Eddy current depth of penetration

 

Many customers come to ask about what is the minimal defect’s depth that our eddy current solutions are able to detect. There is always a magic number: 30 microns. In other words: Is your technology able to detect defects of 30 microns depth?

Before giving an answer, let’s write a little about what penetration depth means. This is not a scientific post but technical based on some scientific aspects, conventions, and finally the experience.

To get a clear understanding of this, first we’ve to see briefly the concept of penetration depth. It’s well known that when an electrical current flows in conductive materials, its density decreases in the material, being maximal on the surface. This decrease follows a curve similar to the image. This is because electrons have less resistance to free movements on the surface that is called the skin effect.

 

 
 

The standard penetration depth is the depth where the density of current (A/m2) is reduced to 37% of the surface’s density, and it’s considered the maximal distance where the effect of eddy current can be significantly appreciated to have good results in nondestructive testing using eddy current. So this is the limit for everybody that we can consider the phenomenon of eddy current enough relevant. So anyone can't assure that they can detect defects beyond the penetration depth of the material, merely because beyond this, there isn’t enough density of current flowing in the material.

 

We said before that this isn’t a scientific post, but we need to take a look of the following equation, just to understand, from a high level point of view, which are the factors that affects the eddy current testing. For a given material:

  • Standard penetration depth depends on the frequency of the testing.  When frequency increases, the standard (ST) depth of penetration decreases. For example, high alloy steel at 20 kHz has a (ST)penetration depth about 120 microns. The same material at 200 kHz has a (ST) penetration depth about 30 microns.
  • (ST) Penetration depth depends on the magnetic permeability of the material. When this permeability increases, penetration depth decreases.
  • (ST)Penetration depth also depends on conductivity of the material. When conductivity increases, penetration depth decreases.

However, there are other environmental aspects that are even more important in eddy current inspection.

Surface conditions. Every material during the production process has a roughness (or more generally surface condition) and vibrations coming from the line. The eddy current equipment manages them as “noise”. So, one of the most important ratios in eddy current testing is noise-signal ratio. Below the level of “noise” isn’t really difficult to detect defects, not impossible but difficult in industrial conditions. For industrial applications, the noise-signal ratio must be enough clear.

Lift off control. Another important variable is the lift off (also called gap) between the test coil and the material. Having a right gap control is essential to detect small defects.

The temperature of the specimen to test, which impacts on conductivity among other physical properties, is, of course very important.

And finally, and not less important, it depends also of the width of the specimen to test. If depth of penetration is bigger than the width, our testing could be altered by the “other side” effect. 

 

We could be writing about this topic for weeks, but taking in account only these few elements, we can already answer the question related to:  “Is ISEND able to detect 30 microns depth defects with its technology?"

The simplest answer is yes, ISEND can do it. Under well controlled conditions (usually laboratory).
But then, you should ask yourself if your industrial conditions allow this.  For instance, if your roughness is bigger than 50 microns,  or you have significant vibrations, or your temperature changes along the testing, or your coil's holder can’t control lift off…, etc, etc. 
Our recommendation, based on all that have been exposed, and mainly, in our experience is: don’t try to identify small defects without a high probability of get true positive; it’s better to start with bigger defects with a reduced probability of false positives.

ISEND designs, manufactures and deploys eddy current solutions for industrial applications.  

 
 

 

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