Eddy Current Testing
Near surface flaw detection
By Jim Cox
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 tools to work with (some wire, a magnet, a compass, and perhaps a crude battery), and yet they were able to derive the theoretical and mathematical relationships that define the eddy current testing process as it is still used today.
The basic rules of how an eddy current test works are well established. If we can create a changing magnetic field near an electrically conductive material (metal) then we will induce (electromagnetic induction) a current (eddy current) to flow in that metal. The relationship between the energy that we use to create the test (primary forces) and the resultant current and magnetic field in the material (secondary forces) allows us to begin to measure certain characteristics of that piece of metal. If the metal is completely homogenous (no cracks, no corrosion), then the induced current will flow unobstructed in the metal. If the metal is somehow degraded or damaged, the eddy currents can't flow in the same way. This change in their flow pattern is detectable and can be displayed by the eddy current test system.
In today's world of electronics, our coils are energized with an alternating current (AC) to create a varying magnetic field around the test probe. Coils are usually designed and optimized for specific tasks. Eddy current testing (ECT) systems allow us to do real-time analysis by watching a trace or dot move on an X-Y screen. We get direct feedback about the coils' energy and how it is affected by the test sample.
Material and Test Parameters
One of the historical arguments against using eddy current testing is that it is too sensitive. The signal changes that we might detect during our inspection could be created by some combination of material changes. The possible variables include the material's electrical conductivity, or its magnetic permeability, and/or the geometrical factors that are encountered while performing the examination. With that many possible responses, how do we figure out what really changed?
Conductivity (s) is an electrical property. It determines how well electrons will move through a material. Metals are normally classified as conductors. Theoretically, we could do an eddy current test on any piece of metal.
Resistivity (r) is another term that we can use to describe the electrical properties of a material.
Permeability (m) is a magnetic property.
Not all metals have significant levels of permeability. The permeability value of a metal determines how it will alter a magnetic field moving through it. In metals with high permeability values (carbon steels) the eddy current test process is only capable of detecting material changes that occur right on the surface immediately in front of the probe. Fortunately, the largest percentage of aircraft structures and surfaces are made from non-ferromagnetic materials (i.e., aluminum). This means that the magnetic field created by the coil moves through aluminum easier than if we were trying to inspect a piece of steel with the same test conditions. In aluminum we can detect changes that are much deeper in the material. The normal limit for this test process is about .2 to .3 inches into the nonferromagnetic materials.
The third major test variable is geometry. In surface scanning operations we typically define our geometry response as Lift-Off. This will occur any time that the coil detects a "change in the spacing" between the coil and the material. This type of signal would be created by the operator if the probe were rocked from side to side by just a few degrees. Geometry responses are considered to be a limiting factor in our ability to detect true material changes. Anything that we can do to limit these geometry responses during the test will improve the quality of the test.
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