?
Knowing approximately how large the target flaws are will help in choosing a probe of the appropriate coverage and resolution for your inspection needs. High-resolution probes are excellent at detecting very small flaws, but flaw location, direction, and surface conditions, as well as the available number of channels may call for compromises that would make such a probe inadequate.
5. Where do you expect to find your flaws?
Near the surface? On the far side of your surface? Somewhere in between? Knowing this also helps in pinpointing the correct operational frequencies and topology for your probe.
Is the surface covered in lubricant, lathered with paint, rusty, rough? Knowing this is essential because it’s going to dictate the type of contact interface you’ll want and its durability. Surface conditions also have an impact on the minimum size of flaws that can be detected by a given ECT/ECA probe. Looking for very small defects on rough surfaces is all about compromise.
8. What are the environmental conditions where you’ll be using the probe?
Is the probe to be used in a briny, radioactive, dusty, or controlled environment? Is the probe to be used near welding equipment, near magnetic sources, underwater? Knowing the type of environment helps to determine what types of mechanical designs are suited to you.
Although not immediately essential, it may come in handy:
- Was the material under test mechanically, thermally, or chemically treated?
- How thick is the surface?
- Was the surface cast or machined?
These are all factors that can modify material properties and impart magnetic properties to materials that shouldn’t have them.
Having the above information on hand makes shopping for that “perfect” probe all the much easier for you or your inspection service provider. It will also make it considerably easier for your probe supplier to accurately assess whether you can use a standard probe or if you need one that is specifically tailored to your inspection needs. The good news is, however, that a wider-than-ever combination of conditions are covered by standard ECA probes, so with the right information, it’s never been easier to shop for off-the-shelf probes.