Eddy Current Testing
Coming of age
By Jim Cox
During the period from 1775 to about 1900, several scientific experimenters investigated the many facets of electromagnetism. If we think about the tools that they must have used in that time frame we will soon realize that this stuff called eddy current testing must be pretty simple. They didn't have computers, electric lights, or even 110 power to plug into. A couple of guys sitting around with some magnets, a compass, a crude battery, and some different pieces of metal were able to figure out what it is and come up with the rules and formulas that we still use today!
Figure 1 shows a model of a magnetic field starting to move across a block of electrically-conductive material. You can do the same experiment with a magnet on a string. This was first documented by a man named Arago. He discovered that the motion of a swinging magnet was rapidly slowed when a nonmagnetic, electrically-conductive block was placed under the magnet. (Hint: try both stainless steel and copper.) This is an eddy current (EC) test. The relative field strength and the number of flux lines cutting into the metal depend upon the position of the magnet.
Over a given time period there is a changing magnetic field at any given point in the metal block. The changing magnetic field strength forces electrons in the material to start moving, i.e., current flow. When a current is flowing in a conductor it will also create a magnetic field around itself (Lenz's Law -— the Right Hand Rule). The (eddy) current flow in the block produces a "secondary" magnetic field that is opposite to the field that created it. These oppositely charged fields (north vs. south) will attract each other. The magnet swing is slowed based on the attractive force between these two fields. This simple model is the basis for some automobile speedometers.
Waving a magnet over the surface of a piece of material is probably not a very efficient EC test process. In today's technology we use specially wound coils. Each is energized with an alternating current (AC) power source to create a changing magnetic field around the coil. Eddy current testing is just a process of monitoring how two magnetic fields react to each other. The primary field (created around a coil) and the secondary field (created in the material by the eddy current flow) oppose each other. This means that when one is a "south" pole the other is a "north" pole. The secondary field is trying to "shut down" the primary field.
If any of the test or material properties (such as conductivity, permeability, or geometry) are changed, then the amount of eddy current flow created in a metal specimen must also change. If the strength of the eddy current flow changes then the strength of the secondary magnetic field created by the current flow must also change. Since these two magnetic fields are oppositely charged, variations in the secondary field strength cause a shift in flux distribution and strength of the primary magnetic field around the coil. This, in turn, changes the coil's electrical parameters. It is a cascade event. As one part of the picture changes, everything else shifts in response.
One of the historical arguments against using eddy current testing is that it is too sensitive. The signal that we detect during our inspection might be created by some combination of material changes. The possible variables include the material's conductivity, or its magnetic permeability, and/or the geometrical factors of how we are performing the examination. 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. Resistivity is more common in the formulas shown here.
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